[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits] [IPR]

Versions: (draft-taylor-midcom-semantics) 00 01 02 03 04 05 06 07 08 RFC 3989

Internet Draft                                            M. Stiemerling
Document: draft-ietf-midcom-semantics-01.txt                  J. Quittek
Expires: August 2003                                     NEC Europe Ltd.
                                                              Tom Taylor
                                                         Nortel Networks

                                                           February 2003


                       MIDCOM Protocol Semantics

                  <draft-ietf-midcom-semantics-01.txt>

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.  Internet-Drafts are
   working documents of the Internet Engineering Task Force (IETF), its
   areas, and its working groups.  Note that other groups may also
   distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html

   Distribution of this document is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.


Abstract


   This memo specifies semantics for a Middlebox Communication (MIDCOM)
   protocol to be used by MIDCOM agents for interacting with
   middleboxes, such as firewalls and NATs.  The semantics discussion
   does not include any specification of a concrete syntax or a
   transport protocol.  However, a concrete protocol is expected to
   implement the specified semantics or - more probably - a superset of
   it.  The MIDCOM protocol semantics is derived from the MIDCOM
   requirements, from the MIDCOM framework, and from working group


Stiemerling, Quittek, Taylor                                    [Page 1]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   decisions.



Table of Contents

   1 Introduction .................................................    2
   1.1 Terminology ................................................    3
   1.2 Transaction Definition Template ............................    4
   2 Semantics Specification ......................................    5
   2.1 General Protocol Design ....................................    5
   2.1.1 Session, Policy Rule, and Policy Rule Group ..............    6
   2.1.2 Atomicity ................................................    7
   2.1.3 Access Control ...........................................    7
   2.1.4 Conformance ..............................................    8
   2.1.5 Middlebox Capabilites ....................................    8
   2.2 Session Control Transactions ...............................    8
   2.2.1 Session Establishment (SE) ...............................    9
   2.2.2 Session Termination (ST) .................................   11
   2.2.3 Asynchronous Session Termination (AST) ...................   12
   2.2.4 Session Termination by Interruption of Connection ........   12
   2.2.5 Session State Machine ....................................   12
   2.3 Policy Rule Transactions ...................................   14
   2.3.1 Overview .................................................   14
   2.3.2 Establishing Policy Rules ................................   15
   2.3.3 Maintaining Policy Rules and Policy Rule Groups ..........   16
   2.3.4 Address Tuples ...........................................   16
   2.3.5 Address Parameter Constraints ............................   17
   2.3.6 Policy Reserve Rule (PRR) ................................   19
   2.3.7 Policy Enable Rule (PER) .................................   22
   2.3.8 Policy Rule Lifetime Change (RLC) ........................   26
   2.3.9 Policy Rule Status (PRS) .................................   28
   2.3.10 Asynchronous Policy Rule Deletion (ARD) .................   30
   2.3.11 Policy Rule State Machine ...............................   30
   2.4 Policy Rule Group Transactions .............................   32
   2.4.1 Overview .................................................   32
   2.4.2 Group Lifetime Change (GLC) ..............................   33
   2.4.3 Group List (GL) ..........................................   34
   2.4.4 Group Status (GS) ........................................   35
   3 Conformance Statements .......................................   36
   3.1 General Implementation Conformance .........................   37
   3.2 Middlebox Conformance ......................................   37
   3.3 Agent Conformance ..........................................   38
   4 Transaction Usage Examples ...................................   38
   4.1 Exploring Policy Rules and Policy Rule Groups ..............   38
   4.2 Enabling a SIP-Signaled Call ...............................   41
   5 Compliance with MIDCOM Requirements ..........................   46
   5.1 Protocol Machinery Requirements ............................   46
   5.1.1 Authorized Association ...................................   46
   5.1.2 Agent connects to Multiple Middleboxes ...................   46


Stiemerling, Quittek, Taylor                                    [Page 2]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   5.1.3 Multiple Agents connect to same Middlebox ................   47
   5.1.4 Deterministic Behavior ...................................   47
   5.1.5 Known and Stable State ...................................   47
   5.1.6 Status Report ............................................   48
   5.1.7 Unsolicited Messages (Asynchronous Notifications) ........   48
   5.1.8 Mutual Authentication ....................................   48
   5.1.9 Session Termination by any Party .........................   48
   5.1.10 Request Result ..........................................   48
   5.1.11 Version Interworking ....................................   49
   5.1.12 Deterministic Handling of Overlapping Rules .............   49
   5.2 Protocol Semantics Requirements ............................   49
   5.2.1 Extensible Syntax and Semantics ..........................   49
   5.2.2 Policy Rules for Different Types of Middleboxes ..........   49
   5.2.3 Ruleset Groups ...........................................   49
   5.2.4 Policy Rule Lifetime Extension ...........................   50
   5.2.5 Robust Failure Modes .....................................   50
   5.2.6 Failure Reasons ..........................................   50
   5.2.7 Multiple Agents Manipulating Same Policy Rule ............   50
   5.2.8 Carrying Filtering Rules .................................   50
   5.2.9 Parity of Port Numbers ...................................   50
   5.2.10 Consecutive Range of Port Numbers .......................   50
   5.2.11 Contradicting Overlapping Policy Rules ..................   51
   5.3 Security Requirements ......................................   51
   5.3.1 Authentication, Confidentiality, Integrity ...............   51
   5.3.2 Optional Confidentiality of Control Messages .............   51
   5.3.3 Operation across Un-trusted Domains ......................   51
   5.3.4 Mitigate Replay Attacks ..................................   51
   6 Security Considerations ......................................   51
   7 Acknowledgements .............................................   52
   8 Open Issues ..................................................   52
   9 Normative References .........................................   53
   10 Informative References ......................................   53
   11 Authors' Addresses ..........................................   53
   12 Full Copyright Statement ....................................   54


1.  Introduction

   The MIDCOM working group has defined a framework [MDC-FRM] for the
   middlebox communication as well as a list of requirements [MDC-REQ].
   The next step towards a MIDCOM protocol is the specification of
   protocol semantics that are constrained, but not complletely implied
   by the documents mentioned above.

   This memo suggests a semantics for the MIDCOM protocol.  It is fully
   compliant with the requirements listed in [MDC-REQ] and with the
   working group's consensus on semantic issues.

   In conformance with the working group charter, the semantics
   description is targeted at packet filters and network address


Stiemerling, Quittek, Taylor                                    [Page 3]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   translators (NATs) and it supports applications that require dynamic
   configuration of these middleboxes.

   The semantics are defined in terms of transactions.  Two basic types
   of transactions are used: request-reply transactions and notification
   transactions. For each transaction the semantics is specified by
   describing (1) the parameters of the transaction, (2) the processing
   (of request messages) at the middlebox, and (3) the state transitions
   at the middlebox caused by the request transactions or indicated by
   the notification transactions, respectively.

   The semantics can be implemented by any protocol that supports these
   two transaction types and that is sufficiently flexible concerning
   transaction parameters.  Different implementations for different
   protocols might need to extend the semantics described below by
   adding further transactions and/or adding further parameters to
   transactions.  Regardless of such extensions, the semantics below
   provide the minimum necessary subset of what must be implemented.

   The reminder of this document is structured as follows. Section 2
   describes the protocol semantics. It is structured in four
   subsections:

      - General Protocol Issues (Section 2.1)
      - Session Control (Section 2.2)
      - Policy Rules (Section 2.3)
      - Policy Rule Groups (Section 2.4)

   Section 3 contains conformance statements for MIDCOM protocol
   definitions and MIDCOM protocol implementations with respect to the
   semantics defined in Section 2.  Section 4 gives two elaborated usage
   examples.  Finally, Section 5 explains how the semantics meets the
   MIDCOM requirements.


1.1.  Terminology

   The terminology in this memo follows the definitions given in the
   framework [MDC-FRM] and requirements [MDC-REQ] document.

   In addition the following terms are used:

   request transaction        A request transaction consists of a
                              request message transfer from the agent to
                              the middlebox, procesing of the message at
                              the middlebox, and a reply message
                              transfer from the middlebox to the agent.
                              A request transaction might cause a state
                              transition at the middlebox.



Stiemerling, Quittek, Taylor                                    [Page 4]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   configuration transaction  A configuration transaction is a request
                              transaction containing a request for state
                              change in the middlebox.  If accepted, it
                              causes a state change at the middlebox.

   monitoring transaction     A monitoring transaction is a request
                              transaction containing a requests for
                              state information from the middlebox.  It
                              does not cause a state transition at the
                              middlebox.

   notification transaction   A notification transaction consists of an
                              asynchronous message transfer from the
                              middlebox and to the agent.  The message
                              indicates a state transistion at the
                              middlebox.

   agent unique               An agent unique value is unique in the
                              context of the agent.  This context
                              includes all MIDCOM session the agent
                              participates in.  An agent unique value is
                              assigned by the agent.

   middlebox unique           A middlebox unique value is unique in the
                              context of the middlebox.  This context
                              includes all MIDCOM session the middlebox
                              participates in.  A middlebox unique value
                              is assigned by the middlebox.

   policy rule                In general, a policy rule is "a basic
                              building block of a policy-based system.
                              It is the binding of a set of actions to a
                              set of conditions - where the conditions
                              are evaluated to determine whether the
                              actions are performed."  [RFC3198].  In
                              the MIDCOM context the condition is a
                              specification of a set of packets to which
                              rules are applied.  The set of actions
                              always contain just a single element per
                              rule, either action "reserve" or action
                              "enable".

   policy reserve rule        A policy rule containing a reserve action.
                              The policy condition of this rule is
                              always true.  The action is the
                              reservation of just an IP address or a
                              combination of an IP address and a range
                              of port numbers on neither, one, or both
                              sides of the middlebox, depending on the
                              latter's configuration.


Stiemerling, Quittek, Taylor                                    [Page 5]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   policy enable rule         A policy rule containing an enable action.
                              The policy condition consists of a
                              descriptor of one or more unidirectional
                              or bidirectional flows, and the policy
                              action enables packets belonging to this
                              flow to traverse the middlebox.  The
                              descriptor identifies the protocol, the
                              flow direction, the source and destination
                              addresses, optionally with a range of port
                              numbers.

1.2.  Transaction Definition Template

   In the following sections, semantics of the MIDCOM protocol is
   specified per transaction.  A transaction specification contains the
   following entries. (Parameter entries are only specified if
   applicable.)

   transaction-name
      A description name for this type of transaction.

   transaction-type
      The transaction type is either 'configuration', 'monitoring', or
       'notification'.  See Section 1.1. for a description of
      transaction types.

   transaction-compliance
      This entry contains either 'mandatory' or 'optional'.  For details
      see Section 2.1.4.

   request-parameters
      This entry lists all parameters that are necessary for this
      request.  A description for each parameter is given.

   reply-parameters (success)
      This entry lists all parameters that are sent back from the
      middlebox to the agent as positive response to the prior request.
      A description for each parameter is given.

   reply-parameters (failure)
      This entry lists all parameters that are sent back from the
      middlebox to the agent as negative response to the prior request.
      A description for each parameter is given.

   notification parameters
      This entry lists all parameters that are used by the middlebox to
      notify the agent about any asynchronous event. A description for
      each parameter is given.




Stiemerling, Quittek, Taylor                                    [Page 6]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   semantics
      This entry describes the actual semantics of the transaction.
      Particularly, it describes the processing of the request by the
      middlebox.


   Each request message contains a parameter identifying the requesting
   agent, and each reply message and each notification message contains
   a parameter identifying the middlebox.  These parameters are not
   explicitly listed in the description of the individual transactions,
   because they are common to all of them and not further referred to in
   the individual semantics descriptions.  Also, they are not
   necessarily passed explicitly as parameters of the midcom protocol,
   but they might be provided by the used underlying (secure) transport
   protocol.


