Network Working Group M Bocci
Internet Draft Alcatel
S.Bryant
Cisco Systems
Expires: January 2006 July 9, 2005
An Architecture for Multi-Segment Pseudo Wire Emulation Edge-to-Edge
draft-bocci-bryant-pwe3-ms-pw-arch-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2005). All Rights Reserved.
Abstract
This document describes an architecture for extending pseudo wire
emulation across multiple packet switched network segments. Scenarios
are discussed where each segment of a given edge-to-edge emulated
service spans a different provider's PSN, and where the emulated
service originates and terminates on the same providers PSN, but may
pass through several PSN tunnel segments in that PSN. It presents an
architectural framework for such multi-segment pseudo wires, defines
terminology, and specifies the various protocol elements and their
functions.
Conventions used in this document
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 [1].
Table of Contents
1. Introduction................................................3
1.1. Motivation.............................................3
1.2. Non-Goals of this Document..............................6
1.3. Terminology............................................6
2. Applicability...............................................7
3. Protocol Layering model......................................7
3.1. Domain of Multi-Segment PWE3............................7
3.2. Payload Types..........................................8
4. Multi-Segment PWE3 Reference Model...........................8
4.1. Intra-Provider Architecture.............................9
4.2. Inter-Provider Architecture.............................9
4.3. PW Switching Models....................................10
4.3.1. Switching using ACs...............................10
4.3.2. Switching using PWs...............................10
5. PE Reference Model.........................................10
5.1. PWE3 Pre-processing....................................10
5.1.1. Forwarding........................................11
5.1.2. Native Service Processing.........................11
6. Protocol Stack reference Model..............................11
7. Maintenance Reference Model.................................12
8. PW Demultiplexer Layer and PSN Requirements.................12
9. Control Plane..............................................12
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10. Fragmentation.............................................13
11. Management and Monitoring..................................13
12. IANA Considerations........................................13
13. Security Considerations....................................13
14. Acknowledgments...........................................13
15. References................................................14
15.1. Normative References..................................14
Author's Addresses............................................14
Intellectual Property Statement................................14
Disclaimer of Validity........................................15
Copyright Statement...........................................15
Acknowledgment................................................15
1. Introduction
RFC 3985 [2] defines the architecture for pseudo wires, where a
pseudo wire (PW) both originates and terminates on the edge of the
same packet switched network (PSN). The PW passes through a maximum
of one PSN tunnel between the originating and terminating PEs.
This document extends the architecture in RFC 3985 to enable pseudo
wires to be extended through multiple PSN tunnels. Use cases for
multi-segment pseudo wires, and the consequent requirements, are
defined in [3].
1.1. Motivation
PWE3 aims to provide point-to-point connectivity between two edges of
a provider network. Requirements for Multi-Segment Pseudo-Wires for
this are specified in [3]. These requirements address three main
problems:
o How to scale PWE3 when the number of PEs grows to many hundreds or
thousands, while minimizing the complexity of the PEs and P
routers.
o How to provide PWE3 across multiple PSN routing domains or areas
in the same provider.
o How to provide PWE3 across multiple provider domains, and
different PSN types.
Consider a single PWE3 domain, such as that shown in Figure 1. There
are 4 PEs, and PWE3 must be provided from any PE to any other PE.
Traditionally, this would be achieved by establishing a full mesh of
PSN tunnels between the PEs. This would also require a full mesh of
LDP signaling adjacencies between the PEs. Pseudo wires could then be
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established between any PE and any other PE via a single, direct
tunnel. PEs must terminate all pseudo wires that are carried on 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 and signaling adjacencies will grow in proportion to
the square of the number of PEs.
A more efficient solution for large numbers of PEs would be to
support a partial mesh of PSN tunnels between the PEs, as shown in
Figure 1. For example, consider a PWE3 service whose endpoints are
PE1 and PE4. Pseudo wires for this can take the path PE1->PE2->PE3,
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-PE3. The end-
to-end PW is known as a multi-segment PW.
,,..--..,,_
.-`` `'.,
+-----+` '+-----+
| PE1 |---------------------| PE2 |
| |---------------------| |
+-----+ PSN Tunnel +-----+
/ || || \
/ || || \
| || || |
| || PSN || |
| || || |
\ || || /
\ || || /
\|| ||/
+-----+ +-----+
| PE3 |---------------------| PE4 |
| |---------------------| |
+-----+`'.,_ ,.'` +-----+
`'''---''``
Figure 1 Single PSN PWE3 Scaling
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.
An example application is shown in Figure 2. Here, the providers
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.
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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 would also represent interworking
points between different types of IP networks e.g. those with MPLS
and those without, and also 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
This model can also be applied to inter-provider services, where they
also rely on a number of separate provider networks to be connected
together.
