Internet Engineering Task Force (IETF) L. Martini
Request for Comments: 6073 C. Metz
Category: Standards Track Cisco Systems, Inc.
ISSN: 2070-1721 T. Nadeau
LucidVision
M. Bocci
M. Aissaoui
Alcatel-Lucent
January 2011
Segmented Pseudowire
Abstract
This document describes how to connect pseudowires (PWs) between
different Packet Switched Network (PSN) domains or between two or
more distinct PW control plane domains, where a control plane domain
uses a common control plane protocol or instance of that protocol for
a given PW. The different PW control plane domains may belong to
independent autonomous systems, or the PSN technology is
heterogeneous, or a PW might need to be aggregated at a specific PSN
point. The PW packet data units are simply switched from one PW to
another without changing the PW payload.
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 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6073.
Martini, et al. Standards Track [Page 1]
RFC 6073 Segmented Pseudowire January 2011
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction ....................................................4
2. Specification of Requirements ...................................5
3. Terminology .....................................................5
4. General Description .............................................6
5. PW Switching and Attachment Circuit Type ........................9
6. Applicability ...................................................9
7. MPLS-PW to MPLS-PW Switching ...................................10
7.1. Static Control Plane Switching ............................10
7.2. Two LDP Control Planes Using the Same FEC Type ............11
7.2.1. FEC 129 Active/Passive T-PE Election Procedure .....11
7.3. LDP Using FEC 128 to LDP Using the Generalized FEC 129 ....12
7.4. LDP SP-PE TLV .............................................12
7.4.1. PW Switching Point PE Sub-TLVs .....................14
7.4.2. Adaptation of Interface Parameters .................15
7.5. Group ID ..................................................16
7.6. PW Loop Detection .........................................16
8. MPLS-PW to L2TPv3-PW Control Plane Switching ...................16
8.1. Static MPLS and L2TPv3 PWs ................................17
8.2. Static MPLS PW and Dynamic L2TPv3 PW ......................17
Martini, et al. Standards Track [Page 2]
RFC 6073 Segmented Pseudowire January 2011
8.3. Static L2TPv3 PW and Dynamic LDP/MPLS PW ..................17
8.4. Dynamic LDP/MPLS and L2TPv3 PWs ...........................17
8.4.1. Session Establishment ..............................18
8.4.2. Adaptation of PW Status message ....................18
8.4.3. Session Tear Down ..................................18
8.5. Adaptation of L2TPv3 AVPs to Interface Parameters .........19
8.6. Switching Point TLV in L2TPv3 .............................20
8.7. L2TPv3 and MPLS PW Data Plane .............................20
8.7.1. Mapping the MPLS Control Word to L2TP ..............21
9. Operations, Administration, and Maintenance (OAM) ..............22
9.1. Extensions to VCCV to Support MS-PWs ......................22
9.2. OAM from MPLS PW to L2TPv3 PW .............................22
9.3. OAM Data Plane Indication from MPLS PW to MPLS PW .........22
9.4. Signaling OAM Capabilities for Switched Pseudowires .......23
9.5. OAM Capability for MS-PWs Demultiplexed Using MPLS ........23
9.5.1. MS-PW and VCCV CC Type 1 ...........................24
9.5.2. MS-PW and VCCV CC Type 2 ...........................24
9.5.3. MS-PW and VCCV CC Type 3 ...........................24
9.6. MS-PW VCCV Operations .....................................24
9.6.1. VCCV Echo Message Processing .......................25
9.6.2. Detailed VCCV Procedures ...........................27
10. Mapping Switched Pseudowire Status ............................31
10.1. PW Status Messages Initiated by the S-PE .................32
10.1.1. Local PW2 Transmit Direction Fault ................33
10.1.2. Local PW1 Transmit Direction Fault ................34
10.1.3. Local PW2 Receive Direction Fault .................34
10.1.4. Local PW1 Receive Direction Fault .................34
10.1.5. Clearing Faults ...................................34
10.2. PW Status Messages and SP-PE TLV Processing ..............35
10.3. T-PE Processing of PW Status Messages ....................35
10.4. Pseudowire Status Negotiation Procedures .................35
10.5. Status Dampening .........................................35
11. Peering between Autonomous Systems ............................35
12. Congestion Considerations .....................................36
13. Security Considerations .......................................36
13.1. Data Plane Security ......................................36
13.1.1. VCCV Security Considerations ......................36
13.2. Control Protocol Security ................................37
14. IANA Considerations ...........................................38
14.1. L2TPv3 AVP ...............................................38
14.2. LDP TLV TYPE .............................................38
14.3. LDP Status Codes .........................................38
14.4. L2TPv3 Result Codes ......................................38
14.5. New IANA Registries ......................................39
15. Normative References ..........................................39
16. Informative References ........................................40
17. Acknowledgments ...............................................42
18. Contributors ..................................................42
Martini, et al. Standards Track [Page 3]
RFC 6073 Segmented Pseudowire January 2011
1. Introduction
The PWE3 Architecture [RFC3985] defines the signaling and
encapsulation techniques for establishing Single-Segment Pseudowires
(SS-PWs) between a pair of terminating PEs. Multi-Segment
Pseudowires (MS-PWs) are most useful in two general cases:
-i. In some cases it is not possible, desirable, or feasible to
establish a PW control channel between the terminating source
and destination PEs. At a minimum, PW control channel
establishment requires knowledge of and reachability to the
remote (terminating) PE IP address. The local (terminating)
PE may not have access to this information because of
topology, operational, or security constraints.
An example is the inter-AS L2VPN scenario where the
terminating PEs reside in different provider networks (ASes)
and it is the practice to cryptographically sign all control
traffic exchanged between two networks. Technically, an
SS-PW could be used but this would require cryptographic
signatures on ALL terminating source and destination PE
nodes. An MS-PW allows the providers to confine key
administration to just the PW switching points connecting the
two domains.
A second example might involve a single AS where the PW setup
path between the terminating PEs is computed by an external
entity. Assume that a full mesh of PWE3 control channels is
established between PE-A, PE-B, and PE-C. A client-layer L2
connection tunneled through a PW is required between
terminating PE-A and PE-C. The external entity computes a PW
setup path that passes through PE-B. This results in two
discrete PW segments being built: one between PE-A and PE-B
and one between PE-B and PE-C. The successful client-layer
L2 connection between terminating PE-A and terminating PE-C
requires that PE-B performs the PWE3 switching process.
A third example involves the use of PWs in hierarchical
IP/MPLS networks. Access networks connected to a backbone
use PWs to transport customer payloads between customer sites
serviced by the same access network and up to the edge of the
backbone where they can be terminated or switched onto a
succeeding PW segment crossing the backbone. The use of PWE3
switching between the access and backbone networks can
potentially reduce the PWE3 control channels and routing
information processed by the access network T-PEs.
Martini, et al. Standards Track [Page 4]
RFC 6073 Segmented Pseudowire January 2011
It should be noted that PWE3 switching does not help in any
way to reduce the amount of PW state supported by each access
network T-PE.
-ii. In some applications, the signaling protocol and
encapsulation on each segment of the PW are different. The
terminating PEs are connected to networks employing different
PW signaling and encapsulation protocols. In this case, it
is not possible to use an SS-PW. An MS-PW with the
appropriate signaling protocol interworking performed at the
PW switching points can enable PW connectivity between the
terminating PEs in this scenario.
A more detailed discussion of the requirements pertaining to MS-PWs
can be found in [RFC5254].
There are four different mechanisms to establish PWs:
-i. Static configuration of the PW (MPLS or Layer 2 Tunneling
Protocol version 3 (L2TPv3))
-ii. LDP using FEC 128 (PWid FEC Element)
-iii. LDP using FEC 129 (Generalized PWid FEC Element)
-iv. L2TPv3
While MS-PWs are composed of PW segments, each PW segment cannot
function independently, as the PW service is always instantiated
across the complete MS-PW. Hence, no PW segments can be signaled or
be operational without the complete MS-PW being signaled at once.
2. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Terminology
- PW Terminating Provider Edge (T-PE). A PE where the customer-
facing attachment circuits (ACs) are bound to a PW forwarder. A
Terminating PE is present in the first and last segments of a
MS-PW. This incorporates the functionality of a PE as defined in
[RFC3985].
- Single-Segment Pseudowire (SS-PW). A PW set up directly between
two T-PE devices. The PW label is unchanged between the
originating and terminating T-PEs.
Martini, et al. Standards Track [Page 5]
RFC 6073 Segmented Pseudowire January 2011
- Multi-Segment Pseudowire (MS-PW). A static or dynamically
configured set of two or more contiguous PW segments that behave
and function as a single point-to-point PW. Each end of an MS-PW
by definition MUST terminate on a T-PE.
- PW Segment. A part of a single-segment or multi-segment PW, which
traverses one PSN tunnel in each direction between two PE devices,
T-PEs and/or S-PEs (switching PE).
- PW Switching Provider Edge (S-PE). A PE capable of switching the
control and data planes of the preceding and succeeding PW segments
in an MS-PW. The S-PE terminates the PSN tunnels of the preceding
and succeeding segments of the MS-PW. It therefore includes a PW
switching point for an MS-PW. A PW switching point is never the
S-PE and the T-PE for the same MS-PW. A PW switching point runs
necessary protocols to set up and manage PW segments with other PW
switching points and terminating PEs. An S-PE can exist anywhere a
PW must be processed or policy applied. It is therefore not
limited to the edge of a provider network.
- MS-PW path. The set of S-PEs that will be traversed in sequence to
form the MS-PW.
