IS-IS Hello Capability

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Table of Contents

1. Introduction

[RFC7981] defines the Router Capability TLV which is used to advertise capabilites of a router to other routers in an area/domain. This allows routers to advertise information necessary for a given router to participate in optional features in an area/domain.¶

There is an analogous need to know what optional features a neighbor supports so as to determine whether those features can be used when establishing/maintaining adjacencies.¶

This document defines a new TLV to be sent in Hello PDUs which supports advertisement of optional features.¶

2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3. Hello Capability Definition

The Hello Capability TLV provides the ability to advertise support for optional capabilities. The TLV MAY be included in the following PDU types:¶

• L1-LAN-HELLO-PDU (15)¶

• L2_LAN-HELLO-PDU (16)¶

• P2P-HELLO-PDU (17)¶

Two classes of information may be advertised in this TLV.¶

Support for capabilities which require no additional information are indicated by setting a bit in a variable length bit mask.¶

Support for capabilites which require additional information are indicated by advertising a sub-TLV. Sub-TLVs are included following the variable length bit mask.¶

4. Hello Capability TLV Definition

This section defines the format and constraints of the Hello Capability TLV.¶

         0 1 2 3 4 5 6 7
         +--+--+--+--+--+
         | TLV Type     |
         +--+--+--+--+--+
         | TLV Length   |
         +--+--+--+--+--+
         | CB Length    |
         +--+--+--+--+--+...
         | CBs ...
         +--+--+--+--+--+...
         | Optional sub-TLVs
         +--+--+--+--+--+...

TLV Type is TBA by IANA (1 octet)¶

TLV Length is total length of the data which follows (1 octet)¶

CB Length is length of the Capability Bits ((Number of bits + 7) / 8 octets (1 octet) May be 0.¶

CBs are the Capability Bits starting with Bit #0. Bit definitions SHOULD be assigned in ascending bit order so as to minimize the number of bits which will need to be transmitted.¶

Undefined bits MUST be transmitted as 0 and MUST be ignored on receipt.¶

Bits which are NOT transmitted MUST be treated as if they are set to 0 on receipt.¶

No capability bits are defined in this document.¶

No sub-TLVs are defined in this document.¶

Multiple Hello Capability TLVs MAY be advertised in a hello PDU. When multiple Hello Capability TLVs are present in a hello PDU only the bit mask in the first such TLV is considered.¶

Subsequent TLVs in the same hello MUST specify a zero length bit mask. Non-zero length bit masks in other than the first Hello Capability TLV MUST be ignored on receipt.¶

sub-TLVs MAY appear in any Hello Capability TLV.¶

5. New Registries

Two new registries are defined by this document.¶

IS-IS Bit Values for the Hello Capabilities TLV is a new registry which defines the bit values advertised in the Capability Flags of the Hello Capability TLV.¶

IS-IS Sub-TLVs for the Hello Capabilites TLV is a new registry which defines the sub-TLV types which may be included in a Hello Capabiily TLV¶

6. IANA Considerations

7. Security Considerations

Security concerns for IS-IS are addressed in [RFC5304] and [RFC5310].¶

Advertisement of additional capability information may introduce new security concerns. Discussion of those security concerns is required in the future documents which defines those capabilities.¶

Authors' Addresses