ISIS Protocol Extensions for Boundary Node Discovery (BND)

Internet-Draft	ISIS BND	August 2012
Dhody & Palle	Expires 4 March 2013	[Page]

Abstract

There are various circumstances where it is highly desirable to be able to dynamically and automatically discover a set of Boundary Nodes (BN) along with their domain information. For that purpose, this document defines extensions to the Intermediate System to Intermediate System(IS-IS) routing protocol for the advertisement of Boundary Node (BN)Discovery information within an IS-IS area or within the entire IS-IS routing domain.¶

Status of This Memo

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This Internet-Draft will expire on 2 February 2013.¶

Copyright Notice

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1. Introduction

This document defines extensions to IS-IS [ISO_10589] to allow a boundary node in an IS-IS routing domain to advertise its location, along with domain information.¶

Generic capability advertisement mechanisms for IS-IS are defined in [RFC4971]. These allow a router to advertise its capabilities within an IS-IS area or an entire IS-IS routing domain. This document leverages this generic capability advertisement mechanism to fully satisfy the dynamic BN discovery.¶

This document defines a new sub-TLV (named the BN Discovery (BND))to be carried within the IS-IS Router Capability TLV ([RFC4971]).¶

The BN information advertised is detailed in Section 3. Protocol extensions and procedures are defined in Section 9 and Section 10.¶

A detailed description about the need for auto discovery of Boundary Nodes (BN) and thier domains is also provided in this document.¶

The IS-IS extensions defined in this document allow for BN discovery within an IS-IS routing domain. Boundary Node can be an ABR or ASBR.¶

This document defines a set of sub-TLVs that are nested within each other. When the degree of nesting TLVs is 2 (a TLV is carried within another TLV) the TLV carried within a TLV is called a sub-TLV. Strictly speaking, when the degree of nesting is 3, a sub-sub-TLV is carried within a sub-TLV that is itself carried within a TLV. For the sake of terminology simplicity, a TLV carried within another TLV is called a sub-TLV regardless of the degree of nesting.¶

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 [RFC2119].¶

2. Terminology

The following terminology is used in this document.¶

ABR:: IS-IS Area Border Router. Routers used to connect two IGP areas.¶
AS:: Autonomous System.¶
ASBR:: Autonomous System Border Router. Router used to connect together ASes of the same or different service providers via one or more inter-AS links¶
BN:: A boundary node is either an ABR in the context of inter-area Traffic Engineering or an ASBR in the context of inter-AS Traffic Engineering.¶
BND:: Boundary Node Discovery¶
BRPC:: Backward Recursive Path Computation¶
Domain:: Any collection of network elements within a common sphere of address management or path computational responsibility. Examples of domains include Interior Gateway Protocol (IGP) areas and Autonomous Systems (ASs).¶
H-PCE:: Hierarchical PCE.¶
IGP:: Interior Gateway Protocol. Either of the two routing protocols, Open Shortest Path First (OSPF) or Intermediate System to Intermediate System (IS-IS).¶
LSA:: Link State Advertisement.¶
IS-IS:: Intermediate System to Intermediate System.¶
PCE:: Path Computation Element. An entity (component, application, or network node) that is capable of computing a network path or route based on a network graph and applying computational constraints.¶
TLV:: Type-Length-Variable data encoding.¶

3. Overview

3.1. Boundary Node (BN) Discovery Information

The BN discovery information is composed of:¶

The BN location: an IPv4 and/or IPv6 address that is used to reach the BN. It is RECOMMENDED to use an address that is always reachable from all connected domains;¶
The set of two or more Domain(s) into which the BN has connectivity;¶

Changes in BN discovery information may occur as a result of BN configuration update or domain status change.¶

3.2. Flooding Scope

The flooding scope for BN information advertised through IS-IS can be a single L1 area, an L1 area and the L2 sub-domain, or the entire IS-IS routing domain.¶

