Re: rfc 1812 third party address on traceroute

[Octavio Alvarez <octalnanog () alvarezp org>]

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).

Octavio.