2.  Semantics Specification

2.1.  General Protocol Design

   The semantics specification aims at a balance between proper support
   of applications that require dynamic configuration of middleboxes and
   simplicity of specification and implementation of the protocol.

   Two kinds of state transitions may occur at the middlebox: state
   transitions are either initiated by a request from the agent to the
   middlebox, or they are initiated by some other event.  In the first
   case the middlebox informs the agent by sending a reply on the actual
   state transition, in latter case the middlebox sends a notification
   to the agent.  Requests and replies contain an agent unique request
   identifier that allows the agent to determine to which sent request a
   received reply corresponds.

   To allow both agents and middleboxes to maintain multiple sessions,
   every message contains information identifying its sender.  In the
   actual protocol, this identifying information may be provided by a
   layer below the middlebox control application.  It is not shown
   explicitly in the message descriptions provided below, but should be
   assumed as a semantic requirement.

   An analysis of the requirements showed that three kinds of
   transactions are required: configuration transactions allowing the
   agent to request state transitions at the middlebox, monitoring
   transaction allowing the agent to request state information from the
   middlebox, and notification transactions allowing the middlebox to
   inform the agent about state transitions not requested by the agent.





Stiemerling, Quittek, Taylor                                    [Page 7]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


2.1.1.  Session, Policy Rule, and Policy Rule Group

   All transactions can further be grouped into transactions concerning
   sessions, transactions concerning policy rules, and transactions
   concerning policy rule groups.  Policy rule groups can be used to
   indicate relationships between policy rules and to simplify
   transactions on a set of policy rules by using a single one per group
   instead of one per policy rule.

   Sessions and policy rules at the middlebox are stateful.  Their
   states are independent of each other and their state machines (one
   per session and one per policy rule) can be separated.  For policy
   rule group are also stateful, by the middlebox does not need to
   maintain state for policy rule groups, because the semantics was
   chosen such that the policy rule group state is implicitly defined by
   the state of all policy rules belonging to the group (see Section
   2.4).

   The separation of session state and policy rule state simplifies the
   specification of the semantics as well as a protocol implementation.
   Therefore, the semantics specification is structured accordingly and
   we use two separated state machines to illustrate the semantics.
   Please note, that state machines of concrete protocol designs and
   implementations will most probably be more complex than the state
   machines presented here.  However, the protocol state machines are
   expected to be a superset of the semantic state machines in this
   document.


2.1.2.  Atomicity

   All request transactions are atomic with respect to each other.  This
   means that processing of a request at the middlebox is never
   interrupted by another arriving or already queued request.  This
   particularly applies when the middlebox concurrently receives
   requests originating in different sessions.  However, asynchronous
   notification transactions may interrupt and terminate processing of a
   request at any time.

   All request transactions are atomic from the point of view of the
   agent.  Processing of a request does not start before the complete
   request arrives at the middlebox.  No intermediate state is stable at
   the middlebox and no intermediate state is reported to any agent.

   The number of transactions specified in this document is rather
   small.  Again for simplicity we reduced it close to a minimal set
   that still meets the requirements.  For a real implementation of the
   protocol, it might be required to split some of the transactions
   specified below into two or more transactions of the respective
   protocol.  Reasons for this might be constraints of the particular


Stiemerling, Quittek, Taylor                                    [Page 8]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   protocol or the desire for more flexibility.  In general this should
   not be a problem. However, it should be considered that this might
   change atomicity of the affected transactions.


2.1.3.  Access Control

   Access to policy rules and policy rule groups is based on ownership.
   When a policy rule is created, a middlebox unique identifier is
   generated for identifying it in further transactions.  Beyond the
   identifier, each policy rule has an owner.  The owner is the
   authenticated agent that established the policy rule.  The middlebox
   uses the owner attribute of a policy rule or group for controlling
   access to it:  each time an authenticated agent requests to modify an
   existing policy rule, the middlebox determines the owner of the
   policy rule and checks if the requesting agent is authorized to
   perform transactions on the owning agent's policy rules.

   The middlebox may be configured to allow specific authenticated
   agents to access and modify policy rules with certain specific
   owners.  Certainly, a reasonable default configuration would be that
   each agent can access its own policy rules.  Also, it might be a good
   idea, to have an agent identity configured to act as administrator
   being allowed to modify all policy rules owned by any agent.  Anyway,
   the configuration of authorization is not subject of the MIDCOM
   protocol semantics.


2.1.4.  Conformance

   The MIDCOM requirements in [MDC-REQ] demand certain capabilities of
   the MIDCOM protocol, which are met by the set of transactions
   specified below.  However, an actual implementation of a middlebox
   may support only a subset of these transactions.  Support limitation
   may be different for different authenticated agents.  At session
   establishment, the middlebox informs the authenticated agent by
   capability exchange, which transactions the agent is authorized to
   perform.  Some transactions need to be offered to every authenticated
   agent.

   Each transaction definition below has a conformance entry which
   contains either 'mandatory' or 'optional'.  A mandatory transaction
   needs to be implemented by every middlebox offering MIDCOM service.
   A mandatory request transaction must be offered to each of the
   authenticated agents.  An optional transaction does not necessarily
   need to be implemented by a middlebox.  An implemented optional
   request transaction does not necessarily need to be offered to every
   authenticated agent.  Whether or not an agent is allowed to use an
   optional request transaction is determined by the middlebox's
   authorization procedure which is not further specified by this


Stiemerling, Quittek, Taylor                                    [Page 9]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   document.


2.1.5.  Middlebox Capabilites

   Editor's note: to be done. (policy rule persistency...)


2.2.  Session Control Transactions

   Before any transaction on policy rules or policy rule groups is
   possible, a valid MIDCOM session must be established.  A MIDCOM
   session is an authorized association between agent and middlebox.
   Sessions are initiated by agents and can be terminated by either the
   agent or the middlebox.  Both agent and middlebox may participate in
   several sessions (with different entities) at the same time.  For
   distinguishing different sessions each party uses local session
   identifiers.

   Session control is supported by three transactions:

      - Session Establishment (SE)
      - Session Termination (ST)
      - Asynchronous Session Termination (AST)

   The first two are request transactions initiated by the agent, the
   last one is a notification transaction initiated by the middlebox.


2.2.1.  Session Establishment (SE)

   transaction-name: session establishment

   transaction-type: configuration

   transaction-compliance: mandatory

   request-parameters:

     - request identifier: an agent unique identifier for matching
       corresponding request and reply at the agent.

     - version: the version of the MIDCOM protocol

     - middlebox authentication challenge (mc): an authentication
       challenge token for authentication of the middlebox.  As seen
       below, this is present only in the first iteration of the
       request.




Stiemerling, Quittek, Taylor                                   [Page 10]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


     - agent authentication (aa): an authentication token to
       authenticate the agent to the middlebox.  As seen below, this is
       updated in the second iteration of the request with material
       responding to the middlebox challenge.

     - encryption method: an identifier of an encryption method.  Also
       'no encryption' may be specified.

   reply-parameters (success):

     - request identifier: an identifier matching the identifier
       request.

     - middlebox authentication (ma): an authentication token to
       authenticate the middlebox to the agent.

     - agent challenge token (ac): an authentication challenge token for
       the agent authentication.

     - middlebox capabilities: a parameter set describing the
       middlebox's capabilities.  The set includes
          - type of the middlebox
            for example: FW, NAT, NATFW, NAPT, NAPTFW, NAT-PT, NAT-PTFW,
            ...
          - IP address wildcard support
          - port wildcard support
          - supported IP version(s) for internal network:
            IPv4, IPv6, or both
          - supported IP version(s) for external network:
            IPv4, IPv6, or both
          - list of supported optional MIDCOM protocol transactions
          - policy rule persistency: persistent or not persistent
          - maximum remaining lifetime of a policy rule or policy rule
            group

   reply-parameters (failure):

     - request identifier: an identifier matching the identifier of the
       request.

     - failure reason: the reason why the session establishment
       transaction failed.  The list of possible reasons includes but is
       not restricted to:
          - authentication failed
          - no authorization
          - protocol version of agent and middlebox do not match
          - encryption method not supported
          - lack of resources

   semantics:


Stiemerling, Quittek, Taylor                                   [Page 11]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


      This session establishment transaction is used to establish a
      MIDCOM session.  For mutual authentication of both parties two
      subsequent session establishment transactions are required as
      shown in Figure 1.

               agent                                       middlebox
                 | session establishment request               |
                 |  (with middlebox challenge mc)              |
                 |-------------------------------------------->|
                 |                                             |
                 | successful reply (with middlebox            |
                 |  authentication ma and agent challenge ac)  |
                 |<--------------------------------------------|
                 |                                             |
                 | session establishment request               |
                 |  (with agent authentication aa)             |
                 |-------------------------------------------->|
                 |                                             |
                 | successful reply                            |
                 |<--------------------------------------------|
                 |                                             |

              Figure 1: Mutual authentication of agent and middlebox

      Session establishment may be simplified by using only a single
      transaction.  In this case server challenge and agent challenge
      are omitted by the sender or ignored by the receiver, and
      authentication must be provided by other means, for example by TLS
      [RFC2246] or IPSEC [RFC2402][RFC2406].

      The middlebox checks with its policy decision point if the
      requesting agent is authorized to open a MIDCOM session.  If not a
      negative reply with 'no authorization' as failure reason is
      generated by the middlebox.  If authentication and authorization
      are successful, the session is established and the agent may start
      with requesting transactions on policy rules and policy rule
      groups.

      Part of the successful reply is an indication of the middlebox's
      capabilities.

      Editor's note: The list of capabilities to be included needs to be
      further elaborated, taking into account how the agent is expected
      to use this information.

      The agent specifies an encryption method for the session but has
      the option of not using encryption.  The middlebox can accept this
      suggestion or reject it.  In case of rejection, the session
      establishment fails and an appropriate failure reason is indicated
      by the middlebox in the reply message.  Then the agent may try


Stiemerling, Quittek, Taylor                                   [Page 12]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


      session setup again with a different encryption method.


2.2.2.  Session Termination (ST)

   transaction-name: session termination

   transaction-type: configuration

   transaction-compliance: mandatory

   request-parameters:

     - request identifier: an agent unique identifier for matching
       corresponding request and reply at the agent.

   reply-parameters (success only):

     - request identifier: an identifier matching the identifier of the
       request.

   semantics:

      This transaction is used to close the MIDCOM session on behalf of
      the agent.  After session termination the middlebox keeps all
      established policy rules until their lifetime expires or until an
      event occurs which causes the middlebox to terminate them.

      The middlebox always generates a successful reply.  After sending
      the reply, the middlebox will not send any further messages to the
      agent within the current session.  It also will not process any
      further request within this session, which it has received while
      it was processing the session termination request, or which it
      receives later.


2.2.3.  Asynchronous Session Termination (AST)

   transaction-name: asynchronous session termination

   transaction-type: notification

   transaction-compliance: mandatory

   notification-parameters:

     - termination reason: The reason why the session is terminated
       without any request from the agent.

   semantics:


Stiemerling, Quittek, Taylor                                   [Page 13]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


      The middlebox may decide at any point in time to terminate a
      MIDCOM session.  Before terminating the actual session the middle
      box generates this notification transaction.  After sending the
      notification, the middlebox will not process any further request
      by the agent, even if it is already queued at the middlebox.

      After session termination the middlebox keeps all established
      policy rules until their lifetime expires or until an event occurs
      on which the middlebox terminates them.


2.2.4.  Session Termination by Interruption of Connection

   If a MIDCOM session is based on an underlying network connection,
   then the session can also be terminated by an interruption of this
   connection.  If the middlebox detects this, it immediately terminates
   the session.  The effect on established policy rules is the same as
   for the Asynchronous Session Termination.


2.2.5.  Session State Machine

   A state machine illustrating the semantics of the session
   transactions is shown in Figure 2.  The used transaction
   abbreviations can be found in the headings of the particular
   transaction section.

