Internet Engineering Task Force (IETF) L. Peterson
Request for Comments: 7336 Akamai Technologies, Inc.
Obsoletes: 3466 B. Davie
Category: Informational VMware, Inc.
ISSN: 2070-1721 R. van Brandenburg, Ed.
TNO
August 2014
Framework for Content Distribution Network Interconnection (CDNI)
Abstract
This document presents a framework for Content Distribution Network
Interconnection (CDNI). The purpose of the framework is to provide
an overall picture of the problem space of CDNI and to describe the
relationships among the various components necessary to interconnect
CDNs. CDNI requires the specification of interfaces and mechanisms
to address issues such as request routing, distribution metadata
exchange, and logging information exchange across CDNs. The intent
of this document is to outline what each interface needs to
accomplish and to describe how these interfaces and mechanisms fit
together, while leaving their detailed specification to other
documents. This document, in combination with RFC 6707, obsoletes
RFC 3466.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7336.
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Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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.
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Table of Contents
1. Introduction ....................................................4
1.1. Terminology ................................................4
1.2. Reference Model ............................................6
1.3. Structure of This Document ................................10
2. Building Blocks ................................................10
2.1. Request Redirection .......................................10
2.1.1. DNS Redirection ....................................10
2.1.2. HTTP Redirection ...................................12
3. Overview of CDNI Operation .....................................12
3.1. Preliminaries .............................................14
3.2. Iterative HTTP Redirect Example ...........................15
3.3. Recursive HTTP Redirection Example ........................21
3.4. Iterative DNS-Based Redirection Example ...................25
3.4.1. Notes on Using DNSSEC ..............................28
3.5. Dynamic Footprint Discovery Example .......................29
3.6. Content Removal Example ...................................31
3.7. Pre-positioned Content Acquisition Example ................32
3.8. Asynchronous CDNI Metadata Example ........................33
3.9. Synchronous CDNI Metadata Acquisition Example .............35
3.10. Content and Metadata Acquisition with Multiple
Upstream CDNs ............................................37
4. Main Interfaces ................................................38
4.1. In-Band versus Out-of-Band Interfaces .....................39
4.2. Cross-Interface Concerns ..................................40
4.3. Request Routing Interfaces ................................40
4.4. CDNI Logging Interface ....................................41
4.5. CDNI Control Interface ....................................43
4.6. CDNI Metadata Interface ...................................43
4.7. HTTP Adaptive Streaming Concerns ..........................44
4.8. URI Rewriting .............................................46
5. Deployment Models ..............................................47
5.1. Meshed CDNs ...............................................47
5.2. CSP Combined with CDN .....................................48
5.3. CSP Using CDNI Request Routing Interface ..................49
5.4. CDN Federations and CDN Exchanges .........................50
6. Trust Model ....................................................53
7. Privacy Considerations .........................................54
8. Security Considerations ........................................55
8.1. Security of CDNI Interfaces ...............................56
8.2. Digital Rights Management .................................56
9. Contributors ...................................................56
10. Acknowledgements ..............................................57
11. Informative References ........................................57
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1. Introduction
This document provides an overview of the various components
necessary to interconnect CDNs, expanding on the problem statement
and use cases introduced in [RFC6770] and [RFC6707]. It describes
the necessary interfaces and mechanisms in general terms and outlines
how they fit together to form a complete system for CDN
Interconnection. Detailed specifications are left to other
documents. This document makes extensive use of message flow
examples to illustrate the operation of interconnected CDNs, but
these examples should be considered illustrative rather than
prescriptive.
[RFC3466] uses different terminology and models for "Content
(distribution) Internetworking (CDI)". It is also less prescriptive
in terms of interfaces. To avoid confusion, this document obsoletes
[RFC3466].
1.1. Terminology
This document uses the core terminology defined in [RFC6707]. It
also introduces the following terms:
CDN-Domain: a hostname (Fully Qualified Domain Name -- FQDN) at the
beginning of a URL (excluding port and scheme), representing a set
of content that is served by a given CDN. For example, in the URL
http://cdn.csp.example/...rest of URL..., the CDN-Domain is
cdn.csp.example. A major role of CDN-Domain is to identify a
region (subset) of the URI space relative to which various CDNI
rules and policies apply. For example, a record of CDNI Metadata
might be defined for the set of resources corresponding to some
CDN-Domain.
Distinguished CDN-Domain: a CDN-Domain that is allocated by a CDN
for the purposes of communication with a peer CDN but that is not
found in client requests. Such CDN-Domains may be used for inter-
CDN acquisition, or as redirection targets, and enable a CDN to
distinguish a request from a peer CDN from an end-user request.
Delivering CDN: the CDN that ultimately delivers a piece of content
to the end user. The last in a potential sequence of Downstream
CDNs.
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Iterative CDNI Request Redirection: When an Upstream CDN elects to
redirect a request towards a Downstream CDN, the Upstream CDN can
base its redirection purely on a local decision (and without
attempting to take into account how the Downstream CDN may in turn
redirect the user agent). In that case, the Upstream CDN
redirects the request to the Request Routing system in the
Downstream CDN, which in turn will decide how to redirect that
request: this approach is referred to as "Iterative" CDNI Request
Redirection.
Recursive CDNI Request Redirection: When an Upstream CDN elects to
redirect a request towards a Downstream CDN, the Upstream CDN can
query the Downstream CDN Request Routing system via the CDNI
Request Routing Redirection interface (or use information cached
from earlier similar queries) to find out how the Downstream CDN
wants the request to be redirected. This allows the Upstream CDN
to factor in the Downstream CDN response when redirecting the user
agent. This approach is referred to as "Recursive" CDNI Request
Redirection. Note that the Downstream CDN may elect to have the
request redirected directly to a Surrogate inside the Downstream
CDN, or to any other element in the Downstream CDN (or in another
CDN), to handle the redirected request appropriately.
Synchronous CDNI operations: operations between CDNs that happen
during the process of servicing a user request, i.e., between the
time that the user agent begins its attempt to obtain content and
the time at which that request is served.
Asynchronous CDNI operations: operations between CDNs that happen
independently of any given user request, such as advertisement of
footprint information or pre-positioning of content for later
delivery.
Trigger Interface: a subset of the CDNI Control interface that
includes operations to pre-position, revalidate, and purge both
metadata and content. These operations are typically called in
response to some action (Trigger) by the Content Service Provider
(CSP) on the Upstream CDN.
We also sometimes use uCDN and dCDN as shorthand for Upstream CDN and
Downstream CDN (see [RFC6707]), respectively.
At various points in this document, the concept of a CDN footprint is
used. For a discussion on what constitutes a CDN footprint, the
reader is referred to [FOOTPRINT-CAPABILITY].
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1.2. Reference Model
This document uses the reference model in Figure 1, which expands the
reference model originally defined in [RFC6707]. (The difference is
that the expanded model splits the Request Routing interface into its
two distinct parts: the Request Routing Redirection interface and the
Footprint & Capabilities Advertisement interface, as described
below.)
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--------
/ \
| CSP |
\ /
--------
*
*
* /\
* / \
---------------------- |CDNI| ----------------------
/ Upstream CDN \ | | / Downstream CDN \
| +-------------+ | | CI | | +-------------+ |
|******* Control |<======|====|=======>| Control *******|
|* +------*----*-+ | | | | +-*----*------+ *|
|* * * | | | | * * *|
|* +------*------+ | | LI | | +------*------+ *|
|* ***** Logging |<======|====|=======>| Logging ***** *|
|* * +-*-----------+ | | | | +-----------*-+ * *|
|* * * * | | | | * * * *|
.....*...+-*---------*-+ | | RI | | +-*---------*-+...*.*...
. |* * | |<======|====|=======>| | * *| .
. |* * | Req-Routing | | |FCI | | | Req-Routing | * *| .
. |* * *** |<======|====|=======>| |** * *| .
. |* * * +-------------+.| | | | +-------------+ * * *| .
. |* * * . | | | * * *| .
. |* * * +-------------+ |. | MI | | +-------------+ * * *| .
. |* * * | Distribution|<==.===|====|=======>| Distribution| * * *| .
. |* * * | | | . \ / | | | * * *| .
. |* * * |+---------+ | | . \/ | | +---------+| * * *| .
. |* * ***| +---------+| | ...Request......+---------+ |*** * *| .
. |* *****+-|Surrogate|***********************|Surrogate|-+***** *| .
. |******* +---------+| | Acquisition | |+----------+ *******| .
. | +-------------+ | | +-------*-----+ | .
. \ / \ * / .
. ---------------------- ---------*------------ .
. * .
. * Delivery .
. * .
. +--*---+ .
...............Request............................| User |..Request..
| Agent|
+------+
<==> interfaces inside the scope of CDNI
**** and .... interfaces outside the scope of CDNI
Figure 1: CDNI Expanded Model and CDNI Interfaces
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While some interfaces in the reference model are "out of scope" for
the CDNI WG (in the sense that there is no need to define new
protocols for those interfaces), we note that we still need to refer
to them in this document to explain the overall operation of CDNI.
We also note that, while we generally show only one Upstream CDN
serving a given CSP, it is entirely possible that multiple uCDNs can
serve a single CSP. In fact, this situation effectively exists today
in the sense that a single CSP can currently delegate its content
delivery to more than one CDN.
The following briefly describes the five CDNI interfaces,
paraphrasing the definitions given in [RFC6707]. We discuss these
interfaces in more detail in Section 4.
o CDNI Control interface (CI): Operations to bootstrap and
parameterize the other CDNI interfaces, as well as operations to
pre-position, revalidate, and purge both metadata and content.
The latter subset of operations is sometimes collectively called
the "Trigger interface".
o CDNI Request Routing interface: Operations to determine what CDN
(and optionally what Surrogate within a CDN) is to serve end-user
requests. This interface is actually a logical bundling of two
separate, but related, interfaces:
* CDNI Footprint & Capabilities Advertisement interface (FCI):
Asynchronous operations to exchange routing information (e.g.,
the network footprint and capabilities served by a given CDN)
that enables CDN selection for subsequent user requests; and
* CDNI Request Routing Redirection interface (RI): Synchronous
operations to select a delivery CDN (Surrogate) for a given
user request.
o CDNI Metadata interface (MI): Operations to communicate metadata
that governs how the content is delivered by interconnected CDNs.
Examples of CDNI Metadata include geo-blocking directives,
availability windows, access control mechanisms, and purge
directives. It may include a combination of:
* Asynchronous operations to exchange metadata that govern
subsequent user requests for content; and
* Synchronous operations that govern behavior for a given user
request for content.
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o CDNI Logging interface (LI): Operations that allow interconnected
CDNs to exchange relevant activity logs. It may include a
combination of:
* Real-time exchanges, suitable for runtime traffic monitoring;
and
* Offline exchanges, suitable for analytics and billing.
The division between the sets of Trigger-based operations in the CDNI
Control interface and the CDNI Metadata interface is somewhat
arbitrary. For both cases, the information passed from the Upstream
CDN to the Downstream CDN can broadly be viewed as metadata that
describes how content is to be managed by the Downstream CDN. For
example, the information conveyed by the CI to pre-position,
revalidate, or purge metadata is similar to the information conveyed
by posting updated metadata via the MI. Even the CI operation to
purge content could be viewed as a metadata update for that content:
purge simply says that the availability window for the named content
ends now. The two interfaces share much in common, so minimally,
there will need to be a consistent data model that spans both.
The distinction we draw has to do with what the uCDN knows about the
successful application of the metadata by the dCDN. In the case of
the CI, the Downstream CDN returning a successful status message
guarantees that the operation has been successfully completed; for
example, the content has been purged or pre-positioned. This implies
that the Downstream CDN accepts responsibility for having
successfully completed the requested operation. In contrast,
metadata passed between CDNs via the MI carries no such completion
guarantee. Returning success implies successful receipt of the
metadata, but nothing can be inferred about precisely when the
metadata will take effect in the Downstream CDN, only that it will
take effect eventually. This is because of the challenge in globally
synchronizing updates to metadata with end-user requests that are
currently in progress (or indistinguishable from currently being in
progress). Clearly, a CDN will not be viewed as a trusted peer if
"eventually" often becomes an indefinite period of time, but the
acceptance of responsibility cannot be as crisply defined for the MI.