4. General Description
A pseudowire (PW) is a mechanism that carries the essential elements
of an emulated service from one PE to one or more other PEs over a
PSN as described in Figure 1 and in [RFC3985]. Many providers have
deployed PWs as a means of migrating existing (or building new) L2VPN
services (e.g., Frame Relay, ATM, or Ethernet) onto a PSN.
PWs may span multiple domains of the same or different provider
networks. In these scenarios, PW control channels (i.e., targeted
LDP, L2TPv3) and PWs will cross AS boundaries.
Inter-AS L2VPN functionality is currently supported, and several
techniques employing MPLS encapsulation and LDP signaling have been
documented [RFC4364]. It is also straightforward to support the same
inter-AS L2VPN functionality employing L2TPv3. In this document, we
define a methodology to switch a PW between different Packet Switched
Network (PSN) domains or between two or more distinct PW control
plane domains.
Martini, et al. Standards Track [Page 6]
RFC 6073 Segmented Pseudowire January 2011
|<-------------- Emulated Service ---------------->|
| |
| |<-------- Pseudowire ------>| |
| | | |
| | |<-- PSN Tunnel -->| | |
| V V V V |
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
There are two methods for switching a PW between two PW domains. In
the first method (Figure 2), the two separate control plane domains
terminate on different PEs.
|<-------Multi-Segment Pseudowire------->|
| PSN PSN |
AC | |<-1->| |<-2->| | AC
| V V V V V V |
| +----+ +-----+ +----+ +----+ |
+----+ | | |=====| | | |=====| | | +----+
| |-------|......PW1.......|--AC1--|......PW2......|-------| |
| CE1| | | | | | | | | | | |CE2 |
| |-------|......PW3.......|--AC2--|......PW4......|-------| |
+----+ | | |=====| | | |=====| | | +----+
^ +----+ +-----+ +----+ +----+ ^
| PE1 PE2 PE3 PE4 |
| ^ ^ |
| | | |
| PW switching points |
| |
| |
|<-------------------- Emulated Service ---------------->|
Figure 2: PW Switching Using AC Reference Model
Martini, et al. Standards Track [Page 7]
RFC 6073 Segmented Pseudowire January 2011
In Figure 2, pseudowires in two separate PSNs are stitched together
using native service attachment circuits. PE2 and PE3 only run the
control plane for the PSN to which they are directly attached. At
PE2 and PE3, PW1 and PW2 are connected using attachment circuit AC1,
while PW3 and PW4 are connected using attachment circuit AC2.
Native |<-----Multi-Segment Pseudowire------>| Native
Service | PSN PSN | Service
(AC) | |<-Tunnel->| |<-Tunnel->| | (AC)
| V V 1 V V 2 V V |
| +----+ +-----+ +----+ |
+----+ | |TPE1|==========|SPE1 |==========|TPE2| | +----+
| |------|.....PW.Seg't1....X....PW.Seg't3.....|-------| |
| CE1| | | | | | | | | |CE2 |
| |------|.....PW.Seg't2....X....PW.Seg't4.....|-------| |
+----+ | | |==========| |==========| | | +----+
^ +----+ +-----+ +----+ ^
| Provider Edge 1 ^ Provider Edge 2 |
| | |
| | |
| PW switching point |
| |
|<----------------- Emulated Service --------------->|
Figure 3: MS-PW Reference Model
In Figure 3, SPE1 runs two separate control planes: one toward TPE1,
and one toward TPE2. The PW switching point (S-PE) is configured to
connect PW Segment 1 and PW Segment 3 together to complete the multi-
segment PW between TPE1 and TPE2. PW Segment 1 and PW Segment 3 MUST
be of the same PW type, but PSN Tunnel 1 and PSN Tunnel 2 need not be
the same technology. In the latter case, if the PW is switched to a
different technology, the PEs must adapt the PDU encapsulation
between the different PSN technologies. In the case where PSN Tunnel
1 and PSN Tunnel 2 are the same technology, the PW PDU does not need
to be modified, and PDUs are then switched between the pseudowires at
the PW label level.
It should be noted that it is possible to adapt one PSN technology to
a different one, for example, MPLS over an IP encapsulation or
Generic Routing Encapsulation (GRE) [RFC4023], but this is outside
the scope of this document. Further, one could perform an
interworking function on the PWs themselves at the S-PE, allowing
conversion from one PW type to another, but this is also outside the
scope of this document.
This document describes procedures for building multi-segment
pseudowires using manual configuration of the switching point PE1.
Martini, et al. Standards Track [Page 8]
RFC 6073 Segmented Pseudowire January 2011
Other documents may build on this base specification to automate the
configuration and selection of S-PE1. All elements of the
establishment of end-to-end MS-PWs including routing and signaling
are out of scope of this document, and any discussion in this
document serves purely as examples. It should also be noted that a
PW can traverse multiple PW switching points along it's path, and the
edge PEs will not require any specific knowledge of how many S-PEs
the PW has traversed (though this may be reported for troubleshooting
purposes).
The general approach taken for MS-PWs is to connect the individual
control planes by passing along any signaling information immediately
upon reception. First, the S-PE is configured to switch a PW segment
from a specific peer to another PW segment destined for a different
peer. No control messages are exchanged yet, as the S-PE does not
have enough information to actually initiate the PW setup messages.
However, if a session does not already exist, a control protocol
(LDP/L2TP) session MAY be setup. In this model, the MS-PW setup is
starting from the T-PE devices. Once the T-PE is configured, it
sends the PW control setup messages. These messages are received by
the S-PE, and immediately used to form the PW setup messages for the
next SS-PW of the MS-PW.
5. PW Switching and Attachment Circuit Type
The PWs in each PSN are established independently, with each PSN
being treated as a separate PW domain. For example, in Figure 2 for
the case of MPLS PSNs, PW1 is setup between PE1 and PE2 using the LDP
targeted session as described in [RFC4447], and at the same time a
separate pseudowire, PW2, is setup between PE3 and PE4. The ACs for
PW1 and PW2 at PE2 and PE3 MUST be configured such that they are the
same PW type, e.g., ATM Virtual Channel Connection (VCC), Ethernet
VLAN, etc.
6. Applicability
The general applicability of MS-PWs and their relationship to L2VPNs
are described in [RFC5659]. The applicability of a PW type, as
specified in the relevant RFC for that encapsulation (e.g., [RFC4717]
for ATM), applies to each segment. This section describes further
applicability considerations.
As with SS-PWs, the performance of any segment will be limited by the
performance of the underlying PSN. The performance may be further
degraded by the emulation process, and performance degradation may be
further increased by traversing multiple PW segments. Furthermore,
the overall performance of an MS-PW is no better than the worst-
performing segment of that MS-PW.
Martini, et al. Standards Track [Page 9]
RFC 6073 Segmented Pseudowire January 2011
Since different PSN types may be able to achieve different maximum
performance objectives, it is necessary to carefully consider which
PSN types are used along the path of an MS-PW.
7. MPLS-PW to MPLS-PW Switching
Referencing Figure 3, T-PE1 set up PW Segment 1 using the LDP
targeted session as described in [RFC4447], at the same time a
separate pseudowire, PW Segment 3, is setup to T-PE2. Each PW is
configured independently on the PEs, but on S-PE1, PW Segment 1 is
connected to PW Segment 3. PDUs are then switched between the
pseudowires at the PW label level. Hence, the data plane does not
need any special knowledge of the specific pseudowire type. A simple
standard MPLS label swap operation is sufficient to connect the two
PWs, and in this case the PW adaptation function cannot be used.
However, when pushing a new PSN label, the Time to Live (TTL) SHOULD
be set to 255, or some other locally configured fixed value.
This process can be repeated as many times as necessary; the only
limitation to the number of S-PEs traversed is imposed by the TTL
field of the PW MPLS label. The setting of the TTL of the PW MPLS
label is a matter of local policy on the originating PE, but SHOULD
be set to 255. However, if the PW PDU contains an Operations,
Administration, and Maintenance (OAM) packet, then the TTL can be set
to the required value as explained later in this document.
There are three different mechanisms for MPLS-to-MPLS PW setup:
-i. Static configuration of the PW
-ii. LDP using FEC 128
-iii. LDP using the generalized FEC 129
This results in four distinct PW switching situations that are
significantly different and must be considered in detail:
-i. Switching between two static control planes
-ii. Switching between a static and a dynamic LDP control plane
-iii. Switching between two LDP control planes using the same FEC
type
-iv. Switching between LDP using FEC 128 and LDP using the
generalized FEC 129
7.1. Static Control Plane Switching
In the case of two static control planes, the S-PE MUST be configured
to direct the MPLS packets from one PW into the other. There is no
control protocol involved in this case. When one of the control
planes is a simple static PW configuration and the other control
Martini, et al. Standards Track [Page 10]
RFC 6073 Segmented Pseudowire January 2011
plane is a dynamic LDP FEC 128 or generalized PW FEC, then the static
control plane should be considered similar to an attachment circuit
(AC) in the reference model of Figure 1. The switching point PE
SHOULD signal the appropriate PW status if it detects a failure in
sending or receiving packets over the static PW segment. In the
absence of a PW status communication mechanism when the PW is
statically configured, the status communicated to the dynamic LDP PW
will be limited to local interface failures. In this case, the S-PE
PE behaves in a very similar manner to a T-PE, assuming an active
signaling role. This means that the S-PE will immediately send the
LDP Label Mapping message if the static PW is deemed to be UP.
7.2. Two LDP Control Planes Using the Same FEC Type
The S-PE SHOULD assume an initial passive role. This means that when
independent PWs are configured on the switching point, the Label
Switching Router (LSR) does not advertise the LDP PW FEC mapping
until it has received at least one of the two PW LDP FECs from a
remote PE. This is necessary because the switching point LSR does
not know a priori what the interface parameter field in the initial
FEC advertisement will contain.
