Network Working Group A. Beck
Request for Comments: 3836 M. Hofmann
Category: Informational Lucent Technologies
H. Orman
Purple Streak Development
R. Penno
Nortel Networks
A. Terzis
Johns Hopkins University
August 2004
Requirements for Open Pluggable Edge Services (OPES)
Callout Protocols
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document specifies the requirements that the OPES (Open
Pluggable Edge Services) callout protocol must satisfy in order to
support the remote execution of OPES services. The requirements are
intended to help evaluate possible protocol candidates, as well as to
guide the development of such protocols.
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Functional Requirements . . . . . . . . . . . . . . . . . . . 3
3.1. Reliability . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Congestion Avoidance . . . . . . . . . . . . . . . . . 3
3.3. Callout Transactions . . . . . . . . . . . . . . . . . 3
3.4. Callout Connections . . . . . . . . . . . . . . . . . . 4
3.5. Asynchronous Message Exchange . . . . . . . . . . . . . 5
3.6. Message Segmentation . . . . . . . . . . . . . . . . . 5
3.7. Support for Keep-Alive Mechanism . . . . . . . . . . . 6
3.8. Operation in NAT Environments . . . . . . . . . . . . . 6
3.9. Multiple Callout Servers . . . . . . . . . . . . . . . 6
3.10. Multiple OPES Processors . . . . . . . . . . . . . . . 6
3.11. Support for Different Application Protocols . . . . . . 7
3.12. Capability and Parameter Negotiations . . . . . . . . . 7
3.13. Meta Data and Instructions . . . . . . . . . . . . . . 8
4. Performance Requirements . . . . . . . . . . . . . . . . . . 9
4.1. Protocol Efficiency . . . . . . . . . . . . . . . . . . 9
5. Security Requirements . . . . . . . . . . . . . . . . . . . . 9
5.1. Authentication, Confidentiality, and Integrity . . . . 9
5.2. Hop-by-Hop Confidentiality. . . . . . . . . . . . . . . 10
5.3. Operation Across Untrusted Domains. . . . . . . . . . . 10
5.4. Privacy . . . . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. References. . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References. . . . . . . . . . . . . . . . . . 10
7.2. Informative References. . . . . . . . . . . . . . . . . 11
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
9. Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . 12
10. Full Copyright Statement. . . . . . . . . . . . . . . . . . . 13
1. Terminology
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 BCP 14, RFC 2119 [2].
2. Introduction
The Open Pluggable Edge Services (OPES) architecture [1] enables
cooperative application services (OPES services) between a data
provider, a data consumer, and zero or more OPES processors. The
application services under consideration analyze, and possibly
transform, application-level messages exchanged between the data
provider and the data consumer.
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The execution of such services is governed by a set of rules
installed on the OPES processor. The enforcement of rules can
trigger the execution of service applications local to the OPES
processor. Alternatively, the OPES processor can distribute the
responsibility of service execution by communicating and
collaborating with one or more remote callout servers. As described
in [1], an OPES processor communicates with and invokes services on a
callout server by using a callout protocol. This document presents
the requirements for such a protocol.
The requirements in this document are divided into three categories -
functional requirements, performance requirements, and security
requirements. Each requirement is presented as one or more
statements, followed by brief explanatory material as appropriate.
3. Functional Requirements
3.1. Reliability
The OPES callout protocol MUST be able to provide ordered reliability
for the communication between an OPES processor and callout server.
Additionally, the callout protocol SHOULD be able to provide
unordered reliability.
In order to satisfy the reliability requirements, the callout
protocol SHOULD specify that it must be used with a transport
protocol that provides ordered/unordered reliability at the
transport-layer, for example TCP [6] or SCTP [7].
3.2. Congestion Avoidance
The OPES callout protocol MUST ensure that congestion avoidance
matching the standard of RFC 2914 [4] is applied on all communication
between the OPES processor and callout server. For this purpose, the
callout protocol SHOULD use a congestion-controlled transport-layer
protocol, presumably either TCP [6] or SCTP [7].
3.3. Callout Transactions
The OPES callout protocol MUST enable an OPES processor and a callout
server to perform callout transactions with the purpose of exchanging
partial or complete application-level protocol messages (or
modifications thereof). More specifically, the callout protocol MUST
enable an OPES processor to forward a partial or complete application
message to a callout server so that one or more OPES services can
process the forwarded application message (or parts thereof). The
result of the service operation may be a modified application
message. The callout protocol MUST therefore enable the callout
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server to return a modified application message or the modified parts
of an application message to the OPES processor. Additionally, the
callout protocol MUST enable a callout server to report the result of
a callout transaction (e.g., in the form of a status code) back to
the OPES processor.
