Subject: [MOBILE-IP] Link-layer/IP destination address

From: Luca Salgarelli <[email-masked]>
Date: Thu, 13 Apr 2000 11:31:26 -0400

Hi.

I have a question regarding router advertisements.

When a MIP agent sends a router advertisement in response to a
solicitation, RFC2002(bis) seems to say that while the dst. link-layer address
of the advertisement should be the unicast source link-layer address of the
solicitation, the IP dst. address should be a MULTICAST or BCAST address.
This seems to be difficult to implement, and I do not see the reason for
this agent behaviour. In addition, RFC1256, cited by RFC2002, seems to
specify the procedure for the selection of the IP dst. addr. of
advertisement differently.

Regarding router advertisements, RFC2002(bis) says, in section 2.1:

    -  Link-Layer Fields

          Destination Address
                   The link-layer destination address of a unicast
                   Agent Advertisement MUST be the same as the source
                   link-layer address of the Agent Solicitation which
                   prompted the Advertisement.

    -  IP Fields

          Destination Address
                   As specified for ICMP Router Discovery [7], the IP
                   destination address of an Agent Advertisement MUST
                   be either the "all systems on this link" multicast
                   address (224.0.0.1) [8] or the "limited broadcast"
                   address (255.255.255.255).  The subnet-directed
                   broadcast address of the form <prefix>.<-1> cannot be
                   used since mobile nodes will not generally know the
                   prefix of the foreign network.

On the other hand, regarding the IP destination address, RFC1256 says, in
section 3:

      Destination Address   The configured AdvertisementAddress or the
                            IP address of a neighboring host.

thus implying that the unicast IP address of the host that solicited the
advertisement should be used, when responding to solicitations.

Is this a bad wording of RFC2002, or the unicast link-layer, but MCAST or
BCAST IP-layer dst address should really be used *when replying to
solicitations*?

Thanks
Luca