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Network Working Group M. Bocci
Request for Comments: 5659 Alcatel-Lucent
Category: Informational S. Bryant
Cisco Systems
October 2009
An Architecture for Multi-Segment Pseudowire Emulation Edge-to-Edge
Abstract
This document describes an architecture for extending pseudowire
emulation across multiple packet switched network (PSN) segments.
Scenarios are discussed where each segment of a given edge-to-edge
emulated service spans a different provider's PSN, as are other
scenarios where the emulated service originates and terminates on the
same provider's PSN, but may pass through several PSN tunnel segments
in that PSN. It presents an architectural framework for such multi-
segment pseudowires, defines terminology, and specifies the various
protocol elements and their functions.
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright and License Notice
Copyright (c) 2009 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 BSD License.
Bocci & Bryant Informational [Page 1]
RFC 5659 Multi-Segment PWE3 Architecture October 2009
Table of Contents
1. Introduction ....................................................3
1.1. Motivation and Context .....................................3
1.2. Non-Goals of This Document .................................6
1.3. Terminology ................................................6
2. Applicability ...................................................8
3. Protocol Layering Model .........................................8
3.1. Domain of MS-PW Solutions ..................................9
3.2. Payload Types ..............................................9
4. Multi-Segment Pseudowire Reference Model ........................9
4.1. Intra-Provider Connectivity Architecture ..................11
4.1.1. Intra-Provider Switching Using ACs .................11
4.1.2. Intra-Provider Switching Using PWs .................11
4.2. Inter-Provider Connectivity Architecture ..................11
4.2.1. Inter-Provider Switching Using ACs .................12
4.2.2. Inter-Provider Switching Using PWs .................12
5. PE Reference Model .............................................13
5.1. Pseudowire Pre-Processing .................................13
5.1.1. Forwarding .........................................13
5.1.2. Native Service Processing ..........................14
6. Protocol Stack Reference Model .................................14
7. Maintenance Reference Model ....................................15
8. PW Demultiplexer Layer and PSN Requirements ....................16
8.1. Multiplexing ..............................................16
8.2. Fragmentation .............................................17
9. Control Plane ..................................................17
9.1. Setup and Placement of MS-PWs .............................17
9.2. Pseudowire Up/Down Notification ...........................18
9.3. Misconnection and Payload Type Mismatch ...................18
10. Management and Monitoring .....................................18
11. Congestion Considerations .....................................19
12. Security Considerations .......................................20
13. Acknowledgments ...............................................23
14. References ....................................................23
14.1. Normative References .....................................23
14.2. Informative References ...................................23
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RFC 5659 Multi-Segment PWE3 Architecture October 2009
1. Introduction
RFC 3985 [1] defines the architecture for pseudowires, where a
pseudowire (PW) both originates and terminates on the edge of the
same packet switched network (PSN). The PW label is unchanged
between the originating and terminating provider edges (PEs). This
is now known as a single-segment pseudowire (SS-PW).
This document extends the architecture in RFC 3985 to enable point-
to-point pseudowires to be extended through multiple PSN tunnels.
These are known as multi-segment pseudowires (MS-PWs). Use cases for
multi-segment pseudowires (MS-PWs), and the consequent requirements,
are defined in RFC 5254 [5].
1.1. Motivation and Context
RFC 3985 addresses the case where a PW spans a single segment between
two PEs. Such PWs are termed single-segment pseudowires (SS-PWs) and
provide point-to-point connectivity between two edges of a provider
network. However, there is now a requirement to be able to construct
multi-segment pseudowires. These requirements are specified in RFC
5254 [5] and address three main problems:
i. How to constrain the density of the mesh of PSN tunnels when the
number of PEs grows to many hundreds or thousands, while
minimizing the complexity of the PEs and P-routers.
ii. How to provide PWs across multiple PSN routing domains or areas
in the same provider.
iii. How to provide PWs across multiple provider domains and
different PSN types.
Consider a single PW domain, such as that shown in Figure 1. There
are 4 PEs, and PWs must be provided from any PE to any other PE.
PWs can be supported by establishing a full mesh of PSN tunnels
between the PEs, requiring a full mesh of LDP signaling adjacencies
between the PEs. PWs can therefore be established between any PE and
any other PE via a single, direct PSN tunnel that is switched only by
intermediate P-routers (not shown in the figure). In this case, each
PW is an SS-PW. A PE must terminate all the pseudowires that are
carried on the PSN tunnels that terminate on that PE, according to
the architecture of RFC 3985. This solution is adequate for small
numbers of PEs, but the number of PEs, PSN tunnels, and signaling
adjacencies will grow in proportion to the square of the number of
PEs.
