Network Working Group Luca Martini
Internet Draft Chris Metz
Expiration Date: January 2006 Thomas D. Nadeau
Cisco Systems Inc.
Vasile Radoaca Mike Duckett
Nortel Networks Bellsouth
Matthew Bocci Florin Balus
Alcatel Nortel Networks
July 2005
Segmented Pseudo Wire
draft-ietf-pwe3-segmented-pw-00.txt
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Abstract
This document describes how to connect pseudo wires (PW) between two
distinct PW control planes or PSN domains. The PW control planes 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.
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Table of Contents
1 Specification of Requirements ........................ 4
2 Terminology .......................................... 4
3 Introduction ......................................... 4
4 General Description .................................. 6
5 PW Switching with Attachment Circuits ................ 9
6 PW-MPLS to PW-MPLS Control Plane Switching ........... 9
6.1 MPLS PW Control Plane Switching ...................... 9
6.1.1 Static Control plane switching ....................... 10
6.1.2 Two LDP control planes using the same FEC type ....... 10
6.1.3 LDP using FEC 128 to LDP using the generalized FEC 129 ...10
6.1.4 PW switching point TLV ............................... 11
6.1.5 Adaptation of Interface Parameters ................... 12
6.1.6 Group ID ............................................. 13
6.1.7 PW Loop Detection .................................... 13
7 PW-MPLS to PW-L2TPv3 Control Plane Switching ......... 13
7.1 Static MPLS and L2TPv3 PWs ........................... 13
7.2 Static MPLS PW and Dynamic L2TPv3 PW ................. 14
7.3 Static L2TPv3 PW and Dynamic LDP/MPLS PW ............. 14
7.4 Dynamic LDP/MPLS and L2TPv3 PWs ...................... 14
7.4.1 Session Establishment ................................ 14
7.4.2 Adaptation of PW Status message ...................... 15
7.4.3 Session Teardown ..................................... 15
7.5 Adaptation of LDP/L2TPv3 AVPs and Interface Parameters ...16
7.6 Switching Point TLV in L2TPv3 ........................ 17
7.7 L2TPv3 and MPLS PW Data Plane ........................ 17
7.7.1 PWE3 Payload Convergence and Sequencing .............. 18
7.7.2 Mapping .............................................. 18
8 Operation And Management ............................. 19
8.1 Pseudo Wire Status ................................... 19
8.2 Pseudowire Status Negotiation Procedures ............. 21
8.3 Status Dampening ..................................... 21
9 Peering Between Autonomous Systems ................... 21
10 Security Considerations .............................. 21
10.1 Data Plane Security .................................. 21
10.2 Control Protocol Security ............................ 22
11 IANA Considerations .................................. 23
12 Intellectual Property Statement ...................... 23
13 Full Copyright Statement ............................. 23
14 Acknowledgments ...................................... 24
15 Normative References ................................. 24
16 Informative References ............................... 24
17 Author Information ................................... 25
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1. 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
2. Terminology
- Ultimate PE (U-PE). A PE where the customer-facing ACs are bound
to a PW forwarder. An ultimate PE is present in the first and
last segments of a MH-PW.
- Single-Hop PW (SH-PW). A PW set up between two PE devices using
standard PWE3 signaling and encapsulation methods.
- Multi-hop PW (MH-PW). Static or dynamically configured set of two
or more contiguous PW segments (SH-PW) that behave and function
as a single point-to-point PW. Each PW segment is setup and
managed using standard PWE3 encapsulation and signaling methods.
- PW Switching Point. (S-PE)A PE capable of switching the control
and data planes of the preceding and succeeding PW segments (SH-
PW) in a MH-PW. A PW Switching Point is never the ultimate PE in
a MH-PW. A PW switching point runs standard PWE3 protocols to
setup and manage PW segments with other PW switching points and
ultimate PEs.
3. Introduction
PWE3 defines the signaling and encapsulation techniques for
establishing SH-PWs between a pair of ultimate PEs and in the vast
majority of cases this will be sufficient. MH-PWs come into play in
two general cases:
-i. When it is not possible, desirable or feasible to establish
a PW control channel between the ultimate source and
destination PEs. At a minimum PW control channel
establishment requires knowledge of and reachability to the
remote (ultimate) PE IP address. The local (ultimate) PE may
not have access to this information related to topology,
operational or security constraints.
