I'm not clear, about this, but presumably the presence of a 6CIO is backward compatible. That is, an implementation that does not support 6CIO but receives one does not crash, and either ignores it or tells the sender that something odd was received. So 6CIO serves much the same purpose as the T-flag and it seems (to me!) you could use on or the other (or both, as you have done!) I tend to disagree with Carsten that putting flags in MBZ fields typically has worse backwards compatibility. They were defined as MBZ (must ignore) specifically to be forward compatible with new usage. Adrian From: Pascal Thubert (pthubert) [mailto:[email protected]] Sent: 26 February 2018 18:01 To: [email protected] Cc: Adrian Farrel ([email protected]) Subject: 6CIO in rfc 6775 update Dear all Quoting Adrian's RTG DIR review: > 7.1.2 > > One alternate way for a 6LN to discover the router's capabilities is > to start by registering... > > You went to a lot of trouble to define the E-flag. You then made the use of the > 6CIO (and hence the E-flag) only a SHOULD, and you defined an alternate > mechanism. (Note: you say "one alternate" implying there are > more!) > > Choice is not good. It complicates the specification and the implementation. > Why go there? Can't you settle on one mechanism and make it necessary and > sufficient? > https://tools.ietf.org/html/draft-ietf-6lo-rfc6775-update-14#section-7.1.1 defines new capability bits for use in the 6CIO, which was introduced by RFC7400 to announce support of the spec. When there is no 6CIO, there's a plan B using EUI-64 as RUID and IID in a NS(EARO), which is fully backward compatible, and see if the response has an EARO or not from the T flag. Adrian indicates that a double mechanism is complexity. For backwards compatibility we need to be able to live without 6CIO. Still 6CIO seems to be a good mechanism to generalize and that's why we used it. So should we keep the CIO mechanism, or should we drop it? Pascal
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