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Versions: 00 01 02 03 04                                                
CDNI                                                          J. Seedorf
Internet-Draft                                                       NEC
Intended status: Informational                               J. Peterson
Expires: August 29, 2013                                         Neustar
                                                              S. Previdi
                                                                   Cisco
                                                      R. van Brandenburg
                                                                     TNO
                                                                   K. Ma
                                                     Azuki Systems, Inc.
                                                       February 25, 2013


       CDNI Request Routing: Footprint and Capabilities Semantics
                draft-spp-cdni-rr-foot-cap-semantics-04

Abstract

   This document tries to capture the semantics of the "Footprint and
   Capabilities Advertisement" part of the CDNI Request Routing
   interface, i.e. the desired meaning and what "Footprint and
   Capabilities Advertisement" is expected to offer within CDNI.  The
   discussion in this document has the goal to facilitate the choosing
   of one or more suitable protocols for "Footprint and Capabilities
   Advertisement" within CDNI Request Routing.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on August 29, 2013.

Copyright Notice

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




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction and scope . . . . . . . . . . . . . . . . . . . .  3
   2.  CDNI FCI in existing CDNI Documents  . . . . . . . . . . . . .  4
   3.  Design Decisions for Footprint and Capabilities  . . . . . . .  7
     3.1.  Advertising Limited Coverage . . . . . . . . . . . . . . .  7
     3.2.  Capabilities and Dynamic Data  . . . . . . . . . . . . . .  8
     3.3.  Advertisement versus Queries . . . . . . . . . . . . . . .  9
     3.4.  Avoiding or Handling 'cheating' dCDNs  . . . . . . . . . .  9
     3.5.  Focus on Main Use Cases may Simplify Things  . . . . . . . 10
   4.  Main Use Case to foster the Clarification of Semantics . . . . 11
   5.  Towards Semantics for Footprint Advertisement  . . . . . . . . 12
   6.  Towards Semantics for Capabilities Advertisement . . . . . . . 14
   7.  Open Issues and Questions  . . . . . . . . . . . . . . . . . . 17
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 19
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 19
   Appendix A.  Acknowledgment  . . . . . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21




















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1.  Introduction and scope

   The CDNI working group is working on a set of protocols to enable the
   interconnection of multiple CDNs to a CDN federation.  This CDN-
   federation should serve multiple purposes, as discussed in [RFC6770],
   for instance, to extend the reach of a given CDN to areas in the
   network which are not covered by this particular CDN.

   The goal of this document is to achieve a clear understanding in the
   CDNI WG about the semantics associated with the CDNI Request Routing
   Footprint & Capabilities Advertisement Interface (from now on
   referred to as FCI), in particular the type of information a
   downstream CDN 'advertises' regarding its footprint and capabilities.
   To narrow down undecided aspects of these semantics, this document
   tries to establish a common understanding of what the FCI should
   offer and accomplish in the context of CDN Interconnection.

   It is explicitly outside the scope of this document to decide on
   specific protocols to use for the FCI.

   General assumptions in this document:

   o  The CDNs participating in the CDN federation have already
      performed a boot strap process, i.e., they have connected to each
      other, either directly or indirectly, and can exchange information
      amongst each other.

   o  The uCDN has received footprint and/or capability advertisements
      from a set of dCDNs.  Footprint advertisement and capability
      advertisement need not use the same underlying protocol.

   o  The upstream CDN (uCDN) receives the initial request-routing
      request from the endpoint requesting the resource.

   This document is organized as follows.  First, a recap of the
   definition of "footprint and capabilities advertisement" in existing
   documents is given, attempting to distill the apparent common
   understanding of what the terms 'footprint' and 'capabilities' mean
   in the context of CDNI.  Then, the detailed semantics of the
   footprint advertisement mechanism and the capability advertisement
   mechanism will be discussed.  Finally, open issues and questions to
   be discussed in the CDNI WG will be listed.









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2.  CDNI FCI in existing CDNI Documents

   In the following section, the existing descriptions of the CDNI FCI
   interface in existing documents will be examined.  After this, the
   apparent common understanding of what this interface is intended to
   offer and accomplish will be carved out.