If one of the S-PEs doesn't accept an LDP Label Mapping message, then
a Label Release message may be sent back to the originator T-PE
depending on the cause of the error. LDP liberal label retention
mode still applies; hence, if a PE is simply not configured yet, the
label mapping is stored for future use. An MS-PW is declared UP only
when all the constituent SS-PWs are UP.
The Pseudowire Identifier (PWid), as defined in [RFC4447], is a
unique number between each pair of PEs. Hence, each SS-PW that forms
an MS-PW may have a different PWid. In the case of the generalized
PW FEC, the Attachment Group Identifier (AGI) / Source Attachment
Identifier (SAI) / Target Attachment Identifier (TAI) may have to
also be different for some, or sometimes all, SS-PWs.
7.2.1. FEC 129 Active/Passive T-PE Election Procedure
When an MS-PW is signaled using FEC 129, each T-PE might
independently start signaling the MS-PW. If the MS-PW path is not
statically configured, in certain cases the signaling procedure could
result in an attempt to set up each direction of the MS-PW through
different S-PEs. If an operator wishes to avoid this situation, one
of the T-PEs MUST start the PW signaling (active role), while the
other waits to receive the LDP label mapping before sending the
respective PW LDP Label Mapping message (passive role). When the
MS-PW path is not statically configured, the active T-PE (the Source
Martini, et al. Standards Track [Page 11]
RFC 6073 Segmented Pseudowire January 2011
T-PE) and the passive T-PE (the Target T-PE) MUST be identified
before signaling is initiated for a given MS-PW.
The determination of which T-PE assumes the active role SHOULD be
done as follows:
The SAII and TAII are compared as unsigned integers; if the SAII is
larger, then the T-PE assumes the active role.
The selection process to determine which T-PE assumes the active role
MAY be superseded by manual provisioning. In this case, one of the
T-PEs MUST be set to the active role, and the other one MUST be set
to the passive role.
7.3. LDP Using FEC 128 to LDP Using the Generalized FEC 129
When a PE is using the generalized FEC 129, there are two distinct
roles that a PE can assume: active and passive. A PE that assumes
the active role will send the LDP PW setup message, while a passive
role PE will simply reply to an incoming LDP PW setup message. The
S-PE will always remain passive until a PWid FEC 128 LDP message is
received, which will cause the corresponding generalized PW FEC LDP
message to be formed and sent. If a generalized FEC PW LDP message
is received while the switching point PE is in a passive role, the
corresponding PW FEC 128 LDP message will be formed and sent.
PWids need to be mapped to the corresponding AGI/TAI/SAI and vice
versa. This can be accomplished by local S-PE configuration, or by
some other means, such as some form of auto discovery. Such other
means are outside the scope of this document.
7.4. LDP SP-PE TLV
The edge-to-edge PW might traverse several switching points, in
separate administrative domains. For management and troubleshooting
reasons, it is useful to record information about the switching
points at the S-PEs that the PW traverses. This is accomplished by
using a PW Switching Point PE TLV (SP-PE TLV).
Sending the SP-PE TLV is OPTIONAL; however, the PE or S-PE MUST
process the TLV upon reception. The "U" bit MUST be set for backward
compatibility with T-PEs that do not support the MS-PW extensions
described in the document. The SP-PE TLV MAY appear only once for
each switching point traversed, and it cannot be of length zero. The
SP-PE TLV is appended to the PW FEC at each S-PE, and the order of
the SP-PE TLVs in the LDP message MUST be preserved. The SP-PE TLV
Martini, et al. Standards Track [Page 12]
RFC 6073 Segmented Pseudowire January 2011
is necessary to support some of the Virtual Circuit Connectivity
Verification (VCCV) functions for MS-PWs. See Section 9.5 for more
details. The SP-PE TLV 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| SP-PE TLV (0x096D) | SP-PE TLV Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLV Type | Length | Variable Length Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Variable Length Value |
| " " " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- SP-PE TLV Length
Specifies the total length of all the following SP-PE TLV fields
in octets.
- Sub-TLV Type
Encodes how the Value field is to be interpreted.
- Length
Specifies the length of the Value field in octets.
- Value
Octet string of Length octets that encodes information to be
interpreted as specified by the Type field.
PW Switching Point PE sub-TLV Types are assigned by IANA according to
the process defined in Section 14 (IANA Considerations) below.
For local policy reasons, a particular S-PE can filter out all
SP-PE TLVs in a Label Mapping message that traverses it and not
include its own SP-PE TLV. In this case, from any upstream PE,
it will appear as if this particular S-PE is the T-PE. This
might be necessary, depending on local policy, if the S-PE is at
the service provider administrative boundary. It should also be
noted that because there are no SP-PE TLVs describing the path
beyond the S-PE that removed them, VCCV will only work as far as
that S-PE.
Martini, et al. Standards Track [Page 13]
RFC 6073 Segmented Pseudowire January 2011
7.4.1. PW Switching Point PE Sub-TLVs
The SP-PE TLV contains sub-TLVs that describe various characteristics
of the S-PE traversed. The SP-PE TLV MUST contain the appropriate
mandatory sub-TLVs specified below. The definitions of the PW
Switching Point PE sub-TLVs are as follows:
- PWid of last PW segment traversed.
This is only applicable if the last PW segment traversed used
LDP FEC 128 to signal the PW. This sub-TLV type contains a PWid
in the format of the PWid described in [RFC4447]. This is just
a 32-bit unsigned integer number.
- PW Switching Point description string.
An OPTIONAL description string of text up to 80 characters long.
Human-readable text MUST be provided in the UTF-8 character set
using the Default Language [RFC2277].
- Local IP address of PW Switching Point.
The local IPv4 or IPv6 address of the PW Switching Point. This
is an OPTIONAL Sub-TLV. In most cases, this will be the local
LDP session IP address of the S-PE.
- Remote IP address of the last PW Switching Point traversed or of
the T-PE.
The IPv4 or IPv6 address of the last PW Switching Point
traversed or of the T-PE. This is an OPTIONAL Sub-TLV. In most
cases, this will be the remote IP address of the LDP session.
This Sub-TLV SHOULD only be included if there are no other SP-PE
TLVs present from other S-PEs, or if the remote IP address of
the LDP session does not correspond to the "Local IP address of
PW Switching Point" TLV value contained in the last SP-PE TLV.
- The FEC element of last PW segment traversed.
This is only applicable if the last PW segment traversed used
LDP FEC 129 to signal the PW.
Martini, et al. Standards Track [Page 14]
RFC 6073 Segmented Pseudowire January 2011
The FEC element of the last PW segment traversed. This is encoded in
the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AGI Type | Length | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ AGI Value (contd.) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AII Type | Length | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ SAII Value (contd.) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AII Type | Length | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ TAII Value (contd.) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- L2 PW address of the PW Switching Point (recommended format).
This sub-TLV type contains an L2 PW address of PW Switching
Point in the format described in Section 3.2 of [RFC5003]. This
includes the AII type field and length, as well as the L2 PW
address with the AC ID field set to zero.
7.4.2. Adaptation of Interface Parameters
[RFC4447] defines several interface parameters, which are used by the
Network Service Processing (NSP) to adapt the PW to the attachment
circuit (AC). The interface parameters are only used at the
endpoints, and MUST be passed unchanged across the S-PE. However,
the following interface parameters MAY be modified as follows:
- 0x03 Optional Interface Description string
This Interface parameter MAY be modified or altogether removed
from the FEC element depending on local configuration policies.
- 0x09 Fragmentation indicator
This parameter MAY be inserted in the FEC by the switching point
if it is capable of re-assembly of fragmented PW frames
according to [RFC4623].
Martini, et al. Standards Track [Page 15]
RFC 6073 Segmented Pseudowire January 2011
- 0x0C VCCV parameter
This Parameter contains the Control Channel (CC) type and
Connectivity Verification (CV) type bit fields. The CV type bit
field MUST be reset to reflect the CV type supported by the
S-PE. The CC type bit field MUST have bit 1 "Type 2: MPLS
Router Alert Label" set to 0. The other bit fields MUST be
reset to reflect the CC type supported by the S-PE.
7.5. Group ID
The Group ID (GR ID) is used to reduce the number of status messages
that need to be sent by the PE advertising the PW FEC. The GR ID has
local significance only, and therefore MUST be mapped to a unique GR
ID allocated by the S-PE.
7.6. PW Loop Detection
A switching point PE SHOULD inspect the PW Switching Point PE TLV, to
verify that its own IP address does not appear in it. If the PE's IP
address appears in a received PW Switching Point PE TLV, the PE
SHOULD break the loop and send a label release message with the
following error code:
Value E Description
0x0000003A 0 PW Loop Detected
If an S-PE along the MS-PW removed all SP-PE TLVs, as mentioned
above, this loop detection method will fail.
8. MPLS-PW to L2TPv3-PW Control Plane Switching
Both MPLS and L2TPv3 PWs may be static or dynamic. This results in
four possibilities when switching between L2TPv3 and MPLS.
-i. Switching between static MPLS and L2TPv3 PWs
-ii. Switching between a static MPLS PW and a dynamic L2TPv3 PW
-iii. Switching between a static L2TPv3 PW and a dynamic LDP/MPLS
PW
-iv. Switching between a dynamic LDP/MPLS PW and a dynamic L2TPv3
PW
Martini, et al. Standards Track [Page 16]
RFC 6073 Segmented Pseudowire January 2011
8.1. Static MPLS and L2TPv3 PWs
In the case of two static control planes, the S-PE MUST be configured
to direct packets from one PW into the other. There is no control
protocol involved in this case. The configuration MUST include which
MPLS PW Label maps to which L2TPv3 Session ID (and associated Cookie,
if present) as well as which MPLS Tunnel Label maps to which PE
destination IP address.
