Hi everybody.
I have one question regarding IPv6 Neighbor Discovery: What happens when a router receives a RS with a valid (possibly tentative) source address, but no SLLAO is included?
This can happen, for instance, with ODAD, where a host with a tentative address may want to solicit a RA with the tentative address. Section 3.2 (Modifications to RFC-mandated behaviour --> Modifications to RFC 2461 Neighbor Discovery) of draft-ietf-ipv6-optimistic-dad-05.txt says:
[...]
* (modifies 6.3.7) A node MUST NOT send a Router Solicitation with a
SLLAO from an Optimistic Address. Router Solicitations SHOULD
be sent from a non-Optimistic or the Unspecified Address,
however they MAY be sent from an Optimistic Address as long as
the SLLAO is not included.
[...]IMO, neither RFC 2461 nor RFC 2461bis is clear on what a router should do in this case. RFC 2461, section 6.2.6 (Router and Prefix Discovery --> Processing Router Solicitations) says:
[...] Router Solicitations in which the Source Address is the unspecified address MUST NOT update the router's Neighbor Cache; solicitations with a proper source address update the Neighbor Cache as follows. If the router already has a Neighbor Cache entry for the solicitation's sender, the solicitation contains a Source Link-Layer Address option, and the received link-layer address differs from that already in the cache, the link-layer address SHOULD be updated in the appropriate Neighbor Cache entry, and its reachability state MUST also be set to STALE. If there is no existing Neighbor Cache entry for the solicitation's sender, the router creates one, installs the link- layer address and sets its reachability state to STALE as specified in Section 7.3.3. Whether or not a Source Link-Layer Address option is provided, if a Neighbor Cache entry for the solicitation's sender exists (or is created) the entry's IsRouter flag MUST be set to FALSE. [...]
The first "If..." doesn't hold because the SLLAO is assumed not to be included in the RS. The second "If..." actually holds, but states that the router "installs the link-layer address and sets its reachability state to STALE". Well, but what is to be done when we do not have the SLLAO? Does the router need to look up the source address from the MAC header? And if it does not and leaves the link-layer address empty, which state should it put the new NC entry in? RFC 2461[bis] doesn't seem to have a state appropriate for this situation. (FreeBSD actually uses its own state; more on this further down.)
The part of section 7.3.3 (NUD --> Node Behavior) which the above text refers to is IMO the following:
[...] A Neighbor Cache entry enters the STALE state when created as a result of receiving packets other than solicited Neighbor Advertisements (i.e., Router Solicitations, Router Advertisements, Redirects, and Neighbor Solicitations). These packets contain the link-layer address of either the sender or, in the case of Redirect, the redirection target. However, receipt of these link-layer addresses does not confirm reachability of the forward-direction path to that node. Placing a newly created Neighbor Cache entry for which the link-layer address is known in the STALE state provides assurance that path failures are detected quickly. In addition, should a cached link-layer address be modified due to receiving one of the above messages the state SHOULD also be set to STALE to provide prompt verification that the path to the new link-layer address is working. [...]
Similar to the first extract, this one seems to assume that a SLLAO is always provided in an RS. So a newly created entry should be put into STALE state. But if there is no link-layer address, then the STALE state doesn't make much sense. The router might attempt to send a packet to that unspecified link-layer address as part of the NUD process.
RFC 2461bis doesn't clarify these things. I might be wrong, but I think there is some inconsistency in RFC 2461bis.
By the way, FreeBSD 5.3 kind of gets around this issue by using an additional NC-entry state, NOSTATE, for new entries for which the link-layer address is unknown. When a packet is to be forwarded (or send) to some neighbor, and the NC has an entry labeled NOSTATE, that entry is put into state INCOMPLETE and address resolution begins as usual.
As far as I can see, the additional state is needed if a router actually creates a NC entry upon receiving a RS without SLLAO. An alternative would be not to create the NC entry. But this may not be appropriate on links layers without source addresses...
Kind regards,
- Christian
-- Christian Vogt, Institute of Telematics, University of Karlsruhe www.tm.uka.de/~chvogt/pubkey/
"No great genius has ever existed without some touch of madness." (Aristotle)
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