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rfc 1812 third party address on traceroute

On Tue 2016-May-31 09:08:42 -0700, Octavio Alvarez <octalnanog at alvarezp.org> wrote:

>On 05/30/2016 10:03 PM, Randy Bush wrote:
>> rfc1812 says
>> 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)


Hugo Slabbert       | email, xmpp/jabber: hugo at slabnet.com
pgp key: B178313E   | also on Signal
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