Network Working Group                                            E. Lear
Internet-Draft                                        Cisco Systems GmbH
Intended status: Standards Track                       February 21, 2013
Expires: August 25, 2013

             A DNS Resource Record for Service Descriptions


   Certain application protocols are highly transactional and require
   substantial care when dealing with latency.  Queries for versioning
   information are, in these circumstances, costly.  In addition, there
   is a desire to allow for a means to specify a lightweight means to
   alternative transport protocols, such as making use of SCTP instead
   of TCP.  This memo specifies a new DNS RR with an eye toward the
   least necessary roundtrips to determine various protocols, port
   numbers, and options for a given service instance.


   [[NOTE: For httpbis, the first goal is to focus down on requirements.
   Consider all this draft through the lens of "http" where one sees the
   words "application protocol" or "service".]]

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 25, 2013.

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Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Design Requirements . . . . . . . . . . . . . . . . . . .   3
     1.2.  Related Work  . . . . . . . . . . . . . . . . . . . . . .   4
       1.2.1.  The DNS SRV Record  . . . . . . . . . . . . . . . . .   4
       1.2.2.  NAPTR . . . . . . . . . . . . . . . . . . . . . . . .   4
       1.2.3.  The URI resource record . . . . . . . . . . . . . . .   4
       1.2.4.  Happy Eyeballs  . . . . . . . . . . . . . . . . . . .   4
     1.3.  Overall approach  . . . . . . . . . . . . . . . . . . . .   5
     1.4.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  The SVCINFO Record Format and Use . . . . . . . . . . . . . .   5
     2.1.  Usage . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Notes . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Backward Compabitibility and Other Interactions . . . . . . .   7
   4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Registration of Profiles  . . . . . . . . . . . . . . . .  11
     6.2.  Initial SVCINFO Profile Registrations . . . . . . . . . .  11
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  SRV example  . . . . . . . . . . . . . . . . . . . .  12
   Appendix B.  Changes  . . . . . . . . . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

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1.  Introduction

   For an application protocol to survive over time, it must include
   version information, and usually must have some sort of capability
   statement or exchange.  How this is done depends on a number of
   characteristics of the protocol, such as whether it is half-duplex
   and what performance characteristics it has.  When using many
   transactional connections, end-to-end latency will occur before new
   capabilities can be used.

   In addition, traditionally application protocol specifications have
   indicated the transport protocol to be used.  It is useful to be able
   to declare in advance what protocol(s) a service instance runs atop.
   For instance, it might be possible to a service instance on both TCP
   [RFC0792] and SCTP [RFC4960].

1.1.  Design Requirements

   We have the following five design requirements for any solution:

   1.  Application protocol version information must be discoverable
       within the DNS.

   2.  Transport protocol version information for a service must be
       discoverable within the DNS.

   3.  Performance of the application must not be unduly impacted.  Some
       additional latency may be tolerated.

   4.  It should be possible for multiple instances of an application to
       be advertised on different ports by the same host.  This is
       particularly common with HTTP [RFC2616].

   5.  While it may be possible to modify URI schema definitions, all
       such modifications MUST be 100% backward compatible.

   One additional potential requirement would be host-level redirection.
   The benefit of host-level redirection in the DNS is that it would
   allow for a virtual server to securely offer TLS for multiple domains
   without the need for multiple IP addresses, as different ports are
   offered for different virtual hosts instead of different IP

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1.2.  Related Work

   There are a number of existing capabilities within the network that
   can address some or all of the requirements above.

1.2.1.  The DNS SRV Record

   It has been possible to make use of a new service name and query the
   DNS for a SRV resource record [RFC2782], again perhaps running a
   race.  SRV provides a mechanism to locate one or more server and port
   for a given service.  There are two concerns with SRV.  First, one
   must indicate the transport protocol as part of the QNAME.  This
   means that discovery of multiple transport protocols requires
   multiple queries.

   In addition, common enterprise deployments create a _TCP zone for
   purposes such as load balancing of SRV responses separate from a
   parent domain.  Keeping in mind that the goal is to reduce the number
   of queries to determine version and protocol parameters, multiple DNS
   queries perform no better than an in-path application protocol
   exchange.  Additional information also cannot always be provided or
   be trusted, because the authoritative name server for the service
   name may not also be authoritative for the target domain.  A detailed
   example of the SRV record's performance is given in Appendix A.

1.2.2.  NAPTR

   Another record that could be considered is NAPTR [RFC3402].  NAPTR is
   a very powerful means for DNS clients to apply search-and-replace
   rules to a given URI.  Building upon the SRV record, NAPTR provides a
   means to specify use of alternate services and transport protocols.
   Because NAPTR may return either an A record or SRV record, one or
   more follow-on query may be needed.  In addition, this leaves us
   without protocol version information.

