Subject: [mpls] Comments on draft-weingarten-mpls-tp-ring-protection-06

From: Jie Dong <[email-masked]>
Date: Fri, 04 Nov 2011 09:45:42 +0000

Hi, 

Here are some comments on this tp ring protection draft. 

1. This draft describes the label stack operation of each protection mechanism. For better clarity, I would recommend using the same protected LSP as example when possible, i.e. using LSR-B as the ingress and LSR-E as the egress in all description of p2p protection, and describes the label stack operation from the ingress to the egress.

2. Label operation in the last paragraph before 2.3.1, "packets arrive at LSR-B with label [LI], and then send out to LSR-A with [[PA(F)|L1]]". Should B swap the inner label LI with LE, which is expected by the egress LSR-F?

3. Label stack operation of wrapping in sec 2.3.2 needs some clarification:

  "1. The data packet arrives at LSR-A with label stack [LI+S] (i.e. top label from the LSP and bottom-of-stack indicator)"

If the section takes the protected LSP in figure 1 as example, LSR-A is the ingress of SPME A-F, NOT the ingress of the ring. If the ingress LSR is B, packets arrive at A may contain the label for SPME B-A, and the inner LSP label LI should be swapped on LSR-B.

  "2. In the normal case (no switching), LSR-A forwards the packet with label stack [PA1(F)|LE+S] (i.e. swap the label for the LSP, to be acceptable to the SPME egress, and push the label for the primary SPME from LSR-A to LSR-F)."

"LE" is defined as "the label of the LSP that is expected at the egress point from the ring", but here it is used as the label expected by SPME egress. Since the inner LSP label needs be swapped each time the SPME label is popped, using "LE" for this operation may be confusing. It would be clearer to define a new term for this.

4. In some specifications "+S" is added to the LSP label. Does it also allow carrying PW label or other labels in the label stack?

Best regards,
Jie