








     An   Advanced  4.4BSD  Interprocess  Communication
                          Tutorial


                     Samuel J. Leffler
                      Robert S. Fabry
                       William N. Joy
                        Phil Lapsley
              Computer Systems Research Group
 Department of Electrical Engineering and Computer Science
             University of California, Berkeley
                Berkeley, California  94720



                        Steve Miller
                        Chris Torek
              Heterogeneous Systems Laboratory
               Department of Computer Science
            University of Maryland, College Park
                College Park, Maryland 20742


                          ABSTRACT

          This document provides an introduction to the
     interprocess communication facilities included  in
     the 4.4BSD release of the UNIX* system.

          It  discusses the overall model for interpro-
     cess communication and introduces the interprocess
     communication  primitives which have been added to
     the system.  The majority of the document  consid-
     ers  the  use  of  these  primitives in developing
     applications.  The reader is expected to be famil-
     iar  with  the C programming language as all exam-
     ples are written in C.











-----------
* UNIX is a trademark of UNIX System Laboratories,
Inc.  in the US and some other countries.









PSD:21-2                        Advanced 4.4BSD IPC Tutorial


                      1. INTRODUCTION


One of the most important additions to UNIX  in  4.2BSD  was
interprocess   communication.   These  facilities  were  the
result of more than two years of  discussion  and  research.
The  facilities  provided in 4.2BSD incorporated many of the
ideas from current research, while trying  to  maintain  the
UNIX  philosophy  of simplicity and conciseness.  The 4.3BSD
release of Berkeley UNIX  improved  upon  some  of  the  IPC
facilities  while  providing an upward-compatible interface.
4.4BSD adds support for ISO protocols and  IP  multicasting.
The  BSD interprocess communication facilities have become a
defacto standard for UNIX.

     UNIX has previously been  very  weak  in  the  area  of
interprocess  communication.   Prior to the 4BSD facilities,
the only standard mechanism which allowed two  processes  to
communicate  were  pipes  (the  mpx files which were part of
Version 7 were experimental).  Unfortunately, pipes are very
restrictive  in that the two communicating processes must be
related through a common ancestor.  Further,  the  semantics
of  pipes makes them almost impossible to maintain in a dis-
tributed environment.

     Earlier attempts at extending  the  IPC  facilities  of
UNIX  have  met  with  mixed  reaction.  The majority of the
problems have been related to the fact that these facilities
have  been tied to the UNIX file system, either through nam-
ing or implementation.   Consequently,  the  IPC  facilities
provided  in  4.2BSD  were designed as a totally independent
subsystem.  The BSD IPC allows processes  to  rendezvous  in
many  ways.   Processes  may  rendezvous through a UNIX file
system-like name space (a space where  all  names  are  path
names)  as  well  as through a network name space.  In fact,
new name spaces may be added at  a  future  time  with  only
minor  changes visible to users.  Further, the communication
facilities have been extended to include more than the  sim-
ple  byte  stream provided by a pipe.  These extensions have
resulted in a completely new part of the system which  users
will need time to familiarize themselves with.  It is likely
that as more use is made of these facilities  they  will  be
refined; only time will tell.

     This  document provides a high-level description of the
IPC facilities in 4.4BSD and their use.  It is  designed  to
complement  the manual pages for the IPC primitives by exam-
ples of their use.  The remainder of this document is  orga-
nized  in  four  sections.   Section  2  introduces the IPC-
related system calls and the basic model  of  communication.
Section  3 describes some of the supporting library routines
users may find useful in constructing  distributed  applica-
tions.   Section 4 is concerned with the client/server model
used in developing applications and includes examples of the









Advanced 4.4BSD IPC Tutorial                        PSD:21-3


two  major types of servers.  Section 5 delves into advanced
topics which sophisticated users  are  likely  to  encounter
when using the IPC facilities.


                         2. BASICS



     The  basic  building  block  for  communication  is the
socket.  A socket is an endpoint of communication to which a
name may be bound.  Each socket in use has a type and one or
more associated processes.  Sockets exist within  communica-
tion  domains.   A  communication  domain  is an abstraction
introduced to bundle common properties of processes communi-
cating  through  sockets.   One  such property is the scheme
used to name sockets.  For example, in the  UNIX  communica-
tion  domain  sockets are named with UNIX path names; e.g. a
socket may be named ``/dev/foo''.  Sockets normally exchange
data  only with sockets in the same domain (it may be possi-
ble to cross domain boundaries, but only if some translation
process  is  performed).   The 4.4BSD IPC facilities support
four separate communication domains: the  UNIX  domain,  for
on-system  communication; the Internet domain, which is used
by processes which communicate using the  Internet  standard
communication  protocols;  the  NS  domain, which is used by
processes which communicate using the Xerox standard  commu-
nication protocols*; and the ISO OSI  protocols,  which  are
not  documented in this tutorial.  The underlying communica-
tion facilities provided by these domains have a significant
influence  on  the internal system implementation as well as
the interface to socket facilities available to a user.   An
example  of the latter is that a socket ``operating'' in the
UNIX domain sees a subset of the error conditions which  are
possible when operating in the Internet (or NS) domain.

2.1.  Socket types

     Sockets  are typed according to the communication prop-
erties visible to a user.  Processes are presumed to  commu-
nicate only between sockets of the same type, although there
is nothing that prevents communication  between  sockets  of
different  types  should the underlying communication proto-
cols support this.

     Four types of sockets  currently  are  available  to  a
user.  A stream socket provides for the bidirectional, reli-
able, sequenced,  and  unduplicated  flow  of  data  without
-----------
*  See  Internet Transport Protocols, Xerox System
Integration Standard (XSIS)028112 for more  infor-
mation.   This  document is almost a necessity for
one trying to write NS applications.









PSD:21-4                        Advanced 4.4BSD IPC Tutorial


record  boundaries.  Aside from the bidirectionality of data
flow, a pair of connected stream sockets provides an  inter-
face nearly identical to that of pipes.

     A datagram socket supports bidirectional flow  of  data
which is not promised to be sequenced, reliable, or undupli-
cated.  That is, a process receiving messages on a  datagram
socket  may  find  messages duplicated, and, possibly, in an
order different from the order in which  it  was  sent.   An
important characteristic of a datagram socket is that record
boundaries in data are preserved.  Datagram sockets  closely
model  the  facilities  found  in  many  contemporary packet
switched networks such as the Ethernet.

     A raw socket provides users access  to  the  underlying
communication  protocols  which support socket abstractions.
These sockets are normally datagram oriented,  though  their
exact  characteristics  are  dependent on the interface pro-
vided by the protocol.  Raw sockets are not intended for the
general  user;  they  have  been  provided  mainly for those
interested in developing new communication protocols, or for
gaining access to some of the more esoteric facilities of an
existing protocol.  The use of raw sockets is considered  in
section 5.

     A  sequenced  packet  socket  is  similar  to  a stream
socket, with the exception that record boundaries  are  pre-
served.   This  interface is provided only as part of the NS
socket abstraction, and is very important in most serious NS
applications.   Sequenced-packet  sockets  allow the user to
manipulate the SPP or IDP headers on a packet or a group  of
packets  either  by  writing  a  prototype header along with
whatever data is to be sent,  or  by  specifying  a  default
header  to  be  used  with all outgoing data, and allows the
user to receive the headers on incoming packets.  The use of
these options is considered in section 5.

     Another  potential  socket  type  which has interesting
properties is the reliably delivered  message  socket.   The
reliably  delivered message socket has similar properties to
a datagram socket, but with  reliable  delivery.   There  is
currently no support for this type of socket, but a reliably
delivered  message  protocol  similar  to   Xerox's   Packet
Exchange  Protocol (PEX) may be simulated at the user level.
More information on this topic can be found in section 5.



-----------
  In  the  UNIX domain, in fact, the semantics are
identical and, as one  might  expect,  pipes  have
been  implemented  internally  as simply a pair of
connected stream sockets.









Advanced 4.4BSD IPC Tutorial                        PSD:21-5


2.2.  Socket creation

     To create a socket the socket system call is used:

     s = socket(domain, type, protocol);

This call requests that the system create a  socket  in  the
specified  domain  and  of the specified type.  A particular
protocol may also be requested.  If  the  protocol  is  left
unspecified (a value of 0), the system will select an appro-
priate protocol from those protocols which comprise the com-
munication  domain  and  which  may  be  used to support the
requested socket type.  The user is returned a descriptor (a
small  integer  number)  which  may  be used in later system
calls which operate on sockets.  The domain is specified  as
one   of   the   manifest  constants  defined  in  the  file
<sys/socket.h>.   For  the  UNIX  domain  the  constant   is
AF_UNIX*;   for  the Internet domain AF_INET; and for the NS
domain, AF_NS.  The socket types are also  defined  in  this
file  and  one  of  SOCK_STREAM,  SOCK_DGRAM,  SOCK_RAW,  or
SOCK_SEQPACKET must be specified.  To create a stream socket
in the Internet domain the following call might be used:

     s = socket(AF_INET, SOCK_STREAM, 0);

This call would result in a stream socket being created with
the TCP protocol providing the underlying communication sup-
port.   To  create  a datagram socket for on-machine use the
call might be:

     s = socket(AF_UNIX, SOCK_DGRAM, 0);


     The default protocol (used when the  protocol  argument
to  the  socket  call is 0) should be correct for most every
situation.  However, it is possible to  specify  a  protocol
other than the default; this will be covered in section 5.

     There  are  several  reasons  a  socket  call may fail.
Aside from the rare occurrence of lack of memory  (ENOBUFS),
a  socket  request  may fail due to a request for an unknown
protocol (EPROTONOSUPPORT), or  a  request  for  a  type  of
socket  for  which  there is no supporting protocol (EPROTO-
TYPE).




-----------
*  The manifest constants are named AF_whatever as
they indicate the ``address  format''  to  use  in
interpreting names.









PSD:21-6                        Advanced 4.4BSD IPC Tutorial


2.3.  Binding local names

     A socket is created without a name.  Until  a  name  is
bound  to  a  socket,  processes have no way to reference it
and, consequently, no messages may be received on it.   Com-
municating  processes  are  bound by an association.  In the
Internet and NS domains, an association is composed of local
and foreign addresses, and local and foreign ports, while in
the UNIX domain, an association is  composed  of  local  and
foreign  path names (the phrase ``foreign pathname'' means a
pathname created by a foreign process, not a pathname  on  a
foreign  system).   In  most  domains,  associations must be
unique.  In the Internet domain there may never be duplicate
<protocol,  local address, local port, foreign address, for-
eign port> tuples.  UNIX domain sockets need not  always  be
bound to a name, but when bound there may never be duplicate
<protocol, local pathname, foreign  pathname>  tuples.   The
pathnames  may  not  refer  to files already existing on the
system in 4.3; the situation may change in future  releases.

     The  bind  system call allows a process to specify half
of an association, <local address, local  port>  (or  <local
pathname>), while the connect and accept primitives are used
to complete a socket's association.

     In the Internet domain, binding names to sockets can be
fairly complex.  Fortunately, it is usually not necessary to
specifically bind an address and port number  to  a  socket,
because  the  connect and send calls will automatically bind
an appropriate address if they  are  used  with  an  unbound
socket.  The process of binding names to NS sockets is simi-
lar in most ways to that of binding names to Internet  sock-
ets.

     The bind system call is used as follows:

     bind(s, name, namelen);

The  bound  name  is  a variable length byte string which is
interpreted by the supporting protocol(s).  Its  interpreta-
tion  may  vary  from  communication domain to communication
domain (this is one of the  properties  which  comprise  the
``domain'').   As  mentioned,  in  the Internet domain names
contain an Internet address  and  port  number.   NS  domain
names  contain  an  NS address and port number.  In the UNIX
domain, names contain a path name and  a  family,  which  is
always AF_UNIX.  If one wanted to bind the name ``/tmp/foo''
to a UNIX domain socket, the following code would be used*:
-----------
* Note that, although the tendency here is to call
the  ``addr''  structure  ``sun'',  doing so would
cause problems if the code were ever ported  to  a









Advanced 4.4BSD IPC Tutorial                        PSD:21-7


     #include <sys/un.h>
      ...
     struct sockaddr_un addr;
      ...
     strcpy(addr.sun_path, "/tmp/foo");
     addr.sun_family = AF_UNIX;
     bind(s, (struct sockaddr *) &addr, strlen(addr.sun_path) +
         sizeof (addr.sun_len) + sizeof (addr.sun_family));

Note  that  in determining the size of a UNIX domain address
null bytes are not counted, which is why strlen is used.  In
the current implementation of UNIX domain IPC, the file name
referred to in addr.sun_path is created as a socket  in  the
system  file  space.  The caller must, therefore, have write
permission  in  the  directory  where  addr.sun_path  is  to
reside,  and  this file should be deleted by the caller when
it is no longer needed.  Future versions  of  4BSD  may  not
create this file.

     In  binding an Internet address things become more com-
plicated.  The actual call is similar,

     #include <sys/types.h>
     #include <netinet/in.h>
      ...
     struct sockaddr_in sin;
      ...
     bind(s, (struct sockaddr *) &sin, sizeof (sin));

but the selection of  what  to  place  in  the  address  sin
requires  some discussion.  We will come back to the problem
of formulating Internet addresses  in  section  3  when  the
library routines used in name resolution are discussed.

     Binding  an  NS address to a socket is even more diffi-
cult, especially since the Internet library routines do  not
work with NS hostnames.  The actual call is again similar:

     #include <sys/types.h>
     #include <netns/ns.h>
      ...
     struct sockaddr_ns sns;
      ...
     bind(s, (struct sockaddr *) &sns, sizeof (sns));

Again,  discussion  of  what  to  place  in a ``struct sock-
addr_ns'' will be deferred to section 3.



-----------
Sun workstation.









PSD:21-8                        Advanced 4.4BSD IPC Tutorial


2.4.  Connection establishment

     Connection establishment is  usually  asymmetric,  with
one  process  a  ``client'' and the other a ``server''.  The
server, when willing to offer its advertised services, binds
a socket to a well-known address associated with the service
and then passively ``listens'' on its socket.   It  is  then
possible  for  an  unrelated  process to rendezvous with the
server.  The client requests services  from  the  server  by
initiating  a ``connection'' to the server's socket.  On the
client side the connect call is used to initiate  a  connec-
tion.  Using the UNIX domain, this might appear as,

     struct sockaddr_un server;
      ...
     connect(s, (struct sockaddr *)&server, strlen(server.sun_path) +
         sizeof (server.sun_family));

while in the Internet domain,

     struct sockaddr_in server;
      ...
     connect(s, (struct sockaddr *)&server, sizeof (server));

and in the NS domain,

     struct sockaddr_ns server;
      ...
     connect(s, (struct sockaddr *)&server, sizeof (server));

where  server  in the example above would contain either the
UNIX pathname, Internet  address  and  port  number,  or  NS
address  and  port  number of the server to which the client
process wishes to speak.  If the client process's socket  is
unbound  at  the  time  of the connect call, the system will
automatically select and bind a name to the socket if neces-
sary;  c.f.  section  5.4.  This is the usual way that local
addresses are bound to a socket.

