Packet Pseudowire Encapsulation over an MPLS PSN
draft-bryant-pwe3-packet-pw-01.txt

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Abstract

This document describes a pseudowire that is used to transport a packet service over an MPLS PSN is the case where the client LSR and the server PE are co-resident in the same equipment. For correct operation these clients require a multi-protocol interface with fate sharing between the client protocol suite. The packet pseudowire may be used to carry all of the required layer 2 and layer 3 protocols between the pair of client LSRs.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

Table of Contents

1.  Introduction
2.  Network Reference Model
3.  Forwarding Model
4.  Encapsulation
5.  Packet Pseudowire Control Word
6.  Status Indication
7.  Setting the load balance label
8.  Client Network Layer Model
9.  Congestion Considerations
10.  Security Considerations
11.  IANA Considerations
12.  References
    12.1.  Normative References
    12.2.  Informative References
§  Authors' Addresses

1.  Introduction

There is a need to provide a method of carrying a packet service over an MPLS PSN in a way that provides isolation between the two networks. The server MPLS network may be an MPLS network or a network conforming to the MPLS-TP [RFC5317] (Bryant, S. and L. Andersson, “Joint Working Team (JWT) Report on MPLS Architectural Considerations for a Transport Profile,” February 2009.). The client may also be either a MPLS network of a network conforming to the MPLS-TP. Considerations as to whether an unrestricted MPLS network can act as a server for an MPLS-TP network are outside the scope of this document.

Where the client equipment is connected to the server equipment via physical interface, the same data-link type MUST be to attach the clients to the PEs, and a pseudowire of the same type as the data-link MUST be used [RFC3985] (Bryant, S. and P. Pate, “Pseudo Wire Emulation Edge-to-Edge (PWE3) Architecture,” March 2005.). The reason that inter-working between different physical and data-link attachment types is specifically disallowed in the pseudowire architecture is because this is a complex task and not a simple bit-mapping exercise. The inter-working is not limited to the physical and data-link interfaces and state-machines it also requires a compatible approach to the formation of the adjacencies between attached client network equipment. As an example the reader should consider the differences between router adjacency formation on a point to point link compared to a multi-point to multi-point interface (e.g. Ethernet).

A further consideration is that two adjacent MPLS LSRs do not simply exchange MPLS packets. They exchange IP packets for adjacency formation, control, routing, label exchange, management and monitoring purposes. In addition they may exchange data-link packets as part of routing (e.g. IS-IS hellos and IS-IS LSPs) and for OAM purposes (e.g. Cisco Discovery Protocol). Thus the two clients require an attachment mechanism that can be used to multiplex a number of protocols. In addition it is essential to the correct operation of the network layer that all of these protocols fate share.

Where the client LSRs and server PEs are co-located in the same equipment the data-link layer can be simplified to a simple protocol identifier (PID) that is used to multiplex the various data-link types onto a pseudowire. This is the method that described in this document.

2.  Network Reference Model

The network reference model for the packet pseudowire operating in an MPLS network is shown in Figure 1. This is an extension of Figure 3 "Pre-processing within the PWE3 Network Reference Model" from [RFC3985] (Bryant, S. and P. Pate, “Pseudo Wire Emulation Edge-to-Edge (PWE3) Architecture,” March 2005.).

               PW                            PW
            End Service                   End Service
                |                            |
                |<------- Pseudowire ------->|
                |                            |
                |          Server            |
                |     |<- PSN Tunnel ->|     |
                |     V                V     |
-------   +-----+-----+                +-----+-----+   -------
       )  |     |     |                |     |     |  (
client  ) | MPLS| PE1 |      PW1       | PE2 | MPLS| ( Client
MPLS PSN )+ LSR1+............................+ LSR2+( MPLS PSN
        ) |     |     |                |     |     | (
       )  |     |     |================|     |     |  (
-------   +-----+-----+                +-----+-----+   --------
                ^                            ^
                |                            |
                |                            |
                |<---- Emulated Service----->|
                |                            |
         Virtual physical             Virtual physical
            termination                  termination

MPLS Pseudowire Network Reference Model

In this model LSRs, LSR1 and LSR2, are part of the client MPLS packet switched network (PSN). The PEs, PE1 and PE2 are part of the server PSN, that is to be used to provide connectivity between the client LSRs. The attachment circuit that is used to connect the MPLS LSRs to the PEs is a virtual interface within the equipment. A packet pseudowire is used to provide connectivity between these virtual interfaces. This packet pseudowire is used to transport all of the required layer 2 and layer 3 between protocols between LSR1 and LSR2.

