GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc3303

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
         Middlebox communication architecture and framework

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 ([REQMTS]) 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.

Srisuresh, et al. Informational [Page 1] RFC 3303 MIDCOM Architecture and Framework August 2002

1. Introduction

 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

Srisuresh, et al. Informational [Page 2] RFC 3303 MIDCOM Architecture and Framework August 2002

 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.  Section 2 describes the terms used in the document.
 Section 3 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.  Section 4
 describes the nature of MIDCOM protocol.  Section 5 identifies
 entities that could potentially host the MIDCOM agent function.
 Section 6 considers the role of Policy server and its function with
 regard to communicating MIDCOM agent authorization policies.  Section
 7 is an illustration of SIP flows using a MIDCOM framework in which
 the MIDCOM agent is co-resident on a SIP proxy server.  Section 8
 addresses operational considerations in deploying a protocol adhering
 to the framework described here.  Section 9 is an applicability
 statement, scoping the location of middleboxes.  Section 11 outlines
 security considerations for the middlebox in view of the MIDCOM
 framework.

2. Terminology

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

2.1. Middlebox function/service

 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).

2.2. Middlebox

 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

Srisuresh, et al. Informational [Page 3] RFC 3303 MIDCOM Architecture and Framework August 2002

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

2.3. Firewall

 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).

2.4. NAT

 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 [NAT-TERM] 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 [NAT-TERM], but is extended beyond IPv4 networks to
 include the IPv4-v6 NAT-PT described in [NAT-PT].  While the IPv4 NAT
 [NAT-TERM] translates one IPv4 address into another IPv4 address to
 provide routing between private V4 and external V4 address realms,
 IPv4-v6 NAT-PT [NAT-PT] 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.

2.5. Proxy

 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.

Srisuresh, et al. Informational [Page 4] RFC 3303 MIDCOM Architecture and Framework August 2002

 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.

2.6. ALG

 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.

2.7. End-Hosts

 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.

2.8. MIDCOM Agents

 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

Srisuresh, et al. Informational [Page 5] RFC 3303 MIDCOM Architecture and Framework August 2002

 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.

2.9. MIDCOM PDP

 MIDCOM Policy Decision Point (PDP) is primarily a Policy Decision
 Point(PDP), as defined in [POL-TERM]; and also acts as a policy
 repository, holding MIDCOM related policy profiles in order to make
 authorization decisions.  [POL-TERM] 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.  Section 6 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

Srisuresh, et al. Informational [Page 6] RFC 3303 MIDCOM Architecture and Framework August 2002

 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.

2.10. Middlebox Communication (MIDCOM) protocol

 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.

2.11. MIDCOM agent registration

 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).

2.12. MIDCOM session

 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.

2.13. Filter

 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 [POL-TERM], which defines a filter as "A set of

Srisuresh, et al. Informational [Page 7] RFC 3303 MIDCOM Architecture and Framework August 2002

 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.

2.14. Policy action (or) Action

 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  [POL-TERM], 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.

2.15. Policy rule(s)

 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.

3.0 Architectural framework for middleboxes

 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.

Srisuresh, et al. Informational [Page 8] RFC 3303 MIDCOM Architecture and Framework August 2002

             +---------------+  +--------------+
             | 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|<-+   |           |              /
   +-------------+  |   |           |              |
                    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

Srisuresh, et al. Informational [Page 9] RFC 3303 MIDCOM Architecture and Framework August 2002

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

4. MIDCOM Protocol

 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.

Srisuresh, et al. Informational [Page 10] RFC 3303 MIDCOM Architecture and Framework August 2002

5.0. MIDCOM Agents

 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

Srisuresh, et al. Informational [Page 11] RFC 3303 MIDCOM Architecture and Framework August 2002

 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       |--->--
      +-----------+                 /---|                    |---<--
     +-----------+|  Data streams  //   +--------------------+
    +-----------+||---------->----//            |
    |end-hosts  ||-----------<-----             .
    +-----------+   (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

Srisuresh, et al. Informational [Page 12] RFC 3303 MIDCOM Architecture and Framework August 2002

5.1. End-hosts as In-Path MIDCOM agents

 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.

6.0. MIDCOM PDP functions

 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.

