| Network Working Group | O. Finkelman |
| Internet-Draft | Qwilt |
| Intended status: Standards Track | S. Mishra |
| Expires: May 23, 2020 | Verizon |
| November 20, 2019 |
CDNI Request Routing Extensions
draft-ietf-cdni-request-routing-extensions-08
Open Caching architecture is a use case of Content Delivery Networks Interconnection (CDNI) in which the commercial Content Delivery Network (CDN) is the upstream CDN (uCDN) and the ISP caching layer serves as the downstream CDN (dCDN). The extensions specified in this document to the CDNI Metadata Interface (MI) and the Footprint and Capabilities Interface (FCI) are derived from requirements raised by Open Caching but are also applicable to CDNI use cases in general.
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The Streaming Video Alliance is a global association that works to solve streaming video challenges in an effort to improve end-user experience and adoption. The Open Caching Working Group of the Streaming Video Alliance is focused on the delegation of video delivery requests from commercial CDNs to a caching layer at the Internet Service Provider's (ISP) network. Open Caching architecture is a specific use case of CDNI where the commercial CDN is the upstream CDN (uCDN) and the ISP caching layer is the downstream CDN (dCDN). The Open Caching Request Routing Specification defines the Request Routing process and the interfaces that are required for its provisioning. This document defines and registers CDNI metadata object [RFC8006] and CDNI Footprint and Capabilities object [RFC8008] that are required for Open Caching Request Routing. For consistency with other CDNI documents this document follows the CDNI convention of uCDN (upstream CDN) and dCDN (downstream CDN) to represent the commercial CDN and ISP caching layer respectively.
This document also registers CDNI Payload Types [RFC7736] for the defined objects:
The following terms are used throughout this document:
Additionally, this document reuses the terminology defined in [RFC6707], [RFC7336], [RFC8006], [RFC8007], and [RFC8008]. Specifically, we use the following CDNI acronyms:
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
Iterative request redirection is defined in Section 1.1 of [RFC7336] and elaborated by examples in Sections 3.2 and 3.4 of [RFC7336]. A Redirection Target (RT) is defined in Section 2 of [RFC7975] for Recursive Request Redirection as:
In this document we adopt the same definition of the RT for the Iterative Request Redirect use case. This use case requires the provisioning of the RT address to be used by the uCDN in order to redirect to the dCDN. RT addresses can vary between different footprints, for example, between different regions, and they may also change over time, for example as a result of network problems. Given this variable and dynamic nature of the redirect target address, it may not be suitable to advertise it during bootstrap. A more dynamic and footprint oriented interface is required. Section 4.3 of [RFC7336] suggests that it could be one of the roles of the FCI [RFC8008]. Following this suggestion, we have therefore, chosen to use the CDNI Footprint and Capabilities interface for redirect target address advertisement.
Use cases
The Redirect Target capability object is used to indicate the target address the uCDN should use in order to redirect a client to the dCDN. A target may be attached to a specific uCDN host, a list of uCDN hosts, or used globally for all the hosts of the uCDN.
When a dCDN is attaching the redirect target to a specific uCDN host or a list of uCDN hosts, the dCDN MUST advertise the hosts within the Redirect Target capability object as "redirecting-hosts". In this case, the uCDN can redirect to that dCDN address, only if the User Agent request was to one of these uCDN hosts.
If the redirect target capability object does not contain a target or the target is empty, the uCDN MUST interpret it as "no target available for these uCDN hosts for the specified footprint". In case such a target was already advertised in a previous FCI object, the uCDN MUST interpret it as an update that deletes the previous redirect target.
A redirect target for DNS redirection is a FQDN used as an alias in a CNAME record response (see [RFC1034]) of the uCDN DNS router. Note that DNS routers make routing decisions based on either the DNS resolver's IP address or the client IP subnet when EDNS0 client-subnet (ECS) is used (see [RFC7871]). The dCDN may choose to advertise redirect targets and footprints to cover both cases, such that the uCDN resolution would route the DNS query to a different dCDN CNAMEs according client subnet or dCDN resolver IP address. This method further allows the dCDN DNS to optimize the resolution by localizing the target CNAMEs. A uCDN implementation SHOULD prefer routing based on client IP subnet when ECS option is present. A dCDN implementation using the ECS option MUST be aware of the privacy drawbacks listed in Section 2 of [RFC7871] and SHOULD follow the guidelines provided in Section 11.1 of [RFC7871].
A redirect target for HTTP redirection is the URI to be used as the value for the Location header of a HTTP redirect 3xx response, typically a 302 (Found) (see Section 7.1.2 of [RFC7231] and section 6.4 of [RFC7231]).