Finally, there is a practical issue that impacts all of the CDNI
interfaces, and that is whether or not to optimize CDNI for HTTP
Adaptive Streaming (HAS). We highlight specific issues related to
delivering HAS content throughout this document, but for a more
thorough treatment of the topic, see [RFC6983].
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1.3. Structure of This Document
The remainder of this document is organized as follows:
o Section 2 describes some essential building blocks for CDNI,
notably the various options for redirecting user requests to a
given CDN.
o Section 3 provides a number of illustrative examples of various
CDNI operations.
o Section 4 describes the functionality of the main CDNI interfaces.
o Section 5 shows how various deployment models of CDNI may be
achieved using the defined interfaces.
o Section 6 describes the trust model of CDNI and the issues of
transitive trust in particular that CDNI raises.
2. Building Blocks
2.1. Request Redirection
At its core, CDNI requires the redirection of requests from one CDN
to another. For any given request that is received by an Upstream
CDN, it will either respond to the request directly, or somehow
redirect the request to a Downstream CDN. Two main mechanisms are
available for redirecting a request to a Downstream CDN. The first
leverages the DNS name resolution process and the second uses
application-layer redirection mechanisms such as the HTTP 302 or
Real-Time Streaming Protocol (RTSP) 302 redirection responses. While
there exists a large variety of application-layer protocols that
include some form of redirection mechanism, this document will use
HTTP (and HTTPS) in its examples. Similar mechanisms can be applied
to other application-layer protocols. What follows is a short
discussion of both DNS- and HTTP-based redirection, before presenting
some examples of their use in Section 3.
2.1.1. DNS Redirection
DNS redirection is based on returning different IP addresses for the
same DNS name, for example, to balance server load or to account for
the client's location in the network. A DNS server, sometimes called
the Local DNS (LDNS), resolves DNS names on behalf of an end user.
The LDNS server in turn queries other DNS servers until it reaches
the authoritative DNS server for the CDN-Domain. The network
operator typically provides the LDNS server, although the user is
free to choose other DNS servers (e.g., OpenDNS, Google Public DNS).
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This latter possibility is important because the authoritative DNS
server sees only the IP address of the DNS server that queries it,
not the IP address of the original end user.
The advantage of DNS redirection is that it is completely transparent
to the end user; the user sends a DNS name to the LDNS server and
gets back an IP address. On the other hand, DNS redirection is
problematic because the DNS request comes from the LDNS server, not
the end user. This may affect the accuracy of server selection that
is based on the user's location. The transparency of DNS redirection
is also a problem in that there is no opportunity to take the
attributes of the user agent or the URI path component into account.
We consider two main forms of DNS redirection: simple and CNAME-
based.
In simple DNS redirection, the authoritative DNS server for the name
simply returns an IP address from a set of possible IP addresses.
The answer is chosen from the set based on characteristics of the set
(e.g., the relative loads on the servers) or characteristics of the
client (e.g., the location of the client relative to the servers).
Simple redirection is straightforward. The only caveats are (1)
there is a limit to the number of alternate IP addresses a single DNS
server can manage; and (2) DNS responses are cached by Downstream
servers so the Time to Live (TTL) on the response must be set to an
appropriate value so as to preserve the freshness of the redirection.
In CNAME-based DNS redirection, the authoritative server returns a
CNAME response to the DNS request, telling the LDNS server to restart
the name lookup using a new name. A CNAME is essentially a symbolic
link in the DNS namespace, and like a symbolic link, redirection is
transparent to the client; the LDNS server gets the CNAME response
and re-executes the lookup. Only when the name has been resolved to
an IP address does it return the result to the user. Note that DNAME
would be preferable to CNAME if it becomes widely supported.
One of the advantages of DNS redirection compared to HTTP redirection
is that it can be cached, reducing load on the redirecting CDN's DNS
server. However, this advantage can also be a drawback, especially
when a given DNS resolver doesn't strictly adhere to the TTL, which
is a known problem in some real-world environments. In such cases,
an end user might end up at a dCDN without first having passed
through the uCDN, which might be an undesirable scenario from a uCDN
point of view.
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2.1.2. HTTP Redirection
HTTP redirection makes use of the redirection response of the HTTP
protocol (e.g.,"302" or "307"). This response contains a new URL
that the application should fetch instead of the original URL. By
changing the URL appropriately, the server can cause the user to
redirect to a different server. The advantages of HTTP redirection
are that (1) the server can change the URL fetched by the client to
include, for example, both the DNS name of the particular server to
use, as well as the original HTTP server that was being accessed; (2)
the client sends the HTTP request to the server, so that its IP
address is known and can be used in selecting the server; and (3)
other attributes (e.g., content type, user agent type) are visible to
the redirection mechanism.
Just as is the case for DNS redirection, there are some potential
disadvantages of using HTTP redirection. For example, it may affect
application behavior; web browsers will not send cookies if the URL
changes to a different domain. In addition, although this might also
be an advantage, results of HTTP redirection are not cached so that
all redirections must go through to the uCDN.
3. Overview of CDNI Operation
To provide a big-picture overview of the various components of CDNI,
we walk through a "day in the life" of a content item that is made
available via a pair of interconnected CDNs. This will serve to
illustrate many of the functions that need to be supported in a
complete CDNI solution. We give examples using both DNS-based and
HTTP-based redirection. We begin with very simple examples and then
show how additional capabilities, such as recursive request
redirection and content removal, might be added.
Before walking through the specific examples, we present a high-level
view of the operations that may take place. This high-level overview
is illustrated in Figure 2. Note that most operations will involve
only a subset of all the messages shown below, and that the order and
number of operations may vary considerably, as the more detailed
examples illustrate.
The following shows Operator A as the Upstream CDN (uCDN) and
Operator B as the Downstream CDN (dCDN), where the former has a
relationship with a content provider and the latter is the CDN
selected by Operator A to deliver content to the end user. The
interconnection relationship may be symmetric between these two CDN
operators, but each direction can be considered as operating
independently of the other; for simplicity, we show the interaction
in one direction only.
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End User Operator B Operator A
| | |
| | |
| | [Async FCI Push] | (1)
| | |
| | [MI pre-positioning] | (2)
| | |
| CONTENT REQUEST | |
|-------------------------------------------------->| (3)
| | |
| | [Sync RI Pull] | (4)
| | |
| CONTENT REQUEST REDIRECTION |
|<--------------------------------------------------| (5)
| | |
| | |
| CONTENT REQUEST | |
|------------------------>| | (6)
| | |
| | [Sync MI Pull] | (7)
| | |
| | ACQUISITION REQUEST |
| X------------------------>| (8)
| X |
| X CONTENT DATA |
| X<------------------------| (9)
| | |
| CONTENT DATA | |
|<------------------------| | (10)
| | |
: : :
: [Other content requests] :
: : :
| | [CI: Content Purge] | (11)
: : :
| | [LI: Log exchange] | (12)
| | |
Figure 2: Overview of Operation
The operations shown in the figure are as follows:
1. The dCDN uses the FCI to advertise information relevant to its
delivery footprint and capabilities prior to any content
requests being redirected.
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2. Prior to any content request, the uCDN uses the MI to pre-
position CDNI Metadata to the dCDN, thereby making that metadata
available in readiness for later content requests.
3. A content request from a user agent arrives at the uCDN.
4. The uCDN may use the RI to synchronously request information
from the dCDN regarding its delivery capabilities to decide if
the dCDN is a suitable target for redirection of this request.
5. The uCDN redirects the request to the dCDN by sending some
response (DNS, HTTP) to the user agent.
6. The user agent requests the content from the dCDN.
7. The dCDN may use the MI to synchronously request metadata
related to this content from uCDN, e.g., to decide whether to
serve it.
8. If the content is not already in a suitable cache in the dCDN,
the dCDN may acquire it from the uCDN.
9. The content is delivered to the dCDN from the uCDN.
10. The content is delivered to the user agent by the dCDN.
11. Some time later, perhaps at the request of the CSP (not shown)
the uCDN may use the CI to instruct the dCDN to purge the
content, thereby ensuring it is not delivered again.
12. After one or more content delivery actions by the dCDN, a log of
delivery actions may be provided to the uCDN using the LI.
The following sections show some more specific examples of how these
operations may be combined to perform various delivery, control, and
logging operations across a pair of CDNs.
3.1. Preliminaries
Initially, we assume that there is at least one CSP that has
contracted with an Upstream CDN (uCDN) to deliver content on its
behalf. We are not particularly concerned with the interface between
the CSP and uCDN, other than to note that it is expected to be the
same as in the "traditional" (non-interconnected) CDN case. Existing
mechanisms such as DNS CNAMEs or HTTP redirects (Section 2) can be
used to direct a user request for a piece of content from the CSP
towards the CSP's chosen Upstream CDN.
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We assume Operator A provides an Upstream CDN that serves content on
behalf of a CSP with CDN-Domain cdn.csp.example. We assume that
Operator B provides a Downstream CDN. An end user at some point
makes a request for URL
http://cdn.csp.example/...rest of URL...
It may well be the case that cdn.csp.example is just a CNAME for some
other CDN-Domain (such as csp.op-a.example). Nevertheless, the HTTP
request in the examples that follow is assumed to be for the example
URL above.
Our goal is to enable content identified by the above URL to be
served by the CDN of Operator B. In the following sections, we will
walk through some scenarios in which content is served as well as
other CDNI operations such as the removal of content from a
Downstream CDN.
3.2. Iterative HTTP Redirect Example
In this section, we walk through a simple, illustrative example using
HTTP redirection from a uCDN to a dCDN. The example also assumes the
use of HTTP redirection inside the uCDN and dCDN; however, this is
independent of the choice of redirection approach across CDNs, so an
alternative example could be constructed still showing HTTP
redirection from the uCDN to dCDN but using DNS for the handling of
the request inside each CDN.
For this example, we assume that Operators A and B have established
an agreement to interconnect their CDNs, with A being Upstream and B
being Downstream.
The operators agree that a CDN-Domain peer-a.op-b.example will be
used as the target of redirections from the uCDN to dCDN. We assume
the name of this domain is communicated by some means to each CDN.
(This could be established out of band or via a CDNI interface.) We
refer to this domain as a "distinguished" CDN-Domain to convey the
fact that its use is limited to the interconnection mechanism; such a
domain is never used directly by a CSP.
We assume the operators also agree on some distinguished CDN-Domain
that will be used for inter-CDN acquisition of the CSP's content from
the uCDN by the dCDN. In this example, we'll use
op-b-acq.op-a.example.
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We assume the operators also exchange information regarding which
requests the dCDN is prepared to serve. For example, the dCDN may be
prepared to serve requests from clients in a given geographical
region or a set of IP address prefixes. This information may again
be provided out of band or via a defined CDNI interface.
We assume DNS is configured in the following way:
o The content provider is configured to make Operator A the
authoritative DNS server for cdn.csp.example (or to return a CNAME
for cdn.csp.example for which Operator A is the authoritative DNS
server).
o Operator A is configured so that a DNS request for
op-b-acq.op-a.example returns a Request Router in Operator A.
o Operator B is configured so that a DNS request for
peer-a.op-b.example/cdn.csp.example returns a Request Router in
Operator B.
Figure 3 illustrates how a client request for
http://cdn.csp.example/...rest of URL...
is handled.
