Subject: Re: [mobile-ip] 0.0.0.0 src address

From: Samita Chakrabarti <[email-masked]>
Date: Fri, 30 Nov 2001 14:26:41 -0800 (PST)

> 
> Section 3.6.1.1 states that the Mobile Node must use 0.0.0.0 as the IP
> source address of the registration request if it does not have a home
> address. Section 3.7.2.3 states that the FA must use the
> IP source of the request as IP destination address when forwarding
> replies. 
> 

Yes, this is a concern. Usually the MNs are supposed to have their assigned
home-addr or a co-located careof addr.  In the case where MN may not
have been assigned home-addr, it could use NAI (rfc2794).

> Thus there might be cases where the FA must forward replies to the
> address 0.0.0.0. This task appears "impossible" since rfc1122 defines
> 0.0.0.0 as something like "this machine on this network", like
> loopback. So a packet with destination address 0.0.0.0 might never go
> out of the FA.
> 

Right, if you send  IP packet with dst=0.0.0.0 out in the network, it  does
not comply with rfc1122. But in some cases, it's possible to have no
home-addr assigned and the solution should be to use NAI. Please check out
rfc2794 and rfc2290.

-Samita

---

From: Marc Hasson <[email-masked]>
Date: Fri, 30 Nov 2001 18:31:45 -0800 (PST)

Alper,

 > The foreign agent should use the link-layer address of the incoming (from
 > MN) packet
 > to send back the responds to the MN. So, FA can send a packet with 0.0.0.0
 > IP destination
 > address and MN's link-layer address as the destination MAC address.
 > 
 > For that, foreign agent should note the source link-layer address of the
 > incoming packet.
 > 
 > The same can also happen with simple rotuer discovery (sending router
 > solicitation, and
 > receiving router advertisement). Please see rfc1256.

I don't see where RFC 1256 supports the concept of *sending* to a
destination address of 0.0.0.0 so let me know if I've missed
some text.   It clearly allows a router solicitation with a
*source* address of 0.0.0.0 from a host that hasn't yet determined its
own address.  But the router advertisement response in such a case
doesn't go to 0.0.0.0, which would be a violation of the RFC as I read
it, but to a configured address.  Here's the text excerpt from the RFC
in section 4.3 that clearly covers the source 0.0.0.0 address case in
the second sentence:

   In addition to the periodic, unsolicited advertisements, a router
   sends advertisements in response to valid solicitations received on
   any of its advertising interfaces.  A router may choose to unicast
   the response directly to the soliciting host's address (if it is not
   zero), or multicast it to the interface's configured
   AdvertisementAddress; in the latter case, the interface's interval
   timer is reset to a new random value, as with unsolicited
   advertisements.  ...

The rationale for a host sending with a source address of 0.0.0.0
appears to be implied in section 5.3 as a mechanism to allow overlap of
the host's address and router discovery processes to occur in
parallel.  Such a host presumably provokes an earlier multicast router
advertisement by sending the solicitation with its 0.0.0.0 source
address rather than waiting for the next periodic advertisement or
until the host has an address.

   -- Marc --