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IETF RFC 4447
Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP)
Last modified on Tuesday, April 25th, 2006
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Network Working Group L. Martini, Ed.
Request for Comments: 4447 E. Rosen
Category: Standards Track Cisco Systems, Inc.
N. El-Aawar
Level 3 Communications, LLC.
T. Smith
Network Appliance, Inc.
G. Heron
Tellabs
April 2006
Pseudowire Setup and Maintenance
Using the Label Distribution Protocol (LDP)
 Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright © The Internet Society (2006).
Abstract
Layer 2 services (such as Frame Relay, Asynchronous Transfer Mode,
and Ethernet) can be "emulated" over an MPLS backbone by
encapsulating the Layer 2 Protocol Data Units (PDU) and transmitting
them over "pseudowires". It is also possible to use pseudowires to
provide low-rate Time Division Multiplexed and a Synchronous Optical
NETworking circuit emulation over an MPLS-enabled network. This
document specifies a protocol for establishing and maintaining the
pseudowires, using extensions to Label Distribution Protocol (LDP).
Procedures for encapsulating Layer 2 PDUs are specified in a set of
companion documents.
Martini, et al. Standards Track PAGE 1 
RFC 4447 PWE3 Using LDP April 2006
Table of Contents
1. Introduction ....................................................3
2. Specification of Requirements ...................................5
3. The Pseudowire Label ............................................5
4. Details Specific to Particular Emulated Services ................7
4.1. IP Layer 2 Transport .......................................7
5. LDP .............................................................7
5.1. LDP Extensions .............................................8
5.2. The PWid FEC Element .......................................8
5.3. The Generalized PWid FEC Element ..........................10
5.3.1. Attachment Identifiers .............................11
5.3.2. Encoding the Generalized ID FEC Element ............13
5.3.2.1. Interface Parameters TLV ..................14
5.3.2.2. PW Grouping TLV ...........................14
5.3.3. Signaling Procedures ...............................15
5.4. Signaling of Pseudowire Status ............................16
5.4.1. Use of Label Mappings Messages .....................16
5.4.2. Signaling PW Status ................................17
5.4.3. Pseudowire Status Negotiation Procedures ...........18
5.5. Interface Parameters Sub-TLV ..............................19
6. Control Word ...................................................20
6.1. PW Types for Which the Control Word is REQUIRED ...........20
6.2. PW Types for Which the Control Word is NOT Mandatory ......21
6.3. LDP Label Withdrawal Procedures ...........................22
6.4. Sequencing Considerations .................................23
6.4.1. Label Advertisements ...............................23
6.4.2. Label Release ......................................24
7. IANA Considerations ............................................24
7.1. LDP TLV TYPE ..............................................24
7.2. LDP Status Codes ..........................................24
7.3. FEC Type Name Space .......................................25
8. Security Considerations ........................................25
8.1. Data-Plane Security .......................................25
8.2. Control-Plane Security ....................................26
9. Acknowledgements ...............................................27
10. Normative References ..........................................27
11. Informative References ........................................27
12. Additional Contributing Authors ...............................28
Appendix A. C-bit Handling Procedures Diagram .....................31
Martini, et al. Standards Track PAGE 2
RFC 4447 PWE3 Using LDP April 2006
1. Introduction
In [FRAME], [ATM], [PPPHDLC], and [ETH], it is explained how to
encapsulate a Layer 2 Protocol Data Unit (PDU) for transmission over
an MPLS-enabled network. Those documents specify that a "pseudowire
header", consisting of a demultiplexor field, will be prepended to
the encapsulated PDU. The pseudowire demultiplexor field is
prepended before transmitting a packet on a pseudowire. When the
packet arrives at the remote endpoint of the pseudowire, the
demultiplexor is what enables the receiver to identify the particular
pseudowire on which the packet has arrived. To transmit the packet
from one pseudowire endpoint to another, the packet may need to
travel through a "Packet Switched Network (PSN) tunnel"; this will
require that an additional header be prepended to the packet.