End User Operator B Operator A
|DNS cdn.csp.example | |
|-------------------------------------------------->|
| | |(1)
|IPaddr of A's Request Router |
|<--------------------------------------------------|
|HTTP cdn.csp.example | |
|-------------------------------------------------->|
| | |(2)
|302 peer-a.op-b.example/cdn.csp.example |
|<--------------------------------------------------|
|DNS peer-a.op-b.example | |
|------------------------>| |
| |(3) |
|IPaddr of B's Request Router |
|<------------------------| |
| | |
|HTTP peer-a.op-b.example/cdn.csp.example |
|------------------------>| |
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| |(4) |
|302 node1.peer-a.op-b.example/cdn.csp.example |
|<------------------------| |
|DNS node1.peer-a.op-b.example |
|------------------------>| |
| |(5) |
|IPaddr of B's Delivery Node |
|<------------------------| |
| | |
|HTTP node1.peer-a.op-b.example/cdn.csp.example |
|------------------------>| |
| |(6) |
| |DNS op-b-acq.op-a.example|
| |------------------------>|
| | |(7)
| |IPaddr of A's Request Router
| |<------------------------|
| |HTTP op-b-acq.op-a.example
| |------------------------>|
| | |(8)
| |302 node2.op-b-acq.op-a.example
| |<------------------------|
| |DNS node2.op-b-acq.op-a.example
| |------------------------>|
| | |(9)
| |IPaddr of A's Delivery Node
| |<------------------------|
| | |
| |HTTP node2.op-b-acq.op-a.example
| |------------------------>|
| | |(10)
| |Data |
| |<------------------------|
|Data | |
|<------------------------| |
Figure 3: Message Flow for Iterative HTTP Redirection
The steps illustrated in the figure are as follows:
1. A DNS resolver for Operator A processes the DNS request for its
customer based on CDN-Domain cdn.csp.example. It returns the IP
address of a Request Router in Operator A.
2. A Request Router for Operator A processes the HTTP request and
recognizes that the end user is best served by another CDN,
specifically one provided by Operator B, and so it returns a 302
redirect message for a new URL constructed by "stacking"
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Operator B's distinguished CDN-Domain (peer-a.op-b.example) on
the front of the original URL. (Note that more complex URL
manipulations are possible, such as replacing the initial CDN-
Domain by some opaque handle.)
3. The end user does a DNS lookup using Operator B's distinguished
CDN-Domain (peer-a.op-b.example). B's DNS resolver returns the
IP address of a Request Router for Operator B. Note that if
request routing within the dCDN was performed using DNS instead
of HTTP redirection, B's DNS resolver would also behave as the
Request Router and directly return the IP address of a delivery
node.
4. The Request Router for Operator B processes the HTTP request and
selects a suitable delivery node to serve the end-user request,
and it returns a 302 redirect message for a new URL constructed
by replacing the hostname with a subdomain of the Operator B's
distinguished CDN-Domain that points to the selected delivery
node.
5. The end user does a DNS lookup using Operator B's delivery node
subdomain (node1.peer-a.op-b.example). B's DNS resolver returns
the IP address of the delivery node.
6. The end user requests the content from B's delivery node. In
the case of a cache hit, steps 6, 7, 8, 9, and 10 below do not
happen, and the content data is directly returned by the
delivery node to the end user. In the case of a cache miss, the
content needs to be acquired by the dCDN from the uCDN (not the
CSP). The distinguished CDN-Domain peer-a.op-b.example
indicates to the dCDN that this content is to be acquired from
the uCDN; stripping the CDN-Domain reveals the original CDN-
Domain cdn.csp.example, and the dCDN may verify that this CDN-
Domain belongs to a known peer (so as to avoid being tricked
into serving as an open proxy). It then does a DNS request for
an inter-CDN acquisition CDN-Domain as agreed above (in this
case, op-b-acq.op-a.example).
7. Operator A's DNS resolver processes the DNS request and returns
the IP address of a Request Router in Operator A.
8. The Request Router for Operator A processes the HTTP request
from Operator B's delivery node. Operator A's Request Router
recognizes that the request is from a peer CDN rather than an
end user because of the dedicated inter-CDN acquisition domain
(op-b-acq.op-a.example). (Note that without this specially
defined inter-CDN acquisition domain, Operator A would be at
risk of redirecting the request back to Operator B, resulting in
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an infinite loop). The Request Router for Operator A selects a
suitable delivery node in uCDN to serve the inter-CDN
acquisition request and returns a 302 redirect message for a new
URL constructed by replacing the hostname with a subdomain of
the Operator A's distinguished inter-CDN acquisition domain that
points to the selected delivery node.
9. Operator A's DNS resolver processes the DNS request and returns
the IP address of the delivery node in Operator A.
10. Operator B requests (acquires) the content from Operator A.
Although not shown, Operator A processes the rest of the URL: it
extracts information identifying the origin server, validates
that this server has been registered, and determines the content
provider that owns the origin server. It may also perform its
own content acquisition steps if needed before returning the
content to dCDN.
The main advantage of this design is that it is simple: each CDN need
only know the distinguished CDN-Domain for each peer, with the
Upstream CDN "pushing" the Downstream CDN-Domain onto the URL as part
of its redirect (step 2), and the Downstream CDN "popping" its CDN-
Domain off the URL to expose a CDN-Domain that the Upstream CDN can
correctly process. Neither CDN need be aware of the internal
structure of the other's URLs. Moreover, the inter-CDN redirection
is entirely supported by a single HTTP redirect; neither CDN need be
aware of the other's internal redirection mechanism (i.e., whether it
is DNS or HTTP based).
One disadvantage is that the end user's browser is redirected to a
new URL that is not in the same domain of the original URL. This has
implications on a number of security or validation mechanisms
sometimes used on endpoints. For example, it is important that any
redirected URL be in the same domain (e.g., csp.example) if the
browser is expected to send any cookies associated with that domain.
As another example, some video players enforce validation of a cross-
domain policy that needs to accommodate the domains involved in the
CDN redirection. These problems are generally solvable, but the
solutions complicate the example, so we do not discuss them further
in this document.
We note that this example begins to illustrate some of the interfaces
that may be required for CDNI, but it does not require all of them.
For example, obtaining information from a dCDN regarding the set of
client IP addresses or geographic regions it might be able to serve
is an aspect of request routing (specifically of the CDNI Footprint &
Capabilities Advertisement interface). Important configuration
information such as the distinguished names used for redirection and
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inter-CDN acquisition could also be conveyed via a CDNI interface
(e.g., perhaps the CDNI Control interface). The example also shows
how existing HTTP-based methods suffice for the acquisition
interface. Arguably, the absolute minimum metadata required for CDNI
is the information required to acquire the content, and this
information was provided "in-band" in this example by means of the
URI handed to the client in the HTTP 302 response. The example also
assumes that the CSP does not require any distribution policy (e.g.,
time window or geo-blocking) or delivery processing to be applied by
the interconnected CDNs. Hence, there is no explicit CDNI Metadata
interface invoked in this example. There is also no explicit CDNI
Logging interface discussed in this example.
We also note that the step of deciding when a request should be
redirected to the dCDN rather than served by the uCDN has been
somewhat glossed over. It may be as simple as checking the client IP
address against a list of prefixes, or it may be considerably more
complex, involving a wide range of factors, such as the geographic
location of the client (perhaps determined from a third-party
service), CDN load, or specific business rules.
This example uses the "iterative" CDNI request redirection approach.
That is, a uCDN performs part of the request redirection function by
redirecting the client to a Request Router in the dCDN, which then
performs the rest of the redirection function by redirecting to a
suitable Surrogate. If request routing is performed in the dCDN
using HTTP redirection, this translates in the end user experiencing
two successive HTTP redirections. By contrast, the alternative
approach of "recursive" CDNI request redirection effectively
coalesces these two successive HTTP redirections into a single one,
sending the end user directly to the right delivery node in the dCDN.
This "recursive" CDNI request routing approach is discussed in the
next section.
While the example above uses HTTP, the iterative HTTP redirection
mechanism would work over HTTPS in a similar fashion. In order to
make sure an end user's HTTPS request is not downgraded to HTTP along
the redirection path, it is necessary for every Request Router along
the path from the initial uCDN Request Router to the final Surrogate
in the dCDN to respond to an incoming HTTPS request with an HTTP
redirect containing an HTTPS URL. It should be noted that using
HTTPS will have the effect of increasing the total redirection
process time and increasing the load on the Request Routers,
especially when the redirection path includes many redirects and thus
many Secure Socket Layer/Transport Layer Security (SSL/TLS) sessions.
In such cases, a recursive HTTP redirection mechanism, as described
in an example in the next section, might help to reduce some of these
issues.
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3.3. Recursive HTTP Redirection Example
The following example builds on the previous one to illustrate the
use of the request routing interface (specifically, the CDNI Request
Routing Redirection interface) to enable "recursive" CDNI request
routing. We build on the HTTP-based redirection approach because it
illustrates the principles and benefits clearly, but it is equally
possible to perform recursive redirection when DNS-based redirection
is employed.
In contrast to the prior example, the operators need not agree in
advance on a CDN-Domain to serve as the target of redirections from
the uCDN to dCDN. We assume that the operators agree on some
distinguished CDN-Domain that will be used for inter-CDN acquisition
of the CSP's content by dCDN. In this example, we'll use
op-b-acq.op-a.example.
We assume the operators also exchange information regarding which
requests the dCDN is prepared to serve. For example, the dCDN may be
prepared to serve requests from clients in a given geographical
region or a set of IP address prefixes. This information may again
be provided out of band or via a defined protocol.
We assume DNS is configured in the following way:
o The content provider is configured to make Operator A the
authoritative DNS server for cdn.csp.example (or to return a CNAME
for cdn.csp.example for which Operator A is the authoritative DNS
server).
o Operator A is configured so that a DNS request for
op-b-acq.op-a.example returns a Request Router in Operator A.
o Operator B is configured so that a request for node1.op-b.example/
cdn.csp.example returns the IP address of a delivery node. Note
that there might be a number of such delivery nodes.
Figure 3 illustrates how a client request for
http://cdn.csp.example/...rest of URL...
is handled.
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End User Operator B Operator A
|DNS cdn.csp.example | |
|-------------------------------------------------->|
| | |(1)
|IPaddr of A's Request Router |
|<--------------------------------------------------|
|HTTP cdn.csp.example | |
|-------------------------------------------------->|
| | |(2)
| |RR/RI REQ cdn.csp.example|
| |<------------------------|
| | |
| |RR/RI RESP node1.op-b.example
| |------------------------>|
| | |(3)
|302 node1.op-b.example/cdn.csp.example |
|<--------------------------------------------------|
|DNS node1.op-b.example | |
|------------------------>| |
| |(4) |
|IPaddr of B's Delivery Node |
|<------------------------| |
|HTTP node1.op-b.example/cdn.csp.example |
|------------------------>| |
| |(5) |
| |DNS op-b-acq.op-a.example|
| |------------------------>|
| | |(6)
| |IPaddr of A's Request Router
| |<------------------------|
| |HTTP op-b-acq.op-a.example
| |------------------------>|
| | |(7)
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| |302 node2.op-b-acq.op-a.example
| |<------------------------|
| |DNS node2.op-b-acq.op-a.example
| |------------------------>|
| | |(8)
| |IPaddr of A's Delivery Node
| |<------------------------|
| | |
| |HTTP node2.op-b-acq.op-a.example
| |------------------------>|
| | |(9)
| |Data |
| |<------------------------|
|Data | |
|<------------------------| |
Figure 4: Message Flow for Recursive HTTP Redirection
The steps illustrated in the figure are as follows:
1. A DNS resolver for Operator A processes the DNS request for its
customer based on CDN-Domain cdn.csp.example. It returns the IP
address of a Request Router in Operator A.
2. A Request Router for Operator A processes the HTTP request and
recognizes that the end user is best served by another CDN --
specifically one provided by Operator B -- so it queries the CDNI
Request Routing Redirection interface of Operator B, providing a
set of information about the request including the URL requested.
Operator B replies with the DNS name of a delivery node.
3. Operator A returns a 302 redirect message for a new URL obtained
from the RI.
