[TUHS] Sockets and the true UNIX

Chris Torek torek at torek.net
Sat Sep 23 04:00:41 AEST 2017

[I said]
>> This also knocks out the need for SO_REUSEADDR ...

>Under TCP/IP I'm not sure you can. The protocol specifies that you must
>wait for a certain period of time (120 sec, if memory serves my right)
>before reusing an address/port combo, so that all in-flight packets have
>disappeared from the network. Only if one is sure that this is not an
>issue one can use SO_REUSEADDR.

That's independent of this issue.  The problem is that when you
call bind(), the system does not know if you are doing it as
the first half of bind()/connect(), or as the first half of
bind()/listen( (followed by an accept() loop).

If you are going to do the latter, you are declaring that you want
to serve *new* SYN requests.  This is when you (currently) must set
SO_REUSEADDR, because bind() checks your local address against the
universe of all local/remote pairs.  But if the system knows that
your call (which, remember, isn't bind() anymore) is your
declaration of intent to be a server, the OS can just check
whether there's already someone declared as "the server".

If you intend to "serve" one specific foreign address (FTP):

   bind_and_connect(fd, local, remote);

the test is: "does <local,remote> specify a unique address"
(and possibly "does <local> match what we already suggested
to you and reserved for you).  If you intend to serve "*":

   bind_and_connect(fd, local, NULL);

the test is: "is there already a <local,*>".

In the server case any existing 2MSL timeouts are irrelevant;
what matters is whether you are competing with another server.

(Note that libc will probably have various convenience wrappers
atop the system call, kind of like libc has system() atop
fork/exec.  Most programs just want to create and bind_and_connect
a socket in one pass.)


More information about the TUHS mailing list