4. Applications

BRPC procedure as defined in [RFC5441], requires Path Computation Element (PCE) [RFC4655] to be aware of the BNs for the inter-domain path computation. This information would be either statically configured at PCE or learned via some mechanism, as listed in Section 5.¶

In case of static configuration, as shown in the Figure 1, incase of ISIS Backbone area(L2), configuration of BNs at PCE5 is extensive. BRPC procedure guarantees a best path only if BNs are selected correctly, any change in BNs at run time may lead to sub-optimal path. Also Administrator need to configure ABR / ASBR ID in such a way that it is reachable from all the domains, BND Tlv can take care of this automatically.¶

  +--------------------------+          +--------------------------+
  |                  +------+|          |                  +------+|
  |      Area1(L1)   | PCE1 ||          |      Area2(L1)   | PCE2 ||
  |                  +--+---+|          |                  +--+---+|
  |                          |          |                          |
  |    +---------------------+----------+--------------------+     |
  |    | +------+    +------+|          |+------+   +------+ |     |
  |    | | BN1  |    | BN2  ||          || BN3  |   | BN4  | |     |
  |    | +--+---+    +--+---+|          |+--+---+   +---+--+ |     |
  +----+----|-----------+----+          +---+-----------+----+-----+
       |    |           |                   |           |    |
       |    +-----------+-------+    +------+-----------+    |
       |                        |    |                       |
       |                        |    |                       |
       |                       ++----++                      |
       |     Backbone(L2)      | PCE5 |                      |
       |                       ++----++                      |
       |                        |    |                       |
       |                        |    |                       |
       |    +-----------+-------+    +------+----------+     |
       |    |           |                   |          |     |
       |    |           |                   |          |     |
  +----+----+-----------+----+          +---+----------+-----+-----+
  |    | +--+---+    +--+---+|          |+--+---+   +--+---+ |     |
  |    | | BN5  |    | BN6  ||          || BN7  |   | BN8  | |     |
  |    | +------+    +------+|          |+------+   +------+ |     |
  |    +---------------------+----------+--------------------+     |
  |                          |          |                          |
  |                  +------+|          |                  +------+|
  |      Area3(L1)   | PCE3 ||          |      Area4(L1)   | PCE4 ||
  |                  +--+---+|          |                  +--+---+|
  +--------------------------+          +--------------------------+

Figure 1: ISIS Area Topology

The problems with existing mechanism to discover Boundary nodes are listed in the next section.¶

5. Existing Mechanisms

5.1. ISIS

As specified in [RFC1142] ABR/ASBR can be identified but not their domain information. As stated in Section 7, Selection of correct BN is based on domain and thus it is ineffective.¶
Selection of ABR/ASBR based on ISIS Database is not a good idea, first it requires PCE to look into ISIS Database thus adding to coupling, second it MAY require BGP routes to be redistributed into ISIS which is also not a good network design principle.¶

5.2. Inter-AS TE Link

[RFC5316] specifies how to advertise TE properties of inter-AS links; through which ASBR and remote AS can be discovered, but ABR and their domain information cannot be discovered via above RFC.¶
AS is made up of multiple Area, there maybe a need to clearly identify a BN by combination of both AS number and Area-id. Refer Section 3.5 of [DOMAIN-SEQ].¶

5.3. ISIS Area Topology

To uniquely identify an ISIS L1 area, an unique area Id MUST be assigned. There is no other way to learn this information. BND can be used to advertise this information.¶

5.4. HPCE

Section 4 of [H-PCE] specifies each child PCE should know the identity of the domains that neighbor its own domain and advertises the same to the parent PCE. No method exist to find the neighbor domain which need to be carried in NEIG-PCE-DOMAIN Sub-TLV and BN discovery along with neighbor domain information can help in generating NEIG-PCE-DOMAIN Sub-TLV.¶

