draft-ietf-isis-ext-lsp-frags-01.txt   draft-ietf-isis-ext-lsp-frags-02.txt 
Network Working Group Amir Hermelin Network Working Group Amir Hermelin
Internet Draft Charlotte's Web Networks Internet Draft Charlotte's Web Networks
Expiration Date: December 2002 Expiration Date: February 2003
Stefano Previdi Stefano Previdi
Mike Shand Mike Shand
Cisco Systems Cisco Systems
Extending the Number of IS-IS LSP Fragments Beyond the 256 Limit Extending the Number of IS-IS LSP Fragments Beyond the 256 Limit
draft-ietf-isis-ext-lsp-frags-01.txt draft-ietf-isis-ext-lsp-frags-02.txt
Status Status
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
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LSP set LSP set
Logical LSP. This term is used only to resolve the ambiguity Logical LSP. This term is used only to resolve the ambiguity
between a logical LSP and an LSP fragment, both of which are between a logical LSP and an LSP fragment, both of which are
sometimes termed "LSP". sometimes termed "LSP".
Extended LSP set Extended LSP set
A group of LSP fragments using an Additional system-id, and A group of LSP fragments using an Additional system-id, and
originated by the Originating System. originated by the Originating System.
Extension-capable IS Extension-capable IS
An IS implementing this extension. An IS implementing the mechanisms described in this document.
1.3 Operation Modes 1.3 Operation Modes
Two administrative operation modes are provided: Two administrative operation modes are provided:
- Operation Mode 1 provides behavior that allows implementations - Operation Mode 1 provides behavior that allows implementations
that don't support this extension, to correctly process the that don't support this extension, to correctly process the
extended fragment information, without any modifications. This extended fragment information, without any modifications. This
mode has some restrictions on what may be advertised in the mode has some restrictions on what may be advertised in the
extended LSP fragments. Namely, only leaf information may be extended LSP fragments. Namely, only leaf information may be
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advertised in the extended LSPs. However, it mandates a change advertised in the extended LSPs. However, it mandates a change
to the way LSPs are considered during the SPF algorithm, in a way to the way LSPs are considered during the SPF algorithm, in a way
that isn't compatible with previous implementations. that isn't compatible with previous implementations.
These modes are configured on a per-level and area basis. That is, These modes are configured on a per-level and area basis. That is,
all LSPs considered in the same SPF instance MUST use the same Mode. all LSPs considered in the same SPF instance MUST use the same Mode.
There is no restriction that an L1/L2 IS operates in the same mode, There is no restriction that an L1/L2 IS operates in the same mode,
for both its L1 and L2 instances. It can use Mode 1 for its L1 LSPs, for both its L1 and L2 instances. It can use Mode 1 for its L1 LSPs,
and Mode 2 for its L2 LSPs, or vice versa. and Mode 2 for its L2 LSPs, or vice versa.
Mode 1 has the only advantage of being backwards compatible with
older implementations. It does have restrictions which are considered
drawbacks. Therefore, routers should operate in Mode 1 only if
backwards compatibility is desired. Otherwise, it is recommended to
run in Mode 2.
Routers MAY implement Operational Mode 2 without supporting running Routers MAY implement Operational Mode 2 without supporting running
in Operational Mode 1. They will still interoperate correctly with in Operational Mode 1. They will still interoperate correctly with
routers that support both modes. routers that support both modes.
1.4 Overview 1.4 Overview
Using Additional system-ids assigned by the administrator, the Using Additional system-ids assigned by the administrator, the
Originating System can advertise the excess link-state information in Originating System can advertise the excess link-state information in
extended LSPs under these Additional system-ids. It would do so as extended LSPs under these Additional system-ids. It would do so as
if other routers, or "Virtual Systems", were advertising this if other routers, or "Virtual Systems", were advertising this
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extended LSPs under numerous Virtual Systems. extended LSPs under numerous Virtual Systems.
For Operation Mode 1, 0-cost adjacencies are advertised from the For Operation Mode 1, 0-cost adjacencies are advertised from the
Originating System to its Virtual System(s). No adjacencies (other Originating System to its Virtual System(s). No adjacencies (other
than back to the Originating System) are advertised in the extended than back to the Originating System) are advertised in the extended
LSPs. As a consequence, the Virtual Systems are 'stub', meaning they LSPs. As a consequence, the Virtual Systems are 'stub', meaning they
can only be reached through their Originating System. Therefore, can only be reached through their Originating System. Therefore,
older implementations do not need modifications in order to correctly older implementations do not need modifications in order to correctly
process these extended LSPs. process these extended LSPs.