1.2.3.  The URI resource record

   The URI resource record provides for a mapping from hostname to URI.
   This record can be used to map a domain to multiple URIs, in fact.
   What it lacks, however, is version information about the application

1.2.4.  Happy Eyeballs

   One final approach is to run a race by initiating the protocol
   exchange using two alternatives, and pursuing the alternative that
   indicates success first, the assumption being that the service
   exists.  An example of this model is Happy Eyeballs [RFC6555], where

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   different versions of IP are tested.  This work specifies that when
   an AAAA record is specified, a race may be performed.  DNS cannot in
   general be used to determine reachability.  Hence, a race may yet be
   appropriate in some circumstances, when a service is advertised.

1.3.  Overall approach

   To allow hosts to advertise a service using multiple versions of
   application protocols or multiple transport protocols, a method is
   needed to efficiently advertise that there exists an equivalence.  To
   accomplish this, we define a new RRtype that states each way to
   connect to the service by using as an initial index the port and
   service that the application intends to connect to, and then
   providing an additional index known as an "InstanceId" that then
   establishes an equivalence between the instant record and any other
   record returned in the query with the same InstanceId.  Examples of
   this approach are found in Section 4.  This additional index is
   useful in circumstances where multiple applications making use of the
   same service (such as HTTP) are running on a single host.  The
   InstanceId can be used to indicate to the client which application is
   instantiated through multiple protocols (such as a database
   application making use of both HTTP 1.1 and HTTP 2.0).  In addition,
   the record references profiles that provide transport protocol,
   version, and other application protocol-specific information.

1.4.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  The SVCINFO Record Format and Use

   The SVCINFO RR is queried with DNS type code TBD.  The format of the
   SVCINFO resource record is as follows:

     domain TTL Class SVCINFO InstanceId Priority Port Profile

   The fields are defined as follows:

   Domain, TTL, Class:   These are specified in [RFC1035].

   SVCINFO:   This is the resource record type.

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   InstanceId:   This textual key identifies an instance of a service.
      If the same InstanceId appears in multiple SVCINFO records, then
      the services on those ports atop those protocols are said by the
      service owner to be equivalent.

   Priority:   This eight-bit number has similar meaning to "Priority"
      as defined in [RFC2782].  Its use and interaction with other
      fields is described below.

   Port:   This is a port number associated with the service running on
      a host.  The value must be a legal value for the assocated
      transport protocol.

   Profile:   This field indicates the name of a profile to be used to
      determine transport protocol and protocol version.  Each profile
      is registered with IANA.  If a host does not recognize the profile
      it MUST ignore the record.  Two initial entries are defined later
      in this memo.

2.1.  Usage

   When a host offers an equivalent service in two different ways it
   will advertise those ways in the DNS with the SVCINFO record, using
   the same InstanceId.  When a host wants to make use of a service, it
   then queries the DNS with QNAME=domain, QCLASS=IN, and QTYPE=SVCINFO.
   It processes the information as follows:

   [[DISCUSS: Is class IN really correct?]]

   1.  First, the host constructs the information required to match
       against.  This will consist of transport protocol, and
       application version that it would use in the absense of a
       matching RR.  This information should uniquely identify a
       profile.  For instance, given a URL of "http://", by default the host would connect via TCP
       port 49992 to host and proceed to use HTTP 1.1.
       This is associated with a profile named TCP-HTTP-1.1.

   2.  If the reply is NOERROR and ANCOUNT=0 is returned, the host
       attempts to connect using the information in the previous step
       (e.g., using profile TCP-HTTP-1.1 which to port 49992).

   3.  If the reply is NOERROR and ANCOUNT > 0, the host searches for
       the port and profile it determined in the first step.  If there
       is no match, it connects as it would have in the previous step.
       Upon a match, it notes the InstanceId of that record, and any
       matching InstanceId of other SVCINFO records returned in the

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   4.  The host then attempts to connect to the profile and port with
       the lowest number priority.  Two records with same InstanceId
       that have the same priority may be tried in any order, or raced.

2.2.  Notes

   Zones MUST NOT publish multiple SVCINFO records for the same domain
   that use the same profile AND port.  Resolvers SHOULD ignore such
   SVCINFO records.

   A client resolver MUST parse all RRs in the reply in order to
   properly determine priority.  Accordingly, clients MUST handle
   truncated responses using the rules described in [RFC2181].