   The CDNI Problem Statement [RFC6707] describes footprint and
   capabilities advertisement as: "[enabling] a Request Routing function
   in an Upstream CDN to query a Request Routing function in a
   Downstream CDN to determine if the Downstream CDN is able (and
   willing) to accept the delegated Content Request".  In addition, the
   draft says "the CDNI Request Routing interface is also expected to
   enable a downstream CDN to provide to the upstream CDN (static or
   dynamic) information (e.g. resources, footprint, load) to facilitate
   selection of the downstream CDN by the upstream CDN request routing
   system when processing subsequent content requests from User Agents".
   It thus considers "resources" and "load" as capabilities to be
   advertised by the downstream CDN.

   The CDNI Use Cases document [RFC6770] describes capabilities as "...
   supported range of devices and User Agents or the supported range of
   delivery technologies".  Examples for such capabilities given are
   specific delivery protocols, technology migration, and meeting a
   certain QoS.

   The CDNI requirements draft [I-D.ietf-cdni-requirements] lists
   several requirements relevant for the "footprint and capabilities
   advertisement" part of the CDNI request routing interface.  In
   summary, the following requirements for the CDNI Request Routing
   Interface and general requirements are relevant for the understanding
   of the semantics of "footprint and capabilities advertisement":

   o  GEN-4 [HIGH], "The CDNI solution shall not require intra-CDN
      information to be exposed to other CDNs for effective and
      efficient delivery of the content.  Examples of intra-CDN
      information include surrogate topology, surrogate status, cached
      content, etc."

   o  GEN-9 [MED], "The CDNI solution should support cascaded CDN
      redirection (CDN1 redirects to CDN2 that redirects to CDN3) to an
      arbitrary number of levels beyond the first level."

   o  GEN-10 [MED], "The CDNI solution should support an arbitrary
      topology of interconnected CDNs (i.e. the CDN topology cannot be
      restricted to a tree, a loop-free topology, etc.)."





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   o  GEN-11 [HIGH], "The CDNI solution shall prevent looping of any
      CDNI information exchange."

   o  REQ-1 [HIGH], allowing the downstream CDN "to communicate to the
      Upstream CDN coarse information about the Downstream CDN ability
      and/or willingness to handle requests from the Upstream CDN.  For
      example, this could potentially include a binary signal
      ("Downstream CDN ready/not-ready to take additional requests from
      Upstream CDN") to be used in case of excessive load or failure
      condition in the Downstream CDN."

   o  REQ-2 [MED], allowing the downstream CDN to communicate
      capabilities such as supported content types and delivery
      protocols, a set of metrics/attributes (e.g.  Streaming bandwidth,
      storage resources, distribution and delivery priority), a set of
      affinities (e.g.  Preferences, indication of distribution/delivery
      fees), information to facilitate request redirection, as well as
      footprint information (e.g. "layer-3 coverage").

   o  REQ-3 [MED], "In the case of cascaded redirection, the CDNI
      Request-Routing interface shall allow the Downstream CDN to also
      include in the information communicated to the Upstream CDN,
      information on the capabilities, resources and affinities of CDNs
      to which the Downstream CDN may (in turn) redirect requests
      received by the Upstream CDN.  In that case, the CDNI Request-
      Routing interface shall prevent looping of such information
      exchange."

   o  REQ-4 [LOW], allowing the downstream CDN to communicate "aggregate
      information on CDNI administrative limits and policy" (e.g. the
      maximum number of requests redirected by the Upstream CDN to be
      served simultaneously by the Downstream CDN or maximum aggregate
      volume of content (e.g. in Terabytes) to be delivered by the
      Downstream CDN over a time period).

   o  REQ-11 [LOW], "The CDNI Request-Routing protocol may support a
      mechanism allowing an Upstream CDN to avoid redirecting a request
      to a Downstream CDN if that is likely to result in the total
      redirection time exceeding some limit."

   Note that in REQ-2 [MED] "Layer-3 coverage" is given as an example of
   what "footprint" information might convey in the CDNI requirements
   draft [I-D.ietf-cdni-requirements].  Also, note that REQ-3 [MED]
   addresses cascaded (transitive) downstream CDNs.  In such a case, a
   downstream CDN needs to include (in its advertisement information
   that it conveys to an upstream CDN) aggregate footprint and
   capabilities information for any further transitive downstream CDNs.
   Such information may be included implicitly (i.e. the cascaded dCDN



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   is oblivious to the uCDN), or explicitly (i.e. the cascaded dCDN of
   the fact that there is a cascaded dCDN is visible to the uCDN).  In
   either case, logic is needed to process incoming footprint
   information from a cascaded dCDN and decide if/how it is to be re-
   advertised/aggregated when advertising footprint to an upstream CDN.