4. The end user does a DNS lookup using the hostname of the URL just
provided (node1.op-b.example). B's DNS resolver returns the IP
address of the corresponding delivery node. Note that, since the
name of the delivery node was already obtained from B using the
RI, there should not be any further redirection here (in contrast
to the iterative method described above.)
5. The end user requests the content from B's delivery node,
potentially resulting in a cache miss. In the case of a cache
miss, the content needs to be acquired from the uCDN (not the
CSP.) The distinguished CDN-Domain op-b.example indicates to the
dCDN that this content is to be acquired from another CDN;
stripping the CDN-Domain reveals the original CDN-Domain
cdn.csp.example. The dCDN may verify that this CDN-Domain
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belongs to a known peer (so as to avoid being tricked into
serving as an open proxy). It then does a DNS request for the
inter-CDN Acquisition "distinguished" CDN-Domain as agreed above
(in this case, op-b-acq.op-a.example).
6. Operator A's DNS resolver processes the DNS request and returns
the IP address of a Request Router in Operator A.
7. The Request Router for Operator A processes the HTTP request from
Operator B's delivery node. Operator A's Request Router
recognizes that the request is from a peer CDN rather than an end
user because of the dedicated inter-CDN acquisition domain
(op-b-acq.op-a.example). (Note that without this specially
defined inter-CDN acquisition domain, Operator A would be at risk
of redirecting the request back to Operator B, resulting in an
infinite loop). The Request Router for Operator A selects a
suitable delivery node in the uCDN to serve the inter-CDN
acquisition request and returns a 302 redirect message for a new
URL constructed by replacing the hostname with a subdomain of the
Operator A's distinguished inter-CDN acquisition domain that
points to the selected delivery node.
8. Operator A recognizes that the DNS request is from a peer CDN
rather than an end user (due to the internal CDN-Domain) and so
returns the address of a delivery node. (Note that without this
specially defined internal domain, Operator A would be at risk of
redirecting the request back to Operator B, resulting in an
infinite loop.)
9. Operator B requests (acquires) the content from Operator A.
Operator A serves content for the requested CDN-Domain to the
dCDN. Although not shown, it is at this point that Operator A
processes the rest of the URL: it extracts information
identifying the origin server, validates that this server has
been registered, and determines the content provider that owns
the origin server. It may also perform its own content
acquisition steps if needed before returning the content to the
dCDN.
Recursive redirection has the advantage (over iterative redirection)
of being more transparent from the end user's perspective but the
disadvantage of each CDN exposing more of its internal structure (in
particular, the addresses of edge caches) to peer CDNs. By contrast,
iterative redirection does not require the dCDN to expose the
addresses of its edge caches to the uCDN.
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This example happens to use HTTP-based redirection in both CDN A and
CDN B, but a similar example could be constructed using DNS-based
redirection in either CDN. Hence, the key point to take away here is
simply that the end user only sees a single redirection of some type,
as opposed to the pair of redirections in the prior (iterative)
example.
The use of the RI requires that the request routing mechanism be
appropriately configured and bootstrapped, which is not shown here.
More discussion on the bootstrapping of interfaces is provided in
Section 4
3.4. Iterative DNS-Based Redirection Example
In this section we walk through a simple example using DNS-based
redirection for request redirection from the uCDN to the dCDN (as
well as for request routing inside the dCDN and the uCDN). As noted
in Section 2.1, DNS-based redirection has certain advantages over
HTTP-based redirection (notably, it is transparent to the end user)
as well as some drawbacks (notably, the client IP address is not
visible to the Request Router).
As before, Operator A has to learn the set of requests that the dCDN
is willing or able to serve (e.g., which client IP address prefixes
or geographic regions are part of the dCDN footprint). We assume
Operator B has and makes known to Operator A some unique identifier
that can be used for the construction of a distinguished CDN-Domain,
as shown in more detail below. (This identifier strictly needs only
to be unique within the scope of Operator A, but a globally unique
identifier, such as an Autonomous System (AS) number assigned to B,
is one easy way to achieve that.) Also, Operator A obtains the NS
records for Operator B's externally visible redirection servers.
Also, as before, a distinguished CDN-Domain, such as
op-b-acq.op-a.example, must be assigned for inter-CDN acquisition.
We assume DNS is configured in the following way:
o The CSP is configured to make Operator A the authoritative DNS
server for cdn.csp.example (or to return a CNAME for
cdn.csp.example for which Operator A is the authoritative DNS
server).
o When uCDN sees a request best served by the dCDN, it returns CNAME
and NS records for "b.cdn.csp.example", where "b" is the unique
identifier assigned to Operator B. (It may, for example, be an AS
number assigned to Operator B.)
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o The dCDN is configured so that a request for "b.cdn.csp.example"
returns a delivery node in the dCDN.
o The uCDN is configured so that a request for
"op-b-acq.op-a.example" returns a delivery node in the uCDN.
Figure 5 depicts the exchange of DNS and HTTP requests. The main
differences from Figure 3 are the lack of HTTP redirection and
transparency to the end user.
End User Operator B Operator A
|DNS cdn.csp.example | |
|-------------------------------------------------->|
| | |(1)
|CNAME b.cdn.csp.example | |
|<--------------------------------------------------|
| | |
|DNS b.cdn.csp.example | |
|-------------------------------------------------->|
| | |(2)
|NS records for b.cdn.csp.example + |
|Glue AAAA/A records for b.cdn.csp.example |
|<--------------------------------------------------|
| | |
|DNS b.cdn.csp.example | |
|------------------------>| |
| |(3) |
|IPaddr of B's Delivery Node |
|<------------------------| |
|HTTP cdn.csp.example | |
|------------------------>| |
| |(4) |
| |DNS op-b-acq.op-a.example|
| |------------------------>|
| | |(5)
| |IPaddr of A's Delivery Node
| |<------------------------|
| |HTTP op-b-acq.op-a.example
| |------------------------>|
| | |(6)
| |Data |
| |<------------------------|
|Data | |
|<------------------------| |
Figure 5: Message Flow for DNS-Based Redirection
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The steps illustrated in the figure are as follows:
1. The Request Router for Operator A processes the DNS request for
CDN-Domain cdn.csp.example and recognizes that the end user is
best served by another CDN. (This may depend on the IP address
of the user's LDNS resolver, or other information discussed
below.) The Request Router returns a DNS CNAME response by
"stacking" the distinguished identifier for Operator B onto the
original CDN-Domain (e.g., b.cdn.csp.example).
2. The end user sends a DNS query for the modified CDN-Domain (i.e.,
b.cdn.csp.example) to Operator A's DNS server. The Request
Router for Operator A processes the DNS request and returns a
delegation to b.cdn.csp.example by sending an NS record plus glue
records pointing to Operator B's DNS server. (This extra step is
necessary since typical DNS implementation won't follow an NS
record when it is sent together with a CNAME record, thereby
necessitating a two-step approach.)
3. The end user sends a DNS query for the modified CDN-Domain (i.e.,
b.cdn.csp.example) to Operator B's DNS server, using the NS and
AAAA/A records received in step 2. This causes B's Request
Router to respond with a suitable delivery node.
4. The end user requests the content from B's delivery node. The
requested URL contains the name cdn.csp.example. (Note that the
returned CNAME does not affect the URL.) At this point, the
delivery node has the correct IP address of the end user and can
do an HTTP 302 redirect if the redirections in steps 2 and 3 were
incorrect. Otherwise, B verifies that this CDN-Domain belongs to
a known peer (so as to avoid being tricked into serving as an
open proxy). It then does a DNS request for an "internal" CDN-
Domain as agreed above (op-b-acq.op-a.example).
5. Operator A recognizes that the DNS request is from a peer CDN
rather than an end user (due to the internal CDN-Domain) and so
returns the address of a delivery node in uCDN.
6. Operator A serves content to dCDN. Although not shown, it is at
this point that Operator A processes the rest of the URL: it
extracts information identifying the origin server, validates
that this server has been registered, and determines the content
provider that owns the origin server.
The advantages of this approach are that it is more transparent to
the end user and requires fewer round trips than HTTP-based
redirection (in its worst case, i.e., when none of the needed DNS
information is cached). A potential problem is that the Upstream CDN
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depends on being able to learn the correct Downstream CDN that serves
the end user from the client address in the DNS request. In standard
DNS operation, the uCDN will only obtain the address of the client's
LDNS resolver, which is not guaranteed to be in the same network (or
geographic region) as the client. If not (e.g., the end user uses a
global DNS service), then the Upstream CDN cannot determine the
appropriate Downstream CDN to serve the end user. In this case, and
assuming the uCDN is capable of detecting that situation, one option
is for the Upstream CDN to treat the end user as it would any user
not connected to a peer CDN. Another option is for the Upstream CDN
to "fall back" to a pure HTTP-based redirection strategy in this case
(i.e., use the first method). Note that this problem affects
existing CDNs that rely on DNS to determine where to redirect client
requests, but the consequences are arguably less serious for CDNI
since the LDNS is likely in the same network as the dCDN serves.
As with the prior example, this example partially illustrates the
various interfaces involved in CDNI. Operator A could learn
dynamically from Operator B the set of prefixes or regions that B is
willing and able to serve via the CDNI Footprint & Capabilities
Advertisement interface. The distinguished name used for acquisition
and the identifier for Operator B that is prepended to the CDN-Domain
on redirection are examples of information elements that might also
be conveyed by CDNI interfaces (or, alternatively, statically
configured). As before, minimal metadata sufficient to obtain the
content is carried "in-band" as part of the redirection process, and
standard HTTP is used for inter-CDN acquisition. There is no
explicit CDNI Logging interface discussed in this example.
3.4.1. Notes on Using DNSSEC
Although it is possible to use DNSSEC in combination with the
Iterative DNS-based Redirection mechanism explained above, it is
important to note that the uCDN might have to sign records on the
fly, since the CNAME returned, and thus the signature provided, can
potentially be different for each incoming query. Although there is
nothing preventing a uCDN from performing such on-the-fly signing,
this might be computationally expensive. In the case where the
number of dCDNs, and thus the number of different CNAMEs to return,
is relatively stable, an alternative solution would be for the uCDN
to pre-generate signatures for all possible CNAMEs. For each
incoming query, the uCDN would then determine the appropriate CNAME
and return it together with the associated pre-generated signature.
Note: In the latter case, maintaining the serial number and signature
of Start of Authority (SOA) might be an issue since, technically, it
should change every time a different CNAME is used. However, since,
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in practice, direct SOA queries are relatively rare, a uCDN could
defer incrementing the serial number and resigning the SOA until it
is queried and then do it on-the-fly.
Note also that the NS record and the glue records used in step 2 in
the previous section should generally be identical to those of their
authoritative zone managed by Operator B. Even if they differ, this
will not make the DNS resolution process fail, but the client DNS
server will prefer the authoritative data in its cache and use it for
subsequent queries. Such inconsistency is a general operational
issue of DNS, but it may be more important for this architecture
because the uCDN (Operator A) would rely on the consistency to make
the resulting redirection work as intended. In general, it is the
administrator's responsibility to make them consistent.
3.5. Dynamic Footprint Discovery Example
There could be situations where being able to dynamically discover
the set of requests that a given dCDN is willing and able to serve is
beneficial. For example, a CDN might at one time be able to serve a
certain set of client IP prefixes, but that set might change over
time due to changes in the topology and routing policies of the IP
network. The following example illustrates this capability. We have
chosen the example of DNS-based redirection, but HTTP-based
redirection could equally well use this approach.
