[ih] UDP Length Field?

[Craig Partridge]

I remember asking Danny Cohen the question about the UDP length decades ago
and for the life of me, I can't recall the answer.  It was about the time I
was trying to learn the origins of UDP, which per Vint's note, created in a
hallway discussion among Jon Postel, Danny Cohen and Dave Reed, at a TCP
group meeting, I think in '78 or '79.

Recall that the creation of UDP meant TCP and IP had to be split apart, so
a distinct TCP was evolving concurrently with UDP in a very short time.

I'm hoping Dave remembers details.

Craig


On Sun, Nov 29, 2020 at 5:33 AM Vint Cerf via Internet-history <
internet-history at elists.isoc.org> wrote:

> the primary proponents of splitting off IP from TCP were Jon Postel, Danny
> Cohen  and David Reed, I believe. Sadly, Jon and Danny are no longer with
> us. My recollection is primarily that UDP was to allow for real-time,
> non-retransmitted, non-sequenced delivery for voice, video, radar in which
> low latency was more important than sequenced and assured delivery. As to
> the length field, it may merely have been habit to include, even if the
> value could have been computed. Sometimes <length> was used to distinguish
> real data from padding to achieve preferred word boundaries.
>
> v
>
>
> On Sat, Nov 28, 2020 at 8:21 PM Brian E Carpenter via Internet-history <
> internet-history at elists.isoc.org> wrote:
>
> > Reverse designing it (a bit like reverse engineering), it seems useful
> > to be able to check that the intended payload length fits inside the
> > actual packet length. If it doesn't, you are exposed to what you might
> > call buffer underrun issues. Conversely, if you don't like covert
> channels,
> > you might want to detect any spare bits after the payload.
> >
> > Regards
> >    Brian Carpenter
> >
> > On 29-Nov-20 12:42, Timothy J. Salo via Internet-history wrote:
> > > Hi,
> > >
> > > Can anyone provide some [historical] insight into why the UDP header
> > > contains a length field?  TCP manages to ascertain the length of data
> in
> > > a packet just fine without a length field, so why couldn't UDP?
> > >
> > > Several people have noted that the UDP length field is redundant,
> > > including for example, the current Internet Draft "Transport Options
> for
> > > UDP",
> > > <https://www.ietf.org/archive/id/draft-ietf-tsvwg-udp-options-09.txt>.
> > >
> > > There are some other opinions, some of which sound to me like
> > > after-the-fact reasoning:
> > >
> > > - So that UDP can run over network protocols other than IP (although
> > >    presumably TCP could do this just fine without a length field).
> But,
> > >    the UDP spec says that an IP-like pseudo header needs to be created,
> > >    in any case.
> > >
> > > - Layering and encapsulation reasons, (although, again, TCP seems like
> > >    a counter example).
> > >
> > > - Word alignment, (there were 16-bits left over, so why not use it for
> > >    the length?).  Personally, this sounds the most likely to me.
> > >
> > > Thanks,
> > >
> > > -tjs
> > >
> > --
> > Internet-history mailing list
> > Internet-history at elists.isoc.org
> > https://elists.isoc.org/mailman/listinfo/internet-history
> >
>
>
> --
> Please send any postal/overnight deliveries to:
> Vint Cerf
> 1435 Woodhurst Blvd
> McLean, VA 22102
> 703-448-0965
>
> until further notice
> --
> Internet-history mailing list
> Internet-history at elists.isoc.org
> https://elists.isoc.org/mailman/listinfo/internet-history
>


-- 
*****
Craig Partridge's email account for professional society activities and
mailing lists.