Network Working Group M. Bocci
Internet Draft A. Zinin
Expires July, 2008 M. Aissaoui
D. Papadimitriou
A. Dolganow
Alcatel-Lucent
Frederic Jounay
France Telecom
Yuji Kamite
NTT Communications
Luca Martini
Cisco
February 4, 2008
OSPF Extensions for Dynamic Multi-segment Pseudo Wires
draft-dolganow-pwe3-ospf-ms-pw-ext-02.txt
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Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
Multi-segment pseudowires have been defined to enable emulated layer
1 and layer 2 services to be delivered from an IP based packet
switched network over a sparse mesh of PSN tunnels and PW control
protocol sessions. MS-PWs can be used to scale PW based networks over
both a single AS, or between multiple ASs, and there is a particular
need to be able to dynamically route MS-PWs through a given AS to
reach PEs at or beyond the edge of the AS, where the route of the PW
through each AS needs to be automatically determined.
This draft proposes extensions to OSPF to enable the automatic
advertisement of summarized PW FECs, thus enabling the dynamic
routing of MS-PWs across an OSPF domain.
Conventions used in this document
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
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. Terminology............................................4
1.2. Architecture...........................................4
2. Applicability...............................................6
3. OSPF Extensions.............................................6
3.1. Attachment Circuit Addressing...........................6
3.2. OSPFv2 LSAs............................................7
3.2.1. Pseudowire Switching LSA...........................7
3.3. OSPFv3 LSAs............................................7
3.3.1. Pseudowire Switching LSA...........................7
3.4. LSA Information Field...................................7
3.4.1. Exterior AII TLV...................................8
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4. LSA Processing Procedures....................................8
4.1. P Routers..............................................8
4.2. PE Routers.............................................8
5. Deployment Considerations....................................9
5.1. Addition and Removal of ACs, S-PEs and T-PEs............9
5.2. Impact on Existing P-Routers............................9
5.3. Congestion in the Underlying PSN Routing...............10
6. Security Considerations.....................................10
7. IANA Considerations........................................10
8. Acknowledgments............................................11
9. References.................................................12
9.1. Normative References...................................12
9.2. Informative References.................................12
Author's Addresses............................................13
Intellectual Property Statement................................14
Disclaimer of Validity........................................14
1. Introduction
Multi-segment pseudowires have been defined to enable emulated layer
2 services to be delivered from an IP based packet switched network
over a sparse mesh of PSN tunnels and PW control protocol
adjacencies. MS-PWs can be used to scale PW based networks over both
a single AS, and multiple ASs. Requirements for MS-PWs are detailed
in [8].
A basic approach to MS-PWs, where the switching points are statically
placed, is described in [10]. This is extended in [11]to allow the
automatic placement of the MS-PWs. This draft uses FEC 129 with AII
type II to summarize the PW end points that are reachable through a
given PE, and to provide a layer 2 address for the S-PEs. MP-BGP is
used to distribute FECs.
The use of MP-BGP is primarily focused on scenarios where each PWE3
domain is a separate AS, and S-PEs are used to switch PWs between
adjacent ASs. MP-BGP; therefore, provides the BGP-enabled T-PE or S-
PE at the ingress of the AS with reachability information for AIIs at
or beyond the border of the AS. This provides sufficient information
to dynamically route the PW across the AS when there is a direct PSN
tunnel between the ingress and egress S-PE or T-PE. When there is no
direct PSN tunnel, a mechanisms must be provided to determine an
indirect route for the PW via some intermediate S-PE within an AS
domain.
A second important case is where MS-PWs are deployed in service
provider access and metro networks. Pseudowires in these networks
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typically span only a single IGP domain or AS. Furthermore, the nodes
contain a minimal routing implementation to cut on the operational
complexity. In such networks MP-BGP is not typically deployed on MTUs
and full MP-BGP functionality may not be required. This prevents
methods defined in [11] to be employed; however, the need to be able
to dynamically route MS-PWs through these topologies still exists.
To enable automatic placement of MS-PWs in the above described cases,
it is possible to leverage the mechanisms of the PSN IGP to
distribute MS-PW endpoint reachability information. This draft
proposes extensions to OSPF to enable the automatic advertisement of
summarized PW layer 2 addresses within a single AS, thus enabling the
automatic placement of MS-PWs across an OSPF domain. The advertised
information is used by T-PEs and S-PEs to derive the MS-PW next hop
route which is used then to signal the next-hop S-PE or T-PE, as
described in [11]. The draft also describes mechanisms to isolate
OSPF advertisements used for MS-PWs from those used for routing in
the underlying PSN to avoid any overloading of the IGP routing by the
mechanisms proposed.