The Redirect Target capability object consists of the following properties:
The following is an example of a Redirect Target capability object serialization that advertises a dCDN target address that is attached to a specific list of uCDN "redirecting-hosts". A uCDN host that is included in that list can redirect to the advertised dCDN redirect target. The capabilities object is serialized as a JSON object as defined in Section 5.1 of [RFC8008]
{
"capabilities": [
{
"capability-type": "FCI.RedirectTarget",
"capability-value": {
"redirecting-hosts": [
"a.service123.ucdn.example.com",
"b.service123.ucdn.example.com"
],
"dns-target": {
"host": "service123.ucdn.dcdn.example.com"
},
"http-target": {
"host": "us-east1.dcdn.example.com",
"path-prefix": "/cache/1/",
"include-redirecting-host": true
}
},
"footprints": [
<Footprint objects>
]
}
]
}
The DnsTarget object gives the target address for the DNS response to delegate from the uCDN to the dCDN.
The following is an example of DnsTarget object:
{
"host": "service123.ucdn.dcdn.example.com"
}
The following is an example of a DNS query for uCDN address "a.service123.ucdn.example.com" and the corresponding CNAME redirection response:
Query:
a.service123.ucdn.example.com:
type A, class IN
Response:
NAME: a.service123.ucdn.example.com, TYPE: CNAME, CLASS: IN,
TTL: 120, RDATA: service123.ucdn.dcdn.example.com
The HttpTarget object gives the necessary information to construct the target Location URI for HTTP redirection.
Example of HttpTarget object with a "scheme", a "path-prefix", and "include-redirecting-host" properties:
{
"host": "us-east1.dcdn.example.com",
"scheme": "https",
"path-prefix": "/cache/1/",
"include-redirecting-host": true
}
Example of a HTTP request for content at uCDN host "a.service123.ucdn.example.com" and the corresponding HTTP response with a Location header, used for redirecting the client to the dCDN, constructed according to the HttpTarget object from the above example:
Request:
GET /vod/1/movie.mp4 HTTP/1.1
Host: a.service123.ucdn.example.com
Response:
HTTP/1.1 302 Found
Location: https://us-east1.dcdn.example.com/cache/1/
a.service123.ucdn.example.com/vod/1/movie.mp4
Before requests can be routed from the uCDN to the dCDN the CDNs must exchange service configurations between them. Using the MI, the uCDN advertises out-of-band its hosts to the dCDN, each host is designated by a hostname and has its own specific metadata (see Section 4.1.2 of [RFC8006]. The dCDN, using the FCI, advertises, also out-of-band, the redirect target address object defined in Section 2.3 for the relevant uCDN hosts. The following is a generalized example of the message flow between an upstream CDN and a downstream dCDN. For simplicity, we focus on the sequence of messages between the uCDN and dCDN and not on how they are passed.
dCDN uCDN
+ +
| |
(1) | MI: host: s123.ucdn.example.com |
| host-metadata: < metadata > |
<-------------------------------------------------------+
| |
(2) | FCI: capability-type: FCI.RedirectTarget |
| redirecting-hosts: s123.ucdn.example.com |
| target host: us-east1.dcdn.example.com |
+------------------------------------------------------->
| |
| |
+ +
Figure 1: Redirect target address advertisement
Once the redirect target has been set, the uCDN can start redirecting user requests to the dCDN. The following is a generic sequence of redirection using the host and redirect target that were advertised in Figure 1 above.
End User dCDN uCDN RR
+ + +
| | |
(1) | Request sent s123.ucdn.example.com |
+-----------------------+----------------------->
| | |
(2) | Redirect to us-east1.dcdn.example.com |
<-----------------------+-----------------------+
| | |
(3) | Request us-east1.dcdn.example.com |
+-----------------------> |
| | |
(4) | Response | |
<-----------------------+ |
| | |
+ + +
Figure 2: Generic requests redirection sequence
Open Caching requires that the uCDN provides a fallback target server to the dCDN, to be used in cases where the dCDN cannot properly handle the request. To avoid redirect loops, the fallback target server's address at the uCDN MUST be different from the original uCDN address from which the client was redirected to the dCDN. The uCDN MUST avoid further redirection when receiving the client request at the fallback target. The fallback target is defined as a generic metadata object (see Section 3.2 of [RFC8006])
Use cases
The Fallback target metadata object is used to indicate the target address the dCDN should redirect a client to when falling back to the uCDN. Fallback target address is represented as an endpoint object as defined in Section 4.3.3 of [RFC8006].
In DNS redirection a CNAME record is used as the fallback target address.
In HTTP redirection a hostname is used as the fallback target address.
When using HTTP redirect to route a client request back to the uCDN, it is the dCDN's responsibility to use the original URL path as the client would have used for the original uCDN request, stripping, if needed, the dCDN path-prefix and/or the uCDN hostname from the redirect URL that may have been used to request the content from the dCDN.
The MI.FallbackTarget Metadata object consists of the following single property:
Example of a MI.FallbackTarget Metadata object that designates the host address the dCDN should use as fallback address to redirect back to the uCDN.