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End User Operator B Operator A
|DNS cdn.csp.example | |
|-------------------------------------------------->|
| | |(1)
| | RI REQ op-b.example |
| |<------------------------|
| | |(2)
| | RI REPLY |
| |------------------------>|
| | |(3)
|CNAME b.cdn.csp.example | |
|NS records for b.cdn.csp.example |
|<--------------------------------------------------|
|DNS b.cdn.csp.example | |
|------------------------>| |
| |(2) |
|IPaddr of B's Delivery Node |
|<------------------------| |
|HTTP cdn.csp.example | |
|------------------------>| |
| |(3) |
| |DNS op-b-acq.op-a.example|
| |------------------------>|
| | |(4)
| |IPaddr of A's Delivery Node
| |<------------------------|
| |HTTP op-b-acq.op-a.example
| |------------------------>|
| | |(5)
| |Data |
| |<------------------------|
|Data | |
|<------------------------| |
Figure 6: Message Flow for Dynamic Footprint Discovery
This example differs from the one in Figure 5 only in the addition of
an RI request (step 2) and corresponding response (step 3). The RI
REQ could be a message such as "Can you serve clients from this IP
Prefix?" or it could be "Provide the list of client IP prefixes you
can currently serve". In either case the response might be cached by
Operator A to avoid repeatedly asking the same question.
Alternatively, or in addition, Operator B may spontaneously advertise
to Operator A information (or changes) on the set of requests it is
willing and able to serve on behalf of Operator A; in that case,
Operator B may spontaneously issue RR/RI REPLY messages that are not
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in direct response to a corresponding RR/RI REQ message. (Note that
the issues of determining the client's subnet from DNS requests, as
described above, are exactly the same here as in Section 3.4.)
Once Operator A obtains the RI response, it is now able to determine
that Operator B's CDN is an appropriate dCDN for this request and
therefore a valid candidate dCDN to consider in its redirection
decision. If that dCDN is selected, the redirection and serving of
the request proceeds as before (i.e., in the absence of dynamic
footprint discovery).
3.6. Content Removal Example
The following example illustrates how the CDNI Control interface may
be used to achieve pre-positioning of an item of content in the dCDN.
In this example, user requests for a particular content, and
corresponding redirection of such requests from Operator A to
Operator B CDN, may (or may not) have taken place earlier. Then, at
some point in time, the uCDN (for example, in response to a
corresponding Trigger from the Content Provider) uses the CI to
request that content identified by a particular URL be removed from
dCDN. The following diagram illustrates the operation. It should be
noted that a uCDN will typically not know whether a dCDN has cached a
given content item; however, it may send the content removal request
to make sure no cached versions remain to satisfy any contractual
obligations it may have.
End User Operator B Operator A
| |CI purge cdn.csp.example/...
| |<------------------------|
| | |
| |CI OK |
| |------------------------>|
| | |
Figure 7: Message Flow for Content Removal
The CI is used to convey the request from the uCDN to the dCDN that
some previously acquired content should be deleted. The URL in the
request specifies which content to remove. This example corresponds
to a DNS-based redirection scenario such as Section 3.4. If HTTP-
based redirection had been used, the URL for removal would be of the
form peer-a.op-b.example/cdn.csp.example/...
The dCDN is expected to confirm to the uCDN, as illustrated by the CI
OK message, the completion of the removal of the targeted content
from all the caches in the dCDN.
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3.7. Pre-positioned Content Acquisition Example
The following example illustrates how the CI may be used to pre-
position an item of content in the dCDN. In this example, Operator A
uses the CDNI Metadata interface to request that content identified
by a particular URL be pre-positioned into Operator B CDN.
End User Operator B Operator A
| |CI pre-position cdn.csp.example/...
| |<------------------------|
| | |(1)
| |CI OK |
| |------------------------>|
| | |
| |DNS op-b-acq.op-a.example|
| |------------------------>|
| | |(2)
| |IPaddr of A's Delivery Node
| |<------------------------|
| |HTTP op-b-acq.op-a.example
| |------------------------>|
| | |(3)
| |Data |
| |<------------------------|
|DNS cdn.csp.example | |
|--------------------------------------------->|
| | |(4)
|IPaddr of A's Request Router |
|<---------------------------------------------|
|HTTP cdn.csp.example| |
|--------------------------------------------->|
| | |(5)
|302 peer-a.op-b.example/cdn.csp.example |
|<---------------------------------------------|
|DNS peer-a.op-b.example |
|------------------->| |
| |(6) |
|IPaddr of B's Delivery Node |
|<-------------------| |
|HTTP peer-a.op-b.example/cdn.csp.example |
|------------------->| |
| |(7) |
|Data | |
|<-------------------| |
Figure 8: Message Flow for Content Pre-Positioning
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The steps illustrated in the figure are as follows:
1. Operator A uses the CI to request that Operator B pre-position a
particular content item identified by its URL. Operator B
responds by confirming that it is willing to perform this
operation.
Steps 2 and 3 are exactly the same as steps 5 and 6 of Figure 3, only
this time those steps happen as the result of the Pre-positioning
request instead of as the result of a cache miss.
Steps 4, 5, 6, and 7 are exactly the same as steps 1, 2, 3, and 4 of
Figure 3, only this time, Operator B's CDN can serve the end-user
request without triggering dynamic content acquisition, since the
content has been pre-positioned in the dCDN. Note that, depending on
dCDN operations and policies, the content pre-positioned in the dCDN
may be pre-positioned to all, or a subset of, dCDN caches. In the
latter case, intra-CDN dynamic content acquisition may take place
inside the dCDN serving requests from caches on which the content has
not been pre-positioned; however, such intra-CDN dynamic acquisition
would not involve the uCDN.
3.8. Asynchronous CDNI Metadata Example
In this section, we walk through a simple example illustrating a
scenario of asynchronously exchanging CDNI Metadata, where the
Downstream CDN obtains CDNI Metadata for content ahead of a
corresponding content request. The example that follows assumes that
HTTP-based inter-CDN redirection and recursive CDNI request routing
are used, as in Section 3.3. However, Asynchronous exchange of CDNI
Metadata is similarly applicable to DNS-based inter-CDN redirection
and iterative request routing (in which cases the CDNI Metadata may
be used at slightly different processing stages of the message
flows).
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End User Operator B Operator A
| | |
| |CI pre-position (Trigger)|
| |<------------------------|(1)
| | |
| |CI OK |
| |------------------------>|(2)
| | |
| |MI pull REQ |
| |------------------------>|(3)
| | |
| |MI metadata REP |(4)
| | |
| | |
| CONTENT REQUEST | |
|-------------------------------------------------->|(5)
| | |
| | RI REQ |
| |<------------------------|(6)
| | |
| | RI RESP |
| |------------------------>|(7)
| | |
| CONTENT REDIRECTION | |
|<--------------------------------------------------|(8)
| | |
| CONTENT REQUEST | |
|------------------------>|(9) |
| | |
: : :
| CONTENT DATA | |
|<------------------------| |(10)
Figure 9: Message Flow for Asynchronous CDNI Metadata
The steps illustrated in the figure are as follows:
1. Operator A uses the CI to trigger the signaling of the
availability of CDNI Metadata to Operator B.
2. Operator B acknowledges the receipt of this Trigger.
3. Operator B requests the latest metadata from Operator A using
the MI.
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4. Operator A replies with the requested metadata. This document
does not constrain how the CDNI Metadata information is actually
represented. For the purposes of this example, we assume that
Operator A provides CDNI Metadata to Operator B indicating that:
* this CDNI Metadata is applicable to any content referenced by
some CDN-Domain.
* this CDNI Metadata consists of a distribution policy
requiring enforcement by the delivery node of a specific per-
request authorization mechanism (e.g., URI signature or token
validation).
5. A Content Request occurs as usual.
6. A CDNI Request Routing Redirection request (RI REQ) is issued by
Operator A's CDN, as discussed in Section 3.3. Operator B's
Request Router can access the CDNI Metadata that are relevant to
the requested content and that have been pre-positioned as per
Steps 1-4, which may or may not affect the response.
7. Operator B's Request Router issues a CDNI Request Routing
Redirection response (RI RESP) as in Section 3.3.
8. Operator B performs content redirection as discussed in
Section 3.3.
9. On receipt of the Content Request by the end user, the delivery
node detects that previously acquired CDNI Metadata is
applicable to the requested content. In accordance with the
specific CDNI metadata of this example, the delivery node will
invoke the appropriate per-request authorization mechanism,
before serving the content. (Details of this authorization are
not shown.)
10. Assuming successful per-request authorization, serving of
Content Data (possibly preceded by inter-CDN acquisition)
proceeds as in Section 3.3.
3.9. Synchronous CDNI Metadata Acquisition Example
In this section we walk through a simple example illustrating a
scenario of Synchronous CDNI Metadata acquisition, in which the
Downstream CDN obtains CDNI Metadata for content at the time of
handling a first request for the corresponding content. As in the
preceding section, this example assumes that HTTP-based inter-CDN
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redirection and recursive CDNI request routing are used (as in
Section 3.3), but dynamic CDNI Metadata acquisition is applicable to
other variations of request routing.
End User Operator B Operator A
| | |
| CONTENT REQUEST | |
|-------------------------------------------------->|(1)
| | |
| | RI REQ |
| (2)|<------------------------|
| | |
| | MI REQ |
| (3)|------------------------>|
| | MI RESP |
| |<------------------------|(4)
| | |
| | RI RESP |
| |------------------------>|(5)
| | |
| | |
| CONTENT REDIRECTION | |
|<--------------------------------------------------|(6)
| | |
| CONTENT REQUEST | |
|------------------------>|(7) |
| | |
| | MI REQ |
| (8)|------------------------>|
| | MI RESP |
| |<------------------------|(9)
| | |
: : :
| CONTENT DATA | |
|<------------------------| |(10)
Figure 10: Message Flow for Synchronous CDNI Metadata Acquisition
The steps illustrated in the figure are as follows:
1. A Content Request arrives as normal.
2. An RI request occurs as in the prior example.
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3. On receipt of the CDNI Request Routing Request, Operator B's CDN
initiates Synchronous acquisition of CDNI Metadata that are
needed for routing of the end-user request. We assume the URI
for the a metadata server is known ahead of time through some
out-of-band means.
4. On receipt of a CDNI Metadata request, Operator A's CDN
responds, making the corresponding CDNI Metadata information
available to Operator B's CDN. This metadata is considered by
Operator B's CDN before responding to the Request Routing
request. (In a simple case, the metadata could simply be an
allow or deny response for this particular request.)
5. Response to the RI request as normal.
6. Redirection message is sent to the end user.
7. A delivery node of Operator B receives the end user request.
8. The delivery node Triggers dynamic acquisition of additional
CDNI Metadata that are needed to process the end-user content
request. Note that there may exist cases where this step need
not happen, for example, because the metadata were already
acquired previously.
9. Operator A's CDN responds to the CDNI Metadata Request and makes
the corresponding CDNI Metadata available to Operator B. This
metadata influence how Operator B's CDN processes the end-user
request.
10. Content is served (possibly preceded by inter-CDN acquisition)
as in Section 3.3.
3.10. Content and Metadata Acquisition with Multiple Upstream CDNs
A single dCDN may receive end-user requests from multiple uCDNs.
When a dCDN receives an end-user request, it must determine the
identity of the uCDN from which it should acquire the requested
content.
Ideally, the acquisition path of an end-user request will follow the
redirection path of the request. The dCDN should acquire the content
from the same uCDN that redirected the request.
Determining the acquisition path requires the dCDN to reconstruct the
redirection path based on information in the end-user request. The
method for reconstructing the redirection path differs based on the
redirection approach: HTTP or DNS.
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With HTTP-redirection, the rewritten URI should include sufficient
information for the dCDN to directly or indirectly determine the uCDN
when the end-user request is received. The HTTP-redirection approach
can be further broken-down based on the how the URL is rewritten
during redirection: HTTP redirection with or without Site
Aggregation. HTTP redirection with Site Aggregation hides the
identity of the original CSP. HTTP redirection without Site
Aggregation does not attempt to hide the identity of the original
CSP. With both approaches, the rewritten URI includes enough
information to identify the immediate neighbor uCDN.
