The MPLS WG Archive

[Ravi Shekhar <ravi_shekhar007@yahoo.com>]

> So then  I think then the base  requirement for the routing  of the response
> from the  egress is that it  be reverse-path-forwarded back  to the ingress,
> i.e., that it follow the path of label distribution from egress to ingress. 


  That seems to be the right way to me. One possible way to achieve this is 
  as a 4 step process - first 2 to gather the hop-by-hop path from ingress to
  egress node and the next 2 to do the ping on LSP:

   1) Initiate a UDP packet at ingress and send it to the next hop for the LSP to
      be traced (i.e. destination IP address is the address of the next hop for 
      LSP and not the egress address itself).
      Have enough information (like ingress router/egress router address/label) 
      in the UDP packet to uniquely identify the LSP.
      Every hop(including ingress/egress) on the way appends its hop information 
      and sends this packet to the next hop of the LSP.
   2) When this UDP packet reaches egress, egress sends it back to the ingress 
      hop-by-hop in the reverse order of the way path was traced(and recorded in
      the packet) in step 1. Sending it this way ensures that it reaches the
      ingress.
   3) At this point ingress has the elaborated hop-by-hop path for the LSP from
      ingress to egress. It now generates a UDP packet at ingress with the
      destination address of the egress. Since the UDP packet's destination is
      egress node, it should traverse the LSP now. Inside the UDP packet fill in
      the path that was traced in step 1 (and received by ingress in step 2). 
   4) If egress receives the UDP packet sent in step 3 via LSP, it can use the
      hop-by-hop information contained in the packet and send a response back 
      hop-by-hop to the ingress (the same way it sent it back in step 2).

   This should work independent of the protocol that sets up the LSP (and across
   multiple protocols). Since a unique return path for response is already
   recorded while traversing in the forward direction of LSP (i.e. from ingress
   to egress) in step 1, it would work for LDP too where same label might have
   been handed over to multiple upstreams.

   Some other useful information can also be gathered. Like protocol that set up
   the LSP on each hop interface / timestamp and number-of-packets-forwarded-on
   -the-LSP in steps 2 and 4 to calculate the rate of forwarding on LSP etc. 
   
   Maximum number of hops to be traced can be specified by the ingress (and
   decremented at each hop) to prevent looping of message in step 1 forever.
   Indication of failure of LSP or reaching maximum number of hops at any hop 
   can be put in the UDP packet and sent back to ingress in step 2.
 
   - Ravi Shekhar.

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