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NSIS Working Group                                        M. Stiemerling
Internet-Draft                                                       NEC
Expires: August 5, 2006                                    H. Tschofenig
                                                                 Siemens
                                                                 C. Aoun
                                                                    ENST
                                                        February 1, 2006


           NAT/Firewall NSIS Signaling Layer Protocol (NSLP)
                     draft-ietf-nsis-nslp-natfw-09

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

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   This Internet-Draft will expire on August 5, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This memo defines the NSIS Signaling Layer Protocol (NSLP) for
   Network Address Translators and firewalls.  This NSLP allows hosts to
   signal along a data path for Network Address Translators and
   firewalls to be configured according to the data flow needs.  The
   network scenarios, problems and solutions for path-coupled Network



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   Address Translator and firewall signaling are described.  The overall
   architecture is given by the framework and requirements defined by
   the Next Steps in Signaling (NSIS) working group.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . .    5
     1.1   Terminology and Abbreviations  . . . . . . . . . . . . .    7
     1.2   Middleboxes  . . . . . . . . . . . . . . . . . . . . . .    9
     1.3   Non-Goals  . . . . . . . . . . . . . . . . . . . . . . .   10
     1.4   General Scenario for NATFW Traversal . . . . . . . . . .   11

   2.  Network Deployment Scenarios using NATFW NSLP  . . . . . . .   13
     2.1   Firewall Traversal . . . . . . . . . . . . . . . . . . .   13
     2.2   NAT with two private Networks  . . . . . . . . . . . . .   14
     2.3   NAT with Private Network on Sender Side  . . . . . . . .   15
     2.4   NAT with Private Network on Receiver Side Scenario . . .   15
     2.5   Both End Hosts behind twice-NATs . . . . . . . . . . . .   16
     2.6   Both End Hosts Behind Same NAT . . . . . . . . . . . . .   17
     2.7   IPv4/v6 NAT with two Private Networks  . . . . . . . . .   18
     2.8   Multihomed Network with NAT  . . . . . . . . . . . . . .   19
     2.9   Multihomed Network with Firewall . . . . . . . . . . . .   19

   3.  Protocol Description . . . . . . . . . . . . . . . . . . . .   21
     3.1   Policy Rules . . . . . . . . . . . . . . . . . . . . . .   21
     3.2   Basic Protocol Overview  . . . . . . . . . . . . . . . .   21
     3.3   Basic Message Processing . . . . . . . . . . . . . . . .   25
     3.4   Protocol Operations  . . . . . . . . . . . . . . . . . .   25
       3.4.1   Creating Sessions  . . . . . . . . . . . . . . . . .   26
       3.4.2   Reserving External Addresses . . . . . . . . . . . .   28
       3.4.3   NATFW Session Refresh  . . . . . . . . . . . . . . .   35
       3.4.4   Deleting Sessions  . . . . . . . . . . . . . . . . .   37
       3.4.5   Reporting Asynchronous Events  . . . . . . . . . . .   37
       3.4.6   Tracing Signaling Sessions . . . . . . . . . . . . .   38
       3.4.7   Proxy Mode for Data Receiver behind NAT  . . . . . .   40
       3.4.8   Proxy Mode for Data Sender behind Middleboxes  . . .   43
       3.4.9   Proxy Mode for Data Receiver behind Firewall . . . .   44
     3.5   Calculation of Session Lifetime  . . . . . . . . . . . .   47
     3.6   Message Sequencing . . . . . . . . . . . . . . . . . . .   49
     3.7   De-Multiplexing at NATs  . . . . . . . . . . . . . . . .   50
     3.8   Selecting Opportunistic Addresses for REA  . . . . . . .   50
     3.9   Session Ownership  . . . . . . . . . . . . . . . . . . .   52
     3.10  Authentication and Authorization . . . . . . . . . . . .   52
     3.11  Reacting to Route Changes  . . . . . . . . . . . . . . .   53
     3.12  Updating Policy Rules  . . . . . . . . . . . . . . . . .   53

   4.  NATFW NSLP Message Components  . . . . . . . . . . . . . . .   55
     4.1   NSLP Header  . . . . . . . . . . . . . . . . . . . . . .   55



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     4.2   NSLP Message Types . . . . . . . . . . . . . . . . . . .   55
     4.3   NSLP Objects . . . . . . . . . . . . . . . . . . . . . .   56
       4.3.1   Session Lifetime Object  . . . . . . . . . . . . . .   57
       4.3.2   External Address Object  . . . . . . . . . . . . . .   57
       4.3.3   Extended Flow Information Object . . . . . . . . . .   58
       4.3.4   Response Code Object . . . . . . . . . . . . . . . .   59
       4.3.5   Proxy Support Object . . . . . . . . . . . . . . . .   60
       4.3.6   Nonce Object . . . . . . . . . . . . . . . . . . . .   60
       4.3.7   Message Sequence Number Object . . . . . . . . . . .   61
       4.3.8   Data Terminal Information Object . . . . . . . . . .   61
       4.3.9   Trace Object . . . . . . . . . . . . . . . . . . . .   63
     4.4   Message Formats  . . . . . . . . . . . . . . . . . . . .   63
       4.4.1   CREATE . . . . . . . . . . . . . . . . . . . . . . .   64
       4.4.2   RESERVE-EXTERNAL-ADDRESS (REA) . . . . . . . . . . .   64
       4.4.3   RESPONSE . . . . . . . . . . . . . . . . . . . . . .   65
       4.4.4   NOTIFY . . . . . . . . . . . . . . . . . . . . . . .   65
       4.4.5   REA-F  . . . . . . . . . . . . . . . . . . . . . . .   66
       4.4.6   TRACE  . . . . . . . . . . . . . . . . . . . . . . .   66

   5.  NATFW NSLP NTLP Requirements . . . . . . . . . . . . . . . .   67

   6.  NSIS NAT and Firewall Transition Issues  . . . . . . . . . .   68

   7.  Security Considerations  . . . . . . . . . . . . . . . . . .   69
     7.1   Trust Relationship and Authorization . . . . . . . . . .   69
       7.1.1   Peer-to-Peer Trust Relationship  . . . . . . . . . .   70
       7.1.2   Intra-Domain Trust Relationship  . . . . . . . . . .   70
       7.1.3   End-to-Middle Trust Relationship . . . . . . . . . .   71
     7.2   Security Threats and Requirements  . . . . . . . . . . .   72
       7.2.1   Attacks related to authentication and authorization    72
       7.2.2   Denial-of-Service Attacks  . . . . . . . . . . . . .   79
       7.2.3   Man-in-the-Middle Attacks  . . . . . . . . . . . . .   80
       7.2.4   Message Modification by non-NSIS on-path node  . . .   81
       7.2.5   Message Modification by malicious NSIS node  . . . .   81
       7.2.6   Session Modification/Deletion  . . . . . . . . . . .   82
       7.2.7   Misuse of unreleased sessions  . . . . . . . . . . .   85
       7.2.8   Data Traffic Injection . . . . . . . . . . . . . . .   86
       7.2.9   Eavesdropping and traffic analysis . . . . . . . . .   88
     7.3   Security Framework for the NAT/Firewall NSLP . . . . . .   89
       7.3.1   Security Protection between neighboring NATFW NSLP
               Nodes  . . . . . . . . . . . . . . . . . . . . . . .   89
       7.3.2   Security Protection between non-neighboring NATFW
               NSLP Nodes . . . . . . . . . . . . . . . . . . . . .   89
       7.3.3   End-to-End Security  . . . . . . . . . . . . . . . .   91

   8.  Open Issues  . . . . . . . . . . . . . . . . . . . . . . . .   92

   9.  Contributors . . . . . . . . . . . . . . . . . . . . . . . .   93



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   10.   References . . . . . . . . . . . . . . . . . . . . . . . .   94
     10.1  Normative References . . . . . . . . . . . . . . . . . .   94
     10.2  Informative References . . . . . . . . . . . . . . . . .   94

       Authors' Addresses . . . . . . . . . . . . . . . . . . . . .   97

   A.  Firewall and NAT Resources . . . . . . . . . . . . . . . . .   98
     A.1   Wildcarding of Policy Rules  . . . . . . . . . . . . . .   98
     A.2   Mapping to Firewall Rules  . . . . . . . . . . . . . . .   99
     A.3   Mapping to NAT Bindings  . . . . . . . . . . . . . . . .   99
     A.4   Mapping for combined NAT and firewall  . . . . . . . . .   99
     A.5   NSLP Handling of Twice-NAT . . . . . . . . . . . . . . .   99

   B.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  100

       Intellectual Property and Copyright Statements . . . . . . .  101



































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1.  Introduction

   Firewalls and Network Address Translators (NAT) have both been used
   throughout the Internet for many years, and they will remain present
   for the foreseeable future.  Firewalls are used to protect networks
   against certain types of attacks from the outside, and in times of
   IPv4 address depletion, NATs provide a virtual extension of the IP
   address space.  Both types of devices may be obstacles to some
   applications, since they only allow traffic created by a limited set
   of applications to traverse them, typically those that use protocols
   with relatively predetermined and static properties (e.g.,  most HTTP
   traffic, and other client/server applications).  Other applications,
   such as IP telephony and most other peer-to-peer applications, which
   have more dynamic properties, create traffic that is unable to
   traverse NATs and firewalls unassisted.  In practice, the traffic of
   many applications cannot traverse autonomous firewalls or NATs, even
   when they have additional functionality which attempts to restore the
   transparency of the network.

   Several solutions to enable applications to traverse such entities
   have been proposed and are currently in use.  Typically, application
   level gateways (ALG) have been integrated with the firewall or NAT to
   configure the firewall or NAT dynamically.  Another approach is
   middlebox communication (MIDCOM, currently under standardization at
   the IETF).  In this approach, ALGs external to the firewall or NAT
   configure the corresponding entity via the MIDCOM protocol [7].
   Several other work-around solutions are available, including STUN
   [26] and TURN [29].  However, all of these approaches introduce other
   problems that are generally hard to solve, such as dependencies on
   the type of NAT implementation (full-cone, symmetric, ...), or
   dependencies on certain network topologies.

   NAT and firewall (NATFW) signaling shares a property with Quality of
   Service (QoS) signaling.  The signaling of both must reach any device
   on the data path that is involved in QoS or NATFW treatment of data
   packets.  This means, that for both, NATFW and QoS, it is convenient
   if signaling travels path-coupled, meaning that the signaling
   messages follow exactly the same path that the data packets take.
   RSVP [13] is an example of a current QoS signaling protocol that is
   path-coupled. [36] proposes the use of RSVP as firewall signaling
   protocol but does not include NATs.

   This memo defines a path-coupled signaling protocol for NAT and
   firewall configuration within the framework of NSIS, called the NATFW
   NSIS Signaling Layer Protocol (NSLP).  The general requirements for
   NSIS are defined in [5].  The general framework of NSIS is outlined
   in [4].  It introduces the split between an NSIS transport layer and
   an NSIS signaling layer.  The transport of NSLP messages is handled



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   by an NSIS Network Transport Layer Protocol (NTLP, with General
   Internet Signaling Transport (GIST) [1] being the implementation of
   the abstract NTLP).  The signaling logic for QoS and NATFW signaling
   is implemented in the different NSLPs.  The QoS NSLP is defined in
   [6], while the NATFW NSLP is defined in this memo.

   The NATFW NSLP is designed to request the dynamic configuration of
   NATs and/or firewalls along the data path.  Dynamic configuration
   includes enabling data flows to traverse these devices without being
   obstructed, as well as blocking of particular data flows at upstream
   firewalls.  Enabling data flows requires the loading of firewall pin
   holes (loading of firewall rules with action allow) and creating NAT
   bindings.  Blocking of data flows requires the loading of firewalls
   rules with action deny/drop.  A simplified example for enabling data
   flows:  A source host sends a NATFW NSLP signaling message towards
   its data destination.  This message follows the data path.  Every
   NATFW NSLP NAT/firewall along the data path intercepts these
   messages, processes them, and configures itself accordingly.
   Thereafter, the actual data flow can traverse all these configured
   firewalls/NATs.

   It is necessary to distinguish between two different basic scenarios
   when operating the NATFW NSLP, independent of the type of middlebox
   to be configured.

   1.  Both, data sender and data receiver, are NSIS NATFW NSLP aware.
       This includes the cases where the data sender is logically
       decomposed from the NSIS initiator or the data receiver logically
       decomposed from the NSIS receiver, but both sides support NSIS.
       This scenario assumes deployment of NSIS all over the Internet,
       or at least at all NATs and firewalls.

   2.  Only one end host or region of the network is NSIS NATFW NSLP
       aware, either data receiver or data sender.

   NATFW NSLP provides two basic modes to cope with various possible
   scenarios likely to be encountered before and after widespread
   deployment of NSIS:

      CREATE mode: The basic mode for configuring a path downstream from
      a data sender

      RESERVE-EXTERNAL-ADDRESS (REA) mode: Used to prime upstream NATs/
      firewalls to expect downstream signaling and at NATs to pre-
      allocate a public address.

   Once there is full deployment of NSIS (in the sense that both end
   hosts support NATFW NSLP signaling), the requisite NAT and firewall



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   state can be created using either just CREATE mode if the data
   receiver resides in a        public addressing realm, or a combination of
   RESERVE-EXTERNAL-ADDRESS and CREATE modes if the data receiver
   resides in a private addressing realm and needs to preconfigure the
   edge-NAT/edge-firewall to provide a (publicly) reachable address for
   use by the data sender.  During the introduction of NSIS, it is
   likely that one or other of the data sender and receiver will not be
   NSIS aware.  In these cases the NATFW NSLP can utilize NSIS aware
   middleboxes on the path between the sender and receiver to provide
   proxy NATFW NSLP services ("proxy mode" services).  Typically these
   boxes will be at the boundaries of the realms in which the end hosts
   are located.  If the data receiver is NSIS unaware, the normal modes
   can be employed but the NSIS signaling terminates at the NSIS aware
   node topologically closest to the receiver which then acts as a proxy
   for the receiver.  If the data sender is unaware a variant of the
   RESERVE-EXTERNAL-ADDRESS mode can be used by a data receiver behind a
   NAT or firewall.

   All modes of operation create NATFW NSLP and NTLP state in NSIS
   entities.  NTLP state allows signaling messages to travel in the
   forward (downstream) and the reverse (upstream) direction along the
   path between a NAT/firewall NSLP sender and a corresponding receiver.
   NAT bindings and firewall rules are NAT/firewall specific state.
   This state is managed using a soft-state mechanism, i.e., it expires
   unless it is refreshed from time to time.

   Section 2 describes the network environment for NATFW NSLP signaling,
   highlighting the trust relationships and authorization required.
   Section 3 defines the NATFW signaling protocol.  Section 4 defines
   the messages and and message components.  In the remaining parts of
   the main body of the document, Section 6 covers transition issues and
   Section 7 addresses security considerations.  Please note that
   readers familiar with firewalls and NATs and their possible location
   within networks can safely skip Section 2.

1.1  Terminology and Abbreviations

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [2].

   This document uses a number of terms defined in [5].  The following
   additional terms are used:

   o  Policy rule: 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"  [28].  In the context of NSIS



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      NATFW NSLP, the condition is a specification of a set of packets
      to which rules are applied.  The set of actions always contains
      just a single element per rule, and is limited to either action
      "deny" or action "allow".

   o  NSLP (rule) directive: Instruction to a NATFW NSLP node as to how
      it should treat the associated policy rule.  Directive 'reserve'
      requests the middlebox to remember the rule and pre-allocate
      addresses where necessary but not install the rule.  Directive
      'install' requests the middle to install an active rule.

   o  Firewall: A packet filtering device that matches packets against a
      set of policy rules and applies the actions.  In the context of
      NSIS NATFW NSLP we refer to this device as a firewall.

   o  Network Address Translator: Network Address Translation is a
      method by which IP addresses are mapped from one IP address realm
      to another, in an attempt to provide transparent routing between
      hosts (see [9]).  Network Address Translators are devices that
      perform this work.

   o  Middlebox: "A middlebox is defined as any intermediate device
      performing functions other than the normal, standard functions of
      an IP router on the datagram path between a source host and a
      destination host" [11].  In the context of this document, the term
      middlebox refers to firewalls and NATs only.  Other types of
      middlebox are currently outside of the scope of this document.

   o  Security Gateway: IPsec-based gateways.

   o  (Data) Receiver (DR or R): The node in the network that is
      receiving the data packets of a flow.

   o  (Data) Sender (DS or S): The node in the network that is sending
      the data packets of a flow.

   o  NATFW NSLP session or signaling session: An application layer flow
      of information for which some network control state information is
      to be manipulated or monitored (as defined in [4]).  The control
      state for NATFW NSLP consists of NSLP state and associated policy
      rules at a middlebox.

   o  NSIS peer or peer: An NSIS node with which an NSIS adjacency has
      been created as defined in [1].

   o  Edge-NAT: An edge-NAT is a NAT device that is reachable from the
      public Internet and that has a globally routable IP address.




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   o  Edge-firewall: An edge-firewall is a firewall device that is
      located on the demarcation line of an administrative domain.

   o  Public Network: "A Global or Public Network is an address realm
      with unique network addresses assigned by Internet Assigned
      Numbers Authority (IANA) or an equivalent address registry.  This
      network is also referred as external network during NAT
      discussions" [9].

   o  Private/Local Network: "A private network is an address realm
      independent of external network addresses.  Private network may
      also be referred alternately as Local Network.  Transparent
      routing between hosts in private realm and external realm is
      facilitated by a NAT router" [9].  IP address space allocation for
      private networks is recommended in [27]

   o  Public/Global IP address: An IP address located in the public
      network according to Section 2.7 of [9].

   o  Private/Local IP address: An IP address located in the private
      network according to Section 2.8 of [9].

   o  Opportunistic Address (OA) or Signaling Destination Address (SDA):
      An IP address out of the public/global IP address range.  The OA/
      SDA may in certain circumstances be part of the private/local IP
      address range.


1.2  Middleboxes

   The term middlebox covers a range of devices which intercept the flow
   of packets between end hosts and perform actions other than standard
   forwarding expected in an IP router.  As such, middleboxes fall into
   a number of categories with a wide range of functionality, not all of
   which is pertinent to the NATFW NSLP.  Middlebox categories in the
   scope of this memo are firewalls that filter data packets against a
   set of filter rules, and NATs that translate packet addresses from
   one address realm to another address realm.  Other categories of
   middleboxes, such as QoS traffic shapers and security gateways, are
   out of scope.

   The term NAT used in this document is a placeholder for a range of
   different NAT flavors.  We consider the following types of NATs:

   o  Traditional NAT (basic NAT and NAPT)

   o  Bi-directional NAT




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   o  Twice-NAT

   o  Multihomed NAT

   For definitions and a detailed discussion about the characteristics
   of each NAT type please see [9].

   All types of middleboxes under consideration here use policy rules to
   make a decision on data packet treatment.  Policy rules consist of a
   flow identifier which selects the packets to which the policy applies
   and an associated action; data packets matching the flow identifier
   are subjected to the policy rule action.  A typical flow identifier
   is the 5-tuple selector which matches the following fields of a
   packet to configured values:

   o  Source and destination IP addresses

   o  Transport protocol number

   o  Transport source and destination port numbers

   For further examples of flow identifiers see Section 5.2.2 of [1].

   Actions for firewalls are usually one or more of:

   o  Allow: forward data packet

   o  Deny: block data packet and discard it

   o  Other actions such as logging, diverting, duplicating, etc

   Actions for NATs include (amongst many others):

   o  Change source IP address and transport port number to a globally
      routeable IP address and associated port number.

   o  Change destination IP address and transport port number to a
      private IP address and associated port number.


1.3  Non-Goals

      Traversal of non-NSIS and non-NATFW NSLP aware NATs and firewalls
      is outside the scope of this document.

      Only firewalls and NATs are considered in this document, any other
      types of devices, for instance, QoS gateways, are out of scope.




