Framework for CDN InterconnectionCisco Systems, Inc.1414 Mass. Ave.BoxboroughMA01719USAbsd@cisco.comVerivue, Inc.2 Research WayPrincetonNJUSA+1 978 303 8032lpeterson@verivue.comThis 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. CDN Interconnection requires the specification of several
interfaces and mechanisms to address issues such as request routing,
metadata exchange, and the acquisition of content by one CDN from
another. 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.The interconnection of Content Distribution Networks (CDNs) is
motivated by several use cases, such as those described in . The overall problem space for
CDN Interconnection is described in . The purpose of this
document is to provide an overview of the various components necessary
to interconnect CDNs. CDN Interconnection requires the specification of
several interfaces and mechanisms to address issues such as request
routing, metadata exchange, and the acquisition of content by one CDN
from another. The intent of this document is to describe how these
interfaces and mechanisms fit together, leaving their detailed
specification to other documents. We make extensive use of message flow
examples to illustrate the operation of interconnected CDNs, but these
examples should be considered illustrative rather than prescriptive.This document draws freely on the terminology defined in and .We also introduce the following terms:CDN Domain: a host name (FQDN) at the beginning of a URL,
representing a set of content that is served by a given CDN. For
example, in the URL http://cdn.csp.com/...rest of url..., the CDN
domain is cdn.csp.com.Distinguished CDN Domain: a CDN domain that is allocated by a CDN
for the purposes of communication with a peer CDN, but which 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.Recursive CDNI request routing: 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 interface (or use information cached from earlier similar
queries) to find out how the Downstream CDN wants the request to be
redirected, which 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 routing. Note that the
Downstream CDN may elect to have the request redirected directly to a
Surrogate inside the Downstream CDN, to the Request-Routing System of
the Downstream CDN, to another CDN, or to any other system that the
Downstream CDN sees as fit for handling the redirected request.Iterative CDNI Request Routing: 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 routing.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.This document uses the reference model in as originally created in .We note that 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 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
uCDN 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 connect to more than one CDN
today. Definitions of the four CDNI interfaces follow. More discussion of
these interfaces appears in . Control Interface: Operations to discover, initialize, and
parameterize the other CDNI interfaces. Once established, all
runtime control over CDNI behavior is under the purview of one of
these other interfaces.Request Routing Interface: Operations to determine what CDN
(and optionally what surrogate within a CDN) is to serve
end-user's requests. May include a
combination of: Asynchronous operations to exchange routing information
(e.g., the network footprint served by a given CDN) that
enables CDN selection for subsequent user requests; andSynchronous operations to select a delivery CDN (surrogate)
for a given user request.Metadata Interface: Operations to communicate metadata that
governs the how content is delivered by interconnected CDNs.
Examples of CDNI metadata include geo-blocking directives, availability
windows, access control mechanisms, and purge directives. May
include a combination of: Asynchronous operations to exchange metadata that govern
subsequent user requests for content; andSynchronous operations that govern behavior for a given
user request for content.Logging Interface: Operations that allow interconnected CDNs to
exchange relevant activity logs. May include a combination of:
Real-time exchanges, suitable for runtime traffic
monitoring; andOff-line exchanges, suitable for analytics and billing.The remainder of this document is organized as follows: describes some essential building
blocks for CDNI, notably the various options for redirecting user
requests to a given CDN. provides a number of
illustrative examples of various CDNI operations. describes the functionality
of the four main CDNI interfaces. shows how various deployment
models of CDNI may be achieved using the defined interfaces. describes the trust model of CDNI
and the issues of transitive trust in particular that CDNI
raises.At its core, CDN Interconnection 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 in-protocol redirection mechanisms such as the
HTTP 302 redirection response. We discuss these below as background
before discussing some examples of their use in .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).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 modify the path component of the URL being
accessed by the client. 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 delivery
nodes a single DNS server can manage; and (2) DNS responses are
cached by downstream servers so the TTL on the response must be set
to an appropriate value so as to preserve the timeliness 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.HTTP redirection makes use of the “302” redirection
response of the HTTP protocol. 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 302 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;
and (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.The disadvantages of HTTP redirection are (1) it is visible to
the application, so it requires application support and may affect
the application behavior (e.g., web browsers will not send cookies
if the URL changes to a different domain); (2) HTTP is a
heavy-weight protocol layered on TCP so it has relatively high
overhead; and (3) the results of HTTP redirection are not cached so
that all redirections must go through to the server.To provide a big-picture overview of the various components of CDN
Interconnection, 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
how additional capabilities, such as recursive request
redirection and content removal, might be added.Before walking through some specific examples, we present a
high-level view of the operations that may take place. This high-level
overview is illustrated in . 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 more detailed examples illustrate below.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 being the best CDN
to deliver content to the end-user. The interconnection relationship may
be symmetric between these two CDN operators, but for simplicity we show