8.2. Static MPLS PW and Dynamic L2TPv3 PW
When a statically configured MPLS PW is switched to a dynamic L2TPv3
PW, the static control plane should be considered identical to an
attachment circuit (AC) in the reference model of Figure 1. The
switching point PE SHOULD signal the appropriate PW status if it
detects a failure in sending or receiving packets over the static PW.
Because the PW is statically configured, the status communicated to
the dynamic L2TPv3 PW will be limited to local interface failures.
In this case, the S-PE behaves in a very similar manner to a T-PE,
assuming an active role.
8.3. Static L2TPv3 PW and Dynamic LDP/MPLS PW
When a statically configured L2TPv3 PW is switched to a dynamic
LDP/MPLS PW, then the static control plane should be considered
identical to an attachment circuit (AC) in the reference model of
Figure 1. The switching point PE SHOULD signal the appropriate PW
status (via an L2TPv3 Set-Link-Info (SLI) message) if it detects a
failure in sending or receiving packets over the static PW. Because
the PW is statically configured, the status communicated to the
dynamic LDP/MPLS PW will be limited to local interface failures. In
this case, the S-PE behaves in a very similar manner to a T-PE,
assuming an active role.
8.4. Dynamic LDP/MPLS and L2TPv3 PWs
When switching between dynamic PWs, the switching point always
assumes an initial passive role. Thus, it does not initiate an
LDP/MPLS or L2TPv3 PW until it has received a connection request
(Label Mapping or Incoming-Call-Request (ICRQ)) from one side of the
node. Note that while MPLS PWs are made up of two unidirectional
Label Switched Paths (LSPs) bonded together by FEC identifiers,
L2TPv3 PWs are bidirectional in nature, setup via a three-message
exchange (ICRQ, Incoming-Call-Reply (ICRP), and Incoming-Call-
Connected (ICCN)). Details of Session Establishment, Tear Down, and
PW Status signaling are detailed below.
Martini, et al. Standards Track [Page 17]
RFC 6073 Segmented Pseudowire January 2011
8.4.1. Session Establishment
When the S-PE receives an L2TPv3 ICRQ message, the identifying AVPs
included in the message are mapped to FEC identifiers and sent in an
LDP Label Mapping message. Conversely, if an LDP Label Mapping
message is received, it is either mapped to an ICRP message or causes
an L2TPv3 session to be initiated by sending an ICRQ.
Following are two example exchanges of messages between LDP and
L2TPv3. The first is a case where an L2TPv3 T-PE initiates an MS-PW;
the second is a case where an MPLS T-PE initiates an MS-PW.
PE 1 (L2TPv3) PW Switching Node PE3 (MPLS/LDP)
AC "Up"
L2TPv3 ICRQ --->
LDP Label Mapping --->
AC "Up"
<--- LDP Label Mapping
<--- L2TPv3 ICRP
L2TPv3 ICCN --->
<-------------------- MS-PW Established ------------------>
PE 1 (MPLS/LDP) PW Switching Node PE3 (L2TPv3)
AC "Up"
LDP Label Mapping --->
L2TPv3 ICRQ --->
<--- L2TPv3 ICRP
<--- LDP Label Mapping
L2TPv3 ICCN --->
AC "Up"
<-------------------- MS-PW Established ------------------>
8.4.2. Adaptation of PW Status Message
L2TPv3 uses the SLI message to indicate an interface status change
(such as the interface transitioning from "Up" or "Down"). MPLS/LDP
PWs either signal this via an LDP Label Withdraw or the PW Status
Notification message defined in Section 4.4 of [RFC4447]. The LDP
status TLV bit SHOULD be mapped to the L2TPv3 equivalent Extended
Circuit Status Values TLV specified in [RFC5641].
8.4.3. Session Tear Down
L2TPv3 uses a single message, Call-Disconnect-Notify (CDN), to tear
down a pseudowire. The CDN message translates to a Label Withdraw
message in LDP. Following are two example exchanges of messages
Martini, et al. Standards Track [Page 18]
RFC 6073 Segmented Pseudowire January 2011
between LDP and L2TPv3. The first is a case where an L2TPv3 T-PE
initiates the termination of an MS-PW; the second is a case where an
MPLS T-PE initiates the termination of an MS-PW.
PE 1 (L2TPv3) PW Switching Node PE3 (MPLS/LDP)
AC "Down"
L2TPv3 CDN --->
LDP Label Withdraw --->
AC "Down"
<-- LDP Label Release
<--------------- MS-PW Data Path Down ------------------>
PE 1 (MPLS LDP) PW Switching Node PE3 (L2TPv3)
AC "Down"
LDP Label Withdraw --->
L2TPv3 CDN -->
<-- LDP Label Release
AC "Down"
<---------------- MS-PW Data Path Down ------------------>
8.5. Adaptation of L2TPv3 AVPs to Interface Parameters
[RFC4447] defines several interface parameters that MUST be mapped to
the equivalent AVPs in L2TPv3 setup messages.
* Interface MTU
The Interface MTU parameter is mapped directly to the L2TP
"Interface Maximum Transmission Unit" AVP defined in [RFC4667].
* Max Number of Concatenated ATM cells
This interface parameter is mapped directly to the L2TP "ATM
Maximum Concatenated Cells AVP" described in Section 6 of
[RFC4454].
* PW Type
The PW Type defined in [RFC4446] is mapped to the L2TPv3
"Pseudowire Type" AVP defined in [RFC3931].
* PWid (FEC 128)
For FEC 128, the PWid is mapped directly to the L2TPv3 "Remote
End ID" AVP defined in [RFC3931].
Martini, et al. Standards Track [Page 19]
RFC 6073 Segmented Pseudowire January 2011
* Generalized FEC 129 SAI/TAI
Section 4.3 of [RFC4667] defines how to encode the SAI and TAI
parameters. These can be mapped directly.
Other interface parameter mappings are unsupported when switching
between LDP/MPLS and L2TPv3 PWs.
8.6. PW Switching Point PE TLV in L2TPv3
When translating between LDP and L2TPv3 control messages, the PW
Switching Point PE TLV described earlier in this document is carried
in a single variable-length L2TP AVP present in the ICRQ and ICRP
messages, and optionally in the ICCN message.
The L2TP "PW Switching Point AVP" is Attribute Type 101. The AVP MAY
be hidden (the L2TP AVP H-bit may be 0 or 1), the length of the AVP
is 6 plus the length of the series of Switching Point PE sub-TLVs
included in the AVP, and the AVP MUST NOT be marked Mandatory (the
L2TP AVP M-bit MUST be 0).
8.7. L2TPv3 and MPLS PW Data Plane
When switching between an MPLS and L2TP PW, packets are sent in their
entirety from one PW to the other, replacing the MPLS label stack
with the L2TPv3 and IP header or vice versa.
Section 5.4 of [RFC3985] discusses the purpose of the various shim
headers necessary for enabling a pseudowire over an IP or MPLS PSN.
For L2TPv3, the Payload Convergence and Sequencing function is
carried out via the Default L2-Specific Sublayer defined in
[RFC3931]. For MPLS, these two functions (together with PSN
Convergence) are carried out via the MPLS Control Word. Since these
functions are different between MPLS and L2TPv3, interworking between
the two may be necessary.
The L2TP L2-Specific Sublayer and MPLS Control Word are shim headers,
which in some cases are not necessary to be present at all. For
example, an Ethernet PW with sequencing disabled will generally not
require an MPLS Control Word or L2TP Default L2-Specific Sublayer to
be present at all. In this case, Ethernet frames are simply sent
from one PW to the other without any modification beyond the MPLS and
L2TP/IP encapsulation and decapsulation.
The following section offers guidelines for how to interwork between
L2TP and MPLS for those cases where the Payload Convergence,
Sequencing, or PSN Convergence functions are necessary on one or both
sides of the switching node.
Martini, et al. Standards Track [Page 20]
RFC 6073 Segmented Pseudowire January 2011
8.7.1. Mapping the MPLS Control Word to L2TP
The MPLS Control Word consists of (from left to right):
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0| Reserved | Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-i. These bits are always zero in an MPLS PW PDU. It is not
necessary to map them to L2TP.
-ii. These six bits may be used for Payload Convergence depending
on the PW type. For ATM, the first four of these bits are
defined in [RFC4717]. These map directly to the bits
defined in [RFC4454]. For Frame Relay, these bits indicate
how to set the bits in the Frame Relay header that must be
regenerated for L2TP as it carries the Frame Relay header
intact.
-iii. L2TP determines its payload length from IP. Thus, this
Length field need not be carried directly to L2TP. This
Length field will have to be calculated and inserted for
MPLS when necessary.
-iv. The Default L2-Specific Sublayer has a sequence number with
different semantics than that of the MPLS Control Word.
This difference eliminates the possibility of supporting
sequencing across the MS-PW by simply carrying the sequence
number through the switching point transparently. As such,
sequence numbers MAY be supported by checking the sequence
numbers of packets arriving at the switching point and
regenerating a new sequence number in the appropriate format
for the PW on egress. If this type of sequence interworking
at the switching node is not supported, and a T-PE requests
sequencing of all packets via the L2TP control channel
during session setup, the switching node SHOULD NOT allow
the session to be established by sending a CDN message with
Result Code set to 31 "Sequencing not supported".
