Internet Engineering Task Force (IETF) L. Martini, Ed.

Internet Engineering Task Force (IETF) L. Martini, Ed.
Request for Comments: 8077 G. Heron, Ed.
STD: 84 Cisco
Obsoletes: 4447, 6723 February 2017
Category: Standards Track
ISSN: 2070-1721

Pseudowire Setup and Maintenance
Using the Label Distribution Protocol (LDP)

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 (PDUs) and then transmitting them
over pseudowires (PWs). It is also possible to use pseudowires to
provide low-rate Time-Division Multiplexed and Synchronous Optical
NETworking circuit emulation over an MPLS-enabled network. This
document specifies a protocol for establishing and maintaining the
pseudowires, using extensions to the Label Distribution Protocol
(LDP). Procedures for encapsulating Layer 2 PDUs are specified in
other documents.

This document is a rewrite of RFC 4447 for publication as an Internet
Standard.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8077.

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RFC 8077 PWE3 Using LDP February 2017

Copyright Notice

Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Without obtaining an adequate license from the person(s) controlling
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not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.

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Table of Contents

1. Introduction ....................................................4
2. Changes from RFC 4447 ...........................................6
3. Specification of Requirements ...................................6
4. The Pseudowire Label ............................................7
5. Details Specific to Particular Emulated Services ................9
5.1. IP Layer 2 Transport .......................................9
6. LDP .............................................................9
6.1. The PWid FEC Element .......................................9
6.2. The Generalized PWid FEC Element ..........................11
6.2.1. Attachment Identifiers .............................12
6.2.2. Encoding the Generalized PWid FEC Element ..........14
6.2.2.1. PW Interface Parameters TLV ...............15
6.2.2.2. PW Group ID TLV ...........................15
6.2.3. Signaling Procedures ...............................16
6.3. Signaling of Pseudowire Status ............................17
6.3.1. Use of Label Mapping Messages ......................17
6.3.2. Signaling PW Status ................................18
6.3.3. Pseudowire Status Negotiation Procedures ...........19
6.4. Interface Parameter Sub-TLV ...............................20
6.5. LDP Label Withdrawal Procedures ...........................21
7. Control Word ...................................................22
7.1. PW Types for Which the Control Word Is REQUIRED ...........22
7.2. PW Types for Which the Control Word Is NOT Mandatory ......22
7.3. Control-Word Renegotiation by Label Request Message .......24
7.4. Sequencing Considerations .................................25
7.4.1. Label Advertisements ...............................25
7.4.2. Label Release ......................................25
8. IANA Considerations ............................................26
8.1. LDP TLV TYPE ..............................................26
8.2. LDP Status Codes ..........................................26
8.3. FEC Type Name Space .......................................26
9. Security Considerations ........................................26
9.1. Data-Plane Security .......................................27
9.2. Control-Plane Security ....................................28
10. Interoperability and Deployment ...............................29
11. References ....................................................29
11.1. Normative References .....................................29
11.2. Informative References ...................................30
Acknowledgments ...................................................31
Contributors ......................................................32
Authors' Addresses ................................................35

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1. Introduction

[RFC4619], [RFC4717], [RFC4618], and [RFC4448] explain 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 demultiplexer field, will be prepended to
the encapsulated PDU. The pseudowire demultiplexer field is
prepended before transmitting a packet on a pseudowire. When the
packet arrives at the remote endpoint of the pseudowire, the
demultiplexer 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.

[RFC4842] and [RFC4553] specify two 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) [ANSI] / 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
demultiplexer and a PSN tunnel header are prepended to this
encapsulation.

[RFC4553] describes methods for transporting low-rate time-division
multiplexing (TDM) digital signals (TDM circuit emulation) over PSNs,
while [RFC4842] 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. [RFC4553] 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) [RFC5036] as a protocol for setting up and maintaining
the pseudowires. In particular, we define new TLVs, Forwarding
Equivalence Class (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 demultiplexer 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. These procedures are intended to be
independent of the underlying version of IP used for LDP signaling.

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In the protocol specified herein, the pseudowire demultiplexer 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

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+-----------------+ +-----------------+
|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 (Provider Edge 1) 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. Changes from RFC 4447

The changes in this document are mostly minor fixes to spelling and
grammar, or clarifications to the text, which were either noted as
errata to [RFC4447] or found by the editors.

Additionally, Section 7.3 ("Control-Word Renegotiation by Label
Request Message") has been added, obsoleting [RFC6723]. The diagram
of C-bit handling procedures has also been removed. A note has been
added in Section 6.3.2 to clarify that the C-bit is part of the FEC.

A reference has also been added to [RFC7358] to indicate the use of
downstream unsolicited mode to distribute PW FEC label bindings,
independent of the negotiated label advertisement mode of the LDP
session.

3. 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].

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4. 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 demultiplexer 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 Adaptation
Layer 5 (AAL5) PDU, the PW label will generally correspond to a
particular ATM Virtual Circuit (VC) at PE2. That is, PE2 needs to be
able to infer from the PW label the outgoing interface and the
VPI/VCI (Virtual Path Identifier / Virtual Circuit Identifier) 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 Data Link Connection Identifier (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 unidirectional and will be repeated independently for

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bidirectional operation. When using the PWid FEC Element, 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 exchange PW FEC
label bindings in downstream unsolicited mode, independent of the
negotiated label advertisement mode of the LDP session according to
the specifications in [RFC7358]. LDP's "liberal label retention"
mode SHOULD be used. However, all the LDP procedures that are
specified in [RFC5036] and that are also applicable to this protocol
specification MUST be implemented.

