<pre>Network Working Group                                       P. Srisuresh
Request for Comments: 3303                               Kuokoa Networks
Category: Informational                                        J. Kuthan
                                              Fraunhofer Institute FOKUS
                                                            J. Rosenberg
                                                             dynamicsoft
                                                              A. Molitor
                                                     Aravox Technologies
                                                               A. Rayhan
                                                      Ryerson University
                                                             August 2002


           <span class="h1">Middlebox communication architecture and framework</span>

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 (2002).  All Rights Reserved.

Abstract

   A principal objective of this document is to describe the underlying
   framework of middlebox communications (MIDCOM) to enable complex
   applications through the middleboxes, seamlessly using a trusted
   third party.  This document and a companion document on MIDCOM
   requirements ([<a href="#ref-REQMTS" title="&quot;Middlebox Communications (midcom) Protocol Requirements&quot;">REQMTS</a>]) have been created as a precursor to
   rechartering the MIDCOM working group.

   There are a variety of intermediate devices in the Internet today
   that require application intelligence for their operation.  Datagrams
   pertaining to real-time streaming applications, such as SIP and
   H.323, and peer-to-peer applications, such as Napster and NetMeeting,
   cannot be identified by merely examining packet headers.  Middleboxes
   implementing Firewall and Network Address Translator services
   typically embed application intelligence within the device for their
   operation.  The document specifies an architecture and framework in
   which trusted third parties can be delegated to assist the
   middleboxes to perform their operation, without resorting to
   embedding application intelligence.  Doing this will allow a
   middlebox to continue to provide the services, while keeping the
   middlebox application agnostic.




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<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>

   Intermediate devices requiring application intelligence are the
   subject of this document.  These devices are referred to as
   middleboxes throughout the document.  Many of these devices enforce
   application specific policy based functions such as packet filtering,
   VPN (Virtual Private Network) tunneling, Intrusion detection,
   security and so forth.  Network Address Translator service, on the
   other hand, provides routing transparency across address realms
   (within IPv4 routing network or across V4 and V6 routing realms),
   independent of applications.  Application Level Gateways (ALGs) are
   used in conjunction with NAT to examine and optionally modify
   application payload so the end-to-end application behavior remains
   unchanged for many of the applications traversing NAT middleboxes.
   There may be other types of services requiring embedding application
   intelligence in middleboxes for their operation.  The discussion
   scope of this document is however limited to Firewall and NAT
   services.  Nonetheless, the MIDCOM framework is designed to be
   extensible to support the deployment of new services.

   Tight coupling of application intelligence with middleboxes makes
   maintenance of middleboxes hard with the advent of new applications.
   Built-in application awareness typically requires updates of
   operating systems with new applications or newer versions of existing
   applications.  Operators requiring support for newer applications
   will not be able to use third party software/hardware specific to the
   application and are at the mercy of their middlebox vendor to make
   the necessary upgrade.  Further, embedding intelligence for a large
   number of application protocols within the same middlebox increases
   complexity of the middlebox and is likely to be error prone and
   degrade in performance.

   This document describes a framework in which application intelligence
   can be moved from middleboxes into external MIDCOM agents.  The
   premise of the framework is to devise a MIDCOM protocol that is
   application independent, so the middleboxes can stay focused on
   services such as firewall and NAT.  The framework document includes
   some explicit and implied requirements for the MIDCOM protocol.
   However, it must be noted that these requirements are only a subset.
   A separate requirements document lists the requirements in detail.

   MIDCOM agents with application intelligence can assist the
   middleboxes through the MIDCOM protocol in permitting applications
   such as FTP, SIP and H.323.  The communication between a MIDCOM agent
   and a middlebox will not be noticeable to the end-hosts that take
   part in the application, unless one of the end-hosts assumes the role
   of a MIDCOM agent.  Discovery of middleboxes or MIDCOM agents in the




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   path of an application instance is outside the scope of this
   document.  Further, any communication amongst middleboxes is also
   outside the scope of this document.

   This document describes the framework in which middlebox
   communication takes place and the various elements that constitute
   the framework.  <a href="#section-2">Section 2</a> describes the terms used in the document.
   <a href="#section-3">Section 3</a> defines the architectural framework of a middlebox for
   communication with MIDCOM agents.  The remaining sections cover the
   components of the framework, illustration using sample flows, and
   operational considerations with the MIDCOM architecture.  <a href="#section-4">Section 4</a>
   describes the nature of MIDCOM protocol.  <a href="#section-5">Section 5</a> identifies
   entities that could potentially host the MIDCOM agent function.
   <a href="#section-6">Section 6</a> considers the role of Policy server and its function with
   regard to communicating MIDCOM agent authorization policies.  <a href="#section-7">Section</a>
   <a href="#section-7">7</a> is an illustration of SIP flows using a MIDCOM framework in which
   the MIDCOM agent is co-resident on a SIP proxy server.  <a href="#section-8">Section 8</a>
   addresses operational considerations in deploying a protocol adhering
   to the framework described here.  <a href="#section-9">Section 9</a> is an applicability
   statement, scoping the location of middleboxes.  <a href="#section-11">Section 11</a> outlines
   security considerations for the middlebox in view of the MIDCOM
   framework.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Terminology</span>

   Below are the definitions for the terms used throughout the document.

<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a>. Middlebox function/service</span>

   A middlebox function or a middlebox service is an operation or method
   performed by a network intermediary that may require application
   specific intelligence for its operation.  Policy based packet
   filtering (a.k.a. firewall), Network address translation (NAT),
   Intrusion detection, Load balancing, Policy based tunneling and IPsec
   security are all examples of a middlebox function (or service).

<span class="h3"><a class="selflink" id="section-2.2" href="#section-2.2">2.2</a>. Middlebox</span>

   A Middlebox is a network intermediate device that implements one or
   more of the middlebox services.  A NAT middlebox is a middlebox
   implementing NAT service.  A firewall middlebox is a middlebox
   implementing firewall service.

   Traditional middleboxes embed application intelligence within the
   device to support specific application traversal.  Middleboxes
   supporting the MIDCOM protocol will be able to externalize
   application intelligence into MIDCOM agents.  In reality, some of the




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   middleboxes may continue to embed application intelligence for
   certain applications and depend on MIDCOM protocol and MIDCOM agents
   for the support of remaining applications.

<span class="h3"><a class="selflink" id="section-2.3" href="#section-2.3">2.3</a>. Firewall</span>

   Firewall is a policy based packet filtering middlebox function,
   typically used for restricting access to/from specific devices and
   applications.  The policies are often termed Access Control Lists
   (ACLs).

<span class="h3"><a class="selflink" id="section-2.4" href="#section-2.4">2.4</a>. NAT</span>

   Network Address Translation is a method by which IP addresses are
   mapped from one address realm to another, providing transparent
   routing to end-hosts.  Transparent routing here refers to modifying
   end-node addresses en-route and maintaining state for these updates
   so that when a datagram leaves one realm and enters another,
   datagrams pertaining to a session are forwarded to the right end-host
   in either realm.  Refer to [<a href="#ref-NAT-TERM" title="&quot;IP Network Address Translator (NAT) Terminology and Considerations&quot;">NAT-TERM</a>] for the definition of
   Transparent routing, various NAT types, and the associated terms in
   use.  Two types of NAT are most common.  Basic-NAT, where only an IP
   address (and the related IP, TCP/UDP checksums) of packets is altered
   and NAPT (Network Address Port Translation), where both an IP address
   and a transport layer identifier, such as a TCP/UDP port (and the
   related IP, TCP/UDP checksums), are altered.

   The term NAT in this document is very similar to the IPv4 NAT
   described in [<a href="#ref-NAT-TERM" title="&quot;IP Network Address Translator (NAT) Terminology and Considerations&quot;">NAT-TERM</a>], but is extended beyond IPv4 networks to
   include the IPv4-v6 NAT-PT described in [<a href="#ref-NAT-PT" title="&quot;Network Address Translation - Protocol Translation (NAT-PT)&quot;">NAT-PT</a>].  While the IPv4 NAT
   [<a href="#ref-NAT-TERM" title="&quot;IP Network Address Translator (NAT) Terminology and Considerations&quot;">NAT-TERM</a>] translates one IPv4 address into another IPv4 address to
   provide routing between private V4 and external V4 address realms,
   IPv4-v6 NAT-PT [<a href="#ref-NAT-PT" title="&quot;Network Address Translation - Protocol Translation (NAT-PT)&quot;">NAT-PT</a>] translates an IPv4 address into an IPv6
   address, and vice versa, to provide routing between a V6 address
   realm and an external V4 address realm.

   Unless specified otherwise, NAT in this document is a middlebox
   function referring to both IPv4 NAT, as well as IPv4-v6 NAT-PT.

<span class="h3"><a class="selflink" id="section-2.5" href="#section-2.5">2.5</a>. Proxy</span>

   A proxy is an intermediate relay agent between clients and servers of
   an application, relaying application messages between the two.
   Proxies use special protocol mechanisms to communicate with proxy
   clients and relay client data to servers and vice versa.  A Proxy
   terminates sessions with both the client and the server, acting as
   server to the end-host client and as client to the end-host server.




