Network Working Group B. Niven-Jenkins, Ed.
Internet-Draft Velocix (Alcatel-Lucent)
Intended status: Informational G. Watson
Expires: December 19, 2011 BT
N. Bitar
Verizon
J. Medved
Juniper Networks
S. Previdi
Cisco Systems
June 17, 2011
Use Cases for ALTO within CDNs
draft-jenkins-alto-cdn-use-cases-01
Abstract
For some time, Content Distribution Networks (CDNs) have been used in
the delivery of some Internet services (e.g. delivery of websites,
software updates and video delivery) as they provide numerous
benefits including reduced delivery cost for cacheable content,
improved quality of experience for end users and increased robustness
of delivery.
In order to derive the optimal benefit from a CDN it is preferable to
deliver content from the servers (caches) that are "closest" to the
End User requesting the content, where "closest" may be as simple as
"geographical or network distance" combined with CDN server load
within a location, but may also consider other more complex
combinations of metrics and CDN or Network Service Provider (NSP)
policies.
There are a number of different ways in which a CDN may obtain the
necessary network topology and/or cost information to allow it to
serve End Users from the most optimal servers/locations, such as
static configuration, passively listening to routing protocols
directly, active probing of underlying network(s), or obtaining
topology and cost by querying an information service such as the ALTO
map & cost services.
This document describes the use cases for a CDN to be able to obtain
network topology and cost information from an ALTO server(s).
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on December 19, 2011.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. CDN overview . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. CDN & ALTO Use Cases . . . . . . . . . . . . . . . . . . . . . 7
3.1. Exposing NSP End User Reachability to a CDN . . . . . . . 8
3.2. Exposing CDN End User Reachability to CSPs . . . . . . . . 9
3.3. CDN deployed within a Broadband network . . . . . . . . . 10
3.4. CDN delivering Over-The-Top of a NSP's network . . . . . . 11
3.5. CDN acquiring content from multiple upstream sources
(Origins) . . . . . . . . . . . . . . . . . . . . . . . . 11
3.6. Additional Use Cases . . . . . . . . . . . . . . . . . . . 12
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
5. Security Considerations . . . . . . . . . . . . . . . . . . . 13
6. Contributing Authors . . . . . . . . . . . . . . . . . . . . . 13
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
8. Normative References . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
For some time, Content Distribution Networks (CDNs) have been used in
the delivery of some Internet services (e.g. delivery of websites,
software updates and video delivery) as they provide numerous
benefits including reduced delivery cost for cacheable content,
improved quality of experience for end users and increased robustness
of delivery.
A CDN typically consists of a network of servers often attached to
Network Service Provider (NSP) networks. The point of attachment is
often as close to content consumers and peering points as
economically or operationally feasible in order to decrease traffic
load on the NSP backbone and to provide better user experience
measured by reduced latency and higher throughput.
As the volume of video and multimedia content delivered over the
Internet is rapidly increasing and expected to continue doing so in
the future, existing CDN providers are scaling up their
infrastructure and many NSPs are deploying their own CDNs. The
result of such deployments is typically that more CDN servers are
being deployed within NSP networks and those CDN servers are being
deployed in locations that are "deeper" (i.e. geographically closer
to the NSP's End Users) than was previously the case.
In order to derive the optimal benefit from a CDN it is preferable to
deliver content from the servers (caches) that are "closest" to the
End User requesting the content, where "closest" may be as simple as
"geographical or network distance" combined with CDN server load
within a location, but may also consider other more complex
combinations of metrics and CDN or NSP policies.
When CDN servers are deployed outside of an NSP's network or in a
small number of central locations within an NSP's network a
simplified view of the NSP's topology or an approximation of
proximity is typically sufficient to enable the CDN to serve End
Users from the optimal server/location. As CDN servers are deployed
deeper within NSP networks it becomes necessary for the CDN to have
more detailed knowledge of the underlying network topology and costs
between network locations in order to enable the CDN to serve End
Users from the most optimal servers for the NSP.
There are a number of different ways in which a CDN may obtain the
necessary network topology and/or cost information to allow it to
serve End Users from the most optimal servers/locations, such as
static configuration, passively listening to routing protocols
directly, active probing of underlying network(s), or obtaining
topology and cost by querying an information service such as the ALTO
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map & cost services.