Accompanying documents [CEP, SAToP] specify methods for transporting
time-division multiplexing (TDM) digital signals (TDM circuit
emulation) over a packet-oriented MPLS-enabled network. The
transmission system for circuit-oriented TDM signals is the
Synchronous Optical Network (SONET)[SDH]/Synchronous Digital
Hierarchy (SDH) [ITUG]. To support TDM traffic, which includes
voice, data, and private leased-line service, the pseudowires must
emulate the circuit characteristics of SONET/SDH payloads. The TDM
signals and payloads are encapsulated for transmission over
pseudowires. A pseudowire demultiplexor and a PSN tunnel header is
prepended to this encapsulation.
[SAToP] describes methods for transporting low-rate time-division
multiplexing (TDM) digital signals (TDM circuit emulation) over PSNs,
while [CEP] similarly describes transport of high-rate TDM
(SONET/SDH). To support TDM traffic, the pseudowires must emulate
the circuit characteristics of the original T1, E1, T3, E3, SONET, or
SDH signals. [SAToP] does this by encapsulating an arbitrary but
constant amount of the TDM data in each packet, and the other methods
encapsulate TDM structures.
In this document, we specify the use of the MPLS Label Distribution
Protocol, LDP [RFC 3036], as a protocol for setting up and maintaining
the pseudowires. In particular, we define new TLVs, FEC elements,
parameters, and codes for LDP, which enable LDP to identify
pseudowires and to signal attributes of pseudowires. We specify how
a pseudowire endpoint uses these TLVs in LDP to bind a demultiplexor
field value to a pseudowire, and how it informs the remote endpoint
of the binding. We also specify procedures for reporting pseudowire
status changes, for passing additional information about the
pseudowire as needed, and for releasing the bindings.
Martini, et al. Standards Track PAGE 3
RFC 4447 PWE3 Using LDP April 2006
In the protocol specified herein, the pseudowire demultiplexor field
is an MPLS label. Thus, the packets that are transmitted from one
end of the pseudowire to the other are MPLS packets, which must be
transmitted through an MPLS tunnel. However, if the pseudowire
endpoints are immediately adjacent and penultimate hop popping
behavior is in use, the MPLS tunnel may not be necessary. Any sort
of PSN tunnel can be used, as long as it is possible to transmit MPLS
packets through it. The PSN tunnel can itself be an MPLS LSP, or any
other sort of tunnel that can carry MPLS packets. Procedures for
setting up and maintaining the MPLS tunnels are outside the scope of
this document.
This document deals only with the setup and maintenance of point-to-
point pseudowires. Neither point-to-multipoint nor multipoint-to-
point pseudowires are discussed.
QoS-related issues are not discussed in this document. The following
two figures describe the reference models that are derived from
[RFC 3985] 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
Martini, et al. Standards Track PAGE 4
RFC 4447 PWE3 Using LDP April 2006
+-----------------+ +-----------------+
|Emulated Service | |Emulated Service |
|(e.g., TDM, ATM) |<==== Emulated Service ===>|(e.g., TDM, ATM) |
+-----------------+ +-----------------+
| Payload | | Payload |
| Encapsulation |<====== Pseudowire =======>| Encapsulation |
+-----------------+ +-----------------+
|PW Demultiplexer | |PW Demultiplexer |
| PSN Tunnel, |<======= PSN Tunnel ======>| PSN Tunnel, |
| PSN & Physical | | PSN & Physical |
| Layers | | Layers |
+-------+---------+ __________ +---------+-------+
| / \ |
+===============/ PSN \================+
\ /
\____________/
Figure 2: PWE3 Protocol Stack Reference Model
For the purpose of this document, PE1 will be defined as the ingress
router, and PE2 as the egress router. A layer 2 PDU will be received
at PE1, encapsulated at PE1, transported and decapsulated at PE2, and
transmitted out of PE2.
2. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC 2119].
3. The Pseudowire Label
Suppose that it is desired to transport Layer 2 PDUs from ingress LSR
PE1 to egress LSR PE2, across an intervening MPLS-enabled network.
