Requirements for Open Pluggable Edge Services (OPES) Callout Protocols
RFC 3836

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

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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