Request for Comments: 4447 E. Rosen
Category: Standards Track Cisco Systems, Inc.
N. El-Aawar
Level 3 Communications, LLC.
T. Smith
Network Appliance, Inc.
G. Heron
Tellabs
April 2006
Pseudowire Setup and Maintenance
Using the Label Distribution Protocol (LDP)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
Layer 2 services (such as Frame Relay, Asynchronous Transfer Mode,
and Ethernet) can be "emulated" over an MPLS backbone by
encapsulating the Layer 2 Protocol Data Units (PDU) and transmitting
them over "pseudowires". It is also possible to use pseudowires to
provide low-rate Time Division Multiplexed and a Synchronous Optical
NETworking circuit emulation over an MPLS-enabled network. This
document specifies a protocol for establishing and maintaining the
pseudowires, using extensions to Label Distribution Protocol (LDP).
Procedures for encapsulating Layer 2 PDUs are specified in a set of
companion documents.
Table of Contents
1. Introduction ....................................................3
2. Specification of Requirements ...................................5
3. The Pseudowire Label ............................................5
4. Details Specific to Particular Emulated Services ................7
4.1. IP Layer 2 Transport .......................................7
5. LDP .............................................................7
5.1. LDP Extensions .............................................8
5.2. The PWid FEC Element .......................................8
5.3. The Generalized PWid FEC Element ..........................10
5.3.1. Attachment Identifiers .............................11
5.3.2. Encoding the Generalized ID FEC Element ............13
5.3.2.1. Interface Parameters TLV ..................14
5.3.2.2. PW Grouping TLV ...........................14
5.3.3. Signaling Procedures ...............................15
5.4. Signaling of Pseudowire Status ............................16
5.4.1. Use of Label Mappings Messages .....................16
5.4.2. Signaling PW Status ................................17
5.4.3. Pseudowire Status Negotiation Procedures ...........18
5.5. Interface Parameters Sub-TLV ..............................19
6. Control Word ...................................................20
6.1. PW Types for Which the Control Word is REQUIRED ...........20
6.2. PW Types for Which the Control Word is NOT Mandatory ......21
6.3. LDP Label Withdrawal Procedures ...........................22
6.4. Sequencing Considerations .................................23
6.4.1. Label Advertisements ...............................23
6.4.2. Label Release ......................................24
7. IANA Considerations ............................................24
7.1. LDP TLV TYPE ..............................................24
7.2. LDP Status Codes ..........................................24
7.3. FEC Type Name Space .......................................25
8. Security Considerations ........................................25
8.1. Data-Plane Security .......................................25
8.2. Control-Plane Security ....................................26
9. Acknowledgements ...............................................27
10. Normative References ..........................................27
11. Informative References ........................................27
12. Additional Contributing Authors ...............................28
Appendix A. C-bit Handling Procedures Diagram .....................31
1. Introduction
In [FRAME], [ATM], [PPPHDLC], and [ETH], it is explained how to
encapsulate a Layer 2 Protocol Data Unit (PDU) for transmission over
an MPLS-enabled network. Those documents specify that a "pseudowire
header", consisting of a demultiplexor field, will be prepended to
the encapsulated PDU. The pseudowire demultiplexor field is
prepended before transmitting a packet on a pseudowire. When the
packet arrives at the remote endpoint of the pseudowire, the
demultiplexor is what enables the receiver to identify the particular
pseudowire on which the packet has arrived. To transmit the packet
from one pseudowire endpoint to another, the packet may need to
travel through a "Packet Switched Network (PSN) tunnel"; this will
require that an additional header be prepended to the packet.
Accompanying documents [CEP, SAToP] specify methods for transporting
time-division multiplexing (TDM) digital signals (TDM circuit
emulation) over a packet-oriented MPLS-enabled network. The
transmission system for circuit-oriented TDM signals is the
Synchronous Optical Network (SONET)[SDH]/Synchronous Digital
Hierarchy (SDH) [ITUG]. To support TDM traffic, which includes
voice, data, and private leased-line service, the pseudowires must
emulate the circuit characteristics of SONET/SDH payloads. The TDM
signals and payloads are encapsulated for transmission over
pseudowires. A pseudowire demultiplexor and a PSN tunnel header is
prepended to this encapsulation.
[SAToP] describes methods for transporting low-rate time-division
multiplexing (TDM) digital signals (TDM circuit emulation) over PSNs,
while [CEP] similarly describes transport of high-rate TDM
(SONET/SDH). To support TDM traffic, the pseudowires must emulate
the circuit characteristics of the original T1, E1, T3, E3, SONET, or
SDH signals. [SAToP] does this by encapsulating an arbitrary but
constant amount of the TDM data in each packet, and the other methods
encapsulate TDM structures.