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   Applications such as FTP, SIP, and RTSP use a control session to
   establish data sessions.  These control and data sessions can take
   divergent paths.  While a proxy can intercept both the control and
   data sessions, it might intercept only the control session.  This is
   often the case with real-time streaming applications such as SIP and
   RTSP.

<span class="h3"><a class="selflink" id="section-2.6" href="#section-2.6">2.6</a>. ALG</span>

   Application Level Gateways (ALGs) are entities that possess the
   application specific intelligence and knowledge of an associated
   middlebox function.  An ALG examines application traffic in transit
   and assists the middlebox in carrying out its function.

   An ALG may be a co-resident with a middlebox or reside externally,
   communicating through a middlebox communication protocol.  It
   interacts with a middlebox to set up state, access control filters,
   use middlebox state information, modify application specific payload,
   or perform whatever else is necessary to enable the application to
   run through the middlebox.

   ALGs are different from proxies.  ALGs are not visible to end-hosts,
   unlike the proxies which are relay agents terminating sessions with
   both end-hosts.  ALGs do not terminate sessions with either end-host.
   Instead, ALGs examine, and optionally modify, application payload
   content to facilitate the flow of application traffic through a
   middlebox.  ALGs are middlebox centric, in that they assist the
   middleboxes in carrying out their function, whereas, the proxies act
   as a focal point for application servers, relaying traffic between
   application clients and servers.

   ALGs are similar to Proxies, in that, both ALGs and proxies
   facilitate Application specific communication between clients and
   servers.

<span class="h3"><a class="selflink" id="section-2.7" href="#section-2.7">2.7</a>. End-Hosts</span>

   End-hosts are entities that are party to a networked application
   instance.  End-hosts referred to in this document, are specifically
   those terminating Real-time streaming Voice-over-IP applications,
   such as SIP and H.323, and peer-to-peer applications such as Napster
   and NetMeeting.

<span class="h3"><a class="selflink" id="section-2.8" href="#section-2.8">2.8</a>. MIDCOM Agents</span>

   MIDCOM agents are entities performing ALG functions, logically
   external to a middlebox.  MIDCOM agents possess a combination of
   application awareness and knowledge of the middlebox function.  This



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   combination enables the agents to facilitate traversal of the
   middlebox by the application's packets.  A MIDCOM agent may interact
   with one or more middleboxes.

   Only "In-Path MIDCOM agents" are considered in this document.  In-
   Path MIDCOM agents are agents which are within the path of those
   datagrams that the agent needs to examine and/or modify in fulfilling
   its role as a MIDCOM agent.  "Within the path" here simply means that
   the packets in question flow through the node that hosts the agent.
   The packets may be addressed to the agent node at the IP layer.
   Alternatively they may not be addressed to the agent node, but may be
   constrained by other factors to flow through it.  In fact, it is
   immaterial to the MIDCOM protocol which of these is the case.  Some
   examples of In-Path MIDCOM agents are application proxies, gateways,
   or even end-hosts that are party to the application.

   Agents not resident on nodes that are within the path of their
   relevant application flows are referred to as "Out-of-Path (OOP)
   MIDCOM agents" and are out of the scope of this document.

<span class="h3"><a class="selflink" id="section-2.9" href="#section-2.9">2.9</a>. MIDCOM PDP</span>

   MIDCOM Policy Decision Point (PDP) is primarily a Policy Decision
   Point(PDP), as defined in [<a href="#ref-POL-TERM" title="&quot;Terminology for Policy-Based Management&quot;">POL-TERM</a>]; and also acts as a policy
   repository, holding MIDCOM related policy profiles in order to make
   authorization decisions.  [<a href="#ref-POL-TERM" title="&quot;Terminology for Policy-Based Management&quot;">POL-TERM</a>] defines a PDP as "a logical
   entity that makes policy decisions for itself or for other network
   elements that request such decisions"; and a policy repository as "a
   specific data store that holds policy rules, their conditions and
   actions, and related policy data".

   A middlebox and a MIDCOM PDP may communicate further if the MIDCOM
   PDP's policy changes or if a middlebox needs further information.
   The MIDCOM PDP may, at anytime, notify the middlebox to terminate
   authorization for an agent.

   The protocol facilitating the communication between a middlebox and
   MIDCOM PDP need not be part of the MIDCOM protocol.  <a href="#section-6">Section 6</a> in the
   document addresses the MIDCOM PDP interface and protocol framework
   independent of the MIDCOM framework.

   Application specific policy data and policy interface between an
   agent or application endpoint and a MIDCOM PDP is out of bounds for
   this document.  The MIDCOM PDP issues addressed in the document are
   focused at an aggregate domain level as befitting the middlebox.  For
   example, a SIP MIDCOM agent may choose to query a MIDCOM PDP for the
   administrative (or corporate) domain to find whether a certain user
   is allowed to make an outgoing call.  This type of application



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   specific policy data, as befitting an end user, is out of bounds for
   the MIDCOM PDP considered in this document.  It is within bounds,
   however, for the MIDCOM PDP to specify the specific end-user
   applications (or tuples) for which an agent is permitted to be an
   ALG.

<span class="h3"><a class="selflink" id="section-2.10" href="#section-2.10">2.10</a>. Middlebox Communication (MIDCOM) protocol</span>

   The protocol between a MIDCOM agent and a middlebox allows the MIDCOM
   agent to invoke services of the middlebox and allow the middlebox to
   delegate application specific processing to the MIDCOM agent.  The
   MIDCOM protocol allows the middlebox to perform its operation with
   the aid of MIDCOM agents, without resorting to embedding application
   intelligence.  The principal motivation behind architecting this
   protocol is to enable complex applications through middleboxes,
   seamlessly using a trusted third party, i.e., a MIDCOM agent.

   This is a protocol yet to be devised.

<span class="h3"><a class="selflink" id="section-2.11" href="#section-2.11">2.11</a>. MIDCOM agent registration</span>

   A MIDCOM agent registration is defined as the process of provisioning
   agent profile information with the middlebox or a MIDCOM PDP.  MIDCOM
   agent registration is often a manual operation performed by an
   operator rather than the agent itself.

   A MIDCOM agent profile may include agent authorization policy (i.e.,
   session tuples for which the agent is authorized to act as ALG),
   agent-hosting-entity (e.g., Proxy, Gateway, or end-host which hosts
   the agent), agent accessibility profile (including any host level
   authentication information), and security profile  (for the messages
   exchanged between the middlebox and the agent).

<span class="h3"><a class="selflink" id="section-2.12" href="#section-2.12">2.12</a>. MIDCOM session</span>

   A MIDCOM session is defined to be a lasting association between a
   MIDCOM agent and a middlebox.  The MIDCOM session is not assumed to
   imply any specific transport layer protocol.  Specifically, this
   should not be construed as referring to a connection-oriented TCP
   protocol.

<span class="h3"><a class="selflink" id="section-2.13" href="#section-2.13">2.13</a>. Filter</span>

   A filter is packet matching information that identifies a set of
   packets to be treated a certain way by a middlebox.  This definition
   is consistent with [<a href="#ref-POL-TERM" title="&quot;Terminology for Policy-Based Management&quot;">POL-TERM</a>], which defines a filter as "A set of





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   terms and/or criteria used for the purpose of separating or
   categorizing.  This is accomplished via single- or multi-field
   matching of traffic header and/or payload data".

   5-Tuple specification of packets in the case of a firewall and 5-
   tuple specification of a session in the case of a NAT middlebox
   function are examples of a filter.

<span class="h3"><a class="selflink" id="section-2.14" href="#section-2.14">2.14</a>. Policy action (or) Action</span>

   Policy action (or Action) is a description of the middlebox
   treatment/service to be applied to a set of packets.  This definition
   is consistent with  [<a href="#ref-POL-TERM" title="&quot;Terminology for Policy-Based Management&quot;">POL-TERM</a>], which defines a policy action as
   "Definition of what is to be done to enforce a policy rule, when the
   conditions of the rule are met.  Policy actions may result in the
   execution of one or more operations to affect and/or configure
   network traffic and network resources".

   NAT Address-BIND (or Port-BIND in the case of NAPT) and firewall
   permit/deny action are examples of an Action.

<span class="h3"><a class="selflink" id="section-2.15" href="#section-2.15">2.15</a>. Policy rule(s)</span>

   The combination of one or more filters and one or more actions.
   Packets matching a filter are to be treated as specified by the
   associated action(s).  The Policy rules may also contain auxiliary
   attributes such as individual rule type, timeout values, creating
   agent, etc.

   Policy rules are communicated through the MIDCOM protocol.

<span class="h3"><a class="selflink" id="section-3.0" href="#section-3.0">3.0</a> Architectural framework for middleboxes</span>

   A middlebox may implement one or more of the middlebox functions
   selectively on multiple interfaces of the device.  There can be a
   variety of MIDCOM agents interfacing with the middlebox to
   communicate with one or more of the middlebox functions on an
   interface.  As such, the middlebox communication protocol must allow
   for selective communication between a specific MIDCOM agent and one
   or more middlebox functions on the interface.  The following diagram
   identifies a possible layering of the service supported by a
   middlebox and a list of MIDCOM agents that might interact with it.









