[mpls] Association of PSC messages to a protection path

[venkatesan mahalingam <[e-mail]>]

Hi,

Please clarify the below query regarding *how to associate the PSC message
with a protection path.*



Below are the details.



Format of the PSC control message is as follows:



0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0 0 0 1|Version|  Reserved     |   Channel Type = MPLS-TP PSC  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          ACH TLV Header                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                         Optional TLVs                         ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Ver|Request|PT |R|  Reserved   |     FPath     |     Path      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: Format of PSC packet with a G-ACh header
(draft-ietf-mpls-tp-linear-protection-02)



*Transmission of PSC control packets:*

The PSC packet will be encapsulated with the MPLS label and next hop mac and
will be sent over the out going interface related to the protection path.



*Reception of PSC control packets:*

The PSC control packets will be ‘label switched’ at the intermediate LSRs
(based on the protection LSP’s label) and thus will reach the LER.



*The questions are:*

*1)       **Associating PSC with protection path based on the label stack:*



Should the LER which receives the PSC packet, associate the PSC message with
the MEG/MEP corresponding to the protection LSP - *based on the label stack?
*



If the answer is *‘yes’*, then how this association be done in case if
Penultimate Hop Popping *(PHP) is enabled* where the label in the PSC packet
will be stripped off when the LER receives the PSC packet.



*(OR)*



*      **Associating PSC with protection path based on the MEG/MEP in
optional TLV :*



A) Should *‘Remote MEG/MEP’* information of the protection LSP be sent in *the
‘Optional TLV’* from the transmitting LER?

If this information is available in the PSC packet, then even if PHP is
enabled, the LER will be able to associate the PSC packet with MEG/MEP and
so with a protection path.



(OR)



B) *‘Source MEP TLV”* should be sent in *the ‘optional TLV’ in the PSC
packet ?*



* 2)       **In case of shared  protection path in 1:n architecture where
n>1:*

Here the understanding is that one protection path is being shared by ‘n’
working transport paths.

In this case, *should the PSC message hold any additional information in the
optional TLVs so as to associate the status conveyed in the PSC message
against a particular working transport path ?*



(eg) if  LSP1, LSP2, LSP3 are being protected by the LSP100, then PSC
messages will be transmitted on LSP100.

In this case, the information such as : PSC request, Fault Path (FPath),
Data path (Path) exchanged in the PSC message should be for LSP1 (or) LSP2
(or) LSP3 ?

Should it be discriminated by exchanging relevant information in the
‘optional TLV’  ?


-- 
Best Regards,
Venkatesan Mahalingam.