     An error is returned if the connection was unsuccessful
(any  name  automatically  bound  by  the  system,  however,
remains).  Otherwise, the  socket  is  associated  with  the
server and data transfer may begin.  Some of the more common
errors returned when a connection attempt fails are:

ETIMEDOUT
     After failing to establish a connection for a period of
     time, the system decided there was no point in retrying
     the connection attempt any more.  This  usually  occurs
     because  the destination host is down, or because prob-
     lems in the network  resulted  in  transmissions  being
     lost.









Advanced 4.4BSD IPC Tutorial                        PSD:21-9


ECONNREFUSED
     The host refused service for some reason.  This is usu-
     ally due to a server process not being present  at  the
     requested name.

ENETDOWN or EHOSTDOWN
     These  operational  errors are returned based on status
     information delivered to the client host by the  under-
     lying communication services.

ENETUNREACH or EHOSTUNREACH
     These  operational  errors can occur either because the
     network or host is unknown (no route to the network  or
     host  is  present),  or  because  of status information
     returned by intermediate gateways or  switching  nodes.
     Many  times  the  status  returned is not sufficient to
     distinguish a network being  down  from  a  host  being
     down,  in  which  case  the system indicates the entire
     network is unreachable.

     For the server to receive a client's connection it must
perform two steps after binding its socket.  The first is to
indicate a willingness to  listen  for  incoming  connection
requests:

     listen(s, 5);

The  second parameter to the listen call specifies the maxi-
mum number of outstanding connections which  may  be  queued
awaiting  acceptance  by the server process; this number may
be limited by the system.  Should a connection be  requested
while the queue is full, the connection will not be refused,
but  rather  the  individual  messages  which  comprise  the
request  will  be ignored.  This gives a harried server time
to make room in  its  pending  connection  queue  while  the
client  retries  the connection request.  Had the connection
been returned with the ECONNREFUSED error, the client  would
be unable to tell if the server was up or not.  As it is now
it is still possible to get the ETIMEDOUT error back, though
this is unlikely.  The backlog figure supplied with the lis-
ten call is currently limited by the system to a maximum  of
5  pending  connections  on  any one queue.  This avoids the
problem of processes hogging system resources by setting  an
infinite backlog, then ignoring all connection requests.

     With  a socket marked as listening, a server may accept
a connection:














PSD:21-10                       Advanced 4.4BSD IPC Tutorial


     struct sockaddr_in from;
      ...
     fromlen = sizeof (from);
     newsock = accept(s, (struct sockaddr *)&from, &fromlen);

(For the UNIX domain, from would be  declared  as  a  struct
sockaddr_un,  and  for the NS domain, from would be declared
as a struct sockaddr_ns, but nothing different would need to
be  done  as  far  as fromlen is concerned.  In the examples
which follow, only Internet routines will be discussed.)   A
new descriptor is returned on receipt of a connection (along
with a new socket).  If the server wishes to  find  out  who
its  client  is,  it  may  supply  a  buffer  for the client
socket's name.  The value-result parameter fromlen  is  ini-
tialized by the server to indicate how much space is associ-
ated with from, then modified on return to reflect the  true
size  of the name.  If the client's name is not of interest,
the second parameter may be a null pointer.

     Accept normally  blocks.   That  is,  accept  will  not
return until a connection is available or the system call is
interrupted by a signal to the process.  Further,  there  is
no  way for a process to indicate it will accept connections
from only a specific individual, or individuals.  It  is  up
to  the  user process to consider who the connection is from
and close down the connection if it does not wish  to  speak
to  the process.  If the server process wants to accept con-
nections on more than one socket, or wants to avoid blocking
on  the  accept  call,  there are alternatives; they will be
considered in section 5.

2.5.  Data transfer

     With a connection established, data may begin to  flow.
To  send  and  receive  data  there are a number of possible
calls.  With the peer entity at each  end  of  a  connection
anchored, a user can send or receive a message without spec-
ifying the peer.  As one might expect, in  this  case,  then
the normal read and write system calls are usable,

     write(s, buf, sizeof (buf));
     read(s, buf, sizeof (buf));

In  addition  to read and write, the new calls send and recv
may be used:

     send(s, buf, sizeof (buf), flags);
     recv(s, buf, sizeof (buf), flags);

While send and recv are  virtually  identical  to  read  and
write,  the  extra  flags argument is important.  The flags,
defined in <sys/socket.h>, may be specified  as  a  non-zero









Advanced 4.4BSD IPC Tutorial                       PSD:21-11


value if one or more of the following is required:


     MSG_OOB         send/receive out of band data
     MSG_PEEK        look at data without reading
     MSG_DONTROUTE   send data without routing packets


Out of band data is a notion specific to stream sockets, and
one which we will not immediately consider.  The  option  to
have data sent without routing applied to the outgoing pack-
ets is currently used only by the routing  table  management
process,  and  is  unlikely  to be of interest to the casual
user.  The ability to preview data is, however, of interest.
When  MSG_PEEK  is  specified  with  a  recv  call, any data
present is returned  to  the  user,  but  treated  as  still
``unread''.   That is, the next read or recv call applied to
the socket will return the data previously previewed.

2.6.  Discarding sockets

     Once a socket is no longer of interest, it may be  dis-
carded by applying a close to the descriptor,

     close(s);

If  data is associated with a socket which promises reliable
delivery (e.g. a stream socket) when a  close  takes  place,
the  system  will  continue to attempt to transfer the data.
However, after a fairly long period of time, if the data  is
still undelivered, it will be discarded.  Should a user have
no use for any pending data, it may perform  a  shutdown  on
the socket prior to closing it.  This call is of the form:

     shutdown(s, how);

where  how is 0 if the user is no longer interested in read-
ing data, 1 if no more data will be sent, or 2 if no data is
to be sent or received.

2.7.  Connectionless sockets

     To  this point we have been concerned mostly with sock-
ets which follow  a  connection  oriented  model.   However,
there  is also support for connectionless interactions typi-
cal of the datagram facilities found in contemporary  packet
switched  networks.   A datagram socket provides a symmetric
interface to  data  exchange.   While  processes  are  still
likely  to be client and server, there is no requirement for
connection establishment.  Instead,  each  message  includes
the destination address.










PSD:21-12                       Advanced 4.4BSD IPC Tutorial


     Datagram  sockets are created as before.  If a particu-
lar local address is needed, the bind operation must precede
the first data transmission.  Otherwise, the system will set
the local address and/or port when data is first  sent.   To
send data, the sendto primitive is used,

     sendto(s, buf, buflen, flags, (struct sockaddr *)&to, tolen);

The s, buf, buflen, and flags parameters are used as before.
The to and tolen values are used to indicate the address  of
the  intended recipient of the message.  When using an unre-
liable datagram interface, it is unlikely  that  any  errors
will be reported to the sender.  When information is present
locally to recognize a message that  can  not  be  delivered
(for  instance when a network is unreachable), the call will
return -1 and the global value errno will contain  an  error
number.

     To  receive messages on an unconnected datagram socket,
the recvfrom primitive is provided:

     recvfrom(s, buf, buflen, flags, (struct sockaddr *)&from, &fromlen);

Once again, the fromlen parameter is  handled  in  a  value-
result  fashion,  initially  containing the size of the from
buffer, and modified on return to indicate the  actual  size
of the address from which the datagram was received.

     In  addition to the two calls mentioned above, datagram
sockets may also use the connect call to associate a  socket
with a specific destination address.  In this case, any data
sent on the socket will automatically be  addressed  to  the
connected  peer,  and only data received from that peer will
be delivered to the user.  Only  one  connected  address  is
permitted for each socket at one time; a second connect will
change the destination address, and  a  connect  to  a  null
address   (family   AF_UNSPEC)   will  disconnect.   Connect
requests on datagram sockets  return  immediately,  as  this
simply  results  in  the system recording the peer's address
(as compared to a stream socket,  where  a  connect  request
initiates  establishment  of  an  end  to  end  connection).
Accept and listen are not used with datagram sockets.

     While a datagram socket  socket  is  connected,  errors
from  recent  send  calls  may  be  returned asynchronously.
These errors may be reported on subsequent operations on the
socket,  or  a  special  socket option used with getsockopt,
SO_ERROR, may be used to interrogate the  error  status.   A
select for reading or writing will return true when an error
indication has  been  received.   The  next  operation  will
return the error, and the error status is cleared.  Other of
the less important details of datagram sockets are described









Advanced 4.4BSD IPC Tutorial                       PSD:21-13


in section 5.

2.8.  Input/Output multiplexing

     One last facility often used in developing applications
is the ability to  multiplex  i/o  requests  among  multiple
sockets and/or files.  This is done using the select call:

     #include <sys/time.h>
     #include <sys/types.h>
      ...

     fd_set readmask, writemask, exceptmask;
     struct timeval timeout;
      ...
     select(nfds, &readmask, &writemask, &exceptmask, &timeout);

Select  takes  as  arguments pointers to three sets, one for
the set of file descriptors for which the caller  wishes  to
be  able to read data on, one for those descriptors to which
data is to be written, and one for which exceptional  condi-
tions  are pending; out-of-band data is the only exceptional
condition currently implemented by the socket If the user is
not  interested in certain conditions (i.e., read, write, or
exceptions), the corresponding argument to the select should
be a null pointer.

     Each set is actually a structure containing an array of
long integer bit masks; the size of the array is set by  the
definition  FD_SETSIZE.  The array is be long enough to hold
one bit for each of FD_SETSIZE file descriptors.

     The macros FD_SET(fd, &mask) and FD_CLR(fd, &mask) have
been  provided for adding and removing file descriptor fd in
the set mask.  The set should be zeroed before use, and  the
macro  FD_ZERO(&mask)  has  been  provided  to clear the set
mask.  The parameter nfds in the select call  specifies  the
range  of  file descriptors  (i.e. one plus the value of the
largest descriptor) to be examined in a set.

     A timeout value may be specified if  the  selection  is
not  to  last  more than a predetermined period of time.  If
the fields in timeout are set to 0, the selection takes  the
form  of a poll, returning immediately.  If the last parame-
ter is a null pointer,  the  selection  will  block  indefi-
nitely*.   Select  normally  returns  the  number  of   file
descriptors  selected; if the select call returns due to the
-----------
*  To  be more specific, a return takes place only
when a descriptor is selectable, or when a  signal
is received by the caller, interrupting the system
call.









PSD:21-14                       Advanced 4.4BSD IPC Tutorial


timeout expiring, then the value  0  is  returned.   If  the
select  terminates because of an error or interruption, a -1
is returned with the error number in  errno,  and  with  the
file descriptor masks unchanged.

     Assuming a successful return, the three sets will indi-
cate which file descriptors are ready to be read from, writ-
ten  to, or have exceptional conditions pending.  The status
of a file descriptor in a select mask may be tested with the
FD_ISSET(fd, &mask) macro, which returns a non-zero value if
fd is a member of the set mask, and 0 if it is not.

     To determine if there  are  connections  waiting  on  a
socket  to  be used with an accept call, select can be used,
followed by a FD_ISSET(fd, &mask) macro to  check  for  read
readiness  on the appropriate socket.  If FD_ISSET returns a
non-zero value, indicating permission to read, then  a  con-
nection is pending on the socket.

     As an example, to read data from two sockets, s1 and s2
as it is available from each and with a one-second  timeout,
the following code might be used:







































Advanced 4.4BSD IPC Tutorial                       PSD:21-15


     #include <sys/time.h>
     #include <sys/types.h>
      ...
     fd_set read_template;
     struct timeval wait;
      ...
     for (;;) {
          wait.tv_sec = 1;         /* one second */
          wait.tv_usec = 0;

          FD_ZERO(&read_template);

          FD_SET(s1, &read_template);
          FD_SET(s2, &read_template);

          nb = select(s1 > s2 ? s1 + 1 : s2 + 1, &read_template, (fd_set *) 0,
              (fd_set *) 0, &wait);
          if (nb <= 0) {
               An error occurred during the select, or
               the select timed out.
          }

          if (FD_ISSET(s1, &read_template)) {
               Socket #1 is ready to be read from.
          }

          if (FD_ISSET(s2, &read_template)) {
               Socket #2 is ready to be read from.
          }
     }


     In  4.2, the arguments to select were pointers to inte-
gers instead of pointers to fd_sets.  This type of call will
still  work  as long as the number of file descriptors being
examined is less than the number of bits in an integer; how-
ever,  the  methods  illustrated above should be used in all
current programs.

     Select  provides  a  synchronous  multiplexing  scheme.
Asynchronous notification of output completion, input avail-
ability, and exceptional conditions is possible through  use
of the SIGIO and SIGURG signals described in section 5.


                3. NETWORK LIBRARY ROUTINES



     The discussion in section 2 indicated the possible need
to locate and construct network  addresses  when  using  the
interprocess   communication  facilities  in  a  distributed









PSD:21-16                       Advanced 4.4BSD IPC Tutorial


environment.  To aid in this task a number of routines  have
been added to the standard C run-time library.  In this sec-
tion we will consider the new routines provided  to  manipu-
late network addresses.  While the 4.4BSD networking facili-
ties support the Internet protocols and the Xerox NS  proto-
cols,  most of the routines presented in this section do not
apply to the NS domain.  Unless otherwise stated, it  should
be  assumed  that  the routines presented in this section do
not apply to the NS domain.

     Locating a service on a remote host requires many  lev-
els  of mapping before client and server may communicate.  A
service is assigned a name which is intended for human  con-
sumption;  e.g.  ``the  login  server on host monet''.  This
name, and the name of the peer host, must then be translated
into  network  addresses  which are not necessarily suitable
for human consumption.  Finally, the address must then  used
in  locating  a  physical location and route to the service.
The specifics of these three mappings  are  likely  to  vary
between  network  architectures.  For instance, it is desir-
able for a network to not require hosts to be named in  such
a  way  that  their physical location is known by the client
host.  Instead, underlying services in the network may  dis-
cover  the  actual location of the host at the time a client
host wishes to communicate.   This  ability  to  have  hosts
named  in  a location independent manner may induce overhead
in connection establishment, as  a  discovery  process  must
take  place, but allows a host to be physically mobile with-
out requiring it to notify  its  clientele  of  its  current
location.