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               +---------------+  +--------------+
               | MIDCOM agent  |  | MIDCOM agent |
               | co-resident on|  | co-resident  |
               | Proxy Server  |  | on Appl. GW  |
               +---------------+  +--------------+
                          ^           ^
                          |           |                     +--------+
                 MIDCOM   |           |                     | MIDCOM |
                 Protocol |           |                   +-|  PDP   |
                          |           |                  /  +--------+
     +-------------+      |           |                 /
     | MIDCOM agent|      |           |                /
     | co-resident |      |           |               /
     | on End-hosts|&lt;-+   |           |              /
     +-------------+  |   |           |              |
                      v   v           v              v
                +-------------------------------------------+
                |  Middlebox Communication      |Policy     |
                |  Protocol (MIDCOM) Interface  |Interface  |
                +----------+--------+-----------+-----------+
     Middlebox  |          |        |           |           |
     Functions  | Firewall |  NAT   |   VPN     | Intrusion |
                |          |        | tunneling | Detection |
                +----------+--------+-----------+-----------+
     Middlebox  | Middlebox function specific policy rule(s)|
     Managed    | and other attributes                      |
     Resources  |                                           |
                +-------------------------------------------+

          Figure 1: MIDCOM agents interfacing with a middlebox

   Firewall ACLs, NAT-BINDs, NAT address-maps and Session-state are a
   few of the middlebox function specific policy rules.  A session state
   may include middlebox function specific attributes, such as timeout
   values, NAT translation parameters (i.e., NAT-BINDS), and so forth.
   As Session-state may be shared across middlebox functions, a
   Session-state may be created by a function, and terminated by a
   different function.  For example, a session-state may be created by
   the firewall function, but terminated by the NAT function, when a
   session timer expires.

   Application specific MIDCOM agents (co-resident on the middlebox or
   external to the middlebox) would examine the IP datagrams and help
   identify the application the datagram belongs to, and assist the
   middlebox in performing functions unique to the application and the
   middlebox service.  For example, a MIDCOM agent, assisting a NAT
   middlebox, might perform payload translations, whereas a MIDCOM agent




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   assisting a firewall middlebox might request the firewall to permit
   access to application specific, dynamically generated, session
   traffic.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. MIDCOM Protocol</span>

   The MIDCOM protocol between a MIDCOM agent and a middlebox allows the
   MIDCOM agent to invoke services of the middlebox and allow the
   middlebox to delegate application specific processing to the MIDCOM
   agent.  The protocol will allow MIDCOM agents to signal the
   middleboxes, to let complex applications using dynamic port based
   sessions through them (i.e., middleboxes) seamlessly.

   It is important to note that an agent and a middlebox can be on the
   same physical device.  In such a case, they may communicate using a
   MIDCOM protocol message formats, but using a non-IP based transport,
   such as IPC messaging (or) they may communicate using well-defined
   API/DLL (or) the application intelligence is fully embedded into the
   middlebox service (as it is done today in many stateful inspection
   firewall devices and NAT devices).

   The MIDCOM protocol will consist of a session setup phase, run-time
   session phase, and a session termination phase.

   Session setup must be preceded by registration of the MIDCOM agent
   with either the middlebox or the MIDCOM PDP.  The MIDCOM agent access
   and authorization profile may either be pre-configured on the
   middlebox (or) listed on a MIDCOM PDP; the middlebox is configured to
   consult.  MIDCOM shall be a client-server protocol, initiated by the
   agent.

   A MIDCOM session may be terminated by either of the parties.  A
   MIDCOM session termination may also be triggered by (a) the middlebox
   or the agent going out of service and not being available for further
   MIDCOM operations, or (b) the MIDCOM PDP notifying the middlebox that
   a particular MIDCOM agent is no longer authorized.

   The MIDCOM protocol data exchanged during run-time is governed
   principally by the middlebox services the protocol supports.
   Firewall and NAT middlebox services are considered in this document.
   Nonetheless, the MIDCOM framework is designed to be extensible to
   support the deployment of other services as well.









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<span class="h3"><a class="selflink" id="section-5.0" href="#section-5.0">5.0</a>. MIDCOM Agents</span>

   MIDCOM agents are logical entities which may reside physically on
   nodes external to a middlebox, possessing a combination of
   application awareness and knowledge of middlebox function.  A MIDCOM
   agent may communicate with one or more middleboxes.  The issues of
   middleboxes discovering agents, or vice versa, are outside the scope
   of this document.  The focus of the document is the framework in
   which a MIDCOM agent communicates with a middlebox using MIDCOM
   protocol, which is yet to be devised.  Specifically, the focus is
   restricted to just the In-Path agents.

   In-Path MIDCOM agents are MIDCOM agents that are located naturally
   within the message path of the application(s) they are associated
   with.  Bundled session applications, such as H.323, SIP, and RTSP
   which have separate control and data sessions, may have their
   sessions take divergent paths.  In those scenarios, In-Path MIDCOM
   agents are those that find themselves in the control path.  In a
   majority of cases, a middlebox will likely require the assistance of
   a single agent for an application in the control path alone.
   However, it is possible that a middlebox function, or a specific
   application traversing the middlebox might require the intervention
   of more than a single MIDCOM agent for the same application, one for
   each sub-session of the application.

   Application Proxies and gateways are a good choice for In-Path MIDCOM
   agents, as these entities by definition, are in the path of an
   application between a client and server.  In addition to hosting the
   MIDCOM agent function, these natively in-path application specific
   entities may also enforce application-specific choices locally, such
   as dropping messages infected with known viruses, or lacking user
   authentication.  These entities can be interjecting both the control
   and data sessions.  For example, FTP control and Data sessions are
   interjected by an FTP proxy server.

   However, proxies may also be interjecting just the control session
   and not the data sessions, as is the case with real-time streaming
   applications, such as SIP and RTSP.  Note, applications may not
   always traverse a proxy and some applications may not have a proxy
   server available.

   SIP proxies and H.323 gatekeepers may be used to host MIDCOM agent
   functions to control middleboxes implementing firewall and NAT
   functions.  The advantage of using in-path entities, as opposed to
   creating an entirely new agent, is that the in-path entities already
   possess application intelligence.  You will need to merely enable the
   use of the MIDCOM protocol to be an effective MIDCOM agent.  Figure 2
   below illustrates a scenario where the in-path MIDCOM agents



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   interface with the middlebox.  Let us say, the MIDCOM PDP has pre-
   configured the in-path proxies as trusted MIDCOM agents on the
   middlebox and the packet filter implements a 'default-deny' packet
   filtering policy.  Proxies use their application-awareness knowledge
   to control the firewall function and selectively permit a certain
   number of voice stream sessions dynamically using MIDCOM protocol.

   In the illustration below, the proxies and the MIDCOM PDP are shown
   inside a private domain.  The intent however, is not to imply that
   they be inside the private boundary alone.  The proxies may also
   reside external to the domain.  The only requirement is that there be
   a trust relationship with the middlebox.

                          +-----------+
                          | MIDCOM    |
                          |  PDP      |~~~~~~~~~~~~~|
                          +-----------+              \
                                                      \
                   +--------+                          \
                   | SIP    |___                        \
           ________| Proxy  |   \            Middlebox   \
          /        +--------+..  |        +--------------------+
         |                    :  | MIDCOM |           |        |
         |  RTSP +---------+  :..|........| MIDCOM    | POLICY |
     SIP |   ____|  RTSP   |.....|........| PROTOCOL  | INTER- |
         |  /    |  Proxy  |___  |        | INTERFACE | FACE   |
         | |     +---------+   \  \       |--------------------|
         | |                    \  \______|                    |__SIP
         | |                     \________|                    |__RTSP
         | |                           ---|     FIREWALL       |---&gt;--
        +-----------+                 /---|                    |---&lt;--
       +-----------+|  Data streams  //   +--------------------+
      +-----------+||----------&gt;----//            |
      |end-hosts  ||-----------&lt;-----             .
      +-----------+   (RTP, RTSP data, etc.)      |
                                                  .  Outside the
             Within a private domain              |  private domain

      Legend: ---- Application data path datagrams
              ____ Application control path datagrams
              .... Middlebox Communication Protocol (MIDCOM)
              ~~~~ MIDCOM PDP Interface
                |
                .  private domain Boundary
                |

       Figure 2: In-Path MIDCOM Agents for middlebox Communication




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<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>. End-hosts as In-Path MIDCOM agents</span>

   End-hosts are another variation of In-Path MIDCOM agents.  Unlike
   Proxies, End-hosts are a direct party to the application and possess
   all the end-to-end application intelligence there is to it.  End-
   hosts presumably terminate both the control and data paths of an
   application.  Unlike other entities hosting MIDCOM agents, end-host
   is able to process secure datagrams.  However, the problem would be
   one of manageability - upgrading all the end-hosts running a specific
   application.

<span class="h3"><a class="selflink" id="section-6.0" href="#section-6.0">6.0</a>. MIDCOM PDP functions</span>

   The functional decomposition of the MIDCOM architecture assumes the
   existence of a logical entity, known as MIDCOM PDP, responsible for
   performing authorization and related provisioning services for the
   middlebox as depicted in figure 1.  The MIDCOM PDP is a logical
   entity which may reside physically on a middlebox or on a node
   external to the middlebox.  The protocol employed for communication
   between the middlebox and the MIDCOM PDP is unrelated to the MIDCOM
   protocol.

