One document matched: draft-ginsberg-isis-extlsp-00.txt
INTERNET DRAFT Simplified Extension of LSP Space Mar 2006
Network Working Group L. Ginsberg
Internet Draft S. Previdi
Expiration Date: Aug 2006 M. Shand
Cisco Systems
March 2006
Simplified Extension of LSP Space for IS-IS
draft-ginsberg-isis-extlsp-00.txt
Status of this Memo
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Abstract
This draft describes a simplified method for extending the LSP space
beyond the 256 Link State PDU (LSP) limit defined in ISO 10589. This
method is intended as a preferred replacement for the method defined
in RFC 3786.
Table of Contents
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1. Conventions used in this document..............................2
2. Overview.......................................................2
3. Definitions of Commonly Used Terms.............................3
4. Utilizing Additional System IDs................................3
4.1 Additional Information in Extended LSPs......................4
4.2 Extended LSP Restrictions....................................4
4.2.1 TLVs Which MUST NOT Appear................................4
4.2.2 Overload, Attached, and Partition Repair Bits.............5
4.3 Originating LSP Restrictions.................................5
4.4 IS Alias ID TLV (IS-Alias)...................................5
5. Comparison with the RFC 3786 Solution..........................6
6. Deployment Considerations......................................7
6.1 Advertising New TLVs in Extended LSPs........................7
6.2 Reachability and TLV Staleness...............................7
6.3 Moving TE Information to Extended LSPs.......................7
7. Security Considerations........................................8
8. IANA Considerations............................................8
9. Normative References...........................................8
10. Acknowledgments...............................................9
11. Authors' Addresses............................................9
12. Full Copyright Statement......................................9
1. 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 [BCP4].
2. Overview
[IS-IS] defines the set of LSP fragments which may be originated by
a system at each level. This set is limited to 256 fragments. [IS-
IS] also defines a maximum value for an LSP fragment
(originatingLxLSPBufferSize) as 1492 bytes. The carrying capacity of
an LSP set is sufficient to advertise the neighbor and leaf node
reachability advertisements of an area/domain in anticipated
deployment scenarios. However, the definition of additional
information to be included in LSPs (e.g. traffic engineering
information, router capabilities, etc.) has the potential to exceed
the carrying capacity of an LSP set.
This issue first drew interest when traffic engineering extensions
were introduced. This interest resulted in the solution defined in
RFC 3786. However, that solution suffers from restrictions required
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to maintain interoperability with systems which do not support the
extensions.
This document defines extensions which allow a system to exceed the
256 fragment limit and do so in a way which has no interoperability
issues with systems which do not support the extension. It is seen
as a simpler and therefore preferred solution to the problem.
3. Definitions of Commonly Used Terms
This section provides definitions for terms that are used throughout
the text. The terminology is consistent with that used in RFC 3786.
Originating System: A router physically running the IS-IS protocol.
As this document describes a method which allows a single IS-IS
instance to originate LSPs using multiple instance identifiers,
the Originating System represents the single "physical" IS-IS
instance.
Normal system-id: The system-id of an Originating System as defined
by [IS-IS].
Additional system-id: A system-id other than the "Normal system-id"
that is assigned by the network administrator to an Originating
System in order to allow the generation of extended LSP fragments.
The Additional system-id, like the Normal system-id, must be
unique throughout the routing area (Level-1) or domain (Level-2).
Original LSP: An LSP using the Normal system-id in its LSP ID.
Extended LSP: An LSP using an Additional system-id in its LSP ID.
LSP set: Logical LSP. This term is used only to resolve the
ambiguity between a logical LSP and an LSP fragment, both of which
are sometimes termed "LSP".
Extended LSP set: A group of LSP fragments using an Additional
system-id, and originated by the Originating System.
Extension-capable IS: An IS implementing the mechanisms described in
this document.
4. Utilizing Additional System IDs
This extension allows an Originating System to be assigned
additional system-ids which may be used to generate additional LSP
sets. The additional system-ids are subject to the same restrictions
as normal system-ids i.e. when used at Level-1 the additional
system-id MUST be unique within the Level-1 area. When used at
Level-2 the additional system-id MUST be unique within the domain.