Bocci & Bryant Informational [Page 3]
RFC 5659 Multi-Segment PWE3 Architecture October 2009
For reasons of economy, the edge PEs that terminate the attachment
circuits (ACs) are often small devices built to very low cost with
limited processing power. Consider an example where a particular PE,
residing at the edge of a provider network, terminates N PWs to/from
N different remote PEs. This needs N PW signaling adjacencies to be
set up and maintained. If the edge PE attaches to a single
intermediate PE that is able to switch the PW, that edge PE only
needs a single adjacency to signal and maintain all N PWs. The
intermediate switching PE (which is a larger device) needs M
signaling adjacencies, but statistically this is less than tN, where
t is the number of edge PEs that it is serving. Similarly, if the
PWs are running over TE PSN tunnels, there is a statistical reduction
in the number of TE PSN tunnels that need to be set up and maintained
between the various PEs.
One possible solution that is more efficient for large numbers of
PEs, in particular for the control plane, is therefore to support a
partial mesh of PSN tunnels between the PEs, as shown in Figure 1.
For example, consider a PW service whose endpoints are PE1 and PE4.
Pseudowires for this can take the path PE1->PE2->PE4 and, rather than
terminating at PE2, be switched between ingress and egress PSN
tunnels on that PE. This requires a capability in PE2 that can
concatenate PW segments PE1-PE2 to PW segments PE2-PE4. The end-to-
end PW is known as a multi-segment PW.
,,..--..,,_
.-`` `'.,
+-----+` '+-----+
| PE1 |---------------------| PE2 |
| |---------------------| |
+-----+ PSN Tunnel +-----+
/ || || \
/ || || \
| || || |
| || PSN || |
| || || |
\ || || /
\ || || /
\|| ||/
+-----+ +-----+
| PE3 |---------------------| PE4 |
| |---------------------| |
+-----+`'.,_ ,.'` +-----+
`'''---''``
Figure 1: PWs Spanning a Single PSN with Partial Mesh of PSN Tunnels
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RFC 5659 Multi-Segment PWE3 Architecture October 2009
Figure 1 shows a simple, flat PSN topology. However, large provider
networks are typically not flat, consisting of many domains that are
connected together to provide edge-to-edge services. The elements in
each domain are specialized for a particular role, for example,
supporting different PSN types or using different routing protocols.
An example application is shown in Figure 2. Here, the provider's
network is divided into three domains: two access domains and the
core domain. The access domains represent the edge of the provider's
network at which services are delivered. In the access domain,
simplicity is required in order to minimize the cost of the network.
The core domain must support all of the aggregated services from the
access domains, and the design requirements here are for scalability,
performance, and information hiding (i.e., minimal state). The core
must not be exposed to the state associated with large numbers of
individual edge-to-edge flows. That is, the core must be simple and
fast.
In a traditional layer 2 network, the interconnection points between
the domains are where services in the access domains are aggregated
for transport across the core to other access domains. In an IP
network, the interconnection points could also represent interworking
points between different types of IP networks, e.g., those with MPLS
and those without, and points where network policies can be applied.
<-------- Edge to Edge Emulated Services ------->
,' . ,-` `', ,' .
/ \ .` `, / \
/ \ / , / \
AC +----+ +----+ +----+ +----+ AC
---| PE |-----| PE |---------------| PE |-------| PE |---
| 1 | | 2 | | 3 | | 4 |
+----+ +----+ +----+ +----+
\ / \ / \ /
\ / \ Core ` \ /
`, ` . ,` `, `
'-'` `., _.` '-'`
Access 1 `''-''` Access 2
Figure 2: Multi-Domain Network Model
A similar model can also be applied to inter-provider services, where
a single PW spans a number of separate provider networks in order to
connect ACs residing on PEs in disparate provider networks. In this
case, each provider will typically maintain their own PE at the
border of their network in order to apply policies such as security
Bocci & Bryant Informational [Page 5]
RFC 5659 Multi-Segment PWE3 Architecture October 2009
and Quality of Service (QoS) to PWs entering their network. Thus,
the connection between the domains will normally be a link between
two PEs on the border of each provider's network.
Consider the application of this model to PWs. PWs use tunneling
mechanisms such as MPLS to enable the underlying PSN to emulate
characteristics of the native service. One solution to the multi-
domain network model above is to extend PSN tunnels edge-to-edge
between all of the PEs in access domain 1 and all of the PEs in
access domain 2, but this requires a large number of PSN tunnels, as
described above, and also exposes the access and the core of the
network to undesirable complexity. An alternative is to constrain
the complexity to the network domain interconnection points (PE2 and
PE3 in the example above). Pseudowires between PE1 and PE4 would
then be switched between PSN tunnels at the interconnection points,
enabling PWs from many PEs in the access domains to be aggregated
across only a few PSN tunnels in the core of the network. PEs in the
access domains would only need to maintain direct signaling sessions
and PSN tunnels, with other PEs in their own domain, thus minimizing
complexity of the access domains.
1.2. Non-Goals of This Document
The following are non-goals for this document:
o The on-the-wire specification of PW encapsulations.
o The detailed specification of mechanisms for establishing and
maintaining multi-segment pseudowires.
1.3. Terminology
The terminology specified in RFC 3985 [1] and RFC 4026 [2] applies.
In addition, we define the following terms:
o 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 an MS-
PW. This incorporates the functionality of a PE as defined in RFC
3985.
o 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.
