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Versions: 00 draft-ietf-dnsext-rfc2671bis-edns0

DNSEXT Working Group                                            P. Vixie
Internet-Draft                                                  M. Graff
Obsoletes: 2671 (if approved)                Internet Systems Consortium
Intended status: Standards Track                           July 13, 2009
Expires: January 14, 2010


                  Extension Mechanisms for DNS (EDNS0)
                     draft-graff-dnsext-edns0bis-00

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 14, 2010.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   The Domain Name System's wire protocol includes a number of fixed
   fields whose range has been or soon will be exhausted and does not



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   allow clients to advertise their capabilities to servers.  This
   document describes backward compatible mechanisms for allowing the
   protocol to grow.

   This document is a starting point to update the EDNS0 RFC after 10
   years of operational experience.


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . . . 3
   3.  Affected Protocol Elements  . . . . . . . . . . . . . . . . . . 3
     3.1.  Message Header  . . . . . . . . . . . . . . . . . . . . . . 3
     3.2.  Label Types . . . . . . . . . . . . . . . . . . . . . . . . 3
     3.3.  UDP Message Size  . . . . . . . . . . . . . . . . . . . . . 3
   4.  Extended Label Types  . . . . . . . . . . . . . . . . . . . . . 4
     4.1.  Extended Label Type . . . . . . . . . . . . . . . . . . . . 4
     4.2.  Reserved Label Type . . . . . . . . . . . . . . . . . . . . 4
   5.  OPT pseudo-RR . . . . . . . . . . . . . . . . . . . . . . . . . 4
     5.1.  OPT Record Behavior . . . . . . . . . . . . . . . . . . . . 4
     5.2.  OPT Record Format . . . . . . . . . . . . . . . . . . . . . 4
       5.2.1.  Fixed Content . . . . . . . . . . . . . . . . . . . . . 4
       5.2.2.  Variable Content  . . . . . . . . . . . . . . . . . . . 5
     5.3.  Sender's Payload Size . . . . . . . . . . . . . . . . . . . 5
       5.3.1.  Reassembly Considerations . . . . . . . . . . . . . . . 6
       5.3.2.  Path MTU  . . . . . . . . . . . . . . . . . . . . . . . 6
       5.3.3.  No Caching  . . . . . . . . . . . . . . . . . . . . . . 6
       5.3.4.  Oversize Requests . . . . . . . . . . . . . . . . . . . 6
       5.3.5.  Be Reasonable . . . . . . . . . . . . . . . . . . . . . 6
     5.4.  Extended RCODE  . . . . . . . . . . . . . . . . . . . . . . 6
   6.  Transport Considerations  . . . . . . . . . . . . . . . . . . . 7
     6.1.  Meaning of OPT Presense . . . . . . . . . . . . . . . . . . 7
     6.2.  Meaning of OPT Absence  . . . . . . . . . . . . . . . . . . 7
     6.3.  Refusing Message with OPT Records . . . . . . . . . . . . . 8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . . . 9
     10.1. Normative References  . . . . . . . . . . . . . . . . . . . 9
     10.2. Informative References  . . . . . . . . . . . . . . . . . . 9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 9









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1.  Introduction

   DNS [RFC1035] specifies a Message Format and within such messages
   there are standard formats for encoding options, errors, and name
   compression.  The maximum allowable size of a DNS Message is fixed.
   Many of DNS's protocol limits are too small for uses which are or
   which are desired to become common.  There is no way for
   implementations to advertise their capabilities.

   Existing clients will not know how to interpret the protocol
   extensions detailed here.  In practice, these clients will be
   upgraded when they have need of a new feature, and only new features
   will make use of the extensions.  We must however take account of
   client behaviour in the face of extra fields, and design a fallback
   scheme for interoperability with these clients.


2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].


3.  Affected Protocol Elements

3.1.  Message Header

   The DNS Message Header's (see RFC 1035, section 4.1.1 [RFC1035])
   second full 16-bit word is divided into a 4-bit OPCODE, a 4-bit
   RCODE, and a number of 1-bit flags.  The original reserved Z bits
   have been allocated to various purposes, and most of the RCODE values
   are now in use.  More flags and more possible RCODEs are needed.

3.2.  Label Types

   The first two bits of a wire format domain label are used to denote
   the type of the label.  RFC 1035, 4.1.4 [RFC1035] allocates two of
   the four possible types and reserves the other two.  Proposals for
   use of the remaining types far outnumber those available.  More label
   types are needed.

