This document describes an extension to IS-IS protocol [ISO10589], [RFC1195] to add an optional operational capability, that allows tagging and grouping of the nodes in an IS-IS domain. This allows simple management and easy control over route and path selection, based on local configured policies.

This document describes the protocol extensions to disseminate per-node administrative tags in IS-IS protocols.

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].

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1. Introduction

This document provides mechanisms to advertise per-node administrative tags in the IS-IS Link State PDU [RFC1195]. In certain path-selection applications like for example in traffic-engineering or LFA [RFC5286] selection there is a need to tag the nodes based on their roles in the network and have policies to prefer or prune a certain group of nodes.

2. Administrative Tag

For the purpose of advertising per-node administrative tags within IS-IS, a new sub-TLV to the IS-IS Router Capability TLV-242 that is defined in [RFC4971] is proposed. Path selection is a functional set which applies both to TE and non-TE applications. Per-node administrative tags are used to advertise an attribute of the node. As such they are independent of the routing protocol used to advertise them. Because per-node administrative tags may be used to advertise many different attributes, associating the advertisement to TLVs specific to a particular use case (e.g. TE extensions to IS-Neighbors TLVs [RFC5305] in the case of TE path selection) is not appropriate.

An administrative Tag is a 32-bit integer value that can be used to identify a group of nodes in the IS-IS domain. The new sub-TLV specifies one or more administrative tag values. An IS-IS router advertises the set of groups it is part of in the specific IS-IS level. As an example, all PE-nodes may be configured with certain tag value, whereas all P-nodes are configured with a different tag value.

The new sub-TLV defined will be carried inside the IS-IS Router Capability TLV-242 [RFC4971]) in the Link State PDUs originated by the router. TLV 242 can be either specified to be flooded within the specific level in which the same has been originated, or they can be specfied to be relayed from originating level to the other as well. Per-node administrative tags that are included in a 'level-specific' TLV 242 have a 'level-wide' flooding scope associated. On the other hand, per-node administrative tags included in a 'domain-wide' TLV 242 have 'domain-wide' flooding scope associated. For details on how TLV 242 are flooded and relayed in the entire network please, refer to [RFC4971].

Choosing the flooding scope to be associated with group tags, is defined by the needs of the operator's usage and is a matter of local policy or configuration. Operator may choose to advertise a set of per-node administrative tags across levels and another set of per-node administrative tags within the specific level. But evidently the same set of per-node administrative tags cannot be advertised both across levels and within a specific level. A receiving IS-IS router will not be able to distinguish between the significance of a per-node administrative tag advertised with 'domain-wide' scope, from that of an administrative tag advertised with 'level-wide' scope, if they have the same value associated but different significance across different scopes.

Implementations SHOULD allow configuring one or more per-node administrative tags to be advertised from a given device along with the flooding scope associated with the same. It SHOULD allow provisioning a set of per-node administrative tags having a 'domain-wide' flooding scope, as well as, a set of per-node administrative tags with 'level-wide' flooding scope only. A given per-node administrative tag MAY be advertised with level-specific scope (Level-1 and/or Level-2) or with domain-wide scope, but MUST NOT be advertised in both scopes. Hence implementations MUST NOT allow configuring the same per-node administrative tag values in both 'domain-wide' and 'level-wide' scopes. However the same administrative tag value MAY be allowed under multiple levels with 'level-wide' scope.

The format of per-node Administrative Tag sub-TLV (see Section 3.1) does not include a topology identifier. Therefore it is not possible to indicate a topology specific context when advertising per-node admin tags. Hence, in deployments using multi-topology routing [RFC5120], advertising a separate set of per-node administrative tags for each topology SHOULD NOT be supported.

3. TLV format

3.1. Per-node Admin Tag sub-TLV

The new Per-node Administrative Tag sub-TLV, like other ISIS Capability sub-TLVs, is formatted as Type/Length/Value (TLV)triplets. Figure 1 below shows the format of the new sub-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      |    Length     |
 +- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                   Administrative Tag #1                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                   Administrative Tag #2                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 //                                                             //
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                   Administrative Tag #N                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 Type :  TBA

 Length: A 8-bit field that indicates the length of the value
         portion in octets and will be a multiple of 4 octets
         dependent on the number of tags advertised.