Martini, et al. Standards Track [Page 21]
RFC 6073 Segmented Pseudowire January 2011
9. Operations, Administration, and Maintenance (OAM)
9.1. Extensions to VCCV to Support MS-PWs
Single-segment pseudowires are signaled using the Virtual Circuit
Connectivity Verification (VCCV) parameter included in the interface
parameter field of the PWid FEC TLV or the interface parameter sub-
TLV of the Generalized PWid FEC TLV as described in [RFC5085]. When
a switching point exists between PE nodes, it is required to be able
to continue operating VCCV end-to-end across a switching point and to
provide the ability to trace the path of the MS-PW over any number of
segments.
This document provides a method for achieving these two objectives.
This method is based on reusing the existing VCCV Control Word (CW)
and decrementing the TTL of the PW label at each S-PE in the path of
the MS-PW.
9.2. OAM from MPLS PW to L2TPv3 PW
When an MS-PW includes SS-PWs that use the L2TPv3, the MPLS PW OAM
MUST be terminated at the S-PE connecting the L2TPv3 and MPLS
segments. Status information received in a particular PW segment can
then be used to generate the appropriate status messages on the
following PW segment. In the case of L2TPV3, the status bits in the
circuit status AVP defined in Section 5.4.5 of [RFC3931] and Extended
Circuit Status Values defined in [RFC5641] can be mapped directly to
the PW status bits defined in Section 5.4.3 of [RFC4447].
VCCV messages are specific to the MPLS data plane and cannot be used
for an L2TPv3 PW segment. Therefore, the S-PE MUST NOT send the VCCV
parameter included in the interface parameter field of the PWid FEC
TLV or the sub-TLV interface parameter of the Generalized PWid FEC
TLV. It might be possible to translate VCCV messages from L2TPv3 PW
segments to MPLS PW segments and vice versa; however, this topic is
left for further study.
9.3. OAM Data Plane Indication from MPLS PW to MPLS PW
As stated above, the S-PE MUST perform a standard MPLS label swap
operation on the MPLS PW label. By the rules defined in [RFC3032],
the PW label TTL MUST be decreased at every S-PE. Once the PW label
TTL reaches the value of 0, the packet is sent to the control plane
to be processed. Hence, by controlling the PW TTL value of the PW
label, it is possible to select exactly which S-PE will respond to
the VCCV packet.
Martini, et al. Standards Track [Page 22]
RFC 6073 Segmented Pseudowire January 2011
9.4. Signaling OAM Capabilities for Switched Pseudowires
Similarly to SS-PW, MS-PW VCCV capabilities are signaled using the
VCCV parameter included in the interface parameter field of the PWid
FEC TLV or the sub-TLV interface parameter of the Generalized PWid
FEC TLV as described in [RFC5085].
In Figure 3, T-PE1 uses the VCCV parameter included in the interface
parameter field of the PWid FEC TLV or the sub-TLV interface
parameter of the Generalized PWid FEC TLV to indicate to the far-end
T-PE2 what VCCV capabilities T-PE1 supports. This is the same VCCV
parameter as would be used if T-PE1 and T-PE2 were connected
directly. S-PE2, which is a PW switching point, as part of the
adaptation function for interface parameters, processes locally the
VCCV parameter then passes it to T-PE2. If there were multiple S-PEs
on the path between T-PE1 and T-PE2, each would carry out the same
processing, passing along the VCCV parameter. The local processing
of the VCCV parameter removes CC Types specified by the originating
T-PE that are not supported on the S-PE. For example, if T-PE1
indicates that it supports CC Types 1, 2, and 3, then the S-PE
removes the Router Alert CC Type 2, leaving the rest of the TLV
unchanged, and passes the modified VCCV parameter to the next S-PE
along the path.
The far end T-PE (T-PE2) receives the VCCV parameter indicating only
the CC Types that are supported by the initial T-PE (T-PE1) and all
S-PEs along the PW path.
9.5. OAM Capability for MS-PWs Demultiplexed Using MPLS
The VCCV parameter ID is defined as follows in [RFC4446]:
Parameter ID Length Description
0x0c 4 VCCV
The format of the VCCV parameter field is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0c | 0x04 | CC Types | CV Types |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Bit 0 (0x01) - Type 1: PWE3 Control Word with 0001b as
first nibble as defined in [RFC4385]
Bit 1 (0x02) - Type 2: MPLS Router Alert Label
Bit 2 (0x04) - Type 3: MPLS Demultiplexor PW Label with
TTL == 1 (Type 3).
Martini, et al. Standards Track [Page 23]
RFC 6073 Segmented Pseudowire January 2011
9.5.1. MS-PW and VCCV CC Type 1
VCCV CC Type 1 can be used for MS-PWs. However, if the CW is enabled
on user packets, VCCV CC Type 1 MUST be used according to the rules
in [RFC5085]. When using CC Type 1 for MS-PWs, the PE transmitting
the VCCV packet MUST set the TTL to the appropriate value to reach
the destination S-PE. However, if the packet is destined for the
T-PE, the TTL can be set to any value that is sufficient for the
packet to reach the T-PE.
9.5.2. MS-PW and VCCV CC Type 2
VCCV CC Type 2 is not supported for MS-PWs and MUST be removed from a
VCCV parameter field by the S-PE.
9.5.3. MS-PW and VCCV CC Type 3
VCCV CC Type 3 can be used for MS-PWs; however, if the CW is enabled,
VCCV Type 1 is preferred according to the rules in [RFC5085]. Note
that for using the VCCV Type 3, TTL method, the PE will set the PW
label TTL to the appropriate value necessary to reach the target PE;
otherwise, the VCCV packet might be forwarded over the AC to the
Customer Premise Equipment (CPE).
9.6. MS-PW VCCV Operations
This document specifies four VCCV operations:
-i. End-to-end MS-PW connectivity verification. This operation
enables the connectivity of the MS-PW to be tested from
source T-PE to destination T-PE. In order to do this, the
sending T-PE must include the FEC used in the last segment
of the MS-PW to the destination T-PE in the VCCV-Ping echo
request. This information is either configured at the
sending T-PE or is obtained by processing the corresponding
sub-TLVs of the optional SP-PE TLV, as described below.
-ii. Partial MS-PW connectivity verification. This operation
enables the connectivity of any contiguous subset of the
segments of an MS-PW to be tested from the source T-PE or a
source S-PE to a destination S-PE or T-PE. Again, the FEC
used on the last segment to be tested must be included in
the VCCV-Ping echo request message. This information is
determined by the sending T-PE or S-PE as in (i) above.
-iii. MS-PW path verification. This operation verifies the path
of the MS-PW, as returned by the SP-PE TLV, against the
actual data path of the MS-PW. The sending T-PE or S-PE
Martini, et al. Standards Track [Page 24]
RFC 6073 Segmented Pseudowire January 2011
iteratively sends a VCCV echo request to each S-PE along the
MS-PW path, using the FEC for the corresponding MS-PW
segment in the SP-PE TLV. If the SP-PE TLV information is
correct, then a VCCV echo reply showing that this is a valid
router for the FEC will be received. However, if the SP-PE
TLV information is incorrect, then this operation enables
the first incorrect switching point to be determined, but
not the actual path of the MS-PW beyond that. This
operation cannot be used when the MS-PW is statically
configured or when the SP-PE TLV is not supported. The
processing of the PW Switching Point PE TLV used for this
operation is described below. This operation is OPTIONAL.
-iv. MS-PW path trace. This operation traces the data path of
the MS-PW using FECs included in the Target FEC stack TLV
[RFC4379] returned by S-PEs or T-PEs in an echo reply
message. The sending T-PE or S-PE uses this information to
recursively test each S-PE along the path of the MS-PW in a
single operation in a similar manner to LSP trace. This
operation is able to determine the actual data path of the
MS-PW, and can be used for both statically configured and
signaled MS-PWs. Support for this operation is OPTIONAL.
Note that the above operations rely on intermediate S-PEs and/or the
destination T-PE to include the PW Switching Point PE TLV as a part
of the MS-PW setup process, or to include the Target FEC stack TLV in
the VCCV echo reply message. For various reasons, e.g., privacy or
security of the S-PE/T-PE, this information may not be available to
the source T-PE. In these cases, manual configuration of the FEC MAY
still be used.
9.6.1. VCCV Echo Message Processing
The challenge for the control plane is to be able to build the VCCV
echo request packet with the necessary information to reach the
desired S-PE or T-PE, for example, the target FEC 128 PW sub-TLV of
the downstream PW segment that the packet is destined for. This
could be even more difficult in situations in which the MS-PW spans
different providers and Autonomous Systems.
For example, in Figure 3, T-PE1 has the FEC 128 of the segment (PW
segment 1), but it does not readily have the information required to
compose the FEC 128 of the following segment (PW segment 3), if a
VCCV echo request is to be sent to T-PE2. This can be achieved by
the methods described in the following subsections.
Martini, et al. Standards Track [Page 25]
RFC 6073 Segmented Pseudowire January 2011
9.6.1.1. Sending a VCCV Echo Request
When performing a partial or end-to-end connectivity or path
verification, the sender of the echo request message requires the FEC
of the last segment to the target S-PE/T-PE node. This information
can either be configured manually or be obtained by inspecting the
corresponding sub-TLVs of the PW Switching Point PE TLV.
The necessary SP-PE sub-TLVs are:
Type Description
0x01 PWid of last PW segment traversed
0x03 Local IP address of PW Switching Point
0x04 Remote IP address of last PW Switching Point traversed or
of the T-PE
When performing an OPTIONAL MS-PW path trace operation, the T-PE will
automatically learn the target FEC by probing, one by one, the S-PEs
of the MS-PW path, using the FEC returned in the Target FEC stack of
the previous VCCV echo reply.