In this document, we specify the use of the MPLS Label Distribution
Protocol, LDP [RFC3036], as a protocol for setting up and maintaining
the pseudowires. In particular, we define new TLVs, FEC elements,
parameters, and codes for LDP, which enable LDP to identify
pseudowires and to signal attributes of pseudowires. We specify how
a pseudowire endpoint uses these TLVs in LDP to bind a demultiplexor
field value to a pseudowire, and how it informs the remote endpoint
of the binding. We also specify procedures for reporting pseudowire
status changes, for passing additional information about the
pseudowire as needed, and for releasing the bindings.
In the protocol specified herein, the pseudowire demultiplexor field
is an MPLS label. Thus, the packets that are transmitted from one
end of the pseudowire to the other are MPLS packets, which must be
transmitted through an MPLS tunnel. However, if the pseudowire
endpoints are immediately adjacent and penultimate hop popping
behavior is in use, the MPLS tunnel may not be necessary. Any sort
of PSN tunnel can be used, as long as it is possible to transmit MPLS
packets through it. The PSN tunnel can itself be an MPLS LSP, or any
other sort of tunnel that can carry MPLS packets. Procedures for
setting up and maintaining the MPLS tunnels are outside the scope of
this document.
This document deals only with the setup and maintenance of point-to-
point pseudowires. Neither point-to-multipoint nor multipoint-to-
point pseudowires are discussed.
QoS-related issues are not discussed in this document. The following
two figures describe the reference models that are derived from
[RFC3985] to support the PW emulated services.
|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudowire ------->| |
| | | |
|Attachment| |<-- PSN Tunnel -->| |Attachment|
| Circuit V V V V Circuit |
V (AC) +----+ +----+ (AC) V
+-----+ | | PE1|==================| PE2| | +-----+
| |----------|............PW1.............|----------| |
| CE1 | | | | | | | | CE2 |
| |----------|............PW2.............|----------| |
+-----+ ^ | | |==================| | | ^ +-----+
^ | +----+ +----+ | | ^
| | Provider Edge 1 Provider Edge 2 | |
| | | |
Customer | | Customer
Edge 1 | | Edge 2
| |
native service native service
Figure 1: PWE3 Reference Model
+-----------------+ +-----------------+
|Emulated Service | |Emulated Service |
|(e.g., TDM, ATM) |<==== Emulated Service ===>|(e.g., TDM, ATM) |
+-----------------+ +-----------------+
| Payload | | Payload |
| Encapsulation |<====== Pseudowire =======>| Encapsulation |
+-----------------+ +-----------------+
|PW Demultiplexer | |PW Demultiplexer |
| PSN Tunnel, |<======= PSN Tunnel ======>| PSN Tunnel, |
| PSN & Physical | | PSN & Physical |
| Layers | | Layers |
+-------+---------+ __________ +---------+-------+
| / \ |
+===============/ PSN \================+
\ /
\____________/
Figure 2: PWE3 Protocol Stack Reference Model
For the purpose of this document, PE1 will be defined as the ingress
router, and PE2 as the egress router. A layer 2 PDU will be received
at PE1, encapsulated at PE1, transported and decapsulated at PE2, and
transmitted out of PE2.
2. Specification of Requirements
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].
3. The Pseudowire Label
Suppose that it is desired to transport Layer 2 PDUs from ingress LSR
PE1 to egress LSR PE2, across an intervening MPLS-enabled network.
We assume that there is an MPLS tunnel from PE1 to PE2. That is, we
assume that PE1 can cause a packet to be delivered to PE2 by
encapsulating the packet in an "MPLS tunnel header" and sending the
result to one of its adjacencies. The MPLS tunnel is an MPLS Label
Switched Path (LSP); thus, putting on an MPLS tunnel encapsulation is
a matter of pushing on an MPLS label.
We presuppose that a large number of pseudowires can be carried
through a single MPLS tunnel. Thus, it is never necessary to
maintain state in the network core for individual pseudowires. We do
not presuppose that the MPLS tunnels are point to point; although the
pseudowires are point to point, the MPLS tunnels may be multipoint to
point. We do not presuppose that PE2 will even be able to determine
the MPLS tunnel through which a received packet was transmitted.
(For example, if the MPLS tunnel is an LSP and penultimate hop
popping is used, when the packet arrives at PE2, it will contain no
information identifying the tunnel.)
When PE2 receives a packet over a pseudowire, it must be able to
determine that the packet was in fact received over a pseudowire, and
it must be able to associate that packet with a particular
pseudowire. PE2 is able to do this by examining the MPLS label that
serves as the pseudowire demultiplexor field shown in Figure 2. Call
this label the "PW label".
When PE1 sends a Layer 2 PDU to PE2, it creates an MPLS packet by
adding the PW label to the packet, thus creating the first entry of
the label stack. If the PSN tunnel is an MPLS LSP, the PE1 pushes
another label (the tunnel label) onto the packet as the second entry
of the label stack. The PW label is not visible again until the MPLS
packet reaches PE2. PE2’s disposition of the packet is based on the
PW label.
If the payload of the MPLS packet is, for example, an ATM AAL5 PDU,
the PW label will generally correspond to a particular ATM VC at PE2.