For both modes, each LSP (set) created by a node will contain on its For both modes, each LSP (set) created by a node will contain in its
fragment-0 a new TLV (IS Alias ID TLV) that contains the Normal fragment-0 a new TLV (IS Alias ID TLV) that contains the Normal
system-id and PN Number of the (first) LSP created by the router. system-id and PN Number of the Original LSP created by the router.
Extension-capable ISs can then use this information and store the Extension-capable ISs can then use this information and store the
original and extended LSPs as one logical LSP. original and extended LSPs as one logical LSP.
2.0 IS Alias ID TLV (IS-A) The only sections that deal only with Mode 1 additions are 3.2,
3.2.1, and 3.2.2. All other sections relate to both modes.
2.0 IS Alias ID TLV (IS-A)
The proposed IS-A TLV allows extension-capable ISs to recognize all The proposed IS-A TLV allows extension-capable ISs to recognize all
LSPs of an Originating System, and combine the original and extended LSPs of an Originating System, and combine the original and extended
LSPs for the purpose of SPF computation. It identifies the Normal LSPs for the purpose of SPF computation. It identifies the Normal
system-id of the Originating System. system-id of the Originating System.
The proposed IS Alias ID TLV is type 24, and its format is as The proposed IS Alias ID TLV is type 24, and its format is as
follows: follows:
x CODE - 24. x CODE - 24.
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Length Length
Total length of the value field Total length of the value field
Value Value
Type-specific TLV payload. Type-specific TLV payload.
For an explanation on sub-TLV handling, see [ISIS-TE]. For an explanation on sub-TLV handling, see [ISIS-TE].
Without sub-TLVs, this structure consumes 8 octets per LSP set. This Without sub-TLVs, this structure consumes 8 octets per LSP set. This
TLV MUST be included in fragment 0 of every LSP set belonging to an TLV MUST be included in fragment 0 of every LSP set belonging to an
Originating System. Currently, there are no sub-TLVs defined. Originating System running in either Mode 1 or Mode 2. Currently,
there are no sub-TLVs defined.
For a complete list of used IS-IS TLV numbers, see [ISIS-CODES]. For a complete list of used IS-IS TLV numbers, see [ISIS-CODES].
3.0 Generating LSPs 3.0 Generating LSPs
3.1 Both Operation Modes 3.1 Both Operation Modes
Under both modes, the Originating System MUST include information Under both modes, the Originating System MUST include information
binding the Original LSP and the Extended ones. It can do this since binding the Original LSP and the Extended ones. It can do this since
it is trivially an extension-capable IS. This is to ensure other it is trivially an extension-capable IS. This is to ensure other
extension-capable routers correctly process the extra information in extension-capable routers correctly process the extra information in
their SPF calculation. This binding is advertised via a new IS Alias their SPF calculation. This binding is advertised via a new IS Alias
ID TLV, which is advertised in all fragment 0, whether they belong to ID TLV, which is advertised in all fragment 0 of Original and
the original LSP or to the extended ones. Extended LSPs.
+---------------------------------------------+ +---------------------------------------------+
| Originating System | | Originating System |
| system-id = S | | system-id = S |
| is-alias-id = S | | is-alias-id = S |
+---------------------------------------------+ +---------------------------------------------+
+-------------------+ +-------------------+ +-------------------+ +-------------------+
| Virtual System | | Virtual System | | Virtual System | | Virtual System |
| system-id = S' | | system-id = S''| | system-id = S' | | system-id = S''|
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| is-alias-id = S | | is-alias-id = S | | is-alias-id = S | | is-alias-id = S |
+-------------------+ +-------------------+ +-------------------+ +-------------------+
Figure 1. Advertising binding between all of a system's LSPs (both Figure 1. Advertising binding between all of a system's LSPs (both
modes). S' and S'' are configured as Additional system-ids. modes). S' and S'' are configured as Additional system-ids.
