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Versions: 00 01                                                         
Network Working Group                                           L. Amini
Internet-Draft                                              IBM Research
Expires: June 23, 2003                                         S. Thomas
                                                         TransNexus, Inc
                                                           O. Spatscheck
                                                               AT&T Labs
                                                           December 2002

         Distribution Requirements for Content Internetworking

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-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

   This Internet-Draft will expire on June 23, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2001). All Rights Reserved.


   This document specifies requirements for interconnecting, or
   peering, the distribution systems of Content Networks (CN).
   Distribution internetworking requires advertising the capabilities
   of a CN offering distribution services, moving content from one CN
   to another, and signaling requirements for consistent storage and
   delivery of content. This document does not address requirements for
   directing user agents to distributed content, nor for aggregating
   access information for distributed content.

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1. Introduction

   Content Internetworking (CI) assumes an architecture wherein the
   resources of multiple CNs are combined so as to achieve a larger
   scale or reach than any of the component CNs could individually [3].
   A Content Distribution Network (CDN) is an example of a CN.

   At the core of CI are three principal architectural elements.  These
   elements are Request Routing, the Distribution and the Accounting.
   The focus of this document, the Internetworking Distribution
   Systems, is responsible for moving content from one Distribution CN
   to another Distribution CN.  Note that the original content provider
   is considered a degenerate case of a Distribution CN.

   In any Distribution Internetworking arrangement, the relationships
   between Distribution CNs can always be decomposed into one or more
   pairs of CNs. Each CN pair comprises one CN which has, or has access
   to, content, and another CN which has, or has access to, systems
   capable of providing distribution and/or delivery functions for
   content. The former CN is referred to as the Content Source, while
   the latter is referred to as the Content Destination.

   This document describes the overall architectural structure and
   building blocks of the internetworked Distribution Systems. It also
   defines the protocol requirements for interconnecting two or more
   Distribution CNs via their respective Content Internetworking
   Gateways (CIG). Specifically, it defines the requirements for:

      Distribution Advertising: announcing the distribution
      capabilities of a Content Destination to potential Content

      Content Signaling: communicating content meta-data to enable
      consistent storage and delivery of content to user agents.

      Content Replication: moving content from a Content Source to a
      Content Destination.

   Although this document does not specifically address requirements
   for communicating within a CN, it is plausible that protocols
   developed to meet inter-CN requirements may also be well-suited for
   intra-CN communications.

   Requirements for the remaining CI architectural elements, the
   Request Routing System, which is responsible for directing user
   agents to the distributed content, and the Accounting System, which
   is responsible for aggregating information related to the access of
   distributed content, are detailed in [6], [7].

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1.1 Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   this document are to be interpreted as described in RFC-2119 [2].

   All other terms in ALL CAPS, except those qualified with explicit
   citations, are defined in [8].

1.2 Change Log

   1.  Fixed terminology to comply with updated Models doc.

   2.  Intro: fixed reach or scale so not synonyms

   3.  Section 1: clarified definition for Content Signaling

   4.  Section 2: clarified that fig 1 is a logical view, need not have
        this strictly hierarchical relationship.

   5.  Section 2.1-2.2: deleted these sections, feedback indicated they
        confused more than claried; material is adequately covered in
        Models doc.

   6.  Section 3: change Use-initiated to surrogate initiated.

   7.  Section 3 (para 2): cig is not necessarily a box, could be
        functionality (protocol conformance) implemented in multiple

   8.  Section 4.1: clarified content signalling example.

   9.  Section 4.3: clarified common base replication vs. content type
        specific replication

   10.  Section 4.4: fixed "extensible model..." wording

   11.  Section 5.1: clarified that content pull for delivery services
        preparation is optional.

   12.  Section 5.3 3-4: modified, added request for feedback

   13.  Section 5.3.5: clarified advertisements as optional

   14.  Section 5.4: dropped requirement for sending data with content

   15.  Section 5.4 6: dropped part about source encryption -- if the
        object is encrypted at source and not decrypted until it

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        reaches the client, then it is just a (downloadable) blob as
        far as the dist system is concerned.