The rest of this document describes the use cases for a CDN to be
able to obtain network topology and cost information from an ALTO
server(s).
1.1. Terminology
The following terms are taken from
[I-D.jenkins-cdni-problem-statement] and repeated here for
completeness.
Content Distribution Network (CDN) / Content Delivery Network (CDN):
Network infrastructure in which the network elements cooperate at
layers 4 through layer 7 for more effective delivery of Content to
User Agents. Typically a CDN consists of a Request Routing system, a
Distribution System (that includes a set of Surrogates), a Logging
System and a CDN control system.
Content Service Provider (CSP): Provides a Content Service to End
Users (which the End Users access via a User Agent). A CSP may own
the Content made available as part of the Content Service, or may
license content rights from another party.
End User (EU): The 'real' user of the system, typically a human but
maybe some combination of hardware and/or software emulating a human
(e.g. for automated quality monitoring etc.)
Network Service Provider (NSP): Provides network-based connectivity/
services to Users.
Surrogate: A device/function that interacts with other elements of
the CDN for the control and distribution of Content within the CDN
and interacts with User Agents for the delivery of the Content.
User Agent (UA): Software (or a combination of hardware and software)
through which the End User interacts with the Content Service. The
User Agent will communicate with the CSP's Service for the selection
of content and one or more CDNs for the delivery of the Content.
Such communication is not restricted to HTTP and may be via a variety
of protocols. Examples of User Agents (non-exhaustive) are:
Browsers, Set Top Boxes (STB), Dedicated content applications (e.g.
media players), etc.
2. CDN overview
This section provides a high level and simplified overview of the
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operation of a CDN to help put the ALTO & CDN use cases into context.
A typical CDN consists of a number of functional components, however
in the context of ALTO three functional components are of interest:
The Request Routing function, the Surrogate (i.e. caching) function
and the Origin function.
The Request Routing function within a CDN is responsible for
receiving content requests from User Agents, obtaining and
maintaining necessary information about a set of candidate
Surrogates, and for selecting and redirecting the User Agent to the
appropriate Surrogate.
The Surrogate function interacts with other elements of the CDN for
the control and distribution of Content within the CDN and interacts
with User Agents for the delivery of the Content.
The figure below shows a high level call flow showing the interaction
between a User Agent, Request Router and Surrogate for the delivery
of content in a single CDN.
User Agent Request Router Surrogate
| | |
| (1) Initial Request | |
+------------------------------>| |
| +--+ |
| | | (2) Surrogate Selection |
| |<-+ |
| (3) Redirection Response | |
|<------------------------------+ |
| | |
| (4) Content Request | |
+------------------------------------------------------------>|
| | |
| | (5) Content |
|<------------------------------------------------------------+
| | |
1. The User Agent makes an initial request to the CDN. Depending on
the type of content being delivered and the configuration of the
CDN this request may be an application (e.g. HTTP, RTMP, etc.)
level request directly from the User Agent or may be a DNS
request via the User Agent's assigned DNS proxy.
2. The Request Router selects an appropriate Surrogate (or set of
Surrogates) based on the User Agent's (or its proxy's) IP
address, the Request Router's knowledge of the network topology
and reachability cost between CDN caches and end users, and any
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additional CDN policies.
3. The Request Router responds to the UA's initial request with an
appropriate response containing a redirection to the selected
cache, for example by returning an appropriate DNS A/AAAA record,
a HTTP 302 redirect, etc.
4. The User Agent uses the information provided in the Redirection
Response to connect directly to the Surrogate and request the
desired content.
5. If CDN policy allows the User Agent to receive the requested
content, the Surrogate delivers the content to the User Agent.
A. [Not Shown] If the Surrogate does not have a copy of the
requested content then it obtains it from the appropriate
Origin Server.
Note: A Surrogate may not communicate with the Origin directly
and instead obtain the requested content from other surrogates
or caching layers in the CDN hierarchy. The details of how
content requests filter through the CDN hierarchy to the
Origin are internal to a specific CDN and are out of scope of
this document.