Bocci & Bryant Informational [Page 6]
RFC 5659 Multi-Segment PWE3 Architecture October 2009
o 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, terminates on a T-PE.
o 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).
o 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.
Note that it was originally anticipated that S-PEs would only be
deployed at the edge of a provider network where they would be used
to switch the PWs of different service providers. However, as the
design of MS-PW progressed, other applications for MS-PW were
recognized. By this time S-PE had become the accepted term for the
equipment, even though they were no longer universally deployed at
the provider edge.
o PW Switching. The process of switching the control and data planes
of the preceding and succeeding PW segments in a MS-PW.
o PW Switching Point. The reference point in an S-PE where the
switching takes place, e.g., where PW label swap is executed.
o Eligible S-PE or T-PE. An eligible S-PE or T-PE is a PE that meets
the security and privacy requirements of the MS-PW, according to
the network operator's policy.
o Trusted S-PE or T-PE. A trusted S-PE or T-PE is a PE that is
understood to be eligible by its next-hop S-PE or T-PE, while a
trust relationship exists between two S-PEs or T-PEs if they
mutually consider each other to be eligible.
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2. Applicability
An MS-PW is a single PW that, for technical or administrative
reasons, is segmented into a number of concatenated hops. From the
perspective of a Layer 2 Virtual Private Network (L2VPN), an MS-PW is
indistinguishable from an SS-PW. Thus, the following are equivalent
from the perspective of the T-PE:
+----+ +----+
|TPE1+--------------------------------------------------+TPE2|
+----+ +----+
|<---------------------------PW----------------------------->|
+----+ +---+ +---+ +----+
|TPE1+--------------+SPE+-----------+SPE+---------------+TPE2|
+----+ +---+ +---+ +----+
Figure 3: MS-PW Equivalence
Although an MS-PW may require services such as node discovery and
path signaling to construct the PW, it should not be confused with an
L2VPN system, which also requires these services. A Virtual Private
Wire Service (VPWS) connects its endpoints via a set of PWs. MS-PW
is a mechanism that abstracts the construction of complex PWs from
the construction of a L2VPN. Thus, a T-PE might be an edge device
optimized for simplicity and an S-PE might be an aggregation device
designed to absorb the complexity of continuing the PW across the
core of one or more service provider networks to another T-PE located
at the edge of the network.
As well as supporting traditional L2VPNs, an MS-PW is applicable to
providing connectivity across a transport network based on packet
switching technology, e.g., the MPLS Transport Profile (MPLS-TP) [6],
[8]. Such a network uses pseudowires to support the transport and
aggregation of all services. This application requires deterministic
characteristics and behavior from the network. The operational
requirements of such networks may need pseudowire segments that can
be established and maintained in the absence of a control plane, and
may also need the operational independence of PW maintenance from the
underlying PSN.
3. Protocol Layering Model
The protocol layering model specified in RFC 3985 applies to MS-PWs
with the following clarification: the pseudowires may be considered
to be a separate layer to the PSN tunnel. That is, although a PW
segment will follow the path of the PSN tunnel between S-PEs, the
Bocci & Bryant Informational [Page 8]
RFC 5659 Multi-Segment PWE3 Architecture October 2009
MS-PW is independent of the PSN tunnel routing, operations,
signaling, and maintenance. The design of PW routing domains should
not imply that the underlying PSN routing domains are the same.
However, MS-PWs will reuse the protocols of the PSN and may, if
applicable, use information that is extracted from the PSN, e.g.,
reachability.
3.1. Domain of MS-PW Solutions
PWs provide the Encapsulation Layer, i.e., the method of carrying
various payload types, and the interface to the PW Demultiplexer
Layer. Other layers provide the following:
o PSN tunnel setup, maintenance, and routing
o T-PE discovery
Not all PEs may be capable of providing S-PE functionality.
Connectivity to the next-hop S-PE or T-PE must be provided by a PSN
tunnel, according to [1]. The selection of which set of S-PEs to use
to reach a given T-PE is considered to be within the scope of MS-PW
solutions.
3.2. Payload Types
MS-PWs are applicable to all PW payload types. Encapsulations
defined for SS-PWs are also used for MS-PW without change. Where the
PSN types for each segment of an MS-PW are identical, the PW types of
each segment must also be identical. However, if different segments
run over different PSN types, the encapsulation may change but the PW
segments must be of an equivalent PW type, i.e., the S-PE must not
need to process the PW payload to provide translation.
4. Multi-Segment Pseudowire Reference Model
The pseudowire emulation edge-to-edge (PWE3) reference architecture
for the single-segment case is shown in [1]. This architecture
applies to the case where a PSN tunnel extends between two edges of a
single PSN domain to transport a PW with endpoints at these edges.