   All sessions start in state CLOSED.  A successful SE transaction can
   cause a state transition to state OPEN, if mutual authentication is
   already provided by other means.  Otherwise, it causes a transition
   to state NOAUTH.  From this state a failed second SE transaction
   returns to state CLOSED.  A successful SE transaction causes a
   transition to state OPEN.  At any time an AST transaction or a
   connection failure may occur causing a transition to state CLOSED.  A
   successful ST transaction from either NOAUTH or OPEN also causes a
   return to CLOSED.
















Stiemerling, Quittek, Taylor                                   [Page 14]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


                                     mc = middlebox challenge
                  SE/failure         ma = middlebox authentication
                  +-------+          ac = agent challenge
                  |       v          aa = agent authentication
                 +----------+
                 |  CLOSED  |----------------+
                 +----------+                | SE(mc!=0)/
                    |   ^  ^                 |  success(ma,ac)
           SE(mc=0, |   |  | AST             |
            aa=OK)/ |   |  | SE/failure      v
            success |   |  | ST/success +----------+
                    |   |  +------------|  NOAUTH  |
                    |   |               +----------+
                    |   | AST                | SE(mc=0,
                    v   | ST/success         |  aa=OK)/
                 +----------+                |  success
                 |   OPEN   |<---------------+
                 +----------+

                 Figure 2: Session State Machine


2.3.  Policy Rule Transactions

   This section describes the semantics for transactions on policy
   rules.  The following transactions are specified:

      - Policy Reserve Rule (PRR)
      - Policy Enable Rule (PER)
      - Policy Rule Lifetime Change (RLC)
      - Policy Rule Status (PRS)
      - Asynchronous Policy Rule Deletion (ARD)

   The first four are request transactions initiated by the agent, the
   last one is a notification transaction initiated by the middlebox.
   The status information transaction (PRS) does not have any effect on
   the policy rule state machine.

   Before any transaction can start, a valid MIDCOM session must be
   established.


2.3.1.  Overview

   Policy Rule transactions PER and RLC constitute the core of the
   MIDCOM protocol.  Both are mandatory and they serve for

      - configuring NAT bindings (PER)
      - configuring firewall pinholes (PER)
      - extending the lifetime of established policy rules (RLC)


Stiemerling, Quittek, Taylor                                   [Page 15]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


      - deleting policy rules (RLC)

   In some cases it is required to know in advance which IP address (and
   port number) would be chosen by NAT in a PER transaction.  This
   information is required before sufficient information for performing
   a complete PER transaction is available (see example in Section 4.2).
   For supporting such cases, the core transactions are extended by the
   Policy Reserve Rule (PRR) transaction serving for

      - reserving addresses and port numbers at NATs (PRR)

   A policy rule contains either a reserve action (established by PRR
   transaction) or an enable action (established by PER transaction).


2.3.2.  Establishing Policy Rules

   The Policy Reserve Rule (PRR) transaction is used to establish an
   address reservation on neither, one, or both sides of the middlebox,
   depending on the latter's configuration.  The transaction returns the
   reserved IP addresses and the optional ranges of port numbers to the
   agent.  No address binding or pinhole configuration is performed at
   the middlebox.  Packet processing at the middlebox remains unchanged.

   On pure firewalls, the PRR transaction is successfully installed
   without any reservation, but the state transition of the midcom
   protocol engine is exactly the same as on NATs.

   On a traditional NAT, just an external address is reserved; on a
   twice-NAT, an internal and an external address is reserved.  In both
   cases the reservation concerns either an IP address or a combination
   of an IP address with a range of port numbers.

   The Policy Enable Rule (PER) transaction is used to establish a
   policy rule that has an effect on packet treatment at the middlebox.
   Depending on its input parameters, it may make use of the reservation
   established by a PRR transaction, or create a new rule from scratch.

   On a NAT, the enable action is interpreted as as bind action
   establishing bindings between internal and external addresses.  At a
   firewall, the enable action is interpreted as one or more allow
   actions configuring pinholes. The number of allow actions depends on
   the parameters of the request and the implementation of the firewall.

   The PRR transaction and the PER transaction are described in more
   detail in Sections 2.3.6. and 2.3.7. below.






Stiemerling, Quittek, Taylor                                   [Page 16]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


2.3.3.  Maintaining Policy Rules and Policy Rule Groups

   Each policy rule has a middlebox unique identifier.

   Each policy rule has an owner.  Access control to the policy rule is
   based on ownership (see section 2.1.3).  Ownership of a policy rule
   does not change during lifetime of the policy rule.

   Each policy rule has its individual lifetime.  If the policy rule
   lifetime expires, the policy rule will be deleted at the middlebox.
   A policy rule lifetime change (RLC) transaction may extend the
   lifetime of the policy rule up to the limit specified by the
   middlebox at session setup. Also a RLC transaction may be used for
   deleting a policy rule by requesting a lifetime of zero. (Please note
   that policy rule lifetimes may also be modified by the group lifetime
   change (GLC) transaction).

   Each policy rule is member of exactly one policy rule group. Group
   membership does not change during the lifetime of a policy rule.
   Selecting the group is part of the transaction establishing the
   policy rule.  This transaction implicitly creates a new group if the
   agent does not specify a group of which the new policy rule should
   become a member.  The new group identifier is chosen by the
   middlebox.  New members are added to a group, if the agents requests
   membership of an already existing group.  A group only exists as long
   as it has member policy rules.  As soon as all policies belonging to
   the group reached the end of their lifetimes, the group does not
   exist anymore.

   Agents can explore the properties and status of all policy rules they
   are allowed to access by using the Policy Rule Status (PRS).


2.3.4.  Address Tuples

   Request and reply messages of the PRR, PER, and PRS transactions
   contain address specifications for IP and transport addresses.  These
   parameters include

      - IP version - IP address - transport protocol - port number -
      port parity - port range

   We refer to the set of these parameters as an address tuple.  An
   address tuple specifies either a communication endpoint at an
   internal or external device or allocated addresses at the middlebox.
   In this document, we distinguish four kinds of address tuples as
   shown in Figure 3.





Stiemerling, Quittek, Taylor                                   [Page 17]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


       +----------+                                 +----------+
       | internal | A0    A1 +-----------+ A2    A3 | external |
       | endpoint +----------+ middlebox +----------+ endpoint |
       +----------+          +-----------+          +----------+

                       Figure 3: Address tuples A0 - A3


     - A0 - internal endpoint: address tuple A0 specifies a
       communication endpoint of a devices within the - with respect to
       the middlebox - internal network.

     - A1 - middlebox inside address: address tuple A1 specifies a
       virtual communication endpoint at the middlebox within the
       internal network.

     - A2 - middlebox outside address: address tuple A2 specifies a
       virtual communication endpoint at the middlebox within the
       external network.

     - A0 - external endpoint: address tuple A3 specifies a
       communication endpoint of a devices within the - with respect to
       the middlebox - external network.

   For a firewall, the inside and outside endpoints are identical to the
   corresponding external or internal endpoints, repectively. In this
   case A0=A2 and A1=A3.

   For a traditional NAT, A0 is different from A2, but the NAT binds
   them.  As for the firewall, A1=A3 at a traditional NAT.

   For a twice-NAT there are two bindings of address tuples: The
   middlebox outside address A2 is bound to the internal endpoint A0 and
   the middlebox inside address A1 is bound to the external endpoint A3.


2.3.5.  Address Parameter Constraints

   For transaction parameters belonging to an address tuple some
   constraints exist which are common for all messages using them.
   Therefore, these constraints are summarized in the following and not
   repeated again when describing the parameters in the transaction
   descriptions.

   The IP version parameter has either the value 'IPv4' or 'IPv6'.  In a
   policy rule, the value of the IP version parameter must be the same
   for address tuples A0 and A1, and it must be the same for A2 and A3.

   The value of the IP address parameter must conform with the specified
   IP version. The specified IP address cannot be wildcarded.


Stiemerling, Quittek, Taylor                                   [Page 18]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   The value of the transport protocol parameter can have one of the
   following values: 'TCP', 'UDP', 'ANY'.  If the transport protocol
   parameter has the value 'ANY', then the values of the parameters port
   number, port range, and port parity are irrelevant.  In a policy rule
   the value of the transport protocol parameter must be the same for
   all address tuples A0, A1, A2, and A3.

   The value of the port number parameter is either zero or a positive
   integer.  A positive integer specifies a concrete UDP or TCP port
   number.  The value zero specifies port wildcarding for the protocol
   specified by the transport protocol parameter.  If the port number
   parameter has the value zero, then the value of the port range
   parameter is irrelevant.  Depending on the value of the transport
   protocol parameter, this parameter may truly refer to ports, or may
   refer to an equivalent concept.

   The port parity parameter is diffently used in the context of policy
   reserve rules (PRR) and policy enable rules (PER).  In the context of
   a PRR, the value of thei parameter may be 'odd', 'even', or 'any'.
   It specifies the parity of the first (lowest) reserved port number.

   In the context of a PER, the port parity parameter indicates to the
   middlebox, whether or not port numbers allocated at the middlebox
   should have the same parity as the corresponding internal or external
   port numbers, respectively.  In this context, the parameter has
   either the value 'same' or 'any'.  If it has the value 'same', then
   the parity of the port number of A0 must be the same as the parity of
   the port number of A2, and the parity of the port number of A1 must
   be the same as the parity of the port number of A3.  If the port
   parity parameter has the value 'any', then there are no contraints on
   the parity of any port number.

   The port range parameter specifies a number of consecutive port
   numbers.  Its value is a positive integer.  Together with the port
   number parameter this parameter defines a set of consecutive port
   numbers starting with the port number specified by the port number
   parameter as the lowest port number and having as many elements as
   specified by the port range parameter.  A value of one specifies just
   a single port number.  The port range parameter must have the same
   value for each address tuple A0, A1, A2, and A3.

   A single policy rule P containing a port range value greater than one
   is equivalent to a set of policy rules containing a number n of
   policies P_1, P_2, ..., P_n that equals the value of the port range
   parameter.  All policy rules P_1, P_2, ..., P_n have a port range
   parameter value of one.  Policy rule P_1 contains a set of address
   tuples A0_1, A1_1, A2_1, and A3_1, that each contain the first port
   number of the respective address tuples in P; policy rule P_2
   contains a set of address tuples A0_2, A1_2, A2_2, and A3_2, that
   each contain the second port number of the respective address tuples


Stiemerling, Quittek, Taylor                                   [Page 19]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   in P; and so on.


2.3.6.  Policy Reserve Rule (PRR)

   transaction-name: policy reserve rule

   transaction-type: configuration

   transaction-compliance: mandatory

   request-parameters:

     - request identifier: an agent unique identifier for matching
       corresponding request and reply at the agent.

     - group identifier: a reference to the group of which the policy
       reserve rule should be a member.

     - internal IP version: requested IP version at the inside of the
       middlebox, see Section 2.3.5.

     - external IP version: requested IP version at the outside of the
       middlebox, see Section 2.3.5.

     - transport protocol: see section 2.3.5.

     - port range: the number of consecutive port numbers to be
       reserved, see Section 2.3.5.

     - port parity: the requested parity of the first (lowest) port
       number to be reserved, Allowed values of this parameter are
       'odd', 'even', and 'any'.  See also Section 2.3.5.

     - policy rule lifetime: a lifetime proposal to the middlebox for
       the requested policy rule.

   reply-parameters (success):

     - request identifier: an identifier matching the identifier of the
       request.

     - policy rule identifier: a middlebox unique policy rule
       identifier.  It is assigned by the middlebox and used as policy
       rule handle in further policy rule transactions, particularly to
       refer to the policy reserve rule in a subsequent PER transaction.

     - group identifier: a reference to the group of which the policy
       reserve rule is a member.



