Internet DRAFT - draft-ietf-midcom-requirements


Midcom Working Group                               R P Swale
Internet Draft                          BTexact Technologies
November 2001                                       P A Mart
Expires May 2002                      Marconi Communications
                                                    P Sijben
                                         Lucent Technologies
                                                  Scott Brim
                                               Melinda Shore
                                               Cisco Systems

  Middlebox Communications (midcom) Protocol Requirements

Status of this Memo

     This document is an Internet-Draft and is in full conformance with
     all provisions of Section 10 of RFC 2026 [RFC2026].

     Internet-Drafts are working documents of the Internet Engineering
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     This document specifies the requirements that the Middlebox Commu-
     nication (midcom) protocol must satisfy in order to meet the needs
     of applications wishing to influence middlebox function.  These
     requirements were developed with a specific focus on network
     address translation and firewall middleboxes.

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1.  Introduction

     This document is one of two developed by the Middlebox Communica-
     tion (midcom) working group to address the requirements and frame-
     work for a protocol between middleboxes and "midcom agents."  This
     document presents midcom requirements; [MCFW] presents the context
     and framework.  That document also presents terminology and defini-
     tions and should be read in tandem with this one.

     These requirements were developed by examining the midcom framework
     and extracting requirements, both explicit and implicit, that
     appeared there.

2.  Requirements

     Each requirement is presented as a statement, followed by brief
     explanatory material as appropriate.  Terminology is defined in
     [MCFW].  There may be overlap between requirements.

2.1.  Protocol machinery


     The Midcom protocol must enable a Midcom agent requiring the ser-
     vices of a middlebox to establish an authorized association between
     itself and the middlebox.

     This states that the protocol must allow the middlebox to identify
     an agent requesting services and make a determination as to whether
     or not the agent will be permitted to do so.


     The Midcom protocol must allow a Midcom agent to communicate with
     more than one middlebox simultaneously.

     In any but the most simple network, an agent is likely to want to
     influence the behavior of more than one middlebox.  The protocol
     design must not preclude the ability to do this.


     The Midcom protocol must allow a middlebox to communicate with more
     than one Midcom agent simultaneously.

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     There may be multiple instances of a single application or multiple
     applications desiring service from a single middlebox, and differ-
     ent agents may represent them.  The protocol design must not pre-
     clude the ability to do so.


     Where a multiplicity of Midcom Agents are interacting with a given
     middlebox, the Midcom protocol must provide mechanisms ensuring
     that the overall behavior is deterministic.

     This states that the protocol must include mechanisms for avoiding
     race conditions or other situations in which the requests of one
     agent may influence the results of the requests of other agents in
     an unpredictable manner.


     The Midcom protocol must enable the middlebox and any associated
     Midcom agents to establish known and stable state.  This must
     include the case of power failure, or other failure, where the pro-
     tocol must ensure that any resources used by a failed element can
     be released.

     This states that the protocol must provide clear identification for
     requests and results and that protocol operations must be atomic
     with respect to the midcom protocol.


     The middlebox must be able to report its status to a Midcom agent
     with which it is associated.


     The protocol must support unsolicited messages from middlebox to
     agent, for reporting conditions detected asynchronously at the mid-

     It may be the case that exceptional conditions or other events at
     the middlebox (resource shortages, intrusion mitigation) will cause
     the middlebox to close pinholes or release resources without con-
     sulting the associated Midcom agent.  In that event the protocol
     must allow the middlebox to notify the agent.

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                           Midcom Requirements              October 2001


     The Midcom protocol must provide for the mutual authentication of
     Midcom agent and middlebox to one another.

     In addition for the more obvious need for the Midcom agent to
     authenticate itself to the middlebox, there are some attacks
     against the protocol which can be mitigated by having the middlebox
     authenticate to the agent.  See [MCFW].


     The Midcom protocol must allow either the Midcom agent or the mid-
     dlebox to terminate the Midcom session between a Midcom Agent and a
     middlebox.  This allows either entity to close the session for
     maintenance, security or other reasons.


     A Midcom agent must be able to determine whether or not a request
     was successful.

     This states that a middlebox must return a success or failure indi-
     cation to a request made by an agent.


     The Midcom protocol must contain version interworking capabilities
     to enable subsequent extensions to support different types of mid-
     dlebox and future requirements of applications not considered at
     this stage.

     We assume that there will be later revisions of this protocol.  The
     initial version will focus on communication with firewalls and
     NATs, and it is possible that the protocol will need to be modified
     as support for other middlebox types is added.  These version
     interworking capabilities may include (but not be limited to) a
     protocol version number.


     It must be possible to deterministically predict the behavior of
     the middlebox in the presence of overlapping rules.