   16.  Section 5.5 8: dropped subscription fee and next hop.

   17.  Section 5.5 8: changed content id to content set id.

   18.  Section 5.5: added information on relationship to WEBI/RUP.

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2. Distribution System Overview

   In Distribution System Internetworking, even the most complex
   communication arrangements can be expressed in terms of simple
   interactions between a Content Source and a Content Destination.
   Figure 1, for example, shows a relationship between four different
   administrative authorities. CN A operates a network of SURROGATES,
   and "CN D" (actually the original content provider, or ORIGIN) has
   content to be distributed. CN A communicates with CN B, which
   communications with CN C, which, in turn, communicates with CN D.

      +------------+   +-----------+   +-----------+   +------------+
      |   CN A     |   |    CN B   |   |    CN C   |   |   "CN D"   |
      |(SURROGATES)|<->|  (agent   |<->|  (agent   |<->| (ORIGIN)   |
      |            |   |   for A)  |   |   for D)  |   |            |
      +------------+   +-----------+   +-----------+   +------------+

                        CONTENT DST <-> CONTENT SRC
                                        CONTENT DST <-> CONTENT SRC

      Figure 1: Distribution System Interworking

   In each case, one of the parties in the communications has the role
   of Content Destination, while the other party is the Content Source.
   Note that a particular CN's role may change, depending on the party
   with whom it is communicating. CN B, for example, is a Content
   Source when communicating with CN A, but a Content Destination when
   communicating with CN C.

   Note that a Content Destination which peers directly with the
   Content ORIGIN, will interface with the ORIGIN just as it interfaces
   with any other Content Source.

   Although Figure 1 provides an example of multiple CNs peered in
   series, a Distribution CN may serve as the Content Source for
   multiple Content Destinations.  Likewise, a Distribution CN may
   serve as the Content Destination for multiple Content Sources.

   Additionally, it is possible for the internetworking relationship
   between a single Source-Destination pair to be reciprocal for
   different content sets.  That is, CN A may request distribution
   services from CN B for Content Set A, while CN B requests
   distribution services from CN A for Content Set B.

   Further, note that Figure 1 is a logical view of the internetworking
   relationship between several Content Source-Destination pairs;
   actual communications are not restricted to this pair-wise

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   hierarchy.  For example, CDN D may specify a single authoritative
   distribution channel from which any distributing CN must retrieve
   the CONTENT.

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3. Replication Models

   Replication of content may take place using a push model, or a pull
   model, or a combination of both.

   o  SURROGATE-initiated replication of CONTENT, where a SURROGATE
      retrieves CONTENT based on a cache miss or on a prefetching
      policy specified at the SURROGATE, represents the pull model.
      This is the model currently used by caching proxies.

   o  ORIGIN-initiated replication of CONTENT to SURROGATEs represents
      the push model. This model is used to preposition CONTENT in
      anticipation of demand.

   Replication, when between the administrative domains of different
   Distribution CNs must adhere to the CI protocol for content
   replicaiton. Replication within a single Distribution CN is an
   intra-CN communication and therefore, need not adhear to CI
   protocols.  Further, the replication model used within a single
   Distribution CN need not be the same as the model used to replicate
   CONTENT between CNs.

   For both ORIGIN- and SURROGATE- initiated replication, the Content
   Source may use replication mechanisms beyond a simple transfer.  For
   example, it may be desirable to have the Content Destination join a
   multicast channel on which a set of content is pushed to all

   Another example is for CONTINUOUS MEDIA.  In the case of live
   broadcasts, the data need not be cached on the SURROGATES.  Instead,
   replication takes the form of "splitting" the live stream at various
   points in the network. Splitting is also referred to as application
   layer multicast.

