[tsvwg] SCTP ABC

[Timo Völker <[e-mail]>]

Hi,

SCTP (RFC4960) limits the increase of its cwnd in slow start to one PMTU (or PMDCS as named in the rfc4960-bis document) as stated in Section 7.2.1. This means the cwnd may be increased less when the receiver sends an ack on every second packet compared to on every packet.

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Ex:
1. A sender in slow start with PMTU=1200 sends two packets with 1000 byte of data each.

> DATA(len=1000, tsn=1)
> DATA(len=1000, tsn=2)

2a. In case the receiver sends an ack on every packet, the sender receives two acks.

< ACK(cum_tsn=1)
--> cwnd += 1000
< ACK(cum_tsn=2)
--> cwnd += 1000

resulting in a cwnd increased by 2000 byte.

2b. In case the receiver sends an ack on every second packet, the sender receives one ack for both packets.

< ACK(cum_tsn=2)
--> cwnd += 1200

resulting in a cwnd increased by only 1200 byte due to the PMTU limit.
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TCP's Appropriate Byte Counting (ABC) specified in RFC3465 addresses that with the parameter L set to 2*SMSS.

Adding a note for that in rfc4960-bis would help, I think.

Timo