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_N... 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