An example is the inter-AS L2VPN scenario where the ultimate
PEs reside in different provider networks (ASes) and it is
the practice to MD5-key all control traffic exchanged
between two networks. Technically a SH-PW could be used but
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this would require MD5-keying on ALL ultimate source and
destination PE nodes. An MH-PW allows the providers to
confine MD5 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 ultimate PEs is computed by an
external entity (i.e. client-layer routing protocol). Assume
a full mesh of PWE3 control channels established between
PE-A, PE-B and PE-C. A client-layer L2 connection tunneled
through a PW is required between ultimate 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 ultimate PE-A
and ultimate 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 U-PEs.
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 U-PE.
-ii. PWE3 signaling and encapsulation protocols are different.
The ultimate PEs are connected to networks employing
different PW signaling and encapsulation protocols. In this
case it is not possible to use a SH-PW. A MH-PW with the
appropriate interworking performed at the PW switching
points can enable PW connectivity between the ultimate PEs
in this scenario.
There are four different signaling protocols that are defined to
signal PWs:
-i. Static configuration of the PW (MPLS or L2TPv3).
-ii. LDP using FEC 128
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-iii. LDP using the generalized FEC 129
-iv. L2TPv3
4. General Description
A pseudo-wire (PW) is a tunnel established between two provider edge
(PE) nodes to transport L2 PDUs across a packet switched network
(PSN) as described in Figure 1 and in [PWE3-ARCH]. Many providers are
looking at PWs as a means of migrating existing (or building new)
L2VPN services (i.e. Frame-Relay, ATM, Ethernet) on top of a PSN by
using PWs. PWs may span multiple autonomous systems 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 [2547BIS]. It is also straightforward to support the same
inter-AS L2VPN functionality employing L2TPv3. In this document we
define methodology to switch a PW between two PW control planes.
|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudo Wire ------>| |
| | | |
| | |<-- 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 control
planes. In the first method (Figure 2), the two control planes
terminate on different PEs.
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|<------------Emulated Service---------->|
| 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 stitching points |
| |
| |
|<-------------------- Emulated Service ---------------->|
Figure 2: PW Switching using ACs Reference Model
In Figure 2, pseudo wires 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 |<-----------Pseudo Wire----------->| Native
Layer2 | | Layer2
Service | |<-PSN1-->| |<--PSN2->| | Service
(AC) V V V V V V (AC)
| +----+ +-----+ +----+ |
+----+ | | PE1|=========| PE2 |=========| PE3| | +----+
| |----------|........PW1.........|...PW3........|----------| |
| CE1| | | | | | | | | |CE2 |
| |----------|........PW2.........|...PW4........|----------| |
+----+ | | |=========| |=========| | | +----+
^ +----+ +-----+ +----+ | ^
| Provider Edge 1 ^ Provider Edge 3 |
| (Ultimate PE) | (Ultimate PE) |
| | |
| PW switching point |
| (Optional PW adaptation function) |
| |
|<------------------- Emulated Service ------------------>|
Figure 3: PW Control Plane Switching Reference Model
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In Figure 3 PE2 runs two separate control planes: one toward PE1, and
one Toward PE3. The PW switching point is at PE2 which is configured
to connect PW1 and PW3 together to complete the multi-hop PW between
PE1 and PE3. PW1 and PW3 MUST be of the same PW type, but PSN1 and
PSN2 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 PSN1 and PSN2 are the same technology the PW PDU does not need
to be modified, and PDUs are then switched between the pseudo-wires
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 or GRE [MPLS-IP-GRE]
encapsulation, but this is outside the scope of this document.
Further, one could perform an interworking function on the PWs
themselves at the PW switching point, allowing conversion from one PW
type to another, but this is also outside the scope of this document.
The pseudowire switching methodology described in this document
assumes manual configuration of the switching point at PE2. 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 PW switching points the PW has traversed
(though this may be reported for troubleshooting purposes).
In general the approach taken is to connect the individual control
planes by passing along any signaling information immediately upon
reception. First the PW switching point is configured to switch a
SH-PW from a specific peer to another SH-PW destined for a different
peer. No control messages are exchanged yet as the PW switching point
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 is setup. In this model the MH-PW setup
is starting from the U-PE devices. Next once the U-PE is configured
it sends the PW control setup messages. These messages are received,
and immediately used to form the PW setup messages for the next SH-PW
of the MH-PW. If one of the Switching PEs doesn't accept an LDP Setup
message then a Label release message is sent back to the originator
U-PE. A MH-PW is declared UP when all the constituent SH-PWs are UP.
