Re: Initial SCTP support for the upcoming 1.6.5 release

[Michael Tuexen <Michael.Tuexen@lurchi.franken.de>]

Kevin,

see my comments in-line.

Best regards
Michael

On Aug 20, 2004, at 3:13 AM, Kevin Ryde wrote:

> Michael Tuexen <Michael.Tuexen@lurchi.franken.de> writes:
>>
>> The argument, to use a function like getprotobyname
>> to perform a lookup in /etc/protocols, is not valid in my opinion.
>> The reason is, that besides I have not seen that, that  the
>> constants above are defined in /usr/include/netinet/in.h and
>> are used.
>
> I don't want to go against the advice of the GNU/Linux man page.  It's
> pretty explicit.
You are not going against it. You are not using them. You are only
providing constants available in /usr/include/netinet/in.h to
the guile user.
>
>> So if, a system (I do not know of any such system),
>> uses a different number for TCP, this number will not only be in
>> /etc/protocols, but also in /usr/include/netinet/in.h. A different
>> story are port numbers. They can (and should) be looked up in
>> /etc/services.
>> There are no constants describing the port number for an echo  
>> server...
>
> Well I guess both protocol and service numbers for standard stuff are
> constants.  I don't know why one reads a file instead of just putting
> numbers in a program.  Maybe it dates right back to a time when such
> things were still in flux.  Or maybe the theory is to avoid numbers
> hard coded in programs (though strings hard coded aren't much better
> :-).
You are right, both port numbers and protocol numbers are kind of
standardized. But there is two differences:
Normally a user of a program does not specify the transport protocol,
but the port (telnet 127.0.0.1 echo, for example). That is why you
need a translation service for these service names.
The second is that there are no predefined constants for port numbers
available on a system like it is for protocols.
I agree with the man page that it is bad to use
s = socket(AF_INET, SOCK_STREAM, 132);
It should be
s = socket(AF_INET, SOCK_STREAM, IPPROTO_SCTP);
because you are not using a number, you are using a constant.

So what is the difference if you use getprotocolbyname? You can move
the binaries to a different host which uses different numbers?
I have not seen that and I'm pretty sure that this is not in tune
with the socket() call.

BTW, for UDP and TCP the calls
s = socket(AF_INET, SOCK_DGRAM,  0);
s = socket(AF_INET, SOCK_STREAM, 0);
are used as acronyms for
s = socket(AF_INET, SOCK_DGRAM,  IPPROTO_UDP);
s = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

So I think the man page and reality match no pretty good for these  
things,
but the man page seems to be pretty old (1995, on
http://www.freebsd.org/cgi/man.cgi? 
query=protocols&apropos=0&sektion=0&manpath=Red+Hat+Linux%2Fi386+9&forma 
t=html

Best regards
Michael

PS.: The usage of the third arg of the socket() call is the one  
described
      in the 3rd edition of Unix Network Programming, so I guess that  
some
      people will try to use it that way...




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