Re: [babel] About ICMP and unnumbered routers

I don't know if I qualify as a "wise person", but I have comments :-)

I may have found a solution from 4rd. They reserved an
"IPv4 dummy address" specifically for the purpose of
filling in the source address of ICMPv4 packets:

https://tools.ietf.org/html/rfc7600#section-4.8
(Text copied below for convenience)

I think this might suit our needs?

Cheers,
David

4.8.  Tunnel-Generated ICMPv6 Error Messages

   If a Tunnel packet is discarded on its way across a 4rd domain
   because of an unreachable destination, an ICMPv6 error message is
   returned to the IPv6 source.  For the IPv4 source of the discarded
   packet to be informed of packet loss, the ICMPv6 message has to be
   converted into an ICMPv4 message.

   R-22: If a CE or BR receives an ICMPv6 error message [RFC4443], it
         MUST synthesize an ICMPv4 error packet [RFC792].  This packet
         MUST contain the first 8 octets of the discarded packet's IP
         payload.  The reserved IPv4 dummy address (192.0.0.8/32; see
         Section 6) MUST be used as its source address.

         As described in [RFC6145], ICMPv6 Type = 1 and Code = 0
         (Destination Unreachable, No route to destination) MUST be
         translated into ICMPv4 Type = 3 and Code = 0 (Destination
         Unreachable, Net unreachable), and ICMPv6 Type = 3 and Code = 0
         (Time Exceeded, Hop limit exceeded in transit) MUST be
         translated into ICMPv4 Type = 11 and Code = 0 (Time Exceeded,
         time to live exceeded in transit).

On Sun, Aug 2, 2020 at 12:08 PM Juliusz Chroboczek <jch@irif.fr> wrote:

> Dear all,
>
> This mail is about the consequences of draft-bastian-babel-v4ov6-00 on
> ICMPv4 generation.
>
> At last week's meeting, Margaret kindly pointed out that we didn't do our
> homework, and that we didn't consider how ICMPv4 is sent out when the
> sending router has no IPv4 addresses.  I've looked at a few RFCs, and here
> are my tentative conclusions.
>
> RFC 1812 Section 2.2.7 says:
>
>    In this scheme, a router that
>    has unnumbered point to point lines also has a special IP address,
>    called a router-id in this memo.  The router-id is one of the
>    router's IP addresses (a router is required to have at least one IP
>    address).  This router-id is used as if it is the IP address of all
>    unnumbered interfaces.
>
> So it looks like it was decided back in 1995 that all IPv4 routers need at
> least one IPv4 address.  Much as I tried, I couldn't find anything that
> would relax this requirement.  As expected, when an ICMPv4 packet is sent
> over an unnumbered interface, the distinguished "router-id" address is
> used as the source address:
>
>    the IP source address in an ICMP message originated by the router MUST
>    be one of the IP addresses associated with the physical interface over
>    which the ICMP message is transmitted.  If the interface has no IP
>    addresses associated with it, the router's router-id (see Section
>    [5.2.5]) is used instead.
>
> (This appears to be a typo -- there's nothing relevant in Section 5.2.5.)
>
> Interestingly, there appears to be nothing in RFC 1812 that specifies that
> the router-id is globally unique; thus, what we could do would be to put
> the same IPv4 address on all routers.  This would make traceroute a little
> confusing (RFC 5837 might help), but it would solve both the problem with
> ICMPv4 blackholes and compliance with RFC 1812.
>
> In the light of the above, and in the immortal words of Lenin, what shall
> we do?
>
> 1. Pick a private IPv4 (say 10.42.42.42) and put it on the loopback
>    interface of all unnumbered routers.
>
> 2. Request a dedicated IPv4 address (the "all-routers anycast address") to
>    be used as the loopback address of unnumbered routers.
>
> 3. Update RFC 1812 to allow unnumbered routers and 0.0.0.0 as the source
>    address of ICMPv4 packets.
>
> I'm interested in the comments of wise people.
>
> -- Juliusz
>
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