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NSIS Working Group                                        M. Stiemerling
Internet-Draft                                                       NEC
Expires: April 25, 2005                                    H. Tschofenig
                                                                 Siemens
                                                               M. Martin
                                                                     NEC
                                                                 C. Aoun
                                                         Nortel Networks
                                                        October 25, 2004


           NAT/Firewall NSIS Signaling Layer Protocol (NSLP)
                     draft-ietf-nsis-nslp-natfw-04

Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

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

   This Internet-Draft will expire on April 25, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

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



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   network scenarios, problems and solutions for path-coupled Network
   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  . . . . . . . . . . . . . . . . . . . . . . .  8
     1.3   Non-Goals  . . . . . . . . . . . . . . . . . . . . . . . .  9
     1.4   General Scenario for NATFW Traversal . . . . . . . . . . .  9

   2.  Network Deployment Scenarios using NATFW NSLP  . . . . . . . . 11
     2.1   Firewall Traversal . . . . . . . . . . . . . . . . . . . . 11
     2.2   NAT with two private Networks  . . . . . . . . . . . . . . 12
     2.3   NAT with Private Network on Sender Side  . . . . . . . . . 12
     2.4   NAT with Private Network on Receiver Side Scenario . . . . 13
     2.5   Both End Hosts behind twice-NATs . . . . . . . . . . . . . 14
     2.6   Both End Hosts Behind Same NAT . . . . . . . . . . . . . . 15
     2.7   IPv4/v6 NAT with two Private Networks  . . . . . . . . . . 15
     2.8   Multihomed Network with NAT  . . . . . . . . . . . . . . . 16

   3.  Protocol Description . . . . . . . . . . . . . . . . . . . . . 18
     3.1   Policy Rules . . . . . . . . . . . . . . . . . . . . . . . 18
     3.2   Basic protocol overview  . . . . . . . . . . . . . . . . . 18
     3.3   Protocol Operations  . . . . . . . . . . . . . . . . . . . 20
       3.3.1   Creating Sessions  . . . . . . . . . . . . . . . . . . 21
       3.3.2   Reserving External Addresses . . . . . . . . . . . . . 23
       3.3.3   NATFW Session refresh  . . . . . . . . . . . . . . . . 27
       3.3.4   Deleting Sessions  . . . . . . . . . . . . . . . . . . 28
       3.3.5   Reporting Asynchronous Events  . . . . . . . . . . . . 29
       3.3.6   QUERY capabilities within the NATFW NSLP protocol  . . 30
       3.3.7   QUERY Message semantics  . . . . . . . . . . . . . . . 31
       3.3.8   Reserving External Addresses and triggering Create
               messages . . . . . . . . . . . . . . . . . . . . . . . 32
       3.3.9   Using CREATE messages to Trigger Reverse Path
               CREATE Messages  . . . . . . . . . . . . . . . . . . . 34
     3.4   Calculation of Session Lifetime  . . . . . . . . . . . . . 36
     3.5   Firewall and NAT Resources . . . . . . . . . . . . . . . . 38
     3.6   De-Multiplexing at NATs  . . . . . . . . . . . . . . . . . 38
     3.7   Selecting Opportunistic Addresses for REA  . . . . . . . . 38

   4.  NATFW NSLP NTLP Requirements . . . . . . . . . . . . . . . . . 40

   5.  NATFW NSLP Message Components  . . . . . . . . . . . . . . . . 41
     5.1   NSLP Header  . . . . . . . . . . . . . . . . . . . . . . . 41
     5.2   NSLP message types . . . . . . . . . . . . . . . . . . . . 41



