Re: rfc 1812 third party address on traceroute

[Marc Storck <mstorck () voipgate com>]

With BCP38 in mind, could therre be situations where Router R is not allowed to source packets with address A out of 
intergace C?

I think that the possibility does exist.

E.g. If interface A and C are upstream interfaces, router R may use an IP address from ISP A on interface A and an 
address from ISP C on interface C.

Obviously BCP38 is not widely deployed but yet...

Regards,

Marc

> On 31 mai 2016, at 07:05, Randy Bush <randy () psg com> 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)
>               |                 |
>               `-----------------'
>
> if the source of a traceroute from S toward D with TTL to expire on R,
> and R's FIB wants to exit via C to get back to S (yes, virginia, the
> internet is highly asymmetric), the source address of the time exceeded
> message should be C.
>
> 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.
>
> ras's preso,
> https://www.nanog.org/meetings/nanog47/presentations/Sunday/RAS_Traceroute_N47_Sun.pdf
> page 10 illustrates this issue with rfc1812
>
> cursory research and talking with C & J seem to indicate that they do
> what i want not what some folk have interpreted 1812 to mean.  at least
> on some models.
>
> is anyone seeing the dreaded rfc1812 behavior in a citable fashion?  how
> common is it?
>
> randy