   Agents are registered with a MIDCOM PDP for authorization to invoke
   services of the middlebox.  The MIDCOM PDP maintains a list of agents
   that are authorized to connect to each of the middleboxes the MIDCOM
   PDP supports.  In the context of the MIDCOM Framework, the MIDCOM PDP
   does not assist a middlebox in the implementation of the services it
   provides.

   The MIDCOM PDP acts in an advisory capacity to a middlebox, to
   authorize or terminate authorization for an agent attempting
   connectivity to the middlebox.  The primary objective of a MIDCOM PDP
   is to communicate agent authorization information, so as to ensure
   that the security and integrity of a middlebox is not jeopardized.
   Specifically, the MIDCOM PDP should associate a trust level with each
   agent attempting to connect to a middlebox and provide a security
   profile.  The MIDCOM PDP should be capable of addressing cases when
   end-hosts are agents to the middlebox.

<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>. Authentication, Integrity and Confidentiality</span>

   Host authenticity and individual message security are two distinct
   types of security considerations.  Host authentication refers to
   credentials required of a MIDCOM agent to authenticate itself to the
   middlebox and vice versa.  When authentication fails, the middlebox
   must not process signaling requests received from the agent that
   failed authentication.  Two-way authentication should be supported.
   In some cases, the 2-way authentication may be tightly linked to the



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   establishment of keys to protect subsequent traffic.  Two-way
   authentication is often required to prevent various active attacks on
   the MIDCOM protocol and secure establishment of keying material.

   Security services such as authentication, data integrity,
   confidentiality and replay protection may be adapted to secure MIDCOM
   messages in an untrusted domain.  Message authentication is the same
   as data origin authentication and is an affirmation that the sender
   of the message is who it claims to be.  Data integrity refers to the
   ability to ensure that a message has not been accidentally,
   maliciously or otherwise altered or destroyed.  Confidentiality is
   the encryption of a message with a key, so that only those in
   possession of the key can decipher the message content.  Lastly,
   replay protection is a form of sequence integrity, so when an
   intruder plays back a previously recorded sequence of messages, the
   receiver of the replay messages will simply drop the replay messages
   into bit-bucket.  Certain applications of the MIDCOM protocol might
   require support for non-repudiation as an option of the data
   integrity service.  Typically, support for non-repudiation is
   required for billing, service level agreements, payment orders, and
   receipts for delivery of service.

   IPsec AH ([IPSEC-AH]) offers data-origin authentication, data
   integrity and protection from message replay.  IPsec ESP ([IPSEC-
   ESP]) provides data-origin authentication to a lesser degree (same as
   IPsec AH if the MIDCOM transport protocol turns out to be TCP or
   UDP), message confidentiality, data integrity and protection from
   replay.  Besides the IPsec based protocols, there are other security
   options as well.  TLS based transport layer security is one option.
   There are also many application-layer security mechanisms available.
   Simple Source-address based security is a minimal form of security
   and should be relied on only in the most trusted environments, where
   those hosts will not be spoofed.

   The MIDCOM message security shall use existing standards, whenever
   the existing standards satisfy the requirements.  Security shall be
   specified to minimize the impact on sessions that do not use the
   security option.  Security should be designed to avoid introducing
   and to minimize the impact of denial of service attacks.  Some
   security mechanisms and algorithms require substantial processing or
   storage, in which case the security protocols should protect
   themselves as well as against possible flooding attacks that
   overwhelm the endpoint (i.e., the middlebox or the agent) with such
   processing.  For connection oriented protocols (such as TCP) using
   security services, the security protocol should detect premature
   closure or truncation attacks.





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<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>. Registration and deregistration of MIDCOM agents</span>

   Prior to allowing MIDCOM agents to invoke services of the middlebox,
   a registration process must take place.  Registration is a different
   process than establishing a MIDCOM session.  The former requires
   provisioning agent profile information with the middlebox or a MIDCOM
   PDP.  Agent registration is often a manual operation performed by an
   operator rather than the agent itself.  Setting up MIDCOM session
   refers to establishing a MIDCOM transport session and exchanging
   security credentials between an agent and a middlebox.  The transport
   session uses the registered information for session establishment.

   Profile of a MIDCOM agent includes agent authorization policy (i.e.,
   session tuples for which the agent is authorized to act as ALG),
   agent-hosting-entity (e.g., Proxy, Gateway or end-host which hosts
   the agent), agent accessibility profile (including any host level
   authentication information) and security profile (i.e., security
   requirements for messages exchanged between the middlebox and the
   agent).

   MIDCOM agent profile may be pre-configured on a middlebox.
   Subsequent to that, the agent may choose to initiate a MIDCOM session
   prior to any data traffic.  For example, MIDCOM agent authorization
   policy for a middlebox service may be preconfigured by specifying the
   agent in conjunction with a filter.  In the case of a firewall, for
   example, the ACL tuple may be altered to reflect the optional Agent
   presence.  The revised ACL may look something like the following.

   (&lt;Session-Direction&gt;, &lt;Source-Address&gt;, &lt;Destination-Address&gt;, &lt;IP-
   Protocol&gt;, &lt;Source-Port&gt;, &lt;Destination-Port&gt;, &lt;Agent&gt;)

   The reader should note that this is an illustrative example and not
   necessarily the actual definition of an ACL tuple.  The formal
   description of the ACL is yet to be devised.  Agent accessibility
   information should also be provisioned.  For a  MIDCOM agent,
   accessibility information includes the IP address, trust level, host
   authentication parameters and message authentication parameters.
   Once a session is established between a middlebox and a MIDCOM agent,
   that session should be usable with multiple instances of the
   application(s), as appropriate.  Note, all of this could be captured
   in an agent profile for ease of management.

   The technique described above is necessary for the pre-registration
   of MIDCOM agents with the middlebox.  The middlebox provisioning may
   remain unchanged, if the middlebox learns of the registered agents
   through a MIDCOM PDP.  In either case, the MIDCOM agent should
   initiate the session prior to the start of the application.  If the
   agent session is delayed until after the application has started, the



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   agent might be unable to process the control stream to permit the
   data sessions.  When a middlebox notices an incoming MIDCOM session,
   and the middlebox has no prior profile of the MIDCOM agent, the
   middlebox will consult its MIDCOM PDP for authenticity,
   authorization, and trust guidelines for the session.

<span class="h3"><a class="selflink" id="section-7.0" href="#section-7.0">7.0</a>. MIDCOM Framework Illustration using an In-Path agent</span>

   In figure 3 below, we consider SIP applications (Refer [<a href="#ref-SIP" title="&quot;SIP: Session Initiation Protocol&quot;">SIP</a>]) to
   illustrate the operation of the MIDCOM protocol.  Specifically, the
   application assumes that a caller, external to a private domain,
   initiates the call.  The middlebox is assumed to be located at the
   edge of the private domain.  A SIP phone (SIP User Agent
   Client/Server) inside the private domain is capable of receiving
   calls from external SIP phones.  The caller uses a SIP Proxy, node
   located external to the private domain, as its outbound proxy.  No
   interior proxy is assumed for the callee.  Lastly, the external SIP
   proxy node is designated to host the MIDCOM agent function.

   Arrows 1 and 8 in the figure below refer to a SIP call setup exchange
   between the external SIP phone and the SIP proxy.  Arrows 4 and 5
   refer to a SIP call setup exchange between the SIP proxy and the
   interior SIP phone, and are assumed to be traversing the middlebox.
   Arrows 2, 3, 6 and 7 below, between the SIP proxy and the middlebox,
   refer to MIDCOM communication.  Na and Nb represent RTP/RTCP media
   traffic (Refer [<a href="#ref-RTP" title="&quot;RTP: A Transport Protocol for Real-Time Applications&quot;">RTP</a>]) path in the external network.  Nc and Nd
   represent media traffic inside the private domain.

                               _________
                          ---&gt;|   SIP   |&lt;-----\
                         /    |  Proxy  |       \
                        |     |_________|       |
                       1|       |^    ^|       4|
                        |       ||    ||        |
                        |8     2||3  7||6       |5
        ______________  |       ||    ||        |    _____________
        |            |&lt;-/      _v|____|v___      \-&gt;|            |
        | External   |    Na   |           |   Nc   | SIP Phone  |
        | SIP phone  |&gt;-------&gt;| Middlebox |&gt;------&gt;| within     |
        |            |&lt;-------&lt;|___________|&lt;------&lt;| Pvt. domain|
        |____________|    Nb                   Nd   |____________|

      Figure 3: MIDCOM framework illustration with In-Path SIP Proxy

   As for the SIP application, we make the assumption that the middlebox
   is pre-configured to accept SIP calls into the private SIP phone.
   Specifically, this would imply that the middlebox implementing
   firewall service is pre-configured to permit SIP calls (destination



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<span class="grey"><a href="./rfc3303">RFC 3303</a>           MIDCOM Architecture and Framework         August 2002</span>


   TCP or UDP port number set to 5060) into the private phone.
   Likewise, middlebox implementing NAPT service would have been pre-
   configured to provide a port binding, to permit incoming SIP calls to
   be redirected to the specific private SIP phone.  I.e., the INVITE
   from the external caller is not made to the private IP address, but
   to the NAPT external address.

