Re: [babel] About ICMP and unnumbered routers

["STARK, BARBARA H" <[e-mail]>]

Has an RFC 5735 169.254.0.0/16 IPv4 link local address been considered? Do the ICMP packets need to cross routed boundaries? [Sorry, I haven't been paying careful attention to the issue, so I haven't taken the time to understand it.]
Barbara

> -----Original Message-----
> From: Donald Eastlake
> 
> Hi Juliusz/David,
> 
> Thanks for the research. See below.
> 
> On Sun, Aug 2, 2020 at 3:08 PM Juliusz Chroboczek <[e-mail]> 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.
> 
> I would recommend against this due to possible collisions.
> 
> > 2. Request a dedicated IPv4 address (the "all-routers anycast address") to
> >    be used as the loopback address of unnumbered routers.
> 
> I think this is the best way forward. Probably the "IPv4 dummy
> address" David Schinazi found would do fine. (If there is any problem
> with that, getting a new special use IPv4 address for this purpose
> would be pretty easy. The assignment requirement is "IETF Review"
> which means any document that goes through IETF Last call will do as
> the basis for a new assignment.
> https://urldefense.proofpoint.com/v2/url?u=https-
> 3A__www.iana.org_assignments_iana-2Dipv4-2Dspecial-2Dregistry_iana-
> 2Dipv4-2Dspecial-2Dregistry.xhtml&d=DwICAg&c=LFYZ-
> o9_HUMeMTSQicvjIg&r=LoGzhC-8sc8SY8Tq4vrfog&m=3AJm3dEuSiQGZ-
> ZV79-GC1BPS_4jpUF5cvaTIlD5ZgI&s=b0I9NcTMzHIVkI9EZQ5GEcPBe-dg-
> xPXFNH5MtgyCH8&e= )
> 
> > 3. Update RFC 1812 to allow unnumbered routers and 0.0.0.0 as the source
> >    address of ICMPv4 packets.
> 
> Updating RFC 1812 seems like a more difficult path and possibly there
> are routers or middle boxes that discard packets with a 0.0.0.0 source
> as bogus. I don't know if the text is still valid but RFC 1122 says of
> 0.0.0.0 source:
> 
>                  This host on this network.  MUST NOT be sent, except as
>                  a source address as part of an initialization procedure
>                  by which the host learns its own IP address.
> 
> Thanks,
> Donald
> ===============================
>  Donald E. Eastlake 3rd   [phone] (cell)
>  2386 Panoramic Circle, Apopka, FL 32703 USA
>  [e-mail]
> 
> > I'm interested in the comments of wise people.
> >
> > -- Juliusz
> 
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