RFC 3226 (rfc3226) - Page 1 of 6

DNSSEC and IPv6 A6 aware server/resolver message size requirements

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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
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