rfc 1812 third party address on traceroute

Randy Bush randy at psg.com
Tue May 31 05:03:33 UTC 2016


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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