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5. PW Switching with Attachment Circuits
In PW switching with attachment circuits, each PSN may be of a
different type e.g. MPLS, L2TPv3. However the details of this are
outside the scope of this document. The PWs and the attachment
circuits MUST be of the same type e.g. ATM, Ethernet, etc.
The PWs in each PSN are established independently, with each PSN
being treated as a separate PWE3 domain. For example, in Figure 2 for
case of MPLS PSNs, PW1 is setup between PE1 and PE2 using the LDP
targeted session as described in [PWE3-MPLS], and at the same time a
separate pseudo wire, 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 native circuit e.g. ATM VCC, Ethernet VLAN, etc. These ACs thus
connect the PWs in PSN1 and PSN2 together.
6. PW-MPLS to PW-MPLS Control Plane Switching
Referencing Figure 3, PE2 sets up a PW1 using the LDP targeted
session as described in [PWE3-MPLS], at the same time a separate
pseudo wire PW3 is setup to PE3. Each PW is configured independently
on the PEs, but on PE2 pseudo wire PW1 is connected to pseudo wire
PW3. PDUs are then switched between the pseudo-wires at the PW label
level. Hence the data plane does not need any special knowledge of
the specific pseudo wire type. A simple standard MPLS label swap
operation is sufficient to connect the two PWs, and in this case the
PW adaptation function is not used.
This process can be repeated as many times as necessary, the only
limitation to the number of PW switching points traversed is imposed
by the TTL field of the PW MPLS Label. The setting of the TTL is a
matter of local policy on the originating PE, but SHOULD be set to
255.
6.1. MPLS PW Control Plane Switching
There are three MPLS to MPLS PW control planes:
-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:
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-i. PW Switching between two static control planes.
-ii. Static Control plane switching to LDP dynamic control plane.
-iii. Two LDP control planes using the same FEC type
-iv. LDP using FEC 128, to LDP using the generalized FEC 129
6.1.1. Static Control plane switching
In the case of two static control planes the PW switching point 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 plane is a dynamic LDP FEC 128 or generalized PW FEC, 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 proper 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 LDP PW will be limited to local interface failures. In this
case, the PW switching point PE behaves in a very similar manner to a
U-PE, assuming an active role.
6.1.2. Two LDP control planes using the same FEC type
As stated in a section above, the PW switching point PE should assume
an initial passive role. This means that once independent PWs are
configured on the switching point, the 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
priory what the interface parameter field in in the initial FEC
advertisement will contain.
The PWID is a unique number between each pair of PEs. Hence Each SH-
PW that forms an MH-PW may have a different PWID. In the case of The
Generalized PW FEC, the AGI/SAI/TAI may have to also be different for
some, or sometimes all, SH-PWs.
6.1.3. 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 PW
switching point PE, will always remain passive until a PWID FEC 128
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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 PW switching point
configuration, or by some other means, such as some form of auto
discovery. Such other means are outside the scope of this document.
6.1.4. PW switching point 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 all the switching points that the PW
traverses. This is accomplished by using a PW switching point TLV.
The OPTIONAL PW switching point TLV is appended to the PW FEC at each
switching point and 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| PW sw TLV (0x096B) | PW sw TLV Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Variable Length Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Variable Length Value |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- PW sw TLV Length
Specifies the total length of all the following PW switching
point TLV fields in octets
- Type
Encodes how the Value field is to be interpreted.
- Length
Specifies the length of the Value field in octets.
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- Value
Octet string of Length octets that encodes information to be
interpreted as specified by the Type field.
Type value 0 is reserved. Type values 1 through 3 are defined in this
document. Values 3 through 63 are to be assigned by IANA using the
"IETF Consensus" policy defined in RFC2434. Type values values 64
through 127 are to be assigned by IANA, using the "First Come First
Served" policy defined in RFC2434. Type values 128 through 255 are
vendor-specific, and values in this range are not to be assigned by
IANA.
The Type Values are assigned as follows:
Type Length Description
0x00 0 Reserved
0x01 4 PW ID of last PW traversed
0x02 variable PW Switching Point description string
0x03 4 IP address of PW Switching Point ( Optional )
The PW switching Point TLV is an OPTIONAL TLV that can appear once
for each switching point traversed.
6.1.5. Adaptation of Interface Parameters
[PWE3-MPLS] 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 end points, and MUST be passed unchanged across the PW switching
point. 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 [PWE3-FRAG].