{
"generic-metadata-type": "MI.FallbackTarget",
"generic-metadata-value":
{
"host": "fallback-a.service123.ucdn.example",
"scheme": "https"
}
}
The uCDN advertises out-of-band the fallback target address to the dCDN, so that the dCDN may redirect a request back to the uCDN in case the dCDN cannot serve it. Using the MI the uCDN advertises its hosts to the dCDN, along with their specific host metadata (see Section 4.1.2 of [RFC8006]. The Fallback Target generic metadata object is encapsulated within the "host-metadata" property of each host. The following is an example of a message flow between an upstream CDN and a downstream dCDN. For simplicity, we focus on the sequence of messages between the uCDN and dCDN, not on how they are passed.
dCDN uCDN
+ +
| |
(1) | MI: host: s123.ucdn.example.com |
| host-metadata: |
| < metadata objects > |
| < MI.FallbackTarget |
| host: fallback-a.service123.ucdn.example > |
| < metadata objects > |
<-------------------------------------------------------+
| |
(2) | FCI: capability-type: FCI.RedirectTarget |
| redirecting-hosts: s123.ucdn.example.com |
| target host: us-east1.dcdn.example.com |
+------------------------------------------------------->
| |
| |
+ +
Figure 3: Advertisement of host metadata with Fallback Target
The following is a generic sequence of redirection using the configurations that were advertised in Figure 3 above. In this case the dCDN redirects back to the uCDN fallback target address.
End User dCDN uCDN fallback uCDN RR
+ + + +
| | | |
(1) | Request sent s123.ucdn.example.com | |
+-------------------+-------------------+------------------->
| | | |
(2) | Redirect to us-east1.dcdn.example.com | |
<-------------------+-------------------+-------------------+
| | | |
(3) | Request us-east1.dcdn.example.com | |
+-------------------> | |
| | | |
(4) | Redirect back to fallback-a.service123.ucdn.example |
<-------------------+ | |
| | | |
(5) | Request fallback-a.service123.ucdn.example |
+---------------------------------------> |
| | | |
(6) | Response | | |
<-------------------+-------------------+ |
| | | |
+ + + +
Figure 4: Redirection to Fallback Target
When advertising fallback addresses to the dCDN the uCDN SHOULD consider the failure use cases that may lead the dCDN to route requests to uCDN fallback. In extreme dCDN network failures or under denial-of-service (DoS) attacks, requests coming from a large segment or multiple segments of the dCDN may be routed back to the uCDN. The uCDN SHOULD therefore design its fallback addressing scheme and its available resources accordingly. A favorable approach would be for the uCDN to use different fallback target address for each uCDN host, enabling it to load balance the requests using the same methods as it would for its original hosts. See Sections 4.1.2 and 4.1.3 of [RFC8006] for a detailed description of how to use GenericMetadata objects within the HostMatch object advertised in the HostIndex of the uCDN.
This document requests the registration of the following CDNI Payload Types under the IANA "CDNI Payload Types" registry defined in [RFC7736]:
| Payload Type | Specification |
|---|---|
| FCI.RedirectTarget | RFCthis |
| MI.FallbackTarget | RFCthis |
[RFC Editor: Please replace RFCthis with the published RFC number for this document.]
Purpose: The purpose of this payload type is to distinguish RedirectTarget FCI objects
Interface: FCI
Encoding: see Section 2.3
Purpose: The purpose of this payload type is to distinguish FallbackTarget MI objects (and any associated capability advertisement)
Interface: MI/FCI
Encoding: see Section 3.1
This specification is in accordance with the CDNI Metadata Interface and the CDNI Request Routing: Footprint and Capabilities Semantics. As such, it is subject to the security and privacy considerations as defined in Section 8 of [RFC8006] and in Section 7 of [RFC8008] respectively.
The Redirect Target FCI object potentially reveals information about the internal structure of the dCDN network. A third party could intercept the FCI transactions and use the information to attack the dCDN. The same is also true for the Fallback Target Metadata object as it may reveal information about the internal structure of the uCDN, exposing it to external exploits. Implementations of the FCI and MI MUST therefore use strong authentication and encryption and strictly follow the directions for securing the interface as defined for the Metadata Interface in Section 8.3 of [RFC8006].
The authors thank Nir B. Sopher for reality checks against production use cases, his contribution is significant to this document. The authors also thank Ben Niven-Jenkins for his review and feedback and Kevin J. Ma for his guidance throughout the development of this document including his regular reviews.
| [OC-RR] | Finkelman, O., Hofmann, J., Klein, E., Mishra, S., Ma, K., Sahar, D. and B. Zurat, "Open Caching - Request Routing Functional Specification", Version 1.1, October 2019. |
| [OCWG] | "Open Caching Home Page" |
| [RFC7736] | Ma, K., "Content Delivery Network Interconnection (CDNI) Media Type Registration", RFC 7736, DOI 10.17487/RFC7736, December 2015. |
| [RFC7871] | Contavalli, C., van der Gaast, W., Lawrence, D. and W. Kumari, "Client Subnet in DNS Queries", RFC 7871, DOI 10.17487/RFC7871, May 2016. |
| [SVA] | "Streaming Video Alliance Home Page" |