   The objective of the MIDCOM agent in the following illustration is to
   merely permit the RTP/RTCP media stream (Refer [<a href="#ref-RTP" title="&quot;RTP: A Transport Protocol for Real-Time Applications&quot;">RTP</a>]) through the
   middlebox, when using the MIDCOM protocol architecture outlined in
   the document.  A SIP session typically establishes two RTP/RTCP media
   streams - one from the callee to the caller and another from the
   caller to the callee.  These media sessions are UDP based and will
   use dynamic ports.  The dynamic ports used for the media stream are
   specified in the SDP section (Refer [<a href="#ref-SDP" title="&quot;SDP: Session Description Protocol&quot;">SDP</a>]) of the SIP payload
   message.  The MIDCOM agent will parse the SDP section and use the
   MIDCOM protocol to (a) open pinholes (i.e., permit RTP/RTCP session
   tuples) in a middlebox implementing firewall service, or (b) create
   PORT bindings and appropriately modify the SDP content to permit the
   RTP/RTCP streams through a middlebox implementing NAT service.  The
   MIDCOM protocol should be sufficiently rich and expressive to support
   the operations described under the timelines.  The examples do not
   show the timers maintained by the agent to keep the middlebox policy
   rule(s) from timing out.

   MIDCOM agent Registration and connectivity between the MIDCOM agent
   and the middlebox are not shown in the interest of restricting the
   focus of the MIDCOM transactions to enabling the middlebox to let the
   media stream through.  MIDCOM PDP is also not shown in the diagram
   below or on the timelines for the same reason.

   The following subsections illustrate a typical timeline sequence of
   operations that transpire with the various elements involved in a SIP
   telephony application path.  Each subsection is devoted to a specific
   instantiation of a middlebox service - NAPT (refer [<a href="#ref-NAT-TERM" title="&quot;IP Network Address Translator (NAT) Terminology and Considerations&quot;">NAT-TERM</a>], [NAT-
   TRAD]), firewall and a combination of both NAPT and firewall are
   considered.

<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>. Timeline flow - Middlebox implementing firewall service</span>

   In the following example, we will assume a middlebox implementing a
   firewall service.  We further assume that the middlebox is pre-
   configured to permit SIP calls (destination TCP or UDP port number
   set to 5060) into the private phone.  The following timeline
   illustrates the operations performed by the MIDCOM agent, to permit
   RTP/RTCP media stream through the middlebox.





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<span class="grey"><a href="./rfc3303">RFC 3303</a>           MIDCOM Architecture and Framework         August 2002</span>


   The INVITE from the caller (external) is assumed to include the SDP
   payload.  You will note that the MIDCOM agent requests the middlebox
   to permit the Private-to-external RTP/RTCP flows before the INVITE is
   relayed to the callee.  This is because, in SIP, the calling party
   must be ready to receive the media when it sends the INVITE with a
   session description.  If the called party (private phone) assumes
   this and sends "early media" before sending the 200 OK response, the
   firewall will have blocked these packets without this initial MIDCOM
   signaling from the agent.

      SIP Phone      SIP Proxy              Middlebox      SIP Phone
      (External)     (MIDCOM agent)         (FIREWALL      (private)
      |                 |                   Service)          |
      |                 |                      |              |
      |----INVITE------&gt;|                      |              |
      |                 |                      |              |
      |&lt;---100Trying----|                      |              |
      |                 |                      |              |
      |              Identify end-2-end        |              |
      |              parameters (from Caller's |              |
      |              SDP) for the pri-to-Ext   |              |
      |              RTP &amp; RTCP sessions.      |              |
      |              (RTP1, RTCP1)             |              |
      |                 |                      |              |
      |                 |+Permit RTP1, RTCP1 +&gt;|              |
      |                 |&lt;+RTP1, RTCP1 OKed++++|              |
      |                 |                      |              |
      |                 |--------INVITE----------------------&gt;|
      |                 |                      |              |
      |                 |&lt;-----180 Ringing--------------------|
      |&lt;--180Ringing----|                      |              |
      |                 |&lt;-------200 OK-----------------------|
      |                 |                      |              |
      |              Identify end-2-end        |              |
      |              parameters (from callee's |              |
      |              SDP) for the Ext-to-Pri   |              |
      |              RTP and RTCP sessions.    |              |
      |              (RTP2, RTCP2)             |              |
      |                 |                      |              |
      |                 |+Permit RTP2, RTCP2 +&gt;|              |
      |                 |&lt;+RTP2, RTCP2 OKed++++|              |
      |                 |                      |              |
      |&lt;---200 OK ------|                      |              |
      |-------ACK------&gt;|                      |              |
      |                 |-----------ACK----------------------&gt;|
      |                 |                      |              |
      |&lt;===================RTP/RTCP==========================&gt;|




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      |                 |                      |              |
      |-------BYE------&gt;|                      |              |
      |                 |--------------------------BYE-------&gt;|
      |                 |                      |              |
      |                 |&lt;----------200 OK--------------------|
      |                 |                      |              |
      |                 |++Cancel permits to   |              |
      |                 |  RTP1, RTCP1, RTP2,  |              |
      |                 |  and RTCP2 +++++++++&gt;|              |
      |                 |&lt;+RTP1, RTP2, RTCP1 &amp; |              |
      |                 |  RTCP2 cancelled ++++|              |
      |                 |                      |              |
      |&lt;---200 OK-------|                      |              |
      |                 |                      |              |

         Legend:      ++++    MIDCOM control traffic
                      ----    SIP control traffic
                      ====    RTP/RTCP media traffic

<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>. Timeline flow - Middlebox implementing NAPT service</span>

   In the following example, we will assume a middlebox implementing
   NAPT service.  We make the assumption that the middlebox is pre-
   configured to redirect SIP calls to the specific private SIP phone
   application.  I.e., the INVITE from the external caller is not made
   to the private IP address, but to the NAPT external address.  Let us
   say, the external phone's IP address is Ea, NAPT middlebox external
   Address is Ma, and the internal SIP phone's private address is Pa.
   SIP calls to the private SIP phone will arrive as TCP/UDP sessions,
   with the destination address and port set to Ma and 5060
   respectively.  The middlebox will redirect these datagrams to the
   internal SIP phone.  The following timeline will illustrate the
   operations necessary to be performed by the MIDCOM agent to permit
   the RTP/RTCP media stream through the middlebox.

   As with the previous example (<a href="#section-7.1">section 7.1</a>), the INVITE from the
   caller (external) is assumed to include the SDP payload.  You will
   note that the MIDCOM agent requests the middlebox to create NAT
   session descriptors for the private-to-external RTP/RTCP flows before
   the INVITE is relayed to the private SIP phone (for the same reasons
   as described in <a href="#section-7.1">section 7.1</a>).  If the called party (private phone)
   sends "early media" before sending the 200 OK response, the NAPT
   middlebox will have blocked these packets without the initial MIDCOM
   signaling from the agent.  Also, note that after the 200 OK is
   received by the proxy from the private phone, the agent requests the
   middlebox to allocate NAT session descriptors for the external-to-
   private RTP2 and RTCP2 flows, such that the ports assigned on the Ma
   for RTP2 and RTCP2 are contiguous.  The RTCP stream does not happen



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   with a non-contiguous port.  Lastly, you will note that even though
   each media stream (RTP1, RTCP1, RTP2 and RTCP2) is independent, they
   are all tied to the single SIP control session, while their NAT
   session descriptors were being created.  Finally, when the agent
   issues a terminate session bundle command for the SIP session, the
   middlebox is assumed to delete all associated media stream sessions
   automagically.

      SIP Phone      SIP Proxy              Middlebox     SIP Phone
      (External)     (MIDCOM agent)         (NAPT         (Private)
      IP Addr:Ea        |                   Service)      IP addr:Pa
      |                 |                   IP addr:Ma        |
      |                 |                      |              |
      |----INVITE------&gt;|                      |              |
      |                 |                      |              |
      |&lt;---100Trying----|                      |              |
      |                 |                      |              |
      |                 |++ Query Port-BIND    |              |
      |                 |   for (Ma, 5060) +++&gt;|              |
      |                 |&lt;+ Port-BIND reply    |              |
      |                 |   for (Ma, 5060) ++++|              |
      |                 |                      |              |
      |                 |++ Query NAT Session  |              |
      |                 |   Descriptor for     |              |
      |                 |   Ea-to-Pa SIP flow+&gt;|              |
      |                 |&lt;+ Ea-to-Pa SIP flow  |              |
      |                 |   Session Descriptor+|              |
      |                 |                      |              |
      |              Determine the Internal    |              |
      |              IP address (Pa)           |              |
      |              of the callee.            |              |
      |                 |                      |              |
      |              Identify UDP port numbers |              |
      |              on Ea (Eport1, Eport1+1)  |              |
      |              for pri-to-ext RTP &amp; RTCP |              |
      |              sessions (RTP1, RTCP1)    |              |
      |                 |                      |              |
      |                 |++Create NAT Session  |              |
      |                 |  descriptors for     |              |
      |                 |  RTP1, RTCP1; Set    |              |
      |                 |  parent session to   |              |
      |                 |  SIP-ctrl session ++&gt;|              |
      |                 |&lt;+RTP1, RTCP1 session |              |
      |                 |  descriptors created+|              |
      |                 |                      |              |
      |                 |                      |..redirected..|
      |                 |--------INVITE--------|-------------&gt;|
      |                 |                      |              |