- 0x0C VCCV parameter The switching point MAY not be able to
inspect the VCCV control channel. In this case the Control
Channel Type indicator MUST be modified to reflect the capability
of the PE acting as the PW switching point.
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6.1.6. 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 PW switching point PE.
6.1.7. PW Loop Detection
A switching point PE SHOULD check the OPTIONAL PW switching Point
TLV, to verify if it's own IP address appears in it. If it's IP
address appears in a received PW switching Point TLV, the PE SHOULD
break the loop, and send a label release message with the following
error code:
0x0000003A "PW Loop Detected"
[ note: error code as defined in [PWE-IANA] pending IANA allocation ]
7. PW-MPLS to PW-L2TPv3 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 MPLS and L2TPv3 static control planes.
-ii. Switching between a static MPLS PW and a dynamic L2TPv3 PW.
-iii. Switching between a static L2TPv3 PW and a dynamic MPLS PW.
-iv. Switching between a dynamic MPLS PW and a dynamic L2TPv3 PW.
7.1. Static MPLS and L2TPv3 PWs
In the case of two static control planes, the PW switching point 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 VC 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.
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7.2. 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 proper 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 PW
switching point PE behaves in a very similar manner to a U-PE,
assuming an active role.
7.3. 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 proper PW status
(via an L2TPv3 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 PW
switching point PE behaves in a very similar manner to a U-PE,
assuming an active role.
7.4. 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 ICRQ) from one side of the node. Note that while
MPLS PWs are made up of two unidirectional LSPs bonded together by
FEC identifiers, L2TPv3 PWs are bidirectional in nature, setup via a
3-message exchange (ICRQ, ICRP and ICCN). Details of Session
Establishment, Teardown, and PW Status signalling are detailed below.
7.4.1. Session Establishment
When the PW switching point 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
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an ICRQ.
Following are two example exchanges of messages between LDP and
L2TPv3. The first is a case where an L2TPv3 U-PE initiates an MH-PW,
the second is a case where an MPLS U-PE initiates an MH-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 --->
<-------------------- MH 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"
<-------------------- MH PW Established ------------------>
7.4.2. Adaptation of PW Status message
L2TPv3 uses the SLI message to indicate a 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
message defined in section 5.3.2 of [PWE3-MPLS].
7.4.3. Session Teardown
L2TPv3 uses a single message, CDN, to tear down a pseudowire. The CDN
message translates to a Label Withdraw message in LDP. Following are
two example exchanges of messages between LDP and L2TPv3. The first
is a case where an L2TPv3 U-PE initiates the termination of an MH-PW,
the second is a case where an MPLS U-PE initiates the termination of
an MH-PW.
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PE 1 (L2TPv3) PW Switching Node PE3 (MPLS/LDP)
AC "Down"
L2TPv3 CDN --->
LDP Label Withdraw --->
AC "Down"
<--------------- MH PW Data Path Down ------------------>
<--- LDP Label Withdraw *
* This LDP Label Withdraw is not necessary to break the end-to-end
MH PW data path.
PE 1 (MPLS LDP) PW Switching Node PE3 (L2TPv3)
AC "Down"
LDP Label Withdraw --->
L2TPv3 CDN -->
AC "Down"
<---------------- MH PW Data Path Down ------------------>
<--- LDP Label Withdraw *
* This LDP Label Withdraw is not necessary to break the end-to-end
MH PW data path.
7.5. Adaptation of LDP/L2TPv3 AVPs and Interface Parameters
[PWE3-MPLS] defines several interface parameters which must be mapped
to similar AVPs in L2TPv3 setup messages.
* Interface MTU
The Interface MTU parameter is mapped directly to the L2TP
Interface MTU AVP defined in [L2TP-L2VPN]
* 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 [L2TP-
ATM].
* Optional Interface Description String
This string may be carried as the "Call-Information AVP"
described in section 2.2 of [L2TP-INFOMSG]
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* "PW Type
The PW Type defined in [PWE3-IANA] is mapped to the L2TPv3 "PW
Type" AVP defined in [L2TPv3].
* PW ID (FEC 128)
For FEC 128, the PW ID is mapped directly to the L2TPv3 "Remote
End ID" AVP defined in [L2TPv3].
* Generalized FEC 129 SAI/TAI
Section 4.3 of [L2TP-L2VPN] defines how to encode the SAI and TAI
parameters. These can be mapped directly.