With DNS-redirection, the dCDN receives the published URI (instead of
a rewritten URI) and does not have sufficient information for the
dCDN to identify the appropriate uCDN. The dCDN may narrow the set
of viable uCDNs by examining the CDNI Metadata from each to determine
which uCDNs are hosting metadata for the requested content. If there
is a single uCDN hosting metadata for the requested content, the dCDN
can assume that the request redirection is coming from this uCDN and
can acquire content from that uCDN. If there are multiple uCDNs
hosting metadata for the requested content, the dCDN may be ready to
trust any of these uCDNs to acquire the content (provided the uCDN is
in a position to serve it). If the dCDN is not ready to trust any of
these uCDNs, it needs to ensure via out of band arrangements that,
for a given content, only a single uCDN will ever redirect requests
to the dCDN.
Content acquisition may be preceded by content metadata acquisition.
If possible, the acquisition path for metadata should also follow the
redirection path. Additionally, we assume metadata is indexed based
on rewritten URIs in the case of HTTP redirection and is indexed
based on published URIs in the case of DNS-redirection. Thus, the RI
and the MI are tightly coupled in that the result of request routing
(a rewritten URI pointing to the dCDN) serves as an input to metadata
lookup. If the content metadata includes information for acquiring
the content, then the MI is also tightly coupled with the acquisition
interface in that the result of the metadata lookup (an acquisition
URL likely hosted by the uCDN) should serve as input to the content
acquisition.
4. Main Interfaces
Figure 1 illustrates the main interfaces that are in scope for the
CDNI WG, along with several others. The detailed specifications of
these interfaces are left to other documents, but see [RFC6707] and
[RFC7337] for some discussion of the interfaces.
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One interface that is not shown in Figure 1 is the interface between
the user and the CSP. While for the purposes of CDNI that interface
is out of scope, it is worth noting that it does exist and can
provide useful functions, such as end-to-end performance monitoring
and some forms of authentication and authorization.
There is also an important interface between the user and the Request
Routing function of both uCDN and dCDN (shown as the "Request"
interface in Figure 1). As we saw in some of the preceding examples,
that interface can be used as a way of passing metadata, such as the
minimum information that is required for dCDN to obtain the content
from the uCDN.
In this section we will provide an overview of the functions
performed by each of the CDNI interfaces and discuss how they fit
into the overall solution. We also examine some of the design trade-
offs, and explore several cross-interface concerns. We begin with an
examination of one such trade-off that affects all the interfaces --
the use of in-band or out-of-band communication.
4.1. In-Band versus Out-of-Band Interfaces
Before getting to the individual interfaces, we observe that there is
a high-level design choice for each, involving the use of existing
in-band communication channels versus defining new out-of-band
interfaces.
It is possible that the information needed to carry out various
interconnection functions can be communicated between peer CDNs using
existing in-band protocols. The use of HTTP 302 redirect is an
example of how certain aspects of request routing can be implemented
in-band (embedded in URIs). Note that using existing in-band
protocols does not imply that the CDNI interfaces are null; it is
still necessary to establish the rules (conventions) by which such
protocols are used to implement the various interface functions.
There are other opportunities for in-band communication beyond HTTP
redirects. For example, many of the HTTP directives used by proxy
servers can also be used by peer CDNs to inform each other of caching
activity. Of these, one that is particularly relevant is the
If-Modified-Since directive, which is used with the GET method to
make it conditional: if the requested object has not been modified
since the time specified in this field, a copy of the object will not
be returned, and instead, a 304 (not modified) response will be
returned.
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4.2. Cross-Interface Concerns
Although the CDNI interfaces are largely independent, there are a set
of conventions practiced consistently across all interfaces. Most
important among these is how resources are named, for example, how
the CDNI Metadata and Control interfaces identify the set of
resources to which a given directive applies or the CDNI Logging
interface identifies the set of resources for which a summary record
applies.
While, theoretically, the CDNI interfaces could explicitly identify
every individual resource, in practice, they name resource aggregates
(sets of URIs) that are to be treated in a similar way. For example,
URI aggregates can be identified by a CDN-Domain (i.e., the FQDN at
the beginning of a URI) or by a URI-Filter (i.e., a regular
expression that matches a subset of URIs contained in some CDN-
Domain). In other words, CDN-Domains and URI-Filters provide a
uniform means to aggregate sets (and subsets) of URIs for the purpose
of defining the scope for some operation in one of the CDNI
interfaces.
4.3. Request Routing Interfaces
The Request Routing interface comprises two parts: the Asynchronous
interface used by a dCDN to advertise footprint and capabilities
(denoted FCI) to a uCDN, allowing the uCDN to decide whether to
redirect particular user requests to that dCDN; and the Synchronous
interface used by the uCDN to redirect a user request to the dCDN
(denoted RI). (These are somewhat analogous to the operations of
routing and forwarding in IP.)
As illustrated in Section 3, the RI part of request routing may be
implemented in part by DNS and HTTP. Naming conventions may be
established by which CDN peers communicate whether a request should
be routed or content served.
We also note that RI plays a key role in enabling recursive
redirection, as illustrated in Section 3.3. It enables the user to
be redirected to the correct delivery node in dCDN with only a single
redirection step (as seen by the user). This may be particularly
valuable as the chain of interconnected CDNs increases beyond two
CDNs. For further discussion on the RI, see [REDIRECTION].
In support of these redirection requests, it is necessary for CDN
peers to exchange additional information with each other, and this is
the role of the FCI part of request routing. Depending on the
method(s) supported, this might include:
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o The operator's unique id (operator-id) or distinguished CDN-Domain
(operator-domain);
o NS records for the operator's set of externally visible Request
Routers;
o The set of requests the dCDN operator is prepared to serve (e.g.,
a set of client IP prefixes or geographic regions that may be
served by the dCDN).
o Additional capabilities of the dCDN, such as its ability to
support different CDNI Metadata requests.
Note that the set of requests that a dCDN is willing to serve could
in some cases be relatively static (e.g., a set of IP prefixes),
could be exchanged off-line, or might even be negotiated as part of a
peering agreement. However, it may also be more dynamic, in which
case the exchange supported by FCI would be helpful. A further
discussion of the Footprint & Capability Advertisement interface can
be found in [FOOTPRINT-CAPABILITY].
4.4. CDNI Logging Interface
It is necessary for the Upstream CDN to have visibility into the
delivery of content that it redirected to a Downstream CDN. This
allows the Upstream CDN to properly bill its customers for multiple
deliveries of content cached by the Downstream CDN, as well as to
report accurate traffic statistics to those content providers. This
is one role of the LI.
Other operational data that may be relevant to CDNI can also be
exchanged by the LI. For example, a dCDN may report the amount of
content it has acquired from uCDN, and how much cache storage has
been consumed by content cached on behalf of uCDN.
Traffic logs are easily exchanged off-line. For example, the
following traffic log is a small deviation from the Apache log file
format, where entries include the following fields:
o Domain - the full domain name of the origin server
o IP address - the IP address of the client making the request
o End time - the ending time of the transfer
o Time zone - any time zone modifier for the end time
o Method - the transfer command itself (e.g., GET, POST, HEAD)
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o URL - the requested URL
o Version - the protocol version, such as HTTP/1.0
o Response - a numeric response code indicating transfer result
o Bytes Sent - the number of bytes in the body sent to the client
o Request ID - a unique identifier for this transfer
o User agent - the user agent, if supplied
o Duration - the duration of the transfer in milliseconds
o Cached Bytes - the number of body bytes served from the cache
o Referrer - the referrer string from the client, if supplied
Of these, only the Domain field is indirect in the Downstream CDN --
it is set to the CDN-Domain used by the Upstream CDN rather than the
actual origin server. This field could then used to filter traffic
log entries so only those entries matching the Upstream CDN are
reported to the corresponding operator. Further discussion of the LI
can be found in [LOGGING].
One open question is who does the filtering. One option is that the
Downstream CDN filters its own logs and passes the relevant records
directly to each Upstream peer. This requires that the Downstream
CDN know the set of CDN-Domains that belong to each Upstream peer.
If this information is already exchanged between peers as part of
another interface, then direct peer-to-peer reporting is
straightforward. If it is not available, and operators do not wish
to advertise the set of CDN-Domains they serve to their peers, then
the second option is for each CDN to send both its non-local traffic
records and the set of CDN-Domains it serves to an independent third
party (i.e., a CDN Exchange), which subsequently filters, merges, and
distributes traffic records on behalf of each participating CDN
operator.
A second open question is how timely traffic information should be.
For example, in addition to offline traffic logs, accurate real-time
traffic monitoring might also be useful, but such information
requires that the Downstream CDN inform the Upstream CDN each time it
serves Upstream content from its cache. The Downstream CDN can do
this, for example, by sending a conditional HTTP GET request
(If-Modified-Since) to the Upstream CDN each time it receives an HTTP
GET request from one of its end users. This allows the Upstream CDN
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to record that a request has been issued for the purpose of real-time
traffic monitoring. The Upstream CDN can also use this information
to validate the traffic logs received later from the Downstream CDN.
There is obviously a trade-off between accuracy of such monitoring
and the overhead of the Downstream CDN having to go back to the
Upstream CDN for every request.
Another design trade-off in the LI is the degree of aggregation or
summarization of data. One situation that lends itself to
summarization is the delivery of HTTP Adaptive Streaming (HAS), since
the large number of individual chunk requests potentially results in
large volumes of logging information. This case is discussed below,
but other forms of aggregation may also be useful. For example,
there may be situations where bulk metrics such as bytes delivered
per hour may suffice rather than the detailed per-request logs
outlined above. It seems likely that a range of granularities of
logging will be needed along with ways to specify the type and degree
of aggregation required.
4.5. CDNI Control Interface
The CDNI Control interface is initially used to bootstrap the other
interfaces. As a simple example, it could be used to provide the
address of the logging server in the dCDN to the uCDN in order to
bootstrap the CDNI Logging interface. It may also be used, for
example, to establish security associations for the other interfaces.
The other role the CI plays is to allow the uCDN to pre-position,
revalidate, or purge metadata and content on a dCDN. These
operations, sometimes collectively called the "Trigger interface",
are discussed further in [CONTROL-TRIGGERS].
4.6. CDNI Metadata Interface
The role of the CDNI Metadata interface is to enable CDNI
distribution metadata to be conveyed to the Downstream CDN by the
Upstream CDN. Such metadata includes geo-blocking restrictions,
availability windows, access-control policies, and so on. It may
also include information to facilitate acquisition of content by a
dCDN (e.g., alternate sources for the content, authorization
information needed to acquire the content from the source). For a
full discussion of the CDNI Metadata interface, see [METADATA]
Some distribution metadata may be partially emulated using in-band
mechanisms. For example, in case of any geo-blocking restrictions or
availability windows, the Upstream CDN can elect to redirect a
request to the Downstream CDN only if that CDN's advertised delivery
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footprint is acceptable for the requested URL. Similarly, the
request could be forwarded only if the current time is within the
availability window. However, such approaches typically come with
shortcomings such as inability to prevent from replay outside the
time window or inability to make use of a Downstream CDN that covers
a broader footprint than the geo-blocking restrictions.
Similarly, some forms of access control may also be performed on a
per-request basis using HTTP directives. For example, being able to
respond to a conditional GET request gives the Upstream CDN an
opportunity to influence how the Downstream CDN delivers its content.
Minimally, the Upstream CDN can invalidate (purge) content previously
cached by the Downstream CDN.
All of these in-band techniques serve to illustrate that uCDNs have
the option of enforcing some of their access control policies
themselves (at the expense of increased inter-CDN signaling load),
rather than delegating enforcement to dCDNs using the MI. As a
consequence, the MI could provide a means for the uCDN to express its
desire to retain enforcement for itself. For example, this might be
done by including a "check with me" flag in the metadata associated
with certain content. The realization of such in-band techniques
over the various inter-CDN acquisition protocols (e.g., HTTP)
requires further investigation and may require small extensions or
semantic changes to the acquisition protocol.