   The CDNI framework draft [I-D.ietf-cdni-framework] describes a
   "footprint" as in [I-D.previdi-cdni-footprint-advertisement],
   consisting of two parts: 1) "a class of end user requests
   (represented, for example, by a set of IP prefixes, or a geographic
   region) that the dCDN is willing and able to serve directly, without
   use of another dCDN", and 2) "the connectivity of the dCDN to other
   CDNs that may be able to serve content to users on behalf of dCDN".
   The term "connectivity" has recently been replaced with
   "reachability" in [I-D.previdi-cdni-footprint-advertisement], and as
   discussed above, "without use of another dCDN" may include aggregated
   transitive dCDNs.  Further examples for capabilities are "the ability
   to handle certain types of content (e.g. specific streaming formats)
   or quality of service (QoS)."  Content handling capabilities
   discussed in [I-D.ma-cdni-capabilities] include delivery and
   acquisition protocols, redirection modes, and metadata related
   capabilities (e.g., authorization algorithm).

   From reading the various draft listed above, it is safe to conclude
   that neither the term 'footprint' nor 'capabilities' has been clearly
   and unambiguously defined in these documents and a very broad range
   of potential capabilities is listed.
























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3.  Design Decisions for Footprint and Capabilities

   A large part of the difficulty in discussing the FCI lies in
   understanding what exactly is meant when trying to define footprint
   in terms of "coverage" or "reachability."  While the operators of
   CDNs pick strategic locations to situate caches, a cache with a
   public IPv4 address is reachable by any endpoint on the Internet
   unless some policy enforcement precludes the use of the cache.

   Some CDNs aspire to cover the entire world, which we will henceforth
   call global CDNs.  The footprint advertised by such a CDN in the CDNI
   environment would, from a coverage or reachability perspective,
   presumably cover all prefixes.  Potentially more interesting for CDNI
   use cases, however, are CDNs that claim a more limited coverage, but
   seek to federate with other CDNs in order to create a single CDN
   fabric which shares resources.

   Futhermore, not all capabilties need be footprint restricted.
   Depending upon the use case, the optimal semantics of "footprints
   with capability attributes" vs. "capabilities with footprint
   restrictions" are not clear.

   The key to understanding the semantics of footprint and capability
   advertisement lies in understand why a dCDN would advertise a limited
   coverage area, and how a uCDN would use such advertisements to decide
   among one of several dCDNs.  The following section will discuss some
   of the trade-offs and design decisions that need to be decided upon
   for the CDNI FCI.

3.1.  Advertising Limited Coverage

   The basic use case that would motivate a dCDN to advertise a limited
   coverage is that the CDN was built to cover only a particular portion
   of the Internet.  For example, an ISP could purpose-build a CDN to
   serve only their own customers by situating caches in close
   topological proximity to high concentrations of their subscribers.
   The ISP knows the prefixes it has allocated to end users and thus can
   easily construct a list of prefixes that its caches were positioned
   to serve.

   When such a purpose-built CDN joins a federation, however, and
   advertises its footprint to a uCDN, the original intended coverage of
   the CDN might not represent its actual value to the federation of
   CDNs.  Consider an ISP-A and ISP-B that both field their own CDNs,
   which they federate through CDNI.  A given user E, who is customer of
   ISP-B, might happen to be topologically closest to a cache fielded by
   ISP-A, if E happens to live in a region where ISP-B has few customers
   and ISP-A has many.  In this case, should ISP-A's CDN "cover" E?  If



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   ISP-B's CDN has a failure condition, should the uCDN understand that
   ISP-A's caches are potentially available back-ups - and if so, how
   does ISP-A advertise itself as a "standby" for E?  What about the
   case where CDNs advertising to the same uCDN express overlapping
   coverage (for example, a federation mixing global and limited CDNs)?

   The answers to these questions greatly depend on how much information
   we want the uCDN to use to make a selection of a dCDN.  If a uCDN has
   three dCDNs to choose from that "cover" the IP address of user E,
   obviously the uCDN might be interested to know how optimal the
   coverage is from each of the dCDNs - coverage need not be binary,
   either provided or not provided. dCDNs could advertise a coverage
   "score," for example, and provided that they all reported scores
   fairly on the same scale, uCDNs could use that to make their
   topological optimality decision.  Alternatively, dCDNs could for
   their footprint advertise the IP addresses of their caches rather
   than prefix "coverage," and let the uCDN decide for itself (based on
   its own topological intelligence) which dCDN has better resources to
   serve a given user.

   In summary, the semantics of advertising footprint depend on whether
   such qualitative metrics for expressing footprint (such as the
   coverage 'score' mentioned above) should be part of the CDNI FCI, or
   if it should focus just on 'binary' footprint.

3.2.  Capabilities and Dynamic Data

   In cases where the apparent footprint of dCDNs overlaps, uCDNs might
   also want to rely on a host of other factors to evaluate the
   respective merits of dCDNs.  These include facts related to the
   caches themselves, to the network where the cache is deployed, to the
   nature of the resource sought and to the administrative policies of
   the respective networks.

   In the absence of network-layer impediments to reaching caches, the
   choice to limit coverage is necessarily an administrative policy.
   Much policy must be agreed upon before CDNs can merge into
   federations, including questions of membership, compensation, volumes
   and so on.  A uCDN certainly will factor these sorts of
   considerations into its decision to select a dCDN, but there is
   probably little need for dCDNs to actually advertise them through an
   interface - they will be settled out of band as a precondition for
   federating.

   Other facts about the dCDN would be expressed through the interface
   to the uCDN.  Some capabilities of a dCDN are static, and some are
   highly dynamic.  Expressing the total storage built into its caches,
   for example, changes relatively rarely, whereas the amount storage in



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   use at any given moment is highly volatile.  Network bandwidth
   similarly could be expressed as either total bandwidth available to a
   cache, or based on the current state of the network.  A cache may at
   one moment lack a particular resource in storage, but have it the
   next.

   The semantics of the capabilities interface will depend on how much
   of the dCDN state needs to be pushed to the uCDN and qualitatively
   how often that information should be updated.

3.3.  Advertisement versus Queries

   In a federated CDN environment, each dCDN shares some of its state
   with the uCDN, which the uCDN uses to build a unified picture of all
   of the dCDNs available to it.  In architectures that share detailed
   capability information, the uCDN could basically perform the entire
   request-routing intelligence down to selecting a particular cache
   before sending the request to the dCDN (note that within the current
   CDNI WG scope, such direct selection of specific caches by the uCDN
   is out of scope).  However, when the uCDN must deal with many
   potential dCDNs, this approach does not scale.  Especially as CDNs
   scale up from dozens or hundreds of caches to thousands or tens of
   thousands, the volume of updates to footprint and capability may
   become onerous.

   Were the volume of updates to exceed the volumes of requests to the
   uCDN, it might make more sense for the uCDN to query dCDNs upon
   receiving requests (as is the case in the recursive redirection mode
   described in [I-D.ietf-cdni-framework]), instead of receiving
   advertisements and tracking the state of dCDNs itself.  The advantage
   of querying dCDNs would be that much of the dynamic data that dCDNs
   cannot share with the uCDN would now be factored into the uCDN's
   decision. dCDNs need not replicate any state to the uCDN - uCDNs
   could effectively operate in a stateless mode.

   The semantics of both footprint and capability advertisement depend
   on the service model here: are there cases where a synchronous query/
   response model would work better for the uCDN decision than a state
   replication model?

3.4.  Avoiding or Handling 'cheating' dCDNs

   In a situation where more than one dCDN is willing to serve a given
   end user request, it might be attractive for a dCDN to 'cheat' in the
   sense that the dCDN provides inaccurate information to the uCDN in
   order to convince the uCDN to select it opposed to 'competing' dCDNs.
   It could therefore be desirable to take away the incentive for dCDNs
   to cheat (in information advertised) as much as possible.  One option



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   here is to make the information the dCDN advertises somehow
   verifiable for the uCDN.  One the other hand, a cheating dCDN might
   be avoided or handled by the fact that there will be strong
   contractual agreements between a uCDN and a dCDN, so that a dCDN
   would risk severe penalties or legal consequences when caught
   cheating.

   Overall, it seems that information a dCDN advertises should (in the
   long run) be somehow qualitatively verifiable by the uCDN, though
   possibly through non-real-time out-of-band audits.  It is probably an
   overly strict requirement to mandate that such verification be
   possible "immediately", i.e. during the request routing process
   itself.  If the uCDN can detect a cheating dCDN at a later stage, it
   should suffice for the uCDN to "de-incentivize" cheating because it
   would negatively affect the long-term business relationship with a
   particular dCDN.

3.5.  Focus on Main Use Cases may Simplify Things

   To narrow down semantics for "footprint" and "capabilities" in the
   CDNI context, it can be useful to initially focus on key use cases to
   be addressed by the CDNI WG that are to be envisioned the main
   deployments in the foreseeable future.  In this regard, a main
   realistic use case is the existence of ISP-owned CDNs, which
   essentially cover a certain operator's network.  At the same time,
   however, the possibility of overlapping footprints should not be
   excluded, i.e. the scenario where more than one dCDN claims it can
   serve a given end user request.  The ISPs may also choose to federate
   with a fallback global CDN.

   It seems reasonable to assume that in most use cases it is the uCDN
   that makes the decision on selecting a certain dCDN for request
   routing based on information the uCDN has received from this
   particular dCDN.  It may be assumed that 'cheating' CDNs will be
   dealt with via means outside the scope of CDNI and that the
   information advertised between CDNs is accurate.  In addition,
   excluding the use of qualitative information (e.g., cache proximity,
   delivery latency, cache load) to to predict the quality of delivery
   would further simplify the use case allowing it to better focus on
   the basic functionality of the FCI.











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4.  Main Use Case to foster the Clarification of Semantics

   Focusing on a main use case that contains a simple (yet somewhat
   challenging), realistic, and generally imaginable scenario can help
   in narrowing down the requirements for the CDNI FCI.  To this end,
   the following (simplified) use case can help in clarifying the
   semantics of footprint and capabilities for CDNI.  In particular, the
   intention of the use case is to clarify what information needs to be
   exchanged on the CDNI FCI, what types of information need to be
   supported in a mandatory fashion (and which should be considered
   optional), and what types of information need to be updated with
   respect to a priori established CDNI contracts.

   In short, one can imagine the following use case: A given uCDN has
   several dCDNs.  It selects one dCDN for delivery protocol A and
   footprint 1 and another dCDN for delivery protocol B and footprint 1.
   The dCDN that serves delivery protocol B has a further, transitive
   (level-2) dCDN, that serves delivery protocol B in a subset of
   footprint 1 where the first-level dCDN cannot serve delivery protocol
   B itself.  What happens if capabilities change in the transitive
   level-2 dCDN that might affect how the uCDN selects a level-1 dCDN
   (e.g. in case the level-2 dCDN cannot serve delivery protocol B
   anymore)?  How will these changes be conveyed to the uCDN?  In
   particular, what information does the uCDN need to be able to select
   a new first-level dCDN, either for all of footprint 1 or only for the
   subset of footprint 1 that the transitive level-2 dCDN served on
   behalf of the first-level dCDN?
























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5.  Towards Semantics for Footprint Advertisement

   Roughly speaking, "footprint" can be defined as "ability and
   willingness to serve" by a downstream CDN.  However, in addition to
   simple "ability and willingness to serve", the uCDN may wish to have
   additional information to make a dCDN selection decision, e.g., "how
   well" a given dCDN can actually serve a given end user request.  The
   "ability and willingness" to serve should be distinguished from the
   subjective qualitative measurement of "how well" it was served.  One
   can imagine that such additonal information is implicitly associated
   with a given footprint, e.g. due to contractual agreements (e.g.
   SLAs), business relationships, or perceived dCDN quality in the past.
   As an alternative, such additional information could also be
   explicitly tagged along with the footprint.

   It is reasonable to assume that a significant part of the actual
   footprint advertisement will happen in contractual agreements between
   participating CDNs, i.e. prior to the advertisement phase using the
   CDNI FCI.  The reason for this assumption is that any contractual
   agreement is likely to contain specifics about the dCDN coverage
   (i.e. the dCDN footprint) the contractual agreement applies to.  In
   particular, additional information to judge the delivery quality
   associated with a given dCDN footprint might be defined in
   contractual agreements (i.e. outside of the CDNI FCI).  Further, one
   can assume that dCDN contractual agreements about the delivery
   quality associated with a given footprint will probably be based on
   high-level aggregated statistics (i.e. not too detailed).