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<span class="grey"><a href="./rfc3303">RFC 3303</a>           MIDCOM Architecture and Framework         August 2002</span>


      |                 |&lt;-----180Ringing---------------------|
      |                 |                      |              |
      |&lt;--180Ringing----|                      |              |
      |                 |&lt;-------200 OK-----------------------|
      |                 |                      |              |
      |              Identify UDP port numbers |              |
      |              on Pa (Pport2, Pport2+1)  |              |
      |              for ext-to-pri RTP &amp; RTCP |              |
      |              sessions (RTP2, RTCP2)    |              |
      |                 |                      |              |
      |                 |++Create consecutive  |              |
      |                 |  port BINDs on Ma    |              |
      |                 |  for (Pa, Pport2),   |              |
      |                 |  (Pa, Pport2+1) ++++&gt;|              |
      |                 |&lt;+Port BINDs created++|              |
      |                 |                      |              |
      |                 |++Create NAT Session  |              |
      |                 |  descriptors for     |              |
      |                 |  RTP2, RTCP2; Set    |              |
      |                 |  parent session to   |              |
      |                 |  SIP-ctrl session ++&gt;|              |
      |                 |&lt;+RTP2, RTCP2 session |              |
      |                 |  descriptors created+|              |
      |                 |                      |              |
      |              Modify the SDP            |              |
      |              parameters in "200 OK"    |              |
      |              with NAPT PORT-BIND       |              |
      |              for the RTP2 port on Ma.  |              |
      |                 |                      |              |
      |&lt;---200 OK ------|                      |              |
      |                 |                      |              |
      |-------ACK------&gt;|                      |              |
      |                 |                      |              |
      |              Modify IP addresses       |              |
      |              appropriately in the SIP  |              |
      |              header (e.g., To, from,   |              |
      |              Via, contact fields)      |              |
      |                 |                      |..redirected..|
      |                 |-----------ACK--------|-------------&gt;|
      |                 |                      |              |
      |                 |                      |              |
      |&lt;===================RTP/RTCP============|=============&gt;|
      |                 |                      |              |
      |-------BYE------&gt;|                      |              |
      |                 |                      |              |
      |                 |----------------------|-----BYE-----&gt;|
      |                 |                      |              |
      |                 |&lt;----------200 OK--------------------|



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      |                 |                      |              |
      |                 |+++Terminate the SIP  |              |
      |                 |   Session bundle +++&gt;|              |
      |                 |&lt;++SIP Session bundle |              |
      |                 |   terminated ++++++++|              |
      |                 |                      |              |
      |&lt;---200 OK-------|                      |              |
      |                 |                      |              |

         Legend:      ++++    MIDCOM control traffic
                      ----    SIP control traffic
                      ====    RTP/RTCP media traffic

<span class="h3"><a class="selflink" id="section-7.3" href="#section-7.3">7.3</a>. Timeline flow - Middlebox implementing NAPT and firewall</span>

   In the following example, we will assume a middlebox implementing a
   combination of a firewall and a stateful NAPT service.  We make the
   assumption that the NAPT function is configured to translate the IP
   and TCP headers of the initial SIP session into the private SIP
   phone, and the firewall function is configured to permit the initial
   SIP session.

   In the following time line, it may be noted that the firewall
   description is based on packet fields on the wire (ex: as seen on the
   external interface of the middlebox).  In order to ensure correct
   behavior of the individual services, you will notice that NAT
   specific MIDCOM operations precede firewall specific operations on
   the MIDCOM agent.  This is noticeable in the time line below when the
   MIDCOM agent processes the "200 OK" from the private SIP phone.  The
   MIDCOM agent initially requests the NAT service on the middlebox to
   set up port-BIND and session-descriptors for the media stream in both
   directions.  Subsequent to that, the MIDCOM agent determines the
   session parameters (i.e., the dynamic UDP ports) for the media
   stream, as viewed by the external interface and requests the firewall
   service on the middlebox to permit those sessions through.

      SIP Phone      SIP Proxy              Middlebox     SIP Phone
      (External)     (MIDCOM agent)         (NAPT &amp;       (Private)
      IP Addr:Ea        |                   firewall      IP addr:Pa
      |                 |                   Services)         |
      |                 |                   IP addr:Ma        |
      |                 |                      |              |
      |----INVITE------&gt;|                      |              |
      |                 |                      |              |
      |&lt;---100Trying----|                      |              |
      |                 |                      |              |
      |                 |++ Query Port-BIND    |              |
      |                 |   for (Ma, 5060) +++&gt;|              |



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      |                 |&lt;+ Port-BIND reply    |              |
      |                 |   for (Ma, 5060) ++++|              |
      |                 |                      |              |
      |                 |++ Query NAT Session  |              |
      |                 |   Descriptor for     |              |
      |                 |   Ea-to-Pa SIP flow+&gt;|              |
      |                 |&lt;+ Ea-to-Pa SIP flow  |              |
      |                 |   Session Descriptor+|              |
      |                 |                      |              |
      |              Determine the Internal    |              |
      |              IP address (Pa)           |              |
      |              of the callee.            |              |
      |                 |                      |              |
      |              Identify UDP port numbers |              |
      |              on Ea (Eport1, Eport1+1)  |              |
      |              for pri-to-ext RTP &amp; RTCP |              |
      |              sessions (RTP1, RTCP1)    |              |
      |                 |                      |              |
      |                 |++Create NAT Session  |              |
      |                 |  descriptors for     |              |
      |                 |  RTP1, RTCP1; Set the|              |
      |                 |  parent session to   |              |
      |                 |  point to SIP flow++&gt;|              |
      |                 |&lt;+RTP1, RTCP1 session |              |
      |                 |  descriptors created+|              |
      |                 |                      |              |
      |                 |++Permit RTP1 &amp; RTCP1 |              |
      |                 |  sessions External to|              |
      |                 |  middlebox, namely   |              |
      |                 |  Ma to Ea:Eport1,    |              |
      |                 |  Ma to Ea:Eport1+1   |              |
      |                 |  sessions ++++++++++&gt;|              |
      |                 |&lt;+Ma to Ea:Eport1,    |              |
      |                 |  Ma to Ea:Eport1+1   |              |
      |                 |  sessions OKed ++++++|              |
      |                 |                      |              |
      |                 |                      |..redirected..|
      |                 |--------INVITE--------|-------------&gt;|
      |                 |                      |              |
      |                 |&lt;-----180Ringing---------------------|
      |                 |                      |              |
      |&lt;--180Ringing----|                      |              |
      |                 |&lt;-------200 OK-----------------------|
      |                 |                      |              |
      |              Identify UDP port numbers |              |
      |              on Pa (Pport2, Pport2+1)  |              |
      |              for ext-to-pri RTP &amp; RTCP |              |
      |              sessions (RTP2, RTCP2)    |              |



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      |                 |                      |              |
      |                 |++Create consecutive  |              |
      |                 |  port BINDs on Ma    |              |
      |                 |  for (Pa, Pport2),   |              |
      |                 |  (Pa, Pport2+1) ++++&gt;|              |
      |                 |&lt;+Port BINDs created  |              |
      |                 |  on Ma as (Mport2,   |              |
      |                 |  Mport2+1) ++++++++++|              |
      |                 |                      |              |
      |                 |++Create NAT Session  |              |
      |                 |  descriptors for     |              |
      |                 |  RTP2, RTCP2; Set the|              |
      |                 |  parent session to   |              |
      |                 |  point to SIP flow++&gt;|              |
      |                 |&lt;+RTP2, RTCP2 session |              |
      |                 |  descriptors created+|              |
      |                 |                      |              |
      |              Modify the SDP            |              |
      |              parameters in "200 OK"    |              |
      |              with NAPT PORT-BIND       |              |
      |              for RTP2 port on Ma.      |              |
      |                 |                      |              |
      |                 |++Permit RTP2 &amp; RTCP2 |              |
      |                 |  sessions External   |              |
      |                 |  middlebox, namely   |              |
      |                 |  Ea to Ma:Mport2,    |              |
      |                 |  Ea to Ma:Mport2+1   |              |
      |                 |  sessions ++++++++++&gt;|              |
      |                 |&lt;+Ea to Ma:Mport2,    |              |
      |                 |  Ea to Ma:Mport2     |              |
      |                 |  sessions OKed ++++++|              |
      |                 |                      |              |
      |&lt;---200 OK ------|                      |              |
      |                 |                      |              |
      |-------ACK------&gt;|                      |              |
      |                 |                      |..redirected..|
      |                 |-----------ACK--------|-------------&gt;|
      |                 |                      |              |
      |                 |                      |              |
      |&lt;===================RTP/RTCP============|=============&gt;|
      |                 |                      |              |
      |-------BYE------&gt;|                      |              |
      |                 |                      |              |
      |                 |----------------------|-----BYE-----&gt;|
      |                 |                      |              |
      |                 |&lt;----------200 OK--------------------|
      |                 |                      |              |
      |                 |+++Terminate the SIP  |              |



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      |                 |   Session bundle +++&gt;|              |
      |                 |&lt;++SIP Session bundle |              |
      |                 |   terminated ++++++++|              |
      |                 |                      |              |
      |                 |++Cancel permits to   |              |
      |                 |  sessions External   |              |
      |                 |  middlebox, namely   |              |
      |                 |  Ma to Ea:Eport1,    |              |
      |                 |  Ma to Ea:Eport1+1   |              |
      |                 |  Ea to Ma:Mport2,    |              |
      |                 |  Ea to Ma:Mport2+1   |              |
      |                 |  sessions ++++++++++&gt;|              |
      |                 |&lt;+Removed permits to  |              |
      |                 |  sessions listed ++++|              |
      |                 |                      |              |
      |&lt;---200 OK-------|                      |              |
      |                 |                      |              |

         Legend:      ++++    MIDCOM control traffic
                      ----    SIP control traffic
                      ====    RTP/RTCP media traffic

<span class="h3"><a class="selflink" id="section-8.0" href="#section-8.0">8.0</a>. Operational considerations</span>

<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>. Multiple MIDCOM sessions between agents and middlebox</span>

   A middlebox cannot be assumed to be a simple device implementing just
   one middlebox function and no more than a couple of interfaces.
   Middleboxes often combine multiple intermediate functions into the
   same device and have the ability to provision individual interfaces
   of the same device with different sets of functions and varied
   provisioning for the same function across the interfaces.