Bocci & Bryant Informational [Page 9]
RFC 5659 Multi-Segment PWE3 Architecture October 2009
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 4: MS-PW Reference Model
Figure 4 extends this architecture to show a multi-segment case. The
PEs that provide services to CE1 and CE2 are Terminating PE1 (T-PE1)
and Terminating PE2 (T-PE2), respectively. A PSN tunnel extends from
T-PE1 to Switching PE1 (S-PE1) across PSN1, and a second PSN tunnel
extends from S-PE1 to T-PE2 across PSN2. PWs are used to connect the
attachment circuits (ACs) attached to PE1 to the corresponding ACs
attached to T-PE2.
Each PW segment on the tunnel across PSN1 is switched to a PW segment
in the tunnel across PSN2 at S-PE1 to complete the multi-segment PW
(MS-PW) between T-PE1 and T-PE2. S-PE1 is therefore the PW switching
point. PW segment 1 and PW segment 3 are segments of the same MS-PW,
while PW segment 2 and PW segment 4 are segments of another MS-PW.
PW segments of the same MS-PW (e.g., PW segment 1 and PW segment 3)
must be of equivalent PW types, as described in Section 3.2, while
PSN tunnels (e.g., PSN1 and PSN2) may be of the same or different PSN
types. An S-PE switches an MS-PW from one segment to another based
on the PW demultiplexer, i.e., a PW label that may take one of the
forms defined in Section 5.4.1 of RFC 3985 [1].
Note that although Figure 4 only shows a single S-PE, a PW may
transit more than one S-PE along its path. This architecture is
applicable when the S-PEs are statically chosen, or when they are
chosen using a dynamic path-selection mechanism. Both directions of
an MS-PW must traverse the same set of S-PEs on a reciprocal path.
Note that although the S-PE path is therefore reciprocal, the path
taken by the PSN tunnels between the T-PEs and S-PEs might not be
reciprocal due to choices made by the PSN routing protocol.
Bocci & Bryant Informational [Page 10]
RFC 5659 Multi-Segment PWE3 Architecture October 2009
4.1. Intra-Provider Connectivity Architecture
There is a requirement to deploy PWs edge-to-edge in large service
provider networks (RFC 5254 [5]). Such networks typically encompass
hundreds or thousands of aggregation devices at the edge, each of
which would be a PE. These networks may be partitioned into separate
metro and core PW domains, where the PEs are interconnected by a
sparse mesh of tunnels.
Whether or not the network is partitioned into separate PW domains,
there is also a requirement to support a partial mesh of traffic-
engineered PSN tunnels.
The architecture shown in Figure 4 can be used to support such cases.
PSN1 and PSN2 may be in different administrative domains or access
regions, core regions, or metro regions within the same provider's
network. PSN1 and PSN2 may also be of different types. For example,
S-PEs may be used to connect PW segments traversing metro networks of
one technology, e.g., statically allocated labels, with segments
traversing an MPLS core network.
Alternatively, T-PE1, S-PE1, and T-PE2 may reside at the edges of the
same PSN.
4.1.1. Intra-Provider Switching Using ACs
In this model, the PW reverts to the native service AC at the domain
boundary PE. This AC is then connected to a separate PW on the same
PE. In this case, the reference models of RFC 3985 apply to each
segment and to the PEs. The remaining PE architectural
considerations in this document do not apply to this case.
4.1.2. Intra-Provider Switching Using PWs
In this model, PW segments are switched between PSN tunnels that span
portions of a provider's network, without reverting to the native
service at the boundary. For example, in Figure 4, PSN1 and PSN2
would be portions of the same provider's network.
4.2. Inter-Provider Connectivity Architecture
Inter-provider PWs may need to be switched between PSN tunnels at the
provider boundary in order to minimize the number of tunnels required
to provide PW-based services to CEs attached to each provider's
network. In addition, the following may need to be implemented on a
per-PW basis at the provider boundary:
Bocci & Bryant Informational [Page 11]
RFC 5659 Multi-Segment PWE3 Architecture October 2009
o Operations, Administration, and Maintenance (OAM). Note that
this is synonymous with 'Operations and Maintenance' referred to
in RFC 5254 [5].
o Authentication, Authorization, and Accounting (AAA)
o Security mechanisms
Further security-related architectural considerations are described
in Section 12.
4.2.1. Inter-Provider Switching Using ACs
In this model, the PW reverts to the native service at the provider
boundary PE. This AC is then connected to a separate PW at the peer
provider boundary PE. In this case, the reference models of RFC 3985
apply to each segment and to the PEs. This is similar to the case in
Section 4.1.1, except that additional security and policy enforcement
measures will be required. The remaining PE architectural
considerations in this document do not apply to this case.
4.2.2. Inter-Provider Switching Using PWs
In this model, PW segments are switched between PSN tunnels in each
provider's network, without reverting to the native service at the
boundary. This architecture is shown in Figure 5. Here, S-PE1 and
S-PE2 are provider border routers. PW segment 1 is switched to PW
segment 2 at S-PE1. PW segment 2 is then carried across an inter-
provider PSN tunnel to S-PE2, where it is switched to PW segment 3 in
PSN2.
