rfc 1812 third party address on traceroute

Wed Jun 1 21:42:02 UTC 2016

On Wed, Jun 1, 2016 at 5:03 PM, Octavio Alvarez <octalnanog at alvarezp.org> wrote:
> On 05/31/2016 11:22 AM, William Herrin wrote:
>>> 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).
>>
>> That doesn't make sense. There may be multiple next hops out A. If the
>> next hop in the FIB is out C, how would the router pick the next hop
>> to send to out A?
>
> Back to the physical address that sent the TTL-offending packet.

Howdy,

That would be an example of a layer violation. The only guarantee that
layer 2 makes to layer 3 is that if you tell the layer 2 stack the
layer 3 next hop address on that lan segment, it can figure out where
to deliver your packet (via arp on ethernet, but this is not
necessarily true of other layer 2s).

Long story short, layer violations break things. Indeed, many of BGP's
thornier problems and the mess that is mobile routing can all be
traced to a single layer violation that TCP commits on IP.

Regards,
Bill Herrin

-- 
William Herrin ................ herrin at dirtside.com  bill at herrin.us
Owner, Dirtside Systems ......... Web: <http://www.dirtside.com/>