9.6.1.2. Receiving a VCCV Echo Request
Upon receiving a VCCV echo request, the control plane on S-PEs (or
the target node of each segment of the MS-PW) validates the request
and responds to the request with an echo reply consisting of a return
code of 8 (label switched at stack depth) indicating that it is an
S-PE and not the egress router for the MS-PW.
S-PEs that wish to reveal their downstream next-hop in a trace
operation should include the FEC of the downstream PW segment in the
Target FEC stack (as per Sections 3.2 and 4.5 of [RFC4379]) of the
echo reply message. FEC 128 PWs MUST use the format shown in Section
3.2.9 of [RFC4379] for the sub-TLV in the Target FEC stack, while FEC
129 PWs MUST use the format shown in Section 3.2.10 of [RFC4379] for
the sub-TLV in the Target FEC stack. Note that an S-PE MUST NOT
include this FEC information in the reply if it has been configured
not to do so for administrative reasons or for reasons explained
previously.
If the node is the T-PE or the egress node of the MS-PW, it responds
to the echo request with an echo reply with a return code of 3
(Egress Router).
Martini, et al. Standards Track [Page 26]
RFC 6073 Segmented Pseudowire January 2011
9.6.1.3. Receiving a VCCV Echo Reply
The operation to be taken by the node receiving the echo reply in
response to an echo request depends on the VCCV mode of operation
described above. See Section 9.5.2 for detailed procedures.
9.6.2. Detailed VCCV Procedures
There are two similar methods of verifying the MS-PW path: Path Trace
and Path Verification. Path Trace does not use the LDP control plane
to obtain information on the path to verify, so this method is well
suited if portions of the MS-PW are statically configured SS-PWs.
The Path Verification method relies on information obtained from the
LDP control plane, and hence offers better verification of the
current forwarding behavior compared to the LDP signaled forwarding
information of the MS-PW path. However, in the case where there are
statically signaled SS-PWs in the MS-PW path, the path information is
unavailable and must be programmed manually.
9.6.2.1. End-to-End Connectivity Verification between T-PEs
In Figure 3, if T-PE1, S-PE, and T-PE2 support Control Word, the PW
control plane will automatically negotiate the use of the CW. VCCV
CC Type 3 will function correctly whether or not the CW is enabled on
the PW. However, VCCV Type 1 (which can be use for end-to-end
verification only) is only supported if the CW is enabled.
At the S-PE, the data path operations include an outer label pop,
inner label swap, and new outer label push. Note that there is no
requirement for the S-PE to inspect the CW. Thus, the end-to-end
connectivity of the multi-segment pseudowire can be verified by
performing all of the following steps:
-i. The T-PE forms a VCCV-Ping echo request message with the FEC
matching that of the last PW segment to the destination
T-PE.
-ii. The T-PE sets the inner PW label TTL to the exact value to
allow the packet to reach the far-end T-PE. (The value is
determined by counting the number of S-PEs from the control
plane information.) Alternatively, if CC Type 1 is
supported, the packet can be encapsulated according to CC
Type 1 in [RFC5085].
-iii. The T-PE sends a VCCV packet that will follow the exact same
data path at each S-PE as that taken by data packets.
Martini, et al. Standards Track [Page 27]
RFC 6073 Segmented Pseudowire January 2011
-iv. The S-PE may perform an outer label pop, if Penultimate Hop
Popping (PHP) is disabled, and will perform an inner label
swap with TTL decrement and a new outer label push.
-v. There is no requirement for the S-PE to inspect the CW.
-vi. The VCCV packet is diverted to VCCV control processing at
the destination T-PE.
-vii. The destination T-PE replies using the specified reply mode,
i.e., reverse PW path or IP path.
9.6.2.2. Partial Connectivity Verification from T-PE
In order to trace part of the multi-segment pseudowire, the TTL of
the PW label may be used to force the VCCV message to 'pop out' at an
intermediate node. When the TTL expires, the S-PE can determine that
the packet is a VCCV packet either by checking the CW or (if the CW
is not in use) by checking for a valid IP header with UDP destination
port 3503. The packet should then be diverted to VCCV processing.
In Figure 3, if T-PE1 sends a VCCV message with the TTL of the PW
label equal to 1, the TTL will expire at the S-PE. T-PE1 can thus
verify the first segment of the pseudowire.
The VCCV packet is built according to [RFC4379], Section 3.2.9 for
FEC 128, or Section 3.2.10 for FEC 129. All the information
necessary to build the VCCV LSP ping packet is collected by
inspecting the S-PE TLVs.
Note that this use of the TTL is subject to the caution expressed in
[RFC5085]. If a penultimate LSR between S-PEs or between an S-PE and
a T-PE manipulates the PW label TTL, the VCCV message may not emerge
from the MS-PW at the correct S-PE.
9.6.2.3. Partial Connectivity Verification between S-PEs
Assuming that all nodes along an MS-PW support the Control Word CC
Type 3, VCCV between S-PEs may be accomplished using the PW label TTL
as described above. In Figure 3, the S-PE may verify the path
between it and T-PE2 by sending a VCCV message with the PW label TTL
set to 1. Given a more complex network with multiple S-PEs, an S-PE
may verify the connectivity between it and an S-PE two segments away
by sending a VCCV message with the PW label TTL set to 2. Thus, an
S-PE can diagnose connectivity problems by successively increasing
the TTL. All the information needed to build the proper VCCV echo
Martini, et al. Standards Track [Page 28]
RFC 6073 Segmented Pseudowire January 2011
request packet (as described in [RFC4379], Sections 3.2.9 or 3.2.10)
is obtained automatically from the LDP label mapping that contains
S-PE TLVs.
9.6.2.4. MS-PW Path Verification
As an example, in Figure 3, VCCV trace can be performed on the MS-PW
originating from T-PE1 by a single operational command. The
following process ensues:
-i. T-PE1 sends a VCCV echo request with TTL set to 1 and a FEC
containing the pseudowire information of the first segment
(PW1 between T-PE1 and S-PE) to S-PE for validation. If FEC
Stack Validation is enabled, the request may also include an
additional sub-TLV such as LDP Prefix and/or RSVP LSP,
dependent on the type of transport tunnel the segmented PW
is riding on.
-ii. S-PE validates the echo request with the FEC. Since it is a
switching point between the first and second segment, it
builds an echo reply with a return code of 8 and sends the
echo reply back to T-PE1.
-iii. T-PE1 builds a second VCCV echo request based on the
information obtained from the control plane (SP-PE TLV). It
then increments the TTL and sends it out to T-PE2. Note
that the VCCV echo request packet is switched at the S-PE
data path and forwarded to the next downstream segment
without any involvement from the control plane.
-iv. T-PE2 receives and validates the echo request with the FEC.
Since T-PE2 is the destination node or the egress node of
the MS-PW, it replies to T-PE1 with an echo reply with a
return code of 3 (Egress Router).
-v. T-PE1 receives the echo reply from T-PE2. T-PE1 is made
aware that T-PE2 is the destination of the MS-PW because the
echo reply has a return code of 3. The trace process is
completed.
If no echo reply is received, or an error code is received from a
particular PE, the trace process MUST stop immediately, and packets
MUST NOT be sent further along the MS-PW.
For more detail on the format of the VCCV echo packet, refer to
[RFC5085] and [RFC4379]. The TTL here refers to that of the inner
(PW) label TTL.
Martini, et al. Standards Track [Page 29]
RFC 6073 Segmented Pseudowire January 2011
9.6.2.5. MS-PW Path Trace
As an example, in Figure 3, VCCV trace can be performed on the MS-PW
originating from T-PE1 by a single operational command. The
following OPTIONAL process ensues:
-i. T-PE1 sends a VCCV echo request with TTL set to 1 and a FEC
containing the pseudowire information of the first segment
(PW1 between T-PE1 and S-PE) to S-PE for validation. If FEC
Stack Validation is enabled, the request may also include an
additional sub-TLV such as LDP Prefix and/or RSVP LSP,
dependent on the type of transport tunnel the segmented PW
is riding on.
-ii. The S-PE validates the echo request with the FEC.
-iii. The S-PE builds an echo reply with a return code of 8 and
sends the echo reply back to T-PE1, appending the FEC 128
information for the next segment along the MS-PW to the VCCV
echo reply packet using the Target FEC stack TLV (as per
Sections 3.2 and 4.5 of [RFC4379]).
-iv. T-PE1 builds a second VCCV echo request based on the
information obtained from the FEC stack TLV received in the
previous VCCV echo reply. It then increments the TTL and
sends it out to T-PE2. Note that the VCCV echo request
packet is switched at the S-PE data path and forwarded to
the next downstream segment without any involvement from the
control plane.
-v. T-PE2 receives and validates the echo request with the FEC.
Since T-PE2 is the destination node or the egress node of
the MS-PW, it replies to T-PE1 with an echo reply with a
return code of 3 (Egress Router).
-vi. T-PE1 receives the echo reply from T-PE2. T-PE1 is made
aware that T-PE2 is the destination of the MS-PW because the
echo reply has a return code of 3. The trace process is
completed.
If no echo reply is received, or an error code is received from a
particular PE, the trace process MUST stop immediately, and packets
MUST NOT be sent further along the MS-PW.
For more detail on the format of the VCCV echo packet, refer to
[RFC5085] and [RFC4379]. The TTL here refers to that of the inner
(PW) label TTL.