Bocci & Bryant Informational [Page 12]
RFC 5659 Multi-Segment PWE3 Architecture October 2009
|<------Multi-Segment Pseudowire------>|
| Provider Provider |
AC | |<----1---->| |<----2--->| | AC
| V V V V V V |
| +----+ +-----+ +----+ +----+ |
+----+ | | |=====| |=====| |=====| | | +----+
| |-------|......PW.....X....PW.....X...PW.......|-------| |
| CE1| | | |Seg 1| |Seg 2| |Seg 3| | | |CE2 |
+----+ | | |=====| |=====| |=====| | | +----+
^ +----+ +-----+ +----+ +----+ ^
| T-PE1 S-PE1 S-PE2 T-PE2 |
| ^ ^ |
| | | |
| PW switching points |
| |
| |
|<------------------- Emulated Service --------------->|
Figure 5: Inter-Provider Reference Model
5. PE Reference Model
5.1. Pseudowire Pre-Processing
Pseudowire pre-processing is applied in the T-PEs as specified in RFC
3985. Processing at the S-PEs is specified in the following
sections.
5.1.1. Forwarding
Each forwarder in the S-PE forwards packets from one PW segment on
the ingress PSN-facing interface of the S-PE to one PW segment on the
egress PSN-facing interface of the S-PE.
The forwarder selects the egress segment PW based on the ingress PW
label. The mapping of ingress to egress PW label may be statically
or dynamically configured. Figure 6 shows how a single forwarder is
associated with each PW segment at the S-PE.
Bocci & Bryant Informational [Page 13]
RFC 5659 Multi-Segment PWE3 Architecture October 2009
+------------------------------------------+
| S-PE Device |
+------------------------------------------+
Ingress | | | | Egress
PW instance | Single | | Single | PW Instance
<==========>X PW Instance + Forwarder + PW Instance X<==========>
| | | |
+------------------------------------------+
Figure 6: Point-to-Point Service
Other mappings of PW-to-forwarder are for further study.
5.1.2. Native Service Processing
There is no native service processing in the S-PEs.
6. Protocol Stack Reference Model
Figure 7 illustrates the protocol stack reference model for multi-
segment PWs.
+-----------+ +-----------+
| Emulated | | Emulated |
| Service | | Service |
|(e.g., ATM)|<======= Emulated Service =======>|(e.g., ATM)|
+-----------+ +-----------+
| Payload | | Payload |
| Encap. |<=== Multi-segment Pseudowire ===>| Encap. |
+-----------+ +--------+ +-----------+
| PW Demux |<PW Segment>|PW Demux|<PW Segment>| PW Demux |
+-----------+ +--------+ +-----------+
|PSN Tunnel,|<PSN Tunnel>| PSN |<PSN Tunnel>|PSN Tunnel,|
| PSN & PHY | |Physical| | PSN & PHY |
| Layers | | Layers | | Layers |
+----+------+ +--------+ +-----+-----+
| .......... | .......... |
| / \ | / \ |
+==========/ PSN \===/ PSN \======+
\ domain 1 / \ domain 2 /
\__________/ \__________/
`````````` ``````````
Figure 7: Multi-Segment PW Protocol Stack
The MS-PW provides the CE with an emulated physical or virtual
connection to its peer at the far end. Native service PDUs from the
CE are passed through an Encapsulation Layer and a PW demultiplexer
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RFC 5659 Multi-Segment PWE3 Architecture October 2009
is added at the sending T-PE. The PDU is sent over PSN domain via
the PSN transport tunnel. The receiving S-PE swaps the existing PW
demultiplexer for the demultiplexer of the next segment and then
sends the PDU over transport tunnel in PSN2. Where the ingress and
egress PSN domains of the S-PE are of the same type, e.g., they are
both MPLS PSNs, a simple label swap operation is performed, as
described in Section 3.13 of RFC 3031 [3]. However, where the
ingress and egress PSNs are of different types, e.g., MPLS and
L2TPv3, the ingress PW demultiplexer is removed (or popped), and a
mapping to the egress PW demultiplexer is performed and then inserted
(or pushed).
Policies may also be applied to the PW at this point. Examples of
such policies include admission control, rate control, QoS mappings,
and security. The receiving T-PE removes the PW demultiplexer and
restores the payload to its native format for transmission to the
destination CE.
Where the encapsulation format is different, e.g., MPLS and L2TPv3,
the payload encapsulation may be translated at the S-PE.
7. Maintenance Reference Model
Figure 8 shows the maintenance reference model for multi-segment
pseudowires.
