Internet Draft David Allan, Nigel Bragg
Document: draft-allan-pw-o-pbt-02.txt Nortel
Category: Standards Track February 2007
Pseudo Wires over Provider Backbone Transport
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Copyright (C) The Internet Trust (2007).
Abstract
This memo describes architecture and procedures for the operation of
pseudo wires over provider backbone transport (PBT).
1. Introduction
Provider backbone transport offers a mechanism to permit scalable
p2p tunnels to be configured or signaled in an Ethernet subnetwork.
Such tunnels are suitable to function in the role of PSN in the PWE3
architecture. The forwarding mechanisms utilized by PBT are
progressing at IEEE 802 under the title "Provider Backbone Bridging-
Traffic Engineering (PBB-TE)".
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
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"OPTIONAL" in this document are to be interpreted as described in
RFC 2119.
In addition to well understood GMPLS terms, this memo uses
terminology from IEEE 802.1 and introduces a few new terms:
B-MAC Backbone MAC
B-VID Backbone VLAN ID
B-VLAN Backbone Virtual LAN
BEB Backbone Edge Bridge
BCB Backbone Core Bridge
C-MAC Customer MAC
C-VID Customer VLAN ID
C-VLAN Customer Virtual LAN
DA Destination Address
LLC Logical Link Control
MAC Media Access Control
PBB Provider Backbone Bridge
PBT Provider Backbone Transport
RTP Real time protocol
SA Source Address
VID VLAN ID
VLAN Virtual LAN
3. Changes since previous version
1) Clarifications with respect to label spaces and protection groups
added.
2) References updated.
4. PWoPBT architecture
PBT permits Ethernet bi-directional p2p tunnels to be configured
across an Ethernet subnetwork. These tunnels can be either
configured by management or signaled as described in [FEDYK].
Frequently more than one diversely routed tunnel is set up to form a
protection group, the most common instantiation being 1:1 protection
switching.
+---------------------+ +-------------------------+
| Payload |------------->| Raw payload if possible |
/=====================\ +-------------------------+
H Payload Convergence H-----------+->| Flags, seq #, etc. |
H---------------------H / +-------------------------+
H Timing H---------/--->| RTP |
H---------------------H / +-------------+ |
H Sequencing H----one of | |
\=====================/ \ | +-----------+
| PW Demultiplexer |---------+--->| PW service label |
+---------------------+ +-------------------------+
| PSN Convergence |------------->| Not needed |
+---------------------+ +-------------------------+
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Pseudo Wires over Provider Backbone Transport
| PSN |------------->| PBT |
+---------------------+ +-------------------------+
| Data-Link |------------->| Data-link |
+---------------------+ +-------------------------+
| Physical |------------->| Physical |
+---------------------+ +-------------------------+
Figure 1. PWE3 architecture illustrating role of PBT
Figure 1 illustrates the role of PBT in the PWE3 architecture [PW-
ARCH]. Where PBT Ethernet PDUs are carried directly over an Ethernet
PHY, the PBT, data-link and physical layers are effectively a single
layer from the point of view of control and management.
The PWoPBT architecture takes advantage of the fact that the
Ethernet LLC permits multiple protocols to be simultaneously
multiplexed over a PBT protection group. This permits both MPLS/PW
payload/PDUs and IP control and OAM PDUs to be multiplexed together.
+-ATM +-PING
+-Ethernet +-BFD
+-FR +-ETHOAM
+-HDLC |
+-PPP |
+-SaTOP |
| (etc.) |
+----------+ +--------+
|PW payload| | PW OAM |
+----------+ +--------+
| |
0000 0001 +--------------+
\ / | LDP |
+-------------------+ +--------------+
| PW CW | | TCP |
+-------------------+ +--------------+ +--------------+
| PW label | | IP | |802.1ag/Y.1731|
+-------------------+ +--------------+ +--------------+
| | |
=0x8847 =0x0800 =TBD
\ | /
/+-------------------------------------------------+\
/ | etype | \
/ +-------------------------------------------------+ \
/ | VLAN | PBT
802.1Q+-------------------------------------------------+ PSN
header| SA-MAC | /
\ +-------------------------------------------------+ /
\ | DA-MAC | /
\+-------------------------------------------------+/
Figure 2: Multiplexing of PW bearer, PW OAM, PW control & tunnel
OAM over PBT tunnel
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Further, control, bearer and OAM PDUs inherently share fate with the
PBT tunnel or (where used) protection group simplifying the job of
proactive monitoring of connectivity. Where a protection group is
used control, OAM and bearer traffic is forwarded on the currently
active path of the protection group. Further the PW service may
directly inherit availability status from the tunnel or protection
group.
In addition to the regular IP Infrastructure that may be established
to support PSN Control Plane (routing, GMPLS signaling) exchanges, a
PBT tunnel may appear as a single IP hop. The IP control channel
allows the mode of operation to be directly analogous to channel
associated signaling. PW labels offered over the signaling channel
are directly bound to the PBT tunnel and inherit the QoS
characteristics of the tunnel directly.
