[ih] UDP Length Field?

[vinton cerf]

1977 - about TCP 3 time...

v


On Sun, Nov 29, 2020 at 9:48 AM Craig Partridge via Internet-history <
internet-history at elists.isoc.org> wrote:

> 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.
> --
> Internet-history mailing list
> Internet-history at elists.isoc.org
> https://elists.isoc.org/mailman/listinfo/internet-history
>