This document requires that a receiving LSR MUST respond to a Label
Request message with either a Label Mapping for the requested label
or a Notification message that indicates why it cannot satisfy the
request. These procedures are specified in [RFC5036], Sections 3.5.7
("Label Mapping Message") and 3.5.8 ("Label Request Message"). Note
that sending these responses is a stricter requirement than is
specified in [RFC5036], but these response messages are REQUIRED to
ensure correct operation of this protocol.

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. PW encapsulation is always
symmetrical in both directions of traffic along a specific PW,
whether or not the PW uses an LDP control plane.

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 identity 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.

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5. Details Specific to Particular Emulated Services

5.1. 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.

6. LDP

The PW label bindings are distributed using the LDP downstream
unsolicited mode described in [RFC5036]. The PEs will establish an
LDP session using the Extended Discovery mechanism described in
Sections 2.4.2 and 2.5 of [RFC5036].

An LDP Label Mapping message contains a 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 Section 3.4.2.1 of [RFC5036].
For setting up and maintaining pseudowires, the Generic Label TLV
MUST be used.

6.1. 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:

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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 |
| " |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

- Control word bit (C)

The C-bit is used to flag the presence of a control word as
follows:

C = 1 control word present on this PW.
C = 0 no control word present on this PW.

Please see Section 7 ("Control Word") for further explanation.

- 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)" [RFC4446].

- PW info length

Length of the PW ID field and the Interface Parameter Sub-TLV
field in octets. If this value is 0, then it references all PWs
using the specified Group ID, and there is no PW ID present, nor
are there any Interface Parameter Sub-TLVs.

- Group ID

An arbitrary 32-bit value that represents a group of PWs that is
used to create groups in the PW space. The Group ID is intended
to be used as a port index or a virtual tunnel index. To simplify
configuration, a particular PW Group ID at ingress could be part
of a Group ID assigned to the virtual tunnel for transport to the
egress router. The Group ID is very useful for sending wildcard
label withdrawals or PW wildcard status Notification messages to
remote PEs upon physical port failure.

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- PW ID

A non-zero, 32-bit connection ID that together with the PW type
identifies a particular PW. Note that the PW ID and the PW type
MUST be the same at both endpoints.

- Interface Parameter Sub-TLV

This variable length TLV is used to provide interface-specific
parameters, such as Attachment Circuit MTU.

Note that as the Interface Parameter Sub-TLV is part of the FEC,
the rules of LDP make it impossible to change the interface
parameters once the pseudowire has been set up. Thus, the
interface parameters field must not be used to pass information,
such as status information, that may change during the life of the
pseudowire. Optional parameter TLVs should be used for that
purpose.

Using the PWid FEC, each of the two pseudowire endpoints
independently initiates the setup of a unidirectional LSP. An
outgoing LSP and an incoming LSP are bound together into a single
pseudowire if they have the same PW ID and PW type.

6.2. The Generalized PWid FEC Element

The PWid FEC Element can be used if a unique 32-bit value has been
assigned to the PW and if each endpoint has been provisioned with
that value. The Generalized PWid FEC Element requires that the PW
endpoints be uniquely identified; the PW itself is identified as a
pair of endpoints. In addition, the endpoint identifiers are
structured to support applications where the identity of the remote
endpoints needs to be auto-discovered rather than statically
configured.

The "Generalized PWid FEC Element" is FEC type 0x81.

The Generalized PWid FEC Element does not contain anything
corresponding to the Group ID of the PWid FEC Element. The
functionality of the Group ID is provided by a separate optional LDP
TLV, the PW Group ID TLV, described in Section 6.2.2.2. The
interface parameters field of the PWid FEC Element is also absent;
its functionality is replaced by the optional PW Interface Parameters
TLV, described in Section 6.2.2.1.

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6.2.1. Attachment Identifiers

As discussed in [RFC3985], a pseudowire can be thought of as
connecting two "forwarders". The protocol used to set up a
pseudowire must allow the forwarder at one end of a pseudowire to
identify the forwarder at the other end. We use the term "Attachment
Identifier", or "AI", to refer to the field that the protocol uses to
identify the forwarders. In the PWid FEC, the PWid field serves as
the AI. In this section, we specify a more general form of AI that
is structured and of variable length.

Every Forwarder in a PE must be associated with an Attachment
Identifier (AI), either through configuration or through some
algorithm. The Attachment Identifier must be unique in the context
of the PE router in which the Forwarder resides. The combination <PE
router IP address, AI> must be globally unique.

It is frequently convenient to regard a set of Forwarders as being
members of a particular "group", where PWs may only be set up among
members of a group. In such cases, it is convenient to identify the
Forwarders relative to the group, so that an Attachment Identifier
would consist of an Attachment Group Identifier (AGI) plus an
Attachment Individual Identifier (AII).

An Attachment Group Identifier may be thought of as a VPN-id, or a
VLAN identifier, some attribute that is shared by all the Attachment
PWs (or pools thereof) that are allowed to be connected.

The details of how to construct the AGI and AII fields identifying
the pseudowire endpoints are outside the scope of this specification.
Different pseudowire applications, and different provisioning models,
will require different sorts of AGI and AII fields. The
specification of each such application and/or model must include the
rules for constructing the AGI and AII fields.

As previously discussed, a (bidirectional) pseudowire consists of a
pair of unidirectional LSPs, one in each direction. If a particular
pseudowire connects PE1 with PE2, the PW direction from PE1 to PE2
can be identified as:

<PE1, <AGI, AII1>, PE2, <AGI, AII2>>,

and the PW direction from PE2 to PE1 can be identified by:

<PE2, <AGI, AII2>, PE1, <AGI, AII1>>.

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Note that the AGI must be the same at both endpoints, but the AII
will in general be different at each endpoint. Thus, from the
perspective of a particular PE, each pseudowire has a local or
"Source AII", and a remote or "Target AII". The pseudowire setup
protocol can carry all three of these quantities:

- Attachment Group Identifier (AGI)

- Source Attachment Individual Identifier (SAII)

- Target Attachment Individual Identifier (TAII)

If the AGI is non-null, then the Source AI (SAI) consists of the AGI
together with the SAII, and the Target AI (TAI) consists of the TAII
together with the AGI. If the AGI is null, then the SAII and TAII
are the SAI and TAI, respectively.

