Re: rfc 1812 third party address on traceroute

[Hugo Slabbert <hugo () slabnet com>]

On Tue 2016-May-31 09:08:42 -0700, Octavio Alvarez <octalnanog () alvarezp org> wrote:

> On 05/30/2016 10:03 PM, Randy Bush wrote:
> > rfc1812 says
> >
> >    4.3.2.4 ICMP Message Source Address
> >
> >    Except where this document specifies otherwise, 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.
> >
> > some folk have interpreted this to mean that, if a router R has three
> > interfaces
> >
> >                .-----------------.
> >                |                 |
> >                |               B |--------- D
> >     S ---------| A      R        |
> >                |               C |--------- (toward S)
> >                |                 |
> >                `-----------------'
> >
> > of course, simpletons such as i would desire the source of the time
> > exceeded message to be A.  after all, this is the interface to which i
> > sent the icmp with the TTL to expire.
>
> Do you mean the source address or the source interface?
>
> I'm not sure if you mean that, if sent through C it should have the
> source addres of A, or that it should actually be sent through A
> regardless of the routing table (which sounds better to me).

How is the latter better?  What guarantees are there that the adjacent L3 
device on R's interface A has a route for S and if such a route exists that 
it doesn't simply point at R?  As Randy so eloquently put it:

> > (yes, virginia, the internet is highly asymmetric)

> Octavio.

--
Hugo Slabbert       | email, xmpp/jabber: hugo () slabnet com
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