Other interface parameter mappings will either be defined in a future
version of this document, or are unsupported when switching between
LDP/MPLS and L2TPv3 PWs.
7.6. Switching Point TLV in L2TPv3
When translating between LDP and L2TPv3 control messages, the PW
Switching Point TLV described earlier in this document is carried in
a single variable length L2TP AVP present in the ICRQ, ICRP messages,
and optionally in the ICCN message.
The L2TP "Switching Point AVP" is Attribute Type TBA-L2TP-AVP-1. 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 sub-
TLVs included in the AVP, and the AVP MUST NOT be marked Mandatory
(the L2TP AVP M-bit MUST be 0).
7.7. 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. There are some
situations where an additional amount of interworking must be
provided between the two data planes at the PW switching node.
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7.7.1. PWE3 Payload Convergence and Sequencing
Section 5.4 of [PWE3-ARCH] 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 [L2TPv3].
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.
7.7.2. Mapping
The MPLS Control Word consists of (from left to right):
-i. These bits are always zero in MPLS are not necessary to be
mapped 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 [PWE3-ATM]. These map directly to the bits
defined in [L2TP-ATM]. For Frame Relay, these bits indicate
how to set the bits in the Frame Relay header which 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.
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-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 MH-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 proper format for
the PW on egress. If this type of sequence interworking at
the switching node is not supported, and a U-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 17 "sequencing not supported" (subject to
IANA Assignment).
8. Operation And Management
8.1. Pseudo Wire Status
In the PW switching with attachment circuits case (Figure 2), PW
status messages indicating PW or attachment circuit faults SHOULD be
mapped to fault indications or OAM messages on the connecting AC as
defined in [PW-MSG-MAP]. If the AC connecting two PWs crosses an
administrative boundary, then the manner in which those OAM messages
are treated at the boundary is out of scope of this draft.
In the PW control plane switching case (Figure 3), there is no
attachment circuit at the PW switching point, but the two PWs are
connected together. Similarly, the status of the PWs are forwarded
unchanged from one PW to the other by the control plane switching
function. However, it may sometimes be necessary to communicate
status of one of the locally attached SH-PW at a PW switching point.
For LDP this can be accomplished by sending an LDP status
notification message containing the PW status TLV, as well as an
OPTIONAL PW switching point TLV.
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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
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| PWId FEC or Generalized ID FEC |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| PW sw TLV (0x096B) | PW sw TLV Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Variable Length Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Only one PW switching point TLV can be present in this message. This
message is then relayed by each PW switching point unchanged. The U-
PE decodes the status message and the included PW switching point TLV
to detect where exactly the fault occurred. At the U-PE if there is
no PW switching point TLV included in the LDP status notification
then the status message can be assumed to have originated at the
remote U-PE. It should also be noted that once a PW status
notification message is initiated at a PW switching point, any
further status message received from an upstream neighbor is
processed locally and not forwarded until the PW switching point
original status message is cleared. When a PW status event, for a
particular PW, occurs at the S-PE, the appropriate PW status
notification MUST be send toward both remote S-PEs or U-PEs attached
to the PW.
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8.2. Pseudowire Status Negotiation Procedures
Pseudowire Status signaling methodology, defined in [PWE3-MPLS],
SHOULD be transparent to the switching point.
8.3. 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 protocol route advertisement
dampening. The details of this OPTIONAL algorithm are a matter of
implementation, and are outside the scope of this document.
9. Peering Between Autonomous Systems
The procedures outlined in this document can be employed to provision
and manage MH-PWs crossing AS boundaries.
The use of more advanced mechanisms involving auto-discovery and
ordered PWE3 MH-PW signaling will be covered ina separate document.
10. Security Considerations
This document specifies the LDP and L2TPv3 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
treat the security considerations for both the data plane and the
control plane.
10.1. Data Plane Security
Data plane security consideration as discussed in [PWE3-MPLS],
[L2TPv3], and [PWE3-ARCH] apply to this extension without any
changes.
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10.2. Control Protocol Security
General security considerations with regard to the use of LDP are
specified in section 5 of RFC 3036. Security considerations with
regard to the L2TPv3 control plane are specified in [L2TPv3]. 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 which 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 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 MD5 authentication key
option, as described in section 2.9 of RFC 3036, or the Control
Message Authentication option of [L2TPv3] 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 are not usually considered to be highly urgent.
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.