The interpretation of the SAI and TAI is a local matter at the
respective endpoint.

The association of two unidirectional LSPs into a single
bidirectional pseudowire depends on the SAI and the TAI. Each
application and/or provisioning model that uses the Generalized PWid
FEC must specify the rules for performing this association.

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6.2.2. Encoding the Generalized PWid FEC Element

FEC element type 0x81 is used. The FEC element is encoded 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Gen PWid (0x81)|C| PW Type |PW info length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AGI Type | Length | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ AGI Value (contd.) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AII Type | Length | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ SAII Value (contd.) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AII Type | Length | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ TAII Value (contd.) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

This document does not specify the AII and AGI type field values;
specification of the type field values to be used for a particular
application is part of the specification of that application. IANA
has assigned these values using the method defined in [RFC4446].

The SAII, TAII, and AGI are simply carried as octet strings. The
Length byte specifies the size of the Value field. The null string
can be sent by setting the Length byte to 0. If a particular
application does not need all three of these sub-elements, it MUST
send all the sub-elements but set the Length to 0 for the unused sub-
elements.

The PW information length field contains the length of the SAII,
TAII, and AGI, combined in octets. If this value is 0, then it
references all PWs using the specific Group ID (specified in the PW
Group ID TLV). In this case, there are no other FEC element fields
(AGI, SAII, etc.) present, nor any PW Interface Parameters TLVs.

Note that the interpretation of a particular field as AGI, SAII, or
TAII depends on the order of its occurrence. The Type field
identifies the type of the AGI, SAII, or TAII. When comparing two

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occurrences of an AGI (or SAII or TAII), the two occurrences are
considered identical if the Type, Length, and Value fields of one are
identical, respectively, to those of the other.

6.2.2.1. PW Interface Parameters TLV

This TLV MUST only be used when sending the Generalized PWid FEC. It
specifies interface-specific parameters. Specific parameters, when
applicable, MUST be used to validate that the PEs and the ingress and
egress ports at the edges of the circuit have the necessary
capabilities to interoperate with each other.

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| PW Intf P. TLV (0x096B) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLV Type | Length | Variable Length Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Variable Length Value |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

A more detailed description of this field can be found in Section 6.4
("Interface Parameter Sub-TLV").

6.2.2.2. PW Group ID TLV

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| PW Group ID TLV (0x096C) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The PW Group ID is an arbitrary 32-bit value that represents an
arbitrary group of PWs. It is used to create group PWs; for example,
a PW Group ID can be used as a port index and assigned to all PWs
that lead to that port. Use of the PW Group ID enables a PE to send
"wildcard" label withdrawals, or "wildcard" status Notification
messages, to remote PEs upon physical port failure.

Note Well: The PW Group ID is different from and has no relation to
the Attachment Group Identifier.

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The PW Group ID TLV is not part of the FEC and will not be advertised
except in the PW FEC advertisement. The advertising PE MAY use the
wildcard withdraw semantics, but the remote PEs MUST implement
support for wildcard messages. This TLV MUST only be used when
sending the Generalized PWid FEC.

To issue a wildcard command (status or withdraw):

- Set the PW Info Length to 0 in the Generalized PWid FEC Element.

- Send only the PW Group ID TLV with the FEC (no AGI/SAII/TAII is
sent).

6.2.3. Signaling Procedures

In order for PE1 to begin signaling PE2, PE1 must know the address of
the remote PE2 and a TAI. This information may have been configured
at PE1, or it may have been learned dynamically via some auto-
discovery procedure.

The egress PE (PE1), which has knowledge of the ingress PE, initiates
the setup by sending a Label Mapping message to the ingress PE (PE2).
The Label Mapping message contains the FEC TLV, carrying the
Generalized PWid FEC Element (type 0x81). The Generalized PWid FEC
Element contains the AGI, SAII, and TAII information.

Next, when PE2 receives such a Label Mapping message, PE2 interprets
the message as a request to set up a PW whose endpoint (at PE2) is
the Forwarder identified by the TAI. From the perspective of the
signaling protocol, exactly how PE2 maps AIs to Forwarders is a local
matter. In some Virtual Private Wire Service (VPWS) provisioning
models, the TAI might, for example, be a string that identifies a
particular Attachment Circuit, such as "ATM3VPI4VCI5", or it might,
for example, be a string, such as "Fred", that is associated by
configuration with a particular Attachment Circuit. In Virtual
Private LAN Service (VPLS), the AGI could be a VPN-id, identifying a
particular VPLS instance.

If PE2 cannot map the TAI to one of its Forwarders, then PE2 sends a
Label Release message to PE1, with a Status Code of
"Unassigned/Unrecognized TAI", and the processing of the Label
Mapping message is complete.

The FEC TLV sent in a Label Release message is the same as the FEC
TLV received in the Label Mapping message being released (but without
the interface parameter TLV). More generally, the FEC TLV is the

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same in all LDP messages relating to the same PW. In a Label Release
message, this means that the SAII is the remote peer's AII and the
TAII is the sender's local AII.

If the Label Mapping message has a valid TAI, PE2 must decide whether
to accept it. The procedures for so deciding will depend on the
particular type of Forwarder identified by the TAI. Of course, the
Label Mapping message may be rejected due to standard LDP error
conditions as detailed in [RFC5036].

