Network Working Group                                            L. Wood
Internet-Draft                                      University of Surrey
Intended status: Experimental                                May 8, 2010
Expires: November 9, 2010

    Specifying transport mechanisms in Uniform Resource Identifiers


   This document describes a simple extension of the Uniform Resource
   Identifier (URI) format that allows preferred transport mechanisms,
   including protocols, ports and interfaces, to be specified as
   parseable additions to the scheme name.  This explicit configuration
   is beneficial for separation of the HyperText Transfer Protocol
   (HTTP) from underlying transports, which has been increasingly
   recognised as useful when a variety of ways of transporting or
   configuring use of HTTP are available and a choice of mechanism to
   use must be indicated.

Status of this Memo

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Table of Contents

   1.  Background and Introduction . . . . . . . . . . . . . . . . . . 3
   2.  Foundations of URI scheme syntax  . . . . . . . . . . . . . . . 4
   3.  Extending the URI scheme to indicate transports and
       interfaces  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   4.  Use of these modifiers  . . . . . . . . . . . . . . . . . . . . 5
   5.  Relevant work . . . . . . . . . . . . . . . . . . . . . . . . . 6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 9

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1.  Background and Introduction

   Desire to separate the Hypertext Transfer Protocol (HTTP) [RFC2616]
   from its traditional transport of the Transmission Control Protocol
   (TCP) is increasing.

   There are environments where TCP is not suitable, or absent, yet HTTP
   can still be used as a method to transfer data.  Being able to
   indicate the desired transport and interface to use in the URI for a
   program to interpret when executing HTTP GETs or PUTs is useful when
   a choice of mechanisms and interfaces are available, and
   infrastructure such as DNS cannot be queried for advice.

   This document outlines how the desired transport and interface can be
   indicated in the Uniform Resource Identifier (URI) format [RFC3986]
   by a simple extension to that format using existing syntax.

   This syntax is useful for carrying HTTP over different transport
   protocols.  HTTP can be thought of a session layer, running over a
   transport layer providing reliable delivery of the HTTP stream.  This
   transport layer has commonly (and almost universally) been TCP in the
   terrestrial Internet, although alternative transport layers, such as
   SCTP, can also be used under HTTP [I-D.natarajan-http-over-sctp].
   For long-delay networks, or for network conditions where TCP or an
   equivalent is not suitable, an alternative transport layer such as
   Saratoga [I-D.wood-tsvwg-saratoga] can be used under HTTP instead in
   hop-by-hop communications between nodes.  This has been described in
   detail [I-D.wood-dtnrg-http-dtn-delivery].

   HTTP requires only reliable streaming that can be used to provide
   ordered delivery to the application; how that reliable streaming is
   provided is up to the local transport layer in the local network.  In
   the examples given above, TCP or SCTP are used to carry HTTP over the
   congestion-sensitive public Internet, while Saratoga would be used
   for HTTP across dedicated private links.

   Steve Deering has often described IP as 'the waist in the hourglass'
   [Deering98] - what is above and touching on IP can be changed, what
   is below and touching on IP can be changed, but provided the new
   elements continue to interface to and work with IP, the hourglass
   remains complete and the network stack remains functional.  Here,
   HTTP is the waist in this particular hourglass; applications can use
   HTTP to communicate, provided HTTP runs over a reliable transport
   stream.  The applications can vary.  The transport stream can be
   changed; HTTP does not even have to run over a TCP/IP stack, but
   could even be made to run directly over something else entirely.
   Given the prevalence of IP in many networks, it is likely that two
   popular waists (layers that other layers interface to) exist: IP and

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   HTTP.  The transport protocol and physical enviroment that IP and
   HTTP are used with will vary more, depending on local conditions and

   Being able to specify how HTTP or other schemes are carried is useful
   when a variety of methods are available to choose from.  The syntax
   described here is useful for local configuration, e.g. in a scripting
   language that is aware of the local host and remote host's shared
   support of a given transport protocol.  It is less useful on the
   public world-wide web, because users (and web page designers) are not
   generally capable of determining which transport protocol(s) are
   supported by their web browser, operating system, or network.
   However, the option to explicitly choose a communication method is

2.  Foundations of URI scheme syntax

   Before describing an extension to URI scheme syntax, it is worthwhile
   to lay out the foundations on which this syntax is based.

