On Tue 2016-May-31 09:08:42 -0700, Octavio Alvarez <octalnanog@alvarezp.org> wrote:
On 05/30/2016 10:03 PM, Randy Bush 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) | | `-----------------'
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.
Do you mean the source address or the source interface?
I'm not sure if you mean that, if sent through C it should have the source addres of A, or that it should actually be sent through A regardless of the routing table (which sounds better to me).
How is the latter better? What guarantees are there that the adjacent L3 device on R's interface A has a route for S and if such a route exists that it doesn't simply point at R? As Randy so eloquently put it:
(yes, virginia, the internet is highly asymmetric)
Octavio.
-- Hugo Slabbert | email, xmpp/jabber: hugo@slabnet.com pgp key: B178313E | also on Signal