3.3.  UDP Message Size

   DNS Messages are limited to 512 octets in size when sent over UDP.
   While the minimum maximum reassembly buffer size still allows a limit
   of 512 octets of UDP payload, most of the hosts now connected to the
   Internet are able to reassemble larger datagrams.  Some mechanism



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   must be created to allow requestors to advertise larger buffer sizes
   to responders.


4.  Extended Label Types

4.1.  Extended Label Type

   The "0 1" label type will now indicate an extended label type, whose
   value is encoded in the lower six bits of the first octet of a label.
   All subsequently developed label types should be encoded using an
   extended label type.

4.2.  Reserved Label Type

   The "1 1 1 1 1 1" extended label type will be reserved for future
   expansion of the extended label type code space.


5.  OPT pseudo-RR

5.1.  OPT Record Behavior

   One OPT pseudo-RR can be added to the additional data section of
   either a request or a response.  An OPT is called a pseudo-RR because
   it pertains to a particular transport level message and not to any
   actual DNS data.  OPT RRs shall never be cached, forwarded, or stored
   in or loaded from master files.  The quantity of OPT pseudo-RRs per
   message shall be either zero or one, but not greater.

5.2.  OPT Record Format

   An OPT RR has a fixed part and a variable set of options expressed as
   {attribute, value} pairs.  The fixed part holds some DNS meta data
   and also a small collection of new protocol elements which we expect
   to be so popular that it would be a waste of wire space to encode
   them as {attribute, value} pairs.

5.2.1.  Fixed Content

   The fixed part of an OPT RR is structured as follows:










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         +------------+--------------+---------------------------+
         | Field Name | Field Type   | Description               |
         +------------+--------------+---------------------------+
         | NAME       | domain name  | empty (root domain)       |
         | TYPE       | u_int16_t    | OPT                       |
         | CLASS      | u_int16_t    | sender's UDP payload size |
         | TTL        | u_int32_t    | extended RCODE and flags  |
         | RDLEN      | u_int16_t    | describes RDATA           |
         | RDATA      | octet stream | {attribute,value} pairs   |
         +------------+--------------+---------------------------+

                               OPT RR Format

5.2.2.  Variable Content

   The variable part of an OPT RR is encoded in its RDATA and is
   structured as zero or more of the following:


                  +0 (MSB)                            +1 (LSB)
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
    0: |                          OPTION-CODE                          |
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
    2: |                         OPTION-LENGTH                         |
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
    4: |                                                               |
       /                          OPTION-DATA                          /
       /                                                               /
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

   OPTION-CODE
         Assigned by IANA.

   OPTION-LENGTH
         Size (in octets) of OPTION-DATA.

   OPTION-DATA
         Varies per OPTION-CODE.

5.3.  Sender's Payload Size

   The sender's UDP payload size (which OPT stores in the RR CLASS
   field) is the number of octets of the largest UDP payload that can be
   reassembled and delivered in the sender's network stack.  Note that
   path MTU, with or without fragmentation, may be smaller than this.






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5.3.1.  Reassembly Considerations

   4.5.1 Note that a 512-octet UDP payload requires a 576-octet IP
   reassembly buffer.  Choosing 1280 on an Ethernet connected requestor
   would be reasonable.  The consequence of choosing too large a value
   may be an ICMP message from an intermediate gateway, or even a silent
   drop of the response message.

5.3.2.  Path MTU

   Both requestors and responders are advised to take account of the
   path's discovered MTU (if already known) when considering message
   sizes.

5.3.3.  No Caching

   4.5.3.  The requestor's maximum payload size can change over time,
   and should therefore not be cached for use beyond the transaction in
   which it is advertised.

5.3.4.  Oversize Requests

   4.5.4.  The responder's maximum payload size can change over time,
   but can be reasonably expected to remain constant between two
   sequential transactions; for example, a meaningless QUERY to discover
   a responder's maximum UDP payload size, followed immediately by an
   UPDATE which takes advantage of this size.  (This is considered
   preferrable to the outright use of TCP for oversized requests, if
   there is any reason to suspect that the responder implements EDNS,
   and if a request will not fit in the default 512 payload size limit.)

5.3.5.  Be Reasonable

   4.5.5.  Due to transaction overhead, it is unwise to advertise an
   architectural limit as a maximum UDP payload size.  Just because your
   stack can reassemble 64KB datagrams, don't assume that you want to
   spend more than about 4KB of state memory per ongoing transaction.