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      The exact implementation of policy rules and their mapping to
      firewall rule sets and NAT bindings or sessions at the middlebox
      is an implementation issue and thus out of scope of this document.
      Some examples are given in Appendix A


1.4  General Scenario for NATFW Traversal

   The purpose of NSIS NATFW signaling is to enable communication
   between endpoints across networks even in the presence of NAT and
   firewall middleboxes that have not been specially engineered to
   facilitate communication with the application protocols used.  This
   removes the need to create and maintain application layer gateways
   for specific protocols that have been commonly used to provide
   transparency in previous generations of NAT and firewall middleboxes.
   It is assumed that these middleboxes will be statically configured in
   such a way that NSIS NATFW signaling messages themselves are allowed
   to reach the locally installed NATFW NSLP daemon.  NSIS NATFW NSLP
   signaling is used to dynamically install additional policy rules in
   all NATFW middleboxes along the data path that will allow
   transmission of the application data flow(s).  Firewalls are
   configured to forward data packets matching the policy rule provided
   by the NSLP signaling.  NATs are configured to translate data packets
   matching the policy rule provided by the NSLP signaling.  An
   additional capability, that is an exception to the primary goal of
   NSIS NATFW signaling, is that the NATFW nodes can request blocking of
   particular data flows instead of enabling these flows at upstream
   firewalls.

   The basic high-level picture of NSIS usage is that end hosts are
   located behind middleboxes, meaning that there is a middlebox on the
   data path from the end host in a private network and the external
   network (NATFW in Figure 1).  Applications located at these end hosts
   try to establish communication with corresponding applications on
   other such end hosts.  They trigger the NSIS entity at the local host
   to control provisioning for middlebox traversal along the prospective
   data path (e.g., via an API call).  The NSIS entity in turn uses NSIS
   NATFW NSLP signaling to establish policy rules along the data path,
   allowing the data to travel from the sender to the receiver
   unobstructed.











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   Application          Application Server (0, 1, or more)   Application

   +----+                        +----+                        +----+
   |    +------------------------+    +------------------------+    |
   +-+--+                        +----+                        +-+--+
     |                                                           |
     |         NSIS Entities                      NSIS Entities  |
   +-+--+        +----+                            +-----+     +-+--+
   |    +--------+    +----------------------------+     +-----+    |
   +-+--+        +-+--+                            +--+--+     +-+--+
     |             |               ------             |          |
     |             |           ////      \\\\\        |          |
   +-+--+        +-+--+      |/               |     +-+--+     +-+--+
   |    |        |    |     |   Internet       |    |    |     |    |
   |    +--------+    +-----+                  +----+    +-----+    |
   +----+        +----+      |\               |     +----+     +----+
                               \\\\      /////
   sender    NATFW (1+)            ------          NATFW (1+) receiver

          Figure 1: Generic View on NSIS in a NAT / Firewall case

   For end-to-end NATFW signaling, it is necessary that each firewall
   and each NAT along the path between the data sender and the data
   receiver implements the NSIS NATFW NSLP.  There might be several NATs
   and FWs in various possible combinations on a path between two hosts.
   Section 2 presents a number of likely scenarios with different
   combinations of NATs and firewalls.
























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2.  Network Deployment Scenarios using NATFW NSLP

   This section introduces several scenarios for middlebox placement
   within IP networks.  Middleboxes are typically found at various
   different locations, including at Enterprise network borders, within
   enterprise networks, as mobile phone network gateways, etc.  Usually,
   middleboxes are placed more towards the edge of networks than in
   network cores.  Firewalls and NATs may be found at these locations
   either alone, or they may be combined; other categories of
   middleboxes may also be found at such locations, possibly combined
   with the NATs and/or firewalls.  Using combined middleboxes typically
   reduces the number of network elements needed.

   NSIS initiators (NI) send NSIS NATFW NSLP signaling messages via the
   regular data path to the NSIS responder (NR).  On the data path,
   NATFW NSLP signaling messages reach different NSIS nodes that
   implement the NATFW NSLP.  Each NATFW NSLP node processes the
   signaling messages according to Section 3 and, if necessary, installs
   policy rules for subsequent data packets.

   Each of the following sub-sections introduces a different scenario
   for a different set of middleboxes and their ordering within the
   topology.  It is assumed that each middlebox implements the NSIS
   NATFW NSLP signaling protocol.

2.1  Firewall Traversal

   This section describes a scenario with firewalls only; NATs are not
   involved.  Each end host is behind a firewall.  The firewalls are
   connected via the public Internet.  Figure 2 shows the topology.  The
   part labeled "public" is the Internet connecting both firewalls.

                  +----+    //----\\       +----+
          NI -----| FW |---|        |------| FW |--- NR
                  +----+    \\----//       +----+

                 private     public        private


             FW: Firewall
             NI: NSIS Initiator
             NR: NSIS Responder

                   Figure 2: Firewall Traversal Scenario

   Each firewall on the data path must provide traversal service for
   NATFW NSLP in order to permit the NSIS message to reach the other end
   host.  All firewalls process NSIS signaling and establish appropriate



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   policy rules, so that the required data packet flow can traverse
   them.

   There are several very different ways to place firewalls in a network
   topology.  To distinguish firewalls located at network borders, such
   as administrative domains, from others located internally, the term
   edge-firewall is used.  A similar distinction can be made for NATs,
   with an edge-NAT fulfilling the equivalent role.

2.2  NAT with two private Networks

   Figure 3 shows a scenario with NATs at both ends of the network.
   Therefore, each application instance, the NSIS initiator and the NSIS
   responder, are behind NATs.  The outermost NAT, known as the edge-
   NAT, at each side is connected to the processing    public Internet.
   The NATs are generically labeled as MB (for middlebox), since those
   devices certainly implement NAT functionality, but can implement
   firewall functionality as well.

   Only two middleboxes MB are shown in Figure 3 at each side, but in
   general, any number of MBs on each side must be considered.

           +----+     +----+    //----\\    +----+     +----+
      NI --| MB |-----| MB |---|        |---| MB |-----| MB |--- NR
           +----+     +----+    \\----//    +----+     +----+

                private          public          private

             MB: Middlebox
             NI: NSIS Initiator
             NR: NSIS Responder

             Figure 3: NAT with two Private Networks Scenario

   Signaling traffic from NI to NR has to traverse all the middleboxes
   on the path, and all the middleboxes must be configured properly to
   allow NSIS signaling to traverse them.  The NATFW signaling must
   configure all middleboxes and consider any address translation that
   will result from this configuration in further signaling.  The sender
   (NI) has to know the IP address of the receiver (NR) in advance,
   otherwise it will not be possible to send any NSIS signaling messages
   towards the responder.  Note that this IP address is not the private
   IP address of the responder.  Instead a NAT binding (including a
   public IP address) has to be previously installed on the NAT that
   subsequently allows packets reaching the NAT to be forwarded to the
   receiver within the private address realm.  This generally requires
   further support from an application layer protocol for the purpose of
   discovering and exchanging information.  The receiver might have a



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   number of ways to learn its public IP address and port number and
   might need to signal this information to the sender using the
   application level signaling protocol.

2.3  NAT with Private Network on Sender Side

   This scenario shows an application instance at the sending node that
   is behind one or more NATs (shown as generic MB, see discussion in
   Section 2.2).  The receiver is located in the public Internet.

             +----+     +----+    //----\\
        NI --| MB |-----| MB |---|        |--- NR
             +----+     +----+    \\----//

                  private          public

             MB: Middlebox
             NI: NSIS Initiator
             NR: NSIS Responder

        Figure 4: NAT with Private Network on Sender Side Scenario

   The traffic from NI to NR has to traverse middleboxes only on the
   sender's side.  The receiver has a public IP address.  The NI sends
   its signaling message directly to the address of the NSIS responder.
   Middleboxes along the path intercept the signaling messages and
   configure the policy rules accordingly.

   Note that the data sender does not necessarily know whether the
   receiver is behind a NAT or not, hence, it is the receiving side that
   has to detect whether itself is behind a NAT or not.  As described in
   Section 3.4.2.1 NSIS can also provide help for this procedure.

2.4  NAT with Private Network on Receiver Side Scenario

   The application instance receiving data is behind one or more NATs
   shown as MB (see discussion in Section 2.2).














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               //----\\    +----+     +----+
        NI ---|        |---| MB |-----| MB |--- NR
               \\----//    +----+     +----+

                public          private


             MB: Middlebox
             NI: NSIS Initiator
             NR: NSIS Responder

          Figure 5: NAT with Private Network on Receiver Scenario

   Initially, the NSIS responder must determine its publicly reachable
   IP address at the external middlebox and notify the NSIS initiator
   about this address.  One possibility is that an application level
   protocol is used, meaning that the public IP address is signaled via
   this protocol to the NI.  Afterwards the NI can start its signaling
   towards the NR and so establish the path via the middleboxes in the
   receiver side private network.

   This scenario describes the use case for the RESERVE-EXTERNAL-ADDRESS
   mode of the NATFW NSLP.

2.5  Both End Hosts behind twice-NATs

   This is a special case, where the main problem arises from the need
   to detect that both end hosts are logically within the same address
   space, but are also in two partitions of the address realm on either
   side of a twice-NAT (see [9] for a discussion of twice-NAT
   functionality).

   Sender and receiver are both within a single private address realm
   but the two partitions potentially have overlapping IP address
   ranges.  Figure 6 shows the arrangement of NATs.  This is a common
   configuration in networks, particularly after the merging of
   companies that have used the same private address space, resulting in
   overlapping address ranges.













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                                   public
             +----+     +----+    //----\\
        NI --| MB |--+--| MB |---|        |
             +----+  |  +----+    \\----//
                     |
                     |  +----+
                     +--| MB |------------ NR
                        +----+

                   private

             MB: Middlebox
             NI: NSIS Initiator
             NR: NSIS Responder

     Figure 6: NAT to Public, Sender and Receiver on either side of a
                            twice-NAT Scenario

   The middleboxes shown in Figure 6 are twice-NATs, i.e., they map IP
   addresses and port numbers on both sides, meaning the mapping of
   source and destination address at the private and public interfaces.

   This scenario requires the assistance of application level gateway,
   such as a DNS server.  The application level gateways must handle
   requests that are based on symbolic names, and configure the
   middleboxes so that data packets are correctly forwarded from NI to
   NR.  The configuration of those middleboxes may require other
   middlebox communication protocols, such as MIDCOM [7].  NSIS
   signaling is not required in the twice-NAT only case, since
   middleboxes of the twice-NAT type are normally configured by other
   means.  Nevertheless, NSIS signaling might by useful when there are
   also firewalls on path.  In this case NSIS will not configure any
   policy rule at twice-NATs, but will configure policy rules at the
   firewalls on the path.  The NSIS signaling protocol must be at least
   robust enough to survive this scenario.  This requires that twice-
   NATs must implement the NATFW NSLP also and participate in NATFW
   sessions but they do not change the configuration of the NAT, i.e.,
   they only read the address mapping information out of the NAT and
   translate the Message Routing Information (MRI, [1])within the NSLP
   and NTLP accordingly.  For more information see Appendix A.5

2.6  Both End Hosts Behind Same NAT

   When NSIS initiator and NSIS responder are behind the same NAT (thus
   being in the same address realm, see Figure 7), they are most likely
   not aware of this fact.  As in Section 2.4 the NSIS responder must
   determine its public IP address in advance and transfer it to the
   NSIS initiator.  Afterwards, the NSIS initiator can start sending the



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   signaling messages to the responder's public IP address.  During this
   process, a public IP address will be allocated for the NSIS initiator
   at the same middlebox as for the responder.  Now, the NSIS signaling
   and the subsequent data packets will traverse the NAT twice: from
   initiator to public IP address of responder (first time) and from
   public IP address of responder to responder (second time).  This is
   the worst case in which both sender and receiver obtain a public IP
   address at the NAT, and the communication path is certainly not
   optimal in this case.

               NI              public
                \  +----+     //----\\
                 +-| MB |----|        |
                /  +----+     \\----//
               NR
                   private

             MB: Middlebox
             NI: NSIS Initiator
             NR: NSIS Responder

            Figure 7: NAT to Public, Both Hosts Behind Same NAT


2.7  IPv4/v6 NAT with two Private Networks

   This scenario combines the use case described in Section 2.2 with the
   IPv4 to IPv6 transition scenario involving address and protocol
   translation, i.e., using Network Address and Protocol Translators
   (NAT-PT, [10]).

   The difference from the other scenarios is the use of IPv6 to IPv4
   (and vice versa) address and protocol translation.  Additionally, the
   base NTLP must support transport of messages in mixed IPv4 and IPv6
   networks where some NSIS peers provide translation.

        +----+  +----+   //---\\   +----+  //---\\   +----+  +----+
   NI --| MB |--| MB |--|       |--| MB |-|       |--| MB |--| MB |-- NR
        +----+  +----+   \\---//   +----+  \\---//   +----+  +----+

             private      public            public       private
                           IPv4              IPv6

             MB: Middlebox
             NI: NSIS Initiator
             NR: NSIS Responder

              Figure 8: IPv4/v6 NAT with two Private Networks



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   This scenario needs the same type of application level support as
   described in Section 2.5, and so the issues relating to twice-NATs
   apply here as well.

   Note that the current form of IPv4/v6 NAT known as the Network
   Address  Translator - Protocol Translator (NAT-PT) [10] is being
   removed from the set of recommended mechanisms for general usage in
   IPv4/IPv6 transitions.  This scenario is therefore not expected to be
   commonly seen.

2.8  Multihomed Network with NAT

   The previous sub-sections sketched network topologies where several
   NATs and/or firewalls are ordered sequentially on the path.  This
   section describes a multihomed scenario with two NATs placed on
   alternative paths to the public network.

             +----+
   NI -------| MB |\
       \     +----+ \  //---\\
        \            -|       |-- NR
         \             \\---//
          \  +----+       |
           --| MB |-------+
             +----+
             private
        private          public

             MB: Middlebox
             NI: NSIS Initiator
             NR: NSIS Responder

                Figure 9: Multihomed Network with Two NATs

   Depending on the destination or load balancing requirements, either
   one or the other middlebox is used for the data flow.  Which
   middlebox is used depends on local policy or routing decisions.
   NATFW NSLP must be able to handle this situation properly, see
   Section 3.4.2.1 for an expanded discussion of this topic with respect
   to NATs.

2.9  Multihomed Network with Firewall

   This section describes a multihomed scenario with two firewalls
   placed on alternative paths to the public network (Figure 10).  The
   routing in the private and public network decided which firewall is
   being taken for data flows.  Depending on the data flow's direction,
   either outbound or inbound, a different firewall could be traversed.



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   This is a challenge for the REA-F mode of the NATFW NSLP where the
   NSIS responder is located behind these firewalls within the private
   network.  The REA-F mode is used to block a particular data flow on
   an upstream firewall.  NSIS must route the REA-F mode message
   upstream from NR to NI probably without knowing which path the data
   traffic will take from NI to NR.

             +----+
   NR -------| MB |\
       \     +----+ \  //---\\
        \            -|       |-- NI
         \             \\---//
          \  +----+       |
           --| MB |-------+
             +----+
             private
        private          public

             MB: Middlebox
             NI: NSIS Initiator
             NR: NSIS Responder

             Figure 10: Multihomed Network with two Firewalls




























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3.  Protocol Description

   This section defines messages, objects, and protocol semantics for
   the NATFW NSLP.  Section 3.1 introduces the base element of a NSLP
   session, the policy rule.  Section 3.2 introduces the protocol and
   the protocol behavior is defined in Section 3.4.  Section 4 defines
   the syntax of the messages and objects.

3.1  Policy Rules

   Policy rules, bound to a session, are the building block of middlebox
   devices considered in the NATFW NSLP.  For firewalls the policy rule
   usually consists of a 5-tuple, source/destination addresses,
   transport protocol, and source/destination port numbers, plus an
   action, such as allow or deny.  For NATs the policy rule consists of
   action 'translate this address' and further mapping information, that
   might be, in the simplest case, internal IP address and external IP
   address.

   Policy rules are usually carried in one piece in signaling
   applications.  In NSIS the policy rule is divided into the flow
   identifier, an allow or deny action, and additional information.  The
   filter specification is carried within NTLP's message routing
   information (MRI) and additional information, including the
   specification of the action, is carried in the NATFW NSLP's objects.
   Additional information is, for example, the lifetime of a policy rule
   or session.

3.2  Basic Protocol Overview

   The NSIS NATFW NSLP is carried over the NSIS Transport Layer Protocol
   (NTLP) defined in [1].  The interworking with the NTLP and other
   components is shown in Figure 54.  NATFW NSLP messages are initiated
   by the NSIS initiator (NI), handled by NSIS forwarders (NF) and
   finally processed by the NSIS responder (NR).  It is required that at
   least NI and NR implement this NSLP, intermediate NFs only implement
   this NSLP when they provide relevant middlebox functions.  NSIS
   forwarders that do not have any NATFW NSLP functions just forward
   these packets as they have no interest in them.

   A Data Sender (DS), intending to send data to a Data Receiver (DR)
   must first initiate NATFW NSLP signaling.  This causes the NI
   associated with the data sender (DS) to launch NSLP signaling towards
   the address of data receiver (DR) (see Figure 11).  Although it is
   expected that the DS and the NATFW NSLP NI will usually reside on the
   same host, this specification does not rule out scenarios where the
   DS and NI reside on different hosts, the so-called proxy mode (see
   Section 1.)



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             +-------+    +-------+    +-------+    +-------+
             | DS/NI |<~~~| MB1/  |<~~~| MB2/  |<~~~| DR/NR |
             |       |--->| NF1   |--->| NF2   |--->|       |
             +-------+    +-------+    +-------+    +-------+


                 ========================================>
                    Data Traffic Direction (downstream)

                  --->  : NATFW NSLP request signaling
                  ~~~>  : NATFW NSLP response signaling
                  DS/NI : Data sender and NSIS initiator
                  DR/NR : Data receiver and NSIS responder
                  MB1   : Middlebox 1 and NSIS forwarder 1
                  MB2   : Middlebox 2 and NSIS forwarder 2


                     Figure 11: General NSIS signaling

   The sequence of NSLP events is as follows:

   o  NSIS initiators generate NATFW NSLP request messages and send
      those towards the NSIS responder.  Note, that the NSIS initiator
      may not necessarily be the data sender but may be the data
      receiver, for instance, when using the RESERVE-EXTERNAL-ADDRESS
      message.

   o  NSLP request messages are processed each time a NF with NATFW NSLP
      support is traversed.  These nodes process the message, check
      local policies for authorization and authentication, possibly
      create policy rules, and forward the signaling message to the next
      NSIS node.  The request message is forwarded until it reaches the
      NSIS responder.

   o  NSIS responders will check received messages and process them if
      applicable.  NSIS responders generate response messages and send
      them hop-by-hop back to the NI via the same chain of NFs
      (traversal of the same NF chain is guaranteed through the
      established reverse message routing state in the NTLP).  Note,
      that the NSIS responder may not necessarily be the data receiver
      but may be any intermediate NSIS node that terminates the
      forwarding, for example, in a proxy mode case where an edge-NAT is
      replying to requests.

   o  The response message is processed at each NF implementing the
      NATFW NSLP.





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   o  Once the NI has received a successful response, the data sender
      can start sending its data flow to the data receiver.

   Because NATFW NSLP signaling follows the data path from DS to DR (see
   Figure 11), this immediately enables communication between both hosts
   for scenarios with only firewalls on the data path or NATs on the
   sender side.  For scenarios with NATs on the receiver side certain
   problems arise, as described in Section 2.

   When the NR and the NI are located in different address realms and
   the NR is located behind a NAT, the NI cannot signal to the NR
   address directly.  The DR and NR are not reachable from the NIs using
   the private address of the NR and thus NATFW signaling messages
   cannot be sent to the NR/DR's address.  Therefore, the NR must first
   obtain a NAT binding that provides an address that is reachable for
   the NI.  Once the NR has acquired a public IP address, it forwards
   this information to the DS via a separate protocol (such as SDP
   within SIP).  This application layer signaling, which is out of scope
   of the NATFW NSLP, may involve third parties that assist in
   exchanging these messages.

   NATFW NSLP signaling supports this scenario by using the RESERVE-
   EXTERNAL-ADDRESS mode of operation

   1.  The NR acquires a public address by signaling on the reverse path
       (NR towards NI) and thus making itself available to other hosts.
       This process of acquiring a public addresses is called
       reservation.  During this process the DR reserves publicly
       reachable addresses and ports suitable for NATFW NSLP signaling,
       but data traffic will not be allowed to use this address/port
       initially.

