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 ***


Relevant Pages

  • Re: ARP - IP but why?
    ... The Network layer keeps track of logical addresses, and Ethernet handles delivery to a specific device. ... Since an IP address can be assigned to any ethernet device, it is utilized to associate an IP address to a real-world computer, printer, whatever. ... Once the network layer builds the TCP packet with a destination IP address, it hands it down to the link layer. ...
    (comp.os.linux.networking)
  • Re: packet vs frame
    ... I.e., a data frame becomes ... a packet by tacking on some control bits fore and aft. ... OSI, and a frame is layer 2 of the OSI. ...
    (alt.internet.wireless)
  • RE: DoS/DDoS Attack
    ... We are now looking into a HA/LB setup of the IPS 5500. ... The attack lasted about ... my favorite rate-based IPS box is Top Layer. ... >header to the packet you're sending, then the kernel just place the packet ...
    (Pen-Test)
  • Re: Latency issues with buf_ring
    ... task anymore. ... packet, go do ... layer having to get its grubby fingers into things. ... By modularization, I mean making the "pieces" threadable. ...
    (freebsd-net)
  • Re: packet vs frame
    ... I.e., a data frame becomes ... a packet by tacking on some control bits fore and aft. ... OSI, and a frame is layer 2 of the OSI. ...
    (alt.internet.wireless)