Martini, et al. Standards Track [Page 30]
RFC 6073 Segmented Pseudowire January 2011
10. Mapping Switched Pseudowire Status
In the PW switching with attachment circuits case (Figure 2), PW
status messages indicating PW or attachment circuit faults MUST be
mapped to fault indications or OAM messages on the connecting AC as
defined in [PW-MSG-MAP].
In the PW control plane switching case (Figure 3), there is no
attachment circuit at the S-PE, but the two PWs are connected
together. Similarly, the status of the PWs is forwarded unchanged
from one PW to the other by the control plane switching function.
However, it may sometimes be necessary to communicate fault status of
one of the locally attached PW segments at an S-PE. For LDP, this
can be accomplished by sending an LDP notification message containing
the PW status TLV, as well as an OPTIONAL PW Switching Point PE TLV
as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Notification (0x0001) | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|1| Status (0x0300) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|1| Status Code=0x00000028 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID=0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type=0 | PW Status TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW Status TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW Status TLV | PWid FEC or Generalized ID FEC|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| PWid FEC or Generalized ID FEC (contd.) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| SP-PE TLV (0x096D) | SP-PE TLV Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Variable Length Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Martini, et al. Standards Track [Page 31]
RFC 6073 Segmented Pseudowire January 2011
Only one SP-PE TLV can be present in this message. This message is
then relayed by each S-PE unchanged. The T-PE decodes the status
message and the included SP-PE TLV to detect exactly where the fault
occurred. At the T-PE, if there is no SP-PE TLV included in the LDP
status notification, then the status message can be assumed to have
originated at the remote T-PE.
The merging of the received LDP status and the local status for the
PW segments at an S-PE can be summarized as follows:
-i. When the local status for both PW segments is UP, the S-PE
passes any received AC or PW status bits unchanged, i.e.,
the status notification TLV is unchanged, but the PWid in
the case of a FEC 128 TLV is set to the value of the PW
segment of the next hop.
-ii. When the local status for any of the PW segments is at
fault, the S-PE always sends the local status bits
regardless of whether the received status bits from the
remote node indicated that an upstream fault has cleared.
AC status bits are passed along unchanged.
10.1. PW Status Messages Initiated by the S-PE
The PW fault directions are defined as follows:
+-------+
---PW1 Receive---->| |-----PW2 Transmit---->
S-PE1 | S-PE2 | S-PE3
<--PW1 Transmit----| |<----PW2 Receive------
+-------+
Figure 4: S-PE and PW Transmission/Reception Directions
When a local fault is detected by the S-PE, a PW status message is
sent in both directions along the PW. Since there are no attachment
circuits on an S-PE, only the following status messages are relevant:
0x00000008 - Local PSN-facing PW (ingress) Receive Fault
0x00000010 - Local PSN-facing PW (egress) Transmit Fault
Each S-PE needs to store only two 32-bit PW status words for each PW
segment: one for local failures and one for remote failures (normally
received from another PE). The first failure will set the
appropriate bit in the 32-bit status word, and each subsequent
failure will be ORed to the appropriate PW status word. In the case
Martini, et al. Standards Track [Page 32]
RFC 6073 Segmented Pseudowire January 2011
that the PW status word stores remote failures, this rule has the
effect of a logical OR operation with the first failure received on
the particular PW segment.
It should be noted that remote failures received on an S-PE are just
passed along the MS-PW unchanged, while local failures detected an
S-PE are signaled on both PW segments.
A T-PE can receive multiple failures from S-PEs along the MS-PW;
however, only the failure from the remote closest S-PE will be stored
(last PW status message received). The PW status word received is
just ORed to any existing remote PW status already stored on the
T-PE.
Given that there are two PW segments at a particular S-PE for a
particular MS-PW (referring to Figure 4), there are four possible
failure cases as follows:
-i. PW2 Transmit direction fault
-ii. PW1 Transmit direction fault
-iii. PW2 Receive direction fault
-iv. PW1 Receive direction fault
Once a PW status notification message is initiated at an S-PE for a
particular PW status bit, any further status message for the same
status bit (and received from an upstream neighbor) is processed
locally and not forwarded until the S-PE original status error state
is cleared.
Each S-PE along the MS-PW MUST store any PW status messages
transiting it. If more than one status message with the same PW
status bit set is received by a T-PE or S-PE, only the last PW status
message is stored.
10.1.1. Local PW2 Transmit Direction Fault
When this failure occurs, the S-PE will take the following actions:
* Send a PW status message to S-PE3 containing "0x00000010 - Local
PSN-facing PW (egress) Transmit Fault".
* Send a PW status message to S-PE1 containing "0x00000008 - Local
PSN-facing PW (ingress) Receive Fault".
* Store 0x00000010 in the local PW status word for the PW segment
toward S-PE3.
Martini, et al. Standards Track [Page 33]
RFC 6073 Segmented Pseudowire January 2011
10.1.2. Local PW1 Transmit Direction Fault
When this failure occurs, the S-PE will take the following actions:
* Send a PW status message to S-PE1 containing "0x00000010 - Local
PSN-facing PW (egress) Transmit Fault".
* Send a PW status message to S-PE3 containing "0x00000008 - Local
PSN-facing PW (ingress) Receive Fault".
* Store 0x00000010 in the local PW status word for the PW segment
toward S-PE1.
10.1.3. Local PW2 Receive Direction Fault
When this failure occurs, the S-PE will take the following actions:
* Send a PW status message to S-PE3 containing "0x00000008 - Local
PSN-facing PW (ingress) Receive Fault".
* Send a PW status message to S-PE1 containing "0x00000010 - Local
PSN-facing PW (egress) Transmit Fault".
* Store 0x00000008 in the local PW status word for the PW segment
toward S-PE3.
10.1.4. Local PW1 Receive Direction Fault
When this failure occurs, the S-PE will take the following actions:
* Send a PW status message to S-PE1 containing "0x00000008 - Local
PSN-facing PW (ingress) Receive Fault".
* Send a PW status message to S-PE3 containing "0x00000010 - Local
PSN-facing PW (egress) Transmit Fault".
* Store 0x00000008 in the local PW status word for the PW segment
toward S-PE1.
10.1.5. Clearing Faults
Remote PW status fault clearing messages received by an S-PE will
only be forwarded if there are no corresponding local faults on the
S-PE. (Local faults always supersede remote faults.)
Once the local fault has cleared, and there is no corresponding (same
PW status bit set) remote fault, a PW status message is sent out to
the adjacent PEs, clearing the fault.
Martini, et al. Standards Track [Page 34]
RFC 6073 Segmented Pseudowire January 2011
When a PW status fault clearing message is forwarded, the S-PE will
always send the SP-PE TLV associated with the PE that cleared the
fault.
10.2. PW Status Messages and SP-PE TLV Processing
When a PW status message is received that includes an SP-PE TLV, the
SP-PE TLV information MAY be stored, along with the contents of the
PW status Word according to the procedures described above. The
SP-PE TLV stored is always the SP-PE TLV that is associated with the
PE that set that particular last fault. If subsequent PW status
messages for the same PW status bit are received, the SP-PE TLV will
overwrite the previously stored SP-PE TLV.
10.3. T-PE Processing of PW Status Messages
The PW switching architecture is based on the concept that the T-PE
should process the PW LDP messages in the same manner as if it were
participating in the setup of a PW segment. However, a T-PE
participating in an MS-PW SHOULD be able to process the SP-PE TLV.
Otherwise, the processing of PW status messages and other PW setup
messages is exactly as described in [RFC4447].
10.4. Pseudowire Status Negotiation Procedures
Pseudowire status signaling methodology, defined in [RFC4447], SHOULD
be transparent to the switching point.
10.5. Status Dampening
When the PW control plane switching methodology is used to cross an
administrative boundary, it might be necessary to prevent excessive
status signaling changes from being propagated across the
administrative boundary. This can be achieved by using a similar
method as commonly employed for the BGP route advertisement
dampening. The details of this OPTIONAL algorithm are a matter of
implementation and are outside the scope of this document.
11. Peering between Autonomous Systems
The procedures outlined in this document can be employed to provision
and manage MS-PWs crossing AS boundaries. The use of more advanced
mechanisms involving auto-discovery and ordered PWE3 MS-PW signaling
will be covered in a separate document.
Martini, et al. Standards Track [Page 35]
RFC 6073 Segmented Pseudowire January 2011
12. Congestion Considerations
Each PSN carrying the PW may be subject to congestion. The
congestion considerations in [RFC3985] apply to PW segments as well.
Each PW segment will handle any congestion experienced by the PW
traffic independently of the other MS-PW segments. It is possible
that passing knowledge of congestion between segments and to the
T-PEs can result in more efficient edge-to-edge congestion mitigation
systems. However, any specific methods of congestion mitigation are
outside the scope of this document and left for further study.
13. Security Considerations
This document specifies the LDP, L2TPv3, and VCCV 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
and transmitted from one end of a pseudowire to the other.
Therefore, we discuss the security considerations for both the data
plane and the control plane in the following sections. The
guidelines and security considerations specified in [RFC5920] also
apply to MS-PW when the PSN is MPLS.
13.1. Data Plane Security
Data plane security considerations as discussed in [RFC4447],
[RFC3931], and [RFC3985] apply to this extension without any changes.
13.1.1. VCCV Security Considerations
The VCCV technology for MS-PW offers a method for the service
provider to verify the data path of a specific PW. This involves
sending a packet to a specific PE and receiving an answer that either
confirms the information contained in the packet or indicates that it
is incorrect. This is a very similar process to the commonly used IP
ICMP ping and TTL expired methods for IP networks. It should be
noted that when using VCCV Type 3 for PW when the CW is not enabled,
if a packet is crafted with a TTL greater than the number of hops
along the MS-PW path, or an S-PE along the path mis-processes the
TTL, the packet could mistakenly be forwarded out of the attachment
circuit as a native PW packet. This packet would most likely be
treated as an error packet by the CE. However, if this possibility
is not acceptable, the CW should be enabled to guarantee that a VCCV
packet will never be mistakenly forwarded to the AC.
