Subject: [tsvwg] question about layered congestion issue - draft-ietf-tsvwg-tunnel-congestion-feedback

From: Joe Touch <[email-masked]>
Date: Thu, 30 Mar 2017 17:28:12 -0700

Hi, all,

Discussing this off-list with my co-author on the
draft-ietf-intarea-tunnels doc (Mark Townsley, cc'd), we was wondering
what has been done to validate this work against the potential for
layered congestion interference?

I.e., propagating ECN from inside the tunnel to outside (RFC 6040) is
safe, but using that information to govern tunnel admission seems like
it's using ECN to determine per-hop forwarding. I would expect that to
be the purview of the router where the tunnel is attached (where the
tunnel ingress and egress act as *interfaces* at either routers or hosts
where the tunnel originates/terminates  - as per
draft-ietf-intarea-tunnels).

IMO, there are two clearly valid approaches (which have already been
addressed by RFCs):

    1) use the intra-tunnel ECN to trigger circuit breakers (RFC 8084)

    2) propagate the intra-tunnel ECN to outside the tunnel (RFC 6040)

However, this document appears to recommend an approach that results in
layered congestion control, but doesn't appear to address decoupling
interactions with E2E congestion control (e.g., TCP, DCCP, SCTP), nor
does it seem to present any analysis (incl. experiments) to demonstrate
that this isn't a concern. In fact, it mentions these protocols only in
passing in the introduction.

Has this been addressed? If not, can it be?

Thanks,

Joe