   2.  The NI signals directly to the NR as the NI would do if there is
       no NAT in between, and creates  policy rules at middleboxes.
       Note, that the reservation  mode will only allow  the forwarding
       of signaling messages but not data flow packets.  Data flow
       packets will be 'activated' by the signaling from NI towards NR.
       The RESERVE-EXTERNAL-ADDRESS mode of operation is detailed in
       Section 3.4.2.1












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             +-------+    +-------+    +-------+    +-------+
             | DS/NI |<~~~| MB1/  |<~~~| NR    |    |   DR  |
             |       |--->| NF1   |--->|       |    |       |
             +-------+    +-------+    +-------+    +-------+


                 ========================================>
                    Data Traffic Direction (downstream)

                  --->  : NATFW NSLP request signaling
                  ~~~>  : NATFW NSLP response signaling
                  DS/NI : Data sender and NSIS initiator
                  DR/NR : Data receiver and NSIS responder
                  MB1   : Middlebox 1 and NSIS forwarder 1
                  MB2   : Middlebox 2 and NSIS forwarder 2


                  Figure 12: A NSIS proxy mode signaling

   The above usage assumes that both ends of a communication support
   NSIS but fail when NSIS is only deployed at one end of the network.
   In this case only the receiving or sending side are NSIS aware and
   not both at the same time (see also Section 1).  NATFW NSLP supports
   this scenario by using a proxy mode, as described in Section 3.4.7
   and Section 3.4.8.  Figure 12 sketches the proxy mode operation for a
   data sender behind a middlebox.

   The basic functionality of the NATFW NSLP provides for opening
   firewall pin holes and creating NAT bindings to enable data flows to
   traverse these devices.  Firewalls are normally expected to work on a
   deny-all policy, meaning that traffic that does not explicitly match
   any firewall filter rule will be blocked.  Similarly, the normal
   behavior of NATs is to block all traffic that does not match any
   already configured/installed binding or session.  However, some
   scenarios require support of firewalls having allow-all policies,
   allowing data traffic to traverse the firewall unless it is blocked
   explicitly.  Data receivers can utilize NATFW NSLP's REA-F message to
   install policy rules at upstream firewalls to block unwanted traffic.

   The protocol works on a soft-state basis, meaning that whatever state
   is installed or reserved on a middlebox will expire, and thus be de-
   installed or forgotten after a certain period of time.  To prevent
   premature removal of state that is needed for ongoing communication,
   the NATFW nodes involved  will have to specifically request a session
   extension.  An explicit NATFW NSLP state deletion capability is also
   provided by the protocol.

   Middleboxes should return an error in case of a failure, such that



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   appropriate actions can be taken; this ability would allow debugging
   and error recovery.

   The next sections define the NATFW NSLP message types and formats,
   protocol operations, and policy rule operations.

3.3  Basic Message Processing

   All NATFW messages are subject to a basic message processing when
   received at a node, independent of request or response messages.
   Initially, the syntax of the NSLP message is checked and a RESPONSE
   message with error code is generated if any problem is detected (for
   instance, the message header could not be read).  After passing this
   check, the NATFW NSLP node MUST first perform the checks defined in
   Section 3.9 and Section 3.10, if applicable, before any further
   processing is executed.

   This section should state this ugly sentence out of all protocol
   operations sections on authentication and authorization.  So get it
   rid of it there.  This section opens an interesting question: What
   happens if a NSLP nodes receives a malformed response message?

3.4  Protocol Operations

   This section defines the protocol operations including, how to create
   sessions, maintain them, and how to reserve addresses.  All the NATFW
   NSLP protocol messages MUST be transported via C-mode handling by the
   NTLP and MUST NOT be piggybacked into D-mode NTLP messages used
   during the NTLP path discovery/refresh phase.  The usage of the NTLP
   by protocol messages is described in detail in Section 4.

   The protocol uses six messages:

   o  CREATE: a request message used for creating, changing, refreshing,
      and deleting CREATE NATFW NSLP sessions.

   o  RESERVE-EXTERNAL-ADDRESS (REA): a request message used for
      reserving an external address and (if applicable) port number,
      depending on the type of NAT.  REA messages are used to change,
      refresh, and delete REA NATFW NSLP sessions.

   o  REA-F: a request message used by data receivers located behind
      firewalls to instruct upstream firewalls to allow or block
      incoming data traffic.  REA-F is also used to inform upstream
      firewalls about incoming NATFW NSLP signaling messages.  REA-F
      messages are used to change, refresh, and delete REA-F NATFW NSLP
      sessions.




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   o  TRACE: a request message to trace all involved NATFW NSLP nodes in
      a particular signaling session.

   o  NOTIFY: an asynchronous message used by NATFW peers to alert
      upstream and/or downstream NATFW peers about specific events
      (especially failures).

   o  RESPONSE: used as a response to CREATE, REA, and REA-F messages
      with Success or Error information.


3.4.1  Creating Sessions

   Allowing two hosts to exchange data even in the presence of
   middleboxes is realized in the NATFW NSLP by the CREATE request
   message.  The data sender generates a CREATE message as defined in
   Section 4.4.1 and hands it to the NTLP.  The NTLP forwards the whole
   message on the basis of the message routing information towards the
   NR.  Each NSIS forwarder along the path that implements NATFW NSLP,
   processes the NSLP message.  Forwarding is thus managed NSLP hop-by-
   hop but may pass transparently through NSIS forwarders which do not
   contain NATFW NSLP functionality and non-NSIS aware routers between
   NSLP hop way points.  When the message reaches the NR, the NR can
   accept the request or reject it.  The NR generates a response to the
   request and this response is transported hop-by-hop towards the NI.
   NATFW NSLP forwarders may reject requests at any time.  Figure 13
   sketches the message flow between NI (DS), a NF (e.g., NAT), and NR
   (DR).



       NI      Private Network        NF    Public Internet        NR
       |                              |                            |
       | CREATE                       |                            |
       |----------------------------->|                            |
       |                              |                            |
       | RESPONSE[Error](if necessary)|                            |
       |<-----------------------------| CREATE                     |
       |                              |--------------------------->|
       |                              |                            |
       |                              | RESPONSE[Success/Error]    |
       |    RESPONSE[Success/Error]   |<---------------------------|
       |<-----------------------------|                            |
       |                              |                            |
       |                              |                            |


                     Figure 13: Creation message flow



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   Since the CREATE message is used for several purposes within the
   lifetime of a session, there are several processing rules for NATFW
   peers when generating and receiving CREATE messages.  The different
   processing methods depend not only on the function which the CREATE
   is performing (to create, modify, refresh or delete a session) but
   also on the node at which the processing happens.  For an initial
   CREATE message that creates a new NSIS session, the processing of
   CREATE messages is different for every NSIS node type:

   o  NSLP initiator:  NI only generates initial CREATE messages and
      hands them over to the NTLP.  After receiving a successful
      response,  the data path is configured and the DS can start
      sending its data to the DR.  After receiving an 'error' response
      message the NI MAY try to generate the CREATE message again or
      give up and report the failure to the application, depending on
      the error condition.

   o  NATFW NSLP forwarder:  NFs receiving an initial CREATE message
      MUST first perform the checks defined in Section 3.9 and
      Section 3.10, if applicable, before any further processing is
      executed.  The NF SHOULD check with its local policies if it can
      accept the desired policy rule given the combination of the NTLP's
      'Message-Routing-Information' (MRI) (the flow description
      information) and the CREATE payload (behavior to be enforced on
      the packet stream).  An initial CREATE is distinguished from
      subsequent CREATE messages by the absence of existing NSLP session
      related to the same session ID.  The NSLP message processing
      depends on the middlebox type:

      *  NAT:  When the initial CREATE message is received at the public
         side of the NAT, it looks for a reservation made in advance, by
         using a REA message (see Section 3.4.2.1), that matches the
         destination address/port of the MRI provided by the NTLP.  If
         no reservation has been made in advance the NSLP SHOULD return
         an error response message of type 'no reservation found' and
         discard the request.  If there is a reservation, NSLP stores
         the data sender's address (and if applicable port number) as
         part of the policy rule to be loaded and forwards the message
         with the address set to the internal (private in most cases)
         address of the next NSIS node.  When the initial CREATE message
         is received at the private side, the NAT binding is allocated,
         but not activated (see also Appendix A.3).  The NSLP message is
         forwarded to the next NSIS hop with source address set to the
         NAT's external address from the newly reserved binding.

      *  Firewall: When the initial CREATE message is received, the NSLP
         just remembers the requested policy rule, but does not install
         any policy rule.  Afterwards, the message is forwarded to the



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         next NSLP hop.  There is a difference between requests from
         trusted (authorized NIs) and un-trusted (un-authorized NIs);
         requests from trusted NIs will be pre-authorized, whereas
         requests from un-trusted NIs will not be pre-authorized.  This
         difference is required to speed-up the protocol operations as
         well as for proxy mode usage (please refer to Section 3.4.7).

      *  Combined NAT and firewall:  Processing at combined firewall and
         NAT middleboxes is the same as in the NAT case.  No policy
         rules are installed.  Implementations MUST take into account
         the order of packet processing in the firewall and NAT
         functions within the device.  This will be referred to as
         'order of functions' and is  generally different depending on
         whether the packet arrives at the external or internal side of
         the middlebox.

   o  NSLP receiver: NRs receiving initial CREATE messages MUST reply
      with a 'success' (response object has success information)
      RESPONSE message if they accept the CREATE request message and
      defined in Section 3.9 and Section 3.10, if applicable, have been
      successful executed.  Otherwise they SHOULD generate a RESPONSE
      message with an error code.  The calculation of session lifetime
      applies here as well (see Section 3.5).  RESPONSE messages are
      sent back NSLP hop-by-hop towards the NI, independently of the
      response codes, either success or error.

   Policy rules at middleboxes MUST be only installed upon receiving a
   successful response.  This is a countermeasure to several problems,
   for example wastage of resources due to loading policy rules at
   intermediate NF when the CREATE message does not reach the final NR
   for some reason.

3.4.2  Reserving External Addresses

   NSIS signaling is intended to travel end-to-end, even in the presence
   of NATs and firewalls on-path.  This works well in cases where the
   data sender is itself behind a NAT or a firewall as described in
   Section 3.4.1.  For scenarios where the data receiver is located
   behind a NAT or a firewall and it needs to receive data flows from
   outside its own network (see Figure 5) the problem is more
   troublesome.  NSIS signaling, as well as subsequent data flows, are
   directed to a particular destination IP address that must be known in
   advance and reachable.  Data receivers must tell the local NSIS
   infrastructure (i.e., the upstream firewalls/NATs) about incoming
   NATFW NSLP signaling and data flows before they can receive these
   flows.  It is necessary to discriminate between data receivers behind
   NATs and behind firewalls for understanding the further NATFW
   procedures.  Data receivers that are just behind firewalls already



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   have a public IP address and they need only to be reachable for NATFW
   signaling.  Data receivers behind NATs do not a have a public IP
   address and are not reachable for NATFW signaling.  We first discuss
   the DR behind a NAT case.

3.4.2.1  Reserving External Addresses at NATs

   Figure 14 describes a typical message sequence in a peer-to-peer
   networking environment whereby the two end points learn of each
   others existence with the help of a third party (referred to as an
   Application Server).  Communication between the application server
   and each of the two end points (data sender and data receiver)
   enables the two end hosts to learn each other's IP addresses.  The
   approach described in this memo supports this peer-to-peer approach,
   but is not limited to it.



                      +-------------+   AS-Data Receiver Communication
            +-------->| Application |<-----------------------------+
            |         | Server (AS) |                              |
            |         +-------------+                              |
            |                                          IP(R-NAT_B) |
            |         NSIS Signaling Message               +-------+--+
            |  +------------------------------------------>| NAT/NAPT |
            |  |                                           | B        |
            |  |                                           +-------+--+
            |  |                                                   |
     AS-Data|  |                                                   |
    Receiver|  |                       +----------+                |
       Comm.|  |                       | NAT/NAPT |                |
            |  |                       | A        |                |
            |  |                       +----------+                |
            |  |                                                   |
            |  |                                                   |
            |  |                                                   |
            |  |                                                   |
            v  |                                             IP(R) v
        +--------+                                          +---------+
        | Data   |                                          | Data    |
        | Sender |                                          | Receiver|
        +--------+                                          +---------+

                IP(R):       Private IP Address of Data Receiver
                IP(R-NAT_B): Public IP Address for Data Receiver
                             provided by binding in NAT/NAPT B





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              Figure 14: The Data Receiver behind NAT Problem

   Some sort of communication between the data sender/data receiver and
   a third party is typically necessary (independently of whether NSIS
   is used).  NSIS signaling messages cannot be used to communicate the
   relevant application  level end point identifiers (in the generic
   case at least) as a replacement for communication with the
   application server.

   If the data receiver is behind a NAT then an NSIS signaling message
   will be addressed to the IP address allocated at the NAT (assuming
   one had already been allocated).  If no corresponding NSIS NAT
   Forwarding State at NAT/NAPT B exists (binding IP(R-NAT B) <-> IP(R))
   then the signaling message will terminate at the NAT device (most
   likely without generating a proper response message).  The signaling
   message transmitted by the data sender cannot install the NAT binding
   or NSIS NAT Forwarding State "on-the-fly" since this would assume
   that the data sender knows the topology at the data receiver side
   (i.e., the number and the arrangement of the NAT and the private IP
   address(es) of the data receiver).  A primary goal of path-coupled
   middlebox communication was to avoid end hosts having to discover and
   use this type of topology knowledge.  Data receivers behind a NAT
   must first reserve an external IP address (and, in many cases, a port
   number as well).



       Public Internet                Private Address
                                           Space
                    Edge
    NI(DS)          NAT                    NAT                   NR(DR)
    NR+                                                          NI+
    |               |                       |                       |
    |               |                       |                       |
    |               |                       |                       |
    |               |      REA[DTInfo]      |      REA[DTInfo]      |
    |               |<----------------------|<----------------------|
    |               |                       |                       |
    |               |RESPONSE[Success/Error]|RESPONSE[Success/Error]|
    |               |---------------------->|---------------------->|
    |               |                       |                       |
    |               |                       |                       |

      ============================================================>
                        Data Traffic Direction


           Figure 15: Reservation message flow for DR behind NAT



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   Figure 15 shows the message flow for reserving an external address/
   port at a NAT.  In this case the roles of the different NSIS entities
   are:

   o  The  data receiver (DR) for the anticipated data traffic is the
      NSIS initiator (NI+) for the RESERVE-EXTERNAL-ADDRESS (REA)
      message, but becomes the NSIS responder (NR) for following CREATE
      messages.

   o  The actual data sender (DS) will be the NSIS initiator (NI) for
      later CREATE messages and may be the NSIS target of the signaling
      (NR+).

   o  The actual target of the REA message, the Opportunistic Address
      (OA) is an arbitrary address, that would force the message to get
      intercepted by the far outermost NAT in the network at the
      boundary between the private address and the public address realm.
      The Opportunistic Address is shown as NR+.  REA messages for NATs
      MUST be transported by using the loose-end message routing method
      (LE-MRM) of the NTLP.  Note that REA messages for firewalls (the
      firewall-REA) must be transported by using the path-coupled
      message routing method (PC-MRM), see Section 3.4.2.2.

   The NI+ (could be on the data receiver DR or on any other host within
   the private network) sends the REA message targeted to the
   Opportunistic Address (OA defined earlier).  The OA selection for
   this message is discussed in Section 3.8.  The message routing for
   the REA message is in the reverse direction to the normal message
   routing used for path-coupled signaling where the signaling is sent
   downstream (as opposed to upstream in this case).  When establishing
   NAT bindings (and a NSIS session) the signaling direction does not
   matter since the data path is modified through route pinning due to
   the external NAT address.  Subsequent NSIS messages (and also data
   traffic) will travel through the same NAT boxes.

   The NI+ MUST include a 'data terminal information' object (DTInfo) in
   the REA message and fill it in appropriately (see Section 4.3.8).
   This information SHOULD include at least the 'dst port number' and
   'protocol' fields, in the DTInfo object as these may be required by
   en-route NATs, depending on the type of the NAT.  These two fields
   are most likely required by NAPTs to perform the address and port
   translation.  All other fields MAY be set by the NI+ to restrict the
   set of possible NIs.  An edge-NAT will use the information provided
   within the DTInfo object ('src IPv4/v6 address', 'src port number',
   'protocol') to only allow hosts falling within the specified range to
   originate NATFW NSLP messages.  The possible range is given by the
   'src port number' field and the combination of 'dst prefix' and 'src
   IP address' (see also Section 4.3.8).



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   The REA signaling message creates a NSIS NATFW session at any
   intermediate NSIS NATFW peer(s) encountered.  Furthermore it has to
   be ensured that the edge-NAT device is discovered as part of this
   process.  The end host cannot be assumed to know this device -
   instead the NAT box itself is assumed to know that it is located at
   the outer perimeter of the private addressing realm.  Forwarding of
   the REA  message beyond this entity is not necessary, and MUST be
   prohibited as it provides information on the capabilities of internal
   hosts.

   The edge-NAT device responds to the REA message with a RESPONSE
   message containing a success object carrying the public reachable IP
   address/port number in an 'external address' object (see
   Section 4.3.2).

   Processing of REA messages is specific to the NSIS node type:

   o  NSLP initiator: NI+ only generate REA messages and should never
      receive them.  When the data sender's address information is known
      in advance the NI+ MAY include a DTInfo object in the REA message.
      When the data sender's IP address is not known, NI+s MUST NOT
      include a DTInfo object.

   o  NSLP forwarder: NSLP forwarders receiving REA messages MUST first
      perform the checks defined in Section 3.9 and Section 3.10, if
      applicable, before any further processing is executed.  The NF
      SHOULD check with its local policies if it can accept the desired
      policy rule given by NTLP's message routing information (MRI).
      Further processing depends on the middlebox type:

      *  NAT:  NATs check whether the message is received at the
         external (public in most cases) address or at the internal
         (private) address.  If received at the external address a NF
         MAY generate a RESPONSE message with an error of type 'REA
         received from outside'.  If received at the internal address,
         an IP address/port is reserved.  If it is an edge-NAT, the NSLP
         message is not forwarded any further and a RESPONSE message is
         generated containing an 'external address' object (either IPv4
         or IPv6 version, as appropriate) holding the translated address
         port information in the binding reserved as a result of the REA
         message.  The RESPONSE message is sent back towards the NI+.
         If it is not an edge-NAT, the NSLP message is forwarded further
         using the translated IP address as signaling source address and
         including the translated IP address/port in the MRI.  The edge-
         NAT MAY reject REA messages not carrying a DTInfo object or if
         the address information within this object is invalid or is not
         comliant with local policies (e.g., the information provided is
         wildcarded but the edge-NAT requires full information about DS'



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         IP address and port).

      *  Firewall:  Firewalls MUST not change their configuration on
         receiving a REA message.  They MUST simply forward the message
         and MUST keep NTLP state.  Firewalls that are configured as
         edge-firewalls SHOULD return an error of type 'no NAT here'.

      *  Combined NAT and firewall:  Processing at combined firewall and
         NAT middleboxes is the same as in the NAT case.

   o  NSLP receiver:  This type of message should never be received by
      any NR+ and it SHOULD be discarded silently.

   Processing of a RESPONSE message with an 'external address' object is
   different for every NSIS node type:

   o  NSLP initiator:  Upon receiving a RESPONSE message with an
      external address object, the NI+ can use the IP address and port
      pairs carried for further application signaling.

   o  NSLP forwarder: NFs simply forward this message as long as they
      keep state for the requested reservation.

   o  NSIS responder:  This type of message should never be received by
      any NR+, unless it also the edge-NAT.  In any other case, it
      SHOULD be discarded silently (EDITOR's note: It can be appropriate
      the return an error message).

   o  Edge-NATs: This type of message should never be received by any
      Edge-NAT and it SHOULD be discarded silently.  (EDITOR's note: It
      can be appropriate the return an error message, btw what means
      drop silenty?  What happens to the NTLP session?)

   Reservations made with REA MUST be enabled by a subsequent CREATE
   message.  A reservation made with REA is kept alive as long as the
   NI+ refreshes the particular signaling session and it can be reused
   for multiple, different CREATE messages.  An NI+ may decide to
   teardown a reservation immediately after receiving a CREATE message.
   Without using CREATE Section 3.4.1 or REA in proxy mode Section 3.4.7
   no data traffic will be forwarded to DR beyond the edge-NAT.  REA is
   just taking care about enabling the forwarding of subsequent CREATE
   messages traveling towards the NR.  Correlation of incoming CREATE
   messages to REA reservation states is described in Section 3.7.