   As such, 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 services on the same middlebox.

<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>. Asynchronous notification to MIDCOM agents</span>

   Asynchronous notification by the middlebox to a MIDCOM agent can be
   useful for events such as Session creation, Session termination,
   MIDCOM protocol failure, middlebox function failure or any other
   significant event.  Independently, ICMP error codes can also be
   useful to notify transport layer failures to the agents.

   In addition, periodic notification of various forms of data, such as
   statistics update, would also be a useful function that would be
   beneficial to certain types of agents.



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<span class="h3"><a class="selflink" id="section-8.3" href="#section-8.3">8.3</a>. Timers on middlebox considered useful</span>

   When supporting the MIDCOM protocol, the middlebox is required to
   allocate dynamic resources, as specified in policy rule(s), upon
   request from agents.  Explicit release of dynamically allocated
   resources happens when the application session is ended or when a
   MIDCOM agent requests the middlebox to release the resource.

   However, the middlebox should be able to recover the dynamically
   allocated resources, even as the agent that was responsible for the
   allocation is not alive.  Associating a lifetime for these dynamic
   resources and using a timer to track the lifetime can be a good way
   to accomplish this.

<span class="h3"><a class="selflink" id="section-8.4" href="#section-8.4">8.4</a>. Middleboxes supporting multiple services</span>

   A middlebox could be implementing a variety of services (e.g. NAT and
   firewall) in the same box.  Some of these services might have inter-
   dependency on shared resources and sequence of operation.  Others may
   be independent of each other.  Generally speaking, the sequence in
   which these function operations may be performed on datagrams is not
   within the scope of this document.

   In the case of a middlebox implementing NAT and firewall services, it
   is safe to state that the NAT operation on an interface will precede
   a firewall on the egress and will follow a firewall on the ingress.
   Further, firewall access control lists, used by a firewall, are
   assumed to be based on session parameters, as seen on the interface
   supporting firewall service.

<span class="h3"><a class="selflink" id="section-8.5" href="#section-8.5">8.5</a>. Signaling and Data traffic</span>

   The class of applications the MIDCOM architecture addresses focus
   around applications that have a combination of, one or more,
   signaling and data traffic sessions.  The signaling may be done out-
   of-band, using a dedicated stand-alone session or may be done in-
   band, within a data session.  Alternately, signaling may also be done
   as a combination of both stand-alone and in-band sessions.

   SIP is an example of an application based on distinct signaling and
   data sessions.  A SIP signaling session is used for call setup
   between a caller and a callee.  A MIDCOM agent may be required to
   examine/modify SIP payload content to administer the middlebox so as
   to let the media streams (RTP/RTCP based) through.  A MIDCOM agent is
   not required to intervene in the data traffic.






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   Signaling and context specific Header information is sent in-band,
   within the same data stream for applications such as HTTP embedded
   applications, sun-RPC (embedding a variety of NFS apps), Oracle
   transactions (embedding oracle SQL+, MS ODBC, Peoplesoft) etc.

   H.323 is an example of an application that sends signaling in both
   dedicated stand-alone sessions, as well as in conjunction with data.
   H.225.0 call signaling traffic traverses middleboxes by virtue of
   static policy, no MIDCOM control needed.  H.225.0 call signaling also
   negotiates ports for an H.245 TCP stream.  A MIDCOM agent is required
   to examine/modify the contents of the H.245 so that H.245 can
   traverse it.

   H.245 traverses the middlebox and also carries Open Logical Channel
   information for media data.  So, the MIDCOM agent is once again
   required to examine/modify the payload content needs to let the media
   traffic flow.

   The MIDCOM architecture takes into consideration, supporting
   applications with independent signaling and data sessions as well as
   applications that have signaling and data communicated over the same
   session.

   In the cases where signaling is done on a single stand-alone session,
   it is desirable to have a MIDCOM agent interpret the signaling stream
   and program the middlebox (that transits the data stream) so as to
   let the data traffic through uninterrupted.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>. Applicability Statement</span>

   Middleboxes may be stationed in a number of topologies.  However, the
   signaling framework outlined in this document may be limited to only
   those middleboxes that are located in a DMZ (De-Militarized Zone) at
   the edge of a private domain, connecting to the Internet.
   Specifically, the assumption is that you have a single middlebox
   (running NAT or firewall) along the application route.  Discovery of
   a middlebox along an application route is outside the scope of this
   document.  It is conceivable to have middleboxes located between
   departments within the same domain or inside the service provider's
   domain and so forth.  However, care must be taken to review each
   individual scenario and determine the applicability on a case-by-case
   basis.

   The applicability may also be illustrated as follows.  Real-time and
   streaming applications, such as Voice-Over-IP, and peer-to-peer
   applications, such as Napster and Netmeeting, require administering
   firewalls and NAT middleboxes to let their media streams reach hosts
   inside a private domain.  The requirements are in the form of



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   establishing a "pin-hole" to permit a TCP/UDP session (the port
   parameters of which are dynamically determined) through a firewall or
   retain an address/port bind in the NAT device to permit sessions to a
   port.  These requirements are met by current generation middleboxes
   using adhoc methods, such as embedding application intelligence
   within a middlebox to identify the dynamic session parameters and
   administering the middlebox internally as appropriate.  The objective
   of the MIDCOM architecture is to create a unified, standard way to
   exercise this functionality, currently existing in an ad-hoc fashion,
   in some of the middleboxes.

   By adopting MIDCOM architecture, middleboxes will be able to support
   newer applications they have not been able to support thus far.
   MIDCOM architecture does not, and must not in anyway, change the
   fundamental characteristic of the services supported on the
   middlebox.

   Typically, organizations shield a majority of their corporate
   resources (such as end-hosts) from visibility to the external network
   by the use of a De-Militarized Zone (DMZ) at the domain edge.  Only a
   portion of these hosts are allowed to be accessed by the external
   world.  The remaining hosts and their names are unique to the private
   domain.  Hosts visible to the external world and the authoritative
   name server that maps their names to network addresses are often
   configured within a DMZ (De-Militarized Zone) in front of a firewall.
   Hosts and middleboxes within DMZ are referred to as DMZ nodes.

   Figure 4 below illustrates the configuration of a private domain with
   a DMZ at its edge.  Actual configurations may vary.  Internal hosts
   are accessed only by users inside the domain.  Middleboxes, located
   in the DMZ may be accessed by agents inside or outside the domain.




















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                                      \ | /
                              +-----------------------+
                              |Service Provider Router|
                              +-----------------------+
                               WAN  |
                  Stub A .........|\|....
                                  |
                        +---------------+
                        | NAT middlebox |
                        +---------------+
                            |
                            |   DMZ - Network
      ------------------------------------------------------------
         |         |              |            |             |
        +--+      +--+           +--+         +--+      +-----------+
        |__|      |__|           |__|         |__|      | Firewall  |
       /____\    /____\         /____\       /____\     | middlebox |
      DMZ-Host1  DMZ-Host2 ...  DMZ-Name     DMZ-Web    +-----------+
                                Server       Server etc.   |
                                                           |
        Internal Hosts (inside the private domain)         |
      ------------------------------------------------------------
          |             |                 |           |
         +--+         +--+               +--+       +--+
         |__|         |__|               |__|       |__|
        /____\       /____\             /____\     /____\
       Int-Host1    Int-Host2  .....   Int-Hostn   Int-Name Server

       Figure 4: DMZ network configuration of a private domain.

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>. Acknowledgements</span>

   The authors wish to thank Christian Huitema, Joon Maeng, Jon
   Peterson, Mike Fisk, Matt Holdrege, Melinda Shore, Paul Sijben,
   Philip Mart, Scott Brim and Richard Swale for their valuable
   critique, advice and input on an earlier rough version of this
   document.  The authors owe special thanks to Eliot Lear for kick-
   starting the e-mail discussion on use-case scenarios with a SIP
   application flow diagram through a middlebox.  Much thanks to Bob
   Penfield, Cedric Aoun, Christopher Martin, Eric Fleischman, George
   Michaelson, Wanqun Bao, and others in the MIDCOM work group for their
   very detailed feedback on a variety of topics and adding clarity to
   the discussion.  Last, but not the least, the authors owe much thanks
   to Mark Duffy, Scott Brim, Melinda Shore and others for their help
   with terminology definition and discussing the embedded requirements
   within the framework document.