   Replication of CONTINUOUS MEDIA streams which are not live, and
   therefore may be stored on SURROGATES, also benefits from mechanisms
   beyond in-line replication. For example, CONTINUOUS MEDIA is often
   delivered to CLIENTS over an unreliable channel.  However, a CN
   distributing this content to many CLIENTS should work with a full
   replica.  Existing proprietary replication protocols enable
   distribution of CONTINUOUS MEDIA objects in which a full or partial
   replica can be propagated, the data may be encrypted and/or
   authenticated, and the SURROGATE can support CONTINUOUS
   MEDIA-related services such as random access and stream

   It may be desirable to replicate content to a Distribution CN which
   has no internal SURROGATES.  For example, a Distribution CN may have
   servers at key exchange points within the network which only serve

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   content to other distribution systems, and peer with other CNs which
   provide SURROGATES which deliver content to CLIENTS.

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4. Distribution Internetworking Requirements

   This section details general requirements for exchange of
   inter-domain distribution information.

4.1 General Requirements

   The goal of the Distribution Internetworking is to interconnect the
   Distribution Systems of multiple CNs.  The intent of this
   interconnection is to effectively position content for fast,
   reliable access by CLIENTs. Generally this is accomplished by
   replicating content on SURROGATEs. While the communications path
   from ORIGIN to CLIENTs may traverse a number of links, some within a
   Distribution CN and some between Distribution CNs, Distribution
   Internetworking is concerned only with those communications between
   Distribution CNs.

   The three main components of Distribution Internetworking are
   advertising, replication, and signaling.

   Advertising: Distribution CNs MAY advertise their capabilities to
      potential Content Source CNs.

   Replication: Distribution CNs MUST be able to move content from a
      Content Source to a Content Destination.

   Content signaling: Distribution CNs MUST be able to propagate
      content meta-data. This meta-data includes information such as
      the immediate invalidation of content or a change in the source
      or distribution method of content.

   Note that these requirements do not necessarily translate directly
   into three distinct Distribution Internetworking protocols.

4.2 Advertising Requirements

   The following list specifies requirements to enable advertising of
   distribution capabilities.

   1.  A common protocol for the Advertisement of distribution

   2.  A common format for the actual distribution capabilities
       Advertisements in the protocol.

   3.  Security mechanisms.

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4.3 Replication Requirements

   The following list specifies requirements to enable content

   1.  A common base protocol for the replication of content.

   2.  This common base protocol should specify:

       *  a common format for the actual content data in the protocol.

       *  A common format for the content meta-data in the protocol.

   3.  Alternate content-specific protocols for the replication of
       content should be enabled.  The replication protocol for a
       particular content set is specified via content signaling.

   4.  Scalable distribution of the content.

   5.  Security mechanisms.

4.4 Content Signaling Requirements

   The following list specifies requirements to enable content

   1.  A common protocol for signaling content meta-data.

   2.  An extenisble format for communicating content metadata.
       Minimally, support is required for "add," "withdraw," and
       "expiration time update."

   3.  Scalable distribution of signals on a scale to enable
       Internet-wide peering.

   4.  Security mechanisms.

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5. Distribution Internetworking Protocol Requirements

   This section defines protocol requirements for each of the
   advertising, replication and content signaling components of
   Distribution Internetworking.  Note that these requirements do not
   necessarily translate directly into either one converged or three
   distinct Distribution Internetworking protocols.

5.1 Overview of Distribution Internetworking Flow

   In a Distribution Internetworking arrangement, the following
   sequence of events is expected:

   1.  A Content Source may receive distribution capabilities
       advertisements from one or more Content Destinations. A Content
       Source may or may not require receipt of distribution
       capabilities advertisements prior to requesting services. For
       example, a Content Source may request services based on a
       contractual agreement negotiated off-line.