We assume that there is an MPLS tunnel from PE1 to PE2. That is, we
assume that PE1 can cause a packet to be delivered to PE2 by
encapsulating the packet in an "MPLS tunnel header" and sending the
result to one of its adjacencies. The MPLS tunnel is an MPLS Label
Switched Path (LSP); thus, putting on an MPLS tunnel encapsulation is
a matter of pushing on an MPLS label.
We presuppose that a large number of pseudowires can be carried
through a single MPLS tunnel. Thus, it is never necessary to
maintain state in the network core for individual pseudowires. We do
not presuppose that the MPLS tunnels are point to point; although the
pseudowires are point to point, the MPLS tunnels may be multipoint to
point. We do not presuppose that PE2 will even be able to determine
the MPLS tunnel through which a received packet was transmitted.
Martini, et al. Standards Track PAGE 5
RFC 4447 PWE3 Using LDP April 2006
(For example, if the MPLS tunnel is an LSP and penultimate hop
popping is used, when the packet arrives at PE2, it will contain no
information identifying the tunnel.)
When PE2 receives a packet over a pseudowire, it must be able to
determine that the packet was in fact received over a pseudowire, and
it must be able to associate that packet with a particular
pseudowire. PE2 is able to do this by examining the MPLS label that
serves as the pseudowire demultiplexor field shown in Figure 2. Call
this label the "PW label".
When PE1 sends a Layer 2 PDU to PE2, it creates an MPLS packet by
adding the PW label to the packet, thus creating the first entry of
the label stack. If the PSN tunnel is an MPLS LSP, the PE1 pushes
another label (the tunnel label) onto the packet as the second entry
of the label stack. The PW label is not visible again until the MPLS
packet reaches PE2. PE2's disposition of the packet is based on the
PW label.
If the payload of the MPLS packet is, for example, an ATM AAL5 PDU,
the PW label will generally correspond to a particular ATM VC at PE2.
That is, PE2 needs to be able to infer from the PW label the outgoing
interface and the VPI/VCI value for the AAL5 PDU. If the payload is
a Frame Relay PDU, then PE2 needs to be able to infer from the PW
label the outgoing interface and the DLCI value. If the payload is
an Ethernet frame, then PE2 needs to be able to infer from the PW
label the outgoing interface, and perhaps the VLAN identifier. This
process is uni-directional and will be repeated independently for
bi-directional operation. It is REQUIRED that the same PW ID and PW
type be assigned for a given circuit in both directions. The group
ID (see below) MUST NOT be required to match in both directions. The
transported frame MAY be modified when it reaches the egress router.
If the header of the transported Layer 2 frame is modified, this MUST
be done at the egress LSR only. Note that the PW label must always
be at the bottom of the packet's label stack, and labels MUST be
allocated from the per-platform label space.
This document does not specify a method for distributing the MPLS
tunnel label or any other labels that may appear above the PW label
on the stack. Any acceptable method of MPLS label distribution will
do. This document specifies a protocol for assigning and
distributing the PW label. This protocol is LDP, extended as
specified in the remainder of this document. An LDP session must be
set up between the pseudowire endpoints. LDP MUST be used in its
"downstream unsolicited" mode. LDP's "liberal label retention" mode
SHOULD be used.
Martini, et al. Standards Track PAGE 6
RFC 4447 PWE3 Using LDP April 2006
In addition to the protocol specified herein, static assignment of PW
labels may be used, and implementations of this protocol SHOULD
provide support for static assignment.
This document specifies all the procedures necessary to set up and
maintain the pseudowires needed to support "unswitched" point-to-
point services, where each endpoint of the pseudowire is provisioned
with the identify of the other endpoint. There are also protocol
mechanisms specified herein that can be used to support switched
services and other provisioning models. However, the use of the
protocol mechanisms to support those other models and services is not
described in this document.
4. Details Specific to Particular Emulated Services
4.1. IP Layer 2 Transport
This mode carries IP packets over a pseudowire. The encapsulation
used is according to [RFC 3032]. 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 [RFC 3985], and is outside the scope of this document.
