Internet DRAFT - draft-xu-isis-global-label-sid-adv
draft-xu-isis-global-label-sid-adv
Network Working Group X. Xu
Internet-Draft M. Chen
Intended status: Standards Track Huawei
Expires: June 28, 2014 December 25, 2013
Advertising Global Labels or SIDs Using IS-IS
draft-xu-isis-global-label-sid-adv-00
Abstract
Segment Routing (SR) is a new MPLS paradigm in which each SR-capable
router is required to advertise global MPLS labels or Segment IDs
(SID ) for its attached prefixes by using link-state IGPs, e.g., IS-
IS. One major challenge associated with such global MPLS label or
SID advertisement mechanism is how to avoid a given global MPLS label
or SID from being allocated by different routers to different
prefixes. Although such global label or SID allocation collision
problem can be addressed through manual allocation , it is error-
prone and nonautomatic therefore may not be suitable in large-scale
SR network environments. This document proposes an alternative
approach for allocating and advertising global MPLS labels or SIDs
via IS-IS so as to eliminate the potential risk of label allocation
collision.
Status of This Memo
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Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Advertising Label Bindings for Prefixes using IS-IS . . . . . 3
4. Advertising SID Bindings for Prefixes using IS-IS . . . . . . 4
5. Requesting Label Bindings for Prefixes using IS-IS . . . . . 5
6. Requesting SID Bindings for Prefixes using IS-IS . . . . . . 5
7. Mapping Server Redundancy . . . . . . . . . . . . . . . . . . 6
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
10. Security Considerations . . . . . . . . . . . . . . . . . . . 7
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
11.1. Normative References . . . . . . . . . . . . . . . . . . 7
11.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
Segment Routing (SR) [I-D.filsfils-rtgwg-segment-routing] is a new
MPLS paradigm in which each SR-capable router is required to
advertise global MPLS labels or Segment IDs (SID) for its attached
prefixes by using link-state IGPs, e.g., IS-
IS[I-D.previdi-isis-segment-routing-extensions] . One major challenge
associated with such global MPLS label or SID advertisement mechanism
is how to avoid a given global MPLS label or SID from being allocated
by different routers to different prefixes. Although such global
label or SID allocation collision problem can be addressed through
manual allocation , it is error-prone and nonautomatic therefore may
not be suitable in large-scale SR network environments.
This document proposes an alternative approach for allocating and
advertising global MPLS labels or SIDs via IS-IS so as to eliminate
the potential risk of label allocation collision. The basic idea of
this approach is to allow a particular IGP router to allocate global
MPLS labels or SIDs for those prefixes attached to each SR-capable
router and meanwhile advertise the corresponding label or SID
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bindings in the IGP domain scope. That particular IGP rouer is
therefore refered to as a mapping server. As for how the mapping
server know which prefixes need to be allocated with global labels or
SIDs, it can be achieved either by configuration on the mapping
server or by advertisement from SR-capable routers. In the multi-
level scenario where route summarization between levels is enabled,
the IP longest-match algorithm SHOULD be used by SR-capable routers
when processing label or SID bindings advertised by the mapping
server, just as the mechanism defined in [RFC5283] .
1.1. Requirements Language
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 [RFC2119].
2. Terminology
This memo makes use of the terms defined in
[I-D.filsfils-rtgwg-segment-routing] and [RFC4971].
3. Advertising Label Bindings for Prefixes using IS-IS
A mapping server could uses one or more of the following TLVs to
advertise global labels for those prefixes which need to be allocated
with global labels.
o TLV-135 (IPv4) [RFC5305]
o TLV-235 (MT-IPv4) [RFC5120]
o TLV-236 (IPv6) [RFC5308]
o TLV-237 (MT-IPv6) [RFC5120]
A Label Binding Sub-TLV (TBD) as shown below is associated with a
prefix which is contained in one of the above TLVs:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=TBD | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P| Reserved | MPLS Label (20 bit) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD
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Length: 4
P-Flag: if set, the penultimate hop router MUST perform PHP action
on the allocated MPLS label. For a given prefix, the P-Flag in
the Label Binding Sub-TLV MUST be set to the same value as that of
the P-Flag in the Label Request Sub-TLV if a label request message
(See Section 5 of this document) for that prefix is received by
the mapping server.
MPLS Label: a global label for the prefix which is carried in the
TLV containing this sub-TLV.
Since the mapping server uses these TLVs for label binding
advertisement purpose other than building the normal IP routing
table, the Metric field MUST be set to a value larger than
MAX_PATH_METRIC (i.e., 0xFE000000) according to the following
specification as defined in [RFC5305] "...If a prefix is advertised
with a metric larger then MAX_PATH_METRIC (0xFE000000, see paragraph
3.0), this prefix MUST NOT be considered during the normal SPF
computation. This allows advertisement of a prefix for purposes
other than building the normal IP routing table...". In addition,
when propagating those TLVs across levels, the Label Binding Sub-TLVs
contained in them MUST be preserved.
4. Advertising SID Bindings for Prefixes using IS-IS
A mapping server could uses one or more of the Extended IP
Reachability TLVs (i.e., TLV-135, TLV-235, TLV-236 and TLV-237) to
advertise SIDs for those prefixes which need to be allocated with
SIDs.