   HTTP is not explicitly tied to use over TCP.  To quote [RFC2616],
   section 1.4:

   "HTTP communication usually takes place over TCP/IP connections.  The
   default port is TCP 80, but other ports can be used.  This does not
   preclude HTTP from being implemented on top of any other protocol on
   the Internet, or on other networks.  HTTP only presumes a reliable
   transport; any protocol that provides such guarantees can be used;
   the mapping of the HTTP/1.1 request and response structures onto the
   transport data units of the protocol in question is outside the scope
   of this specification."

   The URI format syntax ([RFC3986], section 3.1) defines the scheme as:

   scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )

   The period (.) is in use in a number of scheme names; the other
   punctuation characters appear unused.

   To quote [RFC2718], section 2.2.2:

   "When a scheme is associated with a network protocol, the
   specification should completely describe how URLs are translated into
   protocol actions in sufficient detail to make the access of the
   network resource unambiguous.  If an implementation of the URL scheme
   requires some configuration, the configuration elements must be
   clearly identified."

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3.  Extending the URI scheme to indicate transports and interfaces

   Within the established foundation, this draft proposes that schemes
   can be extended to include configuration elements that indicate
   transport and interface.  These modify use of the http request, and
   are in the format as given in the following figure:

     extended-scheme = scheme ["-+" port-behaviour ]
                              ["++" transport ]
                              ["+-" interface ]
                              ["--" dynamic-configuration]

                                 Figure 1

   where the optionally-included -+port is a description mapping to the
   default IANA-assigned port number, or the equivalent name, indicating
   the desired behaviour over a transport.  This information is
   specified locally as service names in e.g. /etc/services on unix

   The optionally-included ++transport is the transport name or IANA
   protocol identifier number that that name maps to.  As we aim to
   separate HTTP from TCP and place it over other transports, nicknaming
   this proposal 'http++' obviously follows.

   The optionally-included +-interface can contain a locally-meaningful
   specifier identifying the local interface to use; useful on multi-
   homed devices.  The local interface identifier may identify a virtual
   or physical interface or one that is only capable of using IPv4,
   IPv6, or another network protocol (in the case of HTTP being run over
   something other than a TCP/IP stack.)  Defining virtual interfaces
   limited to one network protocol allows the choice of network protocol
   to be made.

   The optionally-included "--" gets configuration information and
   transport/services to use dynamically via some method, e.g. via DNS
   records, but can be partly overriden by other locally-provided
   configuration from the other three modifier types described above.
   This is described more in the 'relevant work' section below.

4.  Use of these modifiers

   Use of these modifiers would permit http++sctp:// or
   http-+saratoga++udp:// for the uses outlined in
   [I-D.natarajan-http-over-sctp] and
   [I-D.wood-dtnrg-http-dtn-delivery].  Port and internet protocol
   numbers assigned by IANA are accepted as equivalent to assigned names

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   for these underlying protocols, so http-+7542++17:// specifies HTTP
   over Saratoga over UDP. http++132 is equivalent to http++sctp in
   specifying HTTP over SCTP.  As is usual, these are case insensitive,
   so that http++sctp, HTTP++sctp, and HtTp++ScTp are all equivalent.

   Adding these optional transport indicators to the scheme name does
   not change the namespace in any way; the URI should still be treated
   as if it began simply http: and is the http namespace.  Similarly,
   'https++' shares the https namespace, only indicating services,
   transports and interfaces for the https request to use.

   This document deliberately does not state what 'http:' by itself
   implies; local use may invoke dynamic configuration, or simultaneous
   attempts to use multiple available transports or interfaces. 'http:'
   is NOT restricted to mean 'http++tcp:'.

   If required, the port the scheme is actually run over, which the
   behaviour of any specified default port is mapped to, is still
   indicated later in the URI as :number, e.g. :80.  When this is not
   specified the default port for that tranport behaviour is used.  For
   example, Saratoga runs over UDP and is assigned default port 7452 by
   IANA.  Saying 'http-+7452:' means do the default behaviour for port
   7452, i.e. use the Saratoga protocol. http-+7452://blah:1024
   indicates that the default behaviour on port 7452 - that is, Saratoga
   - should be carried over to port 1024, i.e. use the Saratoga protocol
   on port 1024 instead.

   Knowing that a '-+' service port behaviour stated in the scheme is
   only associated with either TCP or UDP would mean that ++tcp or ++udp
   can be omitted when that -+behaviour is given.  That is, -+saratoga
   is known to mean -+saratoga++udp.

   Being able to specify the local interface to initiate a transaction
   on when a choice of interfaces is available on a multihomed device is
   useful, e.g. http-+saratoga+-serial0://.

