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
Pseudowire Setup and Maintenance
Using the Label Distribution Protocol (LDP)
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
This document is a rewrite of RFC 4447 for publication as an Internet
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
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Table of Contents
1. Introduction ....................................................42. Changes from RFC 4447 ...........................................63. Specification of Requirements ...................................64. The Pseudowire Label ............................................75. Details Specific to Particular Emulated Services ................95.1. IP Layer 2 Transport .......................................96. LDP .............................................................96.1. The PWid FEC Element .......................................96.2. The Generalized PWid FEC Element ..........................116.2.1. Attachment Identifiers .............................126.2.2. Encoding the Generalized PWid FEC Element ..........18.104.22.168. PW Interface Parameters TLV ...............22.214.171.124. PW Group ID TLV ...........................156.2.3. Signaling Procedures ...............................166.3. Signaling of Pseudowire Status ............................176.3.1. Use of Label Mapping Messages ......................176.3.2. Signaling PW Status ................................186.3.3. Pseudowire Status Negotiation Procedures ...........196.4. Interface Parameter Sub-TLV ...............................206.5. LDP Label Withdrawal Procedures ...........................217. Control Word ...................................................227.1. PW Types for Which the Control Word Is REQUIRED ...........227.2. PW Types for Which the Control Word Is NOT Mandatory ......227.3. Control-Word Renegotiation by Label Request Message .......247.4. Sequencing Considerations .................................257.4.1. Label Advertisements ...............................257.4.2. Label Release ......................................258. IANA Considerations ............................................268.1. LDP TLV TYPE ..............................................268.2. LDP Status Codes ..........................................268.3. FEC Type Name Space .......................................269. Security Considerations ........................................269.1. Data-Plane Security .......................................279.2. Control-Plane Security ....................................2810. Interoperability and Deployment ...............................2911. References ....................................................2911.1. Normative References .....................................2911.2. Informative References ...................................30
Authors' Addresses ................................................35
[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
[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.
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 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 (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
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].
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
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
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.
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.
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 126.96.36.199 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
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 |
| " |
| " |
- Control word bit (C)
The C-bit is used to flag the presence of a control word as
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.
- 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
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
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 188.8.131.52. 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 184.108.40.206.
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>>.
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
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.
6.2.2. Encoding the Generalized PWid FEC Element
FEC element type 0x81 is used. The FEC element is encoded as
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-
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
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.
220.127.116.11. 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").
18.104.22.168. 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.
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
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-
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
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.