Stiemerling, Quittek, Taylor                                   [Page 20]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


     - reserved inside IP address: The reserved IPv4 or IPv6 address on
       the internal side of the middlebox.  For an outbound flow, this
       will be the destination to which the internal endpoint sends its
       packets (A1 in Figure 3).  For an inbound flow, this will be the
       apparent source address of the packets as forwarded to the
       internal endpoint (A0 in Figure 3).  The middlebox reserves and
       reports an internal address only in the case where twice-NAT is
       in effect.  Otherwise, the value of 0.0.0.0 (IPv4) or :: (IPv6)
       indicates that no internal reservation was made.  See also
       Section 2.3.5.

     - reserved inside port number: see section 2.3.5.

     - reserved outside IP address: The reserved IPv4 or IPv6 address on
       the external side of the middlebox.  For an inbound flow, this
       will be the destination to which the external endpoint sends its
       packets (A2 in Figure 4).  For an outbound flow, this will be the
       apparent source address of the packets as forwarded to the
       external endpoint(A3 in Figure 3).  If the middlebox is
       configured as a pure firewall, the value of this reply parameter
       is of 0.0.0.0 (IPv4) or :: (IPv6) indicating that no external
       reservation was made.  See also Section 2.3.5.

     - reserved outside port number: see section 2.3.5.

     - policy rule lifetime: the policy rule lifetime granted by the
       middlebox, after which the reservation will be revoked if it has
       not been replaced already by a policy enable rule in a PER
       transaction.

   reply-parameters (failure):

     - an identifier matching the identifier of the request.

     - failure reason: the reason why the reserve request was rejected.
       The list of possible reasons includes but is not restricted to:
          - agent not authorized for this transaction
          - agent not authorized for adding members to this group
          - no such group
          - lack of IP addresses
          - lack of port numbers
          - lack of resources

   semantics:

      The agent can use this transaction type to reserve an IP address
      or a combination of IP address, transport type, port number and
      port range at neither, one, or both sides of the middlebox as
      required to support the enabling of a flow.  Typically the PRR
      will be used in scenarios where it is required to perform such a


Stiemerling, Quittek, Taylor                                   [Page 21]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


      reservation before sufficient parameters for a complete policy
      enable rule transaction are available.  See section 4.2 for an
      example.

      When receiving the request, the middlebox determines how many
      address (and port) reservations are required based on its
      configuration.  If it provides only packet filter services, it
      does not perform any reservation and just returns empty values for
      the reserved inside and outside IP addresses and port numbers. If
      it is configured for twice-NAT , it reserves both inside and
      outside IP adresses (and an optional range of port numbers) and
      returns them. Otherwise, it reserves and returns an outside IP
      address (and an optional range of port numbers) and retuns empty
      values for the reserved inside address and port range.

      If there is a lack of resources, such as avaliable IP addresses,
      port numbers, or storage for further policy rules, then the
      reservation fails and an appropriate failure reply is generated.

      If a non-existing policy rule group was specified, or if an
      existing policy rule group was specified that is not owned by the
      requesting agent, then no new policy rule is established and an
      appropriate failure reply is generated.

      In case of success, this transaction creates a new policy reserve
      rule.  If the specified group exists already, then the new policy
      rule becomes a member of it. If no policy group is specified a new
      group is created with the new policy rule as its only member.  The
      middlebox generates a middlebox unique identifier for the new
      policy rule.  The owner of the new policy rule is the
      authenticated agent that sent the request.  The middlebox chooses
      a lifetime value that is greater than zero and less than or equal
      to the minimum of the requested value and the maximum lifetime
      specified by the middlebox at session startup, i.e.:

              0 <= lt_granted <= MINIMUM(lt_requested, lt_maximum)

      whereas:
          - lt_granted is the actually granted lifetime by the middlebox
          - lt_requested is the requested lifetime of the agent
          - lt_maximum is the maximum liftime specified at session setup

   If the protocol identifier is 'IP', then the middlebox reserves
   available inside and/or outside IP address(es) only.  The reserved
   address(es) are returned to the agent.  In this case the request-
   parameters port range and port parity as well as reply-parameters
   inside port number and outside port number are irrelevant.

   If the protocol identifier is 'UDP' or 'TCP', then a combination of
   an IP address and a consecutive sequence of port numbers, starting


Stiemerling, Quittek, Taylor                                   [Page 22]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   with the specified parity, is reserved, on neither, one, or both
   sides of the middlebox as appropriate.  The IP address(es) and the
   first (lowest) reserved port number(s) of the consecutive sequence
   are returned to the agent. (This also applies to other protocols
   supporting ports or the equivalent.)


2.3.7.  Policy Enable Rule (PER)

   transaction-name: policy enable rule

   transaction-type: configuration

   transaction-compliance: mandatory

   request-parameters:

     - request identifier: an agent unique identifier for matching
       corresponding request and reply at the agent.

     - group identifier: a reference to the group of which the policy
       enable rule should be a member.

     - policy reserve rule identifier: a reference to an already
       existing policy reserve rule created by a PRR transaction.  The
       reference may be empty, in which case the middlebox must assign
       any necessary addresses and port numbers within this PER
       transaction.  If it is not empty, then the following request
       parameters are irrelevant: transport protocol, port range, port
       parity, internal IP version, external IP version.

     - transport protocol: see section 2.3.5.

     - port range: the number of consecutive port numbers to be
       reserved, see Section 2.3.5.

     - port parity: the requested parity of the port number(s) to be
       mapped.  Allowed values of this parameter are 'same' and 'any'.
       See also Section 2.3.5.

     - direction of flow: this parameter specifies the direction of
       enabled communication, either 'inbound', 'outbound', or 'bi-
       directional'.

     - internal IP version: requested IP version at the inside of the
       middlebox, see Section 2.3.5.

     - internal IP address: the IP address of the internal communication
       endpoint (A0 in Fig. 3), see Section 2.3.5.



Stiemerling, Quittek, Taylor                                   [Page 23]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


     - internal port number: the port number of the internal
       communication endpoint (A0 in Fig. 3), see Section 2.3.5.

     - external IP version: requested IP version at the outside of the
       middlebox, see Section 2.3.5.

     - external IP address: the IP address of the external communication
       endpoint (A3 in Fig. 3), see Section 2.3.5.

     - external port number: the port number of the external
       communication endpoint (A3 in Fig. 4), see Section 2.3.5.

     - policy rule lifetime: a lifetime proposal to the middlebox for
       the requested policy rule.

   reply-parameters (success):

     - request identifier: an identifier matching the identifier of the
       request.

     - policy rule identifier: a middlebox unique policy rule
       identifier.  It is assigned by the middlebox and used as policy
       rule handle in further policy rule transactions.  If a reserved
       policy rule identifier was provided in the request, then the
       returned policy rule identifier has the same value.

     - group identifier: a reference to the group of which the policy
       enable rule is a member.

     - inside IP address: the IP address provided at the inside of the
       middlebox (A1 in Fig. 3).  In case of a twice-NAT, this parameter
       will be an internal IP address reserved at the inside of the
       middlebox.  In all other cases, this reply-parameter will be
       identical with the external IP address passed with the request.
       If the policy reserve rule identifier parameter was supplied in
       the request and if the respective PRR transaction reserved an
       inside IP address, then the inside IP address provided in the PER
       response will be the identical value to that returned by the
       response to the PRR request.  See also Section 2.3.5.

     - inside port number: the internal port number provided at the
       inside of the middlebox (A1 in Fig. 3),  see also Section 2.3.5.

     - outside IP address: the external IP address provided at the
       outside of the middlebox (A2 in Fig. 4).  In case of a pure
       firewall, this parameter will be identical with the internal IP
       address passed with the request.  In all other cases, this reply-
       parameter will be an external IP address reserved at the outside
       of the middlebox. See also Section 2.3.5.



Stiemerling, Quittek, Taylor                                   [Page 24]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


     - outside port number: the external port number provided at the
       outside of the NAT (A2 in Fig. 3), see Section 2.3.5..

     - policy rule lifetime: the policy rule lifetime granted by the
       middlebox.

   reply-parameters (failure):

     - an identifier matching the identifier of the request.

     - failure reason: the reason why the policy enable rule was
       rejected.  The list of possible reasons includes but is not
       restricted to:
          - agent not authorized for this transaction
          - no such group
          - agent not authorized for adding members to this group
          - no such policy reserve rule
          - agent not authorized for replacing this policy reserve rule
          - conflict with already existing policy rule (e.g. the same
            internal address-port is being mapped to different outside
            address-port pairs)
          - lack of IP addresses
          - lack of port numbers
          - lack of resources

   semantics:

      This transactions can be used by an agent for enabling
      communication between an internal endpoint and an external
      endpoint independent of the type of middlebox (NAT, NAPT,
      firewall, NAT-PT, combined devices, ... ) for uni-directional or
      bi-directional traffic.

      The agent sends an enable request specifying the endpoints
      (optionally including wildcards) and the direction of
      communication (inbound, outbound, bi-directional).  The
      communication endpoints are displayed in Figure 3.  The basic
      operation of the PER transaction can be described by

    1. the agent sending A0 and A3 to the middlebox,

    2. the middlebox reserving A1 and A2 or using A1 and A2 from a
       previous PRR transaction

    3. the middlebox enabling packet transfer between A0 and A3 by
       binding A0-A2 and A1-A3 and/or by opening the corresponding
       pinholes, both according to the specified direction,

    4. the middlebox returning A1 and A2 to the agent.



Stiemerling, Quittek, Taylor                                   [Page 25]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   In case of a pure packet filtering firewall, the returned address
   tuples are the same as the ones in the request: A2=A0 and A1=A3.
   Each partner uses the other one's real address.  In case of a
   traditional NAT the internal endpoint may use the real address of the
   external endpoint (A1=A3), but the external endpoint uses an address
   tuple provided by the NAT (A2!=A0).  In case of a twice-NAT device,
   both endpoints uses address tuples provided by the NAT for addressing
   their communication partner (A3!=A1 and A2!=A0).

   If a firewall is combined with a NAT or a twice-NAT, the replied
   address tuples will be the same as for pure traditional NAT or twice-
   NAT, respectively, but the middlebox will configure its packet filter
   in addition to the performed NAT bindings.  In case of a firewall
   combined with a traditional NAT, more than one enable action might be
   required for the firewall configuration, because incoming and
   outgoing packets may use different source-destination pairs.

   If the reservation identifier is not empty, then the middlebox checks
   whether or not the reference policy rule exists and whether or not
   the agent is authorized to replace this policy rule.

   If a non-existing policy rule group was specified, or if an existing
   policy rule group was specified that is not owned by the requesting
   agent, then no new policy rule is established and an appropriate
   failure reply is generated.

   In case of success, this transaction creates a new policy enable
   rule.  If a policy reserve rule was referenced, then this policy rule
   is terminated without an explicit notification sent to the agent
   (besides the successful PER reply).

   If a policy rule group was specified, then the new policy rule
   becomes a member of it. If no policy group is specified a new group
   is created with the new policy rule as its only member.

   The middlebox generates a middlebox unique identifier for the new
   policy rule. If a policy reserve rule was referenced, then the
   identifier of the policy reserve rule may be re-used.

   The owner of the new policy rule is the authenticated agent that sent
   the request.

   If the transport protocol parameter value is 'any', then the
   middlebox enables communication between the specified external IP
   address and the specified internal IP address.  The addresses to be
   used by the communication partners in order to address each other are
   returned to the agent as inside IP address and outside IP address.
   If the reservation identifier is not empty and if the reservation
   used the same transport protocol type, then the reserved IP addresses
   are is used.


Stiemerling, Quittek, Taylor                                   [Page 26]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   For the transport protocol parameter values 'UDP' and 'TCP' the
   middlebox acts analogously to 'IP' with additionally mapping ranges
   of port numbers and keeping the port parity if requested.

   The configuration of the middlebox may fail because of lack of
   resources, such as available IP addresses, port numbers, or storage
   for further policy rules.  Also it may fail because of a conflict
   with an already established policy rule.  In case of a conflict,  the
   first come first serve mechanism is applied.  Already existing policy
   rules remain unchanged and arriving new ones are rejected.  However,
   in case of a non-conflicting overlap of policy rules (including
   identical policy rules), all policy rules are accepted.