     Standard  routines are provided for: mapping host names
to network addresses, network names to network numbers, pro-
tocol  names  to protocol numbers, and service names to port
numbers and the appropriate protocol to use in communicating
with  the  server  process.   The  file  <netdb.h>  must  be
included when using any of these routines.

3.1.  Host names

     An Internet host name to address mapping is represented
by the hostent structure:



















Advanced 4.4BSD IPC Tutorial                       PSD:21-17


     struct    hostent {
          char *h_name;  /* official name of host */
          char **h_aliases;   /* alias list */
          int  h_addrtype;    /* host address type (e.g., AF_INET) */
          int  h_length; /* length of address */
          char **h_addr_list; /* list of addresses, null terminated */
     };

     #define   h_addr    h_addr_list[0]      /* first address, network byte order */

The  routine  gethostbyname(3N)  takes an Internet host name
and returns a hostent structure, while the routine  gethost-
byaddr(3N)  maps  Internet  host  addresses  into  a hostent
structure.

     The official name of the host and  its  public  aliases
are  returned by these routines, along with the address type
(family) and a  null  terminated  list  of  variable  length
address.   This  list of addresses is required because it is
possible for a host to have many addresses, all  having  the
same  name.   The h_addr definition is provided for backward
compatibility, and is defined to be the first address in the
list of addresses in the hostent structure.

     The  database for these calls is provided either by the
file /etc/hosts (hosts(5)),  or  by  use  of  a  nameserver,
named(8).  Because of the differences in these databases and
their access protocols, the information returned may differ.
When using the host table version of gethostbyname, only one
address will be returned, but all  listed  aliases  will  be
included.   The  nameserver  version  may  return  alternate
addresses, but will not provide any aliases other  than  one
given as argument.

     Unlike  Internet names, NS names are always mapped into
host addresses by the use of  a  standard  NS  Clearinghouse
service,  a distributed name and authentication server.  The
algorithms for mapping NS names to addresses via a Clearing-
house  are rather complicated, and the routines are not part
of the standard  libraries.   The  user-contributed  Courier
(Xerox  remote  procedure  call  protocol) compiler contains
routines to accomplish this mapping; see  the  documentation
and  examples  provided therein for more information.  It is
expected that almost all software that  has  to  communicate
using NS will need to use the facilities of the Courier com-
piler.

     An NS host address is represented by the following:













PSD:21-18                       Advanced 4.4BSD IPC Tutorial


     union ns_host {
          u_char    c_host[6];
          u_short   s_host[3];
     };

     union ns_net {
          u_char    c_net[4];
          u_short   s_net[2];
     };

     struct ns_addr {
          union ns_net   x_net;
          union ns_host  x_host;
          u_short   x_port;
     };

The following code fragment inserts a known NS address  into
a ns_addr:

     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netns/ns.h>
      ...
     u_long netnum;
     struct sockaddr_ns dst;
      ...
     bzero((char *)&dst, sizeof(dst));

     /*
      * There is no convenient way to assign a long
      * integer to a ``union ns_net'' at present; in
      * the future, something will hopefully be provided,
      * but this is the portable way to go for now.
      * The network number below is the one for the NS net
      * that the desired host (gyre) is on.
      */
     netnum = htonl(2266);
     dst.sns_addr.x_net = *(union ns_net *) &netnum;
     dst.sns_family = AF_NS;

     /*
      * host 2.7.1.0.2a.18 == "gyre:Computer Science:UofMaryland"
      */
     dst.sns_addr.x_host.c_host[0] = 0x02;
     dst.sns_addr.x_host.c_host[1] = 0x07;
     dst.sns_addr.x_host.c_host[2] = 0x01;
     dst.sns_addr.x_host.c_host[3] = 0x00;
     dst.sns_addr.x_host.c_host[4] = 0x2a;
     dst.sns_addr.x_host.c_host[5] = 0x18;
     dst.sns_addr.x_port = htons(75);











Advanced 4.4BSD IPC Tutorial                       PSD:21-19


3.2.  Network names

     As  for  host names, routines for mapping network names
to numbers, and back, are provided.  These routines return a
netent structure:

     /*
      * Assumption here is that a network number
      * fits in 32 bits -- probably a poor one.
      */
     struct netent {
            char      *n_name;             /* official name of net */
            char      **n_aliases;         /* alias list */
            int       n_addrtype;          /* net address type */
            int       n_net;               /* network number, host byte order */
     };

The  routines getnetbyname(3N), getnetbynumber(3N), and get-
netent(3N) are the network counterparts to the host routines
described  above.   The  routines  extract their information
from /etc/networks.

     NS network numbers are determined either by asking your
local Xerox Network Administrator (and hardcoding the infor-
mation into your code), or by querying the Clearinghouse for
addresses.  The internetwork router is the only process that
needs to manipulate network numbers on a regular basis; if a
process  wishes to communicate with a machine, it should ask
the Clearinghouse for that  machine's  address  (which  will
include the net number).

3.3.  Protocol names

     For protocols, which are defined in /etc/protocols, the
protoent structure defines the  protocol-name  mapping  used
with  the routines getprotobyname(3N), getprotobynumber(3N),
and getprotoent(3N):

     struct protoent {
            char      *p_name;             /* official protocol name */
            char      **p_aliases;         /* alias list */
            int       p_proto;             /* protocol number */
     };


     In the  NS  domain,  protocols  are  indicated  by  the
"client  type"  field of a IDP header.  No protocol database
exists; see section 5 for more information.













PSD:21-20                       Advanced 4.4BSD IPC Tutorial


3.4.  Service names

     Information regarding services is a  bit  more  compli-
cated.   A  service  is  expected  to  reside  at a specific
``port'' and employ  a  particular  communication  protocol.
This view is consistent with the Internet domain, but incon-
sistent with other network architectures.  Further,  a  ser-
vice  may  reside  on  multiple  ports.  If this occurs, the
higher level library routines will have to  be  bypassed  or
extended.   Services  available  are  contained  in the file
/etc/services.  A service mapping is described by  the  ser-
vent structure,

     struct servent {
            char      *s_name;             /* official service name */
            char      **s_aliases;         /* alias list */
            int       s_port;              /* port number, network byte order */
            char      *s_proto;            /* protocol to use */
     };

The  routine  getservbyname(3N) maps service names to a ser-
vent structure by specifying a service name and, optionally,
a qualifying protocol.  Thus the call

     sp = getservbyname("telnet", (char *) 0);

returns  the service specification for a telnet server using
any protocol, while the call

     sp = getservbyname("telnet", "tcp");

returns only that telnet server which uses the TCP protocol.
The  routines  getservbyport(3N) and getservent(3N) are also
provided.  The getservbyport routine has an interface  simi-
lar  to that provided by getservbyname; an optional protocol
name may be specified to qualify lookups.

     In the NS domain, services are  handled  by  a  central
dispatcher  provided as part of the Courier remote procedure
call facilities.  Again,  the  reader  is  referred  to  the
Courier compiler documentation and to  the  Xerox  standard*
for further details.

3.5.  Miscellaneous

     With  the support routines described above, an Internet
application program should rarely have to deal directly with
addresses.   This allows services to be developed as much as
possible in a network independent  fashion.   It  is  clear,
-----------
*  Courier:  The  Remote  Procedure Call Protocol,
XSIS 038112.









Advanced 4.4BSD IPC Tutorial                       PSD:21-21


however,  that purging all network dependencies is very dif-
ficult.  So long as the user is required to  supply  network
addresses when naming services and sockets there will always
some network dependency in a program.  For example, the nor-
mal  code  included  in  client programs, such as the remote
login program, is of the form  shown  in  Figure  1.   (This
example will be considered in more detail in section 4.)

     If  we wanted to make the remote login program indepen-
dent of the Internet  protocols  and  addressing  scheme  we
would  be forced to add a layer of routines which masked the
network dependent aspects from the  mainstream  login  code.
For the current facilities available in the system this does
not appear to be worthwhile.

     Aside from  the  address-related  data  base  routines,
there  are  several other routines available in the run-time
library which are of interest to users.  These are  intended
mostly to simplify manipulation of names and addresses.  Ta-
ble 1 summarizes  the  routines  for  manipulating  variable
length  byte  strings  and handling byte swapping of network
addresses and values.


+-----------------+---------------------------------------------------------+
|Call             | Synopsis                                                |
+-----------------+---------------------------------------------------------+
|bcmp(s1, s2, n)  | compare byte-strings; 0 if same, not 0 otherwise        |
|bcopy(s1, s2, n) | copy n bytes from s1 to s2                              |
|bzero(base, n)   | zero-fill n bytes starting at base                      |
|htonl(val)       | convert 32-bit quantity from host to network byte order |
|htons(val)       | convert 16-bit quantity from host to network byte order |
|ntohl(val)       | convert 32-bit quantity from network to host byte order |
|ntohs(val)       | convert 16-bit quantity from network to host byte order |
+-----------------+---------------------------------------------------------+

               Table 1.  C run-time routines.

     The byte swapping routines  are  provided  because  the
operating system expects addresses to be supplied in network
order  (aka  ``big-endian''  order).   On  ``little-endian''
architectures, such as Intel x86 and VAX, host byte ordering
is different than network byte ordering.  Consequently, pro-
grams  are  sometimes required to byte swap quantities.  The
library routines which return network addresses provide them
in  network order so that they may simply be copied into the
structures provided  to  the  system.   This  implies  users
should  encounter the byte swapping problem only when inter-
preting network addresses.  For example, if an Internet port
is to be printed out the following code would be required:











PSD:21-22                       Advanced 4.4BSD IPC Tutorial


     printf("port number %d\n", ntohs(sp->s_port));

On  machines  where  unneeded  these routines are defined as
null macros.

     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netinet/in.h>
     #include <stdio.h>
     #include <netdb.h>
      ...
     main(argc, argv)
            int argc;
            char *argv[];
     {
            struct sockaddr_in server;
            struct servent *sp;
            struct hostent *hp;
            int s;
            ...
            sp = getservbyname("login", "tcp");
            if (sp == NULL) {
                   fprintf(stderr, "rlogin: tcp/login: unknown service\n");
                   exit(1);
            }
            hp = gethostbyname(argv[1]);
            if (hp == NULL) {
                   fprintf(stderr, "rlogin: %s: unknown host\n", argv[1]);
                   exit(2);
            }
            bzero((char *)&server, sizeof (server));
            bcopy(hp->h_addr, (char *)&server.sin_addr, hp->h_length);
            server.sin_family = hp->h_addrtype;
            server.sin_port = sp->s_port;
            s = socket(AF_INET, SOCK_STREAM, 0);
            if (s < 0) {
                   perror("rlogin: socket");
                   exit(3);
            }
            ...
            /* Connect does the bind() for us */

            if (connect(s, (char *)&server, sizeof (server)) < 0) {
                   perror("rlogin: connect");
                   exit(5);
            }
            ...
     }

            Figure 1.  Remote login client code.











Advanced 4.4BSD IPC Tutorial                       PSD:21-23


                   4. CLIENT/SERVER MODEL



     The most commonly used paradigm  in  constructing  dis-
tributed  applications  is the client/server model.  In this
scheme client applications request services  from  a  server
process.  This implies an asymmetry in establishing communi-
cation between the client and server which has been examined
in  section 2.  In this section we will look more closely at
the interactions between client  and  server,  and  consider
some  of the problems in developing client and server appli-
cations.

     The client and server require a well known set of  con-
ventions  before  service  may  be  rendered (and accepted).
This set of conventions comprises a protocol which  must  be
implemented  at both ends of a connection.  Depending on the
situation, the protocol may be symmetric or asymmetric.   In
a  symmetric  protocol,  either  side may play the master or
slave  roles.   In  an  asymmetric  protocol,  one  side  is
immutably  recognized  as  the master, with the other as the
slave.  An example of a symmetric  protocol  is  the  TELNET
protocol used in the Internet for remote terminal emulation.
An example of an asymmetric protocol is  the  Internet  file
transfer protocol, FTP.  No matter whether the specific pro-
tocol used in obtaining a service is symmetric  or  asymmet-
ric,  when accessing a service there is a ``client process''
and a ``server process''.  We will first consider the  prop-
erties of server processes, then client processes.

     A  server  process  normally  listens  at  a well known
address for service requests.  That is, the  server  process
remains  dormant  until  a  connection  is  requested  by  a
client's connection to the server's address.  At such a time
the  server  process  ``wakes  up'' and services the client,
performing whatever appropriate actions the client  requests
of it.

     Alternative  schemes  which use a service server may be
used to eliminate a flock of server processes  clogging  the
system while remaining dormant most of the time.  For Inter-
net servers in 4.4BSD, this scheme has been implemented  via
inetd,  the so called ``internet super-server.''  Inetd lis-
tens at a variety of ports, determined at start-up by  read-
ing a configuration file.  When a connection is requested to
a port on which  inetd  is  listening,  inetd  executes  the
appropriate  server program to handle the client.  With this
method, clients are unaware that  an  intermediary  such  as
inetd  has played any part in the connection.  Inetd will be
described in more detail in section 5.










PSD:21-24                       Advanced 4.4BSD IPC Tutorial


     A similar alternative scheme is used by most Xerox ser-
vices.   In  general, the Courier dispatch process (if used)
accepts connections from processes  requesting  services  of
some  sort  or another.  The client processes request a par-
ticular <program number, version number,  procedure  number>
triple.   If  the  dispatcher knows of such a program, it is
started to handle the request; if not, an error is  reported
to  the  client.   In this way, only one port is required to
service a large variety of different requests.   Again,  the
Courier  facilities  are  not  available without the use and
installation of the Courier compiler.  The information  pre-
sented  in  this section applies only to NS clients and ser-
vices that do not use Courier.

4.1.  Servers

     In 4.4BSD most  servers  are  accessed  at  well  known
Internet  addresses  or UNIX domain names.  For example, the
remote login server's main loop is of the form shown in Fig-
ure 2.

     The  first step taken by the server is look up its ser-
vice definition:

     sp = getservbyname("login", "tcp");
     if (sp == NULL) {
            fprintf(stderr, "rlogind: tcp/login: unknown service\n");
            exit(1);
     }

The result of the getservbyname call is used in  later  por-
tions  of  the  code to define the Internet port at which it
listens for service requests (indicated by a connection).




