7. Importance of Domain Information along with BNs

There are methods to learn BNs dynamically from IGP, but the knowledge of neighboring-domains is not possible to obtain. Without this the correct BN based on the domain-path can't be selected. [RFC5441] mentions:¶

"Note that PCE(i) only considers the entry BNs of domain(i), i.e., only the BNs that provide connectivity from domain(i-1). In other words, the set BN-en(k,i) is only made of those BNs that provide connectivity from domain (i-1) to domain(i). "¶

This selection of correct BNs providing connectivity between correct domains cannot be made by the information obtained from IGP. Without the correct selection we would not be following [RFC5441].¶

9. The IS-IS BND Sub-TLV

The IS-IS BND sub-TLV contains a non-ordered set of sub-TLVs.¶

The format of the IS-IS BND sub-TLV and its sub-TLVs is identical to the TLV format used by the Traffic Engineering Extensions to IS-IS [RFC5305]. That is, the TLV is comprised of 1 octet for the type, 1 octet specifying the TLV length, and a value field. The Length field defines the length of the value portion in octets.¶

The IS-IS BND sub-TLV has the following format:¶

   TYPE:   To be assigned by IANA (suggested value 6)
   LENGTH: Variable
   VALUE:  Set of sub-TLVs

Two sub-TLVs are defined:

     Sub-TLV type   Length              Name
           1        variable       BN-ADDRESS sub-TLV
           2        variable       BN-DOMAIN sub-TLV

The BN-ADDRESS and BN-DOMAIN sub-TLVs MUST always be present within the BND sub-TLV.¶

Any unrecognized sub-TLV MUST be silently ignored.¶

The BND sub-TLV is carried within an IS-IS CAPABILITY TLV defined in [RFC4971].¶

The following sub-sections describe the sub-TLVs.¶

9.1. BN-ADDRESS Sub-TLV

The BN-ADDRESS sub-TLV specifies an IP address that can be used to reach the BN. It is RECOMMENDED to make use of an address that is always reachable, provided the BN is alive and reachable.¶

The BN-ADDRESS sub-TLV is mandatory; it MUST be present within the BND sub-TLV. It MAY appear twice, when the BN has both an IPv4 and IPv6 address. It MUST NOT appear more than once for the same address type. If it appears more than once for the same address type, only the first occurrence is processed and any others MUST be ignored.¶

The BN-ADDRESS sub-TLV has the following format:¶

   TYPE:   1
      LENGTH: 5 for an IPv4 address or 17 for an IPv6 address.
      VALUE:  This comprises one octet indicating the address-type and 4
              or 16 octets encoding the IPv4 or IPv6 address to be used
              to reach the BN.

   Address-type:
                  1   IPv4
                  2   IPv6

9.2. BN-DOMAIN Sub-TLV

The BN-DOMAIN sub-TLV specifies a BN-Domain (area and/or AS) where the BN has topology connectivity.¶

The BN-DOMAIN sub-TLV is mandatory; it MUST be present within the BND TLV.¶

A BND sub-TLV MUST include two or more BN-DOMAIN sub-TLVs as the BN has connectivity into multiple BN-Domains.¶

The BN-DOMAIN sub-TLV has the following format:¶

   TYPE:   2
      LENGTH: Variable
      VALUE:  This is composed of one octet indicating the domain-type
              (area ID or AS Number) and a variable length IS-IS area ID
              or a 32-bit AS number, identifying a BN-Domain where the
              BN has visibility and can compute paths.