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|<------------- CE (end-to-end) Signaling ------------>|
| |
| |<-------- MS-PW/T-PE Maintenance ----->| |
| | |<---PW Seg't-->| |<--PW Seg't--->| | |
| | | Maintenance | | Maintenance | | |
| | | | | | | |
| | | PSN | | PSN | | |
| | | |<-Tunnel1->| | | |<-Tunnel2->| | | |
| V V V Signaling V V V V Signaling V V V |
V +----+ +-----+ +----+ V
+----+ |TPE1|===========|SPE1 |===========|TPE2| +----+
| |-------|......PW.Seg't1....X....PW Seg't3......|------| |
| CE1| | | | | | | |CE2 |
| |-------|......PW.Seg't2....X....PW Seg't4......|------| |
+----+ | |===========| |===========| | +----+
^ +----+ +-----+ +----+ ^
| Terminating ^ Terminating |
| Provider Edge 1 | Provider Edge 2 |
| | |
| PW switching point |
| |
|<--------------------- Emulated Service ------------------->|
Figure 8: MS-PW Maintenance Reference Model
RFC 3985 specifies the use of CE (end-to-end) and PSN tunnel
signaling as well as PW/PE maintenance. CE and PSN tunnel signaling
is as specified in RFC 3985. However, in the case of MS-PWs,
signaling between the PEs now has both an edge-to-edge and a hop-by-
hop context. That is, signaling and maintenance between T-PEs and
S-PEs and between adjacent S-PEs is used to set up, maintain, and
tear down the MS-PW segments, which includes the coordination of
parameters related to each switching point as well as to the MS-PW
endpoints.
8. PW Demultiplexer Layer and PSN Requirements
8.1. Multiplexing
The purpose of the PW Demultiplexer Layer at the S-PE is to
demultiplex PWs from ingress PSN tunnels and to multiplex them into
egress PSN tunnels. Although each PW may contain multiple native
service circuits, e.g., multiple ATM virtual circuits (VCs), the
S-PEs do not have visibility of, and hence do not change, this level
of multiplexing because they contain no Native Service Processor
(NSP).
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8.2. Fragmentation
If fragmentation is to be used in an MS-PW, T-PEs and S-PEs must
satisfy themselves that fragmented PW payloads can be correctly
reassembled for delivery to the destination attachment circuit.
An S-PE is not required to make any attempt to reassemble a
fragmented PW payload. However, it may choose to do so if, for
example, it knows that a downstream PW segment does not support
reassembly.
An S-PE may fragment a PW payload using [4].
9. Control Plane
9.1. Setup and Placement of MS-PWs
For multi-segment pseudowires, the intermediate PW switching points
may be statically provisioned or chosen dynamically.
For the static case, there are two options for exchanging the PW
labels:
o By configuration at the T-PEs or S-PEs.
o By signaling across each segment using a dynamic maintenance
protocol.
A multi-segment pseudowire may thus consist of segments where the
labels are statically configured and segments where the labels are
signaled.
For the case of dynamic choice of the PW switching points, there are
two options for selecting the path of the MS-PW:
o T-PEs determine the full path of the PW through intermediate
switching points. This may be either static or based on a dynamic
PW path-selection mechanism.
o Each T-PE and S-PE makes a local decision as to which next-hop S-PE
to choose to reach the target T-PE. This choice is made either
using locally configured information or by using a dynamic PW
path-selection mechanism.
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9.2. Pseudowire Up/Down Notification
Since a multi-segment PW consists of a number of concatenated PW
segments, the emulated service can only be considered as being up
when all of the constituting PW segments and PSN tunnels are
functional and operational along the entire path of the MS-PW.
If a native service requires bi-directional connectivity, the
corresponding emulated service can only be signaled as being up when
the PW segments and PSN tunnels (if used), are functional and
operational in both directions.
RFC 3985 describes the architecture of failure and other status
notification mechanisms for PWs. These mechanisms are also needed in
multi-segment pseudowires. In addition, if a failure notification
mechanism is provided for consecutive segments of the same PW, the
S-PE must propagate such notifications between the consecutive
concatenated segments.
9.3. Misconnection and Payload Type Mismatch
Misconnection and payload type mismatch can occur with PWs.
Misconnection can breach the integrity of the system. Payload
mismatch can disrupt the customer network. In both instances, there
are security and operational concerns.
The services of the underlying tunneling mechanism or the PW control
and OAM protocols can be used to ensure that the identity of the PW
next hop is as expected. As part of the PW setup, a PW-TYPE
identifier is exchanged. This is then used by the forwarder and the
NSP of the T-PEs to verify the compatibility of the ACs. This can
also be used by S-PEs to ensure that concatenated segments of a given
MS-PW are compatible or that an MS-PW is not misconnected into a
local AC. In addition, it is possible to perform an end-to-end
connection verification to check the integrity of the PW, to verify
the identity of S-PEs and check the correct connectivity at S-PEs,
and to verify the identity of the T-PE.
10. Management and Monitoring
The management and monitoring as described in RFC 3985 applies here.
The MS-PW architecture introduces additional considerations related
to management and monitoring, which need to be reflected in the
design of maintenance tools and additional management objects for
MS-PWs.
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The first is that each S-PE is a new point at which defects may occur
along the path of the PW. In order to troubleshoot MS-PWs,
management and monitoring should be able to operate on a subset of
the segments of an MS-PW, as well as edge-to-edge. That is,
connectivity verification mechanisms should be able to troubleshoot
and differentiate the connectivity between T-PEs and intermediate
S-PEs, as well as the connectivity between T-PE and T-PE.