   In all cases when an SVCINFO record is returned, all A and AAAA
   record for the domain SHOULD be returned in the additional
   information section.  This eliminates excess queries without adding
   additional risk of cache poisoning.

   When the application loses communication with the other side, it
   SHOULD re-apply the rules above in attempting to re-establish
   connectivity.  When doing so, applications making use of the SVCINFO
   record MUST observe DNS caching semantics.

3.  Backward Compabitibility and Other Interactions

   There are several side effects of using SVCINFO.  The first, is that
   by its nature, SVCINFO requires backward compatibility.  A service
   must always run on a port that is advertised, be that as an IANA-
   assigned port, through specification within a URI, or some other
   means.  At the same time, when SVCINFO *is* used by the client, an
   observer may not see the client connect to a server on a port
   specified by a given URI.

4.  Examples

   Case 1: Different versions of HTTP, same transport protocol

   Consider the case of host, which is running an
   HTTP1.1 server on port 80 and an HTTP2.0 server on port 49080, both
   using TCP, both serving the same content.  A records for this service
   might appear as follows:
 86400 in svcinfo homepage 10 80 TCP-HTTP-1.1 86400 in svcinfo homepage 5  49080 TCP-HTTP-2.0

         Additional Information: 86400  in a

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 86400  in aaaa 2001:db8::1     604800 in ns

   When a web client attempted to connect to, it
   would construct the expected profile based on transport protocol TCP
   and assuming HTTP 1.1.  It would also know that it is expected to
   connect to port 80.  When it queries for the svcinfo record of, it receives the above response.  It then sees that
   the InstanceId on port 80 for TCP-HTTP-1.1 is "homepage".  It also
   sees that the InstanceId for port 49080 is also "homepage".  It now
   knows that these two services are equivalent, and sees that it should
   attempt to connect to tcp/49080 if it understands the TCP-HTTP-2.0.
   If the client doesn't understand that profile, it, it uses TCP-
   HTTP-1.1, connecting again to 80/tcp and using HTTP 1.1.

   Case two: Same service running on multiple transport protocols

   In the next case, we change the case slightly from above by allowing
   for the idea that http2.0 will make use of SCTP in addition to TCP.
   We allow for the existence of another profile, SCTP-HTTP-2.0.
   Therefore the records might appear as follows:
 86400 in svcinfo homepage 10 80 TCP-HTTP-1.1 86400 in svcinfo homepage 5  49080 TCP-HTTP-2.0 86400 in svcinfo homepage 1  80 SCTP-HTTP-2.0

         Additional Information: 86400  in a 86400  in aaaa 2001:db8::1     604800 in ns

   Now when the web client attempts to connect to http://, after seeing that all three responses for have an InstanceId of "homepage" (including that of
   port 80, where the client has constructed a default profile of TCP-
   HTTP-1.1), if recognizes STCP-HTTP-2.0, it should first try to
   connect to port 80 using sctp, otherwise it behaves as it did the
   previous example.

   Case 3: Multiple services

   In this case the client is attempting to connect to http://, where runs multiple http
   servers.  The DNS configuration appears as follows:

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 86400 in svcinfo homepage 10 80 TCP-HTTP-1.1 86400 in svcinfo homepage 5  49080 TCP-HTTP-2.0 86400 in svcinfo homepage 1  80 SCTP-HTTP-2.0 86400 in svcinfo database 10 8080 TCP-HTTP-1.1 86400 in svcinfo database 5  8081 TCP-HTTP-2.0 86400 in svcinfo database 1   8080 SCTP-HTTP-2.0

        Additional Information: 86400  in a 86400  in aaaa 2001:db8::1     604800 in ns

   When the client queries for and receives the above
   information, it notes that port 8080 has an InstanceId of "database",
   and it knows that it already understands TCP-HTTP-1.1.  It matches
   that InstanceId with the records that contain the other profiles.  It
   ignores all other records that contain InstanceIds other than
   "database".  The client then attempts to connect first using SCTP-
   HTTP-2.0, if it understands that profile.  If it doesn't understand
   either SCTP-HTTP-2.0 or TCP-HTTP-2.0, it will connect on port 8080
   and proceed with HTTP 1.1.

5.  Security Considerations

   Absent the use of DNSSEC [RFC4035], it is important that alternatives
   offered by this service have the same security properties, lest a
   downgrade attack be introduced.

   When published to the world, this record would divulge that the
   likely presence of services running on a particular set of ports.  It
   may not be all that difficult to divine what is running, either based
   on a simple probe, or the version number in the response.