5. LDP
The PW label bindings are distributed using the LDP downstream
unsolicited mode described in [RFC 3036]. The PEs will establish an
LDP session using the Extended Discovery mechanism described in [LDP,
sections 2.4.2 and 2.5].
An LDP Label Mapping message contains an FEC TLV, a Label TLV, and
zero or more optional parameter TLVs.
The FEC TLV is used to indicate the meaning of the label. In the
current context, the FEC TLV would be used to identify the particular
pseudowire that a particular label is bound to. In this
specification, we define two new FEC TLVs to be used for identifying
pseudowires. When setting up a particular pseudowire, only one of
these FEC TLVs is used. The one to be used will depend on the
particular service being emulated and on the particular provisioning
model being supported.
LDP allows each FEC TLV to consist of a set of FEC elements. For
setting up and maintaining pseudowires, however, each FEC TLV MUST
contain exactly one FEC element.
Martini, et al. Standards Track PAGE 7
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The LDP base specification has several kinds of label TLVs, including
the Generic Label TLV, as specified in [RFC 3036], section 3.4.2.1.
For setting up and maintaining pseudowires, the Generic Label TLV
MUST be used.
5.1. LDP Extensions
This document specifies no new LDP messages.
This document specifies the following new TLVs to be used with LDP:
TLV Specified in Section Defined for Message
===================================================================
PW Status TLV 5.4.2 Notification
PW Interface Parameters TLV 5.3.2.1 FEC
PW Grouping ID TLV 5.3.2.2 FEC
Additionally, the following new FEC element types are defined:
FEC Element Type Specified in Section Defined for Message
===================================================================
0x80 5.2 FEC
0x81 5.3 FEC
The following new LDP error codes are also defined:
Status Code Specified in Section
====================================================================
"Illegal C-Bit" 6.1
"Wrong C-Bit" 6.2
"Incompatible bit-rate" [CEP]
"CEP/TDM mis-configuration" [CEP]
"PW status" 5.4.2
"Unassigned/Unrecognized TAI" 5.3.3
"Generic Misconfiguration Error" [SAToP]
"Label Withdraw PW Status Method Not Supported" 5.4.1
5.2. The PWid FEC Element
The PWid FEC element may be used whenever both pseudowire endpoints
have been provisioned with the same 32-bit identifier for the
pseudowire.
Martini, et al. Standards Track PAGE 8
RFC 4447 PWE3 Using LDP April 2006
For this purpose, a new type of FEC element is defined. The FEC
element type is 0x80 and is defined as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PWid (0x80) |C| PW type |PW info Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface Parameter Sub-TLV |
| " |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- PW type
A 15-bit quantity containing a value that represents the type of
PW. Assigned values are specified in "IANA Allocations for
Pseudowire Edge to Edge Emulation (PWE3)" [IANA].
- Control word bit (C)
The bit (C) is used to flag the presence of a control word as
follows:
C = 1 Control word present on this PW.
C = 0 No control word present on this PW.
Please see the section "C-Bit Handling Procedures" for further
explanation.
- PW information length
Length of the PW ID field and the interface parameters sub-TLV 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 ID at ingress could be
part of the virtual tunnel for transport to the egress router.
Martini, et al. Standards Track PAGE 9
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The Group ID is very useful for sending wild card label
withdrawals, or PW wild card 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
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.
5.3. 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 Grouping TLV", described below. The Interface
Parameters field of the PWid FEC element is also absent; its
functionality is replaced by the optional Interface Parameters TLV,
described below.
Martini, et al. Standards Track PAGE 10
RFC 4447 PWE3 Using LDP April 2006
5.3.1. Attachment Identifiers
As discussed in [RFC 3985], 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>>,
The PW direction from PE2 to PE1 can be identified by:
<PE2, <AGI, AII2>, PE1, <AGI, AII1>>.
Martini, et al. Standards Track PAGE 11
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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 ID
FEC element must specify the rules for performing this association.
Martini, et al. Standards Track PAGE 12
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5.3.2. Encoding the Generalized ID 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 the [IANA]
document.