The second is that the set of S-PEs and P-routers along the MS-PW
path may be less optimal than a path between the T-PEs chosen solely
by the underlying PSN routing protocols. This is because the S-PEs
are chosen by the MS-PW path selection mechanism and not by the PSN
routing protocols. Troubleshooting mechanisms should therefore be
provided to verify the set of S-PEs that are traversed by an MS-PW to
reach a T-PE.
Some of the S-PEs and the T-PEs for an MS-PW may reside in a
different service provider's PSN domain from that of the operator who
initiated the establishment of the MS-PW. These situations may
necessitate the use of remote management of the MS-PW, which is able
to securely operate across provider boundaries.
11. Congestion Considerations
The following congestion considerations apply to MS-PWs. These are
in addition to the considerations for PWs described in RFC 3985 [1],
[7], and the respective RFCs specifying each PW type.
The control plane and the data plane fate-share in traditional IP
networks. The implication of this is that congestion in the data
plane can cause degradation of the operation of the control plane.
Under quiescent operating conditions, it is expected that the network
will be designed to avoid such problems. However, MS-PW mechanisms
should also consider what happens when congestion does occur, when
the network is stretched beyond its design limits, for example,
during unexpected network failure conditions.
Although congestion within a single provider's network can be
mitigated by suitable engineering of the network so that the traffic
imposed by PWs can never cause congestion in the underlying PSN, a
significant number of MS-PWs are expected to be deployed for inter-
provider services. In this case, there may be no way of a provider
who initiates the establishment of an MS-PW at a T-PE guaranteeing
that it will not cause congestion in a downstream PSN. A specific
PSN may be able to protect itself from excess PW traffic by policing
all PWs at the S-PE at the provider border. However, this may not be
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effective when the PSN tunnel across a provider utilizes the transit
services of another provider that cannot distinguish PW traffic from
ordinary, TCP-controlled IP traffic.
Each segment of an MS-PW therefore needs to implement congestion
detection and congestion control mechanisms where it is not possible
to explicitly provision sufficient capacity to avoid congestion.
In many cases, only the T-PEs may have sufficient information about
each PW to fairly apply congestion control. Therefore, T-PEs need to
be aware of which of their PWs are causing congestion in a downstream
PSN and of their native service characteristics, and to apply
congestion control accordingly. S-PEs therefore need to propagate
PSN congestion state information between their downstream and
upstream directions. If the MS-PW transits many S-PEs, it may take
some time for congestion state information to propagate from the
congested PSN segment to the source T-PE, thus delaying the
application of congestion control. Congestion control in the S-PE at
the border of the congested PSN can enable a more rapid response and
thus potentially reduce the duration of congestion.
In addition to protecting the operation of the underlying PSN,
consistent QoS and traffic engineering mechanisms should be used on
each segment of an MS-PW to support the requirements of the emulated
service. The QoS treatment given to a PW packet at an S-PE may be
derived from context information of the PW (e.g., traffic or QoS
parameters signaled to the S-PE by an MS-PW control protocol) or from
PSN-specific QoS flags in the PSN tunnel label or PW demultiplexer,
e.g., TC bits in either the label switched path (LSP) or PW label for
an MPLS PSN or the DS field of the outer IP header for L2TPv3.
12. Security Considerations
The security considerations described in RFC 3985 [1] apply here.
Detailed security requirements for MS-PWs are specified in RFC 5254
[5]. This section describes the architectural implications of those
requirements.
The security implications for T-PEs are similar to those for PEs in
single-segment pseudowires. However, S-PEs represent a point in the
network where the PW label is exposed to additional processing. An
S-PE or T-PE must trust that the context of the MS-PW is maintained
by a downstream S-PE. OAM tools must be able to verify the identity
of the far end T-PE to the satisfaction of the network operator.
Additional consideration needs to be given to the security of the
S-PEs, both at the data plane and the control plane, particularly
when these are dynamically selected and/or when the MS-PW transits
the networks of multiple operators.
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An implicit trust relationship exists between the initiator of an
MS-PW, the T-PEs, and the S-PEs along the MS-PW's path. That is, the
T-PE trusts the S-PEs to process and switch PWs without compromising
the security or privacy of the PW service. An S-PE should not select
a next-hop S-PE or T-PE unless it knows it would be considered
eligible, as defined in Section 1.3, by the originator of the MS-PW.
For dynamically placed MS-PWs, this can be achieved by allowing the
T-PE to explicitly specify the path of the MS-PW. When the MS-PW is
dynamically created by the use of a signaling protocol, an S-PE or
T-PE should determine the authenticity of the peer entity from which
it receives the request and the compliance of that request with
policy.
Where an MS-PW crosses a border between one provider and another
provider, the MS-PW segment endpoints (S-PEs or T-PEs) or, for the
PSN tunnel, P-routers typically reside on the same nodes as the
Autonomous System Border Router (ASBRs) interconnecting the two
providers. In either case, an S-PE in one provider is connected to a
limited number of trusted T-PEs or S-PEs in the other provider. The
number of such trusted T-PEs or S-PEs is bounded and not anticipated
to create a scaling issue for the control plane authentication
mechanisms.
