Hello Vicky,
Follow-up below.
On 4/16/2015 7:03 AM, Victoria Mercieca wrote:
Hi all,
This one is about Precursors (Section 12.2 in Version 9a, or Section 10.2 in Version 9b).
* Should we even mention precursors in the main text if its an
optional feature? It is only mentioned at part of the route table
entry but we can easily remove this, and in the Precursors section
state that the list would be part of the route table entry.
1. How would you know which router forwarded traffic to you for a
particular destination? You'd only know the packet source and the
interface it came in on.
Option 1: you could save PktSource as the precursor. If the route
then broke, look up the route to PktSource, and if there is one,
send the RERR to the next hop toward PktSource. If there was no
route, you'd still have to multicast the RERR.
Option 2: look up the route to PktSource and (if it exists) save
the next hop as a precursor. If the route breaks, send the RERR to
the precursor address. If our next hop to PktSource changed before
the RERR was sent, this might go to the wrong router. Also if
there was no route to PktSource, we cant save a precursor, so
would we assume no RERR was needed?
I think Option 1 is better.
However, if the next hop we send the RERR to doesn't use us as a
next hop to the destination, they will just ignore the RERR.
Consider:
source -----> A ---------> B ----------> C ----------> destination
| | ^
| | |
V V |
E --------> D -------------------------|
C's next hop to the source is B. When C receives a packet from
source, C saves B as a precursor. However, the packet might have
come via D.
If C's route to the source changes to use next hop D, which router
should be told when the route to the destination breaks at C?
The source could be using the route via EDC. If the RERR goes to
B, B might regenerate it to A, then A might regenerate it to
source, but the source will say "thats not my next hop, I don't
care". It will carry on sending packets, C will say "I've got no
route, send a RERR to PktSource" (and the same thing happens again).
Either way, just because C's next hop to PktSource is B, doesnt
necessarily mean that PktSource uses the route via B and C to the
destination. If the RERR goes back a different way to the route
PktSource is using, it will ignore the RERR. This affects both the
use of precursors, and the unicast RERR in response to a packet
that cant be forwarded.