If PE2 decides to accept the Label Mapping message, then it has to
make sure that a PW LSP is set up in the opposite (PE1-->PE2)
direction. If it has already signaled for the corresponding PW LSP
in that direction, nothing more needs to be done. Otherwise, it must
initiate such signaling by sending a Label Mapping message to PE1.
This is very similar to the Label Mapping message PE2 received, but
the SAI and TAI are reversed.

Thus, a bidirectional PW consists of two LSPs, where the FEC of one
has the SAII and TAII reversed with respect to the FEC of the other.

6.3. Signaling of Pseudowire Status

6.3.1. Use of Label Mapping Messages

The PEs MUST send Label Mapping messages to their peers as soon as
the PW is configured and administratively enabled, regardless of the
Attachment Circuit state. The PW label should not be withdrawn
unless the operator administratively configures the pseudowire down
(or the PW configuration is deleted entirely). Using the procedures
outlined in this section, a simple label withdraw method MAY also be
supported as a legacy means of signaling PW status and AC status. In
any case, if the label-to-PW binding is not available, the PW MUST be
considered in the down state.

Once the PW status negotiation procedures are completed, if they
result in the use of the label withdraw method for PW status
communication, and this method is not supported by one of the PEs,
then that PE must send a Label Release message to its peer with the
following error:

"Label Withdraw PW Status Method Not Supported"

If the label withdraw method for PW status communication is selected
for the PW, it will result in the Label Mapping message being
advertised only if the Attachment Circuit is active. The PW status
signaling procedures described in this section MUST be fully
implemented.

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6.3.2. Signaling PW Status

The PE devices use an LDP TLV to indicate status to their remote
peers. This PW Status TLV contains more information than the
alternative simple Label Withdraw message.

The format of the PW Status TLV is:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| PW Status (0x096A) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The status code is a 4-octet bit field as specified in "IANA
Allocations for Pseudowire Edge to Edge Emulation (PWE3)" [RFC4446].
The Length field specifies the length of the Status Code field in
octets (equal to 4).

Each bit in the Status Code field can be set individually to indicate
more than a single failure at once. Each fault can be cleared by
sending an appropriate Notification message in which the respective
bit is cleared. The presence of the lowest bit (PW Not Forwarding)
acts only as a generic failure indication when there is a link-down
event for which none of the other bits apply.

The Status TLV is transported to the remote PW peer via the LDP
Notification message as described in [RFC5036]. The format of the
Notification message for carrying the PW Status is 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Notification (0x0001) | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status (TLV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW Status TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PWid FEC TLV or Generalized ID FEC TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW Group ID TLV (Optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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The Status TLV status code is set to 0x00000028, "PW status", to
indicate that PW status follows. Since this notification does not
refer to any particular message, the Message ID field is set to 0.

The PW FEC TLV SHOULD NOT include the Interface Parameter Sub-TLVs,
as they are ignored in the context of this message. However, the PW
FEC TLV MUST include the C-bit, where applicable, as it is part of
the FEC. When a PE's Attachment Circuit encounters an error, use of
the PW Notification message allows the PE to send a single "wildcard"
status message, using a PW FEC TLV with only the Group ID set, to
denote this change in status for all affected PW connections. This
status message contains either the PW FEC TLV with only the Group ID
set, or else it contains the Generalized FEC TLV with only the PW
Group ID TLV.

As mentioned above, the Group ID field of the PWid FEC Element, or
the PW Group ID TLV used with the Generalized PWid FEC Element, can
be used to send a status notification for all arbitrary sets of PWs.
This procedure is OPTIONAL, and if it is implemented, the LDP
Notification message should be as follows: If the PWid FEC Element is
used, the PW information length field is set to 0, the PW ID field is
not present, and the Interface Parameter Sub-TLVs are not present.
If the Generalized FEC Element is used, the AGI, SAII, and TAII are
not present, the PW information length field is set to 0, the PW
Group ID TLV is included, and the PW Interface Parameters TLV is
omitted. For the purpose of this document, this is called the
"wildcard PW status notification procedure", and all PEs implementing
this design are REQUIRED to accept such a Notification message but
are not required to send it.

6.3.3. Pseudowire Status Negotiation Procedures

When a PW is first set up, the PEs MUST attempt to negotiate the
usage of the PW Status TLV. This is accomplished as follows: A PE
that supports the PW Status TLV MUST include it in the initial Label
Mapping message following the PW FEC and the Interface Parameter Sub-
TLVs. The PW Status TLV will then be used for the lifetime of the
pseudowire. This is shown in the following diagram:

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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 FEC or Generalized PWid FEC +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface Parameters |
| " |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| Generic Label (0x0200) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| PW Status (0x096A) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

If a PW Status TLV is included in the initial Label Mapping message
for a PW, then if the Label Mapping message from the remote PE for
that PW does not include a PW Status TLV, or if the remote PE does
not support the PW Status TLV, the PW will revert to the label
withdraw method of signaling PW status. Note that if the PW Status
TLV is not supported by the remote peer, the peer will automatically
ignore it, since the I (ignore) bit is set in the TLV. The PW Status
TLV, therefore, will not be present in the corresponding FEC
advertisement from the remote LDP peer, which results in exactly the
above behavior.

If the PW Status TLV is not present following the FEC TLV in the
initial PW Label Mapping message received by a PE, then the PW Status
TLV will not be used, and both PEs supporting the pseudowire will
revert to the label withdraw procedure for signaling status changes.

If the negotiation process results in the usage of the PW Status TLV,
then the actual PW status is determined by the PW Status TLV that was
sent within the initial PW Label Mapping message. Subsequent updates
of PW status are conveyed through the Notification message.

6.4. Interface Parameter Sub-TLV

This field specifies interface-specific parameters. When applicable,
it MUST be used to validate that the PEs and the ingress and egress
ports at the edges of the circuit have the necessary capabilities to
interoperate with each other. The field structure is defined as
follows:

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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLV Type | Length | Variable Length Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Variable Length Value |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Length field is defined as the length of the interface parameter
including the Sub-TLV Type and Length field itself. Processing of
the interface parameters should continue when unknown interface
parameters are encountered, and they MUST be silently ignored.

