path: root/doc/rfc/rfc3226.txt

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 [RFC1034, RFC1035], DNS Security Extensions
   [RFC2535], EDNS0 [RFC2671] and A6 [RFC2874] 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



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   server also needs to keep a state of the connection during this
   transaction.  Many DNS servers answer thousands of queries per
   second, requiring them to use TCP will cause significant overhead and
   delays.

1.1.  Requirements

   The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", and "MAY"
   in this document are to be interpreted as described in RFC 2119.

2.  Motivating factors

2.1.  DNSSEC motivations

   DNSSEC [RFC2535] secures DNS by adding a Public Key signature on each
   RR set.  These signatures range in size from about 80 octets to 800
   octets, most are going to be in the range of 80 to 200 octets.  The
   addition of signatures on each or most RR sets in an answer
   significantly increases the size of DNS answers from secure zones.

   For performance reasons and to reduce load on DNS servers, it is
   important that security aware servers and resolvers get all the data
   in Answer and Authority section in one query without truncation.
   Sending Additional Data in the same query is helpful when the server
   is authoritative for the data, and this reduces round trips.

   DNSSEC OK[OK] specifies how a client can, using EDNS0, indicate that
   it is interested in receiving DNSSEC records.  The OK bit does not
   eliminate the need for large answers for DNSSEC capable clients.

2.1.1.  Message authentication or TSIG motivation

   TSIG [RFC2845] allows for the light weight authentication of DNS
   messages, but increases the size of the messages by at least 70
   octets.  DNSSEC specifies for computationally expensive message
   authentication SIG(0) using a standard public key signature.  As only
   one TSIG or SIG(0) can be attached to each DNS answer the size
   increase of message authentication is not significant, but may still
   lead to a truncation.

2.2.  IPv6 Motivations

   IPv6 addresses [RFC2874] are 128 bits and can be represented in the
   DNS by multiple A6 records, each consisting of a domain name and a
   bit field.  The domain name refers to an address prefix that may
   require additional A6 RRs to be included in the answer.  Answers
   where the queried name has multiple A6 addresses may overflow a 512-
   octet UDP packet size.



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2.3.  Root server and TLD server motivations

   The current number of root servers is limited to 13 as that is the
   maximum number of name servers and their address records that fit in
   one 512-octet answer for a SOA record.  If root servers start
   advertising A6 or KEY records then the answer for the root NS records
   will not fit in a single 512-octet DNS message, resulting in a large
   number of TCP query connections to the root servers.  Even if all
   client resolver query their local name server for information, there
   are millions of these servers.  Each name server must periodically
   update its information about the high level servers.

   For redundancy, latency and load balancing reasons, large numbers of
   DNS servers are required for some zones.  Since the root zone is used
   by the entire net, it is important to have as many servers as
   possible.  Large TLDs (and many high-visibility SLDs) often have
   enough servers that either A6 or KEY records would cause the NS
   response to overflow the 512 byte limit.  Note that these zones with
   large numbers of servers are often exactly those zones that are
   critical to network operation and that already sustain fairly high
   loads.

2.4.  UDP vs TCP for DNS messages

   Given all these factors, it is essential that any implementation that
   supports DNSSEC and or A6 be able to use larger DNS messages than 512
   octets.

   The original 512 restriction was put in place to reduce the
   probability of fragmentation of DNS responses.  A fragmented UDP
   message that suffers a loss of one of the fragments renders the
   answer useless and the query must be retried.  A TCP connection
   requires a larger number of round trips for establishment, data
   transfer and tear down, but only the lost data segments are
   retransmitted.

   In the early days a number of IP implementations did not handle
   fragmentation well, but all modern operating systems have overcome
   that issue thus sending fragmented messages is fine from that
   standpoint.  The open issue is the effect of losses on fragmented
   messages.  If connection has high loss ratio only TCP will allow
   reliable transfer of DNS data, most links have low loss ratios thus
   sending fragmented UDP packet in one round trip is better than
   establishing a TCP connection to transfer a few thousand octets.







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2.5.  EDNS0 and large UDP messages

   EDNS0 [RFC2671] allows clients to declare the maximum size of UDP
   message they are willing to handle.  Thus, if the expected answer is
   between 512 octets and the maximum size that the client can accept,
   the additional overhead of a TCP connection can be avoided.

3.  Protocol changes:

   This document updates RFC 2535 and RFC 2874, by adding new
   requirements.

   All RFC 2535 compliant servers and resolvers MUST support EDNS0 and
   advertise message size of at least 1220 octets, but SHOULD advertise
   message size of 4000.  This value might be too low to get full
   answers for high level servers and successor of this document may
   require a larger value.

   All RFC 2874 compliant servers and resolver MUST support EDNS0 and
   advertise message size of at least 1024 octets, but SHOULD advertise
   message size of 2048.  The IPv6 datagrams should be 1024 octets,
   unless the MTU of the path is known.  (Note that this is smaller than
   the minimum IPv6 MTU to allow for some extension headers and/or
   encapsulation without exceeding the minimum MTU.)

   All RFC 2535 and RFC 2874 compliant entities MUST be able to handle
   fragmented IPv4 and IPv6 UDP packets.

   All hosts supporting both RFC 2535 and RFC 2874 MUST use the larger
   required value in EDNS0 advertisements.

4.  Acknowledgments

   Harald Alvestrand, Rob Austein, Randy Bush, David Conrad, Andreas
   Gustafsson, Jun-ichiro itojun Hagino, Bob Halley, Edward Lewis
   Michael Patton and Kazu Yamamoto were instrumental in motivating and
   shaping this document.

5.  Security Considerations:

   There are no additional security considerations other than those in
   RFC 2671.

6.  IANA Considerations:

   None





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7.  References

   [RFC1034]  Mockapetris, P., "Domain Names - Concepts and Facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain Names - Implementation and
              Specification", STD 13, RFC 1035, November 1987.

   [RFC2535]  Eastlake, D. "Domain Name System Security Extensions", RFC
              2535, March 1999.

   [RFC2671]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
              2671, August 1999.

   [RFC2845]  Vixie, P., Gudmundsson, O., Eastlake, D. and B.
              Wellington, "Secret Key Transaction Authentication for DNS
              (TSIG)", RFC 2845, May 2000.

   [RFC2874]  Crawford, M. and C. Huitema, "DNS Extensions to Support
              IPv6 Address Aggregation and Renumbering", RFC 2874, July
              2000.

   [RFC3225]  Conrad, D., "Indicating Resolver Support of DNSSEC", RFC
              3225, December 2001.

8.  Author Address

   Olafur Gudmundsson
   3826 Legation Street, NW
   Washington, DC 20015
   USA

   EMail: ogud@ogud.com


















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9.  Full Copyright Statement

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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