When new extended LSP fragments are generated, these fragments should When new extended LSP fragments are generated, these fragments should
be generated as specified in ISO 10589 [ISIS-ISO]. Furthermore, a be generated as specified in ISO 10589 [ISIS-ISO]. Furthermore, a
system SHOULD treat its extended LSPs the same as it treats its system SHOULD treat its extended LSPs the same as it treats its
original LSPs, with the exceptions noted in the following sections. original LSPs, with the exceptions noted in the following sections.
Specifically, creating, flooding, renewing, purging and all other Specifically, creating, flooding, renewing, purging and all other
operations are similar for both original and extended LSPs, unless operations are similar for both Original and Extended LSPs, unless
stated otherwise. The extended LSPs will use one of the Additional stated otherwise. The Extended LSPs will use one of the Additional
system-ids configured for the router, in their LSP ID. system-ids configured for the router, in their LSP ID.
Extended LSPs fragment zero should be regarded in the same special Extended LSPs fragment zero should be regarded in the same special
manner as specified in 10589 for LSPs with number zero, and should manner as specified in 10589 for LSPs with number zero, and should
include the same type of extra information as specified in 10589 and include the same type of extra information as specified in 10589 and
RFC 1195 [ISIS-IP]. So, for example, when a system reissues its LSP RFC 1195 [ISIS-IP]. So, for example, when a system reissues its LSP
fragemnt zero due to an area address change, it should reissue all fragemnt zero due to an area address change, it should reissue all
extended LSPs fragment zero as well. extended LSPs fragment zero as well.
An extended LSP fragment zero MUST be generated for every extended An extended LSP fragment zero MUST be generated for every extended
LSP set, to allow a router's SPF calculation to consider those LSP set, to allow a router's SPF calculation to consider those
fragments in that set. fragments in that set. See section 5 for details.
3.1.1 The Attached Bits 3.1.1 The Attached Bits
The Attached (ATT) bits SHOULD be set to zero for all four metric The Attached (ATT) bits SHOULD be set to zero for all four metric
types, on all extended LSPs. This is due to the following: if a types, on all Extended LSPs. This is due to the following: if a
Virtual System is reachable, so is its Originating System. It is Virtual System is reachable, so is its Originating System. It is
preferable, then, that an L1 IS chooses the Originating System and preferable, then, that an L1 IS chooses the Originating System and
not the Virtual System as its nearest L2 exit point, as connectivity not the Virtual System as its nearest L2 exit point, as connectivity
to the Virtual System has a higher probability of being lost (a to the Virtual System has a higher probability of being lost (as a
result of the extended LSP no longer being advertised). This could result of the extended LSP no longer being advertised). This could
cause unnecessary computations on some implementations. cause unnecessary computations on some implementations.
3.1.2 The Partition Repair Bit 3.1.2 The Partition Repair Bit
The Partition Repair (P) bit SHOULD be set to zero on all extended The Partition Repair (P) bit SHOULD be set to zero on all extended
LSPs. This is for the same reasons as for the Attached bits. LSPs. This is for the same reasons as for the Attached bits.
3.1.3 ES Neighbors TLV 3.1.3 ES Neighbors TLV
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ISO 10589 [ISIS-ISO] section 7.3.7 specifies inserting an ES Neighbor ISO 10589 [ISIS-ISO] section 7.3.7 specifies inserting an ES Neighbor
TLV in L1 LSPs, with the system ID of the router. RFC 1195 [ISIS-IP] TLV in L1 LSPs, with the system ID of the router. RFC 1195 [ISIS-IP]
relieves IP-only routers of this requirement. However, for routers relieves IP-only routers of this requirement. However, for routers
that do insert this ESN TLV in L1 LSPs (whether IP-only or OSI- that do insert this ESN TLV in L1 LSPs (whether IP-only or OSI-
capable), then in an extended LSP, the ESN TLV should include the capable), then in an extended LSP, the ESN TLV should include the
relevant Additional system-id. Furthermore, OSI-capable routers relevant Additional system-id. Furthermore, OSI-capable routers
should accept packets destined for this Additional system-id. should accept packets destined for this Additional system-id.
3.1.4 Overload Bit 3.1.4 Overload Bit
The overload bit should be set consistently across all LSPs, original The overload bit should be set consistently across all LSPs, original
and extended, belonging to an originating system, and should reflect and extended, belonging to an Originating System, and should reflect
the originating system's overload state. the Originating System's overload state.
