Subject: [PWE3] draft-ronc-ptp-mpls-00

From: Stewart Bryant <[email-masked]>
Date: Tue, 10 Jul 2007 10:34:26 +0100

Ron

It is not clear from this document whether you are proposing to
run PTP over the normal (IPv4/IPv6) LDP/RSVP FEC between PEs, or 
over a special one.

An LSR using LDP normally inherits the next hop from IP routing
and IP routing has no reciprocal path requirements, indeed it
would need to take special (cpu intensive) action to cause 
reciprocity.

Also when running with first nibble = 0, no information is
provided as to the payload. This is explicitly stated in RFC4385.
So I do not see how an LSR is able to determine that the payload
is PTP to perform transparent clock actions.

To support transparent clock, PTP needs to either run an a special 
FEC that only carries PTP packets (with reciprocal path properties), 
or it needs to run over a normal FEC (signaled by RSVP) using an 
ACH, or using a reserved label so that PTP packets can be identified.

- Stewart

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From: "Ron Cohen" <[email-masked]>
Date: Tue, 10 Jul 2007 17:27:53 +0300

Stewart,

Thanks for your comments. 

You raised two issues; setup of reciprocal LSPs and identification of
PTP messages. Please see my comments below:

1. The draft explains the advantage of reciprocal (symmetric) LSPs as
quoted below. As you point out, in order to setup reciprocal LSPs
special means must be used, e.g. RSVP-TE. The draft does not elaborate
on how symmetric LSPs are signaled. This should probably be addressed in
next versions of the draft. 

   All PTP LSPs MUST be bi-directional.  PTP protocol is sensitive to
   asymmetry between the propagation delays across the master-slave and
   slave-master paths.  If the asymmetry is known, the PTP protocol has
   means to compensate for the known asymmetry.  If the asymmetry is not
   known, the time recovered at the slaves will have a fixed offset
   equal to half the propagation delay asymmetry.  Therefore, whenever
   possible, the tunnel and PTP LSPs SHOULD be setup in a symmetric way.

2. Identification of PTP messages: The draft proposes that PTP messages
will be sent over a specific FEC identified by the vc-label. Transparent
clocks know in advance that a specific vc-label is associated with a PTP
PW type through LDP signaling as described in section 6 of the draft.

Best,
Ron

-----Original Message-----
From: Stewart Bryant [mailto:[email-masked]] 
Sent: Tuesday, July 10, 2007 12:34 PM
To: pwe3; Ron Cohen
Subject: draft-ronc-ptp-mpls-00

Ron

- Stewart

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