4.7. HTTP Adaptive Streaming Concerns
We consider HTTP Adaptive Streaming (HAS) and the impact it has on
the CDNI interfaces because large objects (e.g., videos) are broken
into a sequence of small, independent chunks. For each of the
following, a more thorough discussion, including an overview of the
trade-offs involved in alternative designs, can be found in RFC 6983.
First, with respect to Content Acquisition and File Management, which
are out of scope for the CDNI interfaces but, nonetheless, relevant
to the overall operation, we assume no additional measures are
required to deal with large numbers of chunks. This means that the
dCDN is not explicitly made aware of any relationship between
different chunks, and the dCDN handles each chunk as if it were an
individual and independent content item. The result is that content
acquisition between uCDN and dCDN also happens on a per-chunk basis.
This approach is in line with the recommendations made in RFC 6983,
which also identifies potential improvements in this area that might
be considered in the future.
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Second, with respect to request routing, we note that HAS manifest
files have the potential to interfere with request routing since
manifest files contain URLs pointing to the location of content
chunks. To make sure that a manifest file does not hinder CDNI
request routing and does not place excessive load on CDNI resources,
either the use of manifest files could be limited to those containing
relative URLs or the uCDN could modify the URLs in the manifest. Our
approach for dealing with these issues is twofold. As a mandatory
requirement, CDNs should be able to handle unmodified manifest files
containing either relative or absolute URLs. To limit the number of
redirects, and thus the load placed on the CDNI interfaces, as an
optional feature uCDNs can use the information obtained through the
CDNI Request Routing Redirection interface to modify the URLs in the
manifest file. Since the modification of the manifest file is an
optional uCDN-internal process, this does not require any
standardization effort beyond being able to communicate chunk
locations in the CDNI Request Routing Redirection interface.
Third, with respect to the CDNI Logging interface, there are several
potential issues, including the large number of individual chunk
requests potentially resulting in large volumes of logging
information, and the desire to correlate logging information for
chunk requests that correspond to the same HAS session. For the
initial CDNI specification, our approach is to expect participating
CDNs to support per-chunk logging (e.g., logging each chunk request
as if it were an independent content request) over the CDNI Logging
interface. Optionally, the LI may include a Content Collection
IDentifier (CCID) and/or a Session IDentifier (SID) as part of the
logging fields, thereby facilitating correlation of per-chunk logs
into per-session logs for applications benefiting from such session
level information (e.g., session-based analytics). This approach is
in line with the recommendations made in RFC 6983, which also
identifies potential improvements in this area that might be
considered in the future.
Fourth, with respect to the CDNI Control interface, and in particular
purging HAS chunks from a given CDN, our approach is to expect each
CDN supports per-chunk content purge (e.g., purging of chunks as if
they were individual content items). Optionally, a CDN may support
content purge on the basis of a "Purge IDentifier (Purge-ID)"
allowing the removal of all chunks related to a given Content
Collection with a single reference. It is possible that this Purge-
ID could be merged with the CCID discussed above for HAS Logging, or
alternatively, they may remain distinct.
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4.8. URI Rewriting
When using HTTP redirection, content URIs may be rewritten when
redirection takes place within a uCDN, from a uCDN to a dCDN, and
within the dCDN. In the case of cascaded CDNs, content URIs may be
rewritten at every CDN hop (e.g., between the uCDN and the dCDN
acting as the transit CDN, and between the transit CDN and the dCDN
serving the request). The content URI used between any uCDN/dCDN
pair becomes a common handle that can be referred to without
ambiguity by both CDNs in all their inter-CDN communications. This
handle allows the uCDN and dCDN to correlate information exchanged
using other CDNI interfaces in both the Downstream direction (e.g.,
when using the MI) and the Upstream direction (e.g., when using the
LI).
Consider the simple case of a single uCDN/dCDN pair using HTTP
redirection. We introduce the following terminology for content URIs
to simplify the discussion:
"u-URI" represents a content URI in a request presented to the
uCDN;
"ud-URI" is a content URI acting as the common handle across uCDN
and dCDN for requests redirected by the uCDN to a specific dCDN;
"d-URI" represents a content URI in a request made within the
delegate dCDN.
In our simple pair-wise example, the "ud-URI" effectively becomes the
handle that the uCDN/dCDN pair use to correlate all CDNI information.
In particular, for a given pair of CDNs executing the HTTP
redirection, the uCDN needs to map the u-URI to the ud-URI handle for
all MI message exchanges, while the dCDN needs to map the d-URI to
the ud-URI handle for all LI message exchanges.
In the case of cascaded CDNs, the transit CDN will rewrite the
content URI when redirecting to the dCDN, thereby establishing a new
handle between the transit CDN and the dCDN, that is different from
the handle between the uCDN and transit CDN. It is the
responsibility of the transit CDN to manage its mapping across
handles so the right handle for all pairs of CDNs is always used in
its CDNI communication.
In summary, all CDNI interfaces between a given pair of CDNs need to
always use the "ud-URI" handle for that specific CDN pair as their
content URI reference.
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5. Deployment Models
In this section, we describe a number of possible deployment models
that may be achieved using the CDNI interfaces described above. We
note that these models are by no means exhaustive and that many other
models may be possible.
Although the reference model of Figure 1 shows all CDN functions on
each side of the CDNI interface, deployments can rely on entities
that are involved in any subset of these functions, and therefore
only support the relevant subset of CDNI interfaces. As already
noted in Section 3, effective CDNI deployments can be built without
necessarily implementing all the interfaces. Some examples of such
deployments are shown below.
Note that, while we refer to Upstream and Downstream CDNs, this
distinction applies to specific content items and transactions. That
is, a given CDN may be Upstream for some transactions and Downstream
for others, depending on many factors such as location of the
requesting client and the particular piece of content requested.
5.1. Meshed CDNs
Although the reference model illustrated in Figure 1 shows a
unidirectional CDN interconnection with a single uCDN and a single
dCDN, any arbitrary CDNI meshing can be built from this, such as the
example meshing illustrated in Figure 11. (Support for arbitrary
meshing may or may not be in the initial scope for the working group,
but the model allows for it.)
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------------- -----------
/ CDN A \<==CDNI===>/ CDN B \
\ / \ /
------------- -----------
/\ \\ /\
|| \\ ||
CDNI \==CDNI===\\ CDNI
|| \\ ||
\/ \/ \/
------------- -----------
/ CDN C \===CDNI===>/ CDN D \
\ / \ /
------------- -----------
/\
||
CDNI
||
\/
-------------
/ CDN E \
\ /
-------------
===> CDNI interfaces, with right-hand side CDN acting as dCDN
to left-hand side CDN
<==> CDNI interfaces, with right-hand side CDN acting as dCDN
to left-hand side CDN and with left-hand side CDN acting
as dCDN to right-hand side CDN
Figure 11: CDNI Deployment Model: CDN Meshing Example
5.2. CSP Combined with CDN
Note that our terminology refers to functional roles and not economic
or business roles. That is, a given organization may be operating as
both a CSP and a fully fledged uCDN when we consider the functions
performed, as illustrated in Figure 12.
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##################################### ##################
# # # #
# Organization A # # Organization B #
# # # #
# -------- ------------- # # ----------- #
# / CSP \ / uCDN \ # # / dCDN \ #
# | | | +----+ | # # | +----+ | #
# | | | | C | | # # | | C | | #
# | | | +----+ | # # | +----+ | #
# | | | +----+ | # # | +----+ | #
# | | | | L | | # # | | L | | #
# | |*****| +----+ |===CDNI===>| +----+ | #
# | | | +----+ | # # | +----+ | #
# | | | | RR | | # # | | RR | | #
# | | | +----+ | # # | +----+ | #
# | | | +----+ | # # | +----+ | #
# | | | | D | | # # | | D | | #
# | | | +----+ | # # | +----+ | #
# \ / \ / # # \ / #
# -------- ------------- # # ----------- #
# # # #
##################################### ##################
===> CDNI interfaces, with right-hand side CDN acting as dCDN
to left-hand side CDN
**** interfaces outside the scope of CDNI
C Control component of the CDN
L Logging component of the CDN
RR Request Routing component of the CDN
D Distribution component of the CDN
Figure 12: CDNI Deployment Model: Organization Combining CSP & uCDN
5.3. CSP Using CDNI Request Routing Interface
As another example, a content provider organization may choose to run
its own Request Routing function as a way to select among multiple
candidate CDN providers; in this case, the content provider may be
modeled as the combination of a CSP and of a special, restricted case
of a CDN. In that case, as illustrated in Figure 13, the CDNI
Request Routing interfaces can be used between the restricted CDN
operated by the content provider Organization and the CDN operated by
the full CDN organization acting as a dCDN in the request routing
control plane. Interfaces outside the scope of the CDNI work can be
used between the CSP functional entities of the content provider
organization and the CDN operated by the full CDN organization acting
as a uCDN) in the CDNI control planes other than the request routing
plane (i.e., Control, Distribution, Logging).
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##################################### ##################
# # # #
# Organization A # # Organization B #
# # # #
# -------- ------------- # # ----------- #
# / CSP \ / uCDN(RR) \ # # / dCDN(RR) \ #
# | | | +----+ | # # | +----+ | #
# | |*****| | RR |==========CDNI=====>| RR | | #
# | | | +----+ | # RR # | +----+ | #
# | | \ / # # | | #
# | | ------------- # # |uCDN(C,L,D)| #
# | | # # | +----+ | #
# | | # # | | C | | #
# | |*******************************| +----+ | #
# | | # # | +----+ | #
# | | # # | | L | | #
# | | # # | +----+ | #
# | | # # | +----+ | #
# | | # # | | D | | #
# | | # # | +----+ | #
# \ / # # \ / #
# -------- # # ----------- #
# # # #
##################################### ##################
===> CDNI Request Routing Interface
**** interfaces outside the scope of CDNI
Figure 13: CDNI Deployment Model: Organization Combining
CSP and Partial CDN
5.4. CDN Federations and CDN Exchanges
There are two additional concepts related to, but distinct from,
CDNI. The first is CDN Federation. Our view is that CDNI is the
more general concept, involving two or more CDNs serving content to
each other's users, while federation implies a multi-lateral
interconnection arrangement, but other CDNI agreements are also
possible (e.g., symmetric bilateral, asymmetric bilateral). An
important conclusion is that CDNI technology should not presume (or
bake in) a particular interconnection agreement, but should instead
be general enough to permit alternative interconnection arrangements
to evolve.
The second concept often used in the context of CDN Federation is CDN
Exchange -- a third-party broker or exchange that is used to
facilitate a CDN federation. Our view is that a CDN exchange offers
valuable machinery to scale the number of CDN operators involved in a
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multi-lateral (federated) agreement, but that this machinery is built
on top of the core CDNI mechanisms. For example, as illustrated in
Figure 14, the exchange might aggregate and redistribute information
about each CDN footprint and capacity, as well as collect, filter,
and redistribute traffic logs that each participant needs for
interconnection settlement, but inter-CDN Request Routing, inter-CDN
content distribution (including inter-CDN acquisition), and inter-CDN
control, which fundamentally involve a direct interaction between an
Upstream CDN and a Downstream CDN -- operate exactly as in a pair-
wise peering arrangement. Turning to Figure 14, we observe that in
this example:
o each CDN supports a direct CDNI Control interface to every other
CDN
o each CDN supports a direct CDNI Metadata interface to every other
CDN
o each CDN supports a CDNI Logging interface with the CDN Exchange
o each CDN supports both a CDNI Request Routing interface with the
CDN Exchange (for aggregation and redistribution of dynamic CDN
footprint discovery information) and a direct RI to every other
CDN (for actual request redirection).