   Two domain types are defined:

                  1   Area ID
                  2   AS Number

The Area ID is the area address as defined in [ISO_10589].¶

When the AS number is coded in two octets, the AS Number field MUST have its first two octets set to 0.¶

10. Elements of Procedure

The BND sub-TLV is advertised within an IS-IS Router Capability TLV defined in [RFC4971]. As such, elements of procedures are inherited from those defined in [RFC4971].¶

The flooding scope is controlled by the S flag in the IS-IS Router Capability TLV (see [RFC4971]). When the scope of the BND sub-TLV is area local, it MUST be carried within an IS-IS Router Capability TLV having the S bit cleared. When the scope of the BND sub-TLV is the entire IS-IS routing domain, it MUST be carried within an IS-IS Router Capability TLV having the S bit set.¶

Note that an L1L2 node may include a BND TLV in a Router Capability TLV with the S bit cleared in both in its L1 and L2 LSPs. This allows the flooding scope to be restricted to the L1 area and the L2 sub-domain.¶

When the BN function is deactivated, the IS-IS speaker MUST originate a new IS-IS LSP that no longer includes the corresponding BND TLV.¶

The BN address (i.e., the address indicated within the BN-ADDRESS sub-TLV) SHOULD be reachable via some prefixes advertised by IS-IS.¶

The BND sub-TLV information regarding a specific BN is only considered current and useable when the router advertising this information is itself reachable via IS-IS calculated paths at the level of the LSP in which the BND sub-TLV appears.¶

A change in the state of a BN (activate, deactivate, domain change) MUST result in a corresponding change in the BND sub-TLV information advertised by an IS-IS router (inserted, removed, updated) in its LSP. The way BNs determine the information they advertise, and how that information is made available to IS-IS, is out of the scope of this document. Some information may be configured and other information may be automatically determined by ISIS.¶

A change in information in the BND sub-TLV MUST NOT trigger any SPF computation at a receiving router.¶

17. References

17.1. Normative References

[RFC2119]: Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.
[RFC4971]: Vasseur, JP., Ed., Shen, N., Ed., and R. Aggarwal, Ed., "Intermediate System to Intermediate System (IS-IS) Extensions for Advertising Router Information", RFC 4971, DOI 10.17487/RFC4971, July 2007, <https://www.rfc-editor.org/info/rfc4971>.
[RFC5304]: Li, T. and R. Atkinson, "IS-IS Cryptographic Authentication", RFC 5304, DOI 10.17487/RFC5304, October 2008, <https://www.rfc-editor.org/info/rfc5304>.
[RFC5305]: Li, T. and H. Smit, "IS-IS Extensions for Traffic Engineering", RFC 5305, DOI 10.17487/RFC5305, October 2008, <https://www.rfc-editor.org/info/rfc5305>.

17.2. Informative References

[RFC1142]: Oran, D., Ed., "OSI IS-IS Intra-domain Routing Protocol", RFC 1142, DOI 10.17487/RFC1142, February 1990, <https://www.rfc-editor.org/info/rfc1142>.
[RFC4655]: Farrel, A., Vasseur, J.-P., and J. Ash, "A Path Computation Element (PCE)-Based Architecture", RFC 4655, DOI 10.17487/RFC4655, August 2006, <https://www.rfc-editor.org/info/rfc4655>.
[RFC5316]: Chen, M., Zhang, R., and X. Duan, "ISIS Extensions in Support of Inter-Autonomous System (AS) MPLS and GMPLS Traffic Engineering", RFC 5316, DOI 10.17487/RFC5316, December 2008, <https://www.rfc-editor.org/info/rfc5316>.
[RFC5441]: Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le Roux, "A Backward-Recursive PCE-Based Computation (BRPC) Procedure to Compute Shortest Constrained Inter-Domain Traffic Engineering Label Switched Paths", RFC 5441, DOI 10.17487/RFC5441, April 2009, <https://www.rfc-editor.org/info/rfc5441>.
[ISO_10589]: 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.
[H-PCE]: King, D. and A. Farrel, "The Application of the Path Computation Element Architecture to the Determination of a Sequence of Domains in MPLS and GMPLS. (draft-ietf-pce-hierarchy-fwk-04)", June 2012.
[DOMAIN-SEQ]: Dhody, D., Palle, U., and R. Casellas, "Standard Representation Of Domain Sequence (draft-ietf-pce-pcep-domain-sequence-01)", July 2012.