   2.  The Content Source will make a decision to request content
       distribution services from a Content Destination.

   3.  The Content Source will send a content signal requesting
       distribution services.

   4.  The Content Destination will accept or reject the request; no
       partial acceptance or negotiation is defined.

       *  If the request is rejected, the error code SHOULD provide
          enough information for the Content Source to determine if it
          should send a request with modified service requirements.

       *  If the request is accepted, the Content Destination will
          prepare for distribution services. Generally, this
          preparation will entail:

          +  optionally retrieving a copy of the object(s),

          +  joining the content update channel (if any), and

          +  preparing to provide access information to the Accounting

       *  Each of the above steps are according to the Content Source's
          specification, and to the Content Destination's policies and

   5.  Once the Content Destination is prepared, it will notify the

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       Request Routing System of the content's availability.

   6.  The Content Destination will terminate service on first
       occurrence of either:

       *  the time frame specified in the Content Source's request for
          distribution services expires or

       *  a content signal requesting withdrawal of the content is

5.2 General Distribution Internetworking Protocol Requirements

   Protocols must be scalable, i.e., support Distribution
   Internetworking on an Internet-wide scale.

   Protocols must prevent looping of advertisements, replication and
   content signaling.

   Protocols must support the ability to optionally conduct
   authenticated and/or encrypted exchanges.

   Protocols must support the ability to optionally exchange

5.3 Advertising Protocol Requirements

   1.  Distribution Internetworking protocols MUST enable a Content
        Destination to advertise the capabilities of its distribution
        service in a common format. This common format for distribution
        service capabilities will be referred to as a Service Profile
        for the remainder of this draft.

   2.  The advertisement protocol must be extensible with the
        restriction that implementation-specific capabilities may be
        safely ignored by Content Source.

   3.  Distribution Internetworking protocols MUST provide
        low-overhead, mechanism to notify in-line elements (e.g.,
        proxies) of CI support.

   4.  [  Editor's Note: prev item can be as simple has having the
        Content Source include a reference to it's CIG so that inline
        systems could contact the CIG and establish an internetworking
        arrangement.  But the feedback has been lukewarm to bad --
        drop?  ]

   5.  Distribution Advertising by a Content Destination must be
        optional. That is, a Content Source may not require real-time

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        advertisement of distribution capabilities in order to
        establish a Distribution Internetworking arrangement.
        Distribution capabilities may be communicated via
        Advertisements or some other agreed upon mechanism such as an
        off-line contract negotiated between the parties.

   6.  Advertised capabilities are those available to the peer,
        potentially based on some off-line contractual agreement, and
        may not necessarily reflect the total capacity of the Content

   7.  The protocol MUST enable a Content Destination to advertise
        multiple service profiles. Each service profile MUST be
        specifiable by a profile identifier.  The profile identifier
        MAY encode Content Source or Content Destination specific
        information, but it has local significance only (i.e., it is
        strictly between the Content Source and Content Destination).

   8.  The protocol MUST enable a Content Destination to advertise
        multiple services profiles to the same or different potential
        Content Sources.

   9.  A Content Destination with regional capabilities SHOULD
        advertise capabilities on a per region basis.  A Content
        Destination which advertises regional capabilities MUST
        minimally be able to identify regions by network

   10.  By default, advertisements are advisory. A Content Destination
        SHOULD be able to specify whether the capabilities are advisory
        or binding.

   11.  The protocol MUST provide the ability to specify distribution
        capabilities in terms of one or more of the following

        Profile ID: Identifier for the service profile being
           advertised.  The profile identifier is to be used by the
           Content Source when requesting service.  This attribute is
           required for all advertisements.  The value need not be
           unique across Distribution CNs, and may be used in
           advertisements to multiple Content Sources.

        FootPrint: The areas served by the CN. Minimally, a Content
           Destination should support expressing footprint according to
           IP network addresses/prefixes.

        Content Type: The type of content (e.g. static Web pages,
           streaming media, etc.) that the CN is able to distribute.