6.  IANA Considerations

   The IANA is requested to allocate a DNS RRTYPE with the following

     A. Submission Date:

     B. Submission Type:
        [X] New RRTYPE
        [ ] Modification to existing RRTYPE

     C. Contact Information for submitter (will be publicly posted):
        Name: TBD
        Email Address: TBD

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        International telephone number: TBD
        Other contact handles: TBD

     D. Motivation for the new RRTYPE application.
        Please keep this part at a high level to inform the Expert and
        reviewers about uses of the RRTYPE.  Most reviewers will be DNS
        experts that may have limited knowledge of your application

        Please see Section 1 of this document.

     E. Description of the proposed RR type.
        This description can be provided in-line in the template, as an
        attachment, or with a publicly available URL.

        Please see Section 1 of this document.

     F. What existing RRTYPE or RRTYPEs come closest to filling that
     need and why are they unsatisfactory?

        Please see Section 1 of this document.

     G. What mnemonic is requested for the new RRTYPE (optional)?
        Note: this can be left blank and the mnemonic decided after the
        template is accepted.


     H. Does the requested RRTYPE make use of any existing IANA registry
        or require the creation of a new IANA sub-registry in DNS
        Parameters?  If so, please indicate which registry is to be used
        or created.  If a new sub-registry is needed, specify the
        allocation policy for it and its initial contents.  Also include
        what the modification procedures will be.

        This RRType refers to the IANA Assigned Internet Protocol
        Numbers registry, but requires no allocations from it.

     I. Does the proposal require/expect any changes in DNS
        servers/resolvers that prevent the new type from being processed
        as an unknown RRTYPE (see [RFC3597])?


     J. Comments:


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6.1.  Registration of Profiles

   The IANA is requested to maintain a registry of profiles for the
   SVCINFO record.  New entries SHALL require review by a designated
   expert.  The following template is to be used:

          Profile Name: <An ASCII string no greater than 20 characters
                that contains no white space>
          Description: A short description of the profile
          Transport Protocol Information: <TCP, UDP, etc.>
          Application information: A pointer to an RFC containing a
                   protocol specification

6.2.  Initial SVCINFO Profile Registrations

   The IANA is requested to register the following profiles in the
   SVCINFO Profiles registry:

   Profile Name: TCP-HTTP-1.1 Description: HTTP 1.1 over TCP, including
   documented extensions Transport Protocol Information: TCP Application
   information: RFC-2616 Profile Name: TCP-HTTP-2.0-a1 Description:
   Early draft of HTTP-2.0 Transport Protocol Information: TCP
   Application information: draft-ietf-httpbis-http2-01.txt

7.  Acknowledgments

   This work largely builds upon the experience gathered with SRV
   records, as originally defined by Paul Vixie.  SRV records are still
   appropriate in many, if not most, circumstances.  Thanks also go to
   Joe Hildebrand, Dan Wing, and Mark Nottingham for their reviews.

8.  References

8.1.  Normative References

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

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

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, July 1997.

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

   [RFC3597]  Gustafsson, A., "Handling of Unknown DNS Resource Record
              (RR) Types", RFC 3597, September 2003.

8.2.  Informative References

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, September 1981.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3402]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part Two: The Algorithm", RFC 3402, October 2002.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, March 2005.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol", RFC
              4960, September 2007.

   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, April 2012.

Appendix A.  SRV example

   Consider the fictional case where http version 2 requires querying
   the DNS for to get to  The SRV
   record might look something like this: 86400 in srv 10 10 49080

   To fully resolve to the necessary components of an
   ip address,transport protocol, and a port, a full service resolver
   must make the following queries:

   1.  A query for to name servers for will either return the SRV record or name servers for

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   2.  A query is made where there is a zone cut for
       We now have the SRV record.  We may also have an A record for if it appears in an additional information
       section in the same zone.

   3.  If we do not have the A record for, we will then
       query for it separately.

   Step 2 is a common step that is necessary today in many enterprise
   deployments, even if the client being served is not within that
   enterprise.  The last step may actually be several steps if the
   target domain is not in the same zone as the name found in the QNAME.
   A check of one highly optimized common news site found ten separate
   and distinct domains.  Risking an average query response time of
   60ms, use of SRV records could inject 600ms on the initial start up
   for that site.

   What we conclude is that the fundamental issue with SRV (and any
   record where a zone split is likely) is that the service name
   requires additional A records for a host to connect to the service
   that may not be possible to provide in a single query.

Appendix B.  Changes

   This section to be removed prior to publication.

   o  00 Initial Revision.

Author's Address

   Eliot Lear
   Cisco Systems GmbH
   Richtistrasse 7
   Wallisellen, ZH  CH-8304

   Phone: +41 44 878 9200

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