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 specified grouping ID. In this case,
there are no other FEC element fields (AGI, SAII, etc.) present, nor
any 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
Martini, et al. Standards Track PAGE 13
RFC 4447 PWE3 Using LDP April 2006
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.
5.3.2.1. Interface Parameters TLV
This TLV MUST only be used when sending the Generalized PW 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 the section
"Interface Parameters Sub-TLV", below.
5.3.2.2. PW Grouping 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 Grouping ID TLV (0x096C)| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The PW Grouping 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 Grouping ID can be used as a port index and assigned to all PWs
that lead to that port. Use of the PW Grouping ID enables one to
send "wild card" label withdrawals, or "wild card" status
notification messages, to remote PEs upon physical port failure.
Note Well: The PW Grouping ID is different from, and has no relation
to, the Attachment Group Identifier.
The PW Grouping ID TLV is not part of the FEC and will not be
advertised except in the PW FEC advertisement. The advertising PE
Martini, et al. Standards Track PAGE 14
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MAY use the wild card withdraw semantics, but the remote PEs MUST
implement support for wild card messages. This TLV MUST only be used
when sending the Generalized PW ID FEC.
To issue a wildcard command (status or withdraw):
- Set the PW Info Length to 0 in the Generalized ID FEC Element.
- Send only the PW Grouping ID TLV with the FEC (no AGI/SAII/TAII is
sent).
5.3.3. 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
autodiscovery 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 Services (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 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 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, this
means that the SAII is the remote peer's AII and the TAII is the
sender's local AII.
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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 [RFC 3036].
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.
5.4. Signaling of Pseudowire Status
5.4.1. Use of Label Mappings 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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5.4.2. 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 the PW IANA
Allocations document [IANA]. The length 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. The general format of the Notification Message
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Notification (0x0001) | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status (TLV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW Status TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PWId FEC TLV or Generalized ID FEC TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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 and Message Type
fields are 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. When a PE's
attachment circuit encounters an error, use of the PW Notification
Message allows the PE to send a single "wild card" 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 Grouping ID TLV.
As mentioned above, the Group ID field of the PWid FEC element, or
the PW Grouping ID TLV used with the Generalized ID 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
Grouping ID TLV is included, and the Interface Parameters TLV is
omitted. For the purpose of this document, this is called the "wild
card 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.
5.4.3. 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 ID 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.
5.5. Interface Parameters 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 interface parameter sub-TLV type values are specified in "IANA
Allocations for Pseudowire Edge to Edge Emulation (PWE3)" [IANA].
The Length field is defined as the length of the interface parameter
including the parameter id and length field itself. Processing of
the interface parameters should continue when unknown interface
parameters are encountered, and they MUST be silently ignored.
- 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. 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 [RFC 2277].
6. Control Word
6.1. 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.
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6.2. 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.
-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).
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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, but do not respond.
Continue to wait for the next control message for the PW.
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 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 is has no extra protocol to execute; it just waits for a Label
Mapping message that has c=0.
The diagram in Appendix A illustrates the above procedure.
6.3. LDP Label Withdrawal Procedures
As mentioned above, the Group ID field of the PWid FEC element, or
the PW Grouping ID TLV used with the Generalized ID 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.
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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
Grouping ID TLV is included, the Interface Parameters TLV is not
present, and the Label TLV is not present. For the purpose of this
document, this is called the "wild card withdraw procedure", and all
PEs implementing this design are REQUIRED to accept such withdrawn
message but are not required to send it. Note that the PW Grouping
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 wild card Label
Release message MUST include only the group ID, or Grouping ID TLV.
A Label Release message initiated by a PE router must always include
the PW ID.
6.4. 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.
6.4.1. 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 (re-use) 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
Martini, et al. Standards Track PAGE 23
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attempting to re-advertise a recently released label. This is only
an issue when sequence number processing is enabled at the
disposition router.
6.4.2. 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 the section on
Label Advertisements.