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        Capacity: The storage capacity that the CN can provide.

        Bandwidth: Maximum outbound bandwidth available from the CN.

        Object Bandwidth: Maximum outbound bandwidth supported for a
           single object.

        Distribution Method: The distribution methods that the CN
           supports; one or more of push, pull, and alm. "alm" refers
           to application layer multicast, or splitting, of CONTINUOUS
           MEDIA; if specified, supported protocols must also be

              [ Editor's Note: Specifying support for splitting
              requires refinement. ]

        Request Routing Type: Type(s) of request routing supported for
           CI Request Routing Systems.

        Accounting System Format: Supported protocol(s) and format(s)
           for sending accounting and access feedback to a specified CI
           Accounting System.

        Time Frame: Time frame for which this advertisement is valid.

        Client Protocols: Indicates the client protocols supported.
           Currently only HTTP and RTSP are valid. However, this field
           must be further qualified than just the transport or
           signalling protocol.  The protocol must clearly define a
           level of support implicated by a given Client Protocol

        Distribution Fee: Indicates the fee charged for distribution
           services.  The value may be expressed in Mbps
           (megabits/second) or in MB (megabytes of storage).

        Advertisement Type: Indicates whether the advertisement is
           advisory or binding.  By default, all advertisements are

        Private Extensions: Additional metrics that the communicating
           parties may agree to use, but are not part of the IETF
           standard. Extensions must be defined such that if not
           understood by the Content Source, they can be safely

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5.3.1 Advertising Examples

   To be provided.

5.4 Replication Protocol Requirements

   1.  A common (base) replication protocol MUST be defined which is
       supported by all CIGs, for any content type which can be used to
       transport meta-data and a full replica of content data.

   2.  Replication MUST support the ability for a Content Source to
       specify a replication channel from which content may be

       1.  [ Editors Note: I am using channel as a generic term which
           would provide a contact point and protocol, and any
           additional info required to establish a connection.  E.g.
           wcips://invalidation.com/content_set for signaling;  will
           provide clarification later ]

   3.  Replication MUST enable specifying optionally supported,
       alternative replication protocols which may be better suited
       than the common base protocol for specific content types or
       configuration scenarios.

   4.  A Content Source SHOULD be able to specify an authoritative
       source for content as well as distribution points.

   5.  The protocol MUST enable replication that is secured (encrypted)
       across the communications channel.

5.4.1 Replication Examples

   To be provided.

5.5 Content Signaling Protocol Requirements

   1.  A Content Source MUST be able to request distribution services
       for one or more content objects.

   2.  A Content Destination MUST explicitly accept or reject a request
       for distribution services.

   3.  A Content Source MUST be able to withdraw (cancel) a request for
       content services for one or multiple content objects.

   4.  Rejected requests for distribution services MUST include error
       codes. Partial rejections or negotiations are not supported.  A
       Content Source may follow a rejection with a request for

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       distribution services under alternate service requirements.

   5.  A Content Source MUST be able to signal consistency meta-data.
       Minimally, Content Sources SHOULD support weak consistency
       mechanisms.  Content Sources MAY support mechanisms for strong

   6.  Content signaling SHOULD include mechanisms to aggregate content

   7.  Content Signaling SHOULD be decoupled from the content ORIGIN.
       I.e., a Content Source should be able to specify a content
       signaling channel.

   8.  The following attributes are defined for content signals:

       Content Set ID: A unique identifier for this specific content
          set, so that future references (e.g. to modify the content or
          to withdraw it from distribution) may be resolved. This value
          can also be used to avoid loops. The Content Set ID MUST be
          global and unique, i.e., a given content set MUST have the
          same ID across all Distribution Systems, and this ID MUST be
          unique across *all* Distribution Systems.

       URI: The uniform resource identifier for the content. It
          identifies how CLIENTS will request delivery services from
          the Distribution CN.  This attr