B.1 DNS RFC procedures general clarifications

29.3033GPPDomain Name System ProceduresRelease 17Stage 3TS

This clause clarifies DNS resolver use of the S-NAPTR procedures in EPC core network nodes.

NOTE: The only EPC core network nodes identified explicitly at this time that employ S-NAPTR procedures are the MME and Release 8 SGSN.

DNS resolvers in EPC core network nodes shall support recursive queries and responses over UDP transport as specified in IETF RFC 1035 [3]. The EPC core network nodes may assume the existence of a local caching DNS server (see GSMA PRD IR.67 [5]) and hence may not need to do iterative queries as specified in IETF RFC 1035 [3]. However, the final deployment decision of local caching DNS servers is up to the operators. It is recommended that the EPC core network nodes support DNS queries and responses over TCP transport up to the 65535 byte maximum. Support of IETF RFC 2671 [13] (EDNS0) is recommended, in order to allow DNS response packets sizes over 512 octets when using UDP transport.

It is recommended that resolvers in EPC core network nodes cache frequently used DNS queries in order to lower load on DNS infrastructure.

EPC core network nodes shall support SRV records as specified in IETF RFC 2782 [8]. However, in the 3GPP scope the ordering of SRV records of the same priority SHALL use the algorithm described in IETF RFC 2782 [8] page 4 instead of the the "SHOULD" requirement in the IETF RFC. This is a 3GPP specific requirement to strengthen the described algorithm and actually allow predictable behavior of the IETF RFC 2782 [8] based load balancing.