When the Generalized ID 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 having just a single set of
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eligible peers associated with the PE as a whole.
11. IANA Considerations
This specification requires IANA to define the following L2TP
Parameters according to [RFC3438]:
Control Message Attribute Value Pairs
TBA-L2TP-AVP-1 - PW Switching Point AVP
12. Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
13. Full Copyright Statement
Copyright (C) The Internet Society (2005).
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
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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.
14. Acknowledgments
The authors wish to acknowledge the contributions of Wei Luo, and
Skip Booth.
15. Normative References
[PWE3-MPLS] "Transport of Layer 2 Frames Over MPLS", Martini, L.,
et al.,draft-ietf-pwe3-control-protocol-16.txt,
( work in progress ), May 2005.
[2547BIS] "BGP/MPLS IP VPNs", Rosen, E, Rekhter, Y.
draft-ietf-l3vpn-rfc2547bis-03.txt ( work in progress ),
October 2004.
[L2TPv3] "Layer Two Tunneling Protocol (Version 3)", J. Lau,
M. Townsley, I. Goyret, RFC3931
16. Informative References
[MPLS-IP-GRE] "Encapsulating MPLS in IP or Generic
Routing Encapsulation (GRE)", Rosen, E, Rekhter, Y.
draft-ietf-mpls-in-ip-or-gre-08.txt, ( work inprogress ),
December 2004.
[PWE3-ARCH] "PWE3 Architecture" Bryant, et al.,
draft-ietf-pwe3-arch-07.txt ( workin progress ), June 2003.
[PWE3-FRAG] "PWE3 Fragmentation and Reassembly", A. Malis,
W. M. Townsley, draft-ietf-pwe3-fragmentation-05.txt
( work in progress ) February 2004
[PWE-IANA] "IANA Allocations for pseudo Wire Edge to
Edge Emulation (PWE3)" Martini, Townsley,
draft-ietf-pwe3-iana-allocation-04.txt (work in progress),
April 2004
[L2TP-L2VPN] "L2VPN Extensions for L2TP", Luo, Wei,
draft-ietf-l2tpext-l2vpn-00.txt, (work in progress), Jan 2004
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[L2TP-INFOMSG] "L2TP Call Information Messages", Mistretta,
Goyret, McGill, Townsley, draft-mistretta-l2tp-infomsg-02.txt,
(work in progress), July 2004
[L2TP-ATM] "ATM Pseudo-Wire Extensions for L2TP", Singh,
Townsley, Lau, draft-ietf-l2tpext-pwe3-atm-00.txt,
(work in progress), March 2004.
[PWE3-ATM] "Encapsulation Methods for Transport of ATM
Over IP and MPLS Networks", Martini, Rosen, Bocci,
"draft-ietf-pwe3-atm-encap-05.txt", (work in progress),
April 2004.
[RFC3438] W. M. Townsley, "Layer Two Tunneling Protocol
(L2TP) Internet"
[BCP68] Assigned Numbers Authority (IANA) Considerations
Update", RFC 3438, BCP 68, November 2002.
[PW-MSG-MAP] "Pseudo Wire (PW) OAM Message Mapping", Nadeau et al,
draft-ietf-pwe3-oam-msg-map-02.txt, (work in progress), February
2005
17. Author Information
Luca Martini
Cisco Systems, Inc.
9155 East Nichols Avenue, Suite 400
Englewood, CO, 80112
e-mail: lmartini@cisco.com
Thomas D. Nadeau
Cisco Systems, Inc.
300 Beaver Brook Road
Boxborough, MA 01719
e-mail: tnadeau@cisco.com
Chris Metz
Cisco Systems, Inc.
e-mail: chmetz@cisco.com
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Internet Draft draft-ietf-pwe3-segmented-pw-00.txt July 2005
Mike Duckett
Bellsouth
Lindbergh Center
D481
575 Morosgo Dr
Atlanta, GA 30324
e-mail: mduckett@bellsouth.net
Vasile Radoaca
Nortel Networks
600 Technology Park,
Billerica, 01821, MA
e-mail: vasile@nortelnetworks.com
Matthew Bocci
Alcatel
Grove House, Waltham Road Rd
White Waltham, Berks, UK. SL6 3TN
e-mail: matthew.bocci@alcatel.co.uk
Florin Balus
Nortel Networks
3500 Carling Ave.
Ottawa, Ontario, CANADA
e-mail: balus@nortel.com
Martini, et al. [Page 26]