Advanced 4.4BSD IPC Tutorial                       PSD:21-25


     main(argc, argv)
          int argc;
          char *argv[];
     {
          int f;
          struct sockaddr_in from;
          struct servent *sp;

          sp = getservbyname("login", "tcp");
          if (sp == NULL) {
               fprintf(stderr, "rlogind: tcp/login: unknown service\n");
               exit(1);
          }
          ...
     #ifndef DEBUG
          /* Disassociate server from controlling terminal */
          ...
     #endif

          sin.sin_port = sp->s_port;    /* Restricted port -- see section 5 */
          ...
          f = socket(AF_INET, SOCK_STREAM, 0);
          ...
          if (bind(f, (struct sockaddr *) &sin, sizeof (sin)) < 0) {
               ...
          }
          ...
          listen(f, 5);
          for (;;) {
               int g, len = sizeof (from);

               g = accept(f, (struct sockaddr *) &from, &len);
               if (g < 0) {
                    if (errno != EINTR)
                         syslog(LOG_ERR, "rlogind: accept: %m");
                    continue;
               }
               if (fork() == 0) {
                    close(f);
                    doit(g, &from);
               }
               close(g);
          }
     }

              Figure 2.  Remote login server.















PSD:21-26                       Advanced 4.4BSD IPC Tutorial


     Step two is to disassociate the server  from  the  con-
trolling terminal of its invoker:

          for (i = 0; i < 3; ++i)
               close(i);

          open("/", O_RDONLY);
          dup2(0, 1);
          dup2(0, 2);

          i = open("/dev/tty", O_RDWR);
          if (i >= 0) {
               ioctl(i, TIOCNOTTY, 0);
               close(i);
          }

This step is important as the server will likely not want to
receive signals delivered to the process group of  the  con-
trolling  terminal.   Note,  however, that once a server has
disassociated itself it can no longer send reports of errors
to a terminal, and must log errors via syslog.

     Once  a  server has established a pristine environment,
it creates a socket and begins accepting  service  requests.
The  bind  call  is required to insure the server listens at
its expected location.  It should be noted that  the  remote
login  server  listens at a restricted port number, and must
therefore be run with a user-id of root.  This concept of  a
``restricted  port number'' is 4BSD specific, and is covered
in section 5.

     The main body of the loop is fairly simple:

     for (;;) {
            int g, len = sizeof (from);

            g = accept(f, (struct sockaddr *)&from, &len);
            if (g < 0) {
                   if (errno != EINTR)
                          syslog(LOG_ERR, "rlogind: accept: %m");
                   continue;
            }
            if (fork() == 0) {   /* Child */
                   close(f);
                   doit(g, &from);
            }
            close(g);            /* Parent */
     }

An accept call blocks the server  until  a  client  requests
service.   This  call  could  return a failure status if the
call is interrupted by a  signal  such  as  SIGCHLD  (to  be









Advanced 4.4BSD IPC Tutorial                       PSD:21-27


discussed  in  section 5).  Therefore, the return value from
accept is checked to insure a connection has  actually  been
established,  and an error report is logged via syslog if an
error has occurred.

     With a connection in hand,  the  server  then  forks  a
child  process and invokes the main body of the remote login
protocol processing.  Note how the socket used by the parent
for  queuing  connection  requests  is  closed in the child,
while the socket created as a result of the accept is closed
in the parent.  The address of the client is also handed the
doit  routine  because  it  requires  it  in  authenticating
clients.

4.2.  Clients

     The  client  side of the remote login service was shown
earlier in Figure 1.  One can see the  separate,  asymmetric
roles  of  the  client  and server clearly in the code.  The
server is a passive entity,  listening  for  client  connec-
tions,  while the client process is an active entity, initi-
ating a connection when invoked.

     Let us consider more closely the  steps  taken  by  the
client  remote login process.  As in the server process, the
first step is to locate the service definition for a  remote
login:

     sp = getservbyname("login", "tcp");
     if (sp == NULL) {
          fprintf(stderr, "rlogin: tcp/login: unknown service\n");
          exit(1);
     }

Next  the destination host is looked up with a gethostbyname
call:

     hp = gethostbyname(argv[1]);
     if (hp == NULL) {
          fprintf(stderr, "rlogin: %s: unknown host\n", argv[1]);
          exit(2);
     }

With this accomplished, all that is required is to establish
a  connection  to the server at the requested host and start
up  the  remote  login  protocol.   The  address  buffer  is
cleared,  then  filled  in  with the Internet address of the
foreign host and the port number at which the login  process
resides on the foreign host:












PSD:21-28                       Advanced 4.4BSD IPC Tutorial


     bzero((char *)&server, sizeof (server));
     bcopy(hp->h_addr, (char *) &server.sin_addr, hp->h_length);
     server.sin_family = hp->h_addrtype;
     server.sin_port = sp->s_port;

A  socket is created, and a connection initiated.  Note that
connect implicitly performs a bind call, since s is unbound.

     s = socket(hp->h_addrtype, SOCK_STREAM, 0);
     if (s < 0) {
          perror("rlogin: socket");
          exit(3);
     }
      ...
     if (connect(s, (struct sockaddr *) &server, sizeof (server)) < 0) {
          perror("rlogin: connect");
          exit(4);
     }

The details of the remote login protocol will not be consid-
ered here.

4.3.  Connectionless servers

     While connection-based services are the norm, some ser-
vices  are  based  on  the use of datagram sockets.  One, in
particular, is the ``rwho''  service  which  provides  users
with  status information for hosts connected to a local area
network.  This service, while predicated on the  ability  to
broadcast information to all hosts connected to a particular
network, is of interest as  an  example  usage  of  datagram
sockets.

     A  user on any machine running the rwho server may find
out the current status of a machine with the ruptime(1) pro-
gram.  The output generated is illustrated in Figure 3.

     Status information for each host is periodically broad-
cast by rwho server processes on  each  machine.   The  same
server process also receives the status information and uses
it to update a database.  This database is then  interpreted
to  generate  the status information for each host.  Servers
operate autonomously, coupled only by the local network  and
its broadcast capabilities.

     Note  that  the  use  of  broadcast  for such a task is
fairly inefficient, as all hosts must process each  message,
whether  or not using an rwho server.  Unless such a service
is  sufficiently  universal  and  is  frequently  used,  the
expense of periodic broadcasts outweighs the simplicity.

     Multicasting  is  an alternative to broadcasting.  Set-
ting up multicast sockets is described in Section 5.10.









Advanced 4.4BSD IPC Tutorial                       PSD:21-29




arpa        up   9:45,       5 users, load   1.15,   1.39,   1.31
cad         up   2+12:04,    8 users, load   4.67,   5.13,   4.59
calder      up   10:10,      0 users, load   0.27,   0.15,   0.14
dali        up   2+06:28,    9 users, load   1.04,   1.20,   1.65
degas       up   25+09:48,   0 users, load   1.49,   1.43,   1.41
ear         up   5+00:05,    0 users, load   1.51,   1.54,   1.56
ernie     down   0:24
esvax     down   17:04
ingres    down   0:26
kim         up   3+09:16,    8 users, load   2.03,   2.46,   3.11
matisse     up   3+06:18,    0 users, load   0.03,   0.03,   0.05
medea       up   3+09:39,    2 users, load   0.35,   0.37,   0.50
merlin    down   19+15:37
miro        up   1+07:20,    7 users, load   4.59,   3.28,   2.12
monet       up   1+00:43,    2 users, load   0.22,   0.09,   0.07
oz        down   16:09
statvax     up   2+15:57,    3 users, load   1.52,   1.81,   1.86
ucbvax      up   9:34,       2 users, load   6.08,   5.16,   3.28


                 Figure 3. ruptime output.

     The rwho server, in a simplified form, is  pictured  in
Figure  4.   There  are  two separate tasks performed by the
server.  The first task is to act as a  receiver  of  status
information  broadcast  by other hosts on the network.  This
job is carried out in the main loop of the program.  Packets
received at the rwho port are interrogated to insure they've
been sent by another rwho  server  process,  then  are  time
stamped  with  their  arrival time and used to update a file
indicating the status of the host.  When a host has not been
heard  from  for  an  extended  period of time, the database
interpretation routines assume the host is down and indicate
such  on  the  status  reports.   This algorithm is prone to
error as a server may be down while a host is  actually  up,
but serves our current needs.

     The  second  task  performed by the server is to supply
information regarding the status of its host.  This involves
periodically  acquiring system status information, packaging
it up in a message and broadcasting it on the local  network
for  other  rwho  servers  to  hear.  The supply function is
triggered by a timer and runs off a  signal.   Locating  the
system status information is somewhat involved, but uninter-
esting.  Deciding where to transmit the resultant packet  is
somewhat problematical, however.

     Status  information must be broadcast on the local net-
work.  For networks which  do  not  support  the  notion  of
broadcast another scheme must be used to simulate or replace
broadcasting.  One possibility is  to  enumerate  the  known









PSD:21-30                       Advanced 4.4BSD IPC Tutorial


neighbors  (based on the status messages received from other
rwho servers).  This,  unfortunately,  requires  some  boot-
strapping  information,  for a server will have no idea what
machines are its neighbors until it receives status messages
from  them.  Therefore, if all machines on a net are freshly
booted, no machine will have any known  neighbors  and  thus
never receive, or send, any status information.  This is the
identical problem faced  by  the  routing  table  management
process  in  propagating  routing  status  information.  The
standard solution, unsatisfactory as it may be, is to inform
one or more servers of known neighbors and request that they
always communicate with these neighbors.  If each server has
at least one neighbor supplied to it, status information may
then propagate through a neighbor to  hosts  which  are  not
(possibly)  directly  neighbors.   If  the server is able to
support networks which provide a  broadcast  capability,  as
well  as those which do not, then networks with an arbitrary
topology may share status information*.





























-----------
* One must, however, be concerned about ``loops''.
That  is,  if a host is connected to multiple net-
works, it will  receive  status  information  from
itself.   This  can  lead to an endless, wasteful,
exchange of information.









Advanced 4.4BSD IPC Tutorial                       PSD:21-31


     main()
     {
            ...
            sp = getservbyname("who", "udp");
            net = getnetbyname("localnet");
            sin.sin_addr = inet_makeaddr(INADDR_ANY, net);
            sin.sin_port = sp->s_port;
            ...
            s = socket(AF_INET, SOCK_DGRAM, 0);
            ...
            on = 1;
            if (setsockopt(s, SOL_SOCKET, SO_BROADCAST, &on, sizeof(on)) < 0) {
                   syslog(LOG_ERR, "setsockopt SO_BROADCAST: %m");
                   exit(1);
            }
            bind(s, (struct sockaddr *) &sin, sizeof (sin));
            ...
            signal(SIGALRM, onalrm);
            onalrm();
            for (;;) {
                   struct whod wd;
                   int cc, whod, len = sizeof (from);

                   cc = recvfrom(s, (char *)&wd, sizeof (struct whod), 0,
                       (struct sockaddr *)&from, &len);
                   if (cc <= 0) {
                          if (cc < 0 && errno != EINTR)
                                 syslog(LOG_ERR, "rwhod: recv: %m");
                          continue;
                   }
                   if (from.sin_port != sp->s_port) {
                          syslog(LOG_ERR, "rwhod: %d: bad from port",
                                 ntohs(from.sin_port));
                          continue;
                   }
                   ...
                   if (!verify(wd.wd_hostname)) {
                          syslog(LOG_ERR, "rwhod: malformed host name from %x",
                                 ntohl(from.sin_addr.s_addr));
                          continue;
                   }
                   (void) sprintf(path, "%s/whod.%s", RWHODIR, wd.wd_hostname);
                   whod = open(path, O_WRONLY | O_CREAT | O_TRUNC, 0666);
                   ...
                   (void) time(&wd.wd_recvtime);
                   (void) write(whod, (char *)&wd, cc);
                   (void) close(whod);
            }
     }

                  Figure 4.  rwho server.











PSD:21-32                       Advanced 4.4BSD IPC Tutorial


     It is important that software operating in  a  distrib-
uted  environment  not  have  any site-dependent information
compiled into it.  This would require a separate copy of the
server  at each host and make maintenance a severe headache.
4.4BSD attempts to isolate  host-specific  information  from
applications by providing system calls which return the nec-
essary information*.  A mechanism exists, in the form of  an
ioctl  call, for finding the collection of networks to which
a host is directly  connected.   Further,  a  local  network
broadcasting  mechanism  has  been implemented at the socket
level.  Combining these two features  allows  a  process  to
broadcast on any directly connected local network which sup-
ports the notion of broadcasting in a site independent  man-
ner.   This  allows  4.4BSD to solve the problem of deciding
how to propagate status information in the case of rwho,  or
more  generally  in broadcasting: Such status information is
broadcast to connected networks at the socket  level,  where
the  connected networks have been obtained via the appropri-
ate ioctl calls.  The specifics of  such  broadcastings  are
complex, however, and will be covered in section 5.


                     5. ADVANCED TOPICS



     A  number  of facilities have yet to be discussed.  For
most users of the IPC the mechanisms already described  will
suffice  in constructing distributed applications.  However,
others will find the need to use some of the features  which
we consider in this section.

5.1.  Out of band data

     The  stream  socket  abstraction includes the notion of
``out of band'' data.  Out of band data is a logically inde-
pendent  transmission  channel  associated with each pair of
connected stream sockets.  Out of band data is delivered  to
the  user  independently  of  normal  data.  The abstraction
defines that the out of band data  facilities  must  support
the reliable delivery of at least one out of band message at
a time.  This message may contain at least one byte of data,
and at least one message may be pending delivery to the user
at any one time.  For communications protocols which support
only in-band signaling (i.e. the urgent data is delivered in
sequence with the normal data), the system normally extracts
the  data  from  the  normal data stream and stores it sepa-
rately.  This allows users to choose between  receiving  the
urgent  data  in  order  and  receiving  it  out of sequence
-----------
* An example of such a system call is the gethost-
name(2) call which returns the host's ``official''
name.









Advanced 4.4BSD IPC Tutorial                       PSD:21-33


without having to buffer all the intervening  data.   It  is
possible to ``peek'' (via MSG_PEEK) at out of band data.  If
the socket has a process group, a SIGURG signal is generated
when  the  protocol is notified of its existence.  A process
can set the process group or process id to  be  informed  by
the  SIGURG  signal  via  the  appropriate  fcntl  call,  as
described below for SIGIO.  If multiple sockets may have out
of  band  data  awaiting  delivery, a select call for excep-
tional conditions may be used  to  determine  those  sockets
with  such  data pending.  Neither the signal nor the select
indicate the actual arrival of  the  out-of-band  data,  but
only notification that it is pending.

     In  addition  to the information passed, a logical mark
is placed in the data stream to indicate the point at  which
the  out of band data was sent.  The remote login and remote
shell applications use this facility  to  propagate  signals
between  client  and server processes.  When a signal flushs
any pending output from the remote process(es), all data  up
to the mark in the data stream is discarded.

