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Internet Engineering Task Force DNS SRv6 A Domain Name System (DNS) Resource Record (RR) Type is specified for storing IPv6 Segment Routing (SRv6) Information in the DNS.

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 all 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 This document specifies a SRV6 RR Type to return a sequence of IPv6 Segment Routing addresses / instructions and optionally other data. 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 if it was in a domain where SRv6 was in use. 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. Segment Routing is intended to be 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.

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
• SRH - Segment Routing (IPv6) Header
• SRv6 - IPv6 Segment Routing
• SRV6 - Mnemonic for the SRv6 RR Type

SRV6 RR Type RDATA The SRV6 RR type enables the storage and retrieval of an ordered sequence of SRv6 quantities each of which is the size of an IPv6 address. The RDATA for this type of RR is simply a sequence of such quantities followed by optional data (see Figure 1) and will be (N*16) bytes long, where N is the number of such quantities present, plus the length of the optional data. This optional data MUST NOT exceed 15 bytes in length or else some of the optional data will be erroneously interpreted as one or more Segements. The RR Type Code for the SRV6 RR is TBD1.

SRV6 RRTYPE Data

The RDATA consists of one or more 128-bit SRv6 segments optionally followed by optional data. If the RDATA length is an exact multiple of 16 bytes, there is no optional data. If optional data is present, the first byte of that optional data consists of flags indicating what further optional data fields are present as decribed below. The optional data fields, if present, appear in the same order as the flags indicating their presence.

• F - If one, indicates that the "SRH Flags" byte is present. This byte is intended for use in the Flags field of the SRH. When this bit is a one, the total optional data MUST be at least 2 bytes long. If it is only one byte long, the SRV6 RR is corrupt and is ignored.
• T - If one, indicates that the "SRH Tag" 2-byte field is present. This field is intended for use in the Tag field of the SRH. If it is one and F is zero, the optional data MUST be at least 3 bytes long. It T is one and F is one, the optional data MUST be at least 4 bytes long. If the optional data is too short, the SRV6 RR is corrupt and is ignored.
• Res - These bits are for future extensibility and MUST be sent as zero and ignored on receipt.
• SRH Flags - See F bit above.
• SRH Tag - See T bit above.
• Other - Later expansion of the SRV6 RR may require additional optional data but the total amount of optional data MUST NOT exceed 15 bytes or the initial 16 bytes of optional data will be incorrectly parsed as a Segment. Other data MUST be ignored on receipt.

Acknowledgements The suggestions and comments of the following persons are gratefully acknowledged: tbd

IANA Considerations IANA is requested to assign an SRV6 RR Type (TBD1) as in the template in Appendix A.

Security Considerations tbd

References Normative References Informative References

SRV6 RR Type Template