3.4.2.2  Signaling Reservation for Firewalls

   Data receivers behind firewalls can experience two basic policy
   settings of their upstream firewalls.  Either the firewall is set to



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   a 'deny all by default' or 'allow all by default' policy.  In the
   'deny all' case, no traffic, neither plain data nor NATFW NSLP
   signaling,is allowed to traverse the firewall.  Vice versa in the
   'allow all' case, all traffic is allowed to traverse.  For 'deny all'
   firewalls, data receivers must be able to notify upstream firewalls
   about their willingness to receive NATFW NSLP signaling (this is
   similar to REA for NATs).  For 'allow all' firewalls, data receivers
   must be able to notify upstream firewalls about unwanted traffic that
   should be blocked.  Data receivers use the RESERVE-EXTERNAL-ADDRESS
   (REA) request message to either allow incoming NATFW NSLP signaling
   messages or to block incoming data traffic, as shown in Figure 16.
   See also the proxy mode of operation for REA-F in Section 3.4.9.



       Public Internet               Protected Address
                                           Space
                    Edge
    NI(DS)          FW                      FW                   NR(DR)
    NR+                                                          NI+
    |               |                       |                       |
    |               |                       |                       |
    |               |                       |                       |
    |               |   REA-F[NATFW_EFI]    |   REA-F[NATFW_EFI]    |
    |               |<----------------------|<----------------------|
    |               |                       |                       |
    |               |RESPONSE[Success/Error]|RESPONSE[Success/Error]|
    |               |---------------------->|---------------------->|
    |               |                       |                       |
    |               |                       |                       |

      ============================================================>
                        Data Traffic Direction


               Figure 16: Signaling reservation message flow

   The processing of REA for firewalls (REA-F) messages is different for
   every NSIS entity:

   o  NSLP initiator (NI+): NI+ MUST always direct REA-F message to the
      address of DS.  NI+ only generates REA-F messages and should never
      receive them.

   o  NSLP forwarder: NSLP forwarders receiving REA-F messages MUST
      first perform the checks defined in Section 3.9 and Section 3.10,
      if applicable, before any further processing is executed.  The NF
      SHOULD check with its local policies if it can accept the desired



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      policy rule given by NTLP's message routing information (MRI).
      Further processing depends on the middlebox type:

      *  NAT:  NATs check whether the message is received at the
         external (public in most cases) address or at the internal
         (private) address.  If received at the internal interface, NATs
         allocated a public IP address and port and forward the message
         further.  Edge-NATs receiving REA-F SHOULD response with error
         RESPONSE indicating 'no edge-firewall'.

      *  Firewall: Non edge-firewalls keep session state and forward the
         message.  Edge-firewalls stop forwarding the check for
         performing the checks defined in Section 3.9 and Section 3.10,
         if applicable.  If the message is accepted, load the specified
         policy rule and generate RESPONSE messages back towards the DR.

      *  Combined NAT and firewall:  Processing at combined firewall and
         NAT middleboxes is the same as in the firewall case.

   o  NSLP receiver:  This type of message should never be received by
      any NR+ and it SHOULD be discarded silently.

   Processing of a RESPONSE message with an external address object is
   different for every NSIS node type:

   o  NSLP initiator (NI+):  The NI+ is ready to received signaling or
      data traffic when receiving a RESPONSE message.

   o  NSLP forwarder: NFs simply forward this message as long as they
      keep state for the requested reservation.

   o  NSIS responder:  This type of message should never be received by
      an NR and it SHOULD be discarded silently.

   o  Edge-NATs/edge-firewall: This type of message should never be
      received by any edge-NAT/edge-firewall and it SHOULD be discarded
      silently.

   EDITOR's note: This section does not explain the operation of the
   NATFW_EFI object.

3.4.3  NATFW Session Refresh

   NATFW NSLP sessions are maintained on a soft-state basis.  After a
   specified timeout, sessions and corresponding policy rules are
   removed automatically by the middlebox, if they are not refreshed.
   Soft-state is created by CREATE, REA, and REA-F and the maintenance
   of this state must be done by these messages.  State created by



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   CREATE must be maintained by CREATE, state created by REA must be
   maintained by REA, and state created by REA-F must be maintained by
   REA-F.  Refresh messages, either CREATE/REA/REA-F, are messages
   carrying the exact MRI and session ID as the initial message and a
   lifetime object with a lifetime greater than zero.  Every refresh
   request message MUST be acknowledged by an appropriate response
   message generated by the NR.  This response message is routed back
   towards the NI, to allow the intermediate NFs to propose a refresh
   period that would align with their local policies.  The NI sends
   refresh messages destined for the NR.  Upon reception by each NSIS
   forwarder, the state for the given session ID is extended by the
   session refresh period, a period of time calculated based on a
   proposed refresh message period.  The lifetime extension of a session
   is calculated as current local time plus proposed lifetime value
   (session refresh period).  Section 3.5  defines the process of
   calculating lifetimes in detail.



   NI      Public Internet        NAT    Private address       NR
      |                              |          space             |
      | CREATE[lifetime > 0]         |                            |
      |----------------------------->|                            |
      |                              |                            |
      | RESPONSE[Error] (if needed)  |                            |
      |<-----------------------------|  CREATE[lifetime > 0]      |
      |                              |--------------------------->|
      |                              |                            |
      |                              |   RESPONSE[Success/Error]  |
      |   RESPONSE[Success/Error]    |<---------------------------|
      |<-----------------------------|                            |
      |                              |                            |
      |                              |                            |



         Figure 17: State Refresh Message Flow, CREATE as example

   Processing of session refresh CREATE/REA/REA-F messages is different
   for every NSIS node type:

   o  NSLP initiator: The NI can generate session refresh CREATE/REA/
      REA-F messages before the session times out.  The rate at which
      the refresh CREATE/REA/REA-F messages are sent and their relation
      to the session state lifetime are further discussed in
      Section 3.5.  The message routing information and the extended
      flow information object MUST be set equal to the values of the
      initial request message.



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   o  NSLP forwarder: NSLP forwarders receiving session refresh messages
      MUST first perform the checks defined in Section 3.9 and
      Section 3.10, if applicable, before any further processing is
      executed.  The NF SHOULD check with its local policies if it can
      accept the desired lifetime extension for the session referred by
      the session ID.  Processing of this message is independent of the
      middlebox type.

   o  NSLP responder: NRs accepting a session refresh CREATE/REA/REA-F
      message generate a RESPONSE message with response object set to
      success.  NRs MUST perform the checks defined in Section 3.9 and
      Section 3.10, if applicable.


3.4.4  Deleting Sessions

   NATFW NSLP sessions may be deleted at any time.  NSLP initiators can
   trigger this deletion by using a CREATE, REA, or REA-F messages with
   a lifetime value set to 0, as shown in Figure 18.




      NI      Public Internet        NAT    Private address       NR
      |                              |          space             |
      |    CREATE[lifetime=0]        |                            |
      |----------------------------->|                            |
      |                              |                            |
      |                              | CREATE[lifetime=0]         |
      |                              |--------------------------->|
      |                              |                            |


             Figure 18: Delete message flow, CREATE as example

   NSLP nodes receiving this message MUST first perform the checks
   defined in Section 3.9 and Section 3.10, if applicable, and
   afterwards MUST delete the session immediately.  Policy rules
   associated with this particular session MUST be deleted immediately.
   This message is forwarded until it reaches the final NR.  The CREATE/
   REA/REA-F request message with a lifetime value of 0, does not
   generate any response, neither positive nor negative, since there is
   no NSIS state left at the nodes along the path.

3.4.5  Reporting Asynchronous Events

   NATFW NSLP forwarders and NATFW NSLP responders must have the ability
   to report asynchronous events to other NATFW NSLP nodes, especially



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   to allow reporting back to the NATFW NSLP initiator.  Such
   asynchronous events may be premature session termination, changes in
   local policies, route change or any other reason that indicates
   change of the NATFW NSLP session state.  Currently, asynchronous
   session termination, re-authorization required and route change
   detected (see Section 3.11) are the only events that are defined, but
   other events may be defined in later revisions of this memo.

   NFs and NRs may generate NOTIFY messages upon asynchronous events,
   with a response object indicating the reason of the event.  NOTIFY
   messages are sentEhop-by-hop upstream towards NI until they reach NI.

   The initial processing when receiving a NOTIFY message is the same
   for all NATFW nodes: NATFW nodes MUST only accept NOTIFY messages
   through already established NTLP messaging associations.  The further
   processing is different for each NATFW NSLP node type and depends on
   the events notified:

   o  NSLP initiator: NIs analyze the notified event and behave
      appropriately based on the event type.  Section 4.3.4 discusses
      the required behavior for each notified event.  NIs MUST NOT
      generate NOTIFY messages.

   o  NSLP forwarder: NFs receiving NOTIFY messages MUST first perform
      the checks defined in Section 3.9 and Section 3.10, if applicable,
      and MUST only accept NOTIFY messages from downstream peers via an
      already existing NTLP messaging association.  After successfully
      doing so, NFs analyze the notified event and behave based on the
      notified events defined in Section 4.3.4.  NFs SHOULD generate
      NOTIFY messages upon asynchronous events and forward them upstream
      towards the NI.  NOTIFY messages are sent further hop-by-hop
      upstream towards the NI.

   o  NSLP responder: NRs SHOULD generate NOTIFY messages upon
      asynchronous events with 'response object(s)' code based on the
      reported event(s).  NRs receiving NOTIFY messages MUST ignore this
      message and discard it.  NOTIFY messages are sent hop-by-hop
      upstream towards NI

   EDITOR's note: The current semantics can result in NOTIFICATION
   storms.  There is a better semantics needed, how to avoid those
   storms and how NOTIFY messages are handled along the path.  Elwyn
   noted: What to do if more than one node detects a failure condition
   along the path.  What happens than?

3.4.6  Tracing Signaling Sessions

   The NATFW NSLP provides a diagnosis capability to session owners (the



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   NI or NI+).  Session owners are able to trace the NSIS nodes being
   involved in a particular signaling session.  The TRACE request
   message is used to trace the involved NSIS nodes along the signaling
   session and to return their identifiers.




      NI      Public Internet        NAT    Private address       NR
      |                              |          space             |
      |           TRACE              |                            |
      |----------------------------->|                            |
      |                              |                            |
      |                              |            TRACE           |
      |                              |--------------------------->|
      |                              |                            |
      |                              |      RESPONSE[IP(NR)]      |
      |                              |<---------------------------|
      | RESPONSE[IP(NR),IP(NAT)]     |                            |
      |<-----------------------------|                            |
      |                              |                            |
      |                              |                            |


         Figure 19: Example for tracing the signaling session path

   The processing when receiving a TRACE message is the different for
   each type of NATFW node:

   o  NSLP initiator: NI generates TRACE request messages.

   o  NSLP forwarder: NFs keep session state and forward the message.

   o  NSLP responder: NRs receiving a TRACE request message terminate
      the forwarding and reply with a RESPONSE message including the
      NATFW_TRACE object.  The NATFW_TRACE object MAY be filled by the
      NR with its IP address.

   Processing of a RESPONSE message to a TRACE request message is
   different for every NSIS node type:

   o  NSLP initiator: The NI terminates the forwarding and checks the
      response message for further internal processing.

   o  NSLP forwarder: NFs MAY include their identifier in the
      NATFW_TRACE object and increment the hop counter by one.  This
      memo defines IPv4 and IPv6 IP addresses as possible node
      identifier.  NFs MUST forward this type of RESPONSE.



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   o  NSLP responder: A NR should never see such a RESPONSE message.  It
      MUST discard the message and reply with an error message.


3.4.7  Proxy Mode for Data Receiver behind NAT

   Comment from Elwyn: The next three sections would benefit from a an
   introductory section .  Some of the common text about NATFW_PROXY
   object and the naming of the xx-PROXY messages, etc.,  could be
   factored out, reducing the size of the text and making the whole
   thing clearer.  Also the sections 3.3.7-3.3.9 should be reordered
   more logically.

   Some migration scenarios need specialized support to cope with cases
   whereonly the receiving side is running NSIS.  End-to-end signaling
   is going to fail without NSIS support at both data sender and data
   receiver, unless the NATFW NSLP also gives the NR the ability to
   install state on the upstream path towards the data sender for
   downstream data packets.  The goal of the method described is to
   trigger the network to generate a CREATE message at the edge-NAT on
   behalf of the data receiver.  In this case, a NR can signal towards
   the Opportunistic Address as is performed in the standard REA message
   handling scenario for NATs as in Section 3.4.2.1.  The message is
   forwarded until the edge-NAT is reached.  A public IP address and
   port number is reserved at an edge-NAT.  As shown in Figure 20,
   unlike the standard REA message handling case, the edge-NAT is
   triggered to send a CREATE message on a new reverse path which
   traverse several firewalls or NATs.  The new reverse path for CREATE
   is necessary to handle routing asymmetries between the edge-NAT and
   DR.  This behavior requires an indication to the edge-NAT within the
   REA message if either the standard behavior (as defined in
   Section 3.4.2.1) is required or a CREATE message is required to be
   sent by the edge-NAT.  This indication is that the REA message
   contains a NATFW_PROXY object.  We distinguish a REA message
   containing a NATFW_PROXY object by calling it a REA-PROXY message.
   In addition when a CREATE message needs to be sent by the edge-NAT,
   the REA message may include the data sender's address (DTInfo), if
   available to the data receiver.  Figure 20 shows this proxy mode REA
   as REA-PROXY.












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      DS       Public Internet       NAT     Private address      NR
     No NI                            NF         space            NI+
      NR+
      |                               |   REA-PROXY[(DTInfo)]     |
      |                               |<------------------------- |
      |                               |  RESPONSE[Error/Success]  |
      |                               | ---------------------- >  |
      |                               |   CREATE                  |
      |                               | ------------------------> |
      |                               |  RESPONSE[Error/Success]  |
      |                               | <----------------------   |
      |                               |                           |
      |                               |                           |



      Figure 20: REA Triggering Sending of CREATE Message on Separate
                               Reverse Path

   The processing of REA-PROXY messages is different for every NSIS
   entity:

   o  NSLP initiator (NI+): When the data sender's address information
      is known in advance the NI+ MAY include a DTInfo object in the
      REA-PROXY request message.  When the data sender's address is not
      known, NI+'s MUST NOT include a DTInfo object.  The NI+ MUST
      choose a random value and include it in the NONCE object.  NI+
      only generate REA-PROXY messages and should never receive them.

   o  NSLP forwarder: NSLP forwarders receiving REA-PROXY messages MUST
      first perform the checks defined in Section 3.9 and Section 3.10,
      if applicable, before any further processing is executed.  The NF
      SHOULD check with its local policies if it can accept the desired
      policy rule given by NTLP's message routing information (MRI).
      Further processing depends on the middlebox type:

      *  NAT:  NATs check whether the message is received at the
         external (public in most cases) address or at the internal
         (private) address.  If received at the external address a NF
         SHOULD generate a RESPONSE message with an  error of type 'REA
         received from outside' and stop forwarding.  If received at the
         internal address, an IP address/port is reserved.  If it is not
         an edge-NAT, the NSLP message is forwarded further with the
         translated IP address/port.  If it is an edge-NAT, the NSLP
         message is not forwarded any further.  The edge-NAT checks
         whether it is willing to send CREATE messages on behalf on NI+
         and if so, it checks the DTInfo object.  The edge-NAT MAY
         reject the REA-PROXY request if there is no DTInfo object or if



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         the address information within DTInfo is not valid or too much
         wildcarded.  If accepted, a RESPONSE message is generated
         containing an External Address Object (either IPv4 or IPv6
         version, as appropriate) holding the translated address port
         information in the binding reserved as a result of the REA
         message.  The RESPONSE message is sent back towards the NI+.
         When the edge-NAT accepts, it generates a CREATE message as
         defined in Section 3.4.1 and includes a NONCE object having the
         same value as of the received NONCE object.  The edge-NAT MUST
         not generate a CREATE-PROXY message (see below xref
         target="proxy_sender"/>).  The edge-NAT MUST refresh the CREATE
         message session only if a REA-PROXY refresh message has been
         received first.

      *  Firewall:  firewalls MUST not change their configuration upon a
         REA message.  They simply MUST forward the message and MUST
         keep NTLP state.  Edge-firewalls SHOULD reply with an error
         RESPONSE indicating 'no egde-NAT here'.

      *  Combined NAT and firewall:  Processing at combined firewall and
         NAT middleboxes is the same as in the NAT case.

   o  NSLP receiver:  This type of message should never be received by
      any NR+ and it SHOULD be discarded silently.

   Processing of a RESPONSE message with an 'external address' object is
   different for every NSIS node type:

   o  NSLP initiator:  Upon receiving a RESPONSE message with an
      external address object, the NI+ can use the IP address and port
      pairs carried for further application signaling.

   o  NSLP forwarder: NFs simply forward this message as long as they
      keep state for the requested reservation.

   o  NSIS responder:  This type of message should never be received by
      an NR and it SHOULD be discarded silently.

   o  Edge-NATs/edge-firewall: This type of message should never be
      received by any Edge-NAT/edge-firewall and it SHOULD be discarded
      silently.

   The scenario described in this section challenges the data receiver
   because it must make a correct assumption about the data sender's
   ability to use NSIS NATFW NSLP signaling.  It is possible for the DR
   to make the wrong assumption in two different ways:





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      a) DS is NSIS unaware but DR assumes DS to be NSIS aware and

      b) DS is NSIS aware but DR assumes DS to be NSIS unaware.

   Case a) will result in middleboxes blocking the data traffic, since
   DS will never send the expected CREATE message.  Case b) will result
   in the DR successfully requesting proxy mode support by the edge-NAT.
   The edge-NAT will send CREATE messages and DS will send CREATE
   messages too.  Both CREATE messages are handled as separated sessions
   and therefore the common rules per session apply.  It is the NR's
   responsibility to decide whether to teardown the REA-PROXY sessions
   in the case where the data sender's side is NSIS aware but was
   incorrectly assumed not to be so by the DR.  It is RECOMMENDED that a
   DR behind NATs uses the proxy mode of operation by default, unless
   the DR knows that the DS is NSIS aware.  The DR MAY cache information
   about data senders which it has found to be NSIS aware in past
   sessions.

   The NONCE object is used to build the relationship between received
   CREATEs and the message initiator.  An NI+ uses the presence of the
   NATFW_NONCE object to correlate it to the particular REA-PROXY
   request.  The absence of an NONCE object indicates a CREATE initiated
   by the DS and not by the edge-NAT.

   There is a possible race condition between the RESPONSE message to
   the REA-PROXY and the CREATE message generated by the edge-NAT.  The
   CREATE message can arrive earlier than the RESPONSE message.  An NI+
   MUST accept CREATE messages generated by the edge-NAT even if the
   RESPONSE message to the REA-PROXY request was not received.

3.4.8  Proxy Mode for Data Sender behind Middleboxes

   As with data receivers behind middleboxes in Section 3.4.7 data
   senders behind middleboxes require proxy mode support.  The issue
   here is that there is no NSIS support at the data receiver's side
   and, by default, there will be no response to CREATE request
   messages.  This scenario requires the last NSIS NATFW NSLP aware node
   to terminate the forwarding and to proxy the response to the CREATE
   message, meaning that this node is generating RESPONSE messages.
   This last node may be an edge-NAT/edge-firewall, or any other NATFW
   NSLP peer, that detects that there is no NR available (probably as a
   result of GIST timeouts but there may be other triggers).  This proxy
   mode handles only data senders behind a middlebox; for receivers
   behind a NAT see Section 3.4.7 and for receivers behind a firewall
   see Section 3.4.9.

   NIs being aware about a NSIS unaware DR, send a CREATE message
   towards DR with a proxy support object (NATFW_PROXY).  We distinguish



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   a CREATE message containing a NATFW_PROXY object by calling it a
   CREATE-PROXY message.  Intermediate NFs can use this additional
   information to decide whether to terminate the message forwarding or
   not.  This proxy support object is an implicit scoping of the CREATE
   message.  Termination of CREATE-PROXY request messages with proxy
   support object included MUST only be done by the outermost egde-NATs/
   edge-firewalls.





