Subject: [mpls-tp] Some queries regarding mpls-tp linear protection draft

From: Suvendu Mozumdar <[email-masked]>
Date: Mon, 13 Sep 2010 23:16:20 -0700

Hi,
          I have following queries over the draft mpls-tp-linear-protection-02:

1. Can you please clarify whether PSC traffic should get stopped flowing in protect path in Unavailable state? I could not find any direct instruction to halt PSC message in protect path, but it does say in section 4.3.3.2 - "Both ends will resume sending the PSC packets over the   protection path, as a result of this recovery." This means that PSC packets were stopped when protection domain moved in Unavailable state. Please clarify the expected behavior regarding transmission of PSC traffic when an LER is in Unavailable state.

2. As per the draft section 4.3.3.4, an LER in remote Protecting Failure state goes to NR(0,0) state when it receives a remote Forced Switch message:

"A remote Forced switch message SHALL cause the LER go into Protecting administrative state and if in protecting failure state due to a local SF condition begin transmitting the SF(1,1)message, otherwise begin transmitting NR(0,0)"

As per my analysis the LER should begin transmitting NR(0,1) not NR(0,0), since in protecting admin state data traffic flows in protect path. Please correct me if my understanding is wrong.

3. Please go through the following state transitions:
        a. An LER goes to Unavailable state due to a local LO.
        b. Next it detects a local SF in working path, and as per section 4.3.3.2, the LER ignores it and remains in Unavailable state.
        c. Next a local Clear is triggered, as a result of which the LER goes to Normal state and starts sending NR(0,0).

     My question is although the LO is cleared, the working path is still under signal failure condition, how can the LER goes to Normal state at step c. It is mainly happening since PSC state machine does not keep track of history of local/remote inputs, it just acts based on the latest local/remote input and the current state. But this can cause the PSC state machine to switch into a state which does not reflect the actual state of affairs of the working/protect paths. There can be multiple such scenarios like this, is there any mechanism to take care of such scenarios?

4. Section 4.3.3.3, talks about processing DNR(0,0) message, please clarify in what scenario an LER can send DNR(0,0) message, since as per definition of DNR state, section 4.3.3.6, "While in Do-not-revert state data traffic continues to be transmitted on the protection path until the administrator sends a command to revert to the Normal state.", which means the data path has to be 1, also as per section 4.3.3.4, regarding processing of local clear SF, "If in local Protecting failure state and the LER is configured for non-revertive behavior then this input SHALL cause the LER to go into Do-not-revert state and begin transmitting a DNR(0,1) message." So I am a bit confused regarding the DNR(0,0) message, please clarify.

Regards,
Suvendu Mozumdar.