 Value:  A sequence of multiple 4 octets defining the
         administrative tags.

Figure 1: IS-IS Per-node Administrative Tag sub-TLV

The 'Per-node Admin Tag' sub-TLV may be generated more than once by an originating router. This MAY happen if a node carries more than 63 per-node administrative groups and a single sub-TLV does not provide sufficient space. As such occurrence of the 'Per-node Admin Tag' sub-TLV does not cancel previous announcements, but rather is cumulative.

4. Elements of Procedure

Meaning of the Per-node administrative tags is generally opaque to IS-IS. Router advertising the per-node administrative tag (or tags) may be configured to do so without knowing (or even explicitly supporting) functionality implied by the tag.

Interpretation of tag values is specific to the administrative domain of a particular network operator. The meaning of a per-node administrative tag is defined by the network local policy and is controlled via the configuration. If a receiving node does not understand the tag value, it ignores the specific tag and floods the Router Capability TLV without any change as defined in [RFC4971].

The semantics of the tag order has no meaning. There is no implied meaning to the ordering of the tags that indicates a certain operation or set of operations that need to be performed based on the ordering.

Each tag SHOULD be treated as an independent identifier that MAY be used in policy to perform a policy action. Tags carried by the administrative tag TLV SHOULD be used to indicate independent characteristics of a node. The TLV SHOULD be considered as an unordered list. Whilst policies may be implemented based on the presence of multiple tags (e.g., if tag A AND tag B are present), they MUST NOT be reliant upon the order of the tags (i.e., all policies should be considered commutative operations, such that tag A preceding or following tag B does not change their outcome).

As mentioned earlier, to avoid incomplete or inconsistent interpretations of the per-node administrative tags the same tag value MUST NOT be advertised by a router in Router Capabilities of different scopes. Implementations MUST NOT allow configuring the same tag value across domain-wide and 'level-wide' scopes. The same tag value MAY be allowed to be configured and advertised under 'level-wide' scope for all levels. A IS-IS Area Border Router (ABR) participating in both levels 1 and 2 MAY advertise the same tag value in the level-specific Router Capability TLVs with 'level-wide' scope generated by it. But the same tag value MUST not be advertised in any of level 1 or level 2 Router-Capability TLV with 'domain-wide' flooding scope (refer to [RFC4971] for more details).

Future IS-IS protocol extensions MUST NOT require use of per-node administrative tags or define well-known tag values to advertise well-known capabilities. Per-node administrative tags are for generic use and do not require IANA registry.

Being part of the Router Capability TLV, the per-node administrative tag sub-TLV MUST be reasonably small and stable. In particular, but not limited to, implementations supporting the per-node administrative tags MUST NOT associate advertised tags to changes in the network topology (both within and outside the IS-IS domain) or reachability of routes.

5. Applications


                    ----------------
                   /                \
                  /                  \
                 /                    \
    +------+   +----+    Access     +----+
    |eNodeB|---|CSG1|    Ring 1     |ASG1|-------------
    +------+   +----+               +----+             \
                 \                    /                 \
                  \                  /                   +----+    +---+
                   \             +----+                  |RSG1|----|RNC|
                    -------------|    |    Aggregate     +----+    +---+
                                 |ASG2|      Ring          |
                    -------------|    |                  +----+    +---+
                   /             +----+                  |RSG2|----|RNC|
                  /                  \                   +----+    +---+
                 /                    \                 /
    +------+   +----+     Access     +----+            /
    |eNodeB|---|CSG2|     Ring 2     |ASG3|------------
    +------+   +----+                +----+
                \                     /
                 \                   /
                  \                 /
                   -----------------

Figure 2: Mobile Backhaul Network


                               +--------------------+
                               |                    |
                               |    +----------+    |
                               |    |          |    |
                               T-10-T          |    |
                              /|   /|          |    |
                             / |  / |          |    |
                          --+  | |  |          |    |
                         /  +--+-+ 100         |    |
                        /  /   |    |          |    |
                       /  /    R-18-R          |    |
                      /  /    /\   /\          |    |
                     /  |    /  \ /  \         |    |
                    /   |   /    x    \        |    |
                   A-25-A  10  10 \    \       |    |
                          /    /   10   10     |    |
                         /    /     \    \     |    |
                        A-25-A       A-25-A    |    |
                         \    \     /    /     |    |
                         201  201  201 201     |    |
                           \    \ /    /       |    |
                            \    x    /        |    |
                             \  / \  /         |    |
                              \/   \/          |    |
                              I-24-I          100  100
                              |    |           |    |
                              |    +-----------+    |
                              |                     |
                              +---------------------+