     The protocol must preclude nondeterministic behavior in the case of
     overlapping rulesets, e.g. by ensuring that some known precedence

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     is imposed.

2.2.  Midcom Protocol Semantics


     The syntax and semantics of the Midcom protocol must be extensible
     to allow the requirements of future applications to be adopted.

     This is related to, but different from, the requirement for ver-
     sioning support.  As support for additional middlebox types is
     added there may be a need to add new message types.


     The Midcom protocol must support the ability of an agent to install
     a ruleset that governs multiple types of middlebox actions (e.g.
     firewall and NAT).

     This states that a the protocol must support rules and actions for
     a variety of types of middleboxes.  A Midcom agent ought to be able
     to have a single Midcom session with a middlebox and use the Midcom
     interface on the middlebox to interface with different middlebox
     functions on the same middlebox interface.


     The protocol must support the concept of a ruleset group comprising
     a multiple of individual rulesets to be treated as an aggregate.

     Applications using more than one data stream may find it more con-
     venient and more efficient to be able to use single messages to
     tear down, extend, and manipulate all middlebox rulesets being used
     by one instance of the application.


     The protocol must allow the midcom agent to extend the lifetime of
     an existing ruleset that otherwise would be deleted by the middle-


     If a peer does not understand an option it must be clear whether
     the action required is to proceed without the unknown attribute
     being taken into account or the request is to be rejected.  Where

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     attributes may be ignored if not understood, a means may be pro-
     vided to inform the client about what has been ignored.

     This states that failure modes must be robust, providing sufficient
     information for the agent or middlebox to be able to accommodate
     the failure or to retry with a new option that is more likely to


     To enable management systems to interact with the Midcom environ-
     ment, the protocol must include failure reasons that allow the Mid-
     com Agent behavior to be modified as a result of the information
     contained in the reason.  Failure reasons need to be chosen such
     that they do not make an attack on security easier.


     The Midcom protocol must not preclude multiple authorized agents
     from working on the same ruleset.


     The Midcom protocol must be able to carry filtering rules, includ-
     ing but not limited to the 5-tuple, from the midcom agent to the

     By "5-tuple" we refer to the standard <source address, source port,
     destination address, destination port, transport protocol> tuple.
     Other filtering elements may be carried, as well.


     When the middlebox performs a port mapping function, the protocol
     should allow the Midcom agent to request that the external port
     number have the same oddity as the internal port.

     This requirement is to support RTP and RTCP [RFC1889] "oddity"


     When the middlebox performs a port mapping function, the protocol
     should allow the Midcom agent to request that a consecutive range
     of external port numbers be mapped to consecutive internal ports.

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                           Midcom Requirements              October 2001

     This requirement is to support RTP and RTCP "sequence" require-


     It should be possible to define rulesets that contain a more spe-
     cific filter spec than an overlapping ruleset.  This should allow
     agents to request actions for the subset that contradict those of
     the overlapping set.

     This should allow Midcom agent to request to a Midcom server con-
     trolling a firewall function that a subset of the traffic that
     would be allowed by the overlapping ruleset be specifically disal-

2.3.  General Security Requirements


     The Midcom protocol must provide for message authentication, confi-
     dentiality, and integrity.


     The Midcom protocol must allow for optional confidentiality protec-
     tion of control messages.  If provided the mechanism should allow a
     choice in the algorithm to be used.


     The Midcom protocol must operate across un-trusted domains between
     the Midcom agent and middlebox in a secure fashion.


     The Midcom protocol must define mechanisms to mitigate replay
     attacks on the control messages.

3.  Security Considerations

     The security requirements for a midcom protocol are discussed in
     section 2.3.

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4.  Normative References

[MCFW] Srisuresh, S. et al.  "Middlebox Communication Architecture and
     framework," work in progress.  October 2001.

[RFC1889] Schulzrinne, H. et al. "RTP: A Transport Protocol for Real-
     Time Applications," RFC 1889.  January 1996.

5.  Informative References

[RFC2026] Bradner, S. "The Internet Standards Process -- Revision 3,"
     RFC 2026. October 1996.

Authors' Addresses

     Richard Swale
     BTexact Technologies
     Callisto House
     Adastral Park
     Ipswich United Kingdom

     Paul Sijben
     Lucent Technologies EMEA BV

     Philip Mart
     Marconi Communications Ltd.
     Edge Lane
     United Kingdom

     Scott Brim
     Cisco Systems
     146 Honness Lane
     Ithaca, NY 14850

     Melinda Shore
     Cisco Systems
     809 Hayts Road
     Ithaca, NY 14850

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                           Midcom Requirements              October 2001



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                           Midcom Requirements              October 2001

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