CDNI Rate Pacing

Rate pacing is a class of network traffic shaping which limits the transmission rate of data over a network. In the context of a Content Delivery Network (CDN), rate pacing provides an important business advantage to a Content Service Provider (CSP) by ensuring that a CDN which is delivering content on behalf of that CSP does not deliver significantly more data than necessary to an end client. For example, suppose an end client is watching some Constant Bit Rate (CBR) video encoded at 1500 kbps. In the absence of rate pacing, the CDN delivering this content may send it to the client at 3000 kbps. If the client chooses to terminate the session before watching the entire video, up to half the transmitted data is wasted. This waste leads to unnecessary cost for the CSP and diminished useful capacity for the CDN. Rate pacing requires configuration on a per-content basis. In order to enable rate pacing in a CDNI environment, the CDNI interfaces need to be extended to optionally support this feature. This document describes: CDNI interface extensions required for supporting rate pacing a token bucket rate pacing algorithm for CDNs

defines the CDNI Footprint and Capabilities semantics. But at the time of writing, no FCI syntax specification has been accepted as a working group document. states that: "The CDNI FCI specification SHOULD define the registry (and the rules for adding new entries to the registry) for the different capability types. Each capability type MAY further have a list of valid values. The individual CDNI interface specifications which define a given capability SHOULD define any necessary registries (and the rules for adding new entries to the registry) for the values advertised for a given capability type." This document defines a new capability type: “RatePacing” to be added to the FCI capability types registry. The value of this capability contains one or more rate pacing algorithm names from the Rate Pacing algorithms registry. For example, the value may be “token-bucket/v1” to indicate that the advertising CDN supports the token bucket algorithm described later in this document. A CDN MAY advertise the “RatePacing” capability in the FCI if it implements this specification and at least one rate pacing algorithm registered in the Rate Pacing algorithms registry.

A new RatePacing metadata object is defined to represent the configuration for rate pacing. The RatePacing object has MIME type “application/cdni.RatePacing.v1”. RatingPacing MAY appear within the metadata list of either HostMetadata or PathMetadata (i.e. may have either host-level scope or a path-level scope). The following section defines the properties of the RatingPacing object.

The presence of the RatePacing Metadata indicates that a dCDN MUST comply with this specification in order to deliver a piece of content. The metadata indicates the rate pacing algorithm name required for delivering the content and the relevant parameters for that algorithm. Property: algo Description: Rate pacing algorithm name from the Rate Pacing Algorithms registry. Dictates the structure of the “params” value. For example, “token-bucket./v1”. Type: String Mandatory-to-Specify: Yes. Property: params Description: An object containing algorithm-specific properties and values which are relevant to the rate pacing algorithm specified by the “algo” property. Each algorithm dictates its own parameters. Type: Object Mandatory-to-Specify: Yes.

The RRI is not impacted by rate pacing. However, if the metadata for a piece of content indicates that rate pacing is required by the uCDN, then a request router should only redirect requests for that content to CDNs which advertise “RatePacing” as a capability. The request router should also limit its choice of dCDNs to those which advertise the same rate pacing algorithm as is specified by the rate pacing metadata. For example, if the metadata for a piece of content includes a GenericMetadata object of type “application/cdni.RatePacing.v1” and the “algo” property in the value of that GenericMetadata is “token-bucket/v1”, then the request router of the uCDN should only redirect requests for that piece of content to dCDNs which advertise a capability type of “RatePacing” and a capability value of “token-bucket/v1”.

The rate at which a piece of content was delivered MAY be indicated via the LI. The “sc-rate” field indicates the rate in bytes per second as a decimal number. The bytes measured should correspond to the sc-entity-bytes field. sc-rate: format: DEC field value: the average rate in bytes per second at which a response was delivered from Surrogate to client. occurence: there MUST be zero or exactly one instances of this field. Note that existing fields defined by include the bytes delivered and the time taken to service a request. However, the time taken includes the acquisition latency which is not relevant to rate pacing.

The CI is not impacted by rate pacing.

Token bucket is one example of a rate pacing algorithm. This document extends the CDNI interfaces with generic support for rate pacing and registers token bucket as a rate pacing algorithm for CDNI. Other algorithms MAY be defined but are beyond the scope of this document. Token bucket is described by . The token bucket algorithm is characterized by two parameters: Rate – the number of tokens added to the bucket per second Size – the maximum number of tokens in the bucket For the purpose of this document, each token represents one byte. The algorithm name “token-bucket/v1” is registered as a Rate Pacing algorithm. This algorithm name may appear as the value of the “RatePacing” capability. This name may also appear as the value of the “algo” property in the “RatePacing” metadata object. If a RatePacing metadata object's “algo” value is “token-bucket/v1” then the metadata object's “params” is an object of type TokenBucketParams, described below.

Property: rate Description: Rate of tokens per second to be added to the bucket as described by the token bucket algorithm. This value MUST be a positive integer. Each token represents one byte. Type: Integer Mandatory-to-Specify: Yes. Property: size Description: Maximum number of tokens per bucket as described by the token bucket algorithm. This value MUST be a positive integer. Type: Integer Mandatory-to-Specify: Yes.

This document requests the following of IANA: Addition of RatePacing in the CDNI Capability Registry defined in TBD. Addition of “RatePacing” to the standard partition of the CDNI GenericMetadata Type Registry defined in : Type name Specification Version MTE STR RatePacing RFCthis 1 true true Addition of “sc-rate” in the CDNI Logging Field Names Registry defined in .

IANA is requested to create new registry, CDNI Rate Pacing Algorithms. The following table defines the initial values of the registry: Algorithm Name Specification Version token-bucket RFCthis 1 New rate pacing algorithm registrations SHOULD specify RatePacing parameter objects as shown in and SHOULD describe the algorithm for rate pacing.

A malicious CSP might attempt to use rate pacing to instruct a dCDN to delivery some content at a very low rate thereby in order to exhaust the resources of a dCDN by forcing connection state to be maintained for longer than usual. The decision to enforce a rate is left to the discretion of a dCDN. An implementation of rate pacing should implement reasonable lower (and upper) bounds to avoid such cases.

The author would like to thank Francois Le Faucheur for his contributions and feedback.