the interaction in one direction only.The operations shown in the Figure are as follows: Prior to any content request, metadata may be
asynchronously pushed from uCDN to dCDN so that it is
available in readiness for later content requests.dCDN may advertise information relevant to its delivery
capabilities (e.g. geographic footprint, reachable address
prefixes) prior to any content requests being
redirected.A content request from a user agent arrives at uCDN.uCDN may synchronously request information from dCDN
regarding its delivery capabilities to decide if dCDN is a
suitable target for redirection of this request.uCDN redirects the request to dCDN by sending some response (DNS,
HTTP) to the user agent.The user agent requests the content from dCDN.dCDN may synchronously request metadata related to this
content from uCDN, e.g. to decide whether to serve it.If the content is not already in a suitable cache in dCDN, dCDN
may acquire it from uCDN.The content is delivered to dCDN from uCDN.The content is delivered to the user agent by dCDN.Some time later, perhaps at the request of the CSP (not shown)
uCDN may instruct dCDN to purge the content to ensure it is not
delivered again.After one or more content delivery actions by dCDN, a log of
delivery actions may be provided to uCDN.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.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 () can be used to direct a user request for a
piece of content from the CSP towards the CSP's chosen upstream
CDN.We use the term "CDN-domain" to refer to the host name (a FQDN) at
the beginning of each URL. We assume Operator A provides an upstream
CDN that serves content on behalf of a CSP with CDN-domain
cdn.csp.com. We assume that Operator B provides a downstream CDN. An
end user at some point makes a request for URLhttp://cdn.csp.com/...rest of url...It may well be the case that cdn.csp.com is just a CNAME for some
other CDN-domain (such as csp.op-a.net). 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.In this section we walk through a simple, illustrative example
using HTTP redirection from uCDN to dCDN. The example also assumes the
use of HTTP redirection inside 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 uCDN to dCDN but using DNS for handling of request
inside each CDN.We assume for this example that Operators A and B have established
an agreement to interconnect their CDNs, with A being upstream and B
being downstream. (It is likely that the agreement would be made in
both directions, but we focus on just one here for clarity.)The operators agree that a CDN-domain peer-a.op-b.net will be used
as the target of redirections from uCDN to dCDN. The name of this
domain must be 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
embedded in URLs that end-users request.The operators must also agree on some distinguished CDN-domain that
will be used for inter-CDN acquisition of CSP's content from uCDN by
dCDN. In this example, we'll use op-b-acq.op-a.net.The operators must also exchange information regarding which
requests dCDN is prepared to serve. For example, 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 interface.DNS must be configured in the following way:The content provider must be configured to make operator A the
authoritative DNS server for cdn.csp.com (or to return a CNAME for
cdn.csp.com for which operator A is the authoritative DNS
server).Operator A must be configured so that a DNS request for
op-b-acq.op-a.net returns a request router in Operator A.Operator B must be configured so that a DNS request for
peer-a.op-b.net/cdn.csp.com returns a request router in Operator
B. illustrates how a client request
forhttp://cdn.csp.com/...rest of url...is handled.The steps illustrated in the figure are as follows:A DNS resolver for Operator A processes the DNS request for its
customer based on CDN-domain cdn.csp.com. It returns the IP
address of a request router in Operator A.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” Operator B’s distinguished CDN-domain
(peer-a.op-b.net) 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.)The end-user does a DNS lookup using Operator B’s
distinguished CDN-domain (peer-a.op-b.net). B’s DNS resolver
returns the IP address of a request router for Operator B. Note
that if request routing within 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.The request router for Operator B processes the HTTP request
and selects a suitable delivery node to serve the end-user
request, and returns a 302 redirect message for a new URL
constructed by replacing the hostname by a subdomain of the
Operator B’s distinguished CDN-domain that points to the
selected delivery node.The end-user does a DNS lookup using Operator B’s
delivery node subdomain (node1.peer-a.op-b.net). B’s DNS
resolver returns the IP address of the delivery node.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 dCDN from uCDN (not the CSP). The
distinguished CDN-domain peer-a.op-b.net indicates to dCDN that
this content is to be acquired from uCDN; stripping the CDN-domain
reveals the original CDN-domain cdn.csp.com and 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.net).Operator A's DNS resolver processes the DNS request and returns
the IP address of a request router in operator A.The request router for Operator A processes the HTTP request
from Operator B delivery node. Operator A 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.net). (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 uCDN to serve the inter-CDN acquisition
request and returns a 302 redirect message for a new URL
constructed by replacing the hostname by a subdomain of the
Operator A’s distinguished inter-CDN acquisition domain that
points to the selected delivery node.Operator A DNS resolver processes the DNS request and returns
the IP address of the delivery node in operator A.Operator A serves content for the requested CDN-domain 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. 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
needs to 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 needs to 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.com) 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 allow for the domains involved in the
CDN redirection. These problems are generally soluble, but the
solutions complicate the example, so we do not discuss them further
in this version of the draft.We note that this example begins to illustrate some of
the interfaces that may be required for CDNI, but does not
require all of them. For example, obtaining information from
dCDN regarding the set of client IP addresses or geographic
regions it might be able to serve is an aspect of the
request routing interface. Important configuration
information such as the distinguished names used for
redirection and inter-CDN acquisition could also be conveyed
via a CDNI interface (e.g., perhaps the 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.