Consider the application of this model to PWE3. PWE3 uses tunneling
mechanisms such as MPLS to enable the underlying IP 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 runs into the scaling issues described
above, and also exposes 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
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example above). Pseudo-wires between PE1 and PE4 would then be
switched between PSN tunnels at the interconnection points, enabling
PWs from may 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 Requirements on multi-segment pseudo-wires.
o The detailed specification of mechanisms for establishing and
maintaining multi-segment pseudo-wires.
1.3. Terminology
The terminology specified in RFC 3985 applies. In addition, we define
the following terms:
o Ultimate PE (U-PE). A PE where the customer-facing attachment
circuits (ACs) are bound to a PW forwarder. An ultimate PE is
present in the first and last segments of a MS-PW.
o Single-Segment PW (SS-PW). A PW setup directly between two U-PE
devices. Each PW in one direction of a SS-PW traverses one PSN
tunnel that connects the two U-PEs.
o Multi-Segment PW (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 a MS-PW by definition MUST
terminate on a U-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 a MS-PW. It is therefore a PW switching point for a
MS-PW. A PW Switching Point is never the S-PE and the U-PE for the
same MS-PW. A PW switching point runs necessary protocols to setup
and manage PW segments with other PW switching points and ultimate
PEs.
o PW Segment. A part of a single-segment or multi-segment PW, which
is set up between two PE devices, U-PEs and/or S-PEs.
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2. Applicability
A 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 U-PE, a MS-PW is indistinguishable from a SS-PW.
Thus, the following are equivalent from the perspective of the UPE
+----+ +----+
|UPE1+--------------------------------------------------+UPE2|
+----+ +----+
|<----------------------PW------------------------>|
+----+ +---+ +---+ +----+
|UPE1+--------------+SPE+-----------+SPE+---------------+UPE2|
+----+ +---+ +---+ +----+
Figure 3 MS-PW Equivalence
Although a MS-PW may require services such as node discovery and path
signaling to construct the PW, it should not be confused with a L2VPN
system, which also requires these services. A 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
U-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 UPE located at the edge of the network.
3. Protocol Layering model
The protocol-layering model specified in RFC 3985 applies to multi-
segment PWE3 with the following clarification: the pseudo-wires may
be considered to be a separate layer to the PSN tunnel. That is, they
are 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-PW will
reuse the protocols of the PSN.
3.1. Domain of Multi-Segment PWE3
PWE3 defines the Encapsulation Layer, the method of carrying various
payload types, and the interface to the PW Demultiplexer Layer. It
is expected that other layers will provide the following:
. PSN tunnel setup, maintenance and routing
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. U-PE discovery
It is assumed that any node that is reachable via a PSN tunnel from
an S-PE or U-PE is a PE, a subset of which may be capable of behaving
as an S-PE. The selection of which S-PEs to use to reach a U-PE is
considered to be in the domain of PWE3.
3.2. Payload Types
Multi-segment PWE3 is applicable to all PWE3 payload types. The same
encapsulations are used in both SS-PW and MH-PW.
4. Multi-Segment PWE3 Reference Model
The PWE3 reference architecture for the single segment case is shown
in [2]. 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.
Native |<-----------Pseudo Wire----------->| Native
Service | | Service
(AC) | |<-PSN1-->| |<-PSN2-->| | (AC)
| V V V V V V |
| +----+ +-----+ +----+
+----+ | |UPE1|=========|SPE1 |=========|UPE2| | +----+
| |-------|....PW.Seg't1........PW Seg't3.....|----------| |
| CE1| | | | | | | | | |CE2 |
| |-------|....PW.Seg't2.......|PW Seg't4.....|----------| |
+----+ | | |=========| |=========| | | +----+
^ +----+ +-----+ +----+ ^
| Provider Edge 1 ^ Provider Edge 2 |
| | |
| | |
| PW switching point |
| |
|<------------------- Emulated Service ------------------>|
Figure 4 PW switching Reference Model
Figure 4 extends this architecture to show a multi-segment case. The
PEs that provide PWE3 to CE1 and CE2 are Ultimate-PE1 (U-PE1) and
Ultimate-PE2 (U-PE2) respectively. A PSN tunnel extends from U-PE1 to
switching-PE1 (S-PE1) across PSN1, and a second PSN tunnel extends
from S-PE1 to S-PE2 across PSN2. PWs are used to connect the
attachment circuits (ACs) attached to PE1 to the corresponding ACs
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attached to PE3. 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 U-PE1 and U-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.,
PW1 and PW3) MAY be of the same PW type or different type, and PSN
tunnels (e.g., PSN1 and PSN2) can be the same or different
technology. This document requires support for MS-PWs with segments
of the same type. An S-PE switches an MS-PW from one segment to
another based on the PW identifiers (e.g., PW label in case of MPLS
PWs).
Note that although Figure 4 only shows a single S-PE, a PW may
transit more 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.
4.1. Intra-Provider Architecture
There is a requirement to deploy PWs edge to edge in large
service provider networks [3]. 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 PWE3 domains, where the PEs are interconnected by a
sparse mesh of tunnels.
