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RFC 4861, "Neighbor Discovery for IP version 6 (IPv6)", September 2007

Source of RFC: ipv6 (int)
See Also: RFC 4861w/ inline errata

Errata ID: 2709
Status: Verified
Type: Technical
Publication Format(s) : TEXT

Reported By: Jan Kramer
Date Reported: 2011-02-09
Verifier Name: Brian Haberman
Date Verified: 2012-10-03

Section Appendix C says:

!INCOMPLETE     NA, Solicited=1,        -                   REACHABLE
                Override=0
                Same link-layer
                address as cached.

!INCOMPLETE     NA, Solicited=any,     Update content of    unchanged
                Override=any, No       IsRouter flag.
                link-layer address

It should say:

!INCOMPLETE     NA, Solicited=1,      -                   REACHABLE
                Override=0
                Same link-layer
                address as cached.

!INCOMPLETE     NA, Solicited=1,     Update content of    REACHABLE
                Override=any, No     IsRouter flag.
                link-layer address

!INCOMPLETE     NA, Solicited=0,     Update content of    unchanged
                Override=any, No     IsRouter flag.
                link-layer address


or 



!INCOMPLETE     NA, Solicited=1,        -                   REACHABLE
                Override=0
                Same link-layer
                address as cached
                or no link-layer 
                address

!INCOMPLETE     NA, Solicited=any,     Update content of    unchanged
                Override=any, No       IsRouter flag.
                link-layer address



Notes:

Section 7.2.4. says:

"If the solicitation's IP Destination Address is
not a multicast address, the Target Link-Layer Address option MAY be
omitted; the neighboring node's cached value must already be current
in order for the solicitation to have been received."

Consider host A has a Neighbor Cache Entry for a unicast address of host B with the state PROBE. If it sends an NS to that address, B will answer with a NA.
If the Target Link-Layer Address is actually omitted, the host which sent the solicitation would only update the IsRouter flag of the Neighbor Cache Entry and leave the state unchanged.
At retransmit timeout host A would send another NS, since the state is still PROBE. After some retransmissions the entry would be discarded, although it was obviously reachable.

With one of the above suggestions, the Neighbor Cache Entry will be marked as REACHABLE, even if no Target Link-Layer Option is included in the NA.

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