Network working group X. Xu
Internet Draft M. Chen
Category: Standard Track Huawei
Expires: January 2014 July 11, 2013
Advertising Global Labels Using IGP
draft-xu-rtgwg-global-label-adv-00
Abstract
Segment Routing (SR) [SR-ARCH] is a new MPLS paradigm in which each
SR-capable router is required to independently advertise global MPLS
labels for its attached prefixes using IGP [SR-ISIS-EXT][SR-OSPF-EXT].
One major challenge associated with such label advertisement
mechanism is how to avoid a given global MPLS label from being
allocated by different routers to different prefixes. Although manual
allocation can address such label allocation collision problem, it is
error-prone and therefore may not be suitable for large SR network
environments. This document proposes an alternative approach for
advertising global labels without any risk of label allocation
collision.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 11, 2014.
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Copyright Notice
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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 [RFC2119].
Table of Contents
1. Introduction ................................................ 3
2. Terminology ................................................. 3
3. Advertising Label Bindings for Prefixes ..................... 3
3.1. Extension to ISIS ...................................... 3
3.2. Extension to OSPFv2 .................................... 4
3.3. Extension to OSPFv3 .................................... 6
4. Requesting Label Bindings for Prefixes ...................... 6
4.1. Extension to ISIS ...................................... 6
4.2. Extension to OSPFv2 .................................... 7
4.3. Extension to OSPFv3 .................................... 7
5. Mapping Server Redundancy and Election ...................... 7
5.1. Extension to ISIS ...................................... 8
5.2. Extension to OSPFv2 .................................... 8
5.3. Extension to OSPFv3 .................................... 8
6. Security Considerations ..................................... 9
7. IANA Considerations ......................................... 9
8. Acknowledgements ............................................ 9
9. References .................................................. 9
9.1. Normative References ................................... 9
9.2. Informative References ................................. 9
Authors' Addresses ............................................ 10
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1. Introduction
Segment Routing (SR) [SR-ARCH] is a new MPLS paradigm in which each
SR-capable router is required to independently advertise global MPLS
labels for its attached prefixes using IGP [SR-ISIS-EXT][SR-OSPF-EXT].
One major challenge associated with such label advertisement
mechanism is how to avoid a given global MPLS label from being
allocated by different routers to different prefixes. Although manual
allocation can address such label allocation collision problem, it is
error-prone therefore may not be suitable for large SR network
environments.
This document proposes an alternative approach for advertising global
labels without any risk of label allocation collision. The basic idea
of this approach is that a single mapping server would, on behalf of
all SR-capable routers within an IGP domain, allocate global labels
for prefixes attached to those SR-capable routers and then advertise
the label bindings in the IGP domain scope. Those prefixes which need
to be allocated with global labels can be manually configured on the
mapping servers or be advertised by the corresponding SR-capable
routers to which those prefixes are attached. In the multi-area/level
scenario where route summary between areas/levels is required, the IP
longest-match algorithm SHOULD be used by SR-capable routers when
processing label bindings advertised by the mapping server.
As for the scenario where the scope of label advertisement is set to
area/level-scoped, it will be discussed in a future version of this
document.
2. Terminology
This memo makes use of the terms defined in [RFC1195] [RFC2328] [SR-
ARCH].
3. Advertising Label Bindings for Prefixes
3.1. Extension to ISIS
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:
TLV-135 (IPv4) [RFC5305]
TLV-235 (MT-IPv4) [RFC5120]
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TLV-236 (IPv6) [RFC5308]
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
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 4 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).
3.2. Extension to OSPFv2
A new Opaque LSA [RFC5250] of type 11 (with domain-wide flooding
scope), referred to as Prefix Opaque LSA, is defined. The opaque type
of this Prefix Opaque LSA is TBD. A mapping server could use one or
more Prefix Opaque LSAs to advertise label bindings for those
prefixes which need to be allocated with global labels.
One or more Prefix TLV (type code=TBD) as shown below could be
contained in a Prefix Opaque LSA.
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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=TBD | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MT-ID | Prefix-Len | Sub-TLV-Len | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Prefix (0-4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Sub-TLVs (Variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
?
| MT-ID | Prefix-Len | Sub-TLV-Len | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Prefix (0-4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Sub-TLVs (Variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD.