The Interface Parameter Sub-TLV Type values are specified in "IANA
Allocations for Pseudowire Edge to Edge Emulation (PWE3)" [RFC4446].

- Interface MTU sub-TLV type

A 2-octet value indicating the MTU in octets. This is the Maximum
Transmission Unit, excluding encapsulation overhead, of the egress
packet interface that will be transmitting the decapsulated PDU
that is received from the MPLS-enabled network. This parameter is
applicable only to PWs transporting packets and is REQUIRED for
these PW types. If this parameter does not match in both
directions of a specific PW, that PW MUST NOT be enabled.

- Optional Interface Description string sub-TLV type

This arbitrary, and OPTIONAL, interface description string is used
to send a human-readable administrative string describing the
interface to the remote PE. This parameter is OPTIONAL and is
applicable to all PW types. The interface description parameter
string length is variable and can be from 0 to 80 octets. Human-
readable text MUST be provided in the UTF-8 charset using the
Default Language [RFC2277].

6.5. LDP Label Withdrawal Procedures

As mentioned above, the Group ID field of the PWid FEC Element, or
the PW Group ID TLV used with the Generalized PWid FEC Element, can
be used to withdraw all PW labels associated with a particular PW
group. This procedure is OPTIONAL, and if it is implemented, the LDP
Label Withdraw message should be as follows: If the PWid FEC Element
is used, the PW information length field is set to 0, the PW ID field
is not present, the Interface Parameter Sub-TLVs are not present, and
the Label TLV is not present. If the Generalized FEC Element is
used, the AGI, SAII, and TAII are not present, the PW information

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length field is set to 0, the PW Group ID TLV is included, the PW
Interface Parameters TLV is not present, and the Label TLV is not
present. For the purpose of this document, this is called the
"wildcard withdraw procedure", and all PEs implementing this design
are REQUIRED to accept such withdraw messages but are not required to
send it. Note that the PW Group ID TLV only applies to PWs using the
Generalized ID FEC Element, while the Group ID only applies to PWid
FEC Element.

The Interface Parameter Sub-TLVs, or TLV, MUST NOT be present in any
LDP PW Label Withdraw or Label Release message. A wildcard Label
Release message MUST include only the Group ID or PW Group ID TLV. A
Label Release message initiated by a PE router must always include
the PW ID.

7. Control Word

7.1. PW Types for Which the Control Word Is REQUIRED

The Label Mapping messages that are sent in order to set up these PWs
MUST have C=1. When a Label Mapping message for a PW of one of these
types is received and C=0, a Label Release message MUST be sent, with
an "Illegal C-bit" status code. In this case, the PW will not be
enabled.

7.2. PW Types for Which the Control Word Is NOT Mandatory

If a system is capable of sending and receiving the control word on
PW types for which the control word is not mandatory, then each such
PW endpoint MUST be configurable with a parameter that specifies
whether the use of the control word is PREFERRED or NOT PREFERRED.
For each PW, there MUST be a default value of this parameter. This
specification does NOT state what the default value should be.

If a system is NOT capable of sending and receiving the control word
on PW types for which the control word is not mandatory, then it
behaves exactly as if it were configured for the use of the control
word to be NOT PREFERRED.

If a Label Mapping message for the PW has already been received but
no Label Mapping message for the PW has yet been sent, then the
procedure is as follows:

-i. If the received Label Mapping message has C=0, send a Label
Mapping message with C=0; the control word is not used.

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-ii. If the received Label Mapping message has C=1, and the PW is
locally configured such that the use of the control word is
preferred, then send a Label Mapping message with C=1; the
control word is used.

-iii. If the received Label Mapping message has C=1, and the PW is
locally configured such that the use of the control word is
not preferred or the control word is not supported, then act
as if no Label Mapping message for the PW had been received
(i.e., proceed to the next paragraph).

If a Label Mapping message for the PW has not already been received
(or if the received Label Mapping message had C=1 and either local
configuration says that the use of the control word is not preferred
or the control word is not supported), then send a Label Mapping
message in which the C-bit is set to correspond to the locally
configured preference for use of the control word. (That is, set C=1
if locally configured to prefer the control word, and set C=0 if
locally configured to prefer not to use the control word or if the
control word is not supported).

The next action depends on what control message is next received for
that PW. The possibilities are as follows:

-i. A Label Mapping message with the same C-bit value as
specified in the Label Mapping message that was sent. PW
setup is now complete, and the control word is used if C=1
but is not used if C=0.

-ii. A Label Mapping message with C=1, but the Label Mapping
message that was sent has C=0. In this case, ignore the
received Label Mapping message and continue to wait for the
next control message for the PW.

-iii. A Label Mapping message with C=0, but the Label Mapping
message that was sent has C=1. In this case, send a Label
Withdraw message with a "Wrong C-bit" status code, followed
by a Label Mapping message that has C=0. PW setup is now
complete, and the control word is not used.

-iv. A Label Withdraw message with the "Wrong C-bit" status code.
Treat as a normal Label Withdraw message, but do not
respond. Continue to wait for the next control message for
the PW.

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If at any time after a Label Mapping message has been received a
corresponding Label Withdraw or Release is received, the action taken
is the same as for any Label Withdraw or Release messages that might
be received at any time.

If both endpoints prefer the use of the control word, this procedure
will cause it to be used. If either endpoint prefers not to use the
control word or does not support the control word, this procedure
will cause it not to be used. If one endpoint prefers to use the
control word but the other does not, the one that prefers not to use
it has no extra protocol to execute; it just waits for a Label
Mapping message that has C=0.