3. CDN & ALTO Use Cases
The primary use case for ALTO in a CDN context is to improve the
selection of a CDN Surrogate or Origin. The CDN makes use of an ALTO
server to choose a better CDN Surrogate or Origin than would
otherwise be the case. In its simplest form an ALTO server would
provide an NSP with the capability to offer a service to a CDN which
provides network map and cost information that the CDN can use to
enhance its surrogate and/or Origin selection.
Although it is possible to obtain raw network map and cost
information in other ways, for example passively listening to the
NSP's routing protocols, the use of an ALTO service to expose that
information may provide additional control to the NSP over how their
network map/cost is exposed. Additionally it may enable the NSP to
maintain a functional separation between their routing plane and
network map computation functions. This may be attractive for a
number of reasons, for example:
o The ALTO service could provide a filtered view of the network
and/or cost map that relates to CDN locations and their proximity
to end users, for example to allow the NSP to control the level of
topology detail they are willing to share with the CDN.
o The ALTO service could apply additional policies to the network
map and cost information to provide a CDN-specific view of the
network map/cost, for example to allow the NSP to encourage the
CDN to use network links that would not ordinarily be preferred by
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a Shortest Path First routing calculation.
o The routing plane may be operated and controlled by a different
operational entity (even within a single NSP) to the CDN and the
ALTO service could provide a layer of separation because:
* The CDN is not able to passively listen to routing protocols.
* The NSP is not willing to allow the CDN to passively listen to
routing protocols, e.g. because the NSP is concerned the CDN
may inadvertently interfere with the routing plane or because
the routing plane and the CDN are operated by different
operational entities/groups (including different entities
within the same NSP).
The use cases in this document are not necessarily specific as to the
relationship between the commercial/operational entity that "owns"
the ALTO service and the commercial/operational entity that "owns"
the CDN service as it is assumed that such relationships will be
deployment specific. Although the ownership of each service may
affect the level of topology detail that the ALTO service will be
permitted to expose, it is assumed that the general requirements a
CDN places on the ALTO service should not change provided that the
ALTO server is able to expose sufficient topology for the CDN to make
appropriate surrogate and/or Origin selection decisions.
In general, the ALTO service is expected to be operated by an entity
or entities that wish to optimize or otherwise influence request
routing decisions. Some, non-exhaustive, examples of such entities
are:
o The entity that operates the CDN's underlying network (e.g. the
"CDN deployed within a Broadband network" described in
Section 3.3).
o An NSP that wishes to optimize over-the-top content delivery from
a CDN that is deployed outside of its network (e.g. the "CDN
delivering Over-The-Top of a NSP" described in Section 3.4).
o An NSP (that may or may not operate a CDN) or a CDN that wishes to
advertise which End Users are reachable via its network/CDN (e.g.
the exposing "End User reachability" use cases described in
Section 3.1 and Section 3.2).
The following sections outline some specific, non-exhaustive, example
use cases, which are subsets of the primary use case outlined above
but applied to specific usage examples to demonstrate how a CDN could
make use of ALTO services.
3.1. Exposing NSP End User Reachability to a CDN
In order for a Request Router to be able to make surrogate selection
decisions, the Request Router needs to have information on which End
User IP subnets are reachable via which networks or network
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locations. The granularity of location information required depends
on the specific deployment of the CDN relative to the End Users. For
example, an Over-The-Top CDN whose surrogates are deployed only
within the Internet "backbone" may only require knowledge of which
End User IP subnets are reachable via which NSPs' networks, whereas a
CDN deployed within a particular NSP's network requires a finer
granularity of knowledge, i.e. which End User IP subnets are
reachable via which regions within that NSP's network.
Such reachability information is often available via dynamic routing
protocols, however it is likely that in a number of deployment
scenarios that peering of the routing plane of the network with a CDN
would be deemed unacceptable (e.g. where the CDN is operated by an
entity other than the NSP(s) operating the underlying network).
Provided that some common mapping between ALTO PIDs and network
locations (or entire networks) is known to both the NSP and the CDN,
the network map services offered by ALTO could be used to expose
which End User IP subnets are reachable via a particular network or
network locations in order to export End User reachability to a
Request Router to enable the NSP to expose End User reachability
while also giving the NSP the ability to control the granularity of
any End User reachability to network location mapping while also
avoiding routing plane peering between the NSP and the CDN.