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---------- ---------
/ CDN A \ / CDN B \
| +----+ | | +----+ |
//========>| C |<==============CDNI============>| C |<==========\\
|| | +----+ | C | +----+ | ||
|| | +----+ | | +----+ | ||
|| //=====>| D |<==============CDNI============>| D |<=======\\ ||
|| || | +----+ | M | +----+ | || ||
|| || | | /------------\ | | || ||
|| || | +----+ | | +--+ CDN Ex| | +----+ | || ||
|| || //==>| RR |<===CDNI==>|RR|<=======CDNI====>| RR |<====\\ || ||
|| || || | +----+ | RR | +--+ | RR | +----+ | || || ||
|| || || | | | /\ | | | || || ||
|| || || | +----+ | | || +---+ | | +----+ | || || ||
|| || || | | L |<===CDNI=======>| L |<=CDNI====>| L | | || || ||
|| || || | +----+ | L | || +---+ | L | +----+ | || || ||
|| || || \ / \ || /\ / \ / || || ||
|| || || ----------- --||----||-- ----------- || || ||
|| || || || || || || ||
|| || || CDNI RR || || || ||
|| || || || CDNI L || || ||
|| || || || || || || ||
|| || || ---||----||---- || || ||
|| || || / \/ || \ || || ||
|| || || | +----+ || | || || ||
|| || \\=====CDNI==========>| RR |<=============CDNI========// || ||
|| || RR | +----+ \/ | RR || ||
|| || | +----+ | || ||
|| || | | L | | || ||
|| || | +----+ | || ||
|| || | +----+ | || ||
|| \\=======CDNI===========>| D |<=============CDNI===========// ||
|| M | +----+ | M ||
|| | +----+ | ||
\\==========CDNI===========>| C |<=============CDNI==============//
C | +----+ | C
\ CDN C /
--------------
<=CDNI RR=> CDNI Request Routing Interface
<=CDNI M==> CDNI Metadata Interface
<=CDNI C==> CDNI Control Interface
<=CDNI L==> CDNI Logging Interface
Figure 14: CDNI Deployment Model: CDN Exchange
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Note that a CDN exchange may alternatively support a different set of
functionality (e.g., Logging only, or Logging and full request
routing, or all the functionality of a CDN including content
distribution). All these options are expected to be allowed by the
IETF CDNI specifications.
6. Trust Model
There are a number of trust issues that need to be addressed by a
CDNI solution. Many of them are in fact similar or identical to
those in a simple CDN without interconnection. In a standard CDN
environment (without CDNI), the CSP places a degree of trust in a
single CDN operator to perform many functions. The CDN is trusted to
deliver content with appropriate quality of experience for the end
user. The CSP trusts the CDN operator not to corrupt or modify the
content. The CSP often relies on the CDN operator to provide
reliable accounting information regarding the volume of delivered
content. The CSP may also trust the CDN operator to perform actions
such as timely invalidation of content and restriction of access to
content based on certain criteria such as location of the user and
time of day, and to enforce per-request authorization performed by
the CSP using techniques such as URI signing.
A CSP also places trust in the CDN not to distribute any information
that is confidential to the CSP (e.g., how popular a given piece of
content is) or confidential to the end user (e.g., which content has
been watched by which user).
A CSP does not necessarily have to place complete trust in a CDN. A
CSP will in some cases take steps to protect its content from
improper distribution by a CDN, e.g., by encrypting it and
distributing keys in some out of band way. A CSP also depends on
monitoring (possibly by third parties) and reporting to verify that
the CDN has performed adequately. A CSP may use techniques such as
client-based metering to verify that accounting information provided
by the CDN is reliable. HTTP conditional requests may be used to
provide the CSP with some checks on CDN operation. In other words,
while a CSP may trust a CDN to perform some functions in the short
term, the CSP is able, in most cases, to verify whether these actions
have been performed correctly and to take action (such as moving the
content to a different CDN) if the CDN does not live up to
expectations.
One of the trust issues raised by CDNI is transitive trust. A CDN
that has a direct relationship with a CSP can now "outsource" the
delivery of content to another (Downstream) CDN. That CDN may in
term outsource delivery to yet another Downstream CDN, and so on.
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The top-level CDN in such a chain of delegation is responsible for
ensuring that the requirements of the CSP are met. Failure to do so
is presumably just as serious as in the traditional single CDN case.
Hence, an Upstream CDN is essentially trusting a Downstream CDN to
perform functions on its behalf in just the same way as a CSP trusts
a single CDN. Monitoring and reporting can similarly be used to
verify that the Downstream CDN has performed appropriately. However,
the introduction of multiple CDNs in the path between CSP and end
user complicates the picture. For example, third-party monitoring of
CDN performance (or other aspects of operation, such as timely
invalidation) might be able to identify the fact that a problem
occurred somewhere in the chain but not point to the particular CDN
at fault.
In summary, we assume that an Upstream CDN will invest a certain
amount of trust in a Downstream CDN, but that it will verify that the
Downstream CDN is performing correctly, and take corrective action
(including potentially breaking off its relationship with that CDN)
if behavior is not correct. We do not expect that the trust
relationship between a CSP and its "top level" CDN will differ
significantly from that found today in single CDN situations.
However, it does appear that more sophisticated tools and techniques
for monitoring CDN performance and behavior will be required to
enable the identification of the CDN at fault in a particular
delivery chain.
We expect that the detailed designs for the specific interfaces for
CDNI will need to take the transitive trust issues into account. For
example, explicit confirmation that some action (such as content
removal) has taken place in a Downstream CDN may help to mitigate
some issues of transitive trust.
7. Privacy Considerations
In general, a CDN has the opportunity to collect detailed information
about the behavior of end users, e.g., by logging which files are
being downloaded. While the concept of interconnected CDNs as
described in this document doesn't necessarily allow any given CDN to
gather more information on any specific user, it potentially
facilitates sharing of this data by a CDN with more parties. As an
example, the purpose of the CDNI Logging interface is to allow a dCDN
to share some of its log records with a uCDN, both for billing
purposes as well as for sharing traffic statistics with the Content
Provider on whose behalf the content was delivered. The fact that
the CDNI interfaces provide mechanisms for sharing such potentially
sensitive user data, shows that it is necessary to include in these
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interface appropriate privacy and confidentiality mechanisms. The
definition of such mechanisms is dealt with in the respective CDN
interface documents.
8. Security Considerations
While there are a variety of security issues introduced by a single
CDN, we are concerned here specifically with the additional issues
that arise when CDNs are interconnected. For example, when a single
CDN has the ability to distribute content on behalf of a CSP, there
may be concerns that such content could be distributed to parties who
are not authorized to receive it, and there are mechanisms to deal
with such concerns. Our focus in this section is on how CDNI
introduces new security issues not found in the single CDN case. For
a more detailed analysis of the security requirements of CDNI, see
Section 9 of [RFC7337].
Many of the security issues that arise in CDNI are related to the
transitivity of trust (or lack thereof) described in Section 6. As
noted above, the design of the various interfaces for CDNI must take
account of the additional risks posed by the fact that a CDN with
whom a CSP has no direct relationship is now potentially distributing
content for that CSP. The mechanisms used to mitigate these risks
may be similar to those used in the single CDN case, but their
suitability in this more complex environment must be validated.
CDNs today offer a variety of means to control access to content,
such as time-of-day restrictions, geo-blocking, and URI signing.
These mechanisms must continue to function in CDNI environments, and
this consideration is likely to affect the design of certain CDNI
interfaces (e.g., metadata, request routing). For more information
on URI signing in CDNI, see [URI-SIGNING].
Just as with a single CDN, each peer CDN must ensure that it is not
used as an "open proxy" to deliver content on behalf of a malicious
CSP. Whereas a single CDN typically addresses this problem by having
CSPs explicitly register content (or origin servers) that are to be
served, simply propagating this information to peer Downstream CDNs
may be problematic because it reveals more information than the
Upstream CDN is willing to specify. (To this end, the content
acquisition step in the earlier examples force the dCDN to retrieve
content from the uCDN rather than go directly to the origin server.)
There are several approaches to this problem. One is for the uCDN to
encode a signed token generated from a shared secret in each URL
routed to a dCDN, and for the dCDN to validate the request based on
this token. Another one is to have each Upstream CDN advertise the
set of CDN-Domains they serve, where the Downstream CDN checks each
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request against this set before caching and delivering the associated
object. Although straightforward, this approach requires operators
to reveal additional information, which may or may not be an issue.
8.1. Security of CDNI Interfaces
It is noted in [RFC7337] that all CDNI interfaces must be able to
operate securely over insecure IP networks. Since it is expected
that the CDNI interfaces will be implemented using existing
application protocols such as HTTP or Extensible Messaging and
Presence Protocol (XMPP), we also expect that the security mechanisms
available to those protocols may be used by the CDNI interfaces.
Details of how these interfaces are secured will be specified in the
relevant interface documents.
8.2. Digital Rights Management
Digital Rights Management (DRM), also sometimes called digital
restrictions management, is often employed for content distributed
via CDNs. In general, DRM relies on the CDN to distribute encrypted
content, with decryption keys distributed to users by some other
means (e.g., directly from the CSP to the end user). For this
reason, DRM is considered out of scope [RFC6707] and does not
introduce additional security issues for CDNI.
9. Contributors
The following individuals contributed to this document:
o Matt Caulfield
o Francois Le Faucheur
o Aaron Falk
o David Ferguson
o John Hartman
o Ben Niven-Jenkins
o Kent Leung
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10. Acknowledgements
The authors would like to thank Huw Jones and Jinmei Tatuya for their
helpful input to this document. In addition, the authors would like
to thank Stephen Farrell, Ted Lemon, and Alissa Cooper for their
reviews, which have helped to improve this document.
11. Informative References
[CONTROL-TRIGGERS]
Murray, R. and B. Niven-Jenkins, "CDNI Control Interface /
Triggers", Work in Progress, July 2014.
[FOOTPRINT-CAPABILITY]
Seedorf, J., Peterson, J., Previdi, S., Brandenburg, R.,
and K. Ma, "CDNI Request Routing: Footprint and
Capabilities Semantics", Work in Progress, July 2014.
[LOGGING] Faucheur, F., Ed., Bertrand, G., Ed., Oprescu, I., Ed.,
and R. Peterkofsky, "CDNI Logging Interface", Work in
Progress, July 2014.
[METADATA]
Niven-Jenkins, B., Murray, R., Caulfield, M., Leung, K.,
and K. Ma, "CDN Interconnection Metadata", Work in
Progress, July 2014.
[REDIRECTION]
Niven-Jenkins, B., Ed. and R. Brandenburg, Ed., "Request
Routing Redirection Interface for CDN Interconnection",
Work in Progress, April 2014.
[RFC3466] Day, M., Cain, B., Tomlinson, G., and P. Rzewski, "A Model
for Content Internetworking (CDI)", RFC 3466, February
2003.
[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.
[RFC6983] van Brandenburg, R., van Deventer, O., Le Faucheur, F.,
and K. Leung, "Models for HTTP-Adaptive-Streaming-Aware
Content Distribution Network Interconnection (CDNI)", RFC
6983, July 2013.
Peterson, et al. Informational [Page 57]
RFC 7336 CDNI Framework August 2014
[RFC7337] Leung, K., Ed. and Y. Lee, Ed., "Content Distribution
Network Interconnection (CDNI) Requirements", RFC 7337,
August 2014.
[URI-SIGNING]
Leung, K., Faucheur, F., Downey, B., Brandenburg, R., and
S. Leibrand, "URI Signing for CDN Interconnection (CDNI)",
Work in Progress, March 2014.
Authors' Addresses
Larry Peterson
Akamai Technologies, Inc.
8 Cambridge Center
Cambridge, MA 02142
USA
EMail: lapeters@akamai.com
Bruce Davie
VMware, Inc.
3401 Hillview Ave.
Palo Alto, CA 94304
USA
EMail: bdavie@vmware.com
Ray van Brandenburg (editor)
TNO
Brassersplein 2
Delft 2612CT
the Netherlands
Phone: +31-88-866-7000
EMail: ray.vanbrandenburg@tno.nl
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