7. IANA Considerations
7.1. LDP TLV TYPE
This document uses several new LDP TLV types; IANA already maintains
a registry of name "TLV TYPE NAME SPACE" defined by RFC 3036. The
following values are suggested for assignment:
TLV type Description
=====================================
0x096A PW Status TLV
0x096B PW Interface Parameters TLV
0x096C Group ID TLV
7.2. LDP Status Codes
This document uses several new LDP status codes; IANA already
maintains a registry of name "STATUS CODE NAME SPACE" defined by RFC
3036. The following values are suggested for assignment:
Range/Value E Description Reference
------------- ----- ---------------------- ---------
0x00000024 0 Illegal C-Bit [RFC 4447]
0x00000025 0 Wrong C-Bit [RFC 4447]
0x00000026 0 Incompatible bit-rate [RFC 4447]
0x00000027 0 CEP-TDM mis-configuration [RFC 4447]
0x00000028 0 PW Status [RFC 4447]
0x00000029 0 Unassigned/Unrecognized TAI [RFC 4447]
0x0000002A 0 Generic Misconfiguration Error [RFC 4447]
0x0000002B 0 Label Withdraw PW Status Method [RFC 4447]
Martini, et al. Standards Track PAGE 24
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7.3. FEC Type Name Space
This document uses two new FEC element types, 0x80 and 0x81, from the
registry "FEC Type Name Space" for the Label Distribution Protocol
(LDP RFC 3036).
8. 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 treat the security considerations for both the data
plane and the control plane.
8.1. 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, as 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 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.
Martini, et al. Standards Track PAGE 25
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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.
8.2. Control-Plane Security
General security considerations with regard to the use of LDP are
specified in section 5 of RFC 3036. 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 pre-configured (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
RFC 3036, 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-
Martini, et al. Standards Track PAGE 26
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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 ID 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 ID 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.
9. Acknowledgements
The authors wish to acknowledge the contributions of Vach Kompella,
Vanson Lim, Wei Luo, Himanshu Shah, and Nick Weeds.
10. Normative References
[RFC 2119] Bradner S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997
[RFC 3036] Andersson, L., Doolan, P., Feldman, N., Fredette, A., and
B. Thomas, "LDP Specification", RFC 3036, January 2001.
[RFC 3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
[IANA] Martini, L., "IANA Allocations for Pseudowire Edge to Edge
Emulation (PWE3)", BCP 116, RFC 4446, April 2006.
11. Informative References
[CEP] Malis, A., Pate, P., Cohen, R., Ed., and D. Zelig,
"SONET/SDH Circuit Emulation Service Over Packet (CEP)",
Work in Progress.
[SAToP] Vainshtein, A., Ed. and Y. Stein, Ed., "Structure-Agnostic
TDM over Packet (SAToP)", Work in Progress.
Martini, et al. Standards Track PAGE 27
RFC 4447 PWE3 Using LDP April 2006
[FRAME] Martini, L., Ed. and C. Kawa, Ed., "Encapsulation Methods
for Transport of Frame Relay Over MPLS Networks", Work in
Progress.
[ATM] Martini, L., Ed., El-Aawar, N., and M. Bocci, Ed.,
"Encapsulation Methods for Transport of ATM Over MPLS
Networks", Work in Progress.
[PPPHDLC] Martini, L., Rosen, E., Heron, G., and A. Malis,
"Encapsulation Methods for Transport of PPP/HDLC Frames
Over IP and MPLS Networks", Work in Progress.
[ETH] Martini, L., Rosen, E., El-Aawar, N., and G. Heron,
"Encapsulation Methods for Transport of Ethernet Over MPLS
Networks", RFC 4448, April 2006.
[SDH] American National Standards Institute, "Synchronous Optical
Network Formats," ANSI T1.105-1995.
[ITUG] ITU Recommendation G.707, "Network Node Interface For The
Synchronous Digital Hierarchy", 1996.
[RFC 3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-Edge
(PWE3) Architecture", RFC 3985, March 2005.
[RFC 2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
12. Additional Contributing Authors
Dimitri Stratton Vlachos
Mazu Networks, Inc.
