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 <ra...@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
