bind9/doc/draft/draft-ietf-dnsext-message-size-01.txt

DNSEXT Working Group                       Olafur Gudmundsson (NAI Labs)
INTERNET-DRAFT                                              October 2000

<draft-ietf-dnsext-message-size-01.txt>

Updates: RFC 2535, RFC 2874



   DNSSEC and IPv6 A6 aware server/resolver message size requirements


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
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   Comments should be sent to the authors or the DNSEXT WG mailing list
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   This draft expires on March 29, 2001.

   Copyright Notice

   Copyright (C) The Internet Society (2000).  All rights reserved.










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Abstract

   This document mandates support for EDNS0 in DNS entities claiming to
   support DNS Security Extensions and 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.




1 - Introduction

   Familiarity with the DNS[RFC1034, RFC1035], DNS Security
   Extensions[RFC2535], EDNS0[RFC2671] and A6[RFC2874] is helpful.

   RFC 1035[RFC1035] 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 size UDP datagrams. Thus, any answer that
   requires more than 512 octets will result in a partial and probably
   useless reply with the Truncation Bit set; in most cases the
   requester will then retry using TCP. Some DNS servers send back an
   answer truncating the message at the last RR boundary before
   truncation, other servers truncate at the previous set, some send
   back empty answer with TC bit set.

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


1.1 - 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..200 octets.  The
   addition of these signatures on each or most RR sets in an answer
   will significantly increase the size of DNS answers from secure
   zones.

   It is important that security aware servers and resolvers get all the
   data in Answer and Authority section in one query without truncation.
   In some cases it is important that some Additional Data be sent



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   along, mainly in delegation cases.

   TSIG[RFC2845] allows for the light weight authentication of DNS
   messages, but increases the size of the messages by at least 70
   octets.  DNSSEC allows for computationally expensive message
   authentication with 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.


1.2 - IPv6 Motivations

   IPv6 addresses[RFC2874] are 128 bits and are 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
   queried name has multiple A6 addresses may overflow a 512-octet UDP
   packet size.


1.3 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 DNS message. If root servers start advertising A6 or
   KEY records then the root zone answer for NS records will not fit in
   an single 512-octet DNS message. Resulting in a large number of TCP
   connections to the root servers.

   It is important that a high level domains have a high number of
   domain name servers for redundancy, latency and load balancing
   reasons.


1.4 UDP vs TCP for DNS messages

   Given all these factors, it is essential that any implementations
   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 avoid fragmentation
   of DNS responses. A fragmented UDP message that suffers a loss off
   one of the fragments renders the answer useless and DNS must
   retransmit the query. TCP connection requires number of round trips
   for establishment, data transfer and tear down, but only the lost
   data segments are retransmitted.




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   In the early days 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 few thousand octets.


1.5 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.

1.7 - Requirements

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


2 - Protocol changes:

   This document updates [RFC2535] and [RFC2874], by adding new
   requirements.

   All RFC2535-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 RFC2874-compliant servers and resolver MUST support EDNS0 and
   advertise message size of at least 1024 octets, but SHOULD advertise
   message size of 2048.

   All RFC2535 and RFC2874 compliant entities MUST be able to handle
   fragmented IP and IPv6 UDP packets.

   All hosts supporting both RFC2535 and RFC2874 MUST use the larger
   required value in EDNS0 advertisements.








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3 Acknowledgments

   Harald Alvestrand, Rob Austein, Randy Bush, David Conrad, Andreas
   Gustafsson, Bob Halley, Edward Lewis and Kazu Yamamoto where
   instrumental in motivating and shaping this document.

4 - Security Considerations:

   There are no additional security considerations other than those in
   RFC2671.


5 - IANA Considerations:

   None

References:


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

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


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


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


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


[RFC2874]  M. Crawford, C. Huitema, S. Thompson, ``DNS Extensions to
           Support IPv6 Address Aggregation and Renumbering'', RFC2874,
           Sometime 2000.









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Author Address


   Olafur Gudmundsson
      NAI Labs
      Network Associates
      3060 Washington Road (Rt. 97)
      Glenwood, MD 21738
      USA
      +1 443 259 2389
      <ogud@tislabs.com>


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