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From: dls@mentor.cc.purdue.edu (David L Stevens)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: IP on a serial line -- which RFC?
Message-ID: <1882@mentor.cc.purdue.edu>
Date: 28 Nov 90 01:45:43 GMT
References: <18466181.ARN13a9@acdhq.north.de>
Distribution: comp
Organization: PUCC UNIX Group
Lines: 37

In article <18466181.ARN13a9@acdhq.north.de>, kaba@acdhq.north.de (Kai Bartels) writes:
> Hy!
> I'm currently workin on an implementation of tcp-ip an during this work
> i realised that i really don't know how fragments are representated on a
> serial line. I don't think it's possible to have it in the same way as
> it is on ethernet, 'cause on a serial line you have to know the length of
> the fragment BEFORE you get it, so you don'y miss the end.
> 
> So the question is: can anybody tell me which RFC to look at?

	RFC 791. :-)

	The IP "length" field is the length of the packet, not the datagram.
That is, for a fragment, the "length" field tells you how big the fragment
is, but you don't know how big the datagram is until you've received the last
of its fragments (RFC 791, page 27, line 1). So, for individual packets, you
can read the min header (20 octets) and it tells you how much header & data
is there.

	But, if you want to make your life easier, you can also add your own
(or someone else's) link-level protocol and encapsulate IP and whatever else
in that. You *will* need this if you do more than just IP stuff on the line.
ARP doesn't make much sense on a 2-node network, but if it's more than just
a toy implementation, it's better to provide self-identifying packets and
while you have that, you might as well add a length field. If you like
Ethernet, just send Ethernet "frames" serially.

	You probably will also want to check out RFC 1055 and maybe 1144.

1055  Romkey, J.L.  Nonstandard for transmission of IP datagrams over serial 
      lines: SLIP.  1988 June; 6 p. (Format: TXT=12911 bytes)

1144  Jacobson, V.  Compressing TCP/IP headers for low-speed serial links.  
      1990 February; 43 p. (Format: TXT=120959, PS=534729 bytes)

-- 
					+-DLS  (dls@mentor.cc.purdue.edu)