     To send an out of band message the MSG_OOB flag is sup-
plied to a send or sendto calls, while  to  receive  out  of
band  data  MSG_OOB  should  be  indicated when performing a
recvfrom or recv call.  To find out if the read  pointer  is
currently  pointing at the mark in the data stream, the SIO-
CATMARK ioctl is provided:

     ioctl(s, SIOCATMARK, &yes);

If yes is a 1 on return, the  next  read  will  return  data
after  the  mark.   Otherwise (assuming out of band data has
arrived), the next read will provide data sent by the client
prior  to  transmission of the out of band signal.  The rou-
tine used in the remote login process  to  flush  output  on
receipt of an interrupt or quit signal is shown in Figure 5.
It reads the normal data up to the  mark  (to  discard  it),
then reads the out-of-band byte.

     A process may also read or peek at the out-of-band data
without first reading up to the mark.  This is  more  diffi-
cult  when  the underlying protocol delivers the urgent data
in-band with the normal data, and only sends notification of
its  presence  ahead of time (e.g., the TCP protocol used to
implement streams in the Internet domain).  With such proto-
cols,  the  out-of-band byte may not yet have arrived when a
recv is done with the MSG_OOB flag.  In that case, the  call
will  return  an  error of EWOULDBLOCK.  Worse, there may be
enough in-band data in the input  buffer  that  normal  flow
control prevents the peer from sending the urgent data until
the buffer is cleared.  The process must then read enough of
the queued data that the urgent data may be delivered.










PSD:21-34                       Advanced 4.4BSD IPC Tutorial



     #include <sys/ioctl.h>
     #include <sys/file.h>
      ...
     oob()
     {
          int out = FWRITE, mark;
          char waste[BUFSIZ];

          /* flush local terminal output */
          ioctl(1, TIOCFLUSH, (char *)&out);
          for (;;) {
               if (ioctl(rem, SIOCATMARK, &mark) < 0) {
                    perror("ioctl");
                    break;
               }
               if (mark)
                    break;
               (void) read(rem, waste, sizeof (waste));
          }
          if (recv(rem, &mark, 1, MSG_OOB) < 0) {
               perror("recv");
               ...
          }
          ...
     }

Figure 5.  Flushing terminal I/O on receipt of out of band data.

     Certain programs that use multiple bytes of urgent data
and must handle multiple urgent signals  (e.g.,  telnet(1C))
need  to  retain  the  position  of  urgent  data within the
stream.  This  treatment  is  available  as  a  socket-level
option,  SO_OOBINLINE;  see  setsockopt(2)  for usage.  With
this option, the position of urgent data (the  ``mark'')  is
retained,  but  the urgent data immediately follows the mark
within the normal data stream returned without  the  MSG_OOB
flag.   Reception  of multiple urgent indications causes the
mark to move, but no out-of-band data are lost.

5.2.  Non-Blocking Sockets

     It is occasionally convenient to make  use  of  sockets
which  do not block; that is, I/O requests which cannot com-
plete immediately and would therefore cause the  process  to
be  suspended  awaiting  completion are not executed, and an
error code is returned.  Once a socket has been created  via
the  socket  call, it may be marked as non-blocking by fcntl
as follows:













Advanced 4.4BSD IPC Tutorial                       PSD:21-35


     #include <fcntl.h>
      ...
     int  s;
      ...
     s = socket(AF_INET, SOCK_STREAM, 0);
      ...
     if (fcntl(s, F_SETFL, FNDELAY) < 0)
          perror("fcntl F_SETFL, FNDELAY");
          exit(1);
     }
      ...


     When performing non-blocking I/O on sockets,  one  must
be careful to check for the error EWOULDBLOCK (stored in the
global variable errno), which occurs when an operation would
normally block, but the socket it was performed on is marked
as non-blocking.   In  particular,  accept,  connect,  send,
recv,  read,  and write can all return EWOULDBLOCK, and pro-
cesses should be prepared to deal with  such  return  codes.
If  an  operation  such  as  a  send  cannot  be done in its
entirety, but partial writes are sensible (for example, when
using  a  stream  socket), the data that can be sent immedi-
ately will be processed, and the return value will  indicate
the amount actually sent.

5.3.  Interrupt driven socket I/O

     The  SIGIO signal allows a process to be notified via a
signal when a socket (or more generally, a file  descriptor)
has  data  waiting  to  be  read.  Use of the SIGIO facility
requires three steps:  First, the  process  must  set  up  a
SIGIO  signal  handler by use of the signal or sigvec calls.
Second, it must set the process id or process group id which
is  to  receive  notification  of  pending  input to its own
process id, or the process group id  of  its  process  group
(note  that  the  default process group of a socket is group
zero).  This is  accomplished  by  use  of  an  fcntl  call.
Third,  it  must enable asynchronous notification of pending
I/O requests with another fcntl call.  Sample code to  allow
a  given  process  to  receive  information  on  pending I/O
requests as they occur for a socket s is given in Figure  6.
With  the  addition  of  a handler for SIGURG, this code can
also be used to prepare for receipt of SIGURG signals.

5.4.  Signals and process groups

     Due to the existence of the SIGURG  and  SIGIO  signals
each  socket  has  an  associated process number, just as is
done for terminals.  This value is initialized to zero,  but
may  be  redefined  at a later time with the F_SETOWN fcntl,
such as was done in the code above for SIGIO.   To  set  the
socket's  process  id for signals, positive arguments should









PSD:21-36                       Advanced 4.4BSD IPC Tutorial



     #include <fcntl.h>
      ...
     int  io_handler();
      ...
     signal(SIGIO, io_handler);

     /* Set the process receiving SIGIO/SIGURG signals to us */

     if (fcntl(s, F_SETOWN, getpid()) < 0) {
          perror("fcntl F_SETOWN");
          exit(1);
     }

     /* Allow receipt of asynchronous I/O signals */

     if (fcntl(s, F_SETFL, FASYNC) < 0) {
          perror("fcntl F_SETFL, FASYNC");
          exit(1);
     }

Figure 6.  Use of asynchronous notification of I/O requests.

be given to the fcntl call.  To  set  the  socket's  process
group  for  signals,  negative arguments should be passed to
fcntl.  Note that the process number  indicates  either  the
associated process id or the associated process group; it is
impossible to specify both at  the  same  time.   A  similar
fcntl,  F_GETOWN,  is  available for determining the current
process number of a socket.

     Another signal which is useful when constructing server
processes is SIGCHLD.  This signal is delivered to a process
when any  child  processes  have  changed  state.   Normally
servers use the signal to ``reap'' child processes that have
exited without  explicitly  awaiting  their  termination  or
periodic  polling  for exit status.  For example, the remote
login server loop shown in Figure  2  may  be  augmented  as
shown in Figure 7.

     If  the  parent  server process fails to reap its chil-
dren, a large number of ``zombie'' processes may be created.

5.5.  Pseudo terminals

     Many programs will not function properly without a ter-
minal for standard input and output.  Since sockets  do  not
provide the semantics of terminals, it is often necessary to
have a process communicating over the network do so  through
a pseudo-terminal.  A pseudo- terminal is actually a pair of
devices, master and slave, which allow a process to serve as
an  active  agent  in  communication  between  processes and
users.  Data written on the slave side of a  pseudo-terminal









Advanced 4.4BSD IPC Tutorial                       PSD:21-37



     int reaper();
      ...
     signal(SIGCHLD, reaper);
     listen(f, 5);
     for (;;) {
          int g, len = sizeof (from);

          g = accept(f, (struct sockaddr *)&from, &len,);
          if (g < 0) {
               if (errno != EINTR)
                    syslog(LOG_ERR, "rlogind: accept: %m");
               continue;
          }
          ...
     }
      ...
     #include <wait.h>
     reaper()
     {
          union wait status;

          while (wait3(&status, WNOHANG, 0) > 0)
               ;
     }


           Figure 7.  Use of the SIGCHLD signal.

is  supplied  as  input to a process reading from the master
side, while data written on the master side are processed as
terminal  input  for  the  slave.   In this way, the process
manipulating the master side of the pseudo-terminal has con-
trol over the information read and written on the slave side
as if it were manipulating  the  keyboard  and  reading  the
screen  on a real terminal.  The purpose of this abstraction
is to preserve terminal semantics  over  a  network  connec-
tion--  that is, the slave side appears as a normal terminal
to any process reading from or writing to it.

     For example, the remote login server uses pseudo-termi-
nals  for  remote  login  sessions.   A user logging in to a
machine across the network is provided a shell with a  slave
pseudo-terminal  as  standard input, output, and error.  The
server process then handles the  communication  between  the
programs  invoked  by  the remote shell and the user's local
client process.  When a user sends a character  that  gener-
ates  an interrupt on the remote machine that flushes termi-
nal output, the pseudo-terminal generates a control  message
for  the  server  process.   The server then sends an out of
band message to the client process to signal a flush of data
at the real terminal and on the intervening data buffered in
the network.









PSD:21-38                       Advanced 4.4BSD IPC Tutorial


     Under 4.4BSD, the name of the slave side of  a  pseudo-
terminal is of the form /dev/ttyxy, where x is a single let-
ter starting at `p' and continuing to `t'.  y is a hexadeci-
mal digit (i.e., a single character in the range 0 through 9
or `a' through `f').  The master side of  a  pseudo-terminal
is /dev/ptyxy, where x and y correspond to the slave side of
the pseudo-terminal.

     In general, the method of obtaining a  pair  of  master
and  slave  pseudo-terminals  is  to  find a pseudo-terminal
which is not currently in use.  The master half of a pseudo-
terminal  is  a single-open device; thus, each master may be
opened in turn until an open succeeds.  The  slave  side  of
the pseudo-terminal is then opened, and is set to the proper
terminal modes if necessary.  The process  then  forks;  the
child  closes  the  master  side of the pseudo-terminal, and
execs the appropriate program.  Meanwhile, the parent closes
the slave side of the pseudo-terminal and begins reading and
writing from the master side.  Sample  code  making  use  of
pseudo-terminals  is  given  in  Figure 8; this code assumes
that a connection on a socket s exists, connected to a  peer
who  wants  a service of some kind, and that the process has
disassociated itself from any previous controlling terminal.

5.6.  Selecting specific protocols

     If  the  third argument to the socket call is 0, socket
will select a default protocol  to  use  with  the  returned
socket  of the type requested.  The default protocol is usu-
ally correct, and alternate choices are not  usually  avail-
able.   However,  when  using ``raw'' sockets to communicate
directly with lower-level protocols or hardware  interfaces,
the protocol argument may be important for setting up demul-
tiplexing.  For example, raw sockets in the Internet  family
may  be  used  to implement a new protocol above IP, and the
socket will receive packets only for the protocol specified.
To  obtain a particular protocol one determines the protocol
number as defined within the communication domain.  For  the
Internet domain one may use one of the library routines dis-
cussed in section 3, such as getprotobyname:

     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netinet/in.h>
     #include <netdb.h>
      ...
     pp = getprotobyname("newtcp");
     s = socket(AF_INET, SOCK_STREAM, pp->p_proto);

This would result in a socket s using a stream based connec-
tion,  but  with  protocol type of ``newtcp'' instead of the
default ``tcp.''










Advanced 4.4BSD IPC Tutorial                       PSD:21-39


     In the NS domain, the available  socket  protocols  are
defined  in  <netns/ns.h>.  To create a raw socket for Xerox
Error Protocol messages, one might use:

     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netns/ns.h>
      ...
     s = socket(AF_NS, SOCK_RAW, NSPROTO_ERROR);





















































PSD:21-40                       Advanced 4.4BSD IPC Tutorial


     gotpty = 0;
     for (c = 'p'; !gotpty && c <= 's'; c++) {
          line = "/dev/ptyXX";
          line[sizeof("/dev/pty")-1] = c;
          line[sizeof("/dev/ptyp")-1] = '0';
          if (stat(line, &statbuf) < 0)
               break;
          for (i = 0; i < 16; i++) {
               line[sizeof("/dev/ptyp")-1] = "0123456789abcdef"[i];
               master = open(line, O_RDWR);
               if (master > 0) {
                    gotpty = 1;
                    break;
               }
          }
     }
     if (!gotpty) {
          syslog(LOG_ERR, "All network ports in use");
          exit(1);
     }

     line[sizeof("/dev/")-1] = 't';
     slave = open(line, O_RDWR);   /* slave is now slave side */
     if (slave < 0) {
          syslog(LOG_ERR, "Cannot open slave pty %s", line);
          exit(1);
     }

     ioctl(slave, TIOCGETP, &b);   /* Set slave tty modes */
     b.sg_flags = CRMOD|XTABS|ANYP;
     ioctl(slave, TIOCSETP, &b);

     i = fork();
     if (i < 0) {
          syslog(LOG_ERR, "fork: %m");
          exit(1);
     } else if (i) {          /* Parent */
          close(slave);
          ...
     } else {        /* Child */
          (void) close(s);
          (void) close(master);
          dup2(slave, 0);
          dup2(slave, 1);
          dup2(slave, 2);
          if (slave > 2)
               (void) close(slave);
          ...
     }

      Figure 8.  Creation and use of a pseudo terminal











Advanced 4.4BSD IPC Tutorial                       PSD:21-41


5.7.  Address binding

     As was mentioned in section  2,  binding  addresses  to
sockets  in  the  Internet and NS domains can be fairly com-
plex.  As a brief reminder, these associations are  composed
of local and foreign addresses, and local and foreign ports.
Port numbers are allocated out of separate spaces,  one  for
each system and one for each domain on that system.  Through
the bind system call, a process may specify half of an asso-
ciation,  the  <local  address,  local port> part, while the
connect  and  accept  primitives  are  used  to  complete  a
socket's  association  by  specifying  the <foreign address,
foreign port> part.  Since the association is created in two
steps  the association uniqueness requirement indicated pre-
viously could be violated unless care is taken.  Further, it
is unrealistic to expect user programs to always know proper
values to use for the local address and local port  since  a
host  may  reside  on multiple networks and the set of allo-
cated port numbers is not directly accessible to a user.

     To simplify  local  address  binding  in  the  Internet
domain  the  notion  of a ``wildcard'' address has been pro-
vided.  When an address is specified as INADDR_ANY (a  mani-
fest  constant defined in <netinet/in.h>), the system inter-
prets the address as ``any valid address''.  For example, to
bind a specific port number to a socket, but leave the local
address unspecified, the following code might be used:

     #include <sys/types.h>
     #include <netinet/in.h>
      ...
     struct sockaddr_in sin;
      ...
     s = socket(AF_INET, SOCK_STREAM, 0);
     sin.sin_family = AF_INET;
     sin.sin_addr.s_addr = htonl(INADDR_ANY);
     sin.sin_port = htons(MYPORT);
     bind(s, (struct sockaddr *) &sin, sizeof (sin));

Sockets with wildcarded local addresses may receive messages
directed  to  the  specified port number, and sent to any of
the possible addresses assigned to a host.  For example,  if
a  host has addresses 128.32.0.4 and 10.0.0.78, and a socket
is bound as above, the process will be able to  accept  con-
nection  requests  which  are  addressed  to  128.32.0.4  or
10.0.0.78.  If a server process wished to only  allow  hosts
on  a given network connect to it, it would bind the address
of the host on the appropriate network.