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Extended LSPs are treated by the IS-IS Update Process in the same
manner as normal LSPs i.e. the same rules as to generation,
flooding, purging, etc. apply. In particular, if the Extended LSP
with LSP Number zero and remaining lifetime > 0 is not present for a
particular additional system-id then none of the extended LSPs in
that LSP set shall be processed.
4.1 Additional Information in Extended LSPs
Fragment 0 of an Extended LSP Set MUST include the new IS alias ID
TLV defined in Section 4.4. This allows the Extended LSP set to be
associated with the Originating System which generated the LSP(s).
4.2 Extended LSP Restrictions
The following restrictions on the information which may appear in an
Extended LSP are defined in order to avoid interoperability issues
with systems which do not support the extensions defined in this
document,
4.2.1 TLVs Which MUST NOT Appear
Information which is directly utilized in the SPF calculation MUST
NOT appear in an Extended LSP. This includes the following TLVs
currently defined in the IANA IS-IS TLV Codepoints Registry:
TLV Name (#)
-----------
Area Addresses (1)
ES Neighbors (3)
Part. DIS (4)
Prefix Neighbors (5)
The extended IS reachability TLV (22)
IP Int. Reach (128)
IP Ext. Address (130)
The extended IP reachability TLV (135)
MT-ISN (222)
M-Topologies (229)
MT IP. Reach (235)
IPv6 IP. Reach (236)
MT IPv6 IP. Reach (237)
If any of the TLVs listed above appear in an Extended LSP, an
Extension Capable IS MUST ignore those TLVs on receipt and SHOULD
report an error. Other TLVs in that extended LSP set MUST be
processed normally.
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4.2.2 Overload, Attached, and Partition Repair Bits
As Extended LSPs do not contain neighbor or area information the use
of the Overload, Attached, and Partition Repair Bits have no
significance. All of these bits MUST be set to 0 in all Extended LSP
fragments by the originating system and MUST be ignored on receipt.
4.3 Originating LSP Restrictions
An originating LSP MUST NOT advertise a neighbor relationship to any
of its additional system-ids.
4.4 IS Alias ID TLV (IS-Alias)
The proposed IS-Alias TLV allows extension-capable ISs to recognize
the Originating System of an Extended LSP set. It identifies the
Normal system-id of the Originating System.
Type 24
Length # of octets in the value field (7 to 255)
Value
No. of octets
+-----------------------+
| Normal System-id | 6
+-----------------------+
| Sub-TLV length | 1
+-----------------------+
| Sub-TLVs (optional) | 0 to 248
+-----------------------+
Normal system-id
The Normal system-id of the Originating System
Sub-TLVs length
Total length of all sub-TLVs.
Sub-TLVs
A series of tuples with the following format:
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No. of Octets
+-------------------+
| Sub-type | 1
+-------------------+
| Length | 1
+-------------------+
| | 0-246
: Value :
: :
| |
+-------------------+
Sub-type
Type of the sub-TLV
Length
Total length of the value field
Value
Type-specific TLV payload.
No subTLVs are defined in this document. Should future extensions
define subTLVs, the subTLVs MUST be formatted as described in RFC
3784.
5. Comparison with the RFC 3786 Solution
This document utilizes the same basic mechanism (additional system-
ids) as RFC 3786 to allow an originating system to generate more
than 256 LSP fragments. It differs from RFC 3786 in that it
restricts the content of Extended LSPs to information which is NOT
utilized in the SPF calculation.
Extended LSPs in this new definition are utilized solely to
accommodate the additional information which various extensions have
defined as new TLV content in LSPs. This distinction allows original
LSPs to be used in the traditional manner and supports
implementation optimizations which ignore extended LSP content when
performing an SPF calculation.
Legacy IS-IS implementations which do not support the extensions
defined in this document see the extended LSPs as information
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associated with an unreachable system. Their SPF calculation is
therefore consistent with that performed by extension capable ISs.
There is therefore no need for the two different operating modes
defined in RFC 3786.
There is also no need for the special handling of the original LSP
set and the extended LSP set(s) as a single Logical LSP during the
SPF as specified in Section 5 of RFC 3786.