Length: Variable.
MT-ID: Multi-Topology ID as defined in [RFC4915].
Prefix-Len: the length of the prefix in bits (i.e., 0-32).
Sub-TLV-Len: the length of Sub-TLVs.
IPv4 Prefix: the prefix is encoded in the minimal number of
octets (i.e., 0-4) for the given number of significant bits.
A Label Binding Sub-TLV (type code=TBD) as shown below is associated
with a prefix which is contained in the Prefix TLV.
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.
Length: 4.
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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 4 of this document) for that prefix
is received by the mapping server.
MPLS Label: a global label which is allocated to the prefix which
is contained in the Prefix TLV.
3.3. Extension to OSPFv3
TBD.
4. Requesting Label Bindings for Prefixes
4.1. Extension to ISIS
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 ISIS routers SHOULD mark those among their
attached prefixes which need to be allocated with a global label by
associating each of these prefixes with a Label Request sub-TLV (type
code=TBD) as shown below.
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 summary between levels is
required, separate Extended IP Reachability TLVs other than those for
IP reachability advertisement purpose SHOULD be used for label
binding advertisement 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).
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4.2. Extension to OSPFv2
SR-capable OSPF routers could use one or more Prefix Opaque LSAs as
defined in section 3.2 of this document to advertise those among
their attached prefixes which need to be allocated with global labels.
A new Sub-TLV of the Prefix TLV, referred to as Label Request Sub-TLV
(type code=TBD) as shown below is associated with a prefix which is
contained in a Prefix TLV.
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.
4.3. Extension to OSPFv3
TBD.
5. Mapping Server Redundancy and Election
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 or OSPF 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 for prefixes on
behalf of SR-capable routers. The comparison of Router ID of ISIS or
OSPF routers 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 is responsible for advertising the same label bindings
as those advertised by the primary mapping server. In this way, it's
possible to avoid unnecessary changes to the data plane (i.e., MPLS
forwarding table) of SR-capable routers in the event of mapping
server failover.
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5.1. Extension to ISIS
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 | Length | Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD
Length: 1
Priority: the priority for mapping server election.
5.2. Extension to OSPFv2
Each candidate mapping server SHOULD advertise its capability of
being mapping servers by using an OSPF Router Informational
Capabilities TLV [RFC4970] contained in an Opaque LSA of type 11
(with domain-wide flooding scope). One of the unreserved OSPF Router
Informational Capabilities Bits is reserved for this purpose.
Furthermore, a sub-TLV (type code=TBD) as shown below is used to
convey the priority value for mapping server election.
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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Priority |
+-+-+-+-+-+-+-+-+
Type: TBD
Length: 1
Priority: the priority for mapping server election.
5.3. Extension to OSPFv3
TBD.
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6. Security Considerations
TBD.
7. IANA Considerations
TBD.
8. Acknowledgements
Thanks to.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[SR-ARCH] 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-00 (work in progress), June 2013.
[SR-ISIS-EXT] Previdi, S., Filsfils, C., and A. Bashandy, "IS-IS
Segment Routing Extensions", May 2013.
[SR-OSPF-EXT] Psenak, P. and S. Previdi, "OSPF Segment Routing
Extensions", May 2013.
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
October 2008.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, October 2008.
[RFC4971] Vasseur, J-P., Ed., Shen, N., Ed., and R. Aggarwal, Ed.,
"Intermediate System to Intermediate System (IS-IS)
Extensions for Advertising Router Information", RFC 4971,
July 2007.
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[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.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem,
"OSPF for IPv6", RFC 5340, July 2008.
[RFC5250] Berger, L., Bryskin, I., Zinin, A., and R. Coltun, "The
OSPF Opaque LSA Option", RFC 5250, July 2008.
[RFC4970] Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
S. Shaffer, "Extensions to OSPF for Advertising Optional
Router Capabilities", RFC 4970, July 2007.
[RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
RFC 4915, June 2007.
Authors' Addresses
Xiaohu Xu
Huawei Technologies,
Beijing, China
Phone: +86-10-60610041
Email: xuxiaohu@huawei.com
Mach(Guoyi) Chen
Huawei Technologies,
Beijing, China
Phone: +86-10-60610041
Email: mach.chen@huawei.com