Hence, there is no explicit metadata interface invoked in
this example. There is also no explicit logging interface
discussed in this example.We also note that the step of deciding when a request
should be redirected to dCDN rather than served by 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 routing
approach. That is, uCDN performs part of the request routing
function to determine that dCDN should serve the request,
and then redirects the client to a request router in dCDN to
perform the rest of the request routing function. 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 routing 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.The following example builds on the previous one to
illustrate the use of the Request Routing 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
uCDN to dCDN. The operators still must agree on some distinguished
CDN-domain that will be used for inter-CDN acquisition of CSP's
content by dCDN. In this example, we'll use op-b-acq.op-a.net.The operators must also exchange information regarding which
requests dCDN is prepared to serve. For example, 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.DNS must be configured in the following way:The content provider must be configured to make operator A the
authoritative DNS server for cdn.csp.com (or to return a CNAME for
cdn.csp.com for which operator A is the authoritative DNS
server).Operator A must be configured so that a DNS request for
op-b-acq.op-a.net returns a request router in Operator A.Operator B must be configured so that a request for
node1.op-b.net/cdn.csp.com returns the IP address of a delivery
node. Note that there might be a number of such delivery
nodes. illustrates how a client
request forhttp://cdn.csp.com/...rest of url...is handled.The steps illustrated in the figure are as follows:A DNS resolver for Operator A processes the DNS request for its
customer based on CDN-domain cdn.csp.com. It returns the IP
address of a Request Router in Operator A.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
queries the CDNI Request Routing 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.Operator A returns a 302 redirect message for a new URL
obtained from the Request Routing Interface.The end-user does a DNS lookup using the host name of the URL
just provided (node1.op-b.net). 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 CDNI Request Routing Interface, there should not be any
further redirection here (in contrast to the iterative method
described above.)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 uCDN (not the CSP.)
