RE: [v6ops] RFC4861 question - short prefixes in PIOs
"Templin (US), Fred L" <Fred.L.Templin@boeing.com> Wed, 26 June 2019 20:22 UTC
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From: "Templin (US), Fred L" <Fred.L.Templin@boeing.com>
To: David Farmer <farmer@umn.edu>, 神明達哉 <jinmei@wide.ad.jp>
CC: 6man <6man@ietf.org>
Subject: RE: [v6ops] RFC4861 question - short prefixes in PIOs
Thread-Topic: [v6ops] RFC4861 question - short prefixes in PIOs
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Date: Wed, 26 Jun 2019 20:22:03 +0000
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Thanks for your response, David: >> RFC 8028 updates RFC4861 and talks about A=L=0, it talks about this in Section 2.1; That does indeed look like the kind of behavior I am after, but how is it different than RFC4191 RIOs? RIOs include a Preference value for each router that advertises the prefix. Do we lose that feature by going with PIOs per RFC8028? Thanks - Fred From: David Farmer [mailto:farmer@umn.edu] Sent: Wednesday, June 26, 2019 12:01 PM To: 神明達哉 <jinmei@wide.ad.jp> Cc: Templin (US), Fred L <Fred.L.Templin@boeing.com>; 6man <6man@ietf.org> Subject: Re: [v6ops] RFC4861 question - short prefixes in PIOs Some additional comments regarding questions #3 and #4 are below; On Wed, Jun 26, 2019 at 11:23 AM 神明達哉 <jinmei@wide.ad.jp<mailto:jinmei@wide.ad.jp>> wrote: (I'm only copying 6man, as I believe it's purely a protocol spec question) At Wed, 26 Jun 2019 15:56:36 +0000, "Templin (US), Fred L" <Fred.L.Templin@boeing.com<mailto:Fred.L.Templin@boeing..com>> wrote: > > I have an RFC4861 question (several actually) on short prefixes in RA PIOs: > > 1) If a PIO includes a prefix with length less than 64 (e.g., 2001:db8::/32) and with L=1, does it > > mean that 2001:db8::/32 should be added to the interface prefix list? In my interpretation (ditto for subsequent questions), yes. +1 > 2) If yes to 1), does it mean that packets forwarded to the interface for any destination covered > > by 2001:db8::/32 will trigger Address Resolution instead of forwarding to a default router? Yes. +1 > 3) If the PIO instead has L=0, does it mean that 2001:db8::/32 is “associated” with the link but > not necessarily “on-link”? I'm not sure how to interpret it (in particular I'm not sure what "associated with the link" means), but my interpretation of L=0 is that the RA doesn't say anything about the on-link-ness of that prefix. See also the description of the L flag in RFC4861: L 1-bit on-link flag. [...] When not set the advertisement makes no statement about on-link or off-link properties of the prefix. In other words, if the L flag is not set a host MUST NOT conclude that an address derived from the prefix is off-link. That is, it MUST NOT update a previous indication that the address is on-link. RFC 8028 updates RFC4861 and talks about A=L=0, it talks about this in Section 2.1; In some circumstances, both L and A might be zero. If SLAAC is not wanted (A=0) and there is no reason to announce an on-link prefix (L=0), a PIO SHOULD be sent to inform hosts that they should use the router in question as the first hop for packets with source addresses in the PIO prefix. An example case is the MIF router in Figure 1, which could send PIOs with A=L=0 for the common prefix. Although this does not violate the existing standard [RFC4861], such a PIO has not previously been common, and it is possible that existing host implementations simply ignore such a PIO or that existing router implementations are not capable of sending such a PIO. Newer implementations that support this mechanism should be updated accordingly: o A host SHOULD NOT ignore a PIO simply because both L and A flags are cleared (extending Section 6.3.4 of [RFC4861]). o A router SHOULD be able to send such a PIO (extending Section 6.2.3 of [RFC4861]). > 4) If yes to 3), does it mean that 2001:db8::/32 should be added to the IPv6 forwarding table > > as a “route-to-interface” with the receiving interface as the next hop? No. See the second MUST NOT of the RFC4861 text cited above. I wouldn't call it a “route-to-interface” it should be a route pointing to the route as the next-hop. > 5) Does A=1 have any meaning for prefixes with length less than 64? Or, must prefixes with > > length less than 64 set A=0? As far as RFC4861 is concerned, the A flag has no meaning, regardless of the prefix length. It only matters in RFC4862. In terms of RFC4862, whether "A=1 has any meaning for prefixes with length less than 64" depends on the length of the IID of the link; if the prefix length != 128-IIDLength, the validation rule 5.5.3 d) of RFC4862 makes the prefix ignored. If non-64 prefix length is invalid in terms of RFC4862 in that sense, it'd be *safe* to avoid setting the A flag, but the protocol specification doesn't say it *must* be so. +1 You may also want to check https://tools.ietf.org/html/draft-jinmei-6man-prefix-clarify-00 I believe it clarifies many of the above questions. -- JINMEI, Tatuya -------------------------------------------------------------------- IETF IPv6 working group mailing list ipv6@ietf.org<mailto:ipv6@ietf.org> Administrative Requests: https://www.ietf.org/mailman/listinfo/ipv6 -------------------------------------------------------------------- -- =============================================== David Farmer Email:farmer@umn.edu<mailto:Email%3Afarmer@umn.edu> Networking & Telecommunication Services Office of Information Technology University of Minnesota 2218 University Ave SE Phone: 612-626-0815 Minneapolis, MN 55414-3029 Cell: 612-812-9952 ===============================================
- RFC4861 question - short prefixes in PIOs Templin (US), Fred L
- Re: [v6ops] RFC4861 question - short prefixes in … 神明達哉
- RE: [v6ops] RFC4861 question - short prefixes in … Templin (US), Fred L
- RE: [v6ops] RFC4861 question - short prefixes in … Dave Thaler
- Re: [v6ops] RFC4861 question - short prefixes in … David Farmer
- RE: [v6ops] RFC4861 question - short prefixes in … Templin (US), Fred L
- RE: [v6ops] RFC4861 question - short prefixes in … Templin (US), Fred L
- RE: [v6ops] RFC4861 question - short prefixes in … Dave Thaler
- Re: [v6ops] RFC4861 question - short prefixes in … David Farmer
- Re: [v6ops] RFC4861 question - short prefixes in … Brian E Carpenter
- Re: [v6ops] RFC4861 question - short prefixes in … Mark Smith
- Re: [v6ops] RFC4861 question - short prefixes in … Alexandre Petrescu
- Re: [v6ops] RFC4861 question - short prefixes in … Mark Smith
- Re: RFC4861 question - short prefixes in PIOs Michael Richardson
- Re: RFC4861 question - short prefixes in PIOs Mark Smith
- Re: [v6ops] RFC4861 question - short prefixes in … Alexandre Petrescu
- Re: [v6ops] RFC4861 question - short prefixes in … Philip Homburg
- Re: [v6ops] RFC4861 question - short prefixes in … Mark Smith
- Re: [v6ops] RFC4861 question - short prefixes in … Pascal Thubert (pthubert)