      DS       Private Address       FW     Public Internet      NR
      NI           Space              NF                         no NR
      |                               |                           |
      |         CREATE-PROXY          |                           |
      |------------------------------>|                           |
      |                               |                           |
      |   RESPONSE[SUCCESS/ERROR]     |                           |
      |<------------------------------|                           |
      |                               |                           |


                 Figure 21: Proxy Mode Create Message Flow

   The processing of CREATE-PROXY messages and RESPONSE messages is
   similar to Section 3.4.1, except that forwarding is stopped at the
   edge-NAT/edge-firewall.  The edge-NAT/edge-firewall responds back to
   NI according the situation (error/success) and will be the NR for
   future NATFW NSLP communication.

3.4.9  Proxy Mode for Data Receiver behind Firewall

   Data receivers behind firewalls would like to use a similar sort of
   proxy mode operation compared to those behind NATs.  While finding an
   upstream edge-NAT is quite easy (it is only required to find some
   edge-NAT as the data traffic will be route-pinned to the NAT),
   locating the appropriate edge-firewall is difficult.  Where a data
   receiver is located in a site network that is multihomed with several
   independently firewalled connections to the public Internet, the
   specific firewall through which the data traffic will be routed has
   to be ascertained.  With this knowledge, proxy mode support that is
   similar to Section 3.3.7 can be used to install appropriate "allow"
   rules in the firewall through which the data traffic will be routed.
   Being able to identify the firewall through which data from a given
   source address will be routed is also essential for implementing the
   capability to install a blocking rule for incoming traffic in a
   firewall which defaults to "allow all".  In the first case the



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   downstream data path must be fully enabled by signaling from the
   edge-firewall towards the data receiver in case there are additional
   firewalls along the path.  This additional signaling is not needed in
   the blocking case as the intention is prevent traffic entering the
   site.

   The REA-F (firewall-REA) is used to locate upstream firewalls and to
   request installation of the appropriate policy rules.  The goal of
   the method described is to trigger the network to generate a CREATE
   message at the edge-firewall on behalf of the data receiver when this
   is needed for an 'allow' rule.  Provided the data sender's IP address
   is known, a NR can signal towards the data sender's address as in the
   standard REA-F message handling scenario for firewalls
   Section 3.4.2.2.  The message is forwarded until it reaches the edge-
   firewall.  As shown in Figure 22, the edge-firewall is triggered to
   send a CREATE message on a new reverse path which traverses through
   internal firewalls or NATs.  The new reverse path for CREATE is
   necessary to handle routing asymmetries between the edge-firewall and
   DR.  REA-F does not install any policy rule but the subsequent CREATE
   message initiated by the edge-firewall does.

   EDITOR's note: The above paragraph describes just the allow case.
   The proxy thing is not needed if a 'deny' rule is requested.





      DS       Public Internet        FW     Private address      NR
     No NI                            NF         space            NI+
      NR+
      |                               |        REA-F-PROXY        |
      |                               |<------------------------- |
      |                               |  RESPONSE[Error/Success]  |
      |                               | ---------------------- >  |
      |                               |   CREATE                  |
      |                               | ------------------------> |
      |                               |  RESPONSE[Error/Success]  |
      |                               | <----------------------   |
      |                               |                           |
      |                               |                           |



     Figure 22: REA-F Triggering Sending of CREATE Message on Separate
                               Reverse Path

   The processing of REA-F-PROXY messages is different for every NSIS



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   entity:

   o  NSLP initiator (NI+): NI+ MUST always direct REA-F-PROXY message
      to the address of DS.  NI+ only generates REA-F messages and
      should never receive them.

   o  NSLP forwarder: NSLP forwarders receiving REA-F messages MUST
      first perform the checks defined in Section 3.9 and Section 3.10,
      if applicable, before any further processing is executed.  The NF
      SHOULD check with its local policies if it can accept the desired
      policy rule given by NTLP's message routing information (MRI).
      Further processing depends on the middlebox type:

      *  NAT:  NATs check whether the message is received at the
         external (public in most cases) address or at the internal
         (private) address.  If received at the internal interface, NATs
         allocated a public IP address and port and forward the message
         further.  Edge-NATs receiving REA-F-PROXY SHOULD response with
         error RESPONSE indicating 'no edge-firewall'

      *  Firewall: Non edge-firewalls keep session state and forward the
         message.  Edge-firewalls stop forwarding the check for
         performing the checks defined in Section 3.9 and Section 3.10,
         if applicable.  If the message is accepted, load the specified
         policy rule and if the policy rule action is "allow", generate
         CREATE messages back towards the DR as  defined in
         Section 3.4.1.  In any case generate a RESPONSE message
         indicating success or failure and send it back towarsd the NI+.

      *  Combined NAT and firewall:  Processing at combined firewall and
         NAT middleboxes is the same as in the firewall case.

   o  NSLP receiver:  This type of message should never be received by
      any NR+ and it SHOULD be discarded silently.

   Processing of a RESPONSE message is different for every NSIS node
   type:

   o  NSLP initiator (NI+):  Upon receiving a RESPONSE message NI+
      should await incoming corresponding CREATE messages if the
      UCREATE-PROXY message was sent with an "allow" rule

   o  NSLP forwarder: NFs simply forward this message as long as they
      keep state for the requested reservation.

   o  NSIS responder:  This type of message should never be received by
      an NR and it SHOULD be discarded silently.




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   o  Edge-NATs/edge-firewall: This type of message should never be
      received by any Edge-NAT/edge-firewall and it SHOULD be discarded
      silently.

   There is a possible race condition between the RESPONSE message to
   the REA-F-PROXY and the CREATE message generated by the edge-
   firewall.  The CREATE message can arrive earlier than the RESPONSE
   message.  An NI+ MUST accept CREATE messages generated by the edge-
   firewall even if the RESPONSE message to the REA-F-PROXY request was
   not received.

3.5  Calculation of Session Lifetime

   NATFW NSLP sessions, and the corresponding policy rules which may
   have been installed, are maintained via soft-state mechanisms.  Each
   session is assigned a lifetime and the session is kept alive as long
   as the lifetime is valid.  After the expiration of the lifetime,
   sessions and policy rules MUST be removed automatically and resources
   bound to them should be freed as well.  Session lifetime is handled
   at every NATFW NSLP node.  The NSLP forwarders and NSLP responder are
   not responsible for triggering lifetime extension refresh messages
   (see Section 3.4.3): this is the task of the NSIS initiator.

   The NSIS initiator MUST choose a session lifetime (expressed in
   seconds) value before sending any message including the initial
   message which creates the session (lifetime is set to zero for
   deleting sessions) to other NSLP nodes.  The session lifetime value
   is calculated based on:

   o  The number of lost refresh messages that NFs should cope with

   o  The end-to-end delay between the NI and NR

   o  Network vulnerability due to session hijacking ([8]).  Session
      hijacking is made easier when the NI does not explicitly remove
      the session.

   o  The user application's data exchange duration, in terms of time
      and networking needs.  This duration is modeled as M x R, with R
      the message refresh period (in seconds) and M a multiplier for R.

   The RSVP specification [13] provides an approriate algorithm for
   calculating the session lifetime as well as means to avoid refresh
   message synchronization between sessions.    [13] recommends:

   1.  The refresh message timer to be randomly set to a value in the
       range [0.5R, 1.5R].




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   2.  To avoid premature loss of state, L (with L being the session
       lifetime) must satisfy L >= (K + 0.5)*1.5*R, where K is a small
       integer.  Then in the worst case, K-1 successive messages may be
       lost without state being deleted.  Currently K = 3 is suggested
       as the default.  However, it may be necessary to set a larger K
       value for hops with high loss rate.  Other algorithms could be
       used to define the relation between the session lifetime and the
       refresh message period, the algorithm provided is only given as
       an example.

   This requested lifetime value is placed in the 'lifetime' object of
   the NSLP message and messages are forwarded to the next NATFW NSLP
   node.

   NATFW NFs processing the request message along the path MAY change
   the requested lifetime to fit their needs and/or local policy.  If an
   NF changes the lifetime value it must also indicate the corresponding
   refresh message period.  NFs MUST NOT increase the lifetime value;
   they MAY reject the requested lifetime immediately and MUST generate
   an error response message of type 'lifetime too big' upon rejection.
   The NSLP request message is forwarded until it reaches the NSLP
   responder.  NSLP responder MAY reject the requested lifetime value
   and MUST generate an error response message of type 'lifetime too
   big' upon rejection.  The NSLP responder MAY also lower the requested
   lifetime to an acceptable value (based on its local policies).  NSLP
   responders generate their appropriate response message for the
   received request message, sets the lifetime value to the above
   granted lifetime and sends the message back hop-by-hop towards NSLP
   initiator.

   Each NSLP forwarder processes the response message, reads and stores
   the granted lifetime value.  The forwarders SHOULD accept the granted
   lifetime, as long as the value is within the tolerable lifetime range
   defined in their local policies.  They MAY reject the lifetime and
   generate a 'lifetime not acceptable' error response message.
   Figure 23 shows the procedure with an example, where an initiator
   requests 60 seconds lifetime in the CREATE message and the lifetime
   is shortened along the path by the forwarder to 20 seconds and by the
   responder to 15 seconds.












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   +-------+ CREATE(lt=60s)  +-----------+ CREATE(lt=20s)  +--------+
   |       |---------------->|    NSLP   |---------------->|        |
   |  NI   |                 |           |                 |  NR    |
   |       |<----------------| forwarder |<----------------|        |
   +-------+ RESPONSE(lt=15s +-----------+ RESPONSE(lt=15s +--------+
                      MRR=3s)                       MRR=3s)

      lt  = lifetime
      MRR = Message Refresh Rate



                  Figure 23: Lifetime Calculation Example


3.6  Message Sequencing

   NATFW NSLP messages need to carry an identifier so that all nodes
   along the path can distinguish messages sent at different points of
   time.  Messages can be lost along the path or duplicated.  So all
   NATFW NSLP nodes should be able to identify either old messages that
   have been received before (duplicated), or the case that messages
   have been lost before (loss).  For message replay protection it is
   necessary to keep information about messages that have already been
   received and requires every NATFW NSLP message to carry a message
   sequence number (MSN), see also Section 4.3.7.

   The MSN MUST be set by the NI and MUST NOT be set or modified by any
   other node.  The initial value for the MSN MUST be generated randomly
   and MUST be unique only within the session for which it is used.  The
   NI MUST increment the MSN by one for every message sent.  Once the
   MSN has reached the maximum value, the next value it takes is zero.

   EDITOR's note: Is it needed to apply this: All NATFW NSLP nodes MUST
   use the algorithm defined in [3] to detect MSN wrap arounds.

   NSIS forwarders and the responder store the MSN from the initial
   CREATE/REA/UCREATE packet which creates the session as start value
   for the session.  NFs and NRs MUST include the received MSN value in
   the corresponding RESPONSE message that they generate.

   When receiving a request message, a NATFW NSLP node uses the MSN
   given in the message to determine whether the state being requested
   is different to the state already installed.  The message MUST be
   discarded if the received MSN value is equal to or lower than the
   stored MSN value.  Such a received MSN value can indicate a
   duplicated and delayed message or replayed message.  If the received
   MSN value is greater than the already stored MSN value, the NATFW



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   NSLP MUST update its stored state accordingly, if permitted by all
   security checks (see Section 3.9 and Section 3.10), and stores the
   updated MSN value accordingly.

   For, example, applying these semantics to a CREATE message exchange
   mean that the first CREATE and carries the initial, randomly
   generated, MSN.  All nodes along the path store this value and the NR
   includes the received value in its response (assuming that the CREATE
   message reaches the NR).  Subsequent CREATE messages, updating the
   request policy rule or lifetime, carry an incremented MSN value, so
   that intermediate nodes can recognize the requested update.

3.7  De-Multiplexing at NATs

   Section 3.4.2.1 describes how NSIS nodes behind NATs can obtain a
   public reachable IP address and port number at a NAT and how it can
   be activated by using CREATE messages (see Section 3.4.1).  The
   information about the public IP address/port number can be
   transmitted via an application level signaling protocol and/or third
   party to the communication partner that would like to send data
   toward the host behind the NAT.  However, NSIS signaling flows are
   sent towards the address of the NAT at which this particular IP
   address and port number is allocated and not directly to the
   allocated IP address and port number.  The NATFW NSLP forwarder at
   this NAT needs to know how the incoming NSLP requests are related to
   reserved addresses, meaning how to de-multiplex incoming NSIS
   requests.

   The de-multiplexing method uses information stored at NATs (such as
   mapping of public IP address to private, transport protocol, port
   numbers), information given by NTLP's message routing information and
   further authentication credentials.

3.8  Selecting Opportunistic Addresses for REA

   As with all other message types, REA messages need a reachable final
   destination IP address.  But as many applications do not provide a
   destination IP address in the first place, there is a need to choose
   a destination address for REA messages.  This destination address can
   be the final target, but for applications which do not provide an
   upfront address, the destination address has to be chosen
   independently.  Choosing the 'correct' destination IP address may be
   difficult and it is possible there is no 'right answer'. [17] shows
   choices for SIP and this section provides some hints about choosing a
   good destination IP address.






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   1.  Public IP address of the data sender:

       *  Assumption:

          +  The data receiver already learned the IP address of the
             data sender (e.g., via a third party).

       *  Problems:

          +  The data sender might also be behind a NAT.  In this case
             the public IP address of the data receiver is the IP
             address allocated at this NAT.

          +  Due to routing asymmetry it might be possible that the
             routes taken by a) the data sender and the application
             server b) the data sender and NAT B might be different,
             this could happen in a network deployment such as in
             Figure 14.  As a consequence it might be necessary to
             advertise a new (and different) external IP address within
             the application (which may or may not allow that) after
             using NSIS to establish a NAT binding.

   2.  Public IP address of the data receiver:

       *  Assumption:

          +  The data receiver already learned his externally visible IP
             address (e.g., based on the third party communication).

       *  Problems:

          +  Communication with a third party is required.

   3.  IP address of the Application Server:

       *  Assumption:

          +  An application server (or a different third party) is
             available.

       *  Problems:

          +  If the NSIS signaling message is not terminated at the NAT
             of the local network then an NSIS unaware application
             server might discard the message.

          +  Routing might not be optimal since the route between a) the
             data receiver and the application server b) the data



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             receiver and the data sender might be different.


3.9  Session Ownership

   Proof of session ownership is a fundamental part of the NATFW NSLP
   signaling protocol.  It is used to validate the origin of a request,
   i.e., invariance of the message sender.  Only request messages
   showing a valid session ownership are processed further.  Within the
   NATFW NSLP, the NSIS initiator (the NI or the NI+) is the ultimate
   session owner for all request messages.  A proof of ownership MUST be
   provided for any request message sent downstream or upstream.  All
   intermediate NATFW NSLP nodes MUST use this proof of ownership to
   validate the message's origin.

   All NATFW nodes along the path must be able to verify that the sender
   of a request is the same entity that initially created the session.
   Generally, the path taken spans different administrative domains and
   cannot rely on using a common authentication scheme.  This
   requirement demands a scheme independent of the local authentication
   scheme in use and administrative requirements being enforced.
   Relying on a public key infrastructure (PKI) for the purpose of prove
   of session ownership is not reasonable due to deployment problems of
   a global PKI.

   The NATFW NSLP relies on the session ID (SID) carried in the NTLP for
   prove of session ownership.  The session ID MUST be generated in a
   random way.  Messages for a particular session are handled by the
   NTLP to the NATFW NSLP for further processing.  Messages carrying a
   different session ID not associated with any NATFW NSLP are subject
   to the regular processing for new NATFW NSLP sessions.

3.10  Authentication and Authorization

   NATFW NSLP nodes receiving signaling messages MUST first check
   whether this message is authenticated and authorized to perform the
   requested action.

   The NATFW NSLP is expected to run in various environments, such as IP
   telephone systems, enterprise networks, home networks, etc.  The
   requirements on authentication and authorization are quite different
   between these use cases.  While a home gateway, or an Internet cafe,
   using NSIS may well be happy with a "NATFW signaling coming from
   inside the network" policy for authorization of signaling, enterprise
   networks are likely to require a stronger authenticated/authorized
   signaling.  This enterprise scenario may require the use of an
   infrastructure and administratively assigned identities to operate
   the NATFW NSLP.



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   EDITOR's note: It is still not clear what are the requirements for
   authentication and authorization in the NATFW case.  This is going to
   be discussed at the next IETF meeting.

3.11  Reacting to Route Changes

   The NATFW NSLP needs to react to route changes in the data path.
   This assumes the capability to detect route changes, to perform NAT
   and firewall configuration on the new path and possibly to tear down
   session state on the old path.  The detection of route changes is
   described in Section 7 of [1] and the NATFW NSLP relies on
   notifications about route changes by the NTLP.  This notification
   will be conveyed by the API between NTLP and NSLP, which is out of
   scope of this memo.

   A NATFW NSLP node other than the NI or NI+ detecting a route change,
   by means described in the NTLP specification or others, generates a
   NOTIFY message indicating this change and sends this upstream towards
   NI.  Intermediate NFs on the way to the NI can use this information
   to decide later if their session can be deleted locally if they do
   not receive an update within a certain time period (EDITOR's note:
   what should be the default value for this time period?).  It is
   important to consider the transport limitations of NOTIFY messages as
   mandate in Section 3.4.5.  NOTIFY messages and therefore route change
   notifications and only accept from downstream peers via existing NTLP
   messaging associations.  (EDITOR's note: double check this measure!
   It might be appropriate to allow NOTIFY messages to be sent up- and
   downstream and just to mandate the MA transport).

   The NI receiving this NOTIFY message SHOULD generate an update
   message and sends it downstream as for the initial exchange.  All the
   remaining processing and message forwarding, such as NSLP next hop
   discovery, is subject to regular NSLP processing as described in the
   particular sections.  Merge points, NFs receiving update request
   messages (see also Section 3.12), can easily use the session ID
   (session ownership information, see also Section 3.9) to update the
   session.

3.12  Updating Policy Rules

   NSIS initiators can request an update of the installed/reserved
   policy rules at any time within a signaling session.  Updates to
   policy rules can be required due to node mobility (NI is moving from
   one IP address to another), route changes (this can result in a
   different NAT mapping at a different NAT device), or the wish of the
   NI to simply change the rule.  NIs can update policy rules in
   existing signaling sessions by sending an appropriate request message
   (similar to Section 3.5) with a different message routing information



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   (MRI) than installed before.  This update request message is treated
   with respect to authorization and authentication exactly as any
   initial request.  Therefore, any node along in the signaling session
   can reject the update with an error response.  A node rejecting the
   update MUST reply with an error message indicating the error reason .

   The request/response message processing and forwarding is executed as
   defined in the particular sections.  The local procedures on how to
   update the MRI in the firewall/NAT is out of scope of this memo.










































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4.  NATFW NSLP Message Components

   A NATFW NSLP message consists of a NSLP header and one or more
   objects following the header.  The NSLP header is common for all
   NSLPs and objects are Type-Length-Value (TLV) encoded using big
   endian (network ordered) binary data representations.  Header and
   objects are aligned to 32 bit boundaries and object lengths that are
   not multiples of 32 bits must be padded to the next higher 32 bit
   multiple.

   The whole NSLP message is carried as payload of a NTLP message.

   Note that the notation 0x is used to indicate hexadecimal numbers.

4.1  NSLP Header

   The NSLP header is common to all NSLPs and is the first part of all
   NSLP messages.  It contains two fields, the NSLP message type and a
   reserved field.  The total length is 32 bits.  The layout of the NSLP
   header is defined by Figure 24.



      0                               16                            31
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   NSLP message type           |       reserved                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                       Figure 24: Common NSLP header

   The reserved field MUST be set to zero in the NATFW NSLP header
   before sending and MUST be ignored during processing of the header.
   Note that other NSLPs use this field as a flag field.

4.2  NSLP Message Types

   The message types identify requests and responses.  Defined messages
   types are:

   o  0x0101 : CREATE

   o  0x0102 : RESERVE-EXTERNAL-ADDRESS(REA)

   o  0x0104 : REA-F





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   o  0x0201 : RESPONSE

   o  0x0301 : NOTIFY


4.3  NSLP Objects

   NATFW NSLP objects use a common header format defined by Figure 25.
   The object header contains two fields, the NSLP object type and the
   object length.  Its total length is 32 bits.