   In each case of failure, an appropriate failure reply is generated.
   The policy reserve rule that is referenced in the PER transaction is
   not affected in case of a failure due to the PER transaction, i.e.
   the policy reserve rule remains.


2.3.8.  Policy Rule Lifetime Change (RLC)

   transaction-name: policy rule lifetime change

   transaction-type: configuration

   transaction-compliance: mandatory

   request-parameters:

     - request identifier: an agent unique identifier for matching
       corresponding request and reply at the agent.

     - policy rule identifier: identifying the policy rule for which the
       lifetime is requested to be changed.  This may identify either a
       policy reserve rule or a policy enable rule.

     - policy rule lifetime: the new lifetime proposal for the policy
       rule.

   reply-parameters (success):

     - request identifier: an identifier matching the identifier of the
       request.

     - policy rule lifetime: The remaining policy rule lifetime granted
       by the middlebox.

   reply-parameters (failure):




Stiemerling, Quittek, Taylor                                   [Page 27]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


     - request identifier: an identifier matching the identifier of the
       request.

     - failure reason: the reason why the lifetime change was rejected.
       The list of possible reasons includes but is not restricted to:
          - agent not authorized for this transaction
          - agent not authorized for changing lifetime of this policy
            rule
          - no such policy rule
          - lifetime cannot be extended

   semantics:

      The agent can use this transaction type to request an extension
      the lifetime of an already established policy rule, to request
      shortening of the life time, or to request policy rule
      termination.  Policy rule termination is requested by suggesting a
      new policy rule lifetime of zero.

      The middlebox first checks whether or not the specified policy
      rule exists and whether or not the agent is authorized to access
      this policy rule.  If one of the checks fails, an appropriate
      failure reply is generated.  If the requested lifetime is longer
      than the current one, the middlebox also checks, whether or not
      the lifetime of the policy rule may be extended and generates an
      appropriate failure message if not.

      A failure reply implies that the lifetime of the policy rule
      remains unchanged.  A success reply is generated by the middlebox,
      if the lifetime of the policy rule was changed in any way.

      The success reply contains the new lifetime of the policy rule.
      The middlebox chooses the lifetime within the interval limited by
      the lifetime of the policy rule at arrival of the request and by
      the suggested lifetime.  The granted remaining lifetime must not
      exceed the maximum lifetime that the middlebox specified at
      session setup together with its other capabilities.  It also must
      not exceed the lifetime of the group of which the policy rule is a
      member.

      Editor's comment: the use of group lifetimes as constraints on
      individual policy rule lifetimes was considered to be not
      necessary in IETF 54 discussion.

      After sending a success reply with a lifetime of zero, the
      middlebox will consider the policy rule to be non-existent.  It
      will not process any further transaction on this policy rule.

      Please note, that policy rule lifetime may also be changed by the
      Group Lifetime Change (GLC) transaction if applied to the group of


Stiemerling, Quittek, Taylor                                   [Page 28]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


      which the policy rule is a member.


2.3.9.  Policy Rule Status (PRS)

   transaction-name: policy rule status

   transaction-type: monitoring

   transaction-compliance: optional

   request-parameters:

     - request identifier: an agent unique identifier for matching
       corresponding request and reply at the agent.

     - policy rule identifier: the middlebox unique policy rule
       identifier.

   reply-parameters (success):

     - request identifier: an identifier matching the identifier of the
       request.

     - policy rule owner: an identifier of the agent owning this policy
       rule.

     - group identifier: a reference to the group of which the policy
       rule is a member.

     - policy rule action: this parameter has either the value 'reserve'
       or the value 'enable'.

     - transport protocol: identifies the protocol for which a
       reservation is requested, see Section 2.3.5.

     - port range: the number of consecutive ports numbers, see Section
       2.3.5.

     - direction: the direction of the communication enabled by the
       middlebox, see Section 2.3.5.

     - internal IP address version: the version of the internal IP
       address (IP version of A0 in Fig. 3)

     - external IP address version: the version of the external IP
       address (IP version of A3 in Fig. 3)

     - internal IP address: the IP address of the internal communication
       endpoint (A0 in Fig. 3), see Section 2.3.5.


Stiemerling, Quittek, Taylor                                   [Page 29]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


     - internal port number: the port number of the internal
       communication endpoint (A0 in Fig. 3), see Section 2.3.5.

     - external IP address: the IP address of the external communication
       endpoint (A3 in Fig. 3), see Section 2.3.5.

     - external port number: the port number of the external
       communication endpoint (A3 in Fig. 3), see Section 2.3.5.

     - inside IP address: the internal IP address provided at the inside
       of the NAT (A1 in Fig. 3), see Section 2.3.5.

     - inside port number: the internal port number provided at the
       inside of the NAT (A1 in Fig. 3), see Section 2.3.5.

     - outside IP address: the external IP address provided at the
       outside of the NAT (A2 in Fig. 3), see Section 2.3.5.

     - outside port number: the external port number provided at the
       outside of the NAT (A2 in Fig. 3), see Section 2.3.5.

     - policy rule lifetime: the remaining lifetime of the policy rule.

   reply-parameters (failure):

     - request identifier: an identifier matching the identifier of the
       request.

     - failure reason: the reason why the request for a status report
       was rejected.  The list of possible reasons includes but is not
       restricted to:
          - transaction not supported
          - agent not authorized for this transaction
          - no such policy rule
          - agent not authorized for accessing this policy rule

   semantics:

      The agent can use this transaction type to list all properties of
      a policy rule.  Usually, the agent has this information already,
      but in special cases (for example after an agent failover) or for
      special agents (for example an administrating agent that can
      access all policy rules) this optional transaction can be helpful.

      The middlebox first checks whether or not the specified policy
      rule exists and whether or not the agent is authorized to access
      this group.  If one of the checks fails, an appropriate failure
      reply is generated.  Otherwise all properties of the policy rule
      are returned to the agent.  Some of the returned parameters may be
      irrelevant, depending on the policy rule action ('reserve' or


Stiemerling, Quittek, Taylor                                   [Page 30]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


      'enable') and depending on other parameters, for example the
      protocol identifier.

      This transaction does not have any effect on the policy rule
      state.


2.3.10.  Asynchronous Policy Rule Deletion (ARD)

   transaction-name: asynchronous policy rule deletion

   transaction-type: notification

   transaction-compliance: mandatory

   notification-parameters:

     - policy rule identifier: the policy rule that will be deleted.

     - deletion reason: the reason why the middlebox will delete the
       policy rule.

   semantics:

      The middlebox may decide at any point in time to delete a policy
      rule.  Particularly, this transaction is triggered by lifetime
      expiration of the policy rule.  Among other events that may cause
      this transaction are changes in the policy rule decision point.

      If this notification is generated, it is sent to all agents that
      are in an open session with the middlebox and that are authorized
      to access the policy rule.  The notification is sent to the agents
      before the middlebox deletes the policy rule.  After sending the
      notification, the middlebox will consider the policy rule to be
      non-existent.  It will not process any further transaction on the
      policy rule.


2.3.11.  Policy Rule State Machine

   The state machine for the policy rule transactions is shown in Figure
   4 with all possible state transitions.  You'll find the used
   transaction abbreviations in the headings of the particular
   transaction section.








Stiemerling, Quittek, Taylor                                   [Page 31]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


                                         PRR/failure
                                         PER/failure
                                        +-----------+
                                        |           v
                        PRR/success   +-+-------------+
                    +-----------------+  PRID UNUSED  |<-+
          +----+    |                 +---------------+  |
          |    |    |                   ^   |            |
          |    v    v        ARD        |   |            |
          |  +-------------+ PER/failure|   | PER/       | ARD
          |  |   RESERVED  +------------+   | success    | RLC(lt=0)/
          |  +-+----+------+ RLC(lt=0)/     |            |  success
          |    |    |         success       |            |
          +----+    |                       v            |
        RLC(lt>0)/  | PER/success     +---------------+  |
         success    +---------------->|    ENABLED    +--+
        RLC/failure                   +-+-------------+
                                        |           ^
                                        +-----------+
            lt = lifetime               RLC(lt>0)/success
                                        RLC/failure

                  Figure 4: Policy Rule State Machine

   This state machine exists per policy rule identifier (PRID).
   Initially, all policy rules are in state PRID UNUSED, which means
   that the policy rule does not exist or is not active.  After
   returning to state RULE UNUSED, the policy rule identifier is no
   longer bound to an existing policy rule and may be re-used by the
   middlebox.

   A successful PRR transaction causes a transition from the initial
   state PRID UNSUSED to state RESERVED, where an address reservation is
   established.  From there, state ENABLED can be entered by a PER
   transaction.  This transaction can also be used for entering state
   ENABLED directly from state PRID UNUSED without a reservation.  In
   state ENABLED the requested communication between the internal and
   the external endpoint is enabled.

   The states RESERVED and ENABLED can be maintained by a successful RLC
   transactions with a requested lifetime greater than 0.  Transitions
   from both of these states back to state PRID UNUSED can be caused by
   an ARD transaction or by a successful RLC transaction with a lifetime
   parameter of 0.  Additionally, a failed PER transaction causes a
   transition from state RESERVED to PRID UNUSED.

   Please note, transitions initiated by RLC transactions may also be
   initiated by GLC transactions.




Stiemerling, Quittek, Taylor                                   [Page 32]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


2.4.  Policy Rule Group Transactions

   This section describes the semantics for transactions on groups of
   policy rules.  These transactions are specified:

      - Group Lifetime Change (GLC)
      - Group List (GL)
      - Group Status (GS)

   All are request transactions initiated by the agent.  The status
   information transactions (GL and GS) do not have any effect on the
   group state machine.


2.4.1.  Overview

   A policy rule group has only one attribute: the list of its members.
   All member policies of a single group must be owned by the same
   authenticated agent.  Therefore, an implicit property of a group is
   its owner, which is the owner of the member policy rules.

   A group is created implicitly, when its first member policy rule is
   established. A group us deleted implicitly, when the last remaining
   member policy rule is deleted.  Consequently, the lifetime of a group
   is the maximum of the lifetimes of all member policy rules.

   A group has a middlebox unique identifier.

   Group transactions are redundant in the sense that they can be
   removed easily from the semantics specification without changing the
   set of possible middlebox configurations an agent can request.
   Therefore, all of them are declared as 'optional' by their respective
   compliance entry in Section 3.  However, they provide some
   functionality, that is not available if only mandatory transactions
   are available.

   The Group Lifetime Change (GLC) transaction is equivalent to
   simultaenously performed Policy Rule Lifetime Change (RLC)
   transactions on all members of the group.  The result of a successful
   GLC transaction is that all member policy rules have the same
   lifetime.  Analogously to the RLC transaction, the GLC transaction
   can be use for deleting all member policy rules by requesting a
   lifetime of zero.

   The status information transactions Group List (GL) and Group Status
   (GS) can be used by the agent for exploring the state of the
   middlebox and for exploring its access rights.  The GL transaction
   lists all groups that the agent may access, including groups owned by
   other agents.  The GS transaction reports the status on an individual
   group and it lists all policy rules of this group by their policy


Stiemerling, Quittek, Taylor                                   [Page 33]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   rule identifiers.  The agent can explore the state of the individual
   policy rules by using the policy rule identifiers in a policy rule
   information transaction (see Section 2.4.7).


2.4.2.  Group Lifetime Change (GLC)

   transaction-name: group lifetime change

   transaction-type: configuration

   transaction-compliance: optional

   request-parameters:

     - request identifier: an agent unique identifier for matching
       corresponding request and reply at the agent.

     - group identifier: a reference to the group for which the lifetime
       is requested to be changed.

     - group lifetime: the new lifetime proposal for the group.

   reply-parameters (success):

     - request identifier: an identifier matching the identifier of the
       request.

