UNIX IPC Datagram Reliability under 4.2BSD

[eric%cit-vax at sri-unix.UUCP]

From:  Eric Holstege <eric at cit-vax>

    It is pointless to require reliable datagrams; the whole point of
    datagrams is that they need not be reliable. If you want a reliable
    channel, use a stream; you can simulate datagrams on a stream.

  It would be convenient to be able to reliably send one message without going
  through the trouble (and overhead) of setting up a stream.

In order to reliably send a datagram, one must have some indication that
the recipient is listening, as well as that he actually receives what he
is sent. This is tantamount to establishing a  two-way  connection,  the
main  inconvenience  of  streams relative to datagrams. Streams are more
general  than  datagram  connections  for  the  same  reason  that  UNIX
byte-seekable files are more general than the record-seekable file types
beloved of VMS,  etc.  As  for  streams  having  more  overhead,  so  do
byte-seekable  files,  but  no  one  proposes  that  another  file type,
512-byte-record files, be added to UNIX for the sake of efficiency.  The
issue  is  the  same,  should the kernel give you the amount of data you
want to read, or should you be forced to read the amount it  feels  like
giving you.
    Besides, pipes in 4.2 are implemented as IPC streams, with  (if  you
believe  the  performance  figures)  a  substantial improvement over 4.1
pipes. Fast enough for me.
    What I'd like to see is a library implementing  something  like  VMS
mailboxes  using  sockets, with calling sequences that aren't as painful
to figure out as the socket stuff.

					    - Eric Holstege
					      eric at cit-vax