The distinguished CDN-domain op-b.net indicates to dCDN that this
content is to be acquired from another CDN; stripping the
CDN-domain reveals the original CDN-domain cdn.csp.com, 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 the inter-CDN Acquisition
“distinguished” CDN-domain as agreed above (in this
case, op-b-acq.op-a.net).Operator A DNS resolver processes the DNS request and returns
the IP address of a request router in operator A.The request router for Operator A processes the HTTP request
from Operator B delivery node. Operator A 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.net). (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 uCDN to serve the inter-CDN acquisition
request and returns a 302 redirect message for a new URL
constructed by replacing the hostname by a subdomain of the
Operator A’s distinguished inter-CDN acquisition domain that
points to the selected delivery node.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.)Operator A serves content for the requested CDN-domain 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. It
may also perform its own content acquisition steps if needed
before returning the content to dCDN.Recursive redirection has the advantage over iterative 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 dCDN to expose the addresses of its edge caches to
uCDN. 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 Request Routing Interface requires that
interface to be appropriately configured and bootstrapped,
which is not shown here. More discussion on the
bootstrapping of interfaces is provided in In this section we walk through a simple example using DNS-based
redirection for request redirection from uCDN to dCDN (as well as for
request routing inside dCDN and uCDN). As noted in , 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 must learn the set of requests that dCDN is
willing or able to serve (e.g. which client IP address prefixes or
geographic regions are part of the dCDN footprint). Operator B must
have and make 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 AS number assigned to B, is one easy way to achieve that.) Also,
Operator A must obtain 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.net, must be assigned for inter-CDN
acquisition.DNS must be configured in the following way:The CSP must be configured to make Operator A the authoritative
DNS server for cdn.csp.com (or to return a CNAME for cdn.csp.com
for which operator A is the authoritative DNS server).When uCDN sees a request best served by dCDN, it returns CNAME
and NS records for "b.cdn.csp.com", where "b" is the unique
identifier assigned to Operator B. (It may, for example, be an AS
number assigned to Operator B.)dCDN must be configured so that a request for "b.cdn.csp.com"
returns a delivery node in dCDN.uCDN must be configured so that a request for
"op-b-acq.op-a.net" returns a delivery node in uCDN. depicts the exchange of DNS and
HTTP requests. The main differences from are the lack of HTTP redirection and
transparency to the end-user.The steps illustrated in the figure are as follows:Request Router for Operator A processes the DNS request for
CDN-domain cdn.csp.com and recognizes that the end-user is best
served by another CDN. (This may depend on the IP address of the
user's local DNS 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.com), plus an NS
record that maps b.cdn.csp.com to B’s Request Router.The end-user does a DNS lookup using the modified CDN-domain
(i.e., b.cdn.csp.com). This causes B’s Request Router to
respond with a suitable delivery node.The end-user requests the content from B’s delivery node.
The requested URL contains the name cdn.csp.com. (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.net).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.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. A potential problem is that
the upstream CDN 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, uCDN
will only obtain the address of the client's local DNS
resolver (LDNS), 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, 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 since the LDNS is likely in the same
network as the dCDN serves. One approach to ensuring that
the client's IP address prefix is correctly determined in
such situations is described in .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 request routing 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 logging
interface discussed in this 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.This example differs from the one in only in the addition of a CDNI Request
Routing Interface request (step 2) and corresponding response (step
3). The RRI 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 RRI REPLY messages that are not in
direct response to a corresponding RRI 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 .)Once Operator A obtains the RRI 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).The following example illustrates how the Metadata interface may be
used to remove an item of content. 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 Metadata Interface to request that content identified by a
particular URL be removed from dCDN. The following diagram illustrates
the operation.The metadata interface is used to convey the request from uCDN to
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 . If HTTP-based redirection had been used, the
URL for removal would be of the form
peer-a.op-b.net/cdn.csp.com/...The dCDN is expected to confirm to the uCDN, as illustrated by the
MI OK message, the completion of the removal of the targeted content
from all the caches in dCDN.The following example illustrates how the metadata interface may be
used to pre-position an item of content in the dCDN. In this example,
Operator A uses the Metadata Interface to request that content
identified by a particular URL be pre-positioned into Operator B
CDN.The steps illustrated in the figure are as follows:Operator A uses the Metadata Interface to request that
Operator B pre-positions 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 , 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, 7 are exactly the same as steps 1, 2, 3, 4 of , only this time Operator B CDN can serve
the end-user request without triggering dynamic content acquisition,
since the content has been pre-positioned in 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-positioning; however, such intra-CDN dynamic
acquisition would not involve the uCDN.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 . 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).The steps illustrated in the figure are as follows:Operator A uses the Metadata Interface to
asynchronously push CDNI metadata to Operator B. The
present 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 "cdn.csp.com/op-b.net/..." (assuming
HTTP redirection is used - it would be applicable to
"cdn.csp.com/..." if DNS redirection were used as in
).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).A Content Request occurs as usual.A CDNI Request Routing Request (RRI REQ) is issued by operator
A CDN, as discussed in . 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
Step 1, which may or may not affect the response.Operator B's request router issues a CDNI Request Routing
Response (RRI RESP) as in .Operator B performs content redirection as discussed in .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.)Assuming successful per-request authorization, serving of
Content Data (possibly preceded by inter-CDN acquisition) proceeds
as in .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
redirection and recursive CDNI request-routing are used (as in ), but dynamic CDNI metadata acquisition is
