RFC 3226 (rfc3226) - Page 1 of 6
DNSSEC and IPv6 A6 aware server/resolver message size requirements
Alternative Format: Original Text Document
Network Working Group O. Gudmundsson Request for Comments: 3226 December 2001 Updates: 2874, 2535 Category: Standards Track DNSSEC and IPv6 A6 aware server/resolver message size requirements Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2001). All Rights Reserved. Abstract This document mandates support for EDNS0 (Extension Mechanisms for DNS) in DNS entities claiming to support either DNS Security Extensions or A6 records. This requirement is necessary because these new features increase the size of DNS messages. If EDNS0 is not supported fall back to TCP will happen, having a detrimental impact on query latency and DNS server load. This document updates RFC 2535 and RFC 2874, by adding new requirements. 1. Introduction Familiarity with the DNS [RFC 1034, RFC 1035], DNS Security Extensions [RFC 2535], EDNS0 [RFC 2671] and A6 [RFC 2874] is helpful. STD 13, RFC 1035 Section 2.3.4 requires that DNS messages over UDP have a data payload of 512 octets or less. Most DNS software today will not accept larger UDP datagrams. Any answer that requires more than 512 octets, results in a partial and sometimes useless reply with the Truncation Bit set; in most cases the requester will then retry using TCP. Furthermore, server delivery of truncated responses varies widely and resolver handling of these responses also varies, leading to additional inefficiencies in handling truncation. Compared to UDP, TCP is an expensive protocol to use for a simple transaction like DNS: a TCP connection requires 5 packets for setup and tear down, excluding data packets, thus requiring at least 3 round trips on top of the one for the original UDP query. The DNS Gudmundsson Standards Track