     In a similar fashion, a local port may be left unspeci-
fied  (specified  as  zero),  in  which case the system will
select an appropriate port number  for  it.   This  shortcut
will work both in the Internet and NS domains.  For example,









PSD:21-42                       Advanced 4.4BSD IPC Tutorial


to bind a specific local address to a socket, but  to  leave
the local port number unspecified:

     hp = gethostbyname(hostname);
     if (hp == NULL) {
          ...
     }
     bcopy(hp->h_addr, (char *) sin.sin_addr, hp->h_length);
     sin.sin_port = htons(0);
     bind(s, (struct sockaddr *) &sin, sizeof (sin));

The system selects the local port number based on two crite-
ria.  The first is that  on  4BSD  systems,  Internet  ports
below  IPPORT_RESERVED  (1024)  (for  the  Xerox  domain,  0
through 3000) are reserved for privileged users  (i.e.,  the
super   user);   Internet  ports  above  IPPORT_USERRESERVED
(50000) are reserved for non-privileged servers.  The second
is that the port number is not currently bound to some other
socket.  In order to find a free Internet port number in the
privileged  range  the rresvport library routine may be used
as follows to return a stream socket in  with  a  privileged
port number:

     int lport = IPPORT_RESERVED - 1;
     int s;
     ...
     s = rresvport(&lport);
     if (s < 0) {
          if (errno == EAGAIN)
               fprintf(stderr, "socket: all ports in use\n");
          else
               perror("rresvport: socket");
          ...
     }

The  restriction  on allocating ports was done to allow pro-
cesses executing in  a  ``secure''  environment  to  perform
authentication  based  on  the  originating address and port
number.  For example, the rlogin(1) command allows users  to
log  in across a network without being asked for a password,
if two conditions hold: First, the name of  the  system  the
user  is  logging in from is in the file /etc/hosts.equiv on
the system he is logging in to (or the system name  and  the
user  name are in the user's .rhosts file in the user's home
directory), and second, that the user's  rlogin  process  is
coming  from  a privileged port on the machine from which he
is logging.  The port number  and  network  address  of  the
machine  from which the user is logging in can be determined
either by the from result of the accept call,  or  from  the
getpeername call.

     In  certain  cases  the algorithm used by the system in
selecting port numbers is  unsuitable  for  an  application.









Advanced 4.4BSD IPC Tutorial                       PSD:21-43


This  is  because  associations  are  created  in a two step
process.  For example, the Internet file transfer  protocol,
FTP,  specifies  that data connections must always originate
from the same local port.  However,  duplicate  associations
are  avoided  by  connecting to different foreign ports.  In
this situation the system would disallow  binding  the  same
local address and port number to a socket if a previous data
connection's socket still existed.  To override the  default
port  selection  algorithm, an option call must be performed
prior to address binding:

      ...
     int  on = 1;
      ...
     setsockopt(s, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on));
     bind(s, (struct sockaddr *) &sin, sizeof (sin));

With the above call, local addresses may be bound which  are
already  in  use.   This  does  not  violate  the uniqueness
requirement as the system still checks at connect time to be
sure  any other sockets with the same local address and port
do not have the same foreign address and port.  If the asso-
ciation already exists, the error EADDRINUSE is returned.  A
related socket option, SO_REUSEPORT, which allows completely
duplicate bindings, is described in the IP multicasting sec-
tion.

5.8.  Socket Options

     It is possible to set and get a number  of  options  on
sockets  via  the  setsockopt  and  getsockopt system calls.
These options include such things as marking  a  socket  for
broadcasting,  not  to  route,  to linger on close, etc.  In
addition, there are protocol-specific  options  for  IP  and
TCP,  as  described  in ip(4), tcp(4), and in the section on
multicasting below.

     The general forms of the calls are:

     setsockopt(s, level, optname, optval, optlen);

and

     getsockopt(s, level, optname, optval, optlen);


     The parameters to the calls are as follows:  s  is  the
socket  on  which the option is to be applied.  Level speci-
fies the protocol  layer  on  which  the  option  is  to  be
applied;  in  most cases this is the ``socket level'', indi-
cated  by  the  symbolic  constant  SOL_SOCKET,  defined  in
<sys/socket.h>.   The actual option is specified in optname,
and is a symbolic constant also defined  in  <sys/socket.h>.









PSD:21-44                       Advanced 4.4BSD IPC Tutorial


Optval  and Optlen point to the value of the option (in most
cases, whether the option is to be turned on  or  off),  and
the  length  of  the value of the option, respectively.  For
getsockopt, optlen is a  value-result  parameter,  initially
set  to  the  size of the storage area pointed to by optval,
and modified upon return to indicate the  actual  amount  of
storage used.

     An example should help clarify things.  It is sometimes
useful to determine the type (e.g., stream, datagram,  etc.)
of  an  existing  socket;  programs  under  inetd (described
below) may need to perform this task.  This  can  be  accom-
plished  as  follows  via  the SO_TYPE socket option and the
getsockopt call:

     #include <sys/types.h>
     #include <sys/socket.h>

     int type, size;

     size = sizeof (int);

     if (getsockopt(s, SOL_SOCKET, SO_TYPE, (char *) &type, &size) < 0) {
          ...
     }

After the getsockopt call, type will be set to the value  of
the  socket  type,  as  defined  in <sys/socket.h>.  If, for
example, the socket were a datagram socket, type would  have
the value corresponding to SOCK_DGRAM.

5.9.  Broadcasting and determining network configuration

     By  using  a  datagram  socket,  it is possible to send
broadcast packets on many networks supported by the  system.
The  network  itself must support broadcast; the system pro-
vides no simulation of  broadcast  in  software.   Broadcast
messages can place a high load on a network since they force
every host on the network to  service  them.   Consequently,
the  ability  to  send broadcast packets has been limited to
sockets which are explicitly marked as  allowing  broadcast-
ing.  Broadcast is typically used for one of two reasons: it
is desired to find a resource on  a  local  network  without
prior  knowledge of its address, or important functions such
as routing require that information be sent to all  accessi-
ble neighbors.

     Multicasting  is  an alternative to broadcasting.  Set-
ting up IP multicast sockets is described in the  next  sec-
tion.

     To  send  a broadcast message, a datagram socket should
be created:









Advanced 4.4BSD IPC Tutorial                       PSD:21-45


     s = socket(AF_INET, SOCK_DGRAM, 0);

or

     s = socket(AF_NS, SOCK_DGRAM, 0);

The socket is marked as allowing broadcasting,

     int  on = 1;

     setsockopt(s, SOL_SOCKET, SO_BROADCAST, &on, sizeof (on));

and at least a port number should be bound to the socket:

     sin.sin_family = AF_INET;
     sin.sin_addr.s_addr = htonl(INADDR_ANY);
     sin.sin_port = htons(MYPORT);
     bind(s, (struct sockaddr *) &sin, sizeof (sin));

or, for the NS domain,

     sns.sns_family = AF_NS;
     netnum = htonl(net);
     sns.sns_addr.x_net = *(union ns_net *) &netnum; /* insert net number */
     sns.sns_addr.x_port = htons(MYPORT);
     bind(s, (struct sockaddr *) &sns, sizeof (sns));

The destination address  of  the  message  to  be  broadcast
depends  on  the  network(s)  on  which the message is to be
broadcast.  The Internet domain supports a  shorthand  nota-
tion  for  broadcast  on  the  local  network,  the  address
INADDR_BROADCAST (defined in <netinet/in.h>.   To  determine
the  list  of addresses for all reachable neighbors requires
knowledge of the networks to which the  host  is  connected.
Since this information should be obtained in a host-indepen-
dent fashion and may be impossible to  derive,  4.4BSD  pro-
vides  a method of retrieving this information from the sys-
tem data structures.  The SIOCGIFCONF ioctl call returns the
interface  configuration  of  a host in the form of a single
ifconf structure; this structure contains  a  ``data  area''
which is made up of an array of of ifreq structures, one for
each network interface  to  which  the  host  is  connected.
These structures are defined in <net/if.h> as follows:



















PSD:21-46                       Advanced 4.4BSD IPC Tutorial


     struct ifconf {
            int    ifc_len;            /* size of associated buffer */
            union {
                   caddr_t             ifcu_buf;
                   struct ifreq *ifcu_req;
            } ifc_ifcu;
     };

     #define       ifc_buf             ifc_ifcu.ifcu_buf/* buffer address */
     #define       ifc_req             ifc_ifcu.ifcu_req/* array of structures returned */

     #define       IFNAMSIZ            16

     struct ifreq {
            char   ifr_name[IFNAMSIZ]; /* if name, e.g. "en0" */
            union {
                   struct sockaddr ifru_addr;
                   struct sockaddr ifru_dstaddr;
                   struct sockaddr ifru_broadaddr;
                   short  ifru_flags;
                   caddr_t             ifru_data;
            } ifr_ifru;
     };

     #define       ifr_addr            ifr_ifru.ifru_addr/* address */
     #define       ifr_dstaddr         ifr_ifru.ifru_dstaddr/* other end of p-to-p link */
     #define       ifr_broadaddr       ifr_ifru.ifru_broadaddr/* broadcast address */
     #define       ifr_flags           ifr_ifru.ifru_flags/* flags */
     #define       ifr_data            ifr_ifru.ifru_data/* for use by interface */

The actual call which obtains the interface configuration is

     struct ifconf ifc;
     char buf[BUFSIZ];

     ifc.ifc_len = sizeof (buf);
     ifc.ifc_buf = buf;
     if (ioctl(s, SIOCGIFCONF, (char *) &ifc) < 0) {
          ...
     }

After this call buf will contain  one  ifreq  structure  for
each network to which the host is connected, and ifc.ifc_len
will have been modified to reflect the number of bytes  used
by the ifreq structures.

     For  each  structure  there exists a set of ``interface
flags'' which tell whether the network corresponding to that
interface  is  up or down, point to point or broadcast, etc.
The SIOCGIFFLAGS ioctl retrieves these flags for  an  inter-
face specified by an ifreq structure as follows:











Advanced 4.4BSD IPC Tutorial                       PSD:21-47


     struct ifreq *ifr;

     ifr = ifc.ifc_req;

     for (n = ifc.ifc_len / sizeof (struct ifreq); --n >= 0; ifr++) {
          /*
           * We must be careful that we don't use an interface
           * devoted to an address family other than those intended;
           * if we were interested in NS interfaces, the
           * AF_INET would be AF_NS.
           */
          if (ifr->ifr_addr.sa_family != AF_INET)
               continue;
          if (ioctl(s, SIOCGIFFLAGS, (char *) ifr) < 0) {
               ...
          }
          /*
           * Skip boring cases.
           */
          if ((ifr->ifr_flags & IFF_UP) == 0 ||
              (ifr->ifr_flags & IFF_LOOPBACK) ||
              (ifr->ifr_flags & (IFF_BROADCAST | IFF_POINTTOPOINT)) == 0)
               continue;


     Once  the  flags  have  been  obtained,  the  broadcast
address must be obtained.  In the case of broadcast networks
this  is done via the SIOCGIFBRDADDR ioctl, while for point-
to-point networks the address of  the  destination  host  is
obtained with SIOCGIFDSTADDR.

     struct sockaddr dst;

     if (ifr->ifr_flags & IFF_POINTTOPOINT) {
          if (ioctl(s, SIOCGIFDSTADDR, (char *) ifr) < 0) {
               ...
          }
          bcopy((char *) ifr->ifr_dstaddr, (char *) &dst, sizeof (ifr->ifr_dstaddr));
     } else if (ifr->ifr_flags & IFF_BROADCAST) {
          if (ioctl(s, SIOCGIFBRDADDR, (char *) ifr) < 0) {
               ...
          }
          bcopy((char *) ifr->ifr_broadaddr, (char *) &dst, sizeof (ifr->ifr_broadaddr));
     }


     After  the appropriate ioctl's have obtained the broad-
cast or destination address (now in dst),  the  sendto  call
may be used:

          sendto(s, buf, buflen, 0, (struct sockaddr *)&dst, sizeof (dst));
     }










PSD:21-48                       Advanced 4.4BSD IPC Tutorial


In  the  above loop one sendto occurs for every interface to
which the host is connected  that  supports  the  notion  of
broadcast  or  point-to-point addressing.  If a process only
wished to send broadcast messages on a given  network,  code
similar  to  that outlined above would be used, but the loop
would need to find the correct destination address.

     Received broadcast messages contain the senders address
and  port, as datagram sockets are bound before a message is
allowed to go out.

5.10.  IP Multicasting

     IP multicasting is the transmission of an  IP  datagram
to a "host group", a set of zero or more hosts identified by
a single IP destination address.  A  multicast  datagram  is
delivered  to all members of its destination host group with
the same "best-efforts" reliability as  regular  unicast  IP
datagrams,  i.e.,  the  datagram is not guaranteed to arrive
intact at all members of the destination  group  or  in  the
same order relative to other datagrams.

     The  membership  of  a  host group is dynamic; that is,
hosts may join and leave groups at any time.   There  is  no
restriction  on  the location or number of members in a host
group.  A host may be a member of more than one group  at  a
time.   A host need not be a member of a group to send data-
grams to it.

     A host group may be permanent or transient.   A  perma-
nent  group  has  a well-known, administratively assigned IP
address.  It is the  address,  not  the  membership  of  the
group,  that is permanent; at any time a permanent group may
have any number of members, even zero.  Those  IP  multicast
addresses  that  are  not  reserved for permanent groups are
available for dynamic assignment to transient  groups  which
exist only as long as they have members.

     In general, a host cannot assume that datagrams sent to
any host group address will reach only the  intended  hosts,
or  that  datagrams received as a member of a transient host
group are intended for the recipient.  Misdelivery  must  be
detected at a level above IP, using higher-level identifiers
or authentication tokens.  Information transmitted to a host
group address should be encrypted or governed by administra-
tive routing controls  if  the  sender  is  concerned  about
unwanted listeners.

     IP  multicasting is currently supported only on AF_INET
sockets of type SOCK_DGRAM and SOCK_RAW, and only on subnet-
works  for  which  the interface driver has been modified to
support multicasting.










Advanced 4.4BSD IPC Tutorial                       PSD:21-49


     The next subsections describe how to send  and  receive
multicast datagrams.