A callout transaction is defined as a message exchange between an
OPES processor and a callout server consisting of a callout request
and a callout response. Both, the callout request and the callout
response MAY each consist of one or more callout protocol messages,
i.e. a series of protocol messages. A callout request MUST always
contain a partial or complete application message. A callout
response MUST always indicate the result of the callout transaction.
A callout response MAY contain a modified application message.
Callout transactions are always initiated by a callout request from
an OPES processor and are typically terminated by a callout response
from a callout server. The OPES callout protocol MUST, however, also
provide a mechanism that allows either endpoint of a callout
transaction to terminate a callout transaction before a callout
request or response has been completely received by the corresponding
callout endpoint. Such a mechanism MUST ensure that a premature
termination of a callout transaction does not result in the loss of
application message data.
A premature termination of a callout transaction is required to
support OPES services, which may terminate even before they have
processed the entire application message. Content analysis services,
for example, may be able to classify a Web object after having
processed just the first few bytes of a Web object.
3.4. Callout Connections
The OPES callout protocol MUST enable an OPES processor and a callout
server to perform multiple callout transactions over a callout
connection. Additionally, the callout protocol MUST provide a method
of associating callout transactions with callout connections. A
callout connection is defined as a logical connection at the
application-layer between an OPES processor and a callout server. A
callout connection MAY have certain parameters associated with it,
for example parameters that control the fail-over behavior of
connection endpoints. Callout connection-specific parameters MAY be
negotiated between OPES processors and callout servers (see Section
3.12).
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The OPES callout protocol MAY choose to multiplex multiple callout
connections over a single transport-layer connection if a flow
control mechanism that guarantees fairness among multiplexed callout
connections is applied.
Callout connections MUST always be initiated by an OPES processor. A
callout connection MAY be closed by either endpoint of the
connection, provided that doing so does not affect the normal
operation of on-going callout transactions associated with the
callout connection.
3.5. Asynchronous Message Exchange
The OPES callout protocol MUST support an asynchronous message
exchange over callout connections.
In order to allow asynchronous processing on the OPES processor and
callout server, it MUST be possible to separate request issuance from
response processing. The protocol MUST therefore allow multiple
outstanding callout requests and provide a method of correlating
callout responses with callout requests.
Additionally, the callout protocol MUST enable a callout server to
respond to a callout request before it has received the entire
request.
3.6. Message Segmentation
The OPES callout protocol MUST allow an OPES processor to forward an
application message to a callout server in a series of smaller
message fragments. The callout protocol MUST further enable the
receiving callout server to re-assemble the fragmented application
message.
Likewise, the callout protocol MUST enable a callout server to return
an application message to an OPES processor in a series of smaller
message fragments. The callout protocol MUST enable the receiving
OPES processor to re-assemble the fragmented application message.
Depending on the application-layer protocol used on the data path,
application messages may be very large in size (for example in the
case of audio/video streams) or of unknown size. In both cases, the
OPES processor has to initiate a callout transaction before it has
received the entire application message to avoid long delays for the
data consumer. The OPES processor MUST therefore be able to forward
fragments or chunks of an application message to a callout server as
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it receives them from the data provider or consumer. Likewise, the
callout server MUST be able to process and return application message
fragments as it receives them from the OPES processor.
Application message segmentation is also required if the OPES callout
protocol provides a flow control mechanism in order to multiplex
multiple callout connections over a single transport-layer connection
(see Section 3.4).
3.7. Support for Keep-Alive Mechanism
The OPES callout protocol MUST provide a keep-alive mechanism which,
if used, would allow both endpoints of a callout connection to detect
a failure of the other endpoint, even in the absence of callout
transactions. The callout protocol MAY specify that keep-alive
messages be exchanged over existing callout connections or a separate
connection between OPES processor and callout server. The callout
protocol MAY also specify that the use of the keep-alive mechanism is
optional.
The detection of a callout server failure may enable an OPES
processor to establish a callout connection with a stand-by callout
server so that future callout transactions do not result in the loss
of application message data. The detection of the failure of an OPES
processor may enable a callout server to release resources which
would otherwise not be available for callout transactions with other
OPES processors.