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<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>. Security Considerations</span>

   Discussed below are security considerations in accessing a middlebox.
   Without MIDCOM protocol support, the premise of a middlebox operation
   fundamentally requires the data to be in the clear, as the middlebox
   needs the ability to inspect and/or modify packet headers and
   payload.  This compromises the confidentiality requirement in some
   environments.  Further, updating transport headers and rewriting
   application payload data, in some cases, by NAT prevents the use of
   integrity protection on some data streams traversing NAT middleboxes.
   Clearly, this can pose a significant security threat to the
   application in an untrusted transport domain.

   The MIDCOM protocol framework removes the need for a middlebox to
   inspect or manipulate transport payload.  This allows applications to
   better protect themselves end-to-end with the aid of a trusted MIDCOM
   agent.  This is especially the case when the agent is a resident on
   the end-host.  When an agent has the same end-to-end ability as the
   end-host to interpret encrypted and integrity protected data,
   transiting a middlebox can be encrypted and integrity protected.  The
   MIDCOM agent will still be able to interpret the data and simply
   notify the middlebox of open holes, install NAT table entries, etc.
   Note, however, the MIDCOM framework does not help with the problem of
   NAT breaking IPsec since in this case the middlebox still modifies IP
   and transport headers.

   Security between a MIDCOM agent and a middlebox has a number of
   components.  Authorization, authentication, integrity and
   confidentiality.  Authorization refers to whether a particular agent
   is authorized to signal a middlebox with requests for one or more
   applications, adhering to a certain policy profile.  Failing the
   authorization process might indicate a resource theft attempt or
   failure due to administrative and/or credential deficiencies.  In
   either case, the middlebox should take the proper measures to
   audit/log such attempts and consult its designated MIDCOM PDP for the
   required action if the middlebox is configured with one.
   Alternatively, the middlebox may resort to a default service deny
   policy when a MIDCOM agent fails to prompt the required credentials.
   <a href="#section-6">Section 6</a> discusses the middlebox to MIDCOM PDP interactions in view
   of policy decisions.

   Authentication refers to confirming the identity of an originator for
   all datagrams received from the originator.  Lack of strong
   credentials for authentication of MIDCOM messages between an agent
   and a middlebox can seriously jeopardize the fundamental service
   rendered by the middlebox.  A consequence of not authenticating an
   agent would be that an attacker could spoof the identity of a
   "legitimate" agent and open holes in the firewall.  Another would be



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   that it could otherwise manipulate the state on a middlebox, creating
   a denial-of-service attack by closing needed pinholes or filling up a
   NAT table.  A consequence of not authenticating the middlebox to an
   agent is that an attacker could pose as a middlebox and respond to
   NAT requests in a manner that would divert data to the attacker.
   Failing to submit the required/valid credentials, once challenged,
   may indicate a replay attack, in which case a proper action is
   required by the middlebox such as auditing, logging, or consulting
   its designated MIDCOM PDP to reflect such failure.  A consequence of
   not protecting the middlebox against replay attacks would be that a
   specific pinhole may be reopened or closed by an attacker at will,
   thereby bombarding end hosts with unwarranted data or causing denial
   of service.

   Integrity is required to ensure that a MIDCOM message has not been
   accidentally or maliciously altered or destroyed.  The result of a
   lack of data integrity enforcement in an untrusted environment could
   be that an imposter will alter the messages sent by an agent and
   bring the middlebox to a halt or cause a denial of service for the
   application the agent is attempting to enable.

   Confidentiality of MIDCOM messages ensure that the signaling data is
   accessible only to the authorized entities.  When a middlebox agent
   is deployed in an untrusted environment, lack of confidentiality will
   allow an intruder to perform traffic flow analysis and snoop the
   middlebox.  The intruder could cannibalize a lesser secure MIDCOM
   session and destroy or compromise the middlebox resources he
   uncovered on other sessions.  Needless to say, the least secure
   MIDCOM session will become the achilles heel and make the middlebox
   vulnerable to security attacks.

   Lastly, there can be security vulnerability to the applications
   traversing a middlebox when a resource on a middlebox is controlled
   by multiple external agents.  A middlebox service may be disrupted
   due to conflicting directives from multiple agents associated with
   different middlebox functions but applied to the same application
   session.  Care must be taken in the protocol design to ensure that
   agents for one function do not abruptly step over resources impacting
   a different function.  Alternately, the severity of such
   manifestations could be lessened when a single MIDCOM agent is
   responsible for supporting all the middlebox services for an
   application, due to the reduced complexity and synchronization effort
   in managing the middlebox resources.








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References

   [<a id="ref-SIP">SIP</a>]       Rosenberg, J., Shulzrinne, H., Camarillo, G., Johnston,
               A., Peterson, J., Sparks, R., Handley, M., Schooler, E.,
               "SIP: Session Initiation Protocol", <a href="./rfc3261">RFC 3261</a>, June 2002.

   [<a id="ref-SDP">SDP</a>]       Handley, M. and V. Jacobson, "SDP: Session Description
               Protocol", <a href="./rfc2327">RFC 2327</a>, April 1998.

   [<a id="ref-H.323">H.323</a>]     ITU-T Recommendation H.323. "Packet-based Multimedia
               Communications Systems," 1998.

   [<a id="ref-RTP">RTP</a>]       Schulzrinne, H., Casner, S., Frederick, R. and V.
               Jacobson, "RTP: A Transport Protocol for Real-Time
               Applications", <a href="./rfc1889">RFC 1889</a>, January 1996.

   [<a id="ref-RTSP">RTSP</a>]      Schulzrinne, H., Rao, A. and R. Lanphier: "Real Time
               Streaming Protocol (RTSP)", <a href="./rfc2326">RFC 2326</a>, April 1998.

   [<a id="ref-FTP">FTP</a>]       Postel, J. and J. Reynolds, "File Transfer Protocol", STD
               9, <a href="./rfc959">RFC 959</a>, October 1985.

   [<a id="ref-NAT-TERM">NAT-TERM</a>]  Srisuresh, P. and M. Holdrege, "IP Network Address
               Translator (NAT) Terminology and Considerations", <a href="./rfc2663">RFC</a>
               <a href="./rfc2663">2663</a>, August 1999.

   [<a id="ref-NAT-TRAD">NAT-TRAD</a>]  Srisuresh, P. and K. Egevang, "Traditional IP Network
               Address Translator (Traditional NAT)", <a href="./rfc3022">RFC 3022</a>, January
               2001.

   [<a id="ref-NAT-PT">NAT-PT</a>]    Tsirtsis, G. and P. Srisuresh, "Network Address
               Translation - Protocol Translation (NAT-PT)", <a href="./rfc2766">RFC 2766</a>,
               February 2000.

   [<a id="ref-IPsec-AH">IPsec-AH</a>]  Kent, S. and R. Atkinson, "IP Authentication Header", <a href="./rfc2402">RFC</a>
               <a href="./rfc2402">2402</a>, November 1998.

   [<a id="ref-IPsec-ESP">IPsec-ESP</a>] Kent, S. and R. Atkinson, "IP Encapsulating Security
               Payload (ESP)", <a href="./rfc2406">RFC 2406</a>, November 1998.

   [<a id="ref-TLS">TLS</a>]       Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
               <a href="./rfc2246">RFC 2246</a>, January 1999.

   [<a id="ref-POL-TERM">POL-TERM</a>]  Westerinen, A., Schnizlein, J., Strassner, J., Scherling,
               M., Quinn, B., Herzog, S., Huynh, A., Carlson, M., Perry,
               J. and S. Waldbusser, "Terminology for Policy-Based
               Management", <a href="./rfc3198">RFC 3198</a>, November 2001.




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   [<a id="ref-REQMTS">REQMTS</a>]    Swale, R. P., Mart, P. A., Sijben, P., Brim, S. and M.
               Shore, "Middlebox Communications (midcom) Protocol
               Requirements", <a href="./rfc3304">RFC 3304</a>, August 2002.

Authors' Addresses

   Pyda Srisuresh
   Kuokoa Networks, Inc.
   475 Potrero Ave.
   Sunnyvale, CA 94085
   EMail: srisuresh@yahoo.com

   Jiri Kuthan
   Fraunhofer Institute FOKUS
   Kaiserin-Augusta-Allee 31
   D-10589 Berlin, Germany
   EMail: kuthan@fokus.fhg.de

   Jonathan Rosenberg
   dynamicsoft
   72 Eagle Rock Avenue
   First Floor
   East Hanover, NJ 07936
   U.S.A.
   EMail: jdrosen@dynamicsoft.com

   Andrew Molitor
   Aravox technologies
   4201 Lexington Avenue North, Suite 1105
   Arden Hills, MN 55126
   U.S.A.
   voice: (651) 256-2700
   EMail: amolitor@visi.com

   Abdallah Rayhan
   WINCORE Lab
   Electrical and Computer Engineering
   Ryerson University
   350 Victoria Street
   Toronto, ON M5B 2K3
   EMail: rayhan@ee.ryerson.ca, ar_rayhan@yahoo.ca










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Full Copyright Statement

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
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