7.3. Control-Word Renegotiation by Label Request Message

It is possible that after the PW C-bit negotiation procedure
described above is complete, the local PE is re-provisioned with a
different control word preference. Therefore, once the control-word
negotiation procedures are complete, the procedure can be restarted
as follows:

-i. If the local PE previously sent a Label Mapping message, it
MUST send a Label Withdraw message to the remote PE and wait
until it has received a Label Release message from the
remote PE.

-ii. The local PE MUST send a Label Release message to the remote
PE for the specific label associated with the FEC that was
advertised for this specific PW. Note: The above-mentioned
steps of the Label Release message and Label Withdraw
message are not required to be executed in any specific
sequence.

-iii. The local PE MUST send a Label Request message to the peer
PE and then MUST wait until it receives a Label Mapping
message containing the remote PE's currently configured
preference for use of the control word.

Once the remote PE has successfully processed the Label Withdraw
message and Label Release messages, it will reset the C-bit
negotiation state machine and its use of the control word with the
locally configured preference.

From this point on, the local and remote PEs will follow the C-bit
negotiation procedures defined in the previous section.

The above C-bit renegotiation process SHOULD NOT be interrupted until
it is completed, or unpredictable results might occur.

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7.4. Sequencing Considerations

In the case where the router considers the sequence number field in
the control word, it is important to note the following details when
advertising labels.

7.4.1. Label Advertisements

After a label has been withdrawn by the output router and/or released
by the input router, care must be taken not to advertise (reuse) the
same released label until the output router can be reasonably certain
that old packets containing the released label no longer persist in
the MPLS-enabled network.

This precaution is required to prevent the imposition router from
restarting packet forwarding with a sequence number of 1 when it
receives a Label Mapping message that binds the same FEC to the same
label if there are still older packets in the network with a sequence
number between 1 and 32768. For example, if there is a packet with
sequence number=n, where n is in the interval [1,32768] traveling
through the network, it would be possible for the disposition router
to receive that packet after it re-advertises the label. Since the
label has been released by the imposition router, the disposition
router SHOULD be expecting the next packet to arrive with a sequence
number of 1. Receipt of a packet with a sequence number equal to n
will result in n packets potentially being rejected by the
disposition router until the imposition router imposes a sequence
number of n+1 into a packet. Possible methods to avoid this are for
the disposition router always to advertise a different PW label, or
for the disposition router to wait for a sufficient time before
attempting to re-advertise a recently released label. This is only
an issue when sequence number processing is enabled at the
disposition router.

7.4.2. Label Release

In situations where the imposition router wants to restart forwarding
of packets with sequence number 1, the router shall 1) send to the
disposition router a Label Release message, and 2) send to the
disposition router a Label Request message. When sequencing is
supported, advertisement of a PW label in response to a Label Request
message MUST also consider the issues discussed in Section 7.4.1
("Label Advertisements").

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8. IANA Considerations

8.1. LDP TLV TYPE

This document uses several new LDP TLV types; IANA already maintains
a registry titled "TLV Type Name Space", defined by RFC 5036. The
following values have been assigned from said registry:

TLV Type Description
=====================================
0x096A PW Status TLV
0x096B PW Interface Parameters TLV
0x096C PW Group ID TLV

8.2. LDP Status Codes

This document uses several new LDP status codes; IANA already
maintains a registry titled "Status Code Name Space", defined by RFC
5036. The following values have been assigned:

Range/Value E Description Reference
------------- ----- ---------------------- ---------
0x00000024 0 Illegal C-Bit [RFC8077]
0x00000025 0 Wrong C-Bit [RFC8077]
0x00000026 0 Incompatible bit-rate [RFC8077]
0x00000027 0 CEP-TDM mis-configuration [RFC8077]
0x00000028 0 PW Status [RFC8077]
0x00000029 0 Unassigned/Unrecognized TAI [RFC8077]
0x0000002A 0 Generic Misconfiguration Error [RFC8077]
0x0000002B 0 Label Withdraw PW Status [RFC8077]
Method Not Supported

8.3. FEC Type Name Space

This document uses two new FEC element types, 0x80 and 0x81, from the
registry "Forwarding Equivalence Class (FEC) Type Name Space" for the
Label Distribution Protocol (LDP) [RFC5036].

9. Security Considerations

This document specifies the LDP extensions that are needed for
setting up and maintaining pseudowires. The purpose of setting up
pseudowires is to enable Layer 2 frames to be encapsulated in MPLS
and transmitted from one end of a pseudowire to the other.
Therefore, we address the security considerations for both the data
plane and the control plane.

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9.1. Data-Plane Security

With regard to the security of the data plane, the following areas
must be considered:

- MPLS PDU inspection
- MPLS PDU spoofing
- MPLS PDU alteration
- MPLS PSN protocol security
- Access Circuit security
- Denial-of-service prevention on the PE routers

When an MPLS PSN is used to provide pseudowire service, there is a
perception that security must be at least equal to the currently
deployed Layer 2 native protocol networks that the MPLS/PW network
combination is emulating. This means that the MPLS-enabled network
SHOULD be isolated from outside packet insertion in such a way that
it SHOULD NOT be possible to insert an MPLS packet into the network
directly. To prevent unwanted packet insertion, it is also important
to prevent unauthorized physical access to the PSN, as well as
unauthorized administrative access to individual network elements.

As mentioned above, an MPLS-enabled network should not accept MPLS
packets from its external interfaces (i.e., interfaces to CE devices
or to other providers' networks) unless the top label of the packet
was legitimately distributed to the system from which the packet is
being received. If the packet's incoming interface leads to a
different Service Provider (SP) (rather than to a customer), an
appropriate trust relationship must also be present, including the
trust that the other SP also provides appropriate security measures.