3.2. Exposing CDN End User Reachability to CSPs
This use case is similar to the use case described in Section 3.1
however in this case it is the CDN that wishes to expose which End
User IP subnets the CDN is capable of delivering services to.
In some deployments a particular CDN may not have reachability to (or
may not wish to offer services to) every End User IP subnet reachable
via the global Internet, for example because the CDN is only deployed
within certain networks or geographic regions and the CDN is either
unable (due to lack of reachability) or unwilling (due to cost or
policy) to serve all End Users reachable via the global Internet.
The reachability offered by a particular CDN may not include all the
End User IP subnets that a particular CSP requires in order to serve
all of that CSP's customers and therefore if the CSP wishes to make
use of the services offered by a CDN that can only serve a subset of
their customers the CSP must have knowledge of which End User IP
subnets a particulart CDN is able to serve, so that they can select
an appropriate CDN to use to deliver their service to particular
subsets of their customers.
In such cases, the network map services offered by ALTO could be used
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to expose to a CSP which End User IP subnets are reachable via a
particular CDN. In the case where the CDN is operated by an NSP
using ALTO in this way could also enable the NSP to separate the
exposure of End User subnets reachable via their CDN from those
reachable via their underlying network.
3.3. CDN deployed within a Broadband network
In this use case an NSP is providing Broadband services to its
customers and has deployed a CDN within its Broadband network to
alleviate the cost and/or improve the User Experience of content
services for its Broadband customers.
The topology of Broadband access/backhaul networks is often much more
constrained than metro/core networks. If CDN surrogates are deployed
within the access/backhaul network, for a given set of End Users, the
NSP is likely to want to utilise the surrogates deployed in the same
access/backhaul region as those End Users in preference to surrogates
deployed within the metro/core or within other access/backhaul
regions.
It is common for Broadband subscribers to obtain their IP addresses
dynamically and in many deployments the IP subnets allocated to a
particular access/backhaul region can change relatively frequently.
For example new IP subnets are added as the subscriber base grows, IP
subnets are moved from one Broadband product in the NSP's portfolio
to another as customers migrate in order to optimise the NSP's IP
address utilisation, or they are simply moved as part of IP address
management, etc.
Additionally, in certain cases, CDN surrogates deployed in a
particular network region may become overloaded, leading to the CDN
selecting alternative surrogates in a different region of the network
for content delivery. If this occurs, an NSP may wish to influence
such a decision, for example because the NSP would prefer a surrogate
to be selected that is deployed in the the next best (cost-wise to
the NSP) location.
In order to meet the NSP's objective of utilising their CDN to
constrain access/backhaul costs and/or improve User Experience it is
important that the CDN is able to select the most appropriate
surrogate for a given set of End User IP subnets. Although the
network topology is often reasonably static, in networks where the IP
subnets allocated to a Broadband region are changing relatively
frequently, static configuration of End User IP Subnets to CDN
surrogates is possible but some NSPs may consider the operational
burden of having to update such static configuration too high and
would prefer the CDN to be able to dynamically obtain network map and
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cost information.
The NSP could make use of an ALTO service to expose a cost mapping/
ranking between End User IP subnets (within that NSP's network) and
CDN surrogate IP subnets/locations to meet its requirements while
avoiding static configuration or direct integration of the CDN into
its IP routing plane and to avoid the CDN being required to implement
network layer routing computations.
3.4. CDN delivering Over-The-Top of a NSP's network
In this use case a CDN is deployed within one or more NSPs' networks
but is delivering content "Over-The-Top" into another NSP's network
(which we will call NSP Z) where the CDN is not deployed.
The CDN is unlikely to have direct visibility of NSP Z's network
topology and may have a choice of entry points into NSP Z's network
from which it could serve content to NSP Z's End Users. For example
because NSP Z has direct peering links with the CDN in a number of
locations or NSP Z has transit and/or peering relationships with
several other NSPs where the CDN is deployed. NSP Z may wish to
influence the locations from which the CDN serves content based on
some factor(s) that it does not wish to expose directly or that might
change over time. For example the available transit/peering capacity
in different locations, the cost of connectivity to different
locations, etc.