     - group lifetime: The group lifetime granted by the middlebox.

   reply-parameters (failure):

     - request identifier: an identifier matching the identifier of the
       request.

     - failure reason: the reason why the lifetime change was rejected.
       The list of possible reasons includes but is not restricted to:
          - transaction not supported
          - agent not authorized for this transaction
          - agent not authorized for changing lifetime of this group
          - no such group
          - lifetime cannot be extended

   semantics:

      The agent can use this transaction type to request an extension of
      the lifetime of all members of a policy rule group, to request
      shortening the lifetime of all members, or to request deletion of
      all member policies (which implies deletion of the group).
      Deletion is requested by suggesting a new group lifetime of zero.


Stiemerling, Quittek, Taylor                                   [Page 34]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


      The middlebox first checks whether or not the specified group
      exists and whether or not the agent is authorized to access this
      group.  If one of the checks fails, an appropriate failure reply
      is generated.  If the requested lifetime is longer than the
      current one, the middlebox also checks whether or not the lifetime
      of the group may be extended and generates an appropriate failure
      message if not.

      A failure reply implies that the lifetime of the group remains
      unchanged.  A success reply is generated by the middlebox if the
      lifetime of the group was changed in any way.

      The success reply contains the new common lifetime of all member
      policy rules of the group.  The middlebox chooses the new lifetime
      less than or equal to the minimum of the requested lifetime and
      the maximum lifetime that the middlebox specified at session setup
      together with its other capabilities, i.e.:

              0 <= lt_granted <= MINIMUM(lt_requested, lt_maximum)

      whereas:
          - lt_granted is the actually granted lifetime by the middlebox
          - lt_requested is the requested lifetime of the agent
          - lt_maximum is the maximum liftime specified at session setup

   After sending a success reply with a lifetime of zero, the member
   policy rules will be deleted without any further notification to the
   agent, and the middlebox will consider the group and all of its
   members to be non-existent.  It will not process any further
   transaction on this group or on any of its members.


2.4.3.  Group List (GL)

   transaction-name: group list

   transaction-type: monitoring

   transaction-compliance: optional

   request-parameters:

     - request identifier: an agent unique identifier for matching
       corresponding request and reply at the agent.

   reply-parameters (success):

     - request identifier: an identifier matching the identifier of the
       request.



Stiemerling, Quittek, Taylor                                   [Page 35]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


     - group list: list of all groups that the agent can access. For
       each listed group the identifier and the owner are indicated.

   reply-parameters (failure):

     - request identifier: an identifier matching the identifier of the
       request.

     - failure reason: the reason why the request for listing groups was
       rejected.  The list of possible reasons includes but is not
       restricted to:
          - transaction not supported
          - agent not authorized for this transaction

   semantics:

      The agent can use this transaction type to list all groups which
      it can access.  Usually, the agent has this information already,
      but in special cases (for example after an agent failover) or for
      special agents (for example an administrating agent that can
      access all groups) this transaction can be helpful.

      The middlebox first checks whether or not the agent is authorized
      to request this transaction.  If the check fails, an appropriate
      failure reply is generated.  Otherwise a list of all groups the
      agent can access is returned indicating the identifier and the
      owner of each group.

      This transaction does not have any effect on the group state.


2.4.4.  Group Status (GS)

   transaction-name: group status

   transaction-type: monitoring

   transaction-compliance: optional

   request-parameters:

     - request identifier: an agent unique identifier for matching
       corresponding request and reply at the agent.

     - group identifier: a reference to the group for which status
       information is requested.

   reply-parameters (success):




Stiemerling, Quittek, Taylor                                   [Page 36]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


     - request identifier: an identifier matching the identifier of the
       request.

     - group owner: an identifier of the agent owning this policy rule
       group.

     - group lifetime: the remaining lifetime of the group.  This is the
       maximum of the remaining lifetime of all members policy rules.

     - member list: list of all policy rules that are members of the
       group.  The policy rules are specified by their middlebox unique
       policy rule identifier.

   reply-parameters (failure):

     - request identifier: an identifier matching the identifier of the
       request.

     - failure reason: the reason why the request for a status report
       was rejected.  The list of possible reasons includes but is not
       restricted to:
          - transaction not supported
          - agent not authorized for this transaction
          - no such group
          - agent not authorized for listing members of this group

   semantics:

      The agent can use this transaction type to list all member policy
      rules of a group.  Usually, the agent has this information
      already, but in special cases (for example after an agent
      failover) or for special agents (for example an administrating
      agent that can access all groups) this transaction can be helpful.

      The middlebox first checks whether or not the specified group
      exists and whether or not the agent is authorized to access this
      group.  If one of the checks fails, an appropriate failure reply
      is generated.  Otherwise a list of all group members is returned
      indicating the identifier of each group.

      This transaction does not have any effect on the group state.


3.  Conformance Statements

   A protocol definition complies with the semantics defined in Section
   2 if the protocol specification includes all specified transactions
   with all their parameters.  However, concrete implementations of the
   protocol may not support some of the optional transactions.  Which
   transactions are required for compliancy is different for agent and


Stiemerling, Quittek, Taylor                                   [Page 37]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   middlebox.

   This section contains conformance statements for MIDCOM protocol
   implementations related to the semantics.  Conformance is specified
   differently for agents and middleboxes.  Most probably these
   conformance statements will be extended by a concrete protocol
   specification.  However, such an extension is expected to extend the
   statements below in a way that all of them still hold.

   The following list shows the transaction-compliance property of all
   transactions as specified in the previous section:

     - Session Control Transactions
         - Session Establishment (SE)              mandatory
         - Session Termination (ST)                mandatory
         - Asynchronous Session Termination (AST)  mandatory

     - Policy Rule Group Transactions
         - Group Lifetime Change (GLC)             optional
         - Group List (GL)                         optional
         - Group Status (GS)                       optional

     - Policy Rule Transactions
         - Policy Reserve Rule (PRR)               mandatory
         - Policy Enable Rule (PER)                mandatory
         - Policy Rule Lifetime Change (RLC)       mandatory
         - Policy Rule Status (PRS)                optional
         - Asynchronous Policy Rule Deletion (ARD) mandatory


3.1.  General Implementation Conformance

   A compliant implementation of a MIDCOM protocol must support all
   mandatory transactions.

   A compliant implementation of a MIDCOM protocol may support none,
   one, or more of the following transactions: GLC, GL, GS, PRS.

3.2.  Middlebox Conformance

   A middlebox implementation of a MIDCOM protocol supports a request
   transaction if it is able to receive and process all possible correct
   message instances of the particular request transaction and if it
   generates a correct reply for any correct request it receives.

   A middlebox implementation of a MIDCOM protocol supports a
   notification transaction if it is able to to generate the
   corresponding notification message properly.

   A compliant middlebox implementation of a MIDCOM protocol must inform


Stiemerling, Quittek, Taylor                                   [Page 38]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   the agent about the list of supported transactions within the SE
   transaction.

3.3.  Agent Conformance

   An agent implementation of a MIDCOM protocol supports a request
   transaction if it is able to generate the corresponding request
   message properly and if it is able to receive and process all
   possible correct replies to the particular request.

   An agent implementation of a MIDCOM protocol supports a notification
   transaction if it is able to receive and process all possible correct
   message instances of the particular transaction.

   A compliant agent implementation of a MIDCOM protocol must not use
   any optional transaction that is not supported by the middlebox.  The
   middlebox informs the agent about the list of supported transactions
   within the SE transaction.


4.  Transaction Usage Examples

   This section gives two usage examples of the transactions specified
   in Section 2.  First it is shown, how an agent can explore all policy
   rules and policy rule groups, which it may access at a middlebox.
   Then the middlebox configuration for enabling a SIP-signaled call is
   demonstrated.


4.1.  Exploring Policy Rules and Policy Rule Groups

   This example precludes an already established session.  It shows how
   an agent can find out

      - which groups it may access and who owns these groups
      - the status and member list of all accessible groups
      - the status and properties of all accessible policy rules

   If there is just a single session, there is no need for any of these
   actions, because the middlebox informs the agent about each state
   transition of any policy rule or policy rule group.  However, after
   the disruption of a session or after an intentional session
   termination, the agent might want to re-establish the session and
   explore, which of the groups and policy rules it established are
   still in place.

   Also an agent system may fail and another one takes over.  Then the
   other one need to find out what has already been configured by the
   failing system and what still needs to be done.



Stiemerling, Quittek, Taylor                                   [Page 39]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   A third situation where exploring policy rules and groups is useful
   is the case of an agent with 'administrator' authorization.  This
   agent may access any policy rule or group created by any other agent
   and modify them.

   All of them probably will start their exploration with the Group List
   (GL) transaction, as shown in Figure 5.  On this request, the
   middlebox returns a list of pairs each containing an agent identifier
   and a group identifier (GID).  The agent gets informed which own
   group and which of other agents' groups it may access.


            agent                                     middlebox
             |                      GL                       |
             |**********************************************>|
             |<**********************************************|
             |   (agent1,GID1) (agent1,GID2) (agent2,GID3)   |
             |                                               |
             |                   GS GID2                     |
             |**********************************************>|
             |<**********************************************|
             |    agent1  lifetime  PID1  PID2  PID3  PID4   |
             |                                               |

               Figure 5: Using the GL and the GS transaction

   In Figure 5 three groups are accessible to the agent, and the agent
   retrieves information about the second group by using the Group
   Status (GS) transaction.  It receives the owner of the group, the
   remaining lifetime, and the list of member policy rules, in this case
   containing four policy rule identifiers (PIDs).

   In the following, the agent explores these four policy rules.  The
   example assumes the middlebox to be a traditional NAPT.  Figure 6
   shows the exploration of the first policy rule.  As reply to a Policy
   Rule Status (PRS) transaction, the middlebox always returns the
   following list of parameters:

      - policy rule owner
      - group identifier
      - policy rule action (reserve or enable)
      - protocol type
      - port range
      - direction
      - internal IP address
      - internal port number
      - external address
      - external port number
      - NAT inside IP address
      - NAT inside port number


Stiemerling, Quittek, Taylor                                   [Page 40]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


      - NAT outside IP address
      - NAT outside port number
      - IP address versions (not printed)


            agent                                     middlebox
             |                   PRS PID1                    |
             |**********************************************>|
             |<**********************************************|
             |    agent1    GID2    RESERVE    UDP    1      |
             | ANY         ANY         ANY         ANY       |
             | ANY         ANY         IPADR_OUT   PORT_OUT1 |
             |                                               |

             Figure 6: Status report for an outside reservation

   The `ANY' parameter printed in Figure 6 is equal to the `zero' IP
   addresses define in section 2.3.5.  The policy rule with PID1 is a
   policy reserve rule for UDP traffic at the outside of the middlebox.
   Since there is no internal or external address involved yet, these
   four fields are wildcarded in the reply.  The same holds for the
   inside NAT address and port number.  The only address information
   given by the reply is the reserved outside IP address of the NAT
   (IPADDR_OUT) and the corresponding port number (PORT_OUT1).  Note,
   that IPADR_OUT and PORT_OUT1 may not be wildcarded, because the
   reserve action does not support this.

