Re: Multiple PDUs in UDP Packet?

From: Barry Margolin (
Date: 05/21/05

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

In article <>,
 Michael B Allen <> 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,
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***