5.4.  Extended RCODE

   4.6.  The extended RCODE and flags (which OPT stores in the RR TTL
   field) are structured as follows:

                  +0 (MSB)                            +1 (LSB)
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
    0: |         EXTENDED-RCODE        |            VERSION            |
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
    2: |                               Z                               |



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       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

   EXTENDED-RCODE
         Forms upper 8 bits of extended 12-bit RCODE.  Note that
         EXTENDED-RCODE value "0" indicates that an unextended RCODE is
         in use (values "0" through "15").

   VERSION
         Indicates the implementation level of whoever sets it.  Full
         conformance with this specification is indicated by version
         ``0.''  Requestors are encouraged to set this to the lowest
         implemented level capable of expressing a transaction, to
         minimize the responder and network load of discovering the
         greatest common implementation level between requestor and
         responder.  A requestor's version numbering strategy should
         ideally be a run time configuration option.
         If a responder does not implement the VERSION level of the
         request, then it answers with RCODE=BADVERS.  All responses
         will be limited in format to the VERSION level of the request,
         but the VERSION of each response will be the highest
         implementation level of the responder.  In this way a requestor
         will learn the implementation level of a responder as a side
         effect of every response, including error responses, including
         RCODE=BADVERS.

   Z
         Set to zero by senders and ignored by receivers, unless
         modified in a subsequent specification.


6.  Transport Considerations

6.1.  Meaning of OPT Presense

   The presence of an OPT pseudo-RR in a request should be taken as an
   indication that the requestor fully implements the given version of
   EDNS, and can correctly understand any response that conforms to that
   feature's specification.

6.2.  Meaning of OPT Absence

   Lack of use of these features in a request must be taken as an
   indication that the requestor does not implement any part of this
   specification and that the responder may make no use of any protocol
   extension described here in its response.






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6.3.  Refusing Message with OPT Records

   Responders who do not understand these protocol extensions are
   expected to send a response with RCODE NOTIMPL, FORMERR, or SERVFAIL.
   Therefore use of extensions should be ``probed'' such that a
   responder who isn't known to support them be allowed a retry with no
   extensions if it responds with such an RCODE.  If a responder's
   capability level is cached by a requestor, a new probe should be sent
   periodically to test for changes to responder capability.


7.  Security Considerations

   Requestor-side specification of the maximum buffer size may open a
   new DNS denial of service attack if responders can be made to send
   messages which are too large for intermediate gateways to forward,
   thus leading to potential ICMP storms between gateways and
   responders.


8.  IANA Considerations

   The IANA has assigned RR type code 41 for OPT.

   It is the recommendation of this document and its working group that
   IANA create a registry for EDNS Extended Label Types, for EDNS Option
   Codes, and for EDNS Version Numbers.

   This document assigns label type 0b01xxxxxx as "EDNS Extended Label
   Type."  We request that IANA record this assignment.

   This document assigns extended label type 0bxx111111 as "Reserved for
   future extended label types."  We request that IANA record this
   assignment.

   This document assigns option code 65535 to "Reserved for future
   expansion."

   This document expands the RCODE space from 4 bits to 12 bits.  This
   will allow IANA to assign more than the 16 distinct RCODE values
   allowed in RFC 1035 [RFC1035].

   This document assigns EDNS Extended RCODE "16" to "BADVERS".

   IESG approval should be required to create new entries in the EDNS
   Extended Label Type or EDNS Version Number registries, while any
   published RFC (including Informational, Experimental, or BCP) should
   be grounds for allocation of an EDNS Option Code.



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9.  Acknowledgements

   Paul Mockapetris, Mark Andrews, Robert Elz, Don Lewis, Bob Halley,
   Donald Eastlake, Rob Austein, Matt Crawford, Randy Bush, and Thomas
   Narten were each instrumental in creating and refining this
   specification.


10.  References

10.1.  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

10.2.  Informative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.


Authors' Addresses

   Paul Vixie
   Internet Systems Consortium
   950 Charter Street
   Redwood City, California  94063
   US

   Phone: +1 650.423.1301
   Email: vixie@isc.org


   Michael Graff
   Internet Systems Consortium
   950 Charter Street
   Redwood City, California  94063
   US

   Phone: +1 650.423.1304
   Email: mgraff@isc.org










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