5.10.1.  Sending IP Multicast Datagrams

     To  send  a multicast datagram, specify an IP multicast
address in the range 224.0.0.0  to  239.255.255.255  as  the
destination address in a sendto(2) call.

     The  definitions  required  for  the  multicast-related
socket  options  are  found  in  <netinet/in.h>.    All   IP
addresses are passed in network byte-order.

     By  default,  IP  multicast  datagrams  are sent with a
time-to-live (TTL) of 1, which prevents them from being for-
warded  beyond  a  single  subnetwork.   A new socket option
allows the TTL for subsequent multicast datagrams to be  set
to any value from 0 to 255, in order to control the scope of
the multicasts:

     u_char ttl;
     setsockopt(sock, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl));

Multicast datagrams with a TTL of 0 will not be  transmitted
on  any  subnet, but may be delivered locally if the sending
host belongs to the destination group and if multicast loop-
back  has  not  been  disabled  on  the  sending socket (see
below).  Multicast datagrams with TTL greater than  one  may
be  delivered  to  more  than one subnet if there are one or
more multicast routers attached to the first-hop subnet.  To
provide meaningful scope control, the multicast routers sup-
port the notion of TTL "thresholds", which prevent datagrams
with  less  than  a certain TTL from traversing certain sub-
nets.  The thresholds enforce the following convention:

       ---------------------------------+-------------
       Scope                            | Initial TTL
       ---------------------------------+-------------
       restricted to the same host      |       0
       restricted to the same subnet    |       1
       restricted to the same site      |      32
       restricted to the same region    |      64
       restricted to the same continent |     128
       unrestricted                     |     255
       ---------------------------------+-------------

"Sites" and "regions" are not strictly  defined,  and  sites
may be further subdivided into smaller administrative units,
as a local matter.

     An application may choose an initial TTL other than the
ones  listed  above.  For example, an application might per-
form an "expanding-ring search" for a  network  resource  by









PSD:21-50                       Advanced 4.4BSD IPC Tutorial


sending  a  multicast query, first with a TTL of 0, and then
with larger and larger TTLs, until a reply is received, per-
haps using the TTL sequence 0, 1, 2, 4, 8, 16, 32.

     The multicast router mrouted(8), refuses to forward any
multicast  datagram  with  a  destination  address   between
224.0.0.0 and 224.0.0.255, inclusive, regardless of its TTL.
This range of addresses is reserved for the use  of  routing
protocols  and other low-level topology discovery or mainte-
nance protocols, such as gateway discovery and group member-
ship reporting.

     The  address 224.0.0.0 is guaranteed not to be assigned
to any group, and 224.0.0.1 is  assigned  to  the  permanent
group of all IP hosts (including gateways).  This is used to
address all multicast hosts on the directly  connected  net-
work.   There  is  no  multicast  address  (or  any other IP
address) for all hosts on the total Internet.  The addresses
of  other  well-known, permanent groups are published in the
"Assigned Numbers" RFC, which is available from  the  Inter-
NIC.

     Each  multicast transmission is sent from a single net-
work interface, even if the host has more  than  one  multi-
cast-capable  interface.   (If the host is also serving as a
multicast router, a multicast may be forwarded to interfaces
other  than  originating interface, provided that the TTL is
greater than 1.)  The default interface to be used for  mul-
ticasting is the primary network interface on the system.  A
socket option is available to override the default for  sub-
sequent transmissions from a given socket:

     struct in_addr addr;
     setsockopt(sock, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr));

where "addr" is the local IP address of the desired outgoing
interface.  An address of INADDR_ANY may be used  to  revert
to the default interface.  The local IP address of an inter-
face can be obtained via the SIOCGIFCONF ioctl.   To  deter-
mine if an interface supports multicasting, fetch the inter-
face flags  via  the  SIOCGIFFLAGS  ioctl  and  see  if  the
IFF_MULTICAST  flag is set.  (Normal applications should not
need to use this option; it is intended primarily for multi-
cast  routers  and  other  system services specifically con-
cerned  with  internet  topology.)   The   SIOCGIFCONF   and
SIOCGIFFLAGS ioctls are described in the previous section.

     If a multicast datagram is sent to a group to which the
sending host itself belongs (on the outgoing  interface),  a
copy  of  the datagram is, by default, looped back by the IP
layer for local delivery.  Another socket option  gives  the
sender explicit control over whether or not subsequent data-
grams are looped back:









Advanced 4.4BSD IPC Tutorial                       PSD:21-51


     u_char loop;
     setsockopt(sock, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop));

where loop is set to 0 to disable loopback, and set to 1  to
enable  loopback.   This  option  improves  performance  for
applications that may have no more than one  instance  on  a
single  host  (such  as  a router demon), by eliminating the
overhead of receiving their own  transmissions.   It  should
generally not be used by applications for which there may be
more than one instance on a single host (such as  a  confer-
encing  program)  or for which the sender does not belong to
the destination group (such as a time querying program).

     A multicast datagram sent with an initial  TTL  greater
than  1  may be delivered to the sending host on a different
interface from that on  which  it  was  sent,  if  the  host
belongs  to  the  destination group on that other interface.
The loopback control option has no effect on such  delivery.

5.10.2.  Receiving IP Multicast Datagrams

     Before  a  host  can receive IP multicast datagrams, it
must become a member of one or more IP multicast groups.   A
process  can ask the host to join a multicast group by using
the following socket option:

     struct ip_mreq mreq;
     setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq))

where "mreq" is the following structure:

     struct ip_mreq {
         struct in_addr imr_multiaddr; /* multicast group to join */
         struct in_addr imr_interface; /* interface to join on */
     }

Every membership is associated with a single interface,  and
it  is  possible  to  join  the  same group on more than one
interface.  "imr_interface" should be INADDR_ANY  to  choose
the  default multicast interface, or one of the host's local
addresses to choose a particular (multicast-capable)  inter-
face.   Up  to IP_MAX_MEMBERSHIPS (currently 20) memberships
may be added on a single socket.

     To drop a membership, use:

     struct ip_mreq mreq;
     setsockopt(sock, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq));

where "mreq" contains the same values as  used  to  add  the
membership.   The  memberships  associated with a socket are
also dropped when the socket is closed or the process  hold-
ing the socket is killed.  However, more than one socket may









PSD:21-52                       Advanced 4.4BSD IPC Tutorial


claim a membership in a particular group, and the host  will
remain  a  member  of  that  group  until  the last claim is
dropped.

     The memberships associated with a socket do not  neces-
sarily  determine  which  datagrams  are  received  on  that
socket.  Incoming multicast packets are accepted by the ker-
nel  IP  layer if any socket has claimed a membership in the
destination group of the datagram; however,  delivery  of  a
multicast  datagram  to  a particular socket is based on the
destination port (or protocol type, for raw  sockets),  just
as  with  unicast datagrams.  To receive multicast datagrams
sent to a particular port, it is necessary to bind  to  that
local  port,  leaving  the  local address unspecified (i.e.,
INADDR_ANY).  To receive multicast datagrams sent to a  par-
ticular  group  and  port,  bind to the local port, with the
local address set to  the  multicast  group  address.   Once
bound  to a multicast address, the socket cannot be used for
sending data.

     More than one process may bind to the  same  SOCK_DGRAM
UDP  port  or  the same multicast group and port if the bind
call is preceded by:

     int on = 1;
     setsockopt(sock, SOL_SOCKET, SO_REUSEPORT, &on, sizeof(on));

All processes sharing the  port  must  enable  this  option.
Every  incoming multicast or broadcast UDP datagram destined
to the shared port is delivered to all sockets bound to  the
port.   For  backwards  compatibility reasons, this does not
apply to incoming unicast datagrams.  Unicast datagrams  are
never  delivered  to more than one socket, regardless of how
many sockets are bound to the datagram's destination port.

     A final multicast-related extension is  independent  of
IP:   two  new  ioctls,  SIOCADDMULTI  and SIOCDELMULTI, are
available to add or delete link-level (e.g., Ethernet)  mul-
ticast  addresses  accepted  by a particular interface.  The
address to be added or  deleted  is  passed  as  a  sockaddr
structure  of  family  AF_UNSPEC,  within the standard ifreq
structure.

     These ioctls are for the use of  protocols  other  than
IP,  and  require superuser privileges.  A link-level multi-
cast address added via  SIOCADDMULTI  is  not  automatically
deleted when the socket used to add it goes away; it must be
explicitly deleted.  It is inadvisable  to  delete  a  link-
level address that may be in use by IP.













Advanced 4.4BSD IPC Tutorial                       PSD:21-53


5.10.3.  Sample Multicast Program

     The following program sends or receives multicast pack-
ets.  If invoked with one argument, it sends a  packet  con-
taining  the current time to an arbitrarily-chosen multicast
group and UDP  port.   If  invoked  with  no  arguments,  it
receives  and prints these packets.  Start it as a sender on
just one host and as a receiver on all the other hosts.

     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netinet/in.h>
     #include <arpa/inet.h>
     #include <time.h>
     #include <stdio.h>

     #define EXAMPLE_PORT    60123
     #define EXAMPLE_GROUP   "224.0.0.250"

     main(argc)
         int argc;
     {
         struct sockaddr_in addr;
         int addrlen, fd, cnt;
         struct ip_mreq mreq;
         char message[50];

         fd = socket(AF_INET, SOCK_DGRAM, 0);
         if (fd < 0) {
             perror("socket");
             exit(1);
         }






























PSD:21-54                       Advanced 4.4BSD IPC Tutorial


         bzero(&addr, sizeof(addr));
         addr.sin_family = AF_INET;
         addr.sin_addr.s_addr = htonl(INADDR_ANY);
         addr.sin_port = htons(EXAMPLE_PORT);
         addrlen = sizeof(addr);
         if (argc > 1) {     /* Send */
             addr.sin_addr.s_addr = inet_addr(EXAMPLE_GROUP);
             while (1) {
                 time_t t = time(0);
                 sprintf(message, "time is %-24.24s", ctime(&t));
                 cnt = sendto(fd, message, sizeof(message), 0,
                         (struct sockaddr *)&addr, addrlen);
                 if (cnt < 0) {
                     perror("sendto");
                     exit(1);
                 }
                 sleep(5);
             }
         } else {            /* Receive */
             if (bind(fd, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
                 perror("bind");
                 exit(1);
             }

             mreq.imr_multiaddr.s_addr = inet_addr(EXAMPLE_GROUP);
             mreq.imr_interface.s_addr = htonl(INADDR_ANY);
             if (setsockopt(fd, IPPROTO_IP, IP_ADD_MEMBERSHIP,
                         &mreq, sizeof(mreq)) < 0) {
                 perror("setsockopt mreq");
                 exit(1);
             }

             while (1) {
                 cnt = recvfrom(fd, message, sizeof(message), 0,
                                 (struct sockaddr *)&addr, &addrlen);
                 if (cnt <= 0) {
                   if (cnt == 0) {
                    break;
                   }
                         perror("recvfrom");
                         exit(1);
                 }
                 printf("%s: message = \"%s\"\n",
                         inet_ntoa(addr.sin_addr), message);
             }
         }
     }


5.11.  NS Packet Sequences

     The semantics of NS connections demand  that  the  user
both  be  able  to look inside the network header associated









Advanced 4.4BSD IPC Tutorial                       PSD:21-55


with any incoming packet and be able to specify what  should
go in certain fields of an outgoing packet.  Using different
calls to setsockopt, it is possible to indicate whether pro-
totype headers will be associated by the user with each out-
going packet (SO_HEADERS_ON_OUTPUT), to indicate whether the
headers  received  by  the system should be delivered to the
user (SO_HEADERS_ON_INPUT), or to indicate default  informa-
tion  that should be associated with all outgoing packets on
a given socket (SO_DEFAULT_HEADERS).

     The contents of a SPP header  (minus  the  IDP  header)
are:

     struct sphdr {
          u_char    sp_cc;         /* connection control */
     #define   SP_SP     0x80      /* system packet */
     #define   SP_SA     0x40      /* send acknowledgement */
     #define   SP_OB     0x20      /* attention (out of band data) */
     #define   SP_EM     0x10      /* end of message */

          u_char    sp_dt;         /* datastream type */
          u_short   sp_sid;        /* source connection identifier */
          u_short   sp_did;        /* destination connection identifier */
          u_short   sp_seq;        /* sequence number */
          u_short   sp_ack;        /* acknowledge number */
          u_short   sp_alo;        /* allocation number */
     };

Here,  the items of interest are the datastream type and the
connection control fields.  The semantics of the  datastream
type  are  defined  by  the  application(s) in question; the
value of this field is, by default, zero, but it can be used
to indicate things such as Xerox's Bulk Data Transfer Proto-
col (in which case it is set to one).  The  connection  con-
trol  field  is  a  mask of the flags defined just below it.
The user may set or clear the end-of-message bit to indicate
that  a given message is the last of a given substream type,
or may set/clear the attention bit as an  alternate  way  to
indicate  that  a  packet should be sent out-of-band.  As an
example, to associate prototype headers  with  outgoing  SPP
packets, consider:





















PSD:21-56                       Advanced 4.4BSD IPC Tutorial


     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netns/ns.h>
     #include <netns/sp.h>
      ...
     struct sockaddr_ns sns, to;
     int s, on = 1;
     struct databuf {
          struct sphdr proto_spp;  /* prototype header */
          char buf[534];      /* max. possible data by Xerox std. */
     } buf;
      ...
     s = socket(AF_NS, SOCK_SEQPACKET, 0);
      ...
     bind(s, (struct sockaddr *) &sns, sizeof (sns));
     setsockopt(s, NSPROTO_SPP, SO_HEADERS_ON_OUTPUT, &on, sizeof(on));
      ...
     buf.proto_spp.sp_dt = 1; /* bulk data */
     buf.proto_spp.sp_cc = SP_EM;  /* end-of-message */
     strcpy(buf.buf, "hello world\n");
     sendto(s, (char *) &buf, sizeof(struct sphdr) + strlen("hello world\n"),
         (struct sockaddr *) &to, sizeof(to));
      ...