PBT tunnels/protection groups may interconnect two U-PEs, a U-PE to
an S-PE, an S-PE to an S-PE. Connecting a U-PE to diverse S-PEs is
for further study.
5. Signaling Procedures
5.1 Adjacency Establishment and Session Initialization
PW control assumes an a-priori existence of a PBT protection group
between a given pair of PEs.
One hello adjacency will be established between any two PEs. The
preferred method of indicating the transport address of the PE is
implicit (source address in the Hello exchange). A PE implements
only one transport IP address. It is common to all PBT tunnel
terminations. This is typically the PE loopback address.
LDP extended discovery is used over the currently active path of the
PBT protection group. In a fault free network this will be the
working path.
The label space indicated in the LDP Link Hello message MUST be the
per-platform label space.
5.2 Signaling Procedures
Once the Hello adjacency has been established, LDP session
initialization proceeds as per [RFC 3036].
Label exchange procedures are as per [PW-CONTROL] for single segment
pseudo wires and as per [MS-PW] for multi-segment pseudo wires.
5.3 Fault scenarios
Failure of a single PBT tunnel in the protection group will not
disrupt either the bearer path or the control adjacency. Failure of
all tunnels in a protection group or the failure of a PE at a
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Pseudo Wires over Provider Backbone Transport
terminating end to a protection group will disrupt the service. If
the network has not been completely severed by link failures, PBT
may be able to recover connectivity prior to expiration of the LDP
hold timer.
5.4 Interworking MS-PWs
PBT introduces no new procedures into the interworking of MS-PWs. It
simply takes the role of a PSN Tunnel in one or more segments. Bi-
directional PBT tunnels are consistent with the requirement for both
directions of an MS-PW to transit common S-PE devices.
6. OAM Procedures
6.1 Capability Indication
OAM capability indication procedures as per [VCCV] and extended in
[MOHAN] is used unmodified.
6.2 Control Channel
In-band VCCV may be used for both SS and MS PWs without
modifications to procedures described in [VCCV] and [MS-PW].
6.3 VCCV BFD
For a single segment PW, use of VCCV BFD is not required as the PW
is 1:1 congruent with the transporting PBT protection group (which
does not implement load spreading such as ECMP) so the PBT OAM
effectively instruments connectivity for the set of PWs carried.
For MS-PWs where a least one segment transits a non PBT network such
as ECMP/LDP, VCCV BFD may be used as PSN OAM congruency with the PW
layer cannot be guaranteed.
6.4 VCCV-PING
Normally the return path for a VCCV-PING reply is the PW in the
reverse direction. This is indicated by LSP-PING reply mode 2. It is
also possible to indicate that the reply traverse each segment of a
MS-PW by indicating a reply mode of 3 (use of router alert in the
reply message) although this only slightly modifies the return path
congruency for pure PBT architectures, and is of primary use in
decoupling the return path from the PW in other PSN types.
6.5 VCCV-ETHOAM
[MOHAN] proposes the use of [802.1ag] and [Y.1731] OAM PDUs in
conjunction with the VCCV channel. In this scenario MEPs are co-
located with the PW end points and for MS-PWs, MIPs are co-located
with the S-PEs.
7. Security Considerations
The use of PBT as a PSN introduces no new security vulnerabilities
to the PWE architecture.
8. References
[FEDYK] GMPLS Control of Ethernet, IETF Internet Draft, draft-
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fedyk-gmpls-ethernet-pbt-01.txt, October 2006
[MOHAN] VCCV Extension for Ethernet OAM, IETF Internet Draft
draft-mohan-pwe3-vccv-eth-01.txt, January 2007
[MS-PW] Dynamic Placement of Multi Segment Pseudo Wires, IETF
Internet Draft, draft-ietf-pwe3-dynamic-ms-pw-02.txt,
October 2006
[PW-ARCH] Pseudo Wire Emulation Edge-to-Edge (PWE3) Architecture,
IETF RFC 3985, March 2005
[PW-CONTROL] Pseudowire Setup and Maintenance using the Label
Distribution Protocol, IETF RFC 4447, April 2006
[RFC 3036] LDP Specification, IETF RFC 3036, January 2001
[VCCV] Pseudo Wire Virtual Circuit Connectivity Verification
(VCCV), IETF Internet Draft, draft-ietf-pwe3-vccv-
12.txt, January 2007
[Y.1731] Y.1731 (2006), ITU-T Recommendation, OAM functions and
mechanisms for Ethernet based networks
[802.1ag] Connectivity Fault Management, IEEE 802.1ag draft 6.1,
work in progress.
9. Author's Address
Dave Allan
Nortel Networks Phone: 1-613-763-6362
3500 Carling Ave. Email: dallan@nortel.com
Ottawa, Ontario, CANADA
Nigel Bragg
Nortel Networks UK Limited Phone +44 (0) 1279 40 2052
London Road, Harlow, Essex, Email: nbragg@nortel.com
CM17 9NA, UK
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13.Acknowledgments
The authors would like to thank Dinesh Mohan for his contributions
to this document.
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