Re: [multipathtcp] Alissa Cooper's Discuss on draft-ietf-mptcp-rfc6824bis-15: (with DISCUSS and COMMENT)

Alan Ford <> Tue, 14 May 2019 17:05 UTC

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From: Alan Ford <>
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Date: Tue, 14 May 2019 18:05:19 +0100
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Subject: Re: [multipathtcp] Alissa Cooper's Discuss on draft-ietf-mptcp-rfc6824bis-15: (with DISCUSS and COMMENT)
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Hi Alissa,

Many thanks for your feedback here. This is a simple typo - the definition of B should affect bits D - H and not C - H. All other sections referring to this (e.g. the IANA considerations) correctly point to D - H, so we will correct this.

The history of the extensibility bit goes back to security area concerns on the original 6824, and wanting to ensure we had a mechanism in the protocol to redefine bits of the handshake to allow an alternative security mechanism to be used in the future, without needing a full new version of the protocol to be defined. Whilst I would be happy to remove it from v1, the security area felt strongly about this in v0, and the flexibility that is potentially available by assigning this bit for this purpose seems to come at a small cost.

Best regards,

> On 13 May 2019, at 21:22, Alissa Cooper via Datatracker <> wrote:
> Alissa Cooper has entered the following ballot position for
> draft-ietf-mptcp-rfc6824bis-15: Discuss
> When responding, please keep the subject line intact and reply to all
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> introductory paragraph, however.)
> Please refer to
> for more information about IESG DISCUSS and COMMENT positions.
> The document, along with other ballot positions, can be found here:
> ----------------------------------------------------------------------
> ----------------------------------------------------------------------
> I have a question about the interaction between the protocol versioning and the
> way the flags B-H are defined. This doc specifies that B MUST be set to 0 and
> that future specs might require it to be set to 1, which could alter the
> meanings of the C-H flags. This is the same way that B is defined in RFC 6824.
> This spec goes on to define C and H differently from how they are defined in
> RFC 6824. Thus, both v0 and v1 implementations will have B set to 0, but they
> will have C and H defined differently. I guess it depends on how you interpret
> "extensibility flag," but my interpretation of this is that the version number
> negotiated using MP_CAPABLE is the actual extensibility mechanism that this
> specification is making use of, because although it changed C and H it did not
> change B. Is this right?
> If so, I'm wondering what the threshold is for defining new versions of this
> protocol versus using this extensibility mechanism based on the B flag. Given
> the way the version negotiation mechanism is defined -- requiring extra
> round-trips in the event of a fallback attempt, and it being subject to a
> downgrade attack -- I'm wondering if some guidance needs to be provided about
> which sorts of protocol changes are expected to be made using the B-flag
> mechanism versus using the version negotiation mechanism. I guess the drawback
> of using the B-flag mechanism is that once a future spec sets it to 1, from
> then on logic within implementations will be required to interpret potentially
> multiple different semantics of C-H. I guess this potentially raises another
> question, which is why the B flag isn't just deprecated given the addition of
> the version negotiation mechanism.
> ----------------------------------------------------------------------
> ----------------------------------------------------------------------
> Section 3.7: s/the lost of MPTCP options/the loss of MPTCP options/
> Appendix E should be titled "Changes from RFC 6824"