   Applying PRS to PID2 (Figure 7) shows that the second policy rule is
   an policy enable rule for inbound UDP packets.  The internal
   destination is fixed concerning IP address, protocol and port number,
   but for the external source, the port number is wildcarded.  The
   outside IP address and port number of the middlebox are the ones the
   external sender needs to use as destination in the original packet it
   sends.  At the middlebox, the destination address is replaced with
   the internal address of the final receiver.  During address
   translation, the source IP address and the source port numbers of the
   packets remain unchanged.  This is indicated by the inside address
   which is identical to the external address.

            agent                                     middlebox
             |                   PRS PID2                    |
             |**********************************************>|
             |<**********************************************|
             |       agent1  GID2  ENABLE  UDP  1  IN        |
             | IPADR_INT   PORT_INT1   IPADR_EXT   ANY       |
             | IPADR_EXT   ANY         IPADR_OUT   PORT_OUT2 |
             |                                               |

            Figure 7: Status report for enabled inbound packets



Stiemerling, Quittek, Taylor                                   [Page 41]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   For traditional NATs the identity of the inside IP address and port
   number with the external IP address and port number always holds
   (A1=A3 in Figure 3).  For a pure firewall, also the outside IP
   address and port number are always identical with the internal IP
   address and port number (A0=A2 in Figure 3).


            agent                                     middlebox
             |                   PRS PID3                    |
             |**********************************************>|
             |<**********************************************|
             |       agent1  GID2  ENABLE  UDP  1  OUT       |
             | IPADR_INT   PORT_INT2   IPADR_EXT   PORT_EXT1 |
             | IPADR_EXT   PORT_EXT1   IPADR_OUT   PORT_OUT3 |
             |                                               |

            Figure 8: Status report for enabled outbound packets

   Figure 8 shows enabled outbound UDP communication between the same
   host.  Here all port numbers are known. Since again A1=A3, the
   internal sender uses the external IP address and port number as
   destination in the original packets.  At the firewall, the internal
   source IP address and port number are replaced by the shown outside
   IP address and port number of the middlebox.


            agent                                     middlebox
             |                   PRS PID4                    |
             |**********************************************>|
             |<**********************************************|
             |       agent1  GID2  ENABLE  TCP  1  BI        |
             |  IPADR_INT   PORT_INT3  IPADR_EXT   PORT_EXT2 |
             |  IPADR_EXT   PORT_EXT2  IPADR_OUT   PORT_OUT4 |
             |                                               |

          Figure 9: Status report for bi-directional TCP traffic

   Finally, Figure 9 shows the status report for enabled bi-directional
   TCP traffic. Please note that still A1=A3: For outbound packets, only
   the source IP address and port number are replaced at the middlebox,
   and for inbound packets, only the destination IP address and port
   number are replaced.


4.2.  Enabling a SIP-Signaled Call

   This elaborated transaction usage example shows the interaction
   between a SIP proxy and a middlebox.  The middlebox itself is a
   traditional NAPT and two user agents communicate with each other via
   the SIP proxy and NAPT as shown in figure 10.


Stiemerling, Quittek, Taylor                                   [Page 42]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


                     +----------+
                     |SIP Proxy |
                     |for domain|
                     |  mb.com  |
                     +----------+
            Private     ^   ^           Public Network
            Network     |   |
          +----------+  |   |  +---------+         +----------+
          |User Agent|<-+   +->|Middlebox|<------->|User Agent|
          |    A     |<#######>|  NAPT   |<#######>|    B     |
          +----------+         +---------+         +----------+


          <--> SIP Signalling
          <##> RTP Traffic

                       Figure 10: Example SIP Scenario


   For the below sequence charts we make these assumptions:

     - The NAPT is statically configured to forward SIP signalling from
       the outside to the SIP proxy server, i.e. traffic to the NAPT's
       external IP address and port 5060 is forwarded to the internal
       SIP proxy.

     - The user agent A, located inside the private network, is
       registered at the SIP proxy with its private IP address.

     - User A knows the general SIP URL of user B.  The URL is B@b.de.
       However, the concrete URL of the SIP User Agent B, which user B
       currently uses, is not known.

     - Only the RTP paths are configured, but not the RTCP paths.

     - The middlebox and the SIP server share an already established
       MIDCOM session.

     - Some parameters are omitted, like the request identifier (RID)


   Furthermore these abbreviations are used:

      - IP_AI: Internal IP address of user agent A
      - P_AI: Internal port number of user agent A to receive RTP data
      - P_AE: External mapped port number of user agent A
      - IP_AE: External IP address of the middlebox
      - IP_B: IP address of user agent B
      - P_B: Port number of user agent B to receive RTP data
      - GID: Group identifier


Stiemerling, Quittek, Taylor                                   [Page 43]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


      - PID: Policy rule identifier

   The abbreviations of the MIDCOM transactions can be found in the
   particular section headings.

   In our example, user A tries to call user B.  Therefore, the user
   agent A sends an INVITE SIP message to the SIP proxy server (see
   Figure 10).  The SDP part of the particular SIP message that is
   relevant for the middlebox configuration is shown in the sequence
   chart as:

       SDP: m=..P_AI..
            c=IP_AI

   where the m tag is the media tag which contains the receiving UDP
   port number and the c tag contains the IP address of the terminal
   receiving the media stream.

   The INVITE message forwarded to user agent B must contain a public IP
   address and a port number to which user agent B can send its RTP
   media stream.  Therefore, the SIP proxy server needs an outside IP
   address and port number at the middlebox (the NAPT) to be available
   for this purpose.  However, since the IP address of user agent B is
   not known yet (it will be sent by user agent B in the reply message),
   the proxy server cannot request an address binding.  Instead it
   reserves an outside IP address and port number with the policy
   reserve rule (PRR).

   The PRR reply reports the reserved IP address, port number and new
   established policy rule group. Now the SIP proxy server replaces in
   the SDP payload of the INVITE message the IP address and port number
   of user agent A by the reserved address and port (see Figure 11).
   Then the SIP INVITE message is forwarded to user agent B with a
   modified SDP body containing the outside address and port number, to
   which user agent B will send its RTP media stream.

















Stiemerling, Quittek, Taylor                                   [Page 44]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   User Agent       SIP                        Middlebox   User Agent
    A              Proxy                          NAPT             B
    |                |                              |              |
    | INIVTE B@B.DE  |                              |              |
    | SDP:m=..P_AI.. |                              |              |
    |     c=IP_AI    |                              |              |
    |--------------->|                              |              |
    |                |                              |              |
    |                |   PRR 0 UDP 1 EVEN 300s      |              |
    |                |*****************************>|              |
    |                |<*****************************|              |
    |                | PRR OK PID1 IP_AE/P_AE 300s  |              |
    |                |                              |              |
    |                | INVITE B@B.DE   SDP:m=..P_AE.. c=IP_AE      |
    |                |-------------------------------------------->|
    |                |<--------------------------------------------|
    |                |       200 OK  SDP:m=..P_B.. c=IP_B          |


           Figure 11: Group establishment and rule reservation

   This SIP `200 OK' reply contains the IP address and port number, at
   which user agent B will receive a media stream. The IP address is
   assumed to be equal to the IP address from which user agent B will
   send its media stream.

   Now, the SIP proxy server has sufficient information for establishing
   the complete NAT binding with a policy enable rule (PER) transaction,
   i.e.  the UDP/RTP data of the call can flow from user agent B to user
   agent A.  For the opposite direction, UDP/RTP data from user agent A
   to B, has to be enabled also.  This is done by a second PER
   transaction with all the necessary parameters (see figure 12). After
   having enabled both UDP/RTP streams the SIP proxy can forward the
   `200 OK' SIP message to user agent A to indicate that the telephone
   call can start.

















Stiemerling, Quittek, Taylor                                   [Page 45]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   User Agent       SIP                        Middlebox   User Agent
    A              Proxy                          NAPT             B
    |                |                              |              |
    |                |  PER GID PID1 UDP 1 EVEN IN  |              |
    |                |   IP_AI P_AI IP_B ANY 300s   |              |
    |                |*****************************>|              |
    |                |<*****************************|              |
    |                |    PER OK PID1 IP_B ANY      |              |
    |                |       IP_AE P_AE1 300s       |              |
    |                |                              |              |
            ...media stream from user agent B to A enabled...
    |                |                              |              |
    |                |  PER GID PID2 UDP 1 EVEN OUT |              |
    |                |    IP_AI ANY IP_B P_B 300s   |              |
    |                |*****************************>|              |
    |                |<*****************************|              |
    |                |    PER OK PID2 IP_B P_B      |              |
    |                |       IP_AE P_AE2 300s       |              |
    |                |                              |              |
             ...media streams from both directions enabled...
    |                |                              |              |
    |    200 OK      |                              |              |
    |<---------------|                              |              |
    | SDP:m=..P_B..  |                              |              |
    |     c=IP_B     |                              |              |

          Figure 12: Policy rule establishment for UDP flows

   User agent B decides to terminate the call and sends its `BYE' SIP
   message to user agent A. The SIP proxy forwards all SIP messages and
   deletes the group afterwards using a group lifetime change (GLC)
   transaction with a requested remaining lifetime of 0 seconds (see
   Figure 13). Deletion of the group includes deleting all member policy
   rules.


















Stiemerling, Quittek, Taylor                                   [Page 46]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   User Agent       SIP                        Middlebox   User Agent
    A              Proxy                          NAPT             B
    |                |                              |              |
    |     BYE        |                     BYE                     |
    |<---------------|<--------------------------------------------|
    |                |                              |              |
    |    200 OK      |                   200 OK                    |
    |--------------->|-------------------------------------------->|
    |                |                              |              |
    |                |         GLC GID 0s           |              |
    |                |*****************************>|              |
    |                |<*****************************|              |
    |                |         GLC OK 0s            |              |
    |                |                              |              |
       ...both NAT bindings for the media streams are removed...

                Figure 13: Deletion of Policy Rule Groups




5.  Compliance with MIDCOM Requirements

   This section explains the compliance of the specified semantics with
   the MIDCOM requirements.  It is structured according to [MDC-REQ]:
      - Compliance with Protocol Machinery Requirements (Section 5.1)
      - Compliance with Protocol Semantics Requirements (Section 5.2)
      - Compliance with Security Requirements (Section 5.3)

   The requirements are referred to using the section number they are
   defined in: "requirement x.y.z" refers to the requirement specified
   in section x.y.z of [MDC-REQ].

5.1.  Protocol Machinery Requirements

5.1.1.  Authorized Association

   The specified semantics enable a MIDCOM agent to establish an
   authorized association between itself and the middlebox.  The agent
   identifies itself by the authentication mechanism of the Session
   Establishment transaction described in Section 2.2.1.  Based on this
   authentication the middlebox can make a determination as to whether
   or not the agent will be permitted to request a service.  Thus,
   requirement 2.1.1 is met.

5.1.2.  Agent connects to Multiple Middleboxes

   As specified in Section 2.2, the MIDCOM protocol allows the agent to
   communicate with more than one middlebox simultaneously.  The
   selection of a mechanism for separating different sessions is left to


Stiemerling, Quittek, Taylor                                   [Page 47]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   the concrete protocol definition.  It must provide a clear mapping of
   protocol messages to open sessions.  Then requirement 2.1.2 is met.

5.1.3.  Multiple Agents connect to same Middlebox

   As specified in Section 2.2, the MIDCOM protocol allows the middlebox
   to communicate with more than one agent simultaneously.  The
   selection of a mechanism for separating different sessions is left to
   the concrete protocol definition.  It must provide a clear mapping of
   protocol messages to open sessions.  Then requirement 2.1.3 is met.

5.1.4.  Deterministic Behavior

   Section 2.1.2 states, that processing a request of an agent may not
   be interrupted by any request of the same or another agent.  This
   provides atomicity among request transactions.  This avoids race
   conditions resulting in an unpredictable behavior of the middlebox.

   Anyway, the behavior of the middlebox can only be predictable in the
   view of its administrators.  In the view of an agent, the middlebox
   behavior is unpredictable, because the administrator can, for example
   at any time modify the authorization of the agent without the agent
   being able to observe this change.  Consequently, the behavior of the
   middlebox is not necessarily deterministic from the point of view of
   any agent.

   Since predictability of the middlebox behavior is given for its
   administrator, requirement 2.1.4 is met.

5.1.5.  Known and Stable State

   Section 2.1.2 states that request transactions are atomic with
   respect to each other and from the point of view of an agent.  All
   transactions are defined clearly as state transitions that either
   leave the current stable and well defined state and enter a new
   stable and well defined one or that remain in the current stable and
   well defined state.  Section 2.1 clearly demands that intermediate
   states are not stable and not reported to any agent.

