Hierarchical IS-IS

The IS-IS routing protocol IS-IS currently supports a two level hierarchy of abstraction. The fundamental unit of abstraction is the 'area', which is a (hopefully) connected set of systems running IS-IS at the same level. Level 1, the lowest level, is abstracted by routers that participate in both Level 1 and Level 2. Practical considerations, such as the size of an area's link state database, cause network designers to restrict the number of routers in any given area. Concurrently, the dominance of scale-out architectures based around small routers has created a situation where the scalability limits of the protocol are going to become critical in the foreseeable future. The goal of this document is to enable additional hierarchy within IS-IS by creating additional hierarchy. Each additional level of hierarchy has a multiplicative effect on scale, so the addtion of six levels should be a significant improvement. While all six levels may not be needed in the short term, it is apparent that the original designers of IS-IS reserved enough space for these levels, and defining six additional levels is only slightly harder than adding a single level, so it makes some sense to expand the design for the future. The modifications described herein are designed to be fully backward compatible. Section references in this document are references to sections of IS-IS.

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.

In this section, we enumerate all of the redefinitions of protocol header fields necessary to add additional levels.

In the fixed header of some IS-IS PDUs, a field is named 'Reserved/Circuit Type' (Section 9.5). The high order six bits are reserved, with the low order two bits indicating Level 1 (bit 1) and Level 2 (bit 2). This field is renamed to be 'Circuit Type'. The bits are redefined as follows: Level 1 Level 2 Level 3 Level 4 Level 5 Level 6 Level 7 Level 8 The value of zero (no bits set) is reserved. PDUs with a Circuit Type of zero SHALL be ignored. The set bits of the Circuit Type MUST be contiguous. If bit n and bit m are set in the Circuit Type, then all bits in the interval [n:m] must be set.

The fixed header of IS-IS PDUs contains an octet with three reserved bits and the 'PDU Type' field. The three reserved bits are transmitted as zero and ignored on receipt. (Section 9.5) To allow for additional PDU space, this entire octet is renamed the 'PDU Type' field.

The 'LAN IS to IS hello PDU' (LAN-HELLO-PDU) is identical in format to the 'Level 2 LAN IS to IS hello PDU' (Section 9.6), except that the PDU Type has value AAA. The LAN-HELLO-PDU MUST be used instead of the 'Level 1 LAN IS to IS hello PDU' (Section 9.5) or the 'Level 2 LAN IS to IS hello PDU' on any circuit that has one or more of Level 3 through Level 8 enabled.

The 'Point-to-point IS to IS hello PDU' can be used on circuits of any Level without modification.

The 'Level n Link State PDU' (Ln-LSP-PDU) has the same format as the 'Level 2 Link State PDU' (Section 9.9), except for the PDU Type. The PDU Types for Levels 3 through 8 are defined as follows: Level 3 (L3-LSP-PDU): BBB Level 4 (L4-LSP-PDU): CCC Level 5 (L5-LSP-PDU): DDD Level 6 (L6-LSP-PDU): EEE Level 7 (L7-LSP-PDU): FFF Level 8 (L8-LSP-PDU): GGG

The 'Level n complete sequence numbers PDU' (Ln-CSNP-PDU) has the same format as the 'Level 2 complete sequence numbers PDU' (Section 9.11), except for the PDU Type. The PDU Types for Levels 3 through 8 are defined as follows: Level 3 (L3-CSNP-PDU): HHH Level 4 (L4-CSNP-PDU): III Level 5 (L5-CSNP-PDU): JJJ Level 6 (L6-CSNP-PDU): KKK Level 7 (L7-CSNP-PDU): LLL Level 8 (L8-CSNP-PDU): MMM

The 'Level 2 partial sequence numbers PDU' (Ln-PSNP-PDU) has the same format as the 'Level 2 partial sequence numbers PDU' (Section 9.13), except for the PDU Type. The PDU Types for Levels 3 through 8 are defined as follows: Level 3 (L3-PSNP-PDU): NNN Level 4 (L4-PSNP-PDU): OOO Level 5 (L5-PSNP-PDU): PPP Level 6 (L6-PSNP-PDU): QQQ Level 7 (L7-PSNP-PDU): RRR Level 8 (L8-PSNP-PDU): SSS

All existing Level 2 TLVs may be used in the corresponding Level 3 through Level 8 PDUs. When used in a Level 3 through Level 8 PDU, the semantics of these TLVs will be applied to the Level of the containing PDU. If the original semantics of the PDU was carrying a reference to Level 1 in a Level 2 TLV, then the semantics of the TLV at level N will be a reference to level N-1. The intent is to retain the original semantics of the TLV at the higher level.

The author would like to thank Dinesh Dutt for inspiring this document.

This document makes many requests to IANA, as follows:

The existing IS-IS PDU registry currently supports values 0-31. This should be expanded to support the values 0-255. The existing value assignments should be retained. Value 255 should be reserved.

IANA is requested to allocate values from the IS-IS PDU registry for the following: LAN-HELLO-PDU: AAA L3-LSP-PDU: BBB L4-LSP-PDU: CCC L5-LSP-PDU: DDD L6-LSP-PDU: EEE L7-LSP-PDU: FFF L8-LSP-PDU: GGG L3-CSNP-PDU: HHH L4-CSNP-PDU: III L5-CSNP-PDU: JJJ L6-CSNP-PDU: KKK L7-CSNP-PDU: LLL L8-CSNP-PDU: MMM L3-PSNP-PDU: NNN L4-PSNP-PDU: OOO L5-PSNP-PDU: PPP L6-PSNP-PDU: QQQ L7-PSNP-PDU: RRR L8-PSNP-PDU: SSS To allow for PDU types to be defined independent of this document, the above values should be allocated from the range 32-254.

This document introduces no new security issues. Security of routing within a domain is already addressed as part of the routing protocols themselves. This document proposes no changes to those security architectures.