applicable to other variations of request routing.The steps illustrated in the figure are as follows:Operator A initially uses the Metadata Interface to
asynchronously push seed metadata to Operator B. For
example, this seed information may include a URI
indicating where CDNI Metadata can later be pulled from
for some content set. (There are alternative ways that
this seeding information may be provided, such as
piggybacking on the CDNI RRI REQ message of Step 3.)A Content Request arrives as normal.A Request Routing Interface request occurs as in the prior
example.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. The seeding
information provided in Step 1 is used to determine how to obtain
the metadata. Note that there may exist cases in which this step
does not occur (e.g., because the CDNI metadata seeding
information indicates CDNI metadata are not needed at that
stage).On receipt of a CDNI Metadata MI 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.)Response to the RRI request as normal.Redirection message is sent to the end user.A delivery node of Operator B receives the end user
request.The delivery node triggers dynamic acquisition of additional
CDNI metadata that are needed to process the end-user content
request. Again the seeding information provided in Step 1 is used
to determine how to acquire the needed CDNI metadata. Note that
there may exist cases where this step need not happen, either
because the metadata were already acquired previously, or because
the seeding information indicates no metadata are required.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.Content is served (possibly preceded by inter-CDN acquisition)
as in . illustrates the four 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
(mostly still to be written, but see and for some discussion of the
interfaces).One interface that is not shown in 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. As we saw in some of the
preceding examples, that interface can be used as a way of passing
information such as the metadata that is required to obtain the content
in dCDN from 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 tradeoffs. We
begin with an examination of one such tradeoff that affects all the
interfaces - the use of in-band or out-of-band communication.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.We may think of the request routing interface as comprising two
parts: the asynchronous advertisement of footprint and capabilities by
a dCDN that allows a uCDN to decide whether to redirect particular
user requests to that dCDN; and the synchronous operation of actually
redirecting a user request. (These are somewhat analogous to the
operations of routing and forwarding in IP.)As illustrated in , the synchronous
part of the request routing interface 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.In support of these exchanges, it is necessary for CDN peers to
exchange additional information with each other. Depending on the
method(s) supported, this includes The operator’s unique id (operator-id) or distinguished
CDN-domain (operator-domain);NS records for the operator’s set of externally visible
request routers;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 dCDN).Of these, the two operator identifiers are fixed, and can be
exchanged off-line as part of a peering agreement. The NS records
potentially change with some frequency, but an existing
protocol—DNS—can be used to dynamically track this
information. That is, a peer can do a DNS lookup on operator-domain to
retrieve the set of NS records corresponding to the peer’s
redirection service.The set of requests that dCDN is willing to serve could in some
cases be relatively static (e.g., a set of IP prefixes) which 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 an
explicit protocol for its exchange would be be helpful.A variety of options exist for the dCDN operator to advertise its
footprint to uCDN. As discussed in , footprint
is comprised of two components: a class of end user requests (represented, for example, by a
set of IP prefixes, or a geographic region) that the dCDN is
willing and able to serve directly, without use of another
dCDN;the connectivity of the dCDN to other CDNs that may be able to
serve content to users on behalf of dCDN. describes an
approach to advertising such footprint information asynchronously
using BGP. In addition to this sort of information, a dCDN
might also advertise "capabilities" such as the ability to
handle certain types of content (e.g. specific streaming
formats) or quality of service (QoS) capabilities. describes
an approach that exchanges CDN "capabilities" over HTTP, while
describes how
ALTO may be used to obtain
request routing information.We also note that the Request Routing interface plays a key role in
enabling recursive redirection, as illustrated in . 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.It is necessary for the upstream CDN to have visibility into the
delivery of content it originates to end-users connected to the
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 Logging interface.Other operational data that may be relevant to CDNI can also be
exchanged by the Logging interface. For example, 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: Domain – the full domain name of the
origin serverIP address – the IP address of the
client making the requestEnd time – the ending time of the
transferTime zone – any time zone modifier for
the end timeMethod – the transfer command itself
(e.g., GET, POST, HEAD)URL – the requested URLVersion – the protocol version, such
as HTTP/1.0Response – a numeric response code
indicating transfer resultBytes Sent – the number of bytes in
the body sent to the clientRequest ID – a unique identifier for
this transferUser agent – the user agent, if
suppliedDuration – the duration of the
transfer in millisecondsCached Bytes – the number of body
bytes served from the cacheReferrer – the referrer string from
the client, if suppliedOf 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.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
knows the set of CDN-domains that belong to each upstream peer. If
this information is already exchanged between peers as part of the
request routing 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 off-line 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 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 tradeoff 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 tradeoff in the Logging interface is the degree of
aggregation or summarization of data. One situation that lends itself
to summarization is the delivery of HTTP-based adaptive bit-rate
video. Most schemes to deliver such video use a large number of
relatively small HTTP requests (e.g. one request per 2-second chunk of
video.) It may be desirable to aggregate logging information so that a
single log entry is provided for the entire video rather than for each
chunk. Note however that such aggregation requires a degree of
application awareness in dCDN to recognize that the many HTTP requests
correspond to a single video.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.The control interface is primarily used for the bootstrapping of
other interfaces. As a simple example, it could be used to provide the
address of the logging server in dCDN to uCDN in order to bootstrap
the logging interface. It may also be used, for example, to establish
security associations for the other interfaces. We discuss the
relationship between the Control and Metadata interfaces in the next
section. The role of the 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
policy information such as the desire to pre-position content rather
than fetch it on demand.Some metadata may be able to be conveyed using in-band mechanisms.