3.8. Operation in NAT Environments
The OPES protocol SHOULD be NAT-friendly, i.e., its operation should
not be compromised by the presence of one or more NAT devices in the
path between an OPES processor and a callout server.
3.9. Multiple Callout Servers
The OPES callout protocol MUST allow an OPES processor to
simultaneously communicate with more than one callout server.
In larger networks, OPES services are likely to be hosted by
different callout servers. Therefore, an OPES processor will likely
have to communicate with multiple callout servers. The protocol
design MUST enable an OPES processor to do so.
3.10. Multiple OPES Processors
The OPES callout protocol MUST allow a callout server to
simultaneously communicate with more than one OPES processor.
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The protocol design MUST support a scenario in which multiple OPES
processors use the services of a single callout server.
3.11. Support for Different Application Protocols
The OPES callout protocol SHOULD be application protocol-agnostic,
i.e., it SHOULD not make any assumptions about the characteristics of
the application-layer protocol used on the data path between the data
provider and data consumer. At a minimum, the callout protocol MUST
be compatible with HTTP [5].
The OPES entities on the data path may use different application-
layer protocols, including, but not limited to, HTTP [5] and RTP [8].
It would be desirable to be able to use the same OPES callout
protocol for any such application-layer protocol.
3.12. Capability and Parameter Negotiations
The OPES callout protocol MUST support the negotiation of
capabilities and callout connection parameters between an OPES
processor and a callout server. This implies that the OPES processor
and the callout server MUST be able to exchange their capabilities
and preferences. Then they MUST be able to engage in a deterministic
negotiation process that terminates either with the two endpoints
agreeing on the capabilities and parameters to be used for future
callout connections/transactions or with a determination that their
capabilities are incompatible.
Capabilities and parameters that could be negotiated between an OPES
processor and a callout server include (but are not limited to):
callout protocol version, fail-over behavior, heartbeat rate for
keep-alive messages, security-related parameters, etc.
The callout protocol MUST NOT use negotiation to determine the
transport protocol to be used for callout connections. The callout
protocol MAY, however, specify that a certain application message
protocol (e.g., HTTP [5], RTP [8]) requires the use of a certain
transport protocol (e.g., TCP [6], SCTP [7]).
Callout connection parameters may also pertain to the characteristics
of OPES callout services if, for example, callout connections are
associated with one or more specific OPES services. An OPES
service-specific parameter may, for example, specify which parts of
an application message an OPES service requires for its operation.
Callout connection parameters MUST be negotiated on a per-callout
connection basis and before any callout transactions are performed
over the corresponding callout connection. Other parameters and
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capabilities, such as the fail-over behavior, MAY be negotiated
between the two endpoints independently of callout connections.
The parties to a callout protocol MAY use callout connections to
negotiate all or some of their capabilities and parameters.
Alternatively, a separate control connection MAY be used for this
purpose.
3.13. Meta Data and Instructions
The OPES callout protocol MUST provide a mechanism for the endpoints
of a particular callout transaction to include metadata and
instructions for the OPES processor or callout server in callout
requests and responses.
Specifically, the callout protocol MUST enable an OPES processor to
include information about the forwarded application message in a
callout request, e.g. in order to specify the type of forwarded
application message or to specify what part(s) of the application
message are forwarded to the callout server. Likewise, the callout
server MUST be able to include information about the returned
application message.
The OPES processor MUST further be able to include an ordered list of
one or more uniquely specified OPES services which are to be
performed on the forwarded application message in the specified
order. However, as the callout protocol MAY also choose to associate
callout connections with specific OPES services, there may not be a
need to identify OPES services on a per-callout transaction basis.
Additionally, the OPES callout protocol MUST allow the callout server
to indicate to the OPES processor the cacheability of callout
responses. This implies that callout responses may have to carry
cache-control instructions for the OPES processor.
The OPES callout protocol MUST further enable the OPES processor to
indicate to the callout server if it has kept a local copy of the
forwarded application message (or parts thereof). This information
enables the callout server to determine whether the forwarded
application message must be returned to the OPES processor, even if
it has not been modified by an OPES service.
The OPES callout protocol MUST also allow OPES processors to comply
with the tracing requirements of the OPES architecture as laid out in
[1] and [3]. This implies that the callout protocol MUST enable a
callout server to convey to the OPES processor information about the
OPES service operations performed on the forwarded application
message.