   Given that a large part of footprint advertisement will actually
   happen in contractual agreements, the semantics of CDNI footprint
   advertisement refer to answering the following question: what exactly
   still needs to be advertised by the CDNI FCI?  For instance, updates
   about temporal failures of part of a footprint can be useful
   information to convey via the CDNI request routing interface.  Such
   information would provide updates on information previously agreed in
   contracts between the participating CDNs.  In other words, the CDNI
   FCI is a means for a dCDN to provide changes/updates regarding a
   footprint it has prior agreed to serve in a contract with a uCDN.

   Generally speaking, one can imagine two categories of footprint to be
   advertised by a dCDN:

   o  Footprint could be defined based on (layer-3) "coverage/
      reachability", where coverage/reachability refers to a set of
      prefixes, a geographic region, or similar boundary.  The dCDN
      claims that it can cover/reach 'end user requests coming from this
      footprint'.




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   o  Footprint could be defined based on "resources", where resources
      refers to surrogates/caches a dCDN claims to have (e.g., the
      location of surrogates/resources).  The dCDN claims that 'from
      this footprint' it can serve incoming end user requests.

   For each of these footprint types, there are capabilities associated
   with a given footprint, i.e. the capabilities (e.g., delivery
   protocol, redirection mode, metadata) supported in the coverage area
   for a "coverage/reachability" defined footprint, or the capabilities
   of resources (e.g., delivery protocol, redirection mode, metadata
   support) for a "resources" defined footprint.  It seems clear that
   "coverage/reachability" types of footprint must be supported within
   CDNI.  It needs to be decided, whether - in addition to "coverage/
   reachability" types - also "resource" types of footprint should be
   supported within CDNI.

   Different concrete candidates for a "coverage/reachability" footprint
   are imaginable.  In particular, the following concrete types of
   footprint seem useful in the CDNI context: 'set of IP-prefixes', 'AS
   number list', and 'geographic region'.  Among these, 'set of IP-
   prefixes' must be able to contain full IP addresses (i.e., a /32 for
   IPv4 and a /128 for IPv6) and also IP prefixes with an arbitrary
   prefix length.  There must also be support for multiple IP address
   versions, i.e., IPv4 and IPv6, in the footprint.

   Independent of the exact type of a footprint, a footprint might also
   include the connectivity of a given dCDN to other CDNs that may be
   able to serve content to users on behalf of that dCDN, to cover cases
   where there is a transitive CDN interconnection.  Further, the
   downstream CDN must be able to express its footprint to an interested
   upstream CDN (uCDN) in a comprehensive form, e.g., as a complete data
   set containing the complete footprint.  Making incremental updates,
   however, to express dynamic changes in state is also desirable.


















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6.  Towards Semantics for Capabilities Advertisement

   In general, the dCDN must be able to express its general capabilities
   to the uCDN.  These general capabilities could express if the dCDN
   supports a given service, for instance, HTTP delivery, RTP/RTSP
   delivery or RTMP.  Furthermore, the dCDN must be able to express
   particular capabilities for the delivery in a particular footprint
   area.  For example, the dCDN might in general offer RTMP but not in
   some specific areas, either for maintenance reasons or because the
   caches covering this particular area cannot deliver this type of
   service.  Hence, in certain cases footprint and capabilities are tied
   together and cannot be interpreted independently from each other.  In
   such cases, i.e. where capabilities must be expressed on a per
   footprint basis, it may be beneficial to combine footprint and
   capabilities advertisement.

   A high-level and very rough semantic for capabilities is thus the
   following: Capabilities are types of information that allow a uCDN to
   determine if a downstream CDN is able (and willing) to accept (and
   properly handle) a delegated content request.  In addition,
   Capabilities are characterized by the fact that this information may
   possibly change over time based on the state of the network or
   caches.

   At a first glance, several broad categories of capabilities seem
   useful to convey via an advertisement interface (and indeed many such
   candidate capabilities have been suggested in CDNI drafts, see
   Section 2).  However, advertising capabilities that change highly
   dynamically (e.g. real-time delivery performance metrics, CDN
   resource load, or other highly dynamically changing QoS information)
   should probably not be in scope for the CDNI FCI.  First, out of the
   multitude of possible metrics and capabilities, it is hard to agree
   on a subset and the precise metrics to be used.  Second, and perhaps
   more importantly, it seems not feasible to specify such highly
   dynamically changing capabilities and the corresponding metrics
   within the CDNI charter time-frame.