That is, PE2 needs to be able to infer from the PW label the outgoing
interface and the VPI/VCI value for the AAL5 PDU. If the payload is
a Frame Relay PDU, then PE2 needs to be able to infer from the PW
label the outgoing interface and the DLCI value. If the payload is
an Ethernet frame, then PE2 needs to be able to infer from the PW
label the outgoing interface, and perhaps the VLAN identifier. This
process is uni-directional and will be repeated independently for
bi-directional operation. It is REQUIRED that the same PW ID and PW
type be assigned for a given circuit in both directions. The group
ID (see below) MUST NOT be required to match in both directions. The
transported frame MAY be modified when it reaches the egress router.
If the header of the transported Layer 2 frame is modified, this MUST
be done at the egress LSR only. Note that the PW label must always
be at the bottom of the packet’s label stack, and labels MUST be
allocated from the per-platform label space.
This document does not specify a method for distributing the MPLS
tunnel label or any other labels that may appear above the PW label
on the stack. Any acceptable method of MPLS label distribution will
do. This document specifies a protocol for assigning and
distributing the PW label. This protocol is LDP, extended as
specified in the remainder of this document. An LDP session must be
set up between the pseudowire endpoints. LDP MUST be used in its
"downstream unsolicited" mode. LDP’s "liberal label retention" mode
SHOULD be used.
In addition to the protocol specified herein, static assignment of PW
labels may be used, and implementations of this protocol SHOULD
provide support for static assignment.
This document specifies all the procedures necessary to set up and
maintain the pseudowires needed to support "unswitched" point-to-
point services, where each endpoint of the pseudowire is provisioned
with the identify of the other endpoint. There are also protocol
mechanisms specified herein that can be used to support switched
services and other provisioning models. However, the use of the
protocol mechanisms to support those other models and services is not
described in this document.
4. Details Specific to Particular Emulated Services
4.1. IP Layer 2 Transport
This mode carries IP packets over a pseudowire. The encapsulation
used is according to [RFC3032]. The PW control word MAY be inserted
between the MPLS label stack and the IP payload. The encapsulation
of the IP packets for forwarding on the attachment circuit is
implementation specific, is part of the native service processing
(NSP) function [RFC3985], and is outside the scope of this document.
5. LDP
The PW label bindings are distributed using the LDP downstream
unsolicited mode described in [RFC3036]. The PEs will establish an
LDP session using the Extended Discovery mechanism described in [LDP,
sections 2.4.2 and 2.5].
An LDP Label Mapping message contains an FEC TLV, a Label TLV, and
zero or more optional parameter TLVs.
The FEC TLV is used to indicate the meaning of the label. In the
current context, the FEC TLV would be used to identify the particular
pseudowire that a particular label is bound to. In this
specification, we define two new FEC TLVs to be used for identifying
pseudowires. When setting up a particular pseudowire, only one of
these FEC TLVs is used. The one to be used will depend on the
particular service being emulated and on the particular provisioning
model being supported.
LDP allows each FEC TLV to consist of a set of FEC elements. For
setting up and maintaining pseudowires, however, each FEC TLV MUST
contain exactly one FEC element.
The LDP base specification has several kinds of label TLVs, including
the Generic Label TLV, as specified in [RFC3036], section 3.4.2.1.
For setting up and maintaining pseudowires, the Generic Label TLV
MUST be used.
5.1. LDP Extensions
This document specifies no new LDP messages.
This document specifies the following new TLVs to be used with LDP:
TLV Specified in Section Defined for Message
===================================================================
PW Status TLV 5.4.2 Notification
PW Interface Parameters TLV 5.3.2.1 FEC
PW Grouping ID TLV 5.3.2.2 FEC
Additionally, the following new FEC element types are defined:
FEC Element Type Specified in Section Defined for Message
===================================================================
0x80 5.2 FEC
0x81 5.3 FEC
The following new LDP error codes are also defined:
Status Code Specified in Section
====================================================================
"Illegal C-Bit" 6.1
"Wrong C-Bit" 6.2
"Incompatible bit-rate" [CEP]
"CEP/TDM mis-configuration" [CEP]
"PW status" 5.4.2
"Unassigned/Unrecognized TAI" 5.3.3
"Generic Misconfiguration Error" [SAToP]
"Label Withdraw PW Status Method Not Supported" 5.4.1
5.2. The PWid FEC Element
The PWid FEC element may be used whenever both pseudowire endpoints
have been provisioned with the same 32-bit identifier for the
pseudowire.
For this purpose, a new type of FEC element is defined. The FEC
element type is 0x80 and is defined as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PWid (0x80) |C| PW type |PW info Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface Parameter Sub-TLV |
| " |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- PW type
A 15-bit quantity containing a value that represents the type of
PW. Assigned values are specified in "IANA Allocations for
Pseudowire Edge to Edge Emulation (PWE3)" [IANA].
- Control word bit (C)
The bit (C) is used to flag the presence of a control word as
follows:
C = 1 Control word present on this PW.