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     5.3   NSLP Objects . . . . . . . . . . . . . . . . . . . . . . . 42
       5.3.1   Session Lifetime Object  . . . . . . . . . . . . . . . 42
       5.3.2   External Address Object  . . . . . . . . . . . . . . . 43
       5.3.3   Extended Flow Information Object . . . . . . . . . . . 44
       5.3.4   Response Code Object . . . . . . . . . . . . . . . . . 45
       5.3.5   Response Type Object . . . . . . . . . . . . . . . . . 45
       5.3.6   Message Sequence Number Object . . . . . . . . . . . . 46
       5.3.7   Scoping Object . . . . . . . . . . . . . . . . . . . . 46
       5.3.8   Bound Session ID Object  . . . . . . . . . . . . . . . 47
       5.3.9   Notify Target Object . . . . . . . . . . . . . . . . . 47
     5.4   Message Formats  . . . . . . . . . . . . . . . . . . . . . 48
       5.4.1   CREATE . . . . . . . . . . . . . . . . . . . . . . . . 48
       5.4.2   RESERVE-EXTERNAL-ADDRESS (REA) . . . . . . . . . . . . 49
       5.4.3   TRIGGER  . . . . . . . . . . . . . . . . . . . . . . . 49
       5.4.4   RESPONSE . . . . . . . . . . . . . . . . . . . . . . . 49
       5.4.5   QUERY  . . . . . . . . . . . . . . . . . . . . . . . . 50
       5.4.6   NOTIFY . . . . . . . . . . . . . . . . . . . . . . . . 50

   6.  NSIS NAT and Firewall Transition Issues  . . . . . . . . . . . 51

   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 52
     7.1   Trust Relationship and Authorization . . . . . . . . . . . 52
       7.1.1   Peer-to-Peer Trust Relationship  . . . . . . . . . . . 53
       7.1.2   Intra-Domain Trust Relationship  . . . . . . . . . . . 54
       7.1.3   End-to-Middle Trust Relationship . . . . . . . . . . . 55

   8.  Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 57

   9.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 58

   10.   References . . . . . . . . . . . . . . . . . . . . . . . . . 59
   10.1  Normative References . . . . . . . . . . . . . . . . . . . . 59
   10.2  Informative References . . . . . . . . . . . . . . . . . . . 59

       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 62

   A.  Problems and Challenges  . . . . . . . . . . . . . . . . . . . 63
     A.1   Missing Network-to-Network Trust Relationship  . . . . . . 63
     A.2   Relationship with routing  . . . . . . . . . . . . . . . . 64
     A.3   Affected Parts of the Network  . . . . . . . . . . . . . . 64
     A.4   NSIS backward compatibility with NSIS unaware NAT and
           Firewalls  . . . . . . . . . . . . . . . . . . . . . . . . 64
     A.5   Authentication and Authorization . . . . . . . . . . . . . 65
     A.6   Directional Properties . . . . . . . . . . . . . . . . . . 65
     A.7   Addressing . . . . . . . . . . . . . . . . . . . . . . . . 66
     A.8   NTLP/NSLP NAT Support  . . . . . . . . . . . . . . . . . . 66
     A.9   Combining Middlebox and QoS signaling  . . . . . . . . . . 66
     A.10  Inability to know the scenario . . . . . . . . . . . . . . 67



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   B.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 68

       Intellectual Property and Copyright Statements . . . . . . . . 69
















































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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 virtually extend the IP address space.
   Both types of devices may be obstacles to many applications, since
   they only allow traffic created by a limited set of applications to
   traverse them (e.g.,  most HTTP traffic, and client/server
   applications), due to the relatively static properties of the
   protocols used.  Other applications, such as IP telephony and most
   other peer-to-peer applications, which have more dynamic properties,
   create traffic which is unable to traverse NATs and Firewalls
   unassisted.  In practice, the traffic from many applications cannot
   traverse autonomous Firewalls or NATs, even when they have added
   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
   [35] and TURN [37].  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 a certain network topology.

   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 [14] is an example of a current QoS signaling protocol that is
   path-coupled.

   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 [2].  The general framework of NSIS is outlined
   in [1].  It introduces the split between an NSIS transport layer and
   an NSIS signaling layer.  The transport of NSLP messages is handled
   by an NSIS Network Transport Layer Protocol (NTLP, with GIMPS [3]



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   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 [4], while the NATFW NSLP is defined in
   this document.

   The NATFW NSLