Internet DRAFT - draft-spp-cdni-rr-foot-cap-semantics
draft-spp-cdni-rr-foot-cap-semantics
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
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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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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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