INTERNET-DRAFT                                 M. Andrews (Nominum Inc.)
   <draft-andrews-http-srv-00.txt>                              T. Kottelin
   Updates: RFC 2782                                           October 2000
             HTTP host and port selection using URIs and SRV RRs
   A generic mechanism for resolving port conflicts between URIs and SRV RRs
   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
      other groups may also distribute working documents as Internet-
      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."
      The list of current Internet-Drafts can be accessed at
      The list of Internet-Draft Shadow Directories can be accessed at
      Comments should be sent to the authors.
      This draft expires on April 2001
   Copyright Notice
      Copyright (C) The Internet Society (2000).  All rights reserved.
      Many of todays HTTP sites are virtual, that is they are hosted on a
      machine that is not known by the name the HTTP site is known by.
      This leads to the problem of how to rationally give these HTTP sites
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   INTERNET-DRAFT                   SRV-URI                    October 2000
      IP addresses.  This has traditionally been done by using CNAMES
      [RFC1034][RFC1035] or by using explicit IP address records where
      CNAMES are illegal due to restrictions in the DNS.
      Both of these solutions have undesired side effects.  CNAMES are not
      protocol specific.  Using IP address records is a logistic nightmare
      for large servers with many virtual sites.  This is becoming a bigger
      problem as companies move away from identifying their HTTP site with
      a ``www'' prefix and just use there delegated domain name, e.g.
      Using SRV [RFC2782] records would seem to be a natural solution to
      this problem in that they are protocol specific and will work where
      CNAMES are illegal in the DNS.  There are problems with doing this
      without thought however in that URIs can specify a port and SRV
      records do specify a port and hence a potential conflict.
      This document addresses the specific requirements of resolving this
      conflict as it relates to the HTTP scheme and provides a general
      framework for other URI schemes.
      The examples below are constructed using the ``http'' scheme.
      However, the logic presented in this document is intended to be used
      with any URI scheme that allows the ``:<port>'' part; examples
      include the ``ftp'' and ``telnet'' schemes.
      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.
   1. URIs without a explicit port specification
      If the URI does not explicitly specify a port to connect to, i.e. the
      URI does not contain a ``:<port>'' part, there is no port conflict.
      In this case a client MUST follow the logic specified in [RFC2782],
      including the server selection mechanism provided by the priority and
      weight fields.  If SRV records do not exist then the client MUST fall
      back to looking for IP address records.
      Single SRV record:
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         SRV RR: SRV   10 0 8080
         A RRs:            A
      Connect to:
      Multiple SRV records:
         SRV RRs: SRV   10 1 8080
          SRV   10 3 8080
          SRV   20 0 8080
         A RRs:            A
      Connect to: or if either is available
      (the probability of being selected should be 25% for,
      and 75% for; otherwise, try
   2. URIs with a explicit port specification
      If the URI does explicitly specify a port to connect to then there is
      a potential conflict in the port specification between the URI and
      the SRV records.  In this case the user agent MUST query for address
      records (instead of SRV records).
      Default port specified:
         SRV RR: SRV   10 1 8080
         A RRs:            A
      Connect to:
      Non-default port specified:
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   INTERNET-DRAFT                   SRV-URI                    October 2000
         SRV RR: SRV   10 1 80
         CNAME RR:            CNAME
         A RRs:      A
      Connect to:
   3. Transitioning Considerations
      When transitioning from using a non-SRV solution to using a SRV based
      solution old, non SRV aware, clients will continue to look for
      address records.  It may be neccessary to use redirection at the HTTP
      layer to direct these clients to the new servers if the SRV records
      point to a different <address, port> tuple.
      It will also be neccessary to continue provide the existing address /
      CNAME records until there is a significant percentage of SRV aware
      clients.  Experience has shown that this should be within one to two
      years of the introduction of the first SRV aware client, for HTTP.
      In cases where you are just trying to replace the A or CNAME record
      refering to a service providers machine with a SRV record the
      following should suffice.
      The service provider is hosting the service on
      and you are
           A   <IP address of> SRV 0 0 80
   Security Considerations
      The authors believe the algorithm described in this document to not
      cause any new security problems.  However care should be taken as SRV
      and non-SRV aware clients may be directed to different locations.
   IANA Considerations
      A well known label has to be allocated for the first label of the
      http SRV record.  This document has used ``_http''.
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   INTERNET-DRAFT                   SRV-URI                    October 2000
      Domain names - concepts and facilities. P.V. Mockapetris.
      Nov-01-1987. STD 0013, RFC 1034.
      Domain names - implementation and specification. P.V. Mockapetris.
      Nov-01-1987. STD 0013, RFC 1035.
      Uniform Resource Locators (URL). T. Berners-Lee, L. Masinter, M. McC-
      ahill. December 1994. RFC 1738.
      A DNS RR for specifying the location of services (DNS SRV). A. Gul-
      brandsen, P. Vixie, L. Esibov. February 2000. RFC 2782.
   Authors' Addresses
         Mark Andrews
            Nominum Inc.
            1 Seymour St.
            Dundas Valley, NSW 2117, Australia
            +61 2 9871 4742
         Thor Kottelin
            Laivalahden puistotie 10 C 37
            FIN-00810 Helsinki, Finland
            +358 400878169
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