Re: Proposal to further clarify prefix length issues in I-D.ietf-6man-rfc4291bis

james woodyatt <> Thu, 09 March 2017 16:26 UTC

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Subject: Re: Proposal to further clarify prefix length issues in I-D.ietf-6man-rfc4291bis
From: james woodyatt <>
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Date: Thu, 9 Mar 2017 08:26:40 -0800
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To: David Farmer <>
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Hi David,

Thank you very much for giving my proposal a fair hearing. This is just a quick response to one point you made. I may have more to say about the rest later.

On Mar 9, 2017, at 06:27, David Farmer <> wrote:
> The conservative version of the proposal at least needs to be adjusted to allow for RFC6164 /127 point-to-point router links as an exception to on-link prefixes always being /64, there seems to be a conflict here in the conservative version.

I agree. While RFC 6164 does use the word “subnet” in a confusing way a few times, it seems reasonable to read it as a standard exception to the rule that on-link prefixes are always subnet prefixes (under my “conservative” proposal).

Hence, here is my revised proposal (conservative) for §2.4.8 Neighbor Discovery.

— Insertion of §2.4.8 Neighbor Discovery

Proposed (conservative):
> Nodes can send packets directly to other nodes with interfaces on the same link, without forwarding by a router, when they can identify the link-layer addresses associated with the unicast IPv6 addresses that share on-link subnet prefixes assigned to the interface. Neighbor discovery is conceptually the process of resolving such associations as necessary.
> Nodes may be configured manually with on-link subnet prefixes for each of their interfaces. Also, routers may inform hosts automatically of the set of on-link subnet prefixes by a process related to neighbor discovery. Nodes sending packets to unicast addresses with subnet prefixes that are not on-link must direct those packets to a router for forwarding to their destination.
> Apart from standard exceptions, every on-link prefix is required to be a subnet prefix, and subnet prefixes for all global unicast addresses beginning with a binary value other than 000 are required according to Section 2.4 to be 64 bits in length. One standard exception to this is Using 127-Bit IPv6 Prefixes on Inter-Router Links [RFC6164].

Reasoning: this text provides clarification that, apart from standard exceptions, on-link prefixes are subnet prefixes for which neighbor discovery can be used by nodes to communicate directly without forwarding packets through a router. It also repeats the requirement that all subnet prefixes other than those beginning with 0b000 are 64 bits long. It provides an explicit reference to RFC6164 as a standard exception.

--james woodyatt <>