   Useful capabilities refer to information that does not change highly
   dynamically and which in many cases is absolutely necessary to decide
   on a particular dCDN for a given end user request.  For instance, if
   an end user request concerns the delivery of a video file with a
   certain protocol (e.g.  RTMP), the uCDN needs to know if a given dCDN
   has the capabiltity of supporting this delivery protocol.

   Similar to footprint advertisement, it is reasonable to assume that a
   significant part of the actual (resource) capabilities advertisement
   will happen in contractual agreements between participating CDNs,
   i.e. prior to the advertisement phase using the CDNI FCI.  The role



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   of capability advertisement is hence rather to enable the dCDN to
   update a uCDN on changes since a contract has been set up (e.g. in
   case a new delivery protocol is suddenly being added to the list of
   supported delivery protocols of a given dCDN, or in case a certain
   delivery protocol is suddenly not being supported anymore due to
   failures).  Capabilities advertisement thus refers to conveying
   information to a uCDN about changes/updates of certain capabilities
   with respect to a given contract.

   Given these semantics, it needs to be decided what exact capabilities
   are useful and how these can be expressed.  Since the details of CDNI
   contracts are not known at the time of this writing (and the CDNI
   interface should probably be agnostic to these contracts anyway), it
   remains to be seen what capabilities will be used to define
   agreements between CDNs in practice.  One implication for
   standardization may be to initially only specify a very limited set
   of mandatory capabilities for advertisement and have on top of that a
   flexible data model that allows exchanging additional capabilities
   when needed.  Still, agreement needs to be found on which
   capabilities (if any) shoudl be mandatory among CDNs.  As discussed
   in Section 3.5, finding the concrete answers to these questions can
   benefit from focusing on a small number of key use cases that are
   highly relevant and contain enough complexity to help in
   understanding what concrete capabilities are needed to facilitate CDN
   Interconnection.

   Under the above considerations, the following capabilities seem
   useful as 'base' capabilities, i.e. ones that are needed in any case
   and therefore constitute mandatory capabilities to be supported by
   the CDNI FCI:

   o  Delivery Protocol (e.g., HTTP vs. RTMP)

   o  Acquisition Protocol (for aquiring content from a uCDN)

   o  Redirection Mode (e.g., DNS Redirection vs. HTTP Redirection as
      discussed in [I-D.ietf-cdni-framework])

   o  Capabilities related to CDNI Logging (e.g., supported logging
      mechanisms)

   o  Capabilities related to CDNI Metadata (e.g., authorization
      algorithms or support for proprietary vendor metadata)

   It is not feasable to enumerate all the possible options for the
   mandatory capabilities listed above (e.g., all the potential delivery
   protocols or metadata options) or anticipate all the future needs for
   additional capabilities.  It would be unreasonable to burden the CDNI



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   FCI specification with defining each supported capability.  The CDNI
   FCI specification should define a generic protocol for conveying any
   capability information.  It would be reasonable to define a registry
   which initially contains the mandatory capabilities listed above, but
   may be extended as needs dictate.  The CDNI FCI specification SHOULD
   define the registry (and the rules for adding new entries to the
   registry) for the different capability types.  Each capability type
   MAY further have a list of valid values.  The individual CDNI
   interface specifications which define a given capability SHOULD
   define any necessary registries (and the rules for adding new entries
   to the registry) for the values advertised for a given capability
   type.

   The mandatory capabilities listed above generally relate to
   information that is configured on a content asset or group of assets
   basis via CDNI metadata.  The capability requirements for acquisition
   and delivery protocol, redirection mode, and other mandatory metadata
   capabilities (e.g. authorization algorithms) are defined in
   [I-D.ietf-cdni-metadata].

   Note: CDNI interface support for logging configuration (i.e., control
   interface vs. metadata interface) has not yet been decided.  Once it
   has been decided, the corresponding CDNI interface specification
   should define the associated capability requirements.



























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7.  Open Issues and Questions

   The following open issues deserve further discussion in the CDNI WG:

   o  What is the service model of this interface: Does the uCDN always
      query the dCDNs?  Or does the dCDN always push information to the
      uCDNs?

   o  In addition to "reachability" types of footprint, should also
      "resource" types of footprint be considered by CDNI (e.g., the
      location of surrogates/resources a dCDN has)?

   o  Does a footprint need to explicitly include the "transitive
      reachability" of a dCDN to further dCDNs that may be able to serve
      content to users on behalf of dCDN?

   o  What is the assumed business relationship between the uCDN and the
      dCDN?  Is the uCDN always the "authoritative" CDN provider which
      transitively has itself contracted several downstream CDN
      providers?

   o  How exactly can a given dCDN derive its footprint?

   o  Should the footprint/capabilities advertisement interface only
      signal the delta with respect to a given contract (between a uCDN
      and a dCDN) or send the whole dCDN state each time?

