3.  Forwarding Model

The packet PW forwarding model is illustrated in Figure 2. The forwarding operation can be likened to a virtual private network (VPN), in which a forwarding decision is first taken at the client layer, an encapsulation is applied and then a second forwarding decision is taken at the server layer.

          +-------------------------------------------------+
          |                                                 |
          |  +--------+                         +--------+  |
          |  |        |   Pkt   +-----+         |        |  |
       ------+        +---------+ PW1 +---------+        +------
          |  | Client |    AC   +-----+         | Server |  |
Client    |  | LSR    |                         | LSR    |  |    Server
Network   |  |        |   Pkt   +-----+         |        |  |    Network
       ------+        +---------+ PW2 +---------+        +------
          |  |        |    AC   +-----+         |        |  |
          |  +--------+                         +--------+  |
          |                                                 |
          +-------------------------------------------------+

Packet PW Forwarding Model

A PW PE comprises two components, the PW processor and an LSR. On receipt of a packet from the attachment circuit (AC) the PW undertakes any header processing, pushes the CW and pushes the PW label. It then passes the packet to an LSR which pushes the label needed to reach the egress PE. At the egress PE, the packet typically arrives with the PW label at top of stack, the packet is thus directed to the correct PW instance by the LSR. The PW instance performs any required reconstruction using the CW and passes the packet directly to the attachment circuit.

In the packet PW case an LSR belonging to the client layer is embedded within the equipment. This determines the next hop in the client layer, pushes the label needed by the client next hop, and passes the packet to the correct PW instance indicating the packet protocol type. The PW instance pushes CW (which included the protocol identifier), pushes the PW label and passes the packet to the server LSR for forwarding to the egress PW PE. At the egress PE, the packet typically arrives with the PW label at the top of stack, the packet is thus directed to the correct PW instance by the server LSR. This removes the CW and passes the packet to the client LSR indicating the protocol type. The client LSR then forwards the packet based on the top label, which is a client layer label.

Note that although the description above is written in terms of the behaviour of an MPLS LSR, the processing would model would be the same for IP.

4.  Encapsulation

The Protocol Stack Reference Model for a packet PW is shown in Figure 3 (PWE3 Protocol Stack Reference Model) below

   +-------------+                                +-------------+
   |  Client     |                                |  Client     |
   |  network    |                                |  network    |
   |  layer      |         Client Service         |  layer      |
   |  service    |<==============================>|  service    |
   +-------------+            Pseudowire          +-------------+
   |Demultiplexer|<==============================>|Demultiplexer|
   +-------------+                                +-------------+
   |   Server    |                                |   Server    |
   |    PSN      |            PSN Tunnel          |    PSN      |
   |   MPLS      |<==============================>|   MPLS      |
   +-------------+                                +-------------+
   |  Physical   |                                |  Physical   |
   +-----+-------+                                +-----+-------+

The encapsulation of a packet PW is shown in Figure 4 (Encapsulation of a pseudowire with a pseudowire load balancing label) below

    +-------------------------------+
    |      MPLS Tunnel label(s)     | n*4 octets (four octets per label)
    +-------------------------------+
    |          PW label             |  4 octets
    +-------------------------------+
    |         Control Word          |  6 octets
    +-------------------------------+
    |            Client             |
    |          Network Layer        |
    |            packet             |  n octets
    |                               |
    +-------------------------------+

5.  Packet Pseudowire Control Word

This section describes the encapsulation of a packet pseudowire. The packet pseudowire always uses the control word. The control word consists of two components: the preferred pseudowire MPLS control word [RFC4385] (Bryant, S., Swallow, G., Martini, L., and D. McPherson, “Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for Use over an MPLS PSN,” February 2006.), immediately followed by a PPP data link layer (DLL) protocol number [RFC1661] (Simpson, W., “The Point-to-Point Protocol (PPP),” July 1994.). The 16 bit format of the PPP DLL protocol number MUST be used.

