rfc2671.txt

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Network Working Group                                            P. Vixie
Request for Comments: 2671                                            ISC
Category: Standards Track                                     August 1999


                  Extension Mechanisms for DNS (EDNS0)

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 (1999).  All Rights Reserved.

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

1 - Rationale and Scope

1.1. DNS (see [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.

1.2. 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.









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RFC 2671          Extension Mechanisms for DNS (EDNS0)       August 1999


2 - Affected Protocol Elements

2.1. The DNS Message Header's (see [RFC1035 4.1.1]) 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.

2.2. The first two bits of a wire format domain label are used to denote
     the type of the label.  [RFC1035 4.1.4] 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.

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

3 - Extended Label Types

3.1. 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.

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

4 - OPT pseudo-RR

4.1. 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.

4.2. 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.





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RFC 2671          Extension Mechanisms for DNS (EDNS0)       August 1999


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

     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

4.4. 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.

4.5. 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.

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.

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





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RFC 2671          Extension Mechanisms for DNS (EDNS0)       August 1999


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.

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.)

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.

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

                 +0 (MSB)                            +1 (LSB)