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4. Performance Requirements
4.1. Protocol Efficiency
The OPES callout protocol SHOULD have minimal latency. For example,
the size and complexity of its headers could be minimized.
Because OPES callout transactions add latency to application protocol
transactions on the data path, callout protocol efficiency is crucial
to overall performance.
5. Security Requirements
In the absence of any security mechanisms, sensitive information
might be communicated between the OPES processor and the callout
server in violation of either endpoint's security and privacy policy,
through misconfiguration or deliberate insider attack. By using
strong authentication, message encryption, and integrity checks, this
threat can be minimized to a smaller set of insiders and/or operator
configuration errors.
The OPES processor and the callout servers SHOULD have enforceable
policies that limit the parties they communicate with and that
determine the protections to use based on identities of the endpoints
and other data (such as enduser policies). In order to enforce the
policies, they MUST be able to authenticate the callout protocol
endpoints using cryptographic methods.
5.1. Authentication, Confidentiality, and Integrity
The parties to the callout protocol MUST have a sound basis for
binding authenticated identities to the protocol endpoints, and they
MUST verify that these identities are consistent with their security
policies.
The OPES callout protocol MUST provide for message authentication,
confidentiality, and integrity between the OPES processor and the
callout server. It MUST provide mutual authentication. For this
purpose, the callout protocol SHOULD use existing security
mechanisms. The callout protocol specification is not required to
specify the security mechanisms, but it MAY instead refer to a
lower-level security protocol and discuss how its mechanisms are to
be used with the callout protocol.
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5.2. Hop-by-Hop Confidentiality
If hop-by-hop encryption is a requirement for the content path, then
this confidentiality MUST be extended to the communication between
the OPES processor and the callout server. While it is recommended
that the communication between the OPES processor and callout server
always be encrypted, encryption MAY be optional if both the OPES
processor and the callout server are co-located together in a single
administrative domain with strong confidentiality guarantees.
In order to minimize data exposure, the callout protocol MUST use a
different encryption key for each encrypted content stream.
5.3. Operation Across Untrusted Domains
The OPES callout protocol MUST operate securely across untrusted
domains between the OPES processor and the callout server.
If the communication channels between the OPES processor and callout
server cross outside of the organization which is responsible for the
OPES services, then endpoint authentication and message protection
(confidentiality and integrity) MUST be used.
5.4. Privacy
Any communication carrying information relevant to privacy policies
MUST protect the data using encryption.
6. Security Considerations
The security requirements for the OPES callout protocol are discussed
in Section 5.
7. References
7.1. Normative References
[1] Barbir, A., Penno, R., Chen, R., Hofmann, M., and H. Orman, "An
Architecture for Open Pluggable Edge Services (OPES)", RFC 3835,
August 2004.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Floyd, S. and L. Daigle, "IAB Architectural and Policy
Considerations for Open Pluggable Edge Services", RFC 3238,
January 2002.
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[4] Floyd, S. and L. Daigle, "IAB Architectural and Policy
Considerations for Open Pluggable Edge Services", RFC 3238,
January 2002.
[5] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.
7.2. Informative References
[6] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[7] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
H., Taylor, T., Rytina, I., Kalla, M., Zhang, L., and V. Paxson,
"Stream Control Transmission Protocol", RFC 2960, October 2000.
[8] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", RFC
3550, July 2003.
8. Acknowledgments
Parts of this document are based on previous work by Anca Dracinschi
Sailer, Volker Hilt, and Rama R. Menon.
The authors would like to thank the participants of the OPES WG for
their comments on this document.
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9. Authors' Addresses
Andre Beck
Lucent Technologies
101 Crawfords Corner Road
Holmdel, NJ 07733
US
EMail: abeck@bell-labs.com
Markus Hofmann
Lucent Technologies
Room 4F-513
101 Crawfords Corner Road
Holmdel, NJ 07733
US
Phone: +1 732 332 5983
EMail: hofmann@bell-labs.com
Hilarie Orman
Purple Streak Development
EMail: ho@alum.mit.edu
URI: http://www.purplestreak.com
Reinaldo Penno
Nortel Networks
600 Technology Park Drive
Billerica, MA 01821
US
EMail: rpenno@nortelnetworks.com
Andreas Terzis
Computer Science Department
Johns Hopkins University
3400 North Charles Street, 224 NEB
Baltimore, MD 21218
Phone: +1 410 516 5847
EMail: terzis@cs.jhu.edu
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10. Full Copyright Statement
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