INTERNET-DRAFT                                                Gary Bajaj
<draft-bajaj-mail-srv-00.txt>                                   May 2003


             Use of SRV records for POP3, POP3S, IMAP and IMAPS.


   Status of this Memo

      This document is an Internet-Draft and is subject to 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
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      Comments should be sent to the author.

      This draft expires in November 2003.


   Abstract

      DNS records for the mail services POP3, POP3S, IMAP and IMAPS do
      not currently provide failover switching as do the DNS MX records
      for SMTP.  This document looks at the issues involved and
      recommends a solution using SRV records.


   Introduction

      Mail servers that require high availability might be multi-homed
      with upstream connectivity to two or more ISPs.  This is
      traditionally accomplished by running BGP4 such that each upstream
      provider would route to the site's own IP block.  If connectivity
      to one ISP fails, incoming connections would seamlessly be routed
      through the other ISP.  For various reasons including IP
      allocation constraints, cost and networking expertise, running
      BGP4 is impratical for most small ISPs.  If using DNS only, SMTP
      can be made fault tolerant by using multiple MX records, one for
      each IP serviced by the mail server so that each MX record is
      tried in turn until an IP responds.  Such failover protection
      using DNS is  not currently possible for POP3 and IMAP connections.

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      Using SRV [RFC2782] records seems to be the obvious solution to
      making POP3, POP3S, IMAP and IMAPS redundant.  SRV records also
      provide for load balancing when using multiple servers that have
      access to mail spool on a shared mass storage device such as NAS.

      Examples:

      SRV RRs:

      _pop3._tcp SRV 1 0 110 host1.example.com.
      _pop3._tcp SRV 1 0 110 host2.example.com.
      _pop3._tcp SRV 0 0 110 host3.example.com.
      _imap._tcp SRV 1 0 143 host1.example.com.
      _imap._tcp SRV 1 0 143 host2.example.com.
      _imap._tcp SRV 0 0 143 host3.example.com.
      _pop3s._tcp SRV 0 3 995 host1.example.com.
      _pop3s._tcp SRV 0 1 995 host2.example.com.
      _imaps._tcp SRV 0 3 993 host1.example.com.
      _imaps._tcp SRV 0 1 993 host2.example.com.

      A RRs:

      host1      A    10.0.0.2
      host2      A    172.16.1.2
      host3      A    172.16.1.3

      host1 and host2 are the same multi-homed host that can accept both
      insecure (pop3, imap) and secure (pop3s, imaps) connections.
      host3 is a separate host that is not multi-homed and does not
      accept secure connections.
      Connect to either 10.0.0.2 or 172.16.1.2 if either is available
      (the probability of being selected is 75% for 10.0.0.2 and 25% for
      172.16.1.2) to download mail over a secure POP3 or IMAP connection
      Connect to 172.16.1.3 and if not available connect to either of
      10.0.0.2 or 172.16.1.2 to download mail over an insecure POP3 or
      IMAP connection.

   Transitioning Considerations
      When transitioning from using a non-SRV solution to using an SRV
      based solution, old non-SRV aware clients (mail user agents) will
      continue to look for A records.  These will not benefit from
      redundancy until updated, but will continue to work.

   IANA Considerations
      Well known labels have to be allocated for the first label of the
      SRV records.  This document has used _pop3, _imap, _pop3s and
      _imaps.


   References

   [RFC2782]
      A DNS RR for specifying the location of services (DNS SRV). A. Gul
      brandsen, P. Vixie, L. Esibov. February 2000. RFC 2782.

Expres November 2003                                            [Page 2]


   Author's Address

         Gary Bajaj
            BITNETS
            58065-12621 118 Ave NW
            Edmonton, Alberta T5L 4Z4
            Canada
            +1 (780) 418-4477
            BEAMERS-Support@BITNETS.com

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