      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |A|B|r|r|   Object Type         |r|r|r|r|   Object Length       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                   Figure 25: Common NSLP object header

   The length is the total length of the object without the object
   header.  The unit is a word, consisting of 4 octets.  The particular
   values of type and length for each NSLP object are listed in the
   subsequent sections that define the NSLP objects.  The two leading
   bits of the NSLP object header are used to signal the desired
   treatment for objects whose treatment has not been defined in this
   memo (see [1], Section A.2.1), i.e., the Object Type has not been
   defined.  NATFW NSLP uses a subset of the categories defined in GIST:

   o  AB=00 ("Mandatory"): If the object is not understood, the entire
      message containing it must be rejected with an error indication.

   o  AB=01 ("Optional"): If the object is not understood, it should be
      deleted and then the rest of the message processed as usual.

   o  AB=10 ("Forward"): If the object is not understood, it should be
      retained unchanged in any message forwarded as a result of message
      processing, but not stored locally.

   The combination AB=11 MUST NOT be used.

   The following sections do not repeat the common NSLP object header,
   they just state the type and the length.





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4.3.1  Session Lifetime Object

   The session lifetime object carries the requested or granted lifetime
   of a NATFW NSLP session measured in seconds.  The Message refresh
   rate value is set by default to 0xFFFF and only set to a specific
   value when an intermediate node changes the message lifetime and
   informs the upstream node about the recommended message refresh rate.

      Type:   NATFW_LT

      Length: 2



     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  NATFW NSLP session lifetime                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  NATFW NSLP message refresh rate              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                        Figure 26: Lifetime object


4.3.2  External Address Object

   The external address object can be included in RESPONSE messages
   (Section 4.4.3) only.

      Type:   NATFW_EXT_IPv4

      Length: 2



     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         port number           |           reserved            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           IPv4 address                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



           Figure 27: External Address Object for IPv4 addresses






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      Type:   NATFW_EXT_IPv6

      Length: 5



     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         port number           |          reserved             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                          IPv6 address                         +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



           Figure 28: External Address Object for IPv6 addresses

   Please note that the field 'port number' MUST be set to 0 if only an
   IP address has been reserved, for instance, by a traditional NAT.  A
   port number of 0 MUST be ignored in processing this object.

4.3.3  Extended Flow Information Object

   In general, flow information is kept in the message routing
   information (MRI) of the NTLP.  Nevertheless, some additional
   information may be required for NSLP operations.  The 'extended flow
   information' object carries this additional information about the
   policy rule's action for firewalls/NATs.

      Type:   NATFW_EFI

      Length: 1



     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           rule action         |           sub_ports           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                   Figure 29: Extended Flow Information




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   This object has two fields, 'rule action' and 'sub_ports'.  The 'rule
   action' field has these meanings:

   o  Allow: A policy rule with this action allows data traffic to
      traverse the middlebox and the NATFW NSLP MUST allow NSLP
      signaling to be forwarded.

   o  Deny: A policy rule with this action blocks data traffic from
      traversing the middlebox and the NATFW NSLP MUST NOT allow NSLP
      signaling to be forwarded.

   o  Accept: A policy rule with this action blocks data traffic from
      traversing the middlebox and the NATFW NSLP MUST allow NSLP
      signaling to be forwarded.

   The 'sub_ports' field contains the number of subsequent transport
   layer ports.  The default value of this field is 0, i.e., only the
   port specified in the NTLP's MRM is used for the policy rule.  A
   value of 1 indicates that additionally to the port specified in the
   NTLP's MRM (port1), a second port (port2) is used.  This port's value
   is calculated as: port2 = port1 + 1.  Other values than 0 or 1 MUST
   NOT be used in this field, but further version of this memo may allow
   other values.  This two subsequent port numbers feature can be used
   by legacy voice over IP equipment.  This legacy equipment assumes two
   subsequent port numbers for its RTP/RTCP flows.

4.3.4  Response Code Object

   This object carries the response code, which may be indications for
   either a successful request or failed request depending on the value
   of the 'response code' field.

      Type:   NATFW_RESPONSE

      Length: 1



     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         response code                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                      Figure 30: Response Code Object

   TBD: Define response classes, success codes and error codes.
   Possible error classes are:



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   o  Policy rule errors

   o  Authentication and Authorization errors

   o  NAT

   Currently errors defined in this memo are:

   o  lifetime too big

   o  lifetime not acceptable

   o  no NAT here

   o  no reservation found

   o  requested external address from outside

   o  re-authorization needed

   o  routing change detected


4.3.5  Proxy Support Object

   This object indicates that proxy mode support is required.  Either in
   a REA message or CREATE message.

      Type:   NATFW_PROXY

      Length: 0


4.3.6  Nonce Object

      Type:   NATFW_NONCE

      Length: 1



     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         nonce                                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                          Figure 31: Nonce Object




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4.3.7  Message Sequence Number Object

   This object carries the MSN value as described in Section 3.6.

      Type:   NATFW_RESP_MSN

      Length: 1



     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    message sequence number                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                 Figure 32: Message Sequence Number Object


4.3.8  Data Terminal Information Object

   The 'data terminal information' object carries additional information
   possibly needed during REA operations.  REA messages are transported
   by the NTLP using the Loose-End message routing method (LE-MRM).  The
   LE-MRM contains only DR's IP address and a signaling destination
   address (destination address).  This destination address is used for
   message routing only and is not necessarily reflecting the address of
   the data sender.  This object contains information about (if
   applicable) DR's port number (the destination port number), DS' port
   number (the source port number), the used transport protocol, the
   prefix length of the IP address, and DS' IP address.

      Type:   NATFW_DSINFO_IPv4

      Length: 3



     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |P|        reserved             |   dest prefix |    protocol   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      dst port number          |      src port number          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       src IPv4 address                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+







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               Figure 33: Data Terminal IPv4 Address Object

      Type:   NATFW_DSINFO_IPv6

      Length: 6



     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |P|        reserved             |   dest prefix |    protocol   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      dst port number          |      src port number          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                      src IPv6 address                         +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



      Figure 34: Data Terminal IPv6 Address Object for IPv6 addresses

   The fields MUST be interpreted according these rules:

   o  dest prefix: This parameter indicates the prefix length of the
      'src IP address' in bits.  For instance, a full IPv4 address
      requires 'dest prefix' to be set to 32.  A value of 0 indicates an
      IP address wildcard.

   o  protocol: The IPv4 protocol field or the last IPv6 header.  This
      field MUST be interpreted if P=1, otherwise it MUST be set to 0
      and MUST be ignored.

   o  dst port number: A value of 0 indicates a port wildcard, i.e., the
      destination port number is not known.  Any other value indicates
      the destination port number.

   o  src port number: A value of 0 indicates a port wildcard, i.e., the
      source port number is not known.  Any other value indicates the
      source port number.

   o  src IPv4/IPv6 address: The source IP address of the data sender.
      This field MUST be set to zero if no IP address is provided, i.e.,
      a complete wildcard is desired (see dest prefix field above).



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4.3.9  Trace Object

   la

      Type:   NATFW_TRACE

      Length: Variable



     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         trace type            |           hop count           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                           IP address                          :
     :              ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



           Figure 35: External Address Object for IPv4 addresses

   The NATFW_TRACE object may contain zero or more identifiers.  The
   type of identifier is given by the value of 'trace type' field.  This
   memo is defining the values for the 'trace type' field: 0x01 for IPv4
   addresses and 0x02 for IPv6 addresses.  The 'hop count' field counts
   the total number of visited NATFW NSLP nodes that are willing to
   include their identifier in this object.  Each node is appending its
   identifier at the end of the object.

4.4  Message Formats

   This section defines the content of each NATFW NSLP message type.
   The message types are defined in Section 4.2.  First, the request
   messages are defined with their respective objects to be included in
   the message.  Second, the response messages are defined with their
   respective objects to be included.

   Basically, each message is constructed of NSLP header and one or more
   NSLP objects.  The order of objects is not defined, meaning that
   objects may occur in any sequence.  Objects are marked either with
   mandatory [M] or optional [O].  Where [M] implies that this
   particular object MUST be included within the message and where [O]
   implies that this particular object is OPTIONAL within the message.

   Each section elaborates the required settings and parameters to be
   set by the NSLP for the NTLP, for instance, how the message routing
   information is set.




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4.4.1  CREATE

   The CREATE request message is used to create NSLP sessions and to
   create policy rules.  Furthermore, CREATE messages are used to
   refresh sessions and to delete them.

   The CREATE message carries these objects:

   o  Lifetime object [M]

   o  Extended flow information object [M]

   o  Message sequence number object [M]

   o  Proxy support object [O]

   o  Nonce object [M if CREATE-PROXY message, otherwise O]

   The message routing information in the NTLP MUST be set to DS as
   source address and DR as destination address.  All other parameters
   MUST be set according the required policy rule.  CREATE messages MUST
   be transported by using the path-coupled MRM.

4.4.2  RESERVE-EXTERNAL-ADDRESS (REA)

   The RESERVE-EXTERNAL-ADDRESS (REA) request message is used to a)
   reserve an external IP address/port at NATs, b) to notify firewalls
   about NSIS capable DRs, or c) to block incoming data traffic at
   upstream firewalls.  All case can be used in proxy mode operations.

   The REA for NATs (case a)) request message carries these objects:

   o  Lifetime object [M]

   o  Message sequence number object [M]

   o  Data terminal information object [M]

   o  Proxy support object [O]

   o  Nonce object [M if proxy support object is included, otherwise O]

   The REA for NATs message needs special NTLP treatment.  First of all,
   REA for NATs messages travel the wrong way, from the DR towards DS.
   Second, the DS' address used during the signaling may be not the
   actual DS (see Section 3.8).  REA for NATs messages MUST be
   transported by using the loose-end MRM defined in Section 5.8.2. of
   [1].



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   The REA for firewalls (case b) and c)) request message carries these
   objects:

   o  Lifetime object [M]

   o  Message sequence number object [M]

   o  Extended flow information object [M]

   o  Proxy support object [O]

   o  Nonce object [M if proxy support object is included, otherwise O]

   The REA for firewalls message needs path-coupled MRM NTLP treatment
   but with messages being sent upstream towards the DS.

4.4.3  RESPONSE

   RESPONSE messages are responses to CREATE, REA, and REA-F messages.

   The RESPONSE message carries these objects:

   o  Lifetime object [M]

   o  Message sequence number object [M]

   o  Response code object [M]

   o  External address object [O]([M] for success responses to REA)

   This message is routed upstream hop-by-hop.

   EDITOR's note:  Text says that this section is defining the behavior
   depending on the response type.

4.4.4  NOTIFY

   The NOTIFY messages is used to report asynchronous events happening
   along the signaled path to other NATFW NSLP nodes.

   The NOTIFY message carries this object:

   o  Response code object with NOTIFY code [M].

   The NOTIFY message is forwarded upstream hop-by-hop using the
   existing upstream node messaging association entry within the node's
   Message Routing State table.




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4.4.5  REA-F

   The REA-F request message is used to install policy rules at upstream
   firewalls

   The REA-F request message carries these objects:

   o  Lifetime object [M]

   o  Message sequence number object [M]

   o  Extended flow information object [M]

   o  Proxy support object [O]

   REA-F messages MUST be sent by using the path-coupled MRM upstream
   towards the data sender's address.

4.4.6  TRACE

   The TRACEF request message is used to trace the involved NATFW NSLP
   nodes along a signal session.

   The TRACE request message carries these objects:

   o  Message sequence number object [M]

   TRACE request messages are sent path-coupled (PC-MRM).























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5.  NATFW NSLP NTLP Requirements

   The NATFW NSLP requires the following capabilities from the NTLP:

   o  Ability to detect that the NSIS Responder does not support NATFW
      NSLP.  This capability is key to launching the proxy mode behavior
      as described in Section 3.4.7 and [15].

   o  Detection of NATs and their support of the NSIS NATFW NSLP.  If
      the NTLP discovers that the NSIS host is behind an NSIS aware NAT,
      the NR will send REA messages to the opportunistic address.  If
      the NTLP discovers that the NSIS host is behind a NAT that does
      not support NSIS then the NSIS host will need to use a separate
      NAT traversal mechanism.

   o  Message origin authentication and message integrity protection

   o  Detection of routing changes

   o  Protection against malicious announcement of fake path changes,
      this is needed to mitigate a threat discussed in Section 7 of [8]






























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6.  NSIS NAT and Firewall Transition Issues

   NSIS NAT and firewall transition issues are premature and will be
   addressed in a separate draft (see [15]).  An update of this section
   will be based on consensus.














































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7.  Security Considerations

   Security is of major concern particularly in case of firewall
   traversal.  This section provides security considerations for the
   NAT/firewall traversal and is organized as follows:

   Section 7.1 describes the framework assumptions with regard to the
   assumed trust relationships between the participating entities.  This
   subsection also motivates a particular authorization model.

   Security threats that focus on NSIS in general are described in [8]
   and they are applicable to this document.  Within Section 7.2 we
   extend this threat investigation by considering NATFW NSLP specific
   threats.  Based on the security threats we list security
   requirements.

   Finally we illustrate how the security requirements that were created
   based on the security threats can be fullfilled by specific security
   mechanisms.  These aspects will be elaborated in Section 7.3.

7.1  Trust Relationship and Authorization

   The NATFW NSLP is a protocol which may involve a number of NSIS nodes
   and is, as such, not a two-party protocol.  This fact requires more
   thoughts about scenarios, trust relationships and authorization
   mechanisms.  Trust relationships and authorization are very important
   for the protocol machinery and they are closely related to each other
   in the sense that a certain degree of trust is required to authorize
   a particular action.  For any action (e.g. create/delete pinholes),
   authorization is very important due to the nature of middleboxes.

   Different types of trust relationships may affect different
   categories of middleboxes.  As explained in [23], establishment of a
   financial relationship is typically very important for QoS signaling,
   whereas financial relationships are less directly of interest for
   NATFW middlebox signaling.  It is therefore not particularly
   surprising that there are differences in the nature and level of
   authorization likely to be required in a QoS signaling environment
   and in NATFW middlebox signaling.  Typically NATFW signaling requires
   authorization to configure firewalls or to modify NAT bindings.  The
   outcome of the authorization is either allowed or disallowed whereas
   QoS signaling might just indicate that a lower QoS reservation is
   allowed.

   Different trust relationships that appear in middlebox signaling
   environments are described in the subsequent sub-sections.  As a
   comparison with other NSIS signaling application it might be
   interesting to mention that QoS signaling relies on peer-to-peer



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   trust relationships and authorization between neighboring nodes or
   neighboring networks.  These type of trust relationships turn out to
   be simpler for a protocol.  However, there are reasons to believe
   that this is not the only type of trust relationship found in today's
   networks.

7.1.1  Peer-to-Peer Trust Relationship

   Starting with the simplest scenario, it is assumed that neighboring
   nodes trust each other.  The required security association to
   authenticate and to protect a signaling message is either available
   (after manual configuration), or has been dynamically established
   with the help of an authentication and key exchange protocol.  If
   nodes are located closely together, it is assumed that security
   association establishment is easier than establishing it between
   distant nodes.  It is, however, difficult to describe this
   relationship generally due to the different usage scenarios and
   environments.  Authorization heavily depends on the participating
   entities, but for this scenario, it is assumed that neighboring
   entities trust each other (at least for the purpose of policy rule
   creation, maintenance, and deletion).  Note that Figure 36 does not
   illustrate the trust relationship between the end host and the access
   network.

   +------------------------+              +-------------------------+
   |Network A               |              |                Network B|
   |              +---------+              +---------+               |
   |        +-///-+ Middle- +---///////----+ Middle- +-///-+         |
   |        |     |  box 1  |   Trust      |  box 2  |     |         |
   |        |     +---------+ Relationship +---------+     |         |
   |        |   Trust       |              |      Trust    |         |
   |        | Relationship  |              |  Relationship |         |
   |        |               |              |               |         |
   |     +--+---+           |              |            +--+---+     |
   |     | Host |           |              |            | Host |     |
   |     |  A   |           |              |            |  B   |     |
   |     +------+           |              |            +------+     |
   +------------------------+              +-------------------------+

                Figure 36: Peer-to-Peer Trust Relationship


7.1.2  Intra-Domain Trust Relationship

   In larger corporations, often more than one middlebox is used to
   protect or serve different departments.  In many cases, the entire
   enterprise is controlled by a security department, which gives
   instructions to the department administrators.  In such a scenario, a



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   peer-to-peer trust-relationship might be prevalent.  Sometimes it
   might be necessary to preserve authentication and authorization
   information within the network.  As a possible solution, a
   centralized approach could be used, whereby an interaction between
   the individual middleboxes and a central entity (for example a policy
   decision point - PDP) takes place.  As an alternative, individual
   middleboxes could exchange the authorization decision with another
   middlebox within the same trust domain.  Individual middleboxes
   within an administrative domain should exploit their trust
   relationship instead of requesting authentication and authorization
   of the signaling initiator again and again.  Thereby complex protocol
   interactions are avoided.  This provides both a performance
   improvement without a security disadvantage since a single
   administrative domain can be seen as a single entity.  Figure 37
   illustrates a network structure which uses a centralized entity.

    +-----------------------------------------------------------+
    |                                               Network A   |
    |                      +---------+                +---------+
    |      +----///--------+ Middle- +------///------++ Middle- +---
    |      |               |  box 2  |                |  box 2  |
    |      |               +----+----+                +----+----+
    | +----+----+               |                          |    |
    | | Middle- +--------+      +---------+                |    |
    | |  box 1  |        |                |                |    |
    | +----+----+        |                |                |    |
    |      |             |           +----+-----+          |    |
    |      |             |           | Policy   |          |    |
    |   +--+---+         +-----------+ Decision +----------+    |
    |   | Host |                     | Point    |               |
    |   |  A   |                     +----------+               |
    |   +------+                                                |
    +-----------------------------------------------------------+

                Figure 37: Intra-domain Trust Relationship


7.1.3  End-to-Middle Trust Relationship

   In some scenarios, a simple peer-to-peer trust relationship between
   participating nodes is not sufficient.  Network B might require
   additional authorization of the signaling message initiator.  If
   authentication and authorization information is not attached to the
   initial signaling message then the signaling message arriving at
   Middlebox 2 would result in an error message being created, which
   indicates the additional authorization requirement.  In many cases
   the signaling message initiator is already aware of the additionally
   required authorization before the signaling message exchange is



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   executed.  Replay protection is a requirement for authentication to
   the non-neighboring middlebox, which might be difficult to accomplish
   without adding additional roundtrips to the signaling protocol (e.g.,
   by adding a challenge/response type of message exchange).

   Figure 38 shows the slightly more complex trust relationships in this
   scenario.

    +--------------------+              +---------------------+
    |          Network A | Trust        |Network B            |
    |                    | Relationship |                     |
    |          +---------+              +---------+           |
    |    +-///-+ Middle- +---///////----+ Middle- +-///-+     |
    |    |     |  box 1  |      +-------+  box 2  |     |     |
    |    |     +---------+      |       +---------+     |     |
    |    |Trust          |      |       |   Trust       |     |
    |    |Relationship   |      |       |   Relationship|     |
    |    |               |      |       |               |     |
    | +--+---+           |      |       |            +--+---+ |
    | | Host +----///----+------+       |            | Host | |
    | |  A   |           |Trust         |            |  B   | |
    | +------+           |Relationship  |            +------+ |
    +--------------------+              +---------------------+


                Figure 38: End-to-Middle Trust Relationship


7.2  Security Threats and Requirements

   This section describes NATFW specific security threats and
   requirements.

7.2.1  Attacks related to authentication and authorization

   The NSIS message which installs policy rules at a middlebox is the
   CREATE message.  The CREATE message travels from the Data Sender (DS)
   toward the Data Receiver (DR).  The packet filter or NAT binding is
   marked as pending by the middleboxes along the path.  If it is
   confirmed with a success RESPONSE message from the DR, the requested
   policy rules on the middleboxes are installed to allow the traversal
   of a data flow.









