[e2e] UDP length field

David Borman dab at BSDI.COM
Wed Apr 11 11:30:27 PDT 2001

> From: Bill Sommerfeld <sommerfeld at orchard.arlington.ma.us>
> To: Hari Balakrishnan <hari at lcs.mit.edu>
> Cc: end2end-interest at postel.org, lln at lcs.mit.edu
> Subject: Re: [e2e] UDP length field 
> Date: Wed, 11 Apr 2001 14:02:26 -0400
> > 	Why does UDP have a length field?
> Same reason tcp does: to preserve an end-to-end length in the presence
> of fragmentation.
> The ip packet length is the length of a particular fragment, and may
> have been set by a router which fragmented the packet.

The TCP header does not have a length field.  The TCP length is part
of the pseudo-header, which is computed from the length of the (reassembled)
IP packet minus the IP headers and options.

			-David Borman, dab at bsdi.com

More information about the end2end-interest mailing list