Note  that  one must be careful when writing headers; if the
prototype header is not written with the data with which  it
is  to  be  associated,  the kernel will treat the first few
bytes of the data as the header, with unpredictable results.
To  turn  off  the  above  association, and to indicate that
packet headers received by the system should be passed up to
the user, one might use:

     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netns/ns.h>
     #include <netns/sp.h>
      ...
     struct sockaddr sns;
     int s, on = 1, off = 0;
      ...
     s = socket(AF_NS, SOCK_SEQPACKET, 0);
      ...
     bind(s, (struct sockaddr *) &sns, sizeof (sns));
     setsockopt(s, NSPROTO_SPP, SO_HEADERS_ON_OUTPUT, &off, sizeof(off));
     setsockopt(s, NSPROTO_SPP, SO_HEADERS_ON_INPUT, &on, sizeof(on));


     Output  is  handled  somewhat  differently  in  the IDP
world.  The header of an IDP-level packet looks like:













Advanced 4.4BSD IPC Tutorial                       PSD:21-57


     struct idp {
          u_short   idp_sum;  /* Checksum */
          u_short   idp_len;  /* Length, in bytes, including header */
          u_char    idp_tc;        /* Transport Control (i.e., hop count) */
          u_char    idp_pt;        /* Packet Type (i.e., level 2 protocol) */
          struct ns_addr idp_dna;  /* Destination Network Address */
          struct ns_addr idp_sna;  /* Source Network Address */
     };

The primary field of interest in an IDP header is the packet
type  field.   The  standard  values  for this field are (as
defined in <netns/ns.h>):

     #define NSPROTO_RI  1         /* Routing Information */
     #define NSPROTO_ECHO     2         /* Echo Protocol */
     #define NSPROTO_ERROR    3         /* Error Protocol */
     #define NSPROTO_PE  4         /* Packet Exchange */
     #define NSPROTO_SPP 5         /* Sequenced Packet */

For SPP connections, the contents of this field are automat-
ically  set  to  NSPROTO_SPP;  for  IDP  packets, this value
defaults to zero, which means ``unknown''.

     Setting the value of that field with SO_DEFAULT_HEADERS
is easy:

     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netns/ns.h>
     #include <netns/idp.h>
      ...
     struct sockaddr sns;
     struct idp proto_idp;         /* prototype header */
     int s, on = 1;
      ...


     s = socket(AF_NS, SOCK_DGRAM, 0);
      ...
     bind(s, (struct sockaddr *) &sns, sizeof (sns));
     proto_idp.idp_pt = NSPROTO_PE;     /* packet exchange */
     setsockopt(s, NSPROTO_IDP, SO_DEFAULT_HEADERS, (char *) &proto_idp,
         sizeof(proto_idp));
      ...


     Using  SO_HEADERS_ON_OUTPUT is somewhat more difficult.
When SO_HEADERS_ON_OUTPUT is turned on for  an  IDP  socket,
the  socket  becomes  (for  all  intents and purposes) a raw
socket.  In this case,  all  the  fields  of  the  prototype
header  (except  the  length  and checksum fields, which are
computed by the kernel) must be filled in correctly in order
for  the  socket  to  send  and  receive  data in a sensible









PSD:21-58                       Advanced 4.4BSD IPC Tutorial


manner.  To be more specific, the source address must be set
to  that  of  the  host  sending  the  data; the destination
address must be set to that of the host for whom the data is
intended;  the  packet type must be set to whatever value is
desired; and the hopcount must be  set  to  some  reasonable
value  (almost  always  zero).  It should also be noted that
simply sending data using write will not work unless a  con-
nect or sendto call is used, in spite of the fact that it is
the destination address in  the  prototype  header  that  is
used,  not  the  one  given  in  either of those calls.  For
almost all IDP applications ,  using  SO_DEFAULT_HEADERS  is
easier and more desirable than writing headers.

5.12.  Three-way Handshake

     The  semantics  of  SPP  connections  indicates  that a
three-way handshake, involving  changes  in  the  datastream
type,  should  --  but is not absolutely required to -- take
place before a SPP connection is  closed.   Almost  all  SPP
connections are ``well-behaved'' in this manner; when commu-
nicating with any process, it is best  to  assume  that  the
three-way  handshake is required unless it is known for cer-
tain that it is not required.  In  a  three-way  close,  the
closing  process  indicates that it wishes to close the con-
nection by sending a zero-length packet with  end-of-message
set  and  with  datastream  type 254.  The other side of the
connection indicates that it is OK to  close  by  sending  a
zero-length  packet  with  end-of-message set and datastream
type 255.  Finally, the closing process replies with a zero-
length  packet  with  substream type 255; at this point, the
connection is considered closed.  The following  code  frag-
ments  are  simplified examples of how one might handle this
three-way handshake at the user level; in the  future,  sup-
port  for  this  type  of close will probably be provided as
part of the C library or as part of the kernel.   The  first
code  fragment  below illustrates how a process might handle
three-way handshake if it sees that the process it is commu-
nicating with wants to close the connection:

     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netns/ns.h>
     #include <netns/sp.h>
      ...
     #ifndef SPPSST_END
     #define SPPSST_END 254
     #define SPPSST_ENDREPLY 255
     #endif
     struct sphdr proto_sp;
     int s;
      ...











Advanced 4.4BSD IPC Tutorial                       PSD:21-59


     read(s, buf, BUFSIZE);
     if (((struct sphdr *)buf)->sp_dt == SPPSST_END) {
          /*
           * SPPSST_END indicates that the other side wants to
           * close.
           */
          proto_sp.sp_dt = SPPSST_ENDREPLY;
          proto_sp.sp_cc = SP_EM;
          setsockopt(s, NSPROTO_SPP, SO_DEFAULT_HEADERS, (char *)&proto_sp,
              sizeof(proto_sp));
          write(s, buf, 0);
          /*
           * Write a zero-length packet with datastream type = SPPSST_ENDREPLY
           * to indicate that the close is OK with us.  The packet that we
           * don't see (because we don't look for it) is another packet
           * from the other side of the connection, with SPPSST_ENDREPLY
           * on it it, too.  Once that packet is sent, the connection is
           * considered closed; note that we really ought to retransmit
           * the close for some time if we do not get a reply.
           */
          close(s);
     }
      ...

To  indicate  to another process that we would like to close
the connection, the following code would suffice:

     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netns/ns.h>
     #include <netns/sp.h>
      ...
     #ifndef SPPSST_END
     #define SPPSST_END 254
     #define SPPSST_ENDREPLY 255
     #endif
     struct sphdr proto_sp;
     int s;
      ...
     proto_sp.sp_dt = SPPSST_END;
     proto_sp.sp_cc = SP_EM;
     setsockopt(s, NSPROTO_SPP, SO_DEFAULT_HEADERS, (char *)&proto_sp,
         sizeof(proto_sp));
     write(s, buf, 0);   /* send the end request */
     proto_sp.sp_dt = SPPSST_ENDREPLY;
     setsockopt(s, NSPROTO_SPP, SO_DEFAULT_HEADERS, (char *)&proto_sp,
         sizeof(proto_sp));
     /*
      * We assume (perhaps unwisely) that the other side will send the ENDREPLY,
      * so we'll just send our final ENDREPLY as if we'd seen theirs already
      */
     write(s, buf, 0);
     close(s);
      ...










PSD:21-60                       Advanced 4.4BSD IPC Tutorial


5.13.  Packet Exchange

     The Xerox standard protocols include a protocol that is
both reliable and datagram-oriented.  This protocol is known
as Packet Exchange (PEX or PE) and, like SPP, is layered  on
top  of  IDP.   PEX  is  important  for  a number of things:
Courier remote procedure calls may be expedited through  the
use  of  PEX,  and many Xerox servers are located by doing a
PEX ``BroadcastForServers'' operation.  Although there is no
implementation  of PEX in the kernel, it may be simulated at
the user level with some clever coding and the  use  of  one
peculiar getsockopt.  A PEX packet looks like:

     /*
      * The packet-exchange header shown here is not defined
      * as part of any of the system include files.
      */
     struct pex {
          struct idp     p_idp;    /* idp header */
          u_short   ph_id[2]; /* unique transaction ID for pex */
          u_short   ph_client;     /* client type field for pex */
     };

The ph_id field is used to hold a ``unique id'' that is used
in duplicate suppression; the ph_client field indicates  the
PEX client type (similar to the packet type field in the IDP
header).  PEX reliability stems from the fact that it is  an
idempotent  (``I  send a packet to you, you send a packet to
me'') protocol.  Processes on each side  of  the  connection
may use the unique id to determine if they have seen a given
packet before (the unique id field differs  on  each  packet
sent)  so  that  duplicates may be detected, and to indicate
which message a given packet is in response to.  If a packet
with  a  given unique id is sent and no response is received
in a given amount of time, the packet is retransmitted until
it  is  decided  that no response will ever be received.  To
simulate PEX, one must be able to  generate  unique  ids  --
something that is hard to do at the user level with any real
guarantee that the id is really unique.  Therefore, a  means
(via  getsockopt)  has  been provided for getting unique ids
from the kernel.  The following code fragment indicates  how
to get a unique id:

     long uniqueid;
     int s, idsize = sizeof(uniqueid);
      ...
     s = socket(AF_NS, SOCK_DGRAM, 0);
      ...
     /* get id from the kernel -- only on IDP sockets */
     getsockopt(s, NSPROTO_PE, SO_SEQNO, (char *)&uniqueid, &idsize);
      ...

The  retransmission  and duplicate suppression code required
to simulate PEX fully is left as an exercise for the reader.










Advanced 4.4BSD IPC Tutorial                       PSD:21-61


5.14.  Inetd

     One  of  the daemons provided with 4.4BSD is inetd, the
so called ``internet super-server.''  Having one daemon lis-
ten  for  requests  for  many daemons instead of having each
daemon listen for its own requests  reduces  the  number  of
idle  daemons and simplies their implementation.  Inetd han-
dles two types of services: standard and TCPMUX.  A standard
service  has  a well-known port assigned to it and is listed
in /etc/services (see services(5)); it may be a service that
implements  an  official  Internet standard or is a BSD-spe-
cific service.  TCPMUX services are nonstandard and  do  not
have  a  well-known port assigned to them.  They are invoked
from inetd when a program connects  to  the  "tcpmux"  well-
known  port  and specifies the service name.  This is useful
for adding locally-developed servers.

     Inetd is invoked at boot time, and determines from  the
file  /etc/inetd.conf the servers for which it is to listen.
Once this information has been read and a pristine  environ-
ment  created,  inetd proceeds to create one socket for each
service it is to listen for, binding  the  appropriate  port
number to each socket.

     Inetd  then  performs a select on all these sockets for
read availability, waiting for somebody wishing a connection
to  the  service  corresponding  to that socket.  Inetd then
performs an accept on the socket in  question,  forks,  dups
the  new  socket to file descriptors 0 and 1 (stdin and std-
out), closes other open  file  descriptors,  and  execs  the
appropriate server.

     Servers  making  use  of inetd are considerably simpli-
fied, as inetd takes care of the majority of  the  IPC  work
required  in  establishing a connection.  The server invoked
by inetd expects the socket connected to its client on  file
descriptors  0 and 1, and may immediately perform any opera-
tions such as read, write, send, or recv.   Indeed,  servers
may  use  buffered  I/O as provided by the ``stdio'' conven-
tions, as long as as they remember to use fflush when appro-
priate.

     One  call which may be of interest to individuals writ-
ing servers under  inetd  is  the  getpeername  call,  which
returns  the  address of the peer (process) connected on the
other end of the socket.  For example, to log  the  Internet
address  in  ``dot  notation''  (e.g.,  ``128.32.0.4'') of a
client connected to a server under inetd, the following code
might be used:















PSD:21-62                       Advanced 4.4BSD IPC Tutorial


     struct sockaddr_in name;
     int namelen = sizeof (name);
      ...
     if (getpeername(0, (struct sockaddr *)&name, &namelen) < 0) {
          syslog(LOG_ERR, "getpeername: %m");
          exit(1);
     } else
          syslog(LOG_INFO, "Connection from %s", inet_ntoa(name.sin_addr));
      ...

While the getpeername call is especially useful when writing
programs to run with inetd, it can be used under other  cir-
cumstances.   Be warned, however, that getpeername will fail
on UNIX domain sockets.

     Standard TCP services are  assigned  unique  well-known
port  numbers  in  the  range  of  0 to 1023 by the Internet
Assigned Numbers Authority (IANA@ISI.EDU).  The limited num-
ber of ports in this range are assigned to official Internet
protocols.  The TCPMUX service allows you  to  add  locally-
developed  protocols  without  needing  an official TCP port
assignment.  The TCPMUX protocol described  in  RFC-1078  is
simple:

     ``A  TCP  client connects to a foreign host on TCP
     port 1.  It sends the service name followed  by  a
     carriage-return  line-feed  <CRLF>.   The  service
     name is never case sensitive.  The server  replies
     with  a single character indicating positive ("+")
     or negative ("-") acknowledgment, immediately fol-
     lowed  by an optional message of explanation, ter-
     minated with a <CRLF>.  If the reply was positive,
     the  selected  protocol begins; otherwise the con-
     nection is closed.''

In 4.4BSD, the TCPMUX service is built into inetd, that  is,
inetd listens on TCP port 1 for requests for TCPMUX services
listed in inetd.conf.  inetd(8) describes the format of TCP-
MUX entries for inetd.conf.

     The  following  is  an  example  TCPMUX  server and its
inetd.conf entry.  More sophisticated servers may want to do
additional processing before returning the positive or nega-
tive acknowledgement.




















Advanced 4.4BSD IPC Tutorial                       PSD:21-63


     #include <sys/types.h>
     #include <stdio.h>

     main()
     {
             time_t t;

             printf("+Go\r\n");
             fflush(stdout);
             time(&t);
             printf("%d = %s", t, ctime(&t));
             fflush(stdout);
     }

The inetd.conf entry is:

     tcpmux/current_time stream tcp nowait nobody /d/curtime curtime

Here's the portion of the client code that handles the  TCP-
MUX handshake:












































PSD:21-64                       Advanced 4.4BSD IPC Tutorial


     char line[BUFSIZ];
     FILE *fp;
      ...

     /* Use stdio for reading data from the server */
     fp = fdopen(sock, "r");
     if (fp == NULL) {
         fprintf(stderr, "Can't create file pointer\n");
         exit(1);
     }

     /* Send service request */
     sprintf(line, "%s\r\n", "current_time");
     if (write(sock, line, strlen(line)) < 0) {
         perror("write");
         exit(1);
     }

     /* Get ACK/NAK response from the server */
     if (fgets(line, sizeof(line), fp) == NULL) {
         if (feof(fp)) {
             die();
         } else {
             fprintf(stderr, "Error reading response\n");
             exit(1);
         }
     }

     /* Delete <CR> */      ')) != NULL) {
     if ((lp = index(line, '
         *lp = ' ';
     }

     switch (line[0]) {
         case '+':
                 printf("Got ACK: %s\n", &line[1]);
                 break;
         case '-':
                 printf("Got NAK: %s\n", &line[1]);
                 exit(0);
         default:
                 printf("Got unknown response: %s\n", line);
                 exit(1);
     }

     /* Get rest of data from the server */
     while ((fgets(line, sizeof(line), fp)) != NULL) {
         fputs(line, stdout);
     }