   Furthermore, for each state transition a message is sent to the
   corresponding agent, either a reply or a notification.  The agent can
   uniquely map each reply to one of the requests that it sent to the
   middlebox, because request agent unique request identifiers are used
   for this purpose.  Notifications are self-explanatory by their
   definition.

   Furthermore, the Group List transaction (Section 2.4.3), the Group
   Status transaction (Section 2.4.4), and the Policy Rule Status
   transaction (Section 2.3.9) allows the agent at any time during a
   session to retrieve information about


Stiemerling, Quittek, Taylor                                   [Page 48]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


      - all policy rule groups it may access,
      - the status and member policy rules of all accessible groups,
      - and the status of all accessible policy rules.

   Therefore, the agent is precisely informed about the state of the
   middlebox (as far as the services requested by the agent are
   affected) and requirement 2.1.5 is met.

5.1.6.  Status Report

   As argued in the previous section, the middlebox unambiguously
   informs the agent about every state transition related to any of the
   services requested by the agent.  Also the agent can at any time
   retrieve full status information about all accessible policy rules
   and policy rule groups.  Thus, requirement 2.1.6 is met.

5.1.7.  Unsolicited Messages (Asynchronous Notifications)

   The semantics include asynchronous notifications from the middlebox
   to the agent, including Asynchronous Session Termination (Section
   2.2.3) and Asynchronous Policy Rule Deletion (Section 2.3.10).  These
   notifications report every change of state, that was not explicitly
   requested by the agent.  Thus, requirement 2.1.7 is met by the
   semantics specified above.

5.1.8.  Mutual Authentication

   As specified in Section 2.2.1, the semantics require mutual
   authentication of agent and middlebox, either by using two subsequent
   Session Establishment transactions or by using mutual authentication
   provided on a lower protocol layer.  Thus, requirement 2.1.8 is met.

5.1.9.  Session Termination by any Party

   The semantics specification states in Section 2.2.2 that the agent
   may request session termination by generating the Session Termination
   request and that the middlebox may not reject this request.  In turn,
   Secion 2.2.3 states that the agent may send the Asynchronous Session
   Termination notification at any time and then terminate the session.
   Thus, requirement 2.1.9 is met.

5.1.10.  Request Result

   Section 2.1 states that each request of an agent is followed by a
   reply of the middlebox indicating either success of failure.  Thus,
   requirement 2.2.10 is met.






Stiemerling, Quittek, Taylor                                   [Page 49]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


5.1.11.  Version Interworking

   Section 2.2.1 states that the agent need to specify the protocol
   version number which it is going to use during the session.  The
   middlebox may accept this and act according to this protocol version
   or reject the session if it does not support this version.  If the
   session setup gets rejected, the agent may try again with another
   version.  Thus, requirement 2.2.11 is met.

5.1.12.  Deterministic Handling of Overlapping Rules

   The only policy rule actions specified are 'reserve' and 'enable'.
   For firewalls, overlapping enable actions or reserve actions do not
   create any conflict, so a firewall will always accept overlapping
   rules as specified in Sections 2.3.1 and 2.3.2 (assuming the required
   authorization is given).

   For NATs reserve and enable may conflict. If a conflicting request
   arrives, it is rejected, as stated in Sections 2.3.1 and 2.3.2.  If
   an overlapping request arrives that does not conflict with the ones
   it overlaps, it is accepted (assuming the required authorization is
   given).

   Therefore, the behavior of the middlebox in the presence of
   overlapping rules can be predicted deterministically, and requirement
   2.1.12 is met.

5.2.  Protocol Semantics Requirements

5.2.1.  Extensible Syntax and Semantics

   Requirement 2.2.1 explicitly requests extensibility of protocol
   syntax.  This needs to be addressed by the concrete protocol
   definition.  The semantics specification is extensible anyway,
   because new transaction may be added.

5.2.2.  Policy Rules for Different Types of Middleboxes

   Section 2.3 explains that the semantics use identical transactions
   for all middlebox types and that the same policy rule can be applied
   to all of them.  Thus requirement 2.2.2 is met.

5.2.3.  Ruleset Groups

   The semantics explicitly supports grouping of policy rules and
   transactions on policy rule groups, as described in Section 2.4.  The
   group transactions can be used for lifetime extension and deletion of
   all policy rules being member of the particular group.  Thus,
   requirement 2.2.3 is met.



Stiemerling, Quittek, Taylor                                   [Page 50]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


5.2.4.  Policy Rule Lifetime Extension

   The semantics include a transaction for explicit lifetime extension
   of policy rules, as described in Section 2.3.3.  Thus requirement
   2.2.4 is met.

5.2.5.  Robust Failure Modes

   The state transitions at the middlebox are clearly specified and
   communicated to the agent.  There is no intermediate state reached by
   a partial processing of a request.  All requests are always processed
   completely, either successful or unsuccessful.  All request
   transaction include a list of failure reasons.  These failure reasons
   cover indication of invalid parameters where applicable.  In case of
   failure one of the specified reasons is returned from the middlebox
   to the agent.  Thus requirement 2.2.5 is met.

5.2.6.  Failure Reasons

   The semantics include a failure reason parameter in each failure
   reply. Thus requirement 2.2.6 is met.

5.2.7.  Multiple Agents Manipulating Same Policy Rule

   As specified in Sections 2.3 and 2.4, each installed policy rule and
   policy rule group has an owner, which is the authenticated agent that
   created the policy rule or group, respectively.  The authenticated
   identity is input to authorization of access to policy rules and
   groups.

   If the middlebox is sufficiently configurable, its administrator can
   configure it such that one authenticated agent is authorized to
   access and modify policy rules and groups owned by another agent.
   Because specified semantics does not preclude this, it meets
   requirement 2.2.7.

5.2.8.  Carrying Filtering Rules

   The Policy Enable Rule transaction specified in Section 2.3.2 can
   carry 5-tuple filtering rules.  It meets requirement 2.2.8.

5.2.9.  Parity of Port Numbers

   As specified in Section 2.3.5, the agent is able to request to keep
   the port parity.  Thus requirement 2.2.9 is met.

5.2.10.  Consecutive Range of Port Numbers

   The Policy Enable Rule transaction (PER, Section 2.3.7) allows the
   agent to specify a range of consecutive port numbers to be mapped.


Stiemerling, Quittek, Taylor                                   [Page 51]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   This can be used for mapping a consecutive range of external port
   numbers to consecutive internal ports.  Thus requirement 2.2.10 is
   met.

5.2.11.  Contradicting Overlapping Policy Rules

   requirement 2.2.11 is based on the assumption that contradicting
   policy rule actions, such as 'enable'/'allow' and
   'disable'/'disallow' are supported.  In conformance with decisions
   made by the working group after finalizing the requirements document,
   this requirement is not met by the semantics, because no
   'disable'/'disallow' action is supported.

5.3.  Security Requirements

5.3.1.  Authentication, Confidentiality, Integrity

   The semantics definition support mutual authentication of agent and
   middlebox and the selection of an encryption method in the Session
   Establishment transaction (Section 2.2.1).  Encryption can be used
   for achieving confidentiality of messages as well as for ensuring
   integrity.  Thus requirement 2.3.1 is met.

5.3.2.  Optional Confidentiality of Control Messages

   The Session Establishment transaction (Section 2.2.1) allows the
   agent to suggest an encryption method (including 'no encryption').
   Thus requirement 2.3.2 is met.

5.3.3.  Operation across Un-trusted Domains

   Operation across un-trusted domains is supported by mutual
   authentication and by encryption.  Thus requirement 2.3.3 is met.

5.3.4.  Mitigate Replay Attacks

   The specified semantics mitigates replay attacks and meets
   requirement 2.3.4 by requiring mutual authentication of agent and
   middlebox, and by supporting message encryption.  Further mitigation
   can be provided as part of a concrete MIDCOM protocol definition, for
   example by requiring consecutively increasing numbers for request
   identifiers.


6.  Security Considerations

   The interaction between a middlebox and an agent is (see [MDC-FRM]) a
   very sensitive point with respect to security. The configuration of
   policy rules from a middlebox external entity appears to be
   contradicting the nature of a middlebox. Therefore, effective means


Stiemerling, Quittek, Taylor                                   [Page 52]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


   have to be used to ensure:
      - mutual authentication between agent and middlebox
      - authorization
      - message integrity
      - message confidentiality

   The semantics define a mechanism to ensure mutual authentication
   between agent and middlebox (see section 2.2.1). In combination with
   the authentication, the middlebox is able to decide whether an agent
   is authorized to request an action at the middlebox or not.  The
   semantics rely on underlying protocols, like TLS or IPSEC, to keep
   the message integrity and confidentiality of the transferred data
   between both entities.


7.  Acknowledgements

   We like to thank all the people contributing to the semantics
   discussion on the mailing list for a lot of valuable comments.


8.  Open Issues

   Here is the list of open issues and to do issues:

     - Is IP wildcarding required? What would be application sceanrios
       for IP wildcarding?

     - Further elaborate the capability information sent from the
       middlebox to the agent at session setup.  What further capability
       information should be sent?

     - Is there a need to support enabling ICMP, IGMP, RSVP, ...?

     - In case of a failure of the SE transaction because the encryption
       method suggested by the agent is not supported by the middlebox:
       Should the middlebox reply with a list of supported encryption
       methods?

     - Further elaborate section on security considerations.

     - Shall the agent be able to specify parameters for protection
       against denial of service attacks? Examples are
          - maximum total number of TCP connection setups allowed
          - maximum number of TCP connection setups per minute
          - maximum number of UDP packets per minute
          - maximum bit rate
          - ...




Stiemerling, Quittek, Taylor                                   [Page 53]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


9.  Normative References

[MDC-FRM]   Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A.,
            Rayhan, A., "Middlebox Communication Architecture and
            framework", RFC 3303, August 2002

[MDC-REQ]   Swale, R.P., Mart, P.A., Sijben, P., Brimm, S., Shore, M.,
            "Middlebox Control (MIDCOM) Protocol Architecture and
            Requirements", RFC 3304, August 2002

[NAT-TERM]  Srisuresh,P., and Holdrege, M., "IP Network Translator (NAT)
            Terminology and Considerations", RFC 2663, August 1999.


10.  Informative References

[RFC3198]   Westerinen, A. et al., "Terminology for Policy-Based
            Management", RFC 3198, November 2001.

[RFC2246]   Dierks, T., Allen, C., "The TLS Protocol Version 1.0", RFC
            2246, January 1999.

[RFC2402]   Kent, S., and Atkinson, R., "IP Authentication Header", RFC
            2402, November 1998.

[RFC2406]   Kent, S., and Atkinson, R., "IP Encapsulating Security
            Payload (ESP)", RFC 2406, November 1998.


11.  Authors' Addresses

     Martin Stiemerling
     NEC Europe Ltd.
     Network Laboratories
     Kurfuersten-Anlage 36
     69115 Heidelberg
     Germany

     Phone: +49 6221 90511-13
     Email: stiemerling@ccrle.nec.de


     Juergen Quittek
     NEC Europe Ltd.
     Network Laboratories
     Kurfuersten-Anlage 36
     69115 Heidelberg
     Germany

     Phone: +49 6221 90511-15


Stiemerling, Quittek, Taylor                                   [Page 54]

Internet-Draft          MIDCOM Protocol Semantics          February 2003


     EMail: quittek@ccrle.nec.de


     Tom Taylor
     Nortel Networks
     1852 Lorraine Ave.
     Ottawa, Ontario
     Canada  K1H 6Z8

     Phone: +1 613 736 0961
     Email: taylor@nortelnetworks.com


12.  Full Copyright Statement

   Copyright (C) The Internet Society (2003). 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.












Stiemerling, Quittek, Taylor                                   [Page 55]
Internet-Draft          MIDCOM Protocol Semantics          February 2003


Html markup produced by rfcmarkup 1.108, available from http://tools.ietf.org/tools/rfcmarkup/