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Versions: 00 01 02 03                                                   
   DNSIND Working Group                                         Paul Vixie
   INTERNET-DRAFT                                        Vixie Enterprises
   <draft-ietf-dnsind-edns-01.txt>                             March, 1998
                           Extensions to DNS (EDNS)
   Status of this Memo
      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
      and may be updated, replaced, or obsoleted by other documents at any
      time.  It is inappropriate to use Internet-Drafts as reference
      material or to cite them other than as ``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 ftp.is.co.za (Africa), nic.nordu.net (Europe),
      munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
      ftp.isi.edu (US West Coast).
      The Domain Name System's wire protocol includes a number of fixed
      fields whose range has been or soon will be exhausted.  This document
      describes backward compatible mechanisms for allowing the protocol to
   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.
   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
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   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 - 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
   types 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
   2.3. Compression pointers are 14 bits in size and are relative to the
   start of the DNS Message, which can be 64KB in length.  14 bits restrict
   pointers to the first 16KB of the message, which makes labels introduced
   in the last 48KB of the message unreachable by compression pointers.  A
   longer pointer format is needed.
   2.4. DNS Messages are limited to 512 octets in size when sent over UDP.
   While the minimum maximum reassembly buffer size is still 512 bytes,
   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.
   2.5. DNS Messages are limited to 65535 octets in size when sent over
   TCP.  This acts as an effective maximum on RRset size and on RR size,
   since multiple TCP messages are only possible in the case of zone
   transfers.  Some mechanism must be created to allow normal DNS responses
   (other than zone transfers) to span multiple DNS Messages when TCP is
   2.6. Multiple queries in a question section have not been supported in
   DNS due the applicability of some DNS Message Header flags (such as AA)
   and of the RCODE field only to a single QNAME, QTYPE, and QCLASS.
   Multiple questions per request are desirable, and some way of asking
   them must be made available.
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   3 - Extended Label Types
   3.1. The ``1 0'' 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.  The ``0 1'' label type should be permanently
   3.2. The ``0 0 0 0 0 0'' extended label type will indicate an extended
   compression pointer, such that the following two octets comprise a
   16-bit compression pointer in network byte order.  Like the normal
   compression pointer, this pointer is relative to the start of the DNS
   3.3. The ``0 0 0 0 0 1'' extended label type will indicate a counted bit
   string label with interior longest-match query matching semantics as
   described in [CRAW98].
   3.4. XXX Any other former contenders for the unextended label types? XXX
   4 - OPT pseudo-RR
   4.1. The 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.
   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.
   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 (XXX code)
   CLASS        u_int16_t      sender's UDP buffer size
   TTL          u_int32_t      extended RCODE and flags
   RDLEN        u_int16_t      describes RDATA
   RDATA        octet stream   {attribute,value} pairs
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   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-LENGTH         |          OPTION-CODE          |
      2: |                                                               |
         /                          OPTION-DATA                          /
         /                                                               /
   OPTION-LENGTH  Size (in octets) of OPTION-DATA.
   OPTION-CODE    Assigned by the IANA.  Value 255 is reserved for future
   4.5. The sender's UDP buffer size is the number of octets of the largest
   UDP payload that is reassemblable and deliverable in the sender's
   network stack.  Note that path MTU, with or without fragmentation, may
   be smaller than this.  Also note that a 512-octet UDP payload requires a
   576-octet IP reassembly buffer.  Choosing 1436 on an Ethernet connected
   requestor would be reasonable.
   4.6. The extended RCODE and flags are structured as follows:
                    +0 (MSB)                            +1 (LSB)
      0: |         EXTENDED-RCODE        |            VERSION            |
      2: |MD |FM |                            Z                          |
   EXTENDED-RCODE  Forms upper 8 bits of extended 12-bit RCODE.
                   (Meaningless in requests.)
   VERSION         Indicates the implementation level of whoever sets it.
                   Full conformance with the draft standard version of this
                   specification is version ``0.''
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   Z               Set to zero by senders and ignored by receivers, unless
                   modified in a subsequent specification.
   MD              ``More data'' flag.  Valid only in TCP streams where
                   message ordering is reliable.  This flag indicates that
                   the current message is not the complete response, and
                   should be aggregated with the following message(s)
                   before being considered complete.  Such responses are
                   called ``segmented responses.''  It is an error for the
                   RCODE (including the EXTENDED-RCODE), AA flag, or DNS
                   Message ID to differ among messages in a segmented
                   response.  It is an error for TC to be set on any
                   message in a segmented response.
   FM              ``First match'' flag.  Notable only when multiple
                   questions are sent.  If set, questions will be processed
                   in wire order and the first question whose answer would
                   be NOERROR AND ANCOUNT>0 is treated as if it were the
                   only question in the query message.  If not set,
                   questions can be processed in any order and all possible
                   answer records will be included in the response.
   5 - Multiple Questions
   5.1. If QDCOUNT>1, multiple questions are present.  All questions must
   be for the same QNAME and QCLASS; only the QTYPE is allowed to vary.  It
   is an error for QDCOUNT>1 and any QTYPE=ANY.
   5.2. RCODE and AA apply to all RRs in the answer section having the
   QNAME that is shared by all questions in the question section.
   5.3. QCLASS=ANY is deprecated, for all queries including those with
   5.4. QTYPE=IQUERY is deprecated, for all queries including those with
   6 - Transport Considerations
   6.1. The presence of an OPT pseudo-RR or any new label type, or
   QDCOUNT>1 in a request should be taken as an indication that the
   requestor fully implements this specification and can correctly
   understand any response that conforms to this specification.  If a new
   label type or QDCOUNT>1 is used in a message that does not have an OPT
   RR, a VERSION of ``0'' shall be imputed.
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   6.2. 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.
   6.2. Responders who do not understand these protocol extensions are
   expected to send a respose with RCODE=NOTIMPL or RCODE=FORMERR or even
   RCODE=SERVFAIL.  Therefore use of these extensions should be ``probed''
   such that a responder who isn't known to support them be allowed a retry
   with no extensions in use if it responds with one of the above mentioned
   RCODEs.  Responder capability with respect to these extensions ought to
   be cached by requestors, but a new probe should be sent periodically to
   test for upgrades to responder capability.
   7 - Security Considerations
   No new problems are added to DNS's security model by this specification.
   Correspondingly, no old problems in DNS security are resolved by this
   8 - Acknowledgements
   Paul Mockapetris, Mark Andrews, Robert Elz, Don Lewis, Bob Halley, and
   Donald Eastlake were each instrumental in creating this specification.
   9 - References
   [RFC1035]  P. Mockapetris, ``Domain Names - Implementation and
              Specification,'' RFC 1035, USC/Information Sciences
              Institute, November 1987.
   [CRAW98]   M. Crawford, ``Binary Labels in the Domain Name System,''
              Draft draft-ietf-dnsind-binary-labels-XX, IETF DNSIND, March
   10 - Author's Address
      Paul Vixie
         Vixie Enterprises
         950 Charter Street
         Redwood City, CA 94063
         +1 650 779 7001
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