rfc 1812 third party address on traceroute
Marc Storck
mstorck at voipgate.com
Wed Jun 1 19:16:08 UTC 2016
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 at 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
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