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Re: SV: [SCTP-Discussion] Initial slowstart behavior of SCTP


Chronological Thread 
  • From: "Brian F. G. Bidulock" <bidulock AT openss7.org>
  • To: Torbjörn Andersson <torbjorn.andersson AT kau.se>
  • Cc: discussion AT sctp.de
  • Subject: Re: SV: [SCTP-Discussion] Initial slowstart behavior of SCTP
  • Date: Wed Dec 4 11:02:40 2002
  • Dsn-notification-to: <bidulock AT openss7.org>
  • List-id: SCTP Discussions <discussion.sctp.de>
  • Organization: http://www.openss7.org/
Torbjörn,

On Wed, 04 Dec 2002, Torbjörn Andersson wrote:

> Thank you for your quick answer.
>
> Does not the cwnd grow with one byte for each ack:ed byte even if the
> bytes are sent within the cwnd limit?

No. RFC 2960 Section 7.2.2:

... cwnd should be incremented by 1*MTU per RTT if the sender has
cwnd or more bytes of data outstanding for the corresponding transport
address.

So, considering fragmentation, SWS avoidance and the length of user data
(likely not exact MTUs in size), an implementation must slop up to an MTU over
cwnd to grow it.

This is covered also in draft-ietf-sctpimplguide-07.txt:

Some implementations were having difficulties growing their cwnd. This
was due to improper enforcement of the congestion control rules.
The rules, as written, provided for a slop over the cwnd value.
Without this slop over the sender would appear to NOT be using its
full cwnd value and thus never increase it.

...

New text: (Section 6.1)

B) At any given time, the sender MUST NOT transmit new data to a
given transport address if it has cwnd or more bytes of data
outstanding to that transport address. The sender may exceed cwnd
by up to (PMTU-1) bytes on a new transmission if the cwnd is not
currently exceeded.

I assure you, the behavior is correct.

--brian

--
Brian F. G. Bidulock ¦ The reasonable man adapts himself to the ¦
bidulock AT openss7.org
¦ world; the unreasonable one persists in ¦
http://www.openss7.org/ ¦ trying to adapt the world to himself. ¦
¦ Therefore all progress depends on the ¦
¦ unreasonable man. -- George Bernard Shaw ¦


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