A SID Binding Sub-TLV (TBD) as shown below is associated with a
prefix which is contained in one of the above TLVs:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=TBD | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD
Length: 4
SID: a SID for the prefix which is carried in the TLV containing
this sub-TLV.
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Since the mapping server uses these TLVs for label binding
advertisement purpose other than building the normal IP routing
table, the Metric field MUST be set to a value larger than
MAX_PATH_METRIC (i.e., 0xFE000000). In addition, when propagating
those TLVs across levels, the SID Binding Sub-TLVs contained in them
MUST be preserved.
5. Requesting Label Bindings for Prefixes using IS-IS
When advertising IP reachability information by using one of the
Extended IP Reachability TLVs (i.e., TLV-135, TLV-235, TLV-236 and
TLV-237), SR-capable IS-IS routers SHOULD mark those attached
prefixes which need to be allocated with global labels by associating
each of these prefixes with a Label Request sub-TLV (type code=TBD)
as shown below. In addition, when propagating those TLVs across
levels, the Label Request Sub-TLVs contained in them MUST be
preserved.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=TBD | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD
Length: 4
P-Flag: if set, the penultimate hop router MUST perform PHP action
on the required label.
In the multi-level scenario where route summarization between levels
is required, separate Extended IP Reachability TLVs other than those
for IP reachability advertisement purpose SHOULD be used for label
binding request purpose. Since these separate TLVs are not used for
the purpose of building the normal IP routing table, the Metric field
MUST be set to a value larger than MAX_PATH_METRIC (i.e.,
0xFE000000). In addition, when propagating those TLVs across levels,
the Label Request Sub-TLVs contained in them MUST be preserved.
6. Requesting SID Bindings for Prefixes using IS-IS
When advertising IP reachability information by using one of the
Extended IP Reachability TLVs (i.e., TLV-135, TLV-235, TLV-236 and
TLV-237), SR-capable IS-IS routers SHOULD mark those attached
prefixes which need to be allocated with SIDs by associating each of
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these prefixes with a SID Request sub-TLV (Type Code=TBD and
Length=0)
In the multi-level scenario where route summarization between levels
is required, separate Extended IP Reachability TLVs other than those
for IP reachability advertisement purpose SHOULD be used for SID
binding request purpose. Since these separate TLVs are not used for
the purpose of building the normal IP routing table, the Metric field
MUST be set to a value larger than MAX_PATH_METRIC (i.e.,
0xFE000000). In addition, when propagating those TLVs across levels,
the SID Request Sub-TLVs contained in them MUST be preserved.
7. Mapping Server Redundancy
For redundancy purpose, more than one router could be configured as
candidates for mapping servers. Each candidate for mapping servers
SHOULD advertise its capability of being a mapping servers by using
IS-IS Router Capability TLV. The one with the highest priority
SHOULD be elected as the primary mapping server which is eligible to
allocate and advertise global labels or SIDs for prefixes on behalf
of SR-capable routers. The comparison of IS-IS System ID breaks the
tie between two or more candidates with the same highest priority.
Meanwhile, the one with the second highest priority SHOULD be elected
as a backup mapping server. This backup mapping server SHOULD
advertise the same label bindings as those advertised by the primary
mapping server. In this way, the unnecessary changes to the data
plane (i.e., MPLS forwarding table) of SR-capable routers can be
avoided in the event of mapping server failover.
Each candidate mapping server SHOULD advertise its capability of
being a mapping server and the corresponding priority for mapping
server election by attaching a Mapping Server Capability Sub-TLV
(type code=TBD) shown as below to an IS-IS Router Capability TLV
[RFC4971] with the S flag set (with domain-wide flooding scope).
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=TBD | Length | Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD
Length: 1
Priority: the priority for mapping server election.
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8. Acknowledgements
The authors would like to thank .
9. IANA Considerations
TBD.
10. Security Considerations
This document does not introduce any new security considerations.
11. References
11.1. Normative References
[I-D.previdi-isis-segment-routing-extensions]
Previdi, S., Filsfils, C., Bashandy, A., Gredler, H., and
S. Litkowski, "IS-IS Extensions for Segment Routing",
draft-previdi-isis-segment-routing-extensions-04 (work in
progress), October 2013.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4971] Vasseur, JP., Shen, N., and R. Aggarwal, "Intermediate
System to Intermediate System (IS-IS) Extensions for
Advertising Router Information", RFC 4971, July 2007.
[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120, February 2008.
[RFC5283] Decraene, B., Le Roux, JL., and I. Minei, "LDP Extension
for Inter-Area Label Switched Paths (LSPs)", RFC 5283,
July 2008.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, October 2008.
[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308, October
2008.
11.2. Informative References
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[I-D.filsfils-rtgwg-segment-routing]
Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
Ytti, S., Henderickx, W., Tantsura, J., and E. Crabbe,
"Segment Routing Architecture", draft-filsfils-rtgwg-
segment-routing-01 (work in progress), October 2013.
Authors' Addresses
Xiaohu Xu
Huawei
Email: xuxiaohu@huawei.com
Mach Chen
Huawei
Email: mach.chen@huawei.com
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