125 Cambridgepark Drive
Cambridge, MA 02140
EMail: d@mazunetworks.com
Jayakumar Jayakumar,
Cisco Systems Inc.
225, E.Tasman, MS-SJ3/3,
San Jose, CA, 95134
EMail: jjayakum@cisco.com
Martini, et al. Standards Track PAGE 28
RFC 4447 PWE3 Using LDP April 2006
Alex Hamilton,
Cisco Systems Inc.
285 W. Tasman, MS-SJCI/3/4,
San Jose, CA, 95134
EMail: tahamilt@cisco.com
Steve Vogelsang
ECI Telecom
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
EMail: stephen.vogelsang@ecitele.com
John Shirron
ECI Telecom
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
EMail: john.shirron@ecitele.com
Andrew G. Malis
Tellabs
90 Rio Robles Dr.
San Jose, CA 95134
EMail: Andy.Malis@tellabs.com
Vinai Sirkay
Redback Networks
300 Holger Way
San Jose, CA 95134
EMail: vsirkay@redback.com
Vasile Radoaca
Nortel Networks
600 Technology Park
Billerica MA 01821
EMail: vasile@nortelnetworks.com
Martini, et al. Standards Track PAGE 29
RFC 4447 PWE3 Using LDP April 2006
Chris Liljenstolpe
Alcatel
11600 Sallie Mae Dr.
9th Floor
Reston, VA 20193
EMail: chris.liljenstolpe@alcatel.com
Dave Cooper
Global Crossing
960 Hamlin Court
Sunnyvale, CA 94089
EMail: dcooper@gblx.net
Kireeti Kompella
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
EMail: kireeti@juniper.net
Dan Tappan
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
EMail: tappan@cisco.com
Martini, et al. Standards Track PAGE 30
RFC 4447 PWE3 Using LDP April 2006
Appendix A. C-bit Handling Procedures Diagram
------------------
Y | Received Label | N
-------| Mapping Msg? |--------------
| ------------------ |
-------------- |
| | |
------- ------- |
| C=0 | | C=1 | |
------- ------- |
| | |
| ---------------- |
| | Control Word | N |
| | Capable? |----------- |
| ---------------- | |
| Y | | |
| | | |
| ---------------- | |
| | Control Word | N | |
| | Preferred? |---- | |
| ---------------- | | |
| Y | | | |
| | | | ----------------
| | | | | Control Word |
| | | | | Preferred? |
| | | | ----------------
| | | | N | Y |
| | | | | |
Send Send Send Send Send Send
C=0 C=1 C=0 C=0 C=0 C=1
| | | |
----------------------------------
| If receive the same as sent, |
| PW setup is complete. If not: |
----------------------------------
| | | |
------------------- -----------
| Receive | | Receive |
| C=1 | | C=0 |
------------------- -----------
| |
Wait for the Send
next message Wrong C-Bit
|
Send Label
Mapping Message
Martini, et al. Standards Track PAGE 31
RFC 4447 PWE3 Using LDP April 2006
Authors' Addresses
Luca Martini
Cisco Systems, Inc.
9155 East Nichols Avenue, Suite 400
Englewood, CO, 80112
EMail: lmartini@cisco.com
Nasser El-Aawar
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO, 80021
EMail: nna@level3.net
Giles Heron
Tellabs
Abbey Place
24-28 Easton Street
High Wycombe
Bucks
HP11 1NT
UK
EMail: giles.heron@tellabs.com
Eric C. Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
EMail: erosen@cisco.com
Toby Smith
Network Appliance, Inc.
800 Cranberry Woods Drive
Suite 300
Cranberry Township, PA 16066
EMail: tob@netapp.com
Martini, et al. Standards Track PAGE 32
RFC 4447 PWE3 Using LDP April 2006
Full Copyright Statement
Copyright © The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
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Copies of IPR disclosures made to the IETF Secretariat and any
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Martini, et al. Standards Track PAGE 33
Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP)
RFC TOTAL SIZE: 76204 bytes
PUBLICATION DATE: Tuesday, April 25th, 2006
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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