The three main security problems faced when using an MPLS-enabled
network to transport PWs are spoofing, alteration, and inspection.
First, there is a possibility that the PE receiving PW PDUs will get
a PDU that appears to be from the PE transmitting the PW into the PSN
but that was not actually transmitted by the PE originating the PW.
(That is, the specified encapsulations do not by themselves enable
the decapsulator to authenticate the encapsulator.) A second problem
is the possibility that the PW PDU will be altered between the time
it enters the PSN and the time it leaves the PSN (i.e., the specified
encapsulations do not by themselves assure the decapsulator of the
packet's integrity.) A third problem is the possibility that the
PDU's contents will be seen while the PDU is in transit through the
PSN (i.e., the specification encapsulations do not ensure privacy.)
How significant these issues are in practice depends on the security
requirements of the applications whose traffic is being sent through
the tunnel and how secure the PSN itself is.

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9.2. Control-Plane Security

General security considerations with regard to the use of LDP are
specified in Section 5 of [RFC5036]. Those considerations also apply
to the case where LDP is used to set up pseudowires.

A pseudowire connects two Attachment Circuits. It is important to
make sure that LDP connections are not arbitrarily accepted from
anywhere, or else a local Attachment Circuit might get connected to
an arbitrary remote Attachment Circuit. Therefore, an incoming LDP
session request MUST NOT be accepted unless its IP source address is
known to be the source of an "eligible" LDP peer. The set of
eligible peers could be preconfigured (either as a list of IP
addresses or as a list of address/mask combinations), or it could be
discovered dynamically via an auto-discovery protocol that is itself
trusted. (Obviously, if the auto-discovery protocol were not
trusted, the set of eligible peers it produces could not be trusted.)

Even if an LDP connection request appears to come from an eligible
peer, its source address may have been spoofed. Therefore, some
means of preventing source address spoofing must be in place. For
example, if all the eligible peers are in the same network, source
address filtering at the border routers of that network could
eliminate the possibility of source address spoofing.

The LDP MD5 authentication key option, as described in Section 2.9 of
[RFC5036], MUST be implemented, and for a greater degree of security,
it must be used. This provides integrity and authentication for the
LDP messages and eliminates the possibility of source address
spoofing. Use of the MD5 option does not provide privacy, but
privacy of the LDP control messages is not usually considered
important. As the MD5 option relies on the configuration of pre-
shared keys, it does not provide much protection against replay
attacks. In addition, its reliance on pre-shared keys may make it
very difficult to deploy when the set of eligible neighbors is
determined by an auto-configuration protocol.

When the Generalized PWid FEC Element is used, it is possible that a
particular LDP peer may be one of the eligible LDP peers but may not
be the right one to connect to the particular Attachment Circuit
identified by the particular instance of the Generalized PWid FEC
Element. However, given that the peer is known to be one of the
eligible peers (as discussed above), this would be the result of a
configuration error rather than a security problem. Nevertheless, it
may be advisable for a PE to associate each of its local Attachment
Circuits with a set of eligible peers rather than have just a single
set of eligible peers associated with the PE as a whole.

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RFC 8077 PWE3 Using LDP February 2017

10. Interoperability and Deployment

Section 2.2 of [RFC6410] specifies four requirements that an Internet
Standard must meet. This section documents how this document meets
those requirements.

The pseudowire technology was first deployed in 2001 and has been
widely deployed by many carriers. [RFC7079] documents the results of
a survey of PW implementations with specific emphasis on control-word
usage. [EANTC] documents a public multi-vendor interoperability test
of MPLS and Carrier Ethernet equipment, which included testing of
Ethernet, ATM, and TDM pseudowires.

The errata against [RFC4447] are generally editorial in nature and
have been addressed in this document.

All features in this specification have been implemented by multiple
vendors.

No IPR disclosures have been made to the IETF related to this
document, to RFCs 4447 or 6723, or to the Internet-Drafts that
resulted in RFCs 4447 and 6723.

11. References

11.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI
10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
October 2007, <http://www.rfc-editor.org/info/rfc5036>.

[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
<http://www.rfc-editor.org/info/rfc3032>.

[RFC4446] Martini, L., "IANA Allocations for Pseudowire Edge to Edge
Emulation (PWE3)", BCP 116, RFC 4446, DOI
10.17487/RFC4446, April 2006,
<http://www.rfc-editor.org/info/rfc4446>.

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RFC 8077 PWE3 Using LDP February 2017

[RFC7358] Raza, K., Boutros, S., Martini, L., and N. Leymann, "Label
Advertisement Discipline for LDP Forwarding Equivalence
Classes (FECs)", RFC 7358, DOI 10.17487/RFC7358, October
2014, <http://www.rfc-editor.org/info/rfc7358>.

11.2. Informative References

[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, DOI 10.17487/RFC2277,
January 1998, <http://www.rfc-editor.org/info/rfc2277>.

[RFC3985] Bryant, S., Ed., and P. Pate, Ed., "Pseudo Wire Emulation
Edge-to-Edge (PWE3) Architecture", RFC 3985, DOI
10.17487/RFC3985, March 2005,
<http://www.rfc-editor.org/info/rfc3985>.

[RFC4842] Malis, A., Pate, P., Cohen, R., Ed., and D. Zelig,
"Synchronous Optical Network/Synchronous Digital Hierarchy
(SONET/SDH) Circuit Emulation over Packet (CEP)", RFC
4842, DOI 10.17487/RFC4842, April 2007,
<http://www.rfc-editor.org/info/rfc4842>.

[RFC4553] Vainshtein, A., Ed., and YJ. Stein, Ed., "Structure-
Agnostic Time Division Multiplexing (TDM) over Packet
(SAToP)", RFC 4553, DOI 10.17487/RFC4553, June 2006,
<http://www.rfc-editor.org/info/rfc4553>.