3.1.5 Other Fields and TLVs 3.1.5 Other Fields and TLVs
Other fields and TLVs not mentioned above remain the same, both for Other fields and TLVs not mentioned above remain the same, both for
original and extended LSPs. original and extended LSPs.
3.2 Operation Mode 1 Additions 3.2 Operation Mode 1 Additions
The following additions apply only to routers generating LSPs in Mode The following additions apply only to routers generating LSPs in Mode
1. Routers, which are configured to operate in Operation Mode 2, 1. Routers, which are configured to operate in Operation Mode 2,
SHOULD NOT apply these additions to their advertisements. SHOULD NOT apply these additions to their advertisements.
Under Operation Mode 1, adjacencies between the normal system and its Under Operation Mode 1, adjacencies from the Originating System to
Virtual Systems are advertised using the standard neighbor TLVs. The its Virtual Systems are advertised using the standard neighbor TLVs.
metric for these connections MUST be zero, since the cost of reaching The metric for these connections MUST be zero, since the cost of
a Virtual System is the same as the cost of reaching its Originating reaching a Virtual System is the same as the cost of reaching its
System. Originating System.
To older implementations, Virtual Systems would appear reachable only To older implementations, Virtual Systems would appear reachable only
through their Originating System, hence loss of connectivity to the through their Originating System, hence loss of connectivity to the
Originating System means loss of connectivity to all of its Originating System means loss of connectivity to all of its
information, including that advertised in its extended LSPs. information, including that advertised in its extended LSPs.
Furthermore, the cost of reaching information advertised in non- Furthermore, the cost of reaching information advertised in non-
extended LSPs is the same as the cost of reaching information extended LSPs is the same as the cost of reaching information
advertised in the new extended LSPs, with an additional hop. advertised in the new extended LSPs, with an additional hop.
+---------------------------------------------+ +---------------------------------------------+
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| is-alias-id = S | | is-alias-id = S | | is-alias-id = S | | is-alias-id = S |
+-------------------+ +-------------------+ +-------------------+ +-------------------+
Figure 2. Advertising connections to Virtual Systems under Operation Figure 2. Advertising connections to Virtual Systems under Operation
Mode 1. S' and S'' are configured as Additional system-ids. Mode 1. S' and S'' are configured as Additional system-ids.
Under Operation Mode 1, only "leaf" information, i.e. information Under Operation Mode 1, only "leaf" information, i.e. information
that serves only as leaves in a shortest path tree, can be advertised that serves only as leaves in a shortest path tree, can be advertised
in extended LSPs. in extended LSPs.
When an extended LSP belonging to Additional system-id S' is first When an Extended LSP belonging to Additional system-id S' is first
created, the original LSP MUST specify S' as a neighbor, with metric created, the Original LSP MUST specify S' as a neighbor, with metric
set to zero. This in order to consider the cost of reaching the set to zero. This in order to consider the cost of reaching the
Virtual System S' the same as the cost of reaching its Originating Virtual System S' the same as the cost of reaching its Originating
System. Furthermore, the extended LSP MUST specify the Normal System. Furthermore, the Extended LSP MUST specify the Normal
system-id as a neighbor, with metric set to (MaxLinkMetric - 1). system-id as a neighbor. The metric SHOULD be set to MaxLinkMetric -
This in order to satisfy the two-way connectivity check on other 1 (this is only for uniformity purpose, any metric greater than zero
routers. Where relevant, this adjacency SHOULD be considered as is ok). This in order to satisfy the two-way connectivity check on
point-to-point. other routers. Where relevant, this adjacency SHOULD be considered
as point-to-point.
Note, that the restriction specified in ISO 10589 section 7.2.5 Note, that the restriction specified in ISO 10589 section 7.2.5
holds: if an LSP Number zero of the Originating System is not holds: if an LSP Number zero of the Originating System is not
present, none of that system's neighbor entries would be processed present, none of that system's neighbor entries would be processed
during SPF, hence none of its extended LSPs would be processed as during SPF, hence none of its extended LSPs would be processed as
well. well.