Whether or not the network is partitioned in to separate PWE3
domains, there is a 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,
core or metro regions within the same providers network.
Alternatively, U-PE1, SPE1 and U-PE2 may reside at the edges of the
same PSN.
4.2. Inter-Provider Architecture
Intra-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 PWE3 services to CEs attached to each providers network.
In addition, AAA and security and mechanisms may need to be
implemented on a per-PW basis at the provider boundary.
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4.3. PW Switching Models
4.3.1. 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 3965
apply to each segment and to the PEs. The remaining PE architectural
considerations in this document do not apply to this case.
4.3.2. Switching using PWs.
In this model, PW segments are switched between PSN tunnels in each
providers network, without reverting to the native service at the
boundary. For example, in Figure 4, PSN 1 and PSN 2 would be
different provider's networks. However, this would require that S-PE1
be a member of both provider networks.
An alternative architecture is shown in Figure 5.
|<--------------Pseudo Wire----------->|
| Provider Provider |
AC | |<----1---->| |<----2--->| | AC
| V V V V V V |
| +----+ +-----+ +----+ +----+ |
+----+ | | |=====| |=====| |=====| | | +----+
| |-------|.....PW.1........PW.2.......PW.3......|-------| |
| CE1| | | | | | | | | | | |CE2 |
+----+ | | |=====| |=====| |=====| | | +----+
^ +----+ +-----+ +----+ +----+ ^
| PE1 PE2 PE3 PE4 |
| ^ ^ |
| | | |
| PW switching points |
| |
| |
|<------------------- Emulated Service --------------->|
Figure 5 Inter-Provider Reference Model
5. PE Reference Model
5.1. PWE3 Pre-processing
PWE3 preprocessing is applied in the U-PEs as specified in RFC 3985.
Processing at the S-PEs is specified in the following sections.
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5.1.1. Forwarding
The forwarders in the S-PE forward packets from one or more PW
segments on the ingress PSN facing interface of the S-PE to one or
more PW segments 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 5 shows how a single forwarder is
associated with each PW segment at the S-PE.
+------------------------------------------+
| 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.
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+----------------+ +----------------+
|Emulated Service| |Emulated Service|
|(e.g., TDM, ATM)|<======= Emulated Service =======>|(e.g., TDM, ATM)|
+----------------+ +----------------+
| Payload | | Payload |
| Encapsulation |<== Multi-segment Pseudo Wire ===>| Encapsulation |
+----------------+ +--------+ +----------------+
|PW Demultiplexer|<PW Segment>|PW Demux|<PW Segment>|PW Demultiplexer|
+----------------+ +--------+ +----------------+
| PSN Tunnel, |<PSN Tunnel>| PSN |<PSN Tunnel>| PSN Tunnel, |
| PSN & Physical | |Physical| | PSN & Physical |
| 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
is added at the sending U-PE. The PDU is sent over PSN domain 1. The
receiving S-PE removes the existing PW demultiplexer, adds a new
demultiplexer, and then sends the PDU over PSN2. Policies may also be
applied to the PW at this point. The receiving U-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 transparently translated at the S-PE.
7. Maintenance Reference Model
To be added in a future version.
8. PW Demultiplexer Layer and PSN Requirements
To be added in a future version.
9. Control Plane
For multi-segment pseudo wires, the intermediate PW switching points
may be statically provisioned, or they may be dynamically signaled.
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For the dynamic case, there are two options for selecting the path of
the PW:
o U-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 The each segment of the PW path is determined locally by each U-PE
or S-PE, either through static configuration or based on a dynamic
PW path selection mechanism.
Further details of the impact of these on the control plane
architecture will be provided in a future revision.
10. Fragmentation
An SPE is not required to make any attempt to reassemble a fragmented PW
payload. An SPE may fragment a PW payload fragment.
11. Management and Monitoring
To be added in a later version.
12. IANA Considerations
To be added in a future version.
13. Security Considerations
To be added in a later version.
14. Acknowledgments
The authors gratefully acknowledge the input of Mustapha Aissaoui,
Dimitri Papadimitrou, and Luca Martini.
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15. References
15.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Bryant, S. and Pate, P. (Editors), "Pseudo Wire Emulation Edge-
to-Edge (PWE3) Architecture", RFC 3985, March 2005
[3] Martini, S. Bitar, N. and Bocci, M (Editors), "Requirements for
inter domain Pseudo-Wires", draft-martini-pwe3-mh-pw-
requirements-01.txt, internet Draft, March 2005
Author's Addresses
Matthew Bocci
Alcatel
Voyager Place,
Shoppenhangers Rd,
Maidenhead, Berks, UK Email: matthew.bocci@alcatel.co.uk
Stewart Bryant
Cisco Systems,
250, Longwater,
Green Park,
Reading, RG2 6GB,
United Kingdom. Email: stbryant@cisco.com
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