6.1. Authentication, Integrity and Confidentiality

 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

Srisuresh, et al. Informational [Page 13] RFC 3303 MIDCOM Architecture and Framework August 2002

 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.

Srisuresh, et al. Informational [Page 14] RFC 3303 MIDCOM Architecture and Framework August 2002

6.2. Registration and deregistration of MIDCOM agents

 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.
 (<Session-Direction>, <Source-Address>, <Destination-Address>, <IP-
 Protocol>, <Source-Port>, <Destination-Port>, <Agent>)
 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

Srisuresh, et al. Informational [Page 15] RFC 3303 MIDCOM Architecture and Framework August 2002

 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.

7.0. MIDCOM Framework Illustration using an In-Path agent

 In figure 3 below, we consider SIP applications (Refer [SIP]) 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 [RTP]) path in the external network.  Nc and Nd
 represent media traffic inside the private domain.
                             _________
                        --->|   SIP   |<-----\
                       /    |  Proxy  |       \
                      |     |_________|       |
                     1|       |^    ^|       4|
                      |       ||    ||        |
                      |8     2||3  7||6       |5
      ______________  |       ||    ||        |    _____________
      |            |<-/      _v|____|v___      \->|            |
      | External   |    Na   |           |   Nc   | SIP Phone  |
      | SIP phone  |>------->| Middlebox |>------>| within     |
      |            |<-------<|___________|<------<| 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

Srisuresh, et al. Informational [Page 16] RFC 3303 MIDCOM Architecture and Framework August 2002

 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 [RTP]) 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 [SDP]) 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 [NAT-TERM], [NAT-
 TRAD]), firewall and a combination of both NAPT and firewall are
 considered.

7.1. Timeline flow - Middlebox implementing firewall service

 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.

Srisuresh, et al. Informational [Page 17] RFC 3303 MIDCOM Architecture and Framework August 2002

 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------>|                      |              |
    |                 |                      |              |
    |<---100Trying----|                      |              |
    |                 |                      |              |
    |              Identify end-2-end        |              |
    |              parameters (from Caller's |              |
    |              SDP) for the pri-to-Ext   |              |
    |              RTP & RTCP sessions.      |              |
    |              (RTP1, RTCP1)             |              |
    |                 |                      |              |
    |                 |+Permit RTP1, RTCP1 +>|              |
    |                 |<+RTP1, RTCP1 OKed++++|              |
    |                 |                      |              |
    |                 |--------INVITE---------------------->|
    |                 |                      |              |
    |                 |<-----180 Ringing--------------------|
    |<--180Ringing----|                      |              |
    |                 |<-------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 +>|              |
    |                 |<+RTP2, RTCP2 OKed++++|              |
    |                 |                      |              |
    |<---200 OK ------|                      |              |
    |-------ACK------>|                      |              |
    |                 |-----------ACK---------------------->|
    |                 |                      |              |
    |<===================RTP/RTCP==========================>|

Srisuresh, et al. Informational [Page 18] RFC 3303 MIDCOM Architecture and Framework August 2002

    |                 |                      |              |
    |-------BYE------>|                      |              |
    |                 |--------------------------BYE------->|
    |                 |                      |              |
    |                 |<----------200 OK--------------------|
    |                 |                      |              |
    |                 |++Cancel permits to   |              |
    |                 |  RTP1, RTCP1, RTP2,  |              |
    |                 |  and RTCP2 +++++++++>|              |
    |                 |<+RTP1, RTP2, RTCP1 & |              |
    |                 |  RTCP2 cancelled ++++|              |
    |                 |                      |              |
    |<---200 OK-------|                      |              |
    |                 |                      |              |
       Legend:      ++++    MIDCOM control traffic
                    ----    SIP control traffic
                    ====    RTP/RTCP media traffic

7.2. Timeline flow - Middlebox implementing NAPT service

 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 (section 7.1), 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 section 7.1).  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

Srisuresh, et al. Informational [Page 19] RFC 3303 MIDCOM Architecture and Framework August 2002