1.1. Terminology
The terminology defined in [9] applies.
1.2. Architecture
+-----+ +-----+
|T-PE1+--------------------------------------------------+T-PE2|
+-----+ +-----+
|<---------------------------PW----------------------------->|
+-----+ +-----+ +-----+ +-----+
|T-PE1+-------------+S-PE1+-----------+S-PE2+------------+T-PE2|
+-----+ +-----+ +-----+ +-----+
<---------------------- Single AS -------------------------->
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Figure 1 MS-PW Routing Model for a Single AS
Figure 1 illustrates the MS-PW routing model for a single AS. ACs
attached to T-PE2 are associated with the OSPF Router_ID or any
locally assigned routable address. Each S-PE / T-PE is also assigned
its own layer 2 address in the form of an AII as described in [11].
The proposed model requires the existence of PSN tunnels between T-
PE/S-PE, S-PE/S-PE and/or S-PE/T-PE. PWs are established on these
tunnels using Targeted LDP signaling [11].
When a PSN tunnel exists and can be used for the establishment of MS-
PW or it segment, T-PE advertises the set of exterior AIIs reachable
via this tunnel. This is done using summarized Type 2 AIIs so that a
separate advertisement is not required for every AC reachable via
that tunnel. AIIs may be summarized using the aggregation rules for
AII Type 2 described in [6]. When an advertisement is received by an
S-PE and the S-PE has a PSN tunnel connectivity to the advertising
PE, the advertisement is installed in the S-PE's routing information
and the S-PE summarizes AIIs at the far end of the tunnel in its own
advertisement for propagation in the AS backbone and further
propagation into other areas. When an advertisement is received by a
T-PE and the T-PE has PSN tunnel connectivity to the advertising PE,
the advertisement is installed in the PE's routing information base.
As an alternative to this summarization-based population of an
advertisement at S-PEs, a static (through configuration) method may
be employed at the S-PE in order to populate the AIIs reachable
through it.
Based on the advertised information, each PE builds a routing
information base containing all exterior AIIs reachable through a
given next hop S-PE. When creating a MS-PW, the PE looks up the AII
and determines the next hop S-PE or T-PE for LDP signaling, as
described in [11].
Figure 1 depicts the simple model of a one-to-one relationship of T-
PE to S-PE and S-PE to S-PE, and of a single S-PE to S-PE segment. In
the general case, multiple S-PE segments will exist, and the
relationship between two S-PEs and/or the T-PE and S-PEs will be one-
to-many or many-to-one. Processing in these cases follows that of the
general case illustrated in Figure 1. Selection of an S-PE from a set
of multiple available next-hop S-PEs may be achieved by comparing the
IGP metrics for the route to the terminating T-PE (TT-PE) via each of
these next-hop S-PEs.
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2. Applicability
The proposed OSPFv2 and OSPFv3 protocol extensions are intended for
domains where MP-BGP is not used and/or only a partial mesh of PSN
tunnels exists. In many cases, this will apply to routing MS-PWs
across a single AS, where the source T-PE (ST-PE), the Terminating T-
PE (TT-PE) and all of the intermediate S-PEs reside in the same AS.
However, the above application may also be used where OSPF is used to
route one portion of a MS-PW across a given AS where the ST-PE and
the TT-PE reside in different ASs. Here, OSPF is used to advertise
the AIIs reachable through S-PEs corresponding to ASBRs. This enables
the ingress S-PEs and intermediate S-PEs in an AS to route MS-PWs to
the correct egress S-PE in the AS to reach a TT-PE in another AS.
This draft does not define how the egress S-PE learns what AIIs are
externally reachable through it, but this could be by configuration,
or by learning the exterior reachable addresses from an exterior
gateway protocol such as BGP.
3. OSPF Extensions
3.1. Attachment Circuit Addressing
As in [11], attachment circuit addressing is derived from AII type 2
[2], as shown in the following figure:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AII Type=02 | Length | Global ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Global ID (contd.) | Prefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix (contd.) | AC ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AC ID (contd.) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 Attachment Circuit Addressing
Implementations of this procedure MUST interpret the AII as described
in [11].
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3.2. OSPFv2 LSAs
This extension makes use of the opaque LSA.
One new LSA is defined: the PW Switching LSA. This LSA describes the
S-PEs/T-PEs, PSN tunnel(s) between peer S-PEs or T-PEs, and AII
addresses reachable.
3.2.1. Pseudowire Switching LSA
OSPFv2 routers behaving as S-PEs or T-PEs advertise the layer 2
addresses reachable through them. This advertisement MUST be in an
AS-scoped or Area-scoped opaque LSA described in [3].
The 'O' bit in the LSA Option field MUST be set to 1. The Opaque LSA
type is TBD.
[Note: IANA will assign the Opaque LSA value]
The LSA Information field is formatted as described in Section 3.4.
below.
3.3. OSPFv3 LSAs
3.3.1. Pseudowire Switching LSA
The OSPFv3 PW switching LSA has a function code of TBD. The S1/S2 bit
are set to indicate an AS flooding scope for the LSA. The U bit is
set indicating the OSPFv3 PW switching LSA should be flooded even if
it is not understood. The LSA Information field is formatted as
described in Section 3.4. below.
3.4. LSA Information Field
The LSA information consists of two or more nested Type/Length/Value
(TLV) triplets.
The LSA MUST contain a TLV for the IP address of the advertising
router. For OSPF v2 routers, this is the Router Address TLV defined
in Section 2.4.1 of RFC 3630 [4]. For OSPF v3 routers, this is the
Router IPv6 Address TLV specified in Section 3 of draft-ietf-ospf-
ospfv3-traffic-07.txt [5]. In each of these, the router address MUST
be set to the IP address of the advertising T-PE/S-PE.
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3.4.1. Exterior AII TLV
The Exterior AII TLV is used to describe addresses attached of
attached T-PEs or those routable through S-PEs to T-PEs in another
AS. If the LSA information is of type AII, then the value field
contains one or more AII Type 2 TLVs, as described above.
Exterior AIIs correspond to the AII (or AC) configured on a T-PE,
which must contain at least the prefix and global ID to be used in
FEC129 for signaling the PW endpoint. The prefix may or may not be
directly related to the loopback address of the T-PE.
4. LSA Processing Procedures
Nodes capable of pseudowire switching on either side of a PSN tunnel
exchange PW switching information using the Pseudowire Switching
opaque LSA. These Pseudowire Switching LSAs are processed and flooded
as described in Section 4.2.
4.1. P Routers
OSPF routers that receive LSAs described in this draft and that are
not S-PEs or T-PEs MUST flood them according to the rules of OSPFv2
or OSPFv3, as applicable.
4.2. PE Routers
S-PEs and T-PEs that are OSPF routers and that receive LSAs described
in this draft MUST flood them according to the rules of OSPFv2 or
OSPFv3, as applicable. These LSAs are also installed in a PW routing
database. This routing database MAY be used by S-PEs and T-PEs to
calculate a PW routing table. The PW routing table has the structure
described in Section 7 of [11], and is used to determine the next
signaling hop when a S-PE receives a PW setup message as described in
that draft. PW static routes may also be provisioned, as described in
[11].
S-PEs that are also OSPF ABR MAY opt to summarize the PW routing
information receive in type 10 area scoped opaque LSA. The
summarization can be done in a similar way as for Ipv4 or ipv6
routes.
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S-PEs that are ASBRs that receive type 10 LSAs described in this
draft MAY summarize Exterior AII TLVs received in non-backbone
advertisements in that S-PEs' own backbone advertisement, and MAY
summarize Exterior AII TLVs received in backbone advertisements in
that S-PEs non-backbone area advertisements.
5. Deployment Considerations
5.1. Addition and Removal of ACs, S-PEs and T-PEs
It is important that the impact of PW switching information
advertised on the underlying OSPF routing is minimized. To achieve
that it is RECOMMENDED that:
1. The Exterior AII TLV only contains the prefix and global ID of the
T-PE. Such summarization allows the content of the Exterior AII
TLV to remain unchanged when an AC is added or removed, thus
removing a need to re-advertise the Exterior AII TLV. Likewise, no
new LSA is advertised when AC connectivity flaps or when a
pseudowire is established/provisioned.
2. S-PEs use AII summarization that minimizes the impact on the S-
PEs' advertisements into backbone on changes to Exterior AII TLV
received in a non-backbone advertisements.
5.2. Impact on Existing P-Routers
P-routers supporting Opaque LSA processing procedures must exist
along the flooding path in the AS to ensure propagation of the
information required for dynamic pseudowire routing. Ideally,
multiple "Opaque LSA" flooding paths exist, so a failure of a router
along a path does not isolate subset of a network.
Routers supporting Opaque LSA processing as described in [3], will
flood the LSAs as specified in [3]. The impact of this additional
Opaque LSA flooding load MAY be constrained through appropriate
levels of aggregation of AIIs as described in Section 5.1. . Section
5.3. describes complementary methods for limiting the impact of any
additional flooding.
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5.3. Congestion in the Underlying PSN Routing
This draft describes the use of the underlying interior gateway
protocol in an IP network to advertise routing information for the
automatic placement of MS-PWs. Congestion may occur in the routing
plane of the PSN if a large amount of pseudowire LSAs are flooded. It
is therefore important to ensure that this does not degrade the
performance of the IGP for the underlying PSN.
Implementations MAY use a number of methods to avoid routing
congestion, including:
o Prioritization of PSN LSAs over PW Switching LSAs.
o Rate limiting PW Switching LSAs so that they do not consume
excessive bandwidth or route processor capacity.
o AII summarization methods as described in section 5.1. above
o OSPF Refresh and Flooding reduction mechanisms as defined in [7].
It is RECOMMENDED that implementations either:
o Use all of the above mentioned techniques to minimize the impact
of pseudowire switching advertisements on the underlying IGP
routing when a single routing instance is used, or
o Use a separate transport instance for pseudowire switching
advertisements.
6. Security Considerations
The security discussion in Section 6 of [3]is also applicable to PW
routing information.
7. IANA Considerations
This draft requests that the following allocations be made from
existing registries:
1. The OSPFv2 opaque LSA type TBD for the PW switching opaque LSA.
2. The OSPFv3 LSA type function code TBD for the PW switching LSA
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8. Acknowledgments
The authors gratefully acknowledge the contributions of Vach
Kompella, Devendra Raut and Yuichi Ikejiri.
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9. References
9.1. Normative References
[1]Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2]Metz, C., et al, "AII Types for Aggregation", Internet Draft,
draft-metz-aii-aggregate-01.txt, October 2006.
[3]Coltun, R., " The OSPF Opaque LSA Option", RFC 2370, July 1998
[4]Katz D. et al., "Traffic Engineering (TE) Extensions to OSPF
Version 2", RFC 3630, September 2003
[5]Ishiguro K. et al., "Traffic Engineering Extensions to OSPF
version 3", Internet Draft, draft-ietf-ospf-ospfv3-traffic-
07.txt, April 2006
[6]Metz C. et al., "Pseudowire Attachment Identifiers for Aggregation
and VPN Autodiscovery", Internet Draft, draft-ietf-pwe3-aii-
aggregate-01.txt, October 2006
[7]Pillay-Esnault P., "OSPF Refresh and Flooding Reduction in Stable
Topologies", RFC 4136, July 2005
9.2. Informative References
[8]Bitar, N., Bocci, M., and Martini, L., "Requirements for inter
domain Pseudo-Wires", Internet Draft, draft-ietf-pwe3-ms-pw-
requirements-02.txt, May 2006
[9]Bocci, M., and Bryant, S.,T., " An Architecture for Multi-Segment
Pseudo Wire Emulation Edge-to-Edge", Internet Draft, draft-
ietf-pwe3-ms-pw-arch-01.txt, May 2006
[10] Martini et al, "Segmented Pseudo Wire", Internet Draft, draft-
ietf-pwe3-segmented-pw-02.txt, March 2006
[11] Martini, L., Bocci, M., Balus, F., et al, " Dynamic Placement
of Multi Segment Pseudo Wires", Internet Draft, draft-ietf-
pwe3-dynamic-ms-pw-00.txt, December 2005
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Author's Addresses
Matthew Bocci
Alcatel-Lucent
Voyager Place,
Shoppenhangers Road
Maidenhead
Berks, UK
Email: matthew.bocci@alcatel-lucent.co.uk
Dimitri Papadimitriou
Alcatel-Lucent
Copernicuslaan 50
2018 ANTWERP
BELGIUM
Email: dimitri.papadimitriou@alcatel-lucent.be
Alex Zinin
ALCATEL-Lucent.
701 East Middlefield Road
M/S MOUNT-HRPB6
MOUNTAIN VIEW, CA 94043
USA
Email: alex.zinin@alcatel-lucent.com
Mustapha Aissaoui
Alcatel-Lucent
600 March Road
OTTAWA, ON K2K 2E6
CANADA
Email: mustapha.aissaoui@alcatel-lucent.com
Andrew Dolganow
Alcatel-Lucent
600 March Road
OTTAWA, ON K2K 2E6
CANADA
Email: andrew.dolganow@alcatel-lucent.com
Yuji Kamite
NTT Communcations
Email: y.kamite@ntt.com
Luca Martini
Cisco
lmartini@cisco.com
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Frederic Jounay
France Telecom
Frederic.jounay@orange-ftgroup.com
Luca Martini
Cisco
lmartini@cisco.com
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This document is subject to the rights, licenses and restrictions
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