Network Working Group S. Bryant, Ed.
Internet-Draft S. Boutros
Intended status: Standards Track L. Martini
Expires: January 9, 2010 S. Sivabalan
G. Swallow
D . Ward
Cisco Systems
A. Malis
Verizon Communications
July 8, 2009
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 [RFC2119].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Network Reference Model . . . . . . . . . . . . . . . . . . . 3
3. Forwarding Model . . . . . . . . . . . . . . . . . . . . . . . 4
4. Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Packet Pseudowire Control Word . . . . . . . . . . . . . . . . 6
6. Status Indication . . . . . . . . . . . . . . . . . . . . . . 7
7. Setting the load balance label . . . . . . . . . . . . . . . . 8
8. Client Network Layer Model . . . . . . . . . . . . . . . . . . 8
9. Congestion Considerations . . . . . . . . . . . . . . . . . . 8
10. Security Considerations . . . . . . . . . . . . . . . . . . . 8
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
12.1. Normative References . . . . . . . . . . . . . . . . . . 9
12.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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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]. 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]. 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
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[RFC3985].
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
Figure 1
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.
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+-------------------------------------------------+
| |
| +--------+ +--------+ |
| | | Pkt +-----+ | | |
------+ +---------+ PW1 +---------+ +------
| | Client | AC +-----+ | Server | |
Client | | LSR | | LSR | | Server
Network | | | Pkt +-----+ | | | Network
------+ +---------+ PW2 +---------+ +------
| | | AC +-----+ | | |
| +--------+ +--------+ |
| |
+-------------------------------------------------+
Packet PW Forwarding Model
Figure 2
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.
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4. Encapsulation
The Protocol Stack Reference Model for a packet PW is shown in
Figure 3 below
+-------------+ +-------------+
| Client | | Client |
| network | | network |
| layer | Client Service | layer |
| service |<==============================>| service |
+-------------+ Pseudowire +-------------+
|Demultiplexer|<==============================>|Demultiplexer|
+-------------+ +-------------+
| Server | | Server |
| PSN | PSN Tunnel | PSN |
| MPLS |<==============================>| MPLS |
+-------------+ +-------------+
| Physical | | Physical |
+-----+-------+ +-----+-------+
Figure 3: PWE3 Protocol Stack Reference Model
The encapsulation of a packet PW is shown in Figure 4 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
| |
+-------------------------------+
Figure 4: Encapsulation of a pseudowire with a pseudowire load
balancing label
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
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word [RFC4385], immediately followed by a PPP data link layer (DLL)
protocol number [RFC1661]. 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].
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
Figure 5
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] and in
section 10 of [I-D.ietf-pwe3-segmented-pw] 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.
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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] 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], [RFC4447]
and [RFC3916].
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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
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
RFC 1661, July 1994.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson,
"Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
Use over an MPLS PSN", RFC 4385, February 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.
12.2. Informative References
[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", draft-ietf-pwe3-fat-pw-00 (work in
progress), July 2009.
[I-D.ietf-pwe3-segmented-pw]
Martini, L., Nadeau, T., Metz, C., Duckett, M., Bocci, M.,
Balus, F., and M. Aissaoui, "Segmented Pseudowire",
draft-ietf-pwe3-segmented-pw-12 (work in progress),
June 2009.
[RFC3916] Xiao, X., McPherson, D., and P. Pate, "Requirements for
Pseudo-Wire Emulation Edge-to-Edge (PWE3)", RFC 3916,
September 2004.
[RFC3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
Edge (PWE3) Architecture", RFC 3985, March 2005.
[RFC5317] Bryant, S. and L. Andersson, "Joint Working Team (JWT)
Report on MPLS Architectural Considerations for a
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Transport Profile", RFC 5317, February 2009.
Authors' Addresses
Stewart Bryant (editor)
Cisco Systems
250, Longwater, Green Park,
Reading, Berks RG2 6GB
UK
Phone: UK
Fax:
Email: stbryant@cisco.com
URI:
Sami Boutros
Cisco Systems
3750 Cisco Way
San Jose, CA 95134
USA
Phone:
Fax:
Email: sboutros@cisco.com
URI:
Luca Martini
Cisco Systems
9155 East Nichols Avenue, Suite 400
Englewood, CO 80112
USA
Phone:
Fax:
Email: lmartini@cisco.com
URI:
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Siva Sivabalan
Cisco Systems
2000 Innovation Drive
Kanata, Ontario K2K 3EB
Canada
Phone:
Fax:
Email: msiva@cisco.com
URI:
George Swallow
Cisco Systems
1414 Massachusetts Ave
Boxborough, MA 01719
USA
Phone:
Fax:
Email: swallow@cisco.com
URI:
David Ward
Cisco Systems
3750 Cisco Way
San Jose, CA 95134
USA
Phone:
Fax:
Email: wardd@cisco.com
URI:
Andy Malis
Verizon Communications
117 West St.
Waltham, MA 02451
USA
Phone:
Fax:
Email: andrew.g.malis@verizon.com
URI:
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