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Versions: 00 01 02                                                      
INTERNET-DRAFT                                   Donald E. Eastlake, 3rd
Updates RFC 1035                                               CyberCash
Expires August 1998                                        February 1998

                 Bigger Domain Name System UDP Replies
                 ------ ------ ---- ------ --- -------

                         Donald E. Eastlake 3rd

Status of This Document

   This draft, file name draft-ietf-dnsind-udp-size-01.txt, is intended
   to be become a Proposed Standard RFC.  Distribution of this document
   is unlimited. Comments should be sent to the DNS mailing list
   <namedroppers@internic.net> or to the author.

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months.  Internet-Drafts may be updated, replaced, or obsoleted by
   other documents at any time.  It is not appropriate to use Internet-
   Drafts as reference material or to cite them other than as a
   ``working draft'' or ``work in progress.''

   To learn the current status of any Internet-Draft, please check the
   1id-abstracts.txt listing contained in the Internet-Drafts Shadow
   Directories on ds.internic.net (East USA), ftp.isi.edu (West USA),
   ftp.nordu.net (North Europe), ftp.nis.garr.it (South Europe),
   munnari.oz.au (Pacific Rim), or ftp.is.co.za (Africa).


   The Domain Name System defaults to using UDP for queries and replies
   with a DNS payload limit of 512 bytes.  Larger replies cause an
   initial truncation indication leading to a subsequent handling via
   TCP with substantially higher overhead.  An extension to DNS UDP
   requests is specified which frequently permits larger UDP responses
   thus reducing the need for use of TCP.

Donald E. Eastlake, 3rd                                         [Page 1]

INTERNET-DRAFT                                    Bigger DNS UDP Replies


   Paul Vixie originated the basic idea specified herein.

   Some errors notice by Chris Thompson in verion -00 have been fixed.

Table of Contents

      Status of This Document....................................1

      Table of Contents..........................................2

      1. Introduction............................................3
      2. Permitting Larger DNS UDP Packets.......................3
      3. Compatibility Discussion................................5
      4. Security Considerations.................................5

      Author's Addresses.........................................6
      Expiration and File Name...................................6

Donald E. Eastlake, 3rd                                         [Page 2]

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1. Introduction

   The global Internet Domain Name System (DNS) is documented in RFC
   1034, 1035, and numerous additional Requests for Comment.  It
   provides a distributed hierarchical database with redundant servers.
   Recently security features have been added to the DNS [RFC 2065].

   DNS can transfer data via both UDP and TCP.  Some requests that are
   very likely to have big responses, most commonly zone transfers, just
   use TCP.  However, the vast majority of requests are initially sent
   via UDP which causes the response to be via UDP.

   DNS over UDP is constrained to one packet for the request, which is
   normally no problem as requests are usually small, and one packet for
   response, which can be a problem.  The DNS data portion of DNS UDP
   packets is currently limited to 512 bytes.  The standard states that
   if the data required to be in the response to a UDP request does not
   fit in 512 bytes, a truncation flag bit is set in the response and
   the resolver must try again using TCP with TCP's substantially higher
   set up and tear down overhead.

   As signatures and/or keys are included in more responses due to DNS
   security [RFC 2065] and average domain names get longer and larger
   addresses for IPv6 [RFC 1886] come into use and there are increasing
   numbers of instances of larger RRsets, the old UDP response size
   limit will increasingly be exceeded. Yet the bulk of the network has
   MTUs on the order of the Ethernet MTU or larger (in some cases
   simulated by link adaptation layers that disguise a smaller physical
   MTU) and all modern IP stacks can handle buffering of that size or

2. Permitting Larger DNS UDP Packets

   Efforts are under way to define an additional information resource
   record that can be used to communicate exact buffer sizes and other
   information. However, many older DNS servers ignore any request with
   a non-empty additional information section.  Pending deployment of
   some such more exact and comprehensive solution, the following change
   is made in the DNS over UDP protocol.

   No change is made in the size limit on UDP queries.  It remains at
   512 bytes.

