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TICTOC                                                       Y(J). Stein
Internet-Draft                                   RAD Data Communications
Intended status: Informational                        September 19, 2010
Expires: March 23, 2011

                 Transport of Timing Packets over MPLS


   The TICTOC charter specifies that the working group is concerned with
   highly accurate time and frequency distribution over native IP and
   MPLS-enabled IP Packet Switched Networks (PSNs).  We discuss here
   issues specific to MPLS PSNs.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on March 23, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
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   publication of this document.  Please review these documents

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   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the BSD License.

Table of Contents

   1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Encapsulation Options . . . . . . . . . . . . . . . . . . . . . 3
     2.1.  Using IP  . . . . . . . . . . . . . . . . . . . . . . . . . 4
     2.2.  Using an Ethernet PW  . . . . . . . . . . . . . . . . . . . 4
     2.3.  Defining a New MPLS Client or a new PW Type . . . . . . . . 4
     2.4.  Using VCCV or the G-ACh . . . . . . . . . . . . . . . . . . 5
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   4.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 6

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1.  Motivation

   The TICTOC charter specifies that the working group is concerned with
   highly accurate time and frequency distribution over native IP and
   MPLS-enabled IP Packet Switched Networks (PSNs).  To date,
   discussions have focused on NTPv4 and 1588-2008 timing distribution
   using UDP/IP encapsulations.  The present document discusses
   transport of timing packets over MPLS PSNs, and is based on material
   presented and discussed in previous TICTOC meetings.

   We first must address the question as to why special treatment is
   needed at all for transport of timing packets over MPLS.  Timing
   packets in IP format can certainly be transported over MPLS without
   the LSRs along the path being aware of them.  However, there
   advantages to being able to recognize, and potentially manipulate,
   timing packets.

   For highly accurate timing distribution, timing packets are required
   to travel through the PSN with minimal, symmetric, and
   quasistationary delay, as well as minimal and uncorrelated packet
   delay variation.  Thus, prioritization and symmetric routing of
   timing packets are minimal requirements.  For the most demanding
   applications, timing distribution mechanisms avail themselves of "on-
   path support", such as Transparent Clock (TC) overwriting of header
   fields.  None of these can be selectively applied to timing packets
   unless they can be recognized by the LSR.

2.  Encapsulation Options

   MPLS as a server layer presently permits three types of clients:
   1. MPLS (via label stacking)
   2. IP (either IPv4 or IPv6)
   3. pseudowires (PWs)
   although we can not rule out defining a new client for timing
   distribution.  Taking into account the two defined associated
   channels that carry non-user traffic, namely the VCCV associated
   channel for PWs [RFC5085] and the GAL G-ACh defined for MPLS-TP
   [RFC5586] any proposal for carrying timing packets over MPLS will
   need to put the timing information in one of the following six
   1. a new MPLS client type
   2. IP
   3. an Ethernet PW

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   4. a new "timing" pseudowire type
   5. a new VCCV channel type
   6. a new G-ACh channel type.
   We will discuss each of these options in turn.

2.1.  Using IP

   Since the two main timing distribution protocols have UDP/IP
   encapsulations, arguably the simplest method of transporting timing
   packets is in this format.  There are several methods for
   intermediate LSRs to recognize the timing packets :
   o  Deep Packet Inspection (i.e., peeking under the MPLS stack and
      identifying well-known port numbers)
   o  using an arbitrary configured or signaled MPLS label
   o  using a new reserved MPLS label
   o  using a specific MPLS Traffic Class (ex-EXP).
   It should be noted that there are very few available reserved labels,
   and that traffic class usage is not standardized.

   If an arbitrary label is required to be signaled, then an extension
   to the signaling protocol will be required.  Such an extension will
   identify the FEC as belonging to a timing flow.

2.2.  Using an Ethernet PW

   The UDP/IP packets of the timing distribution protocols of the
   previous subsection may be contained in Ethernet frames, that can be
   transported over an MPLS network in an Ethernet PW.  In addition,
   IEEE 1588-2008 has a native Ethernet (non-IP) encapsulation.

   Methods for identifying timing packets inside an Ethernet PW are
   similar to those of the previous subsection.

2.3.  Defining a New MPLS Client or a new PW Type

   IEEE 1588-2008 allows for different transport layers to be defined,
   with annexes to the standard defining UDP/IPv4, UDP/IPv6, Ethernet,
   and several other transport networks.  It is possible to define a new
   transport mechanism for MPLS, which entails specifying how to
   encapsulate a PTP PDU in an MPLS packet.  This can be done in the
   context of the PWE3 architecture (this was proposed in
   draft-ronc-ptp-mpls-00.txt, now expired) or without reference to that
   architecture.  If the PWE3 architecture is used, then PWE3 features,
   such as the control word and the control protocol, may be used.

   Whether the MPLS encapsulation is a PW or not, the timing packet will
   be recognized by virtue of its bottom-of-stack label.  When
   additional labels are present, the LSR will need to search for the

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   label with S=1.  This technique is technically a violation of the
   MPLS architecture, but is presently performed for other purposes,
   e.g., ECMP avoidance [RFC4928].

   The particular label(s) used to signify timing packets may be
   distributed by manual configuration or a signaling protocol.  If the
   latter is employed, a new FEC type will need to be defined.  If a PW
   is used, a new MPLS Pseudowire Type [RFC4446] will be needed for use
   in the PWE3 control protocol [RFC4447].

2.4.  Using VCCV or the G-ACh

   Timing is often distributed in order to enable proper functioning of
   applications that already define PWs between the end points of the
   timing flow.  In this case, the timing information may be placed in
   the VCCV associated channel of the application's PW.  The associated
   channel is typically identified by having the same PW (bottom-of-
   stack) label as the application, but a control word with 0001 in its
   initial nibble.  The timing packets may then be identified by a new
   channel type, or may use the IP channel type and then would be
   identified by the well-known port numbers.  It is recognized that
   this requires the LSR to perform relatively deep packet inspection.

   If there is no application PW between the timing end points, then we
   may still use the Generic Associated CHannel (G-ACh) defined in
   [RFC5586] in a similar manner.  The G-ACh is identified by the G-ACh
   Label (GAL), which is a reserved MPLS label of value 13, at its
   bottom-of-stack.  The format after the GAL is the same as that of
   VCCV, and thus the considerations of the previous paragraph apply.

3.  IANA Considerations

   This document requires no IANA actions.

4.  References

   [RFC4446]  Martini, L., "IANA Allocations for Pseudowire Edge to Edge
              Emulation (PWE3)", BCP 116, RFC 4446, April 2006.

   [RFC4447]  Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G.
              Heron, "Pseudowire Setup and Maintenance Using the Label
              Distribution Protocol (LDP)", RFC 4447, April 2006.

   [RFC4928]  Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal
              Cost Multipath Treatment in MPLS Networks", BCP 128,
              RFC 4928, June 2007.

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   [RFC5085]  Nadeau, T. and C. Pignataro, "Pseudowire Virtual Circuit
              Connectivity Verification (VCCV): A Control Channel for
              Pseudowires", RFC 5085, December 2007.

   [RFC5586]  Bocci, M., Vigoureux, M., and S. Bryant, "MPLS Generic
              Associated Channel", RFC 5586, June 2009.

Author's Address

   Yaakov (Jonathan) Stein
   RAD Data Communications
   24 Raoul Wallenberg St., Bldg C
   Tel Aviv  69719

   Email: yaakov_s@rad.com

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