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    +-----+               +-----+               +-----+
    | DS  |               | MB  |               | DR  |
    +-----+               +-----+               +-----+
       |                     |                     |
       |       CREATE        | CREATE              |
       |-------------------->+-------------------->|
       |                     |                     |
       |   Succeeded/Error   |   Succeeded/Error   |
       |<--------------------+<--------------------|
       |                     |                     |
        ==========================================>
                      Direction of data traffic

                          Figure 39: CREATE Mode

   In this section we will consider some simple scenarios for middlebox
   configuration:

   o  Data Sender (DS) behind a firewall

   o  Data Sender (DS) behind a NAT

   o  Data Receiver (DR) behind a firewall

   o  Data Receiver (DR) behind a NAT

   A real-world scenario could include a combination of these firewall/
   NAT placements, such as, a DS and/or a DR behind a chain of NATs and
   firewalls.

   Figure 40 shows one possible scenario:




















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        +-------------------+                  +--------------------+
        |    Network A      |                  |       Network B    |
        |                   |                  |                    |
        |    +-----+        |    //-----\\     |        +-----+     |
        |    | MB2 |--------+----|  INET  |----+--------| MB3 |     |
        |    +-----+        |    \\-----//     |        +-----+     |
        |       |           |                  |           |        |
        |    +-----+        |                  |        +-----+     |
        |    | MB1 |        |                  |        | MB4 |     |
        |    +-----+        |                  |        +-----+     |
        |       |           |                  |           |        |
        |    +-----+        |                  |        +-----+     |
        |    | DS  |        |                  |        | DR  |     |
        |    +-----+        |                  |        +-----+     |
        +-------------------+                  +--------------------+

        MB: Middle box (NAT or firewall or a combination)
        DS: Data Sender
        DR: Data Receiver

                Figure 40: Several middleboxes per network


7.2.1.1  Data Sender (DS) behind a firewall

           +------------------------------+
           |                              |
           |   +-----+     create      +-----+
           |   | DS  | --------------> | FW  |
           |   +-----+                 +-----+
           |                              |
           +------------------------------+

   DS sends a CREATE message to request the traversal of a data flow.

   It is up to network operators to decide how far they can trust users
   inside their networks.  However, there are several reasons why they
   should not.

   The following attacks are possible:

   o  DS could open a firewall pinhole with a source address different
      from its own host.

   o  DS could open firewall pinholes for incoming data flows that are
      not supposed to enter the network.





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   o  DS could request installation of any policy rules and allow all
      traffic go through.

   SECURITY REQUIREMENT: The middlebox MUST authenticate and authorize
      the neighboring NAT/FW NSLP node which requests an action.
      Authentication and authorization of the initiator SHOULD be
      provided to NATs and firewalls along the path.


7.2.1.2  Data Sender (DS) behind a NAT

   The case 'DS behind a NAT' is analogous to the case 'DS behind a
   firewall'.

   Figure 42 illustrates such a scenario:

                   +------------------------------+
                   |                              |
                   |   +------+     CREATE        |
                   |   | NI_1 | ------\         +-----+ CREATE  +-----+
                   |   +------+        \------> | NAT |-------->| MB  |
                   |                            +-----+         +-----+
                   |   +------+                   |
                   |   | NI_2 |                   |
                   |   +------+                   |
                   +------------------------------+

                    Figure 42: Several NIs behind a NAT

   In this case the middlebox MB does not know who is the NSIS Initiator
   since both NI_1 and NI_2 are behind a NAT (which is also NSIS aware).
   Authentication needs to be provided by other means such as the NSLP
   or the application layer.

   SECURITY REQUIREMENT: The middlebox MUST authenticate and ensure that
      the neighboring NAT/FW NSLP node is authorized to request an
      action.  Authentication and authorization of the initiator (which
      is the DR in this scenario) to the middleboxes (via another NSIS
      aware middlebox) SHOULD be provided.


7.2.1.3  Data Receiver (DR) behind a firewall

   In this case a CREATE message comes from an entity DS outside the
   network towards the DR inside the network.






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                                 +------------------------------+
                                 |                              |
       +-----+    CREATE      +-----+    CREATE      +-----+    |
       | DS  | -------------> | FW  | -------------> | DR  |    |
       +-----+ <------------- +-----+ <------------- +-----+    |
               success RESPONSE  |     success RESPONSE         |
                                 |                              |
                                 +------------------------------+

   Since policy rules at middleboxes must only be installed after
   receiving a successful response it is necessary that the middlebox
   waits until the Data Receiver DR confirms the request of the Data
   Sender DS with a success RESPONSE message.  This is, however, only
   necessary

   o  if the action requested with the CREATE message cannot be
      authorized and

   o  if the middlebox is still forwarding the signaling message towards
      the end host (without state creation/deletion/modification).

   This confirmation implies that the data receiver is expecting the
   data flow.

   At this point we differentiate two cases:

   1.  DR knows the IP address of the DS (for instance because of some
       previous application layer signaling) and is expecting the data
       flow.

   2.  DR might be expecting the data flow (for instance because of some
       previous application layer signaling) but does not know the IP
       address of the Data Sender DS.

   For the second case, Figure 44 illustrates a possible attack: an
   adversary Mallory M could be sniffing the application layer signaling
   and thus knows the address and port number where DR is expecting the
   data flow.  Thus it could pretend to be DS and send a CREATE message
   towards DR with the data flow description (M -> DR).  Since DR does
   not know the IP address of DS, it is not able to recognize that the
   request is coming from the "wrong guy".  It will send a success
   RESPONSE message back and the middlebox will install policy rules
   that will allow Mallory M to inject its data into the network.








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                    Application Layer signaling
              <------------------------------------>
             /                                      \
            /                      +-----------------\------------+
           /                       |                  \           |
       +-----+                  +-----+                +-----+    |
       | DS  |              ->  | FW  |                | DR  |    |
       +-----+             /    +-----+                +-----+    |
                  CREATE  /       |                               |
       +-----+           /        +-------------------------------+
       | M   |----------
       +-----+

             Figure 44: DR behind a firewall with an adversary

   Network administrators will probably not rely on a DR to check the IP
   address of the DS.  Thus we have to assume the worst case with an
   attack such as in Figure 44.  Many operators might not allow NSIS
   signaling message to traverse the firewall in Figure 44 without
   proper authorization.  In this case the threat is not applicable.

   SECURITY REQUIREMENT: A binding between the application layer and the
      NSIS signaling SHOULD be provided.


7.2.1.4  Data Receiver (DR) behind a NAT

   When a data receiver DR behind a NAT sends a RESERVE-EXTERNAL-ADDRESS
   (REA) message to get a public reachable address that can be used as a
   contact address by an arbitrary data sender if the DR was unable to
   restrict the future data sender.  The NAT reserves an external
   address and port number and sends them back to DR.  The NAT adds an
   address mapping entry in its reservation list which links the public
   and private addresses as follows:

   (DR_ext <=> DR_int) (*).

   The NAT sends a RESPONSE message with the external address' object
   back to the DR with the address DR_ext.  DR informs DS about the
   public address that it has recently received, for instance, by means
   of application layer signaling.

   When a data sender sends a CREATE message towards DR_ext then the
   message will be forwarded to the DR.  The data sender might want to
   update the NAT binding stored at the edge-NAT to make it more
   restrictive.

   We assume that the adversary Mallory M obtains the contact address



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   (i.e., external address and port) allocated at the NAT possibly by
   eavesdropping on the application layer signaling and sends a CREATE
   message.  As a consequence Mallory would be able to communicate with
   DR (if M is authorized by the edge-NAT and if the DR accepts CREATE
   and returns a RESPONSE.


                    Application Layer signaling
              <------------------------------------>
             /                                      \
            /                      +-----------------\------------+
           /                       |       REA        \           |
       +-----+                  +-----+  <-----------  +-----+    |
       | DS  |              ->  | NAT |  ----------->  | DR  |    |
       +-----+             /    +-----+  rtn_ext_addr  +-----+    |
                  CREATE  /       |                               |
       +-----+           /        +-------------------------------+
       | M   |----------
       +-----+

   SECURITY REQUIREMENT: The DR MUST be able to specify which data
      sender are allowed to traverse the NAT in order to be forwarded to
      DRs address.


7.2.1.5  NSLP Message Injection

   Malicious hosts, located either off-path or on-path, could inject
   arbitrary NATFW NSLP messages into the signaling path, causing
   several problems.  These problems apply when no proper authorization
   and authentication scheme is available.

   By injecting a bogus CREATE message with lifetime set to zero, a
   malicious host could try to teardown NATFW NSLP session state
   partially or completely on a data path, causing a service
   interruption.

   By injecting a bogus responses or NOTIFY message, for instance,
   timeout, a malicious host could try to teardown NATFW NSLP session
   state as well.  This could affect the data path partially or totally,
   causing a service interruption.

   SECURITY REQUIREMENT: Messages, such as TRIGGER, can be misused by
      malicious hosts, and therefore need to be authorized.







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7.2.2  Denial-of-Service Attacks

   In this section we describe several ways how an adversary could
   launch a Denial of service (DoS) attack on networks running NSIS for
   middlebox configuration to exhaust their resources.

7.2.2.1  Flooding with CREATE messages from outside

7.2.2.1.1  Attacks due to NSLP state

   A CREATE message requests the NSLP to store state information such as
   a NAT binding or a policy rule.

   The policy rules requested in the CREATE message will be installed at
   the arrival of a confirmation from the Data Receiver with a success
   RESPONSE message.  A successful RESPONSE message includes the session
   ID.  So the NSLP looks up the NSIS session and installs the requested
   policy rules.

   An adversary from outside could launch a DoS attack with arbitrary
   CREATE messages.  For each of these messages the middlebox needs to
   store state information such as the policy rules to be loaded, i.e.,
   the middlebox could run out of memory.  This kind of attack is also
   mentioned in [8] Section 4.8.

   SECURITY REQUIREMENT: A NAT/FW NSLP node MUST authorize the 'create-
      session' message before storing state information.


7.2.2.1.2  Attacks due to authentication complexity

   This kind of attack is possible if authentication is based on
   mechanisms that require computing power, for example, digital
   signatures.

   For a more detailed treatment of this kind of attack, the reader is
   encouraged to see [8].

   SECURITY REQUIREMENT: A NAT/FW NSLP node MUST NOT introduce new
      denial of service attacks based on authentication or key
      management mechanisms.


7.2.2.1.3  Attacks to the endpoints

   The NATFW NSLP requires firewalls to forward NSLP messages, a
   malicious node may keep sending NSLP messages to a target.  This may
   consume the access network resources of the victim, drain the battery



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   of the victim's terminal and may force the victim to pay for the
   received although undesired data.

   This threat may be more particularly be relevant in networks where
   access link is a limited resource, for instance in cellular networks,
   and where the terminal capacities are limited.

   SECURITY REQUIREMENT: A NATFW NSLP aware firewall or NAT MUST be able
      to block unauthorized signaling message, if this threat is a
      concern.


7.2.2.2  Flooding with REA messages from inside

   Although we are more concerned with possible attacks from outside the
   network, we need also to consider possible attacks from inside the
   network.

   An adversary inside the network could send arbitrary RESERVE-
   EXTERNAL-ADDRESS messages.  At a certain point the NAT will run out
   of port numbers and the access for other users to the outside will be
   disabled.

   SECURITY REQUIREMENT: The NAT/FW NSLP node MUST authorize state
      creation for the RESERVE-EXTERNAL-ADDRESS message.  Furthermore,
      the NAT/FW NSLP implementation MUST prevent denial of service
      attacks involving the allocation of an arbitrary number of NAT
      bindings or the installation of a large number of packet filters.


7.2.3  Man-in-the-Middle Attacks

   Figure 46 illustrates a possible man-in-the-middle attack using the
   RESERVE-EXTERNAL-ADDRESS (REA) message.  This message travels from DR
   towards the public Internet.  The message might not be intercepted
   because there are no NSIS aware middleboxes.

   Imagine such an NSIS signaling message is then intercepted by an
   adversary Mallory (M).  M returns a faked RESPONSE message whereby
   the adversary pretends that a NAT binding was created.  This NAT
   binding is returned with the RESPONSE message.  Malory might insert
   it own IP address in the response, the IP address of a third party or
   the address of a black hole.  In the first case, the DR thinks that
   the address of Mallory M is its public address and will inform the DS
   about it.  As a consequence, the DS will send the data traffic to
   Mallory M.

   The data traffic from the DS to the DR will re-directed to Mallory M.



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   M will be able to read, modify or block the data traffic (if the end-
   to-end communication itself does not experience protection).
   Eavesdropping and modification is only possible if the data traffic
   is itself unprotected.


     +-----+          +-----+               +-----+
     | DS  |          |  M  |               | DR  |
     +-----+          +-----+               +-----+
        |                |                     |
        |                |       REA           |
        |                | <------------------ |
        |                |                     |
        |                |      RESPONSE       |
        |                | ------------------> |
        |                |                     |
        |  data traffic  |                     |
        |===============>|        data traffic |
        |                |====================>|

     Figure 46: MITM attack using the RESERVE-EXTERNAL-ADDRESS message

   SECURITY REQUIREMENT: Mutual authentication between neighboring NATFW
      NSLP MUST be provided.  To ensure that only legitimate nodes along
      the path act as NSIS entities the initiator MUST authorize the
      responder.  In the example in Figure 46 the firewall FW must
      perform an authorization with the neighboring entities.


7.2.4  Message Modification by non-NSIS on-path node

   An unauthorized on-path node along the path towards the destination
   could easily modify, inject or just drop an NSIS message.  It could
   also hijack or disrupt the communication.

   SECURITY REQUIREMENT: Message integrity, replay protection and data
      origin authentication between neighboring NAT/FW NSLPs MUST be
      provided.


7.2.5  Message Modification by malicious NSIS node

   Message modification by a NSIS node that became malicious is more
   serious.  An adversary could easily create arbitrary pinholes or NAT
   bindings.  For example:






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   o  NATs need to modify the source/destination of the data flow in the
      'create session' message.

   o  Each middlebox along the path may change the requested lifetime in
      the CREATE message to fit their needs and/or local policy.

   SECURITY REQUIREMENT: None.  Malicious NSIS NATs and firewalls will
      not be addressed.


7.2.6  Session Modification/Deletion

   The Session ID is included in signaling messages as a reference to
   the established state.  If an adversary is able to obtain the Session
   Identifier for example by eavesdropping on signaling messages, it
   would be able to add the same Session Identifier to a new signaling
   message and effect some modifications.

   Consider the scenario described in Figure 47.  Here an adversary
   pretends to be 'DS in mobility'.  The signaling messages start from
   the DS and go through a series of routers towards the DR.  We assume
   that an off-path adversary is connected to one of the routers along
   the old path (here Router 3).  We also assume that the adversary
   knows the Session ID of the NSIS session initiated by the DS.
   Knowing the Session ID, the adversary now sends signaling messages
   towards the DR.  When the signaling message reaches Router3 then
   existing state information can be modified or even deleted.  The
   adversary can modify or delete the established reservation causing
   unexpected behavior for the legitimate user.  The source of the
   problem is that the Router 3 (cross-over router) is unable to decide
   whether the new signaling message was initiated from the owner of the
   session.  In this scenario, the adversary need not even be located in
   the DS-DR path.  This problem and the solution approaches are
   described in more detail in [25].

















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                                   Session ID(SID-x)
                                          +--------+         +--------+
                        +-------->--------+ Router +-------->+   DR   |
       Session ID(SID-x)|                 |   4    |         |        |
                    +---+----+            +--------+         +--------+
                    | Router |
             +------+   3    +*******
             |      +---+----+      *
             |                      *
             | Session ID(SID-x)    * Session ID(SID-x)
         +---+----+             +---+----+
         | Access |             | Access |
         | Router |             | Router |
         |   1    |             |   2    |
         +---+----+             +---+----+
             |                      *
             | Session ID(SID-x)    * Session ID(SID-x)
        +----+------+          +----+------+
        |    DS     |          | Adversary |
        |           |          |           |
        +-----------+          +-----------+

            Figure 47: State Modification by off-path adversary

   As a summary, an off-path adversary's knowledge of Session-ID could
   cause session modification/deletion.

   SECURITY REQUIREMENT: The initiator MUST be able to demonstrate
      ownership of the session it wishes to modify.


7.2.6.1  Misuse of mobility in NAT handling

   Another kind of session modification is related to mobility
   scenarios.  NSIS allows end hosts to be mobile, it is possible that
   an NSIS node behind a NAT needs to update its NAT binding in case of
   address change.  Whenever a host behind a NAT initiates a data
   transfer, it is assigned an external IP and port number.  In typical
   mobility scenarios, the DR might also obtain a new address according
   to the topology and it should convey its new IP address to the NAT.
   The NAT is assumed to modify these NAT bindings based on the new IP
   address conveyed by the endhost.









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     Public                       Private Address
     Internet                     space

                   +----------+                  +----------+
        +----------|  NAT     |------------------|End host  |
                   |          |                  |          |
                   +----------+                  +----------+
                            |
                            |                    +----------+
                            \--------------------|Malicious |
                                                 |End host  |
                                                 +----------+
                         data traffic
                    <========================

               Figure 48: Misuse of mobility in NAT binding

   A NAT binding can be changed with the help of NSIS signaling.  When a
   DR moves to a new location and obtains a new IP address, it sends an
   NSIS signaling message to modify the NAT binding.  It would use the
   Session-ID and the new flow-id to update the state.  The NAT updates
   the binding and the DR continues to receive the data traffic.
   Consider the scenario in Figure 48 where an the endhost(DR) and the
   adversary are behind a NAT.  The adversary pretending that it is the
   end host could generate a spurious signaling message to update the
   state at the NAT.  This could be done for these purposes:

      Connection hijacking by redirecting packets to the attacker as in
      Figure 49

      Third party flooding by redirecting packets to arbitrary hosts

      Service disruption by redirecting to non-existing hosts


















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       +----------+        +----------+          +----------+
       |  NAT     |        |End host  |          |Malicious |
       |          |        |          |          |End host  |
       +----------+        +----------+          +----------+
            |                    |                     |
            | Data Traffic       |                     |
            |--------->----------|                     |
            |                    |      Spurious       |
            |                    | NAT binding update  |
            |---------<----------+--------<------------|
            |                    |                     |
            | Data Traffic       |                     |
            |--------->----------+-------->------------|
            |                    |                     |

                      Figure 49: Connection Hijacking

   SECURITY REQUIREMENT: A NAT/FW signaling message MUST be
      authenticated, authorized, integrity protected and replay
      protected between neighboring NAT/FW NSLP nodes.


7.2.7  Misuse of unreleased sessions

   Assume that DS (N1) initiates NSIS session with DR (N2) through a
   series of middleboxes as in Figure 50.  When the DS is sending data
   to DR, it might happen that the DR disconnects from the network
   (crashes or moves out of the network in mobility scenarios).  In such
   cases, it is possible that another node N3 (which recently entered
   the network protected by the same firewall) is assigned the same IP
   address that was previously allocated to N2.  The DS could take
   advantage of the firewall policies installed already, if the refresh
   interval time is very high.  The DS can flood the node (N3), which
   will consume the access network resources of the victim forcing it to
   pay for unwanted traffic as shown in Figure 51.  Note that here we
   make the assumption that the data receiver has to pay for receiving
   data packets.














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       Public Internet
                                         +--------------------------+
                                         |                          |
       +-------+    CREATE           +---+-----+        +-------+   |
       |       |-------------->------|         |---->---|       |   |
       |  N1   |--------------<------|   FW    |----<---|  N2   |   |
       |       |  success RESPONSE   |         |        |       |   |
       |       |==============>======|         |====>===|       |   |
       +-------+    Data Traffic     +---+-----+        +-------+   |
                                         |                          |
                                         +--------------------------+

                        Figure 50: Before mobility


    Public Internet
                                      +--------------------------+
                                      |                          |
    +-------+                     +---+-----+        +-------+   |
    |       |                     |         |        |       |   |
    |  N1   |==============>======|   FW    |====>===|  N3   |   |
    |       |    Data Traffic     |         |        |       |   |
    +-------+                     +---+-----+        +-------+   |
                                      |                          |
                                      +--------------------------+

                         Figure 51: After mobility

   Also, this threat is valid for the other direction as well.  The DS
   which is communicating with the DR may disconnect from the network
   and this IP address may be assigned to a new node that had recently
   entered the network.  This new node could pretend to be the DS and
   send data traffic to the DR in conformance with the firewall policies
   and cause service disruption.

   SECURITY REQUIREMENT: Data origin authentication is needed to
      mitigate this threat.  In order to allow firewalls to verify that
      a legitimate end host transmitted the data traffic data origin
      authentication is required.  This is, however, outside the scope
      of this document.  Hence, there are no security requirements
      imposed by this section which will be addressed by the NATFW NSLP.


