[tsvwg] draft-stewart-tsvwg-sctpecn

["Scheffenegger, Richard" <[e-mail]>]

Hi,


While looking as to how other protocol do ECN feedback signaling, I found that this draft is missing (at least not mentioning) an interesting detail.

The existence of a CWR Chunk indicates, that the feedback signal is supposed to be reliable.

However, the ECN Echo chunk defines the Number of CE marked Packets as 32-bit unsigned int, and does not specify how a receiver should behave it never received a CWR chunk...

The sheer size of the ECN Echo chunk would lend itself to be a simple feedback signal, without any explicit signal from the sender.

Historically, in RFC4960, it appears that this chunk effectively contained only a single bit (no counter; only the presence of the chunk). In order to establish a reliable feedback for at most one ECN signal per RTT, a handshake mechanism (CWR chunk) is required. That mechanism was very much alike what TCP does with ECE / CWR.



IMHO, this draft should be more clear around the mechanism:

If a handshake is required, the 32-bit counter MUST NOT roll over at 2^32-1, but remain at that value until CWR is received.

The sheer size of the counter however makes it highly unlikely that the sender will not receive a signal (before the session collapses for lack of feedback from the receiver). So a handshake-less (CWR chunk optional on the sender?) is also feasible, but then the ECN counter MUST roll over...


As it stands now, the counter rolls over, and is reset via CWR...

Are there any specific reasons for this chimera mechanism? 

Does it allow for special use cases, which cannot be addressed by either handshake or simple counter alone?

I'm very interested in the answers to the last two questions, as I am one of the authors of an Accurate ECN feedback draft for TCP, and TCPM has a milestone to define an appropriate (accurate) feedback mechanism that allows more than 1 bit per RTT... 

Best regards,

Richard Scheffenegger

> draft-stewart-tsvwg-sctpecn-02
> Partially replaces (draft-stewart-tsvwg-sctp-nonce)
> IETF-78 suggested there was interest in this topic.
> Discussed at IETF-83, Paris.
> DUE: ECN is within the TSVWG Charter,
> will call for adoption on the list.
> Please comment on list.