   The presently unused RCODE field in UDP queries is redefined to
   specify the resolver imposed limit on the DNS data in the UDP
   response.  This four bit field is presently specified as zero.
   Values from 1 through 7 are defined as follows:

Donald E. Eastlake, 3rd                                         [Page 3]

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             RCODE   DNS data limit in bytes

              0        512 (current default)
              1        768
              2       1280 (new default)
              3       1920
              4       3200 (appropriate for all FDDI UDP)
              5       4800
              6       8000
              7      12000
             8-15     -reserved

   A resolver should take into account its local buffer space and any
   knowledge it has about the local network MTU (maximum transmission
   unit) or the PMTU (path MTU) to the server it is querying.  Making a
   reasonable allowance for IP headers that may be added by the server,
   the resolver should then pick an RCODE value from the above table. A
   value that might be expected to cause a reply packet to fragment into
   two pieces is still preferable to using TCP. In the absence of any
   information, the value 2 should be used.

   The resolver should not do PMTU discovery just to provide a more
   accurate RCODE.  The additional packets required for PMTU discovery
   would defeat the purpose of avoiding the additional packets required
   by TCP.

   A server, on receiving a query with a non-zero RCODE, MUST limit its
   DNS response message to the size specified but may need to limit it
   to a lower amount due to buffer space available.  It SHOULD also
   limit it based on local network MTU or the PMTU to the resolver, if
   known, less a reasonable allowance for IP headers.

   1280 bytes of DNS data is chosen as the new default to provide a
   generous allowance for IP headers and still be within the highly
   prevalent approximately Ethernet size or larger MTU and buffering
   generally available today. (It should also be noted that
   consideration is being given to increasing the original IPv6 minimum
   MTU from 576 to 1280 bytes.)

   An IPv6 server should enable fragmentation on UDP replies.  While
   fragmentation will not be frequent if the above guidelines are
   followed, it may occur on occasion. In principle, IPv6 headers and
   options could be huge, resulting in a very large UDP packet even
   though the DNS payload is limited, but this should not occur in

Donald E. Eastlake, 3rd                                         [Page 4]

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3. Compatibility Discussion

   No cases are known where the above change will cause a problem for
   non-recursive queries.  Old servers will ignore the RCODE field of
   the UDP query and should return 512 or fewer bytes, possibly with a
   truncation indication.  Servers with this feature included should use
   the RCODE value to determine a ceiling on the size of response they
   will send.  Non-zero values of RCODE will permit them to send larger
   UDP responses if local conditions are appropriate.

   There is a potential problem with recursive queries.  If (1) an
   updated recursive query specifies bigger UDP responses with a non-
   zero RCODE to an old server and (2) that server in turn issues a
   corresponding query into which it blindly copies the RCODE field and
   (3) this corresponding query goes to an updated server that honors
   the non-zero RCODE field and (4) the updated server response DNS data
   is actually larger than 512 bytes as permitted by the RCODE in the
   query, then the intermediate old DNS server may be confused by the
   larger than 512 byte DNS response it receives.  However, there are
   already DNS implementations out there on the Internet that send back
   larger than 512 byte responses in violation of the old standard and
   DNS implementations are being deployed which protect themselves
   against and are not confused by larger than expected responses.

   Should the above problem manifest itself, it can be cured by making
   the queries be TCP based or non-recursive or by upgrading the
   intermediate DNS server to which the recursive queries are being sent
   to implement this bigger UDP packet feature.  There are cases, such
   as resolvers behind a firewall that can only get outside DNS
   information via a recursive server and changing to non-recursive
   queries is not possible.  Upgrading the DNS server is the strongly
   recommended solution.

4. Security Considerations

   General DNS security issues are considered in RFC 2065.

   In the absence of request security [RFC 2065], the request RCODE
   could be modified in transit. If set lower, this might result in
   unnecessary TCP.  If set higher, this might result in unnecessary

   Larger packets may make it easier to cause some forms of denial of
   service due to fragment loss.

Donald E. Eastlake, 3rd                                         [Page 5]

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   RFC 1034 - P. Mockapetris, "Domain names - concepts and facilities",

   RFC 1035 - P. Mockapetris, "Domain names - implementation and
   specification", 11/01/1987.

   RFC 2065 - D. Eastlake, C. Kaufman,  "Domain Name System Security
   Extensions", 01/03/1997.

Author's Addresses

   Donald E. Eastlake 3rd
   CyberCash, Inc.
   318 Acton Street
   Carlisle, MA 01741 USA

   Telephone:   +1 978 287 4877
                +1 703 620-4200 (main office, Reston, VA)
   FAX:         +1 978 371 7148
   EMail:       dee@cybercash.com

Expiration and File Name

   This draft expires August 1998.

   Its file name is draft-ietf-dnsind-udp-size-01.txt.

Donald E. Eastlake, 3rd                                         [Page 6]