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8.  Security Considerations

   Security considerations will be discussed in a future version of this
   document.















































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9.  References

9.1.  Normative References

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

9.2.  Informative References

   [I-D.ietf-cdni-framework]
              Peterson, L. and B. Davie, "Framework for CDN
              Interconnection", draft-ietf-cdni-framework-03 (work in
              progress), February 2013.

   [I-D.ietf-cdni-metadata]
              Niven-Jenkins, B., Murray, R., Watson, G., Caulfield, M.,
              Leung, K., and K. Ma, "CDN Interconnect Metadata",
              draft-ietf-cdni-metadata-00 (work in progress),
              October 2012.

   [I-D.ietf-cdni-requirements]
              Leung, K. and Y. Lee, "Content Distribution Network
              Interconnection (CDNI) Requirements",
              draft-ietf-cdni-requirements-05 (work in progress),
              February 2013.

   [I-D.ma-cdni-capabilities]
              Ma, K., "Content Distribution Network Interconnection
              (CDNI) Capabilities Interface",
              draft-ma-cdni-capabilities-01 (work in progress),
              February 2013.

   [I-D.previdi-cdni-footprint-advertisement]
              Previdi, S., Faucheur, F., Faucheur, F., Medved, J., and
              L. Faucheur, "CDNI Footprint Advertisement",
              draft-previdi-cdni-footprint-advertisement-02 (work in
              progress), September 2012.

   [RFC6707]  Niven-Jenkins, B., Le Faucheur, F., and N. Bitar, "Content
              Distribution Network Interconnection (CDNI) Problem
              Statement", RFC 6707, September 2012.

   [RFC6770]  Bertrand, G., Stephan, E., Burbridge, T., Eardley, P., Ma,
              K., and G. Watson, "Use Cases for Content Delivery Network
              Interconnection", RFC 6770, November 2012.






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Appendix A.  Acknowledgment

   Jan Seedorf is partially supported by the CHANGE project (CHANGE:
   Enabling Innovation in the Internet Architecture through Flexible
   Flow-Processing Extensions, http://www.change-project.eu/), a
   research project supported by the European Commission under its 7th
   Framework Program (contract no. 257422).  The views and conclusions
   contained herein are those of the authors and should not be
   interpreted as necessarily representing the official policies or
   endorsements, either expressed or implied, of the CHANGE project or
   the European Commission.

   Jan Seedorf has been partially supported by the COAST project
   (COntent Aware Searching, retrieval and sTreaming,
   http://www.coast-fp7.eu), a research project supported by the
   European Commission under its 7th Framework Program (contract no.
   248036).  The views and conclusions contained herein are those of the
   authors and should not be interpreted as necessarily representing the
   official policies or endorsements, either expressed or implied, of
   the COAST project or the European Commission.

   Martin Stiemerling provided initial input to this document and
   valuable comments to the ongoing discussions among the authors of
   this document.



























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Authors' Addresses

   Jan Seedorf
   NEC
   Kurfuerstenanlage 36
   Heidelberg  69115
   Germany

   Phone: +49 6221 4342 221
   Fax:   +49 6221 4342 155
   Email: seedorf@neclab.eu


   Jon Peterson
   NeuStar
   1800 Sutter St Suite 570
   Concord  CA 94520
   USA

   Phone:
   Fax:
   Email: jon.peterson@neustar.biz


   Stefano Previdi
   Cisco Systems
   Via Del Serafico 200
   Rome  0144
   Italy

   Phone:
   Fax:
   Email: sprevidi@cisco.com


   Ray van Brandenburg
   TNO
   Brassersplein 2
   Delft  2612CT
   The Netherlands

   Phone: +31-88-866-7000
   Email: ray.vanbrandenburg@tno.nl








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   Kevin J. Ma
   Azuki Systems, Inc.
   43 Nagog Park
   Acton  MA 01720
   USA

   Phone: +1 978-844-5100
   Email: kevin.ma@azukisystems.com











































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