7.2.8  Data Traffic Injection

   In some environments, such as enterprise networks, it is still common
   to perform authorization for access to a service based on the source
   IP address of the service requester.  There is no doubt that this by



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   itself represents a security weakness.  Hence by spoofing a
   connection, an attacker is able to reach the target machines, using
   the existing firewall rules.

   The adversary is able to inject its own data traffic in conformance
   with the firewall policies simultaneously along with the genuine DS.

   SECURITY REQUIREMENT: Since IP spoofing is a general limitation of
      non-cryptographic packet filters no security requirement needs to
      be created for the NAT/FW NSLP.  Techniques such as ingress
      filtering (described below) and data origin authentication (such
      as provided with IPsec based VPNs) can help mitigate this threat.
      This issue is, however, outside the scope of this document.

   Ingress Filtering: Consider the scenario shown in Figure 52.  In this
   scenario the DS is behind a router (R1) and a malicious node (M) is
   behind another router (R2).  The DS communicates with the DR through
   a firewall (FW).  The DS initiates NSIS signaling and installs
   firewall policies at FW.  But the malicious node is also able to send
   data traffic using DS's source address.  If R2 implements ingress
   filtering, these spoofed packets will be blocked.  But this ingress
   filtering may not work in all scenarios.  If both the DS and the
   malicious node are behind the same router, then the ingress filter
   will not be able to detect the spoofed packets as both the DS and the
   malicious node are in the same address range.


























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       +-----------------------------------+
       | +------------------+              |
       | |  +-------+   +---+---+          |
       | |  |  DS   +>--+  R1   +->+       |
       | |  |       |   |       |  |       |
       | |  +-------+   +---+---+  |       |
       | |                  |      |       |
       | +------------------+      |   +---+---+     +-------+
       |                           |   |       |     |       |
       |                           +---+  FW   +-->--|  DR   |
       | +------------------+      ****|       |*****|       |
       | |                  |      *   +---+---+     +-------+
       | |  +-------+   +---+---+  *       |
       | |  |   M   |   |  R2   |  *       |
       | |  |       |***|       |***       |
       | |  +-------+   +---+---+          |
       | +------------------+              |
       +-----------------------------------+

   ---->---- = genuine data traffic
   ********* = spoofed data traffic

                       Figure 52: Ingress filtering


7.2.9  Eavesdropping and traffic analysis

   By collecting NSLP messages, an adversary is able to learn policy
   rules for packet filters and knows which ports are open.  It can use
   this to inject its own data traffic due to the IP spoofing capability
   as already mentioned in Section 7.2.8.

   An adversary could learn authorization tokens included in CREATE
   messages and use them to launch replay-attacks or to create a session
   with its own address as source address. (cut-and-paste attack)

   As shown in Section 4.3 of [25] one possible solution for the session
   ownership problem is confidentiality protection of signaling messages

   SECURITY REQUIREMENT: The threat of eavesdropping itself does not
      mandate the usage of confidentiality protection since an adversary
      can also eavesdrop on data traffic.  In the context of a
      particular security solutions (e.g., authorization tokens) it
      might be necessary to offer confidentiality protection.
      Confidentiality protection also needs to be offered to the refresh
      period.





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7.3  Security Framework for the NAT/Firewall NSLP

   Based on the identified threats a list of security requirements has
   been created.

7.3.1  Security Protection between neighboring NATFW NSLP Nodes

   Based on the analyzed threats it is necessary to provide, between
   neighboring NATFW NSLP nodes, the following mechanism: provide

   o  data origin authentication

   o  replay protection

   o  integrity protection and

   o  optionally confidentiality protection

   To consider the aspect of authentication and key exchange the
   security mechanisms provided in [1] between neighboring nodes MUST be
   enabled when sending NATFW signaling messages.  The proposed security
   mechanisms at GIST provide support for authentication and key
   exchange in addition to denial of service protection.  Depending on
   the chosen protocol, support for flexible authentication protocols
   could be provided.  The mandatory support for security, demands the
   usage of C-MODE for the delivery of data packets and the usage of
   D-MODE only to discover the next NATFW NSLP aware node along the
   path.

7.3.2  Security Protection between non-neighboring NATFW NSLP Nodes

   Based on the security threats and the listed requirements it was
   noted that some scenarios also demand authentication and
   authorization of a NATFW signaling entity (including the initiator)
   towards a non-neighboring node.  This mechanism mainly demands entity
   authentication.  Additionally, security protection of certain
   payloads MAY be required between non-neighboring signaling entities
   and the Cryptographic Message Syntax (CMS) [19] SHOULD be used.  CMS
   can be used

   o  This might be, for example, useful to authenticate and authorize a
      user towards a middlebox and vice versa.

   o  If objects have to be protected between certain non-neighboring
      NATFW NSLP nodes.

   Details about the identifiers, replay protection and the usage of a
   dynamic key management with the help of CMS is for further study.  In



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   some scenarios it is also required to use authorization token.  Their
   purpose is to associate two different signaling protocols (e.g., SIP
   and NSIS) and their authorization decision.  These tokens are
   obtained by non-NSIS protocols, such as SIP or as part of network
   access authentication.  When a NAT or firewall along the path
   receives the token it might be verified locally or passed to the AAA
   infrastructure.

   Examples of authorization tokens or assertions can be found in RFC
   3520 [31] and RFC 3521 [32].  More recent work on authorization token
   alike mechanisms is Security Assertion Markup Language (SAML).  For
   details about SAML see [33], [34] and [35].  Figure 53 shows an
   example of this protocol interaction.  An authorization token is
   provided by the SIP proxy, which acts as the assertion generating
   entity and gets delivered to the end host with proper authentication
   and authorization.  When the NATFW signaling message is transmitted
   towards the network, the authorization token is attached to the
   signaling messages to refer to the previous authorization decision.
   The assertion verifying entity needs to process the token or it might
   be necessary to interact with the assertion granting entity using
   HTTP (or other protocols).  As a result of a successful authorization
   by a NATFW NSLP node, the requested action is executed and later a
   RESPONSE message is generated.




























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    +----------------+   Trust Relationship    +----------------+
    | +------------+ |<.......................>| +------------+ |
    | | Protocol   | |                         | | Assertion  | |
    | | requesting | |    HTTP, SIP Request    | | Granting   | |
    | | authz      | |------------------------>| | Entity     | |
    | | assertions | |<------------------------| +------------+ |
    | +------------+ |    Artifact/Assertion   |  Entity Cecil  |
    |       ^        |                         +----------------+
    |       |        |                          ^     ^|
    |       |        |                          .     || HTTP,
    |       |        |              Trust       .     || other
    |   API Access   |              Relationship.     || protocols
    |       |        |                          .     ||
    |       |        |                          .     ||
    |       |        |                          v     |v
    |       v        |                         +----------------+
    | +------------+ |                         | +------------+ |
    | | Protocol   | |  NSIS NATFW CREATE +    | | Assertion  | |
    | | using authz| |  Assertion/Artifact     | | Verifying  | |
    | | assertion  | | ----------------------- | | Entity     | |
    | +------------+ |                         | +------------+ |
    |  Entity Alice  | <---------------------- |  Entity Bob    |
    +----------------+   RESPONSE              +----------------+

                   Figure 53: Authorization Token Usage

   Threats against the usage of authorization tokens have been mentioned
   in [8] and also in  Section 7.2.  Hence, it is required to provide
   confidentiality protection to avoid allowing an eavesdropper to learn
   the token and to use it in another session (replay attack).  The
   token itself also needs to be protected against tempering.

7.3.3  End-to-End Security

   As part of the threat analysis we concluded that end-to-end security
   is not required and, if used, would be difficult to deploy.
   Furthermore, it might be difficult to use the suitable identifiers
   and to establish the necessary infrastructure for this propose.

   The only reasonable end-to-end security protection needed within NSIS
   seems to be a binding between an NSIS signaling session and
   application layer session.  This aspect is, however, for further
   study.

   In order to solicit feedback from the IETF community on some hard
   security problems for path-coupled NATFW signaling a more detailed
   description in [22] is available.




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8.  Open Issues

   The NATFW NSLP has a series of related documents discussing several
   other aspects of path-coupled NATFW signaling, including security
   [22], migration (i.e., traversal of NSIS unaware NATs) [15], intra-
   realm signaling [16], and inter-working with SIP [17].  Summaries of
   the outcomes from these documents may be added, depending on WG
   feedback, to a later version of this draft.

   A more detailed list of open issue can be found at:
   https://kobe.netlab.nec.de/roundup/nsis-natfw-nslp/index

   It is intended to add an overview figure for all NATFW NSLP building
   blocks into the next version of this memo.  Figure 54 sketches the
   overview



                                 +------------------+
                                 |Security Policies |
                                 |   Server         |
                                 +--------^---------+
                                          |
         +--------------------------------|----------------------+
         | +---------+        +-----------V----+        +-------+|
         | |firewall |<-----> |                |<------>| NAT   ||
         | |Engine   |        | Security policy|        | Engine||
         | +----^----+        | Table/Cache    |        +-^-----+|
         |      |             |             ^  |          |      |
         |      |             +---- --------|--+          |      |
         |   +--|---------------------------|-------------|--+   |
         |   |  V               NATFW NSLP  V             V  |   |
         |   |                                               |   |
         |   +-----------------------------------------------+   |
         |   +--------------------------------------------------+|
         |   |                       GIST                      ||
         |   |                                                  ||
         |   +--------------------------------------------------+|
         |   +---------+ +-------+  +------+  +-------+  +------+|
         |   | TCP     | |  UDP  |  | DCCP |  |  SCTP |  | ICMP ||
         |   +---------+ +-------+  +------+  +-------+  +------+|
         |  +-----------------------------+ +--------------------|
         |  |      IPv4                   | |     IPv6           |
         |  +-----------------------------+ +--------------------|
         +-------------------------------------------------------+


                   Figure 54: NATFW NSLP Building Blocks



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9.  Contributors

   We would like to thank the following individuals for their
   contributions to this document:

   o  Marcus Brunner and Henning Schulzrinne for work on work on IETF
      drafts which lead us to start with this document,

   o  Miquel Martin for his help on the initial version of this
      document,

   o  Srinath Thiruvengadam and Ali Fessi work for their work on the
      NAT/firewall threats draft,

   o  Elywn Davies for his help to make this document more readable,

   o  and the NSIS working group.


































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10.  References

10.1  Normative References

   [1]  Schulzrinne, H. and R. Hancock, "GIST: General Internet
        Signaling Transport", draft-ietf-nsis-ntlp-08 (work in
        progress), September 2005.

   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [3]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
        August 1996.

10.2  Informative References

   [4]   Hancock, R., Karagiannis, G., Loughney, J., and S. Van den
         Bosch, "Next Steps in Signaling (NSIS): Framework", RFC 4080,
         June 2005.

   [5]   Brunner, M., "Requirements for Signaling Protocols", RFC 3726,
         April 2004.

   [6]   Bosch, S., "NSLP for Quality-of-Service signalling",
         draft-ietf-nsis-qos-nslp-08 (work in progress), October 2005.

   [7]   Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and A.
         Rayhan, "Middlebox communication architecture and framework",
         RFC 3303, August 2002.

   [8]   Tschofenig, H. and D. Kroeselberg, "Security Threats for NSIS",
         draft-ietf-nsis-threats-06 (work in progress), October 2004.

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

   [10]  Tsirtsis, G. and P. Srisuresh, "Network Address Translation -
         Protocol Translation (NAT-PT)", RFC 2766, February 2000.

   [11]  Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and Issues",
         RFC 3234, February 2002.

   [12]  Srisuresh, P., Tsirtsis, G., Akkiraju, P., and A. Heffernan,
         "DNS extensions to Network Address Translators (DNS_ALG)",
         RFC 2694, September 1999.

   [13]  Braden, B., Zhang, L., Berson, S., Herzog, S., and S. Jamin,
         "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional



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         Specification", RFC 2205, September 1997.

   [14]  Yadav, S., Yavatkar, R., Pabbati, R., Ford, P., Moore, T.,
         Herzog, S., and R. Hess, "Identity Representation for RSVP",
         RFC 3182, October 2001.

   [15]  Aoun, C., Brunner, M., Stiemerling, M., Martin, M., and H.
         Tschofenig, "NAT/Firewall NSLP Migration Considerations",
         draft-aoun-nsis-nslp-natfw-migration-02 (work in progress),
         July 2004.

   [16]  Aoun, C., "NATFirewall NSLP Intra-realm considerations",
         draft-aoun-nsis-nslp-natfw-intrarealm-01 (work in progress),
         July 2004.

   [17]  Martin, M., "SIP NSIS Interactions for NAT/Firewall Traversal",
         draft-martin-nsis-nslp-natfw-sip-01 (work in progress),
         July 2004.

   [18]  Tschofenig, H., "Extended QoS Authorization for the QoS NSLP",
         draft-tschofenig-nsis-qos-ext-authz-00 (work in progress),
         July 2004.

   [19]  Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3369,
         August 2002.

   [20]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
         Session Initiation Protocol", RFC 3261, June 2002.

   [21]  Ohba, Y., "Problem Statement and Usage Scenarios for PANA",
         draft-ietf-pana-usage-scenarios-06 (work in progress),
         April 2003.

   [22]  Tschofenig, H., "Path-coupled NAT/firewall Signaling Security
         Problems",
         DRAFT draft-tschofenig-nsis-natfw-security-problems-00.txt,
         July 2004.

   [23]  Tschofenig, H., Buechli, M., Van den Bosch, S., and H.
         Schulzrinne, "NSIS Authentication, Authorization and Accounting
         Issues", March 2003.

   [24]  Adrangi, F. and H. Levkowetz, "Problem Statement: Mobile IPv4
         Traversal of VPN Gateways",
         DRAFT draft-ietf-mobileip-vpn-problem-statement-req-02.txt,
         April 2003.




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   [25]  Tschofenig, H., "Security Implications of the Session
         Identifier", draft-tschofenig-nsis-sid-00 (work in progress),
         June 2003.

   [26]  Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN
         - Simple Traversal of User Datagram Protocol (UDP) Through
         Network Address Translators (NATs)", RFC 3489, March 2003.

   [27]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E.
         Lear, "Address Allocation for Private Internets", BCP 5,
         RFC 1918, February 1996.

   [28]  Westerinen, A., Schnizlein, J., Strassner, J., Scherling, M.,
         Quinn, B., Herzog, S., Huynh, A., Carlson, M., Perry, J., and
         S. Waldbusser, "Terminology for Policy-Based Management",
         RFC 3198, November 2001.

   [29]  Rosenberg, J., "Traversal Using Relay NAT (TURN)",
         draft-rosenberg-midcom-turn-08 (work in progress),
         September 2005.

   [30]  Tschofenig, H., "Using SAML for SIP",
         draft-tschofenig-sip-saml-04 (work in progress), July 2005.

   [31]  Hamer, L-N., Gage, B., Kosinski, B., and H. Shieh, "Session
         Authorization Policy Element", RFC 3520, April 2003.

   [32]  Hamer, L-N., Gage, B., and H. Shieh, "Framework for Session
         Set-up with Media Authorization", RFC 3521, April 2003.

   [33]  Maler, E., Philpott, R., and P. Mishra, "Bindings and Profiles
         for the OASIS Security Assertion Markup Language (SAML) V1.1",
         September 2003.

   [34]  Maler, E., Philpott, R., and P. Mishra, "Assertions and
         Protocol for the OASIS Security Assertion Markup  Language
         (SAML) V1.1", September 2003.

   [35]  Maler, E. and J. Hughes, "Technical Overview of the OASIS
         Security Assertion Markup Language  (SAML) V1.1", March 2004.

   [36]  Roedig, U., Goertz, M., Karten, M., and R. Steinmetz, "RSVP as
         firewall Signalling Protocol", Proceedings of the 6th IEEE
         Symposium on Computers and Communications, Hammamet,
         Tunisia pp. 57 to 62, IEEE Computer Society Press, July 2001.






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Authors' Addresses

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

   Phone: +49 (0) 6221 905 11 13
   Email: stiemerling@netlab.nec.de
   URI:   http://www.stiemerling.org


   Hannes Tschofenig
   Siemens AG
   Otto-Hahn-Ring 6
   Munich  81739
   Germany

   Phone:
   Email: Hannes.Tschofenig@siemens.com
   URI:   http://www.tschofenig.com


   Cedric Aoun
   Ecole Nationale Superieure des Telecommunications
   Paris
   France

   Email: cedric@caoun.net





















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Appendix A.  Firewall and NAT Resources

   The NATFW NSLP carries, in conjunction with the NTLP's Message
   Routing Information (MRI), the policy rules to be installed at NATFW
   peers.  This policy rule is an abstraction with respect to the real
   policy rule to be installed at the respective firewall or NAT.  It
   conveys the initiator's request and must be mapped to the possible
   configuration on the particular used NAT and/or firewall.  For pure
   firewalls a filter rule must be created and for pure NATs a NAT
   binding must be created.  In mixed firewall and NAT boxes, the policy
   rule must be mapped in filter rules and bindings observing the
   ordering of the firewall and NAT engine.  Depending on the ordering,
   NAT before firewall or vice versa, the firewall rules must carry
   private or public IP addresses.  However, the exact mapping depends
   on the implementation of the firewall or NAT which is different for
   each vendor.  The remainder of this section gives thus only an
   abstract mapping of NATFW NSLP policy rules to firewall rules and NAT
   bindings, without going into the specifics on single configuration
   parameter possibilities.

   A policy rule consists out of the message routing information (MRI),
   carried in the NTLP, and information available in the NATFW NSLP.
   The information of the NSLP is stored in the extend flow information
   object and the message type, in particular the flow direction.
   Additional information, such as the external IP address and port
   number, stored in the NAT or firewall will be used as well.

A.1  Wildcarding of Policy Rules

   The policy rule/MRI to be installed can be wildcarded to some degree.
   Wildcarding applies to IP address, transport layer port numbers, and
   the IP payload (or next header in IPv6).  Processing of wildcarding
   splits into the NTLP and the NATFW NSLP layer.  The processing at the
   NTLP layer is independent of the NSLP layer processing and per layer
   constraints apply.  For wildcarding in the NTLP see Section 7.2 of
   [1].

   Wildcarding at the NATFW NSLP level is always a node local policy
   decision.  A signaling message carrying a wildcarded MRI (and thus
   policy rule) arriving at an NSLP node can be rejected if the local
   policy does not allow the request.  For instance, a MRI with IP
   addresses set (not wildcarded), transport protocol TCP, and TCP port
   numbers completely wildcarded.  Now the local policy allows only
   requests for TCP with all ports set and not wildcarded.  The request
   is going to be rejected.






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A.2  Mapping to Firewall Rules

   EDITOR's NOTE: This section is to be done (CREATE, REA-F).

A.3  Mapping to NAT Bindings

   EDITOR's NOTE: This section is to be done (CREATE, REA).

A.4  Mapping for combined NAT and firewall

   EDITOR's NOTE: This section is to be done.

A.5  NSLP Handling of Twice-NAT

   The dynamic configuration of twice-NATs requires application level
   support, as stated in Section 2.5.  The NATFW NSLP cannot be used for
   configuring twice-NATs if application level support is needed.
   Assuming application level support performing the configuration of
   the twice-NAT and the NATFW NSLP being installed at this devices, the
   NATFW NSLP must be able to traverse it.  The NSLP is probably able to
   traverse the twice-NAT, as any other data traffic, but the flow
   information stored in the NTLP's MRI will be invalidated through the
   translation of source and destination address.  The NATFW NSLP
   implementation on the twice-NAT MUST intercept NATFW NSLP and NTLP
   signaling messages as any other NATFW NSLP node does.  For the given
   signaling flow, the NATFW NSLP node MUST look up the corresponding IP
   address translation and modify the NTLP/NSLP signaling accordingly.
   The modification results in an updated MRI with respect to the source
   and destination IP addresses.






















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Appendix B.  Acknowledgments

   We would like to acknowledge: Vishal Sankhla and Joao Girao for their
   input to this draft; and Reinaldo Penno for his comments on the
   initial version of the document.  Furthermore, we would like to
   especially thank Elwyn Davies for his valuable help and input.













































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Stiemerling, et al.      Expires August 5, 2006               [Page 101]


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