[RFC4619] Martini, L., Ed., Kawa, C., Ed., and A. Malis, Ed.,
"Encapsulation Methods for Transport of Frame Relay over
Multiprotocol Label Switching (MPLS) Networks", RFC 4619,
DOI 10.17487/RFC4619, September 2006,
<http://www.rfc-editor.org/info/rfc4619>.

[RFC4717] Martini, L., Jayakumar, J., Bocci, M., El-Aawar, N.,
Brayley, J., and G. Koleyni, "Encapsulation Methods for
Transport of Asynchronous Transfer Mode (ATM) over MPLS
Networks", RFC 4717, DOI 10.17487/RFC4717, December 2006,
<http://www.rfc-editor.org/info/rfc4717>.

[RFC4618] Martini, L., Rosen, E., Heron, G., and A. Malis,
"Encapsulation Methods for Transport of PPP/High-Level
Data Link Control (HDLC) over MPLS Networks", RFC 4618,
DOI 10.17487/RFC4618, September 2006,
<http://www.rfc-editor.org/info/rfc4618>.

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RFC 8077 PWE3 Using LDP February 2017

[RFC4448] Martini, L., Ed., Rosen, E., El-Aawar, N., and G. Heron,
"Encapsulation Methods for Transport of Ethernet over MPLS
Networks", RFC 4448, DOI 10.17487/RFC4448, April 2006,
<http://www.rfc-editor.org/info/rfc4448>.

[RFC4447] Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T., and
G. Heron, "Pseudowire Setup and Maintenance Using the
Label Distribution Protocol (LDP)", RFC 4447, DOI
10.17487/RFC4447, April 2006,
<http://www.rfc-editor.org/info/rfc4447>.

[RFC6410] Housley, R., Crocker, D., and E. Burger, "Reducing the
Standards Track to Two Maturity Levels", BCP 9, RFC 6410,
DOI 10.17487/RFC6410, October 2011,
<http://www.rfc-editor.org/info/rfc6410>.

[RFC6723] Jin, L., Ed., Key, R., Ed., Delord, S., Nadeau, T., and S.
Boutros, "Update of the Pseudowire Control-Word
Negotiation Mechanism", RFC 6723, DOI 10.17487/RFC6723,
September 2012, <http://www.rfc-editor.org/info/rfc6723>.

[RFC7079] Del Regno, N., Ed., and A. Malis, Ed., "The Pseudowire
(PW) and Virtual Circuit Connectivity Verification (VCCV)
Implementation Survey Results", RFC 7079, DOI
10.17487/RFC7079, November 2013,
<http://www.rfc-editor.org/info/rfc7079>.

[ANSI] American National Standards Institute, "Telecommunications
- Synchronous Optical Network (SONET) - Basic Description
Including Multiplex Structures, Rates, and Formats", ANSI
T1.105, October 1995.

[ITUG] International Telecommunications Union, "Network node
interface for the synchronous digital hierarchy (SDH)",
ITU-T Recommendation G.707, May 1996.

[EANTC] European Advanced Networking Test Center, "MPLS and
Carrier Ethernet: Service - Connect - Transport. Public
Multi-Vendor Interoperability Test", February 2009.

Acknowledgments

The authors wish to acknowledge the contributions of Vach Kompella,
Vanson Lim, Wei Luo, Himanshu Shah, and Nick Weeds. The authors wish
to also acknowledge the contribution of the authors of RFC 6723,
whose work has been incorporated in this document: Lizhong Jin,
Raymond Key, Simon Delord, Tom Nadeau, and Sami Boutros.

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Contributors

The following individuals were either authors or contributing authors
for RFC 4447. They are listed here in recognition of their work on
that document.

Nasser El-Aawar
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO 80021
United States of America

Email: [email protected]

Eric C. Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
United States of America

Email: [email protected]

Dan Tappan
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
United States of America

Email: [email protected]

Toby Smith
Google
6425 Penn Ave. #700
Pittsburgh, PA 15206
United States of America

Email: [email protected]

Dimitri Vlachos
Riverbed Technology

Email: [email protected]

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RFC 8077 PWE3 Using LDP February 2017

Jayakumar Jayakumar
Cisco Systems Inc.
3800 Zanker Road, MS-SJ02/2
San Jose, CA 95134
United States of America

Email: [email protected]

Alex Hamilton,
Cisco Systems Inc.
485 East Tasman Drive, MS-SJC07/3
San Jose, CA 95134
United States of America

Email: [email protected]

Steve Vogelsang
ECI Telecom
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
United States of America

Email: [email protected]

John Shirron
ECI Telecom
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
United States of America

Email: [email protected]

Andrew G. Malis
Verizon
60 Sylvan Rd.
Waltham, MA 02451
United States of America

Email: [email protected]

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RFC 8077 PWE3 Using LDP February 2017

Vinai Sirkay
Reliance Infocomm
Dhirubai Ambani Knowledge City
Navi Mumbai 400 709
India

Email: [email protected]

Vasile Radoaca
Nortel Networks
600 Technology Park
Billerica MA 01821
United States of America

Email: [email protected]

Chris Liljenstolpe
149 Santa Monica Way
San Francisco, CA 94127
United States of America

Email: [email protected]

Dave Cooper
Global Crossing
960 Hamlin Court
Sunnyvale, CA 94089
United States of America

Email: [email protected]

Kireeti Kompella
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
United States of America

Email: [email protected]

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RFC 8077 PWE3 Using LDP February 2017

Authors' Addresses

Luca Martini (editor)
Cisco Systems, Inc.
1899 Wynkoop Street, Suite 600
Denver, CO 80202
United States of America

Email: [email protected]

Giles Heron (editor)
Cisco Systems
10 New Square
Bedfont Lakes
Feltham
Middlesex
TW14 8HA
United Kingdom

Email: [email protected]

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