Directly interconnecting the S-PEs/T-PEs using a physically secure
link and enabling signaling and routing authentication between the
S-PEs/T-PEs eliminates the possibility of receiving an MS-PW
signaling message or packet from an untrusted peer. The S-PEs/T-PEs
represent security policy enforcement points for the MS-PW, while the
ASBRs represent security policy enforcement points for the provider's
PSNs. This architecture is illustrated in Figure 9.
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|<------------- MS-PW ---------------->|
| Provider Provider |
AC | |<----1---->| |<----2--->| | AC
| V V V V V V |
| +----+ +-----+ +----+ +----+ |
+---+ | | |=====| |=====| |=====| | | +---+
| |-------|......PW.....X....PW.....X...PW.......|-------| |
|CE1| | | |Seg 1| |Seg 2| |Seg 3| | | |CE2|
+---+ | | |=====| |=====| |=====| | | +---+
^ +----+ +-----+ ^ +----+ +----+ ^
| T-PE1 S-PE1 | S-PE2 T-PE2 |
| ASBR | ASBR |
| | |
| Physically secure link |
| |
| |
|<------------------- Emulated Service --------------->|
Figure 9: Directly Connected Inter-Provider Reference Model
Alternatively, the P-routers for the PSN tunnel may reside on the
ASBRs, while the S-PEs or T-PEs reside behind the ASBRs within each
provider's network. A limited number of trusted inter-provider PSN
tunnels interconnect the provider networks. This is illustrated in
Figure 10.
|<-------------- MS-PW -------------------->|
| Provider Provider |
AC | |<------1----->| |<-----2------->| | AC
| V V V V V V |
| +---+ +---+ +--+ +--+ +---+ +---+ |
+---+ | | |=====| |===============| |=====| | | +---+
| |-----|.....PW....X.......PW..............PW....X.|------| |
|CE1| | | |Seg 1| | Seg 2 | |Seg 3| | | |CE2|
+---+ | | |=====| |===============| |=====| | | +---+
^ +---+ +---+ +--+ ^ +--+ +---+ +---+ ^
| T-PE1 S-PE1 ASBR | ASBR S-PE2 T-PE2 |
| | |
| | |
| Trusted Inter-AS PSN Tunnel |
| |
| |
|<------------------- Emulated Service ----------------->|
Figure 10: Indirectly Connected Inter-Provider Reference Model
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Particular consideration needs to be given to Quality of Service
requests because the inappropriate use of priority may impact any
service guarantees given to other PWs. Consideration also needs to
be given to the avoidance of spoofing the PW demultiplexer.
Where an S-PE provides interconnection between different providers,
security considerations that are similar to the security
considerations for ASBRs apply. In particular, peer entity
authentication should be used.
Where an S-PE also supports T-PE functionality, mechanisms should be
provided to ensure that MS-PWs are switched correctly to the
appropriate outgoing PW segment, rather than to a local AC. Other
mechanisms for PW endpoint verification may also be used to confirm
the correct PW connection prior to enabling the attachment circuits.
13. Acknowledgments
The authors gratefully acknowledge the input of Mustapha Aissaoui,
Dimitri Papadimitrou, Sasha Vainshtein, and Luca Martini.
14. References
14.1. Normative References
[1] Bryant, S., Ed., and P. Pate, Ed., "Pseudo Wire Emulation Edge-
to-Edge (PWE3) Architecture", RFC 3985, March 2005.
[2] Andersson, L. and T. Madsen, "Provider Provisioned Virtual
Private Network (VPN) Terminology", RFC 4026, March 2005.
[3] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol Label
Switching Architecture", RFC 3031, January 2001.
[4] Malis, A. and M. Townsley, "Pseudowire Emulation Edge-to-Edge
(PWE3) Fragmentation and Reassembly", RFC 4623, August 2006.
14.2. Informative References
[5] Bitar, N., Ed., Bocci, M., Ed., and L. Martini, Ed.,
"Requirements for Multi-Segment Pseudowire Emulation Edge-to-Edge
(PWE3)", RFC 5254, October 2008.
[6] Niven-Jenkins, B., Ed., Brungard, D., Ed., Betts, M., Ed.,
Sprecher, N., and S. Ueno, "Requirements of an MPLS Transport
Profile", RFC 5654, September 2009.
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[7] Bryant, S., Davie, B., Martini, L., and E. Rosen, "Pseudowire
Congestion Control Framework", Work in Progress, June 2009.
[8] Bocci, M., Bryant, S., and L. Levrau, "A Framework for MPLS in
Transport Networks", Work in Progress, August 2009.
Authors' Addresses
Matthew Bocci
Alcatel-Lucent
Voyager Place, Shoppenhangers Road,
Maidenhead, Berks, UK
Phone: +44 1633 413600
EMail: matthew.bocci@alcatel-lucent.com
Stewart Bryant
Cisco Systems
250, Longwater,
Green Park,
Reading, RG2 6GB,
United Kingdom
EMail: stbryant@cisco.com
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