 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------>|                      |              |
    |                 |                      |              |
    |<---100Trying----|                      |              |
    |                 |                      |              |
    |                 |++ Query Port-BIND    |              |
    |                 |   for (Ma, 5060) +++>|              |
    |                 |<+ Port-BIND reply    |              |
    |                 |   for (Ma, 5060) ++++|              |
    |                 |                      |              |
    |                 |++ Query NAT Session  |              |
    |                 |   Descriptor for     |              |
    |                 |   Ea-to-Pa SIP flow+>|              |
    |                 |<+ 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 & RTCP |              |
    |              sessions (RTP1, RTCP1)    |              |
    |                 |                      |              |
    |                 |++Create NAT Session  |              |
    |                 |  descriptors for     |              |
    |                 |  RTP1, RTCP1; Set    |              |
    |                 |  parent session to   |              |
    |                 |  SIP-ctrl session ++>|              |
    |                 |<+RTP1, RTCP1 session |              |
    |                 |  descriptors created+|              |
    |                 |                      |              |
    |                 |                      |..redirected..|
    |                 |--------INVITE--------|------------->|
    |                 |                      |              |

Srisuresh, et al. Informational [Page 20] RFC 3303 MIDCOM Architecture and Framework August 2002

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

Srisuresh, et al. Informational [Page 21] RFC 3303 MIDCOM Architecture and Framework August 2002

    |                 |                      |              |
    |                 |+++Terminate the SIP  |              |
    |                 |   Session bundle +++>|              |
    |                 |<++SIP Session bundle |              |
    |                 |   terminated ++++++++|              |
    |                 |                      |              |
    |<---200 OK-------|                      |              |
    |                 |                      |              |
       Legend:      ++++    MIDCOM control traffic
                    ----    SIP control traffic
                    ====    RTP/RTCP media traffic

7.3. Timeline flow - Middlebox implementing NAPT and firewall

 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 &       (Private)
    IP Addr:Ea        |                   firewall      IP addr:Pa
    |                 |                   Services)         |
    |                 |                   IP addr:Ma        |
    |                 |                      |              |
    |----INVITE------>|                      |              |
    |                 |                      |              |
    |<---100Trying----|                      |              |
    |                 |                      |              |
    |                 |++ Query Port-BIND    |              |
    |                 |   for (Ma, 5060) +++>|              |

Srisuresh, et al. Informational [Page 22] RFC 3303 MIDCOM Architecture and Framework August 2002

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

Srisuresh, et al. Informational [Page 23] RFC 3303 MIDCOM Architecture and Framework August 2002

    |                 |                      |              |
    |                 |++Create consecutive  |              |
    |                 |  port BINDs on Ma    |              |
    |                 |  for (Pa, Pport2),   |              |
    |                 |  (Pa, Pport2+1) ++++>|              |
    |                 |<+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++>|              |
    |                 |<+RTP2, RTCP2 session |              |
    |                 |  descriptors created+|              |
    |                 |                      |              |
    |              Modify the SDP            |              |
    |              parameters in "200 OK"    |              |
    |              with NAPT PORT-BIND       |              |
    |              for RTP2 port on Ma.      |              |
    |                 |                      |              |
    |                 |++Permit RTP2 & RTCP2 |              |
    |                 |  sessions External   |              |
    |                 |  middlebox, namely   |              |
    |                 |  Ea to Ma:Mport2,    |              |
    |                 |  Ea to Ma:Mport2+1   |              |
    |                 |  sessions ++++++++++>|              |
    |                 |<+Ea to Ma:Mport2,    |              |
    |                 |  Ea to Ma:Mport2     |              |
    |                 |  sessions OKed ++++++|              |
    |                 |                      |              |
    |<---200 OK ------|                      |              |
    |                 |                      |              |
    |-------ACK------>|                      |              |
    |                 |                      |..redirected..|
    |                 |-----------ACK--------|------------->|
    |                 |                      |              |
    |                 |                      |              |
    |<===================RTP/RTCP============|=============>|
    |                 |                      |              |
    |-------BYE------>|                      |              |
    |                 |                      |              |
    |                 |----------------------|-----BYE----->|
    |                 |                      |              |
    |                 |<----------200 OK--------------------|
    |                 |                      |              |
    |                 |+++Terminate the SIP  |              |

Srisuresh, et al. Informational [Page 24] RFC 3303 MIDCOM Architecture and Framework August 2002

    |                 |   Session bundle +++>|              |
    |                 |<++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 ++++++++++>|              |
    |                 |<+Removed permits to  |              |
    |                 |  sessions listed ++++|              |
    |                 |                      |              |
    |<---200 OK-------|                      |              |
    |                 |                      |              |
       Legend:      ++++    MIDCOM control traffic
                    ----    SIP control traffic
                    ====    RTP/RTCP media traffic

8.0. Operational considerations

8.1. Multiple MIDCOM sessions between agents and middlebox

 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.