For example, to inform the downstream CDN of any geo-blocking
restrictions or availability windows, the upstream can elect to
redirect a request to the downstream CDN only if that CDN's advertised
delivery footprint is acceptable for the requested URL. Similarly, the
request could be forwarded only if the current time is within the
availability window.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.Fine-grain control over how the downstream CDN delivers content on
behalf of the upstream CDN is also possible. For example, by including
the X-Forwarded-For HTTP header with the conditional GET request, the
downstream CDN can report the end-user’s IP address to the
upstream CDN, giving it an opportunity to control whether the
downstream CDN should serve the content to this particular end-user.
The upstream CDN would communicate its directive through its response
to the conditional GET. The downstream CDN can cache information for a
period of time specified by the upstream CDN, thereby reducing control
overhead.Thinking beyond what metadata operations can be done in-line, we
note that all CDNs already export a “content purge“
operation to their customers. The CDNI metadata interface could
support a similar "content purge" API call. When a CSP invokes purge
on the upstream CDN, that CDN in turn invokes purge on all downstream
CDNs that might be caching the content. Of course, agreement as to the
syntax and semantics of this call is required.One open question is how to distinguish between what functionality
is supported by the Metadata interface and what functionality is
supported by the Control interface. The approach taken in this
document is to assume a minimal Control interface that is used to
bootstrap the other interfaces. We assume all information that governs
peer CDN behavior at the granularity of individual content items is
exchanged via the Metadata interface. We note that some other
documents have suggested that the purge operation should be part of
the Control Interface. The authors' view is that purging a piece of
content is just another form of metadata, similar to an availability
window. In effect, a purge is equivalent to a statement that the
availability window for that content has now expired. The timeliness
requirements for purge operations may affect the detailed design of
the metadata interface.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 may other models
may be possible. Although the reference model of 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 , effective CDNI
deployments can be built without necessarily implementing all four
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.Although the reference model illustrated in 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 . (Support for arbitrary meshing may or may
not be in the initial scope for the working group, but the model allows
for it.)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 .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 , the CDNI Request Routing interface 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).There are two additional concepts related to, but distinct from CDN
Interconnection. 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 CDN interconnection 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
multi-lateral (federated) agreement, but that this machinery is built on
top of the core CDNI interconnection mechanisms. For example, as
illustrated in , the exchange might
aggregate and redistribute information about each CDN footprint and
capacity, as well as collect, filter, and re-distribute 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 , we observe that in this
example:each CDN supports a direct CDNI Control interface to every other
CDNeach CDN supports a direct CDNI Metadata interface to every other
CDNeach CDN supports a CDNI Logging interface with the CDN
Exchangeeach 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 CDNI Request Routing
interface to every other CDN (for actual request redirection).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.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.The main trust issue raised by CDNI is that it introduces 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.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.This memo includes no request to IANA.While there is 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 CDN interconnection
introduces new security issues not found in the single CDN case.Many of the security issues that arise in CDNI are related to the
transitivity of trust (or lack thereof) described in . 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.Another concern that arises in any CDN is that information about the
behavior of users (what content they access, how much content they
consume, etc.) may be gathered by the CDN. This risk certainly exists in
inter-connected CDNs, but it should be possible to apply the same
techniques to mitigate it as in the single CDN case.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.)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 is 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
encoded 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 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.It is noted in
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 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.
Issues of 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 for the CDNI
WG
and does not introduce additional security issues for CDNI.
The following individuals contributed to this document:Francois le FaucheurBen Niven-JenkinsDavid FergusonJohn HartmanWe thank Aaron Falk and Huw Jones for their helpful input to the draft.