Re: Multiple PDUs in UDP Packet?

From: Barry Margolin (barmar_at_alum.mit.edu)
Date: 05/21/05


Date: Sat, 21 May 2005 17:07:49 -0400

In article <pan.2005.05.21.20.49.39.346149@ioplex.com>,
 Michael B Allen <mba2000@ioplex.com> wrote:

> On Sat, 21 May 2005 12:39:22 +0200, Måns Rullgård wrote:
>
> >> A PDU is a particular layer's message.
> >
> > If that's what it means, it doesn't really make sense talking about
> > putting several of them in one packet.
>
> Why not? When you call sendto(2) you give it a buffer not a packet. Just
> because it's one packet on the wire doesn't mean an application layer
> protocol encoder couldn't put multiple PDUs into it.
>
> The term "Protocol Data Unit" is nice because "request", "response",
> "client", and "server" infer an orientation and some semantics about
> protocol. For example, it is possible for a server to send a client
> a request.

My understanding is that the real purpose of the term PDU was to embody
the commonalities among layers. So the standard could make general
statements like "a layer-N PDU is encapsulated in a layer-(N-1) PDU."
Otherwise, many things would have to be restated for each layer, to use
their appropriate terminology: "A datagram is encapsulated in a series
of frames", "A command/request is encapsulated in a datagram", and so on.

-- 
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***