An IPv6 Segment Routing (SRv6) Domain Name System (DNS) Resource Record

Introduction The Domain Name System (DNS) is a hierarchical, distributed, highly available database with a variety of security features used for bi-directional mapping between domain names and addresses, for email routing, and for other information . This data is formatted into resource records (RRs) whose content type and structure are indicated by the RR Type field. General familiarity with the DNS and its terminology is assumed in this document.

IPv6 Segment Routing Internet Protocol versions 4 (IPv4,) and 6 (IPv6, ) have long provided header options to include an ordered sequence of addresses in a packet header so the packet travels in order through the nodes specified by that sequence of addresses. This is sometimes referred to as "source routing" because the route or path the packet follows is set, at least in part, when the sequence of addresses is added to the packet, usually at the packet's source, rather than being dynamically determined as the packet proceeds through the network. IPv6 Segment Routing (SRv6, ) extends "source routing" by generalizing the IPv6 sized "address" quantities in a sequence to be "instructions". specifies a particular Segment Routing Header (SRH) that may be use used as part of the headers of an IPv6 packet to indicate an IPv6 Segment Routing sequence of addresses/instructions. And further specifies the structuring of an IPv6 address size quantity such that it is composed of addressing information followed by a function designation which is optionally further followed by arguments to that function. Thus, segment routing might encode a series of operations to be performed on a packet. Furthermore, because a sequence of SRv6 instructions may start with the same constant addressing prefix, methods of compression have been suggested to represent this addressing prefix less often and pack an increased number of quantities into a Segment Routing Header where each quantity may consist optionally of additional address information and/or function designation and/or function arguments.

The SRV6 RR Type The idea is for the SRV6 RR Type to return a sequence of IPv6 Segment Routing addresses/instructions. In many ways, the data returned for an SRV6 DNS RR is like an address. For example, it would be reasonable for an application using SRv6 to do a type SRV DNS query followed by an SRV6 query at the resulting domain name. Furthermore, as a fall back, if no SRV6 RR is present in the DNS at a domain name, an application could query for the AAAA IPv6 address RR type. It is usually the case that Segment Routing is used in a relatively localized domain compared with the global Internet and the DNS is commonly thought of as the source for global Internet addressing. However, DNS servers can be configured in a network so that some names are only visible locally and some RRs are only delivered locally. And there may be other circumstances in which the SRV6 RR Type specified below is useful.

Terminology 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 when, and only when, they appear in all capitals, as shown here. The following acronyms are used in this document:

DNS - Domain Name System
IANA - Internet Assigned Number Authority
RR - DNS Resource Record
SRv6 - IPv6 Segment Routing
SRV6 - Mnemonic for the SRv6 RR Type