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List:       linux-ppp
Subject:    Re: multilink fragmentation??
From:       James Carlson <carlson () workingcode ! com>
Date:       2003-11-15 6:05:50
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Abdul J writes:
> But, what is the smallest packet that the MLPPP code
> will fragment. Say I have a 100 byte packet and 4
> channels in the bundle that can accept the packet at
> the moment. Since the 40 byte TCP/IP header should not
> be fragmented (as per RFC 1717), will the resulting
> fragments be of 40, 20,20,20 bytes?

I think you might have MP (which is currently described in RFC 1990,
not 1717) confused with IP (which is RFC 791).

MP has nothing to do with IP.  It is a self-contained fragmentation
and reassembly mechanism that operates at the link layer (below IP).
It never reorders packets and fragments can be as small as desired.
There is no such 40 byte limit.

(As far as I can tell, RFC 1717 says nothing about avoiding
fragmenting TCP/IP headers.  Can you point out where you saw that?)

> OR would ppp
> consider 100 byte to be too small for fragmentation
> and just send one packet?

This is entirely implementation-dependent.  Implementations may do
anything they like -- including not fragmenting at all, regardless of
packet size, or fragmenting down to individual octets.  The latter
would probably have too much overhead to be practical, but it's
certainly not prohibited.

The usual way to do this is to break the packet up among the available
links, unless it's "very small," since the best MP reassembly latency
is achieved when the fragments sent on all the links are of fairly
even and equal size.

The Linux 2.4.17 implementation seems to have a minimum fragment size
of 64 octets, but (as best I can tell) this is essentially arbitrary.
The real issue for implementors here is the tradeoff between
per-packet overhead and performance, not protocol restrictions.

-- 
James Carlson                                  <carlson@workingcode.com>
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