[189496] in North American Network Operators' Group
Re: rfc 1812 third party address on traceroute
daemon@ATHENA.MIT.EDU (Hugo Slabbert)
Tue May 31 12:52:04 2016
X-Original-To: nanog@nanog.org
Date: Tue, 31 May 2016 09:52:00 -0700
From: Hugo Slabbert <hugo@slabnet.com>
To: nanog@nanog.org
In-Reply-To: <574DB70A.2010209@alvarezp.org>
Errors-To: nanog-bounces@nanog.org
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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=
=20
device on R's interface A has a route for S and if such a route exists that=
=20
it doesn't simply point at R? As Randy so eloquently put it:
>> (yes, virginia, the internet is highly asymmetric)
>
>Octavio.
--=20
Hugo Slabbert | email, xmpp/jabber: hugo@slabnet.com
pgp key: B178313E | also on Signal
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