[e2e] overlay over TCP

[Armando L. Caro, Jr.]

On Thu, 20 Jan 2005, Joe Touch wrote:

> > Basically to get unreliable service (equiviant to udp)
> > all one has to do is set a lifetime parameter less than
> > the 1 second aka RTO.min.
>
> That's perfectly clear! Wait - no, maybe not. So there's no "unreliable"
> flag, ala the unordered flag?
>
> And what does a 'lifetime less than 1 second' mean, exactly - to
> send-side buffers, receive-side, etc? Will it stall other packets sent?
> Others received? If so, it's not UDP semantics per se...

Well, PR-SCTP was not exactly meant to match UDP semantics. It was meant
to offer a richer feature set. It can provide UDP-like reliability service
(ie, unreliable), but it can also provide a range of reliability between
UDP and TCP/SCTP. You cannot have a simple unreliable flag to provide
_that_ service. You _do_ have a partially reliable flag (MSG_PR_SCTP_TTL)
and a corresponding lifetime parameter whose units are in milliseconds.

> > If one does that then you will
> > have the same symantic as UDP. One can choose to
> > use a smaller time then RTO.min too I suppose but
> > if you are after a unreliable service that would
> > be how one would do it.
>
> You 'suppose'? It's not that simple, is it?

Sure it is. You can put a 1 in the lifetime parameter to get a 1ms
lifetime for a data chunk. UNP's Section 9.9 explains in one paragraph how
to use the sctp_sendmsg to get partially reliable service.

> > Some implementations actually
> > have a "SEND_ONLY_ONCE" like flag, this can also
> > be used as well, but of course thats an implemenation
> > specific thing.
>
> So the protocol MIGHT support UDP semantics, IF the API provides access
> to enough of SCTP's knobs that you can figure out how to set it? I'm not
> surprised I didn't notice _that_ in the spec...

PR-SCTP _does_ provide unreliable service. I don't think it is
unreasonable for the application developer to read a man page to determine
how to use an API to get unreliable service from PR-SCTP.

~armando

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| Armando L. Caro, Jr. |  Protocol Engineering Lab |
| www.armandocaro.net  |    University of Delaware |
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