Martini, et al. Standards Track [Page 36]
RFC 6073 Segmented Pseudowire January 2011
13.2. Control Protocol Security
General security considerations with regard to the use of LDP are
specified in Section 5 of RFC 5036. Security considerations with
regard to the L2TPv3 control plane are specified in [RFC3931]. These
considerations apply as well to the case where LDP or L2TPv3 is used
to set up PWs.
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
session request MUST NOT be accepted unless its IP source address is
known to be the source of an "eligible" 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.
(Note that if the auto-discovery protocol were not trusted, the set
of "eligible peers" it produces could not be trusted.)
Even if a connection request appears to come from an eligible peer,
its source address may have been spoofed. So 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.
For a greater degree of security, the LDP authentication option, as
described in Section 2.9 of [RFC5036], or the Control Message
Authentication option of [RFC3931], MAY be used. This provides
integrity and authentication for the control messages, and eliminates
the possibility of source address spoofing. Use of the message
authentication option does not provide privacy, but privacy of
control messages is not usually considered to be highly important.
Both the LDP and L2TPv3 message authentication options rely on the
configuration of pre-shared keys, making it difficult to deploy when
the set of eligible neighbors is determined by an auto-configuration
protocol.
The protocol described in this document relies on the LDP MD5
authentication key option, as described in Section 2.9 of [RFC5036],
to provide integrity and authentication for the LDP messages and
protect against source address spoofing. This mechanism relies on
the configuration of pre-shared keys, which typically introduces some
fragility. In the specific case of MS-PW, the number of links that
leave an organization will be limited in practice, so the reliance on
pre-shared keys should be manageable.
Martini, et al. Standards Track [Page 37]
RFC 6073 Segmented Pseudowire January 2011
When the Generalized PWid FEC Element is used, it is possible that a
particular peer may be one of the eligible 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.
14. IANA Considerations
14.1. L2TPv3 AVP
This document uses a new L2TP parameter; IANA already maintains the
registry "Control Message Attribute Value Pairs" defined by
[RFC3438]. The following new value has been assigned:
101 PW Switching Point AVP
14.2. LDP TLV TYPE
This document uses a new LDP TLV type; IANA already maintains the
registry "TLV TYPE NAME SPACE" defined by RFC 5036. The following
value has been assigned:
TLV type Description
0x096D Pseudowire Switching Point PE TLV
14.3. LDP Status Codes
This document uses a new LDP status code; IANA already maintains the
registry "STATUS CODE NAME SPACE" defined by RFC 5036. The following
value has been assigned:
Assignment E Description
0x0000003A 0 PW Loop Detected
14.4. L2TPv3 Result Codes
This document uses a new L2TPv3 Result Code for the CDN message, as
assigned by IANA in the "Result Code AVP (Attribute Type 1) Values"
registry.
Martini, et al. Standards Track [Page 38]
RFC 6073 Segmented Pseudowire January 2011
Registry Name: Result Code AVP (Attribute Type 1) Values Defined
Result Code values for the CDN message are:
Assignment Description
31 Sequencing not supported
14.5. New IANA Registries
IANA has set up a registry named "Pseudowire Switching Point PE sub-
TLV Type". These are 8-bit values. Type values 1 through 6 are
defined in this document. Type values 7 through 64 are to be
assigned by IANA using the "Expert Review" policy defined in
[RFC5226]. Type values 65 through 127, as well as 0 and 255, are to
be allocated using the IETF consensus policy defined in RFC 5226.
Type values 128 through 254 are reserved for vendor proprietary
extensions and are to be assigned by IANA, using the "First Come
First Served" policy defined in RFC 5226.
The Type Values are assigned as follows:
Type Length Description
0x01 4 PWid of last PW segment traversed
0x02 variable PW Switching Point description string
0x03 4/16 Local IP address of PW Switching Point
0x04 4/16 Remote IP address of last PW Switching Point
traversed or of the T-PE
0x05 variable FEC Element of last PW segment traversed
0x06 12 L2 PW address of PW Switching Point
15. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[RFC3931] Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed.,
"Layer Two Tunneling Protocol - Version 3 (L2TPv3)", RFC
3931, March 2005.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC 4379,
February 2006.
Martini, et al. Standards Track [Page 39]
RFC 6073 Segmented Pseudowire January 2011
[RFC4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson,
"Pseudowire Emulation Edge-to-Edge (PWE3) Control Word
for Use over an MPLS PSN", RFC 4385, February 2006.
[RFC4446] Martini, L., "IANA Allocations for Pseudowire Edge to
Edge Emulation (PWE3)", BCP 116, RFC 4446, April 2006.
[RFC4447] Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T.,
and G. Heron, "Pseudowire Setup and Maintenance Using
the Label Distribution Protocol (LDP)", RFC 4447, April
2006.
[RFC5003] Metz, C., Martini, L., Balus, F., and J. Sugimoto,
"Attachment Individual Identifier (AII) Types for
Aggregation", RFC 5003, September 2007.
[RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", RFC 5036, October 2007.
[RFC5085] Nadeau, T., Ed., and C. Pignataro, Ed., "Pseudowire
Virtual Circuit Connectivity Verification (VCCV): A
Control Channel for Pseudowires", RFC 5085, December
2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5641] McGill, N. and C. Pignataro, "Layer 2 Tunneling Protocol
Version 3 (L2TPv3) Extended Circuit Status Values", RFC
5641, August 2009.
16. Informative References
[PW-MSG-MAP] Aissaoui, M., Busschbach, P., Morrow, M., Martini, L.,
Stein, Y(J)., Allan, D., and T. Nadeau, "Pseudowire (PW)
OAM Message Mapping", Work in Progress, October 2010.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
[RFC3438] Townsley, W., "Layer Two Tunneling Protocol (L2TP)
Internet Assigned Numbers Authority (IANA)
Considerations Update", BCP 68, RFC 3438, December 2002.
Martini, et al. Standards Track [Page 40]
RFC 6073 Segmented Pseudowire January 2011
[RFC3985] Bryant, S., Ed., and P. Pate, Ed., "Pseudo Wire
Emulation Edge-to-Edge (PWE3) Architecture", RFC 3985,
March 2005.
[RFC4023] Worster, T., Rekhter, Y., and E. Rosen, Ed.,
"Encapsulating MPLS in IP or Generic Routing
Encapsulation (GRE)", RFC 4023, March 2005.
[RFC4454] Singh, S., Townsley, M., and C. Pignataro, "Asynchronous
Transfer Mode (ATM) over Layer 2 Tunneling Protocol
Version 3 (L2TPv3)", RFC 4454, May 2006.
[RFC4623] Malis, A. and M. Townsley, "Pseudowire Emulation Edge-
to-Edge (PWE3) Fragmentation and Reassembly", RFC 4623,
August 2006.
[RFC4667] Luo, W., "Layer 2 Virtual Private Network (L2VPN)
Extensions for Layer 2 Tunneling Protocol (L2TP)", RFC
4667, September 2006.
[RFC4717] Martini, L., Jayakumar, J., Bocci, M., El-Aawar, N.,
Brayley, J., and G. Koleyni, "Encapsulation Methods for
Transport of Asynchronous Transfer Mode (ATM) over MPLS
Networks", RFC 4717, December 2006.
[RFC5254] Bitar, N., Ed., Bocci, M., Ed., and L. Martini, Ed.,
"Requirements for Multi-Segment Pseudowire Emulation
Edge-to-Edge (PWE3)", RFC 5254, October 2008.
[RFC5659] Bocci, M. and S. Bryant, "An Architecture for Multi-
Segment Pseudowire Emulation Edge-to-Edge", RFC 5659,
October 2009.
[RFC5920] Fang, L., Ed., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, July 2010.
Martini, et al. Standards Track [Page 41]
RFC 6073 Segmented Pseudowire January 2011
17. Acknowledgments
The authors wish to acknowledge the contributions of Satoru
Matsushima, Wei Luo, Neil Mcgill, Skip Booth, Neil Hart, Michael Hua,
and Tiberiu Grigoriu.
18. Contributors
The following people also contributed text to this document:
Florin Balus
Alcatel-Lucent
701 East Middlefield Rd.
Mountain View, CA 94043
US
EMail: florin.balus@alcatel-lucent.com
Mike Duckett
Bellsouth
Lindbergh Center, D481
575 Morosgo Dr
Atlanta, GA 30324
US
EMail: mduckett@bellsouth.net
Martini, et al. Standards Track [Page 42]
RFC 6073 Segmented Pseudowire January 2011
Authors' Addresses
Luca Martini
Cisco Systems, Inc.
9155 East Nichols Avenue, Suite 400
Englewood, CO 80112
US
EMail: lmartini@cisco.com
Chris Metz
Cisco Systems, Inc.
EMail: chmetz@cisco.com
Thomas D. Nadeau
EMail: tnadeau@lucidvision.com
Matthew Bocci
Alcatel-Lucent
Grove House, Waltham Road Rd
White Waltham, Berks SL6 3TN
UK
EMail: matthew.bocci@alcatel-lucent.co.uk
Mustapha Aissaoui
Alcatel-Lucent
600, March Road,
Kanata, ON
Canada
EMail: mustapha.aissaoui@alcatel-lucent.com
Martini, et al. Standards Track [Page 43]