8.2. Asynchronous notification to MIDCOM agents

 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.

Srisuresh, et al. Informational [Page 25] RFC 3303 MIDCOM Architecture and Framework August 2002

8.3. Timers on middlebox considered useful

 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.

8.4. Middleboxes supporting multiple services

 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.

8.5. Signaling and Data traffic

 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.

Srisuresh, et al. Informational [Page 26] RFC 3303 MIDCOM Architecture and Framework August 2002

 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.

9. Applicability Statement

 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

Srisuresh, et al. Informational [Page 27] RFC 3303 MIDCOM Architecture and Framework August 2002

 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.

Srisuresh, et al. Informational [Page 28] RFC 3303 MIDCOM Architecture and Framework August 2002

                                    \ | /
                            +-----------------------+
                            |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.

10. Acknowledgements

 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.

Srisuresh, et al. Informational [Page 29] RFC 3303 MIDCOM Architecture and Framework August 2002

11. Security Considerations

 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.
 Section 6 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

Srisuresh, et al. Informational [Page 30] RFC 3303 MIDCOM Architecture and Framework August 2002

 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.

Srisuresh, et al. Informational [Page 31] RFC 3303 MIDCOM Architecture and Framework August 2002

References

 [SIP]       Rosenberg, J., Shulzrinne, H., Camarillo, G., Johnston,
             A., Peterson, J., Sparks, R., Handley, M., Schooler, E.,
             "SIP: Session Initiation Protocol", RFC 3261, June 2002.
 [SDP]       Handley, M. and V. Jacobson, "SDP: Session Description
             Protocol", RFC 2327, April 1998.
 [H.323]     ITU-T Recommendation H.323. "Packet-based Multimedia
             Communications Systems," 1998.
 [RTP]       Schulzrinne, H., Casner, S., Frederick, R. and V.
             Jacobson, "RTP: A Transport Protocol for Real-Time
             Applications", RFC 1889, January 1996.
 [RTSP]      Schulzrinne, H., Rao, A. and R. Lanphier: "Real Time
             Streaming Protocol (RTSP)", RFC 2326, April 1998.
 [FTP]       Postel, J. and J. Reynolds, "File Transfer Protocol", STD
             9, RFC 959, October 1985.
 [NAT-TERM]  Srisuresh, P. and M. Holdrege, "IP Network Address
             Translator (NAT) Terminology and Considerations", RFC
             2663, August 1999.
 [NAT-TRAD]  Srisuresh, P. and K. Egevang, "Traditional IP Network
             Address Translator (Traditional NAT)", RFC 3022, January
             2001.
 [NAT-PT]    Tsirtsis, G. and P. Srisuresh, "Network Address
             Translation - Protocol Translation (NAT-PT)", RFC 2766,
             February 2000.
 [IPsec-AH]  Kent, S. and R. Atkinson, "IP Authentication Header", RFC
             2402, November 1998.
 [IPsec-ESP] Kent, S. and R. Atkinson, "IP Encapsulating Security
             Payload (ESP)", RFC 2406, November 1998.
 [TLS]       Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
             RFC 2246, January 1999.
 [POL-TERM]  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", RFC 3198, November 2001.

Srisuresh, et al. Informational [Page 32] RFC 3303 MIDCOM Architecture and Framework August 2002

 [REQMTS]    Swale, R. P., Mart, P. A., Sijben, P., Brim, S. and M.
             Shore, "Middlebox Communications (midcom) Protocol
             Requirements", RFC 3304, 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

Srisuresh, et al. Informational [Page 33] RFC 3303 MIDCOM Architecture and Framework August 2002

Full Copyright Statement

 Copyright (C) The Internet Society (2002).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

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

Srisuresh, et al. Informational [Page 34]

/data/webs/external/dokuwiki/data/pages/rfc/rfc3303.txt · Last modified: 2002/08/06 23:12 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki