Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim: SuggestedFragmentation/Reassembly text

[Jonathan Morton <[e-mail]>]

> On 3 Dec, 2020, at 4:06 pm, Bob Briscoe <[e-mail]> wrote:
> 
> I see that all of Jonathan, David and you Markku are happy with reassembling a mix of ECT(0) and ECT(1) to result in either ECT(0) or ECT(1). (for now). I think we can go one better than that, still without precluding a more specific RFC later. Here's proposed text:
> 
> After the following para:
> 
>    During reassembly of outer fragments [I-D.ietf-intarea-tunnels], if
>    the ECN fields of the outer headers being reassembled into a single
>    packet consist of a mixture of Not-ECT and other ECN codepoints, the
>    packet MUST be discarded.
> 
> Add:
> 
> If there is mix of ECT(0) and ECT(1) fragments, then the reassembled packet MUST be set to either ECT(0) or ECT(1). In this case, reassembly SHOULD take into account that the RFC series has so far ensured that ECT(0) and ECT(1) can either be considered equivalent [RFC3168], or they can provide 2 levels of congestion severity, where the ranking of severity from highest to lowest is CE, ECT(1), ECT(0) [RFC6040]. 
> 
> Rationale: This avoids constraining future RFCs, but at least lays out all the interoperabilityrequirements we already have for handling this mixture. Then if an implementer wants to just default to choosing one, it hints that they should choose ECT(1).

This seems reasonable to me.  The text could do with a proofreading pass, but the technical content makes sense.

 - Jonathan Morton