5.  Relevant work

   [I-D.natarajan-http-over-sctp] proposes carrying http over sctp.
   This can be indicated with http++sctp:.

   [I-D.wood-dtnrg-http-dtn-delivery] proposes separating HTTP from the
   underlying transport entirely, and running it over other transports,
   such as Saratoga [I-D.wood-tsvwg-saratoga].  This can be specified
   locally with http-+saratoga++udp: or simply http-+saratoga:.

   We can also use '--' to indicate that information on resolving the

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   URI must be sought from the network.

   [I-D.jennings-http-srv] has proposed using DNS lookup of a SRV record
   to return a dynamic port value, as well as an address, and could
   indicate this using http--srv:// and http--srv:// in line with the
   use of a service name as indicated above.  [Ed note: need to double-
   check Cullen's current draft.]

   Alternatively, some new DNS record type returning address, port and
   other access information could be explicitly accessed via e.g.
   http--dns:// or some other indication of method.  This could take
   advantage of DNS Name Authority Pointers (NAPTR) via S-NAPTR and
   U-NAPTR [RFC3958] [RFC4848], and encourage the use of and testing
   with those protocols before wider deployment.  [I-D.faltstrom-uri]
   expands further on this approach.  Both SRV and NAPTR can be used by
   SIP in allocating SIP servers [RFC3263].

   Other work on evolving the URI format to enable service discovery
   with DNS for different transport protocols is in e.g.  [Uruena05].

   It should be possible to combine the static configuration in the
   parseable scheme format described here with getting other
   configuration information that is not explicitly given, but that is
   needed to access the URI dyamically, from DNS records when
   appropriate.  Any static information explicitly provided should
   override information from dynamic configuration, just as explicitly
   indicating a port with e.g. :80 in the URI explicitly overrides the
   port returned by http--srv:.

6.  Security Considerations

   No additional security concerns have been thought of at this time.

7.  IANA Considerations

   No additional IANA considerations have been thought of at this time.

8.  Acknowledgements

   Thanks go to Fred Baker, Leslie Daigle, Alfred Hoenes, Cullen
   Jennings, Jonathan Leighton, Preethi Natarajan, Chip Sharp and Dan
   Wing for discussion on points of this draft.

9.  References

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9.1.  Normative References

   [RFC2718]  Masinter, L., Alvestrand, H., Zigmond, D., and R. Petke,
              "Guidelines for new URL Schemes", RFC 2718, November 1999.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

9.2.  Informative References

              Deering, S., "Watching the Waist of the Protocol
              Hourglass", keynote, IEEE International Conference on
              Network Protocols (ICNP), Austin Texas, October 1998.

              Faltstrom, P. and O. Kolkman, "The Uniform Resource
              Identifier (URI) DNS Resource Record",
              draft-faltstrom-uri-04 (work in progress), May 2009.

              Jennings, C., "DNS SRV Records for HTTP",
              draft-jennings-http-srv-05 (work in progress), March 2009.

              Natarajan, P., Amer, P., Leighton, J., and F. Baker,
              "Using SCTP as a Transport Layer Protocol for HTTP",
              draft-natarajan-http-over-sctp-02 (work in progress),
              July 2009.

              Wood, L. and P. Holliday, "Using HTTP for delivery in
              Delay/Disruption-Tolerant Networks",
              draft-wood-dtnrg-http-delivery-05 (work in progress) ,
              May 2010.

              Wood, L., McKim, J., Eddy, W., Ivancic, W., and C.
              Jackson, "Saratoga: A Scalable File Transfer Protocol",
              draft-wood-tsvwg-saratoga-05 (work in progress) ,
              May 2010.

   [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.

   [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation

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              Protocol (SIP): Locating SIP Servers", RFC 3263,
              June 2002.

   [RFC3958]  Daigle, L. and A. Newton, "Domain-Based Application
              Service Location Using SRV RRs and the Dynamic Delegation
              Discovery Service (DDDS)", RFC 3958, January 2005.

   [RFC4848]  Daigle, L., "Domain-Based Application Service Location
              Using URIs and the Dynamic Delegation Discovery Service
              (DDDS)", RFC 4848, April 2007.

              Uruena, M. and D. Larrabeiti, "Nested Uniform Resource
              Identifiers", Proceedings of the 31st EUROMICRO Conference
              on Software Engineering and Advanced Applications, pp.
              380-385 , August 2005.

Author's Address

   Lloyd Wood
   Centre for Communication Systems Research, University of Surrey
   Guildford, Surrey  GU2 7XH
   United Kingdom

   Phone: +44-1483-698123

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