For example, a CSP is using NSP A's CDN and another NSP (NSP Z) has
peering with NSP A in Los Angeles and New York. NSP Z would like to
influence which peering location NSP A's CDN delivers content out of
for NSP Z's end users by using their knowledge of the peering
capacity they have deployed in LA & NY and the capacity they have
between those peering locations and groups of end users without
directly exposing their internal topology to NSP A.
An NSP could make use of an ALTO service to expose a cost mapping/
ranking between End User IP subnets (within that NSP's network) and
entry points into that NSP's network in order to try to influence the
locations from which the CDN serves content into that NSP's network.
3.5. CDN acquiring content from multiple upstream sources (Origins)
Before a surrogate within a CDN is able to deliver content to an End
User it must first have a copy of the content that the End User is
requesting. Content may be obtained by surrogates in advance of it
being requested (pre-positioned) by End Users or it may be obtained
by surrogates dynamically in response to End User requests for the
content (on-demand).
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The ultimate source of the content (i.e. where the 'master' copy is
permanently stored) is typically referred to as the content's Origin
(or Origin Server), however CDNs often employ an internal hierarchy
of caching layers so that surrogates do not necessarily obtain
content directly from the Origin. Such a hierarchy provides a number
of benefits, for example reducing the number of requests for content
received by the Origin (and therefore reducing the scaling
requirements on the Origin), more efficient use of the underlying
network as fewer copies of the content is required to traverse the
same network links, etc.
For a particular CSP's content service multiple, possibly
independently addressable, Origins may be used for resiliency and the
Origin(s) may be deployed in a distributed manner across multiple
geographic locations.
For the rest of this use case "upstream source" is used to mean
either the Origin itself as well as other sources of the content, for
example another caching layer within the CDN that has (or will obtain
on demand) a copy of the content but is not the actual Origin.
Therefore, for a particular item of content, a surrogate may have a
choice of upstream sources (both internal to the CDN and external
Origins) from which it could obtain the content.
When presented with a choice of upstream sources, a surrogate may
utilise some combination of policy and heuristics to decide which
upstream sources (and in which order) it should attempt to use to
obtain the content. A CDN may wish to utilise network topology &
cost information as one of the inputs into such a content source
selection process, for example to weight upstream sources that are
topologically close to the surrogate that requires the content.
Additionally, where the CDN is deployed within one or more NSP
networks, an NSP may want to try to influence the choice of upstream
sources, for example the NSP may prefer the CDN to use content
sources that are deployed within that NSP's network or within
networks with which it has direct peering agreements with over other
content sources.
An NSP (or a CSP) could provide an ALTO service which a CDN could use
to obtain network topology and/or cost/ranking information to use as
an input into surrogates' selection decisions for content sources.
3.6. Additional Use Cases
The following additional use case may be relevant to ALTO and will be
described in more detail in a future version of this document:
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o Inter-provider CDN / CDN Interconnect.
4. IANA Considerations
This document makes no specific request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
5. Security Considerations
TBD.
6. Contributing Authors
Reinaldo Penno
Juniper Networks
Email: rpenno@juniper.net
Richard Alimi
Google
Email: ralimi@google.com
Richard Yang
Yale University
Email: ryr@yale.edu
7. Acknowledgements
The authors would like to thank Vijay Gurbani and Volker Hilt for
their review comments and contributions to the text.
8. Normative References
[I-D.jenkins-cdni-problem-statement]
Niven-Jenkins, B., Faucheur, F., and N. Bitar, "Content
Distribution Network Interconnection (CDNI) Problem
Statement", draft-jenkins-cdni-problem-statement-02 (work
in progress), March 2011.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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Authors' Addresses
Ben Niven-Jenkins (editor)
Velocix (Alcatel-Lucent)
326 Cambridge Science Park
Milton Road, Cambridge CB4 0WG
UK
Email: ben@velocix.com
Grant Watson
BT
Email: grant.watson@bt.com
Nabil Bitar
Verizon
40 Sylvan Road
Waltham, MA 02145
USA
Email: nabil.bitar@verizon.com
Jan Medved
Juniper Networks
Email: jmedved@juniper.net
Stefano Previdi
Cisco Systems
Email: sprevidi@cisco.com
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