Figure 3: Explicit Routing topology

This section lists several examples of how implementations might use the Per-node administrative tags. These examples are given only to demonstrate generic usefulness of the router tagging mechanism. Implementation supporting this specification is not required to implement any of the use cases. It is also worth noting that in some described use cases routers configured to advertise tags help other routers in their calculations but do not themselves implement the same functionality.

Auto-discovery of Services

Router tagging may be used to automatically discover group of routers sharing a particular service.

For example, service provider might desire to establish full mesh of MPLS TE tunnels between all PE routers in the area of MPLS VPN network. Marking all PE routers with a tag and configuring devices with a policy to create MPLS TE tunnels to all other devices advertising this tag will automate maintenance of the full mesh. When new PE router is added to the area, all other PE devices will open TE tunnels to it without the need of reconfiguring them.
Policy-based Fast-Reroute

Increased deployment of Loop Free Alternates (LFA) as defined in [RFC5286] poses operation and management challenges. [I-D.ietf-rtgwg-lfa-manageability] proposes policies which, when implemented, will ease LFA operation concerns.

One of the proposed refinements is to be able to group the nodes in IGP domain with administrative tags and engineer the LFA based on configured policies.
(a)
Administrative limitation of LFA scope

Service provider access infrastructure is frequently designed in layered approach with each layer of devices serving different purposes and thus having different hardware capabilities and configured software features. When LFA repair paths are being computed, it may be desirable to exclude devices from being considered as LFA candidates based on their layer.

For example, if the access infrastructure is divided into the Access, Distribution and Core layers it may be desirable for a Distribution device to compute LFA only via Distribution or Core devices but not via Access devices. This may be due to features enabled on Access routers; due to capacity limitations or due to the security requirements. Managing such a policy via configuration of the router computing LFA is cumbersome and error prone.

With the Per-node administrative tags it is possible to assign a tag to each layer and implement LFA policy of computing LFA repair paths only via neighbors which advertise the Core or Distribution tag. This requires minimal per-node configuration and network automatically adapts when new links or routers are added.
(b)
Optimizing LFA calculations

Calculation of LFA paths may require significant resources of the router. One execution of Dijkstra algorithm is required for each neighbor eligible to become next hop of repair paths. Thus a router with a few hundreds of neighbors may need to execute the algorithm hundreds of times before the best (or even valid) repair path is found. Manually excluding from the calculation neighbors which are known to provide no valid LFA (such as single-connected routers) may significantly reduce number of Dijkstra algorithm runs.

LFA calculation policy may be configured so that routers advertising certain tag value are excluded from LFA calculation even if they are otherwise suitable.
Controlling Remote LFA tunnel termination

[I-D.ietf-rtgwg-remote-lfa] proposed method of tunneling traffic after connected link failure to extend the basic LFA coverage and algorithm to find tunnel tail-end routers fitting LFA requirement. In most cases proposed algorithm finds more than one candidate tail-end router. In real life network it may be desirable to exclude some nodes from the list of candidates based on the local policy. This may be either due to known limitations of the per-node (the router does accept targeted LDP sessions required to implement Remote LFA tunneling) or due to administrative requirements (for example, it may be desirable to choose tail-end router among co-located devices).

The Per-node administrative tag delivers simple and scalable solution. Remote LFA can be configured with a policy to accept during the tail-end router calculation as candidates only routers advertising certain tag. Tagging routers allows to both exclude nodes not capable of serving as Remote LFA tunnel tail-ends and to define a region from which tail-end router must be selected.
Mobile backhaul network service deployment

The topology of mobile backhaul network usually adopts ring topology to save fiber resource and it is divided into the aggregate network and the access network. Cell Site Gateways(CSGs) connects the eNodeBs and RNC(Radio Network Controller) Site Gateways(RSGs)connects the RNCs. The mobile traffic is transported from CSGs to RSGs. The network takes a typical aggregate traffic model that more than one access rings will attach to one pair of aggregate site gateways(ASGs) and more than one aggregate rings will attach to one pair of RSGs.

