CDNI Rate Pacing
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Rate pacing is a class of network traffic shaping which limits the
transmission rate of data over a network. This document defines CDNI
extensions for downstream CDNs to support rate pacing on behalf of
upstream CDNs.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
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:
a rate pacing algorithm for CDNs
CDNI interface extensions required for supporting rate pacing
A token bucket algorithm SHOULD be used for implementing rate pacing.
Other algorithms MAY be used but the traffic shape MUST fit within the
same envelope of a token bucket algorithm. 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.
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. No value needs to be advertised for this capability and
therefore no additional value registry need to be defined.
A CDN MAY advertise the “RatePacing” capability in the
FCI if it implements this specification.
A new RatePacing metadata object is defined to represent the
configuration for rate pacing. The RatePacing object has MIME type
“application/cdni.RatePacing”. 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.
A new RatePacing metadata object is defined to represent the
configuration for rate pacing. The RatePacing object has MIME type
“application/cdni.RatePacing”. 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 the Rate Pacing Algorithm defined by this
specification in order to deliver a piece of content.
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.
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.
The CI is not impacted by rate pacing.
This document requests the following of IANA:
Addition of RatePacing in the CDNI Capability Registry defined in
???.
Addition of RatePacing in the CDNI GenericMetadata Type Registry
defined in .
Addition of sc-rate in the CDNI Logging Field Names Registry defined
in .
A malicious CSP might attempt to use rate pacing to instruct a dCDN
to delivery some content at a very high rate thereby amplifying a DDOS
attack. The decision to enforce a rate is left to the discretion of a
dCDN. An implementation of rate pacing should implement reasonable upper
bound to avoid such cases.
The author would like to thank Francois Le Faucheur for his
contributions and feedback.