The MPLS pseudowire control word is shown in Figure 5. Definitions of the fragmentation (FRG), length and sequence number fields are to be found in [RFC4385] (Bryant, S., Swallow, G., Martini, L., and D. McPherson, “Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for Use over an MPLS PSN,” February 2006.).

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 0| Flags |FRG|  Length   | Sequence Number               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       PPP PID                 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Packet Pseudowire Control Word

Note that the PPP link control protocol is not used.

Where the packet service being supported is a network layer service such as MPLS or IP, only a best effort packet ordering is normally required. The sequence number would therefore normally be set to zero (no PW sequence number processing).

6.  Status Indication

A pseudowire status indicating a fault can be considered equivalent to interface down and SHOULD be passed across the virtual interface to the local LSR. This improves scaling in PE with large numbers of c-resident LSRs and with LSRs that have large numbers of interfaces mapped to pseudowires.

The mechanism described for the mapping of pseudowire status to the virtual interface state that are described in [RFC4447] (Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G. Heron, “Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP),” April 2006.) and in section 10 of [I‑D.ietf‑pwe3‑segmented‑pw] (Martini, L., Nadeau, T., Metz, C., Bocci, M., Aissaoui, M., Balus, F., and M. Duckett, “Segmented Pseudowire,” April 2010.) apply to the packet pseudowire. Pseudowire status messages indicating pseudowire or remote virtual interface faults MUST be mapped to a fault indication on the local virtual interface.

7.  Setting the load balance label

The client service may wish the packet PW to take advantage of any Equal Cost Multi-Path (ECMP) support in the server layer. In this a load balance label as described in [I‑D.ietf‑pwe3‑fat‑pw] (Bryant, S., Filsfils, C., Drafz, U., Kompella, V., Regan, J., and S. Amante, “Flow Aware Transport of Pseudowires over an MPLS PSN,” January 2010.) may be included in the MPLS label stack. Where the client service is MPLS it would be appropriate to copy the client layer, bottom of stack MPLS label into the load balance label. Where the client layer is IP the load balance label would typically be calculated by hashing on the source and destination addresses, the protocol ID and higher-layer flow-dependent fields such as TCP/UDP ports, L2TPv3 Session ID’s etc.

The exact specification of the method of selecting an appropriate load balance label value is outside the scope of this document.

8.  Client Network Layer Model

The packet PW appears as a single point to point link to the client layer. Network Layer adjacency formation and maintenance between the client equipments will the follow normal practice needed to support the required relationship in the client layer. The assignment of metrics for this point to point link is a matter for the client layer. In a hop by hop routing network the metrics would normally be assigned by appropriate configuration of the embedded client network layer equipment (e.g. the embedded client LSR). Where the client was using the packet PW as part of a traffic engineered path, it is up to the operator of the client network to ensure that the server layer operator provides the necessary service layer agreement.

9.  Congestion Considerations

This pseudowire is being used to carry MPLS and its associated support protocols over an MPLS network. There are no congestion considerations beyond those that ordinarily apply to an MPLS network.

10.  Security Considerations

The packet pseudowire provides no means of protecting the contents or delivery of the pseudowire packets on behalf of the client packet service. The packet pseudowire may, however, leverage security mechanisms provided by the MPLS Tunnel Layer. A more detailed discussion of pseudowire security is given in [RFC3985] (Bryant, S. and P. Pate, “Pseudo Wire Emulation Edge-to-Edge (PWE3) Architecture,” March 2005.), [RFC4447] (Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G. Heron, “Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP),” April 2006.) and [RFC3916] (Xiao, X., McPherson, D., and P. Pate, “Requirements for Pseudo-Wire Emulation Edge-to-Edge (PWE3),” September 2004.).

11.  IANA Considerations

IANA are requested to allocate a new pseudowire type for packet pseudowire in the MPLS Pseudowire Types Registry. The next available value is requested.

12.  References

12.1. Normative References

12.2. Informative References

Authors' Addresses