A typical mobile backhaul network with access rings and aggregate links is shown in figure above. The mobile backhaul networks deploy traffic engineering due to the strict Service Level Agreements(SLA). The TE paths may have additional constraints to avoid passing via different access rings or to get completely disjoint backup TE paths. The mobile backhaul networks towards the access side change frequently due to the growing mobile traffic and addition of new eNodeBs. It's complex to satisfy the requirements using cost, link color or explicit path configurations. The per-node administrative tag defined in this document can be effectively used to solve the problem for mobile backhaul networks. The nodes in different rings can be assigned with specific tags. TE path computation can be enhanced to consider additional constraints based on per-node administrative tags.
Policy-based Explicit Routing

Partially meshed network provides multiple paths between any two nodes in the network. In a data center environment, the topology is usually highly symmetric with many/all paths having equal cost. In a long distance network, this is usually less the case for a variety of reasons (e.g. historic, fiber availability constraints, different distances between transit nodes, different roles ...). Hence between a given source and destination, a path is typically preferred over the others, while between the same source and another destination, a different path may be preferred.

In the above topology, operator may want to enforce the following high level explicitly routed policies: - Traffic from A nodes to A nodes must not go through I nodes - Traffic from A nodes to I nodes must not go through R and T nodes with per-node administrative tag, tag A can be configured on all A nodes, (similarly I, R, T), and then configure this single CSPF policy on all A nodes to avoid I nodes for path calculation.

6. Security Considerations

This document does not introduce any further security issues other than those discussed in [ISO10589] and [RFC1195].

7. IANA Considerations

IANA maintains the registry for the Router Capability sub-TLVs. IS-IS Administrative Tags will require new type code for the following new sub-TLV defined in this document.

i) Per-Node-Admin-Tag Sub-TLV, Type: TBD

8. Acknowledgments

Many thanks to Les Ginsberg, Dhruv Dhody, Uma Chunduri and Chris Bowers for providing useful inputs.

9. References

9.1. Normative References

, "

[ISO10589]	Intermediate system to Intermediate system intra-domain 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.", Nov 2002.
[RFC2119]	Bradner, S., Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.

9.2. Informative References

[I-D.ietf-rtgwg-lfa-manageability]	Litkowski, S., Decraene, B., Filsfils, C., Raza, K., Horneffer, M. and p. psarkar@juniper.net, "Operational management of Loop Free Alternates", Internet-Draft draft-ietf-rtgwg-lfa-manageability-04, August 2014.
[I-D.ietf-rtgwg-remote-lfa]	Bryant, S., Filsfils, C., Previdi, S., Shand, M. and N. So, "Remote LFA FRR", Internet-Draft draft-ietf-rtgwg-remote-lfa-09, December 2014.
[RFC1195]	Callon, R., "Use of OSI IS-IS for routing in TCP/IP and dual environments", RFC 1195, December 1990.
[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.
[RFC5286]	Atlas, A. and A. Zinin, "Basic Specification for IP Fast Reroute: Loop-Free Alternates", RFC 5286, September 2008.
[RFC5305]	Li, T. and H. Smit, "IS-IS Extensions for Traffic Engineering", RFC 5305, October 2008.

Authors' Addresses

Pushpasis Sarkar (editor) Juniper Networks, Inc. Electra, Exora Business Park Bangalore, KA 560103 India EMail: psarkar@juniper.net

Hannes Gredler Juniper Networks, Inc. 1194 N. Mathilda Ave. Sunnyvale, CA 94089 US EMail: hannes@juniper.net

Shraddha Hegde Juniper Networks, Inc. Electra, Exora Business Park Bangalore, KA 560103 India EMail: shraddha@juniper.net

Stephane Litkowski Orange EMail: stephane.litkowski@orange.com

Bruno Decraene Orange EMail: bruno.decraene@orange.com

Li Zhenbin Huawei Technologies Huawei Bld. No.156 Beiqing Rd Beijing, KA 100095 China EMail: lizhenbin@huawei.com

Ebben Aries Facebook EMail: exa@fb.com

Rafael Rodriguez Facebook EMail: rafael@fb.com

Harish Raghuveer EMail: harish.r.prabhu@gmail.com

Abstract

Requirements Language