3.2.1 IS Neighbors TLV 3.2.1 IS Neighbors TLV (Mode 1 Only)
An Extended LSP must specify only the Originating System as a An Extended LSP must specify only the Originating System as a
neighbor, with the metric set to (MaxLinkMetric - 1). Where neighbor, with the metric set to (MaxLinkMetric - 1). Where
relevant, this adjacency should be considered as point-to-point. relevant, this adjacency should be considered as point-to-point.
Other neighbors MUST NOT be specified in an Extended LSP, because Other neighbors MUST NOT be specified in an Extended LSP, because
those other neighbors would only specify the Originating System and those other neighbors would only specify the Originating System and
not the additional system, and hence would not satisfy the bi- not the Virtual System, and hence would not satisfy the bi-
directionality check in the SPF computation. directionality check in the SPF computation.
3.2.2 Originating System in the Overload State in (Mode 1 Only)
If the Originating System is in the overload state, information in
the extended LSPs will not be processed by other routers in their SPF
computation. This is because in Mode 1, extended LSPs are reachable
only through adjacencies from the Original LSP. If this LSP has set
its OL bit, adjacencies will not be processed in the SPF computation.
This side effect should be taken into consideration when operating in
Mode 1.
4. Purging Extended LSP Fragments 4. Purging Extended LSP Fragments
ISO 10589 [ISIS-ISO] section 7.3.4.4 note 21 suggests that an ISO 10589 [ISIS-ISO] section 7.3.4.4 note 21 suggests that an
implementation keeps the number of LSP fragments within a certain implementation keeps the number of LSP fragments within a certain
limit based on the optimal (minimal) number of fragments needed. limit based on the optimal (minimal) number of fragments needed.
Section 7.3.4.6 also recommends that an IS purge its empty LSPs to Section 7.3.4.6 also recommends that an IS purge its empty LSPs to
conserve resources. These recommendations hold for the extended LSP conserve resources. These recommendations hold for the extended LSP
fragments as well. However, an extended LSP fragment zero should not fragments as well. However, an extended LSP fragment zero should not
be purged until all of the fragments in its set (i.e. belonging to a be purged until all of the fragments in its set (i.e. belonging to a
particular Additional system-id), are empty as well. This is to particular Additional system-id), are empty as well. This is to
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When considering LSPs of an extension-capable IS (identified by the When considering LSPs of an extension-capable IS (identified by the
inclusion of the IS Alias ID TLV), the original and extended LSPs are inclusion of the IS Alias ID TLV), the original and extended LSPs are
combined to form one large logical LSP. If the LSP belongs to an IS combined to form one large logical LSP. If the LSP belongs to an IS
running Operational Mode 1, there might be adjacencies between the running Operational Mode 1, there might be adjacencies between the
original and extended LSPs. These are trivially ignored (since when original and extended LSPs. These are trivially ignored (since when
processing them the large logical LSP is already on PATHS), and processing them the large logical LSP is already on PATHS), and
doesn't complicate the SPF. Furthermore, this check should already doesn't complicate the SPF. Furthermore, this check should already
be implemented (this scenario could occur on error, without this be implemented (this scenario could occur on error, without this
extension). extension).
If LSP fragment 0 of the original LSP set is missing or its If LSP fragment 0 of the Original LSP set is missing or its
RemainingLifetime is zero, all of the LSPs generated by that RemainingLifetime is zero, all of the LSPs generated by that
Originating System (extended as well) MUST NOT be considered in the Originating System (Extended as well) MUST NOT be considered in the
SPF. That is, the large logical LSP isn't considered in the SPF. SPF. That is, the large logical LSP isn't considered in the SPF.
The original LSP fragments are identified when the is-alias-id value The original LSP fragments are identified when the is-alias-id value
is the same as the system-id of those LSPs. If an LSP fragment 0 of is the same as the system-id of those LSPs. If an LSP fragment 0 of
an extended LSP set is missing or its RemainingLifetime is zero, only an extended LSP set is missing or its RemainingLifetime is zero, only
that LSP set MUST NOT be considered in the SPF. These rules are that LSP set MUST NOT be considered in the SPF. These rules are
present to ensure consistent SPF results on Mode 1 and Mode 2 LSPs. present to ensure consistent SPF results on Mode 1 and Mode 2 LSPs.
Note, that the above behavior is consistent with how previous Note, that the above behavior is consistent with how previous
implementations will interpret Mode 1 LSPs. implementations will interpret Mode 1 LSPs.
 End of changes. 

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