6. Deployment Considerations
There are a number of deployment considerations which limit the
usefulness of extended LSPs unless all systems are extension-capable
ISs.
6.1 Advertising New TLVs in Extended LSPs
As extended LSPs will be utilized to advertise TLVs associated with
other protocol extensions (definition of which is outside the scope
of this document), it is obvious that the utilization of the
information in extended LSPs by legacy IS-IS implementations will be
limited. The implication of this is that as implementations are
revised to support the protocol extensions which define new TLVs
that MAY be advertised in extended LSPs, the implementation SHOULD
also be revised to support the extensions defined in this document
so that they are capable of processing the new TLVs whether they
appear in normal or extended LSPs.
6.2 Reachability and TLV Staleness
In cases where non-SPF information is advertised in LSPs, it is
necessary to determine whether the system which originated the
advertisement is reachable in order to guarantee that a receiving
router does not use or leak stale information. In the case of TLV
information found in extended LSPs, the reachability test MUST be to
the originating system, not to the additional system ID associated
with the extended LSP. Systems which do NOT support the extensions
defined in this document will always see the extended LSPs as coming
from an unreachable system. They will therefore NOT use information
in extended LSPs in cases where reachability is a requirement.
6.3 Moving TE Information to Extended LSPs
One of the major sources of non-SPF related additional information
in LSPs is the Traffic Engineering (TE) information carried in the
extended IS reachability TLV (22) as defined in RFC 3784 and RFC
4205. The restrictions defined in this document prohibit the
presence of TLV 22 in Extended LSPs. In the event that there is a
need to advertise TE information in Extended LSPs, it would be
necessary to define a new TLV to carry the TE information.
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Utilization of a new TLV for TE information would provide additional
benefits which include:
. Elimination of the need for redundant IS neighbor TLVs to be
processed as part of the SPF.
. Easier support for a set of TE information associated with a
single link which exceeds the 255 byte TLV limit by allowing
the interpretation of multiple TLVs to be considered additive
rather than mutually exclusive.
Such an extension would require all routers on which the TE
information is processed to support the new TLV as well as the
extensions defined in this document. Definition of a new TLV to
advertise TE information is outside the scope of this document.
7. Security Considerations
This document raises no new security issues for IS-IS.
8. IANA Considerations
This document defines the following new ISIS TLV that needs to be
reflected in the ISIS TLV code-point registry:
Type Description IIH LSP SNP
---- ----------------------------------- --- --- ---
24 IS Alias ID n y n
9. Normative References
[IS-IS] ISO, "Intermediate system to Intermediate system routeing
information exchange protocol for use in conjunction with the
Protocol for providing the Connectionless-mode Network Service
(ISO 8473)," ISO/IEC 10589:2002, Second Edition.
[RFC 3784] Smit, H. and T. Li, "Intermediate System to
Intermediate System (IS-IS) Extensions for Traffic Engineering
(TE)", RFC 3784, June 2004.
[RFC 3786] Hermelin, A., Previdi, S. and Shand, M., "Extending the
Number of Intermediate to Intermediate (IS-IS) Link State PDU
(LSP) Fragments Beyond the 256 Limit," RFC 3786, May 2004.
[RFC 4205] Kompella, K. and Rehkter, Y., "Intermediate System to
Intermediate System (IS-IS) Extensions in Support of Generalized
Multi-Protocol Label Switching (GMPLS)", RFC 4205, October 2005.
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[BCP9] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
[BCP14] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997
[BCP26] Narten, T. and Alvestrand, H., "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26 , RFC 2434, October
1998
10. Acknowledgments
11. Authors' Addresses
Les Ginsberg
Cisco Systems
510 McCarthy Blvd.
Milpitas, Ca. 95035 USA
Email: ginsberg@cisco.com
Stefano Previdi
CISCO Systems, Inc.
Via Del Serafico 200
00142 - Roma
ITALY
Email: sprevidi@cisco.com
Mike Shand
Cisco Systems
250 Longwater Avenue,
Reading,
Berkshire,
RG2 6GB
UK
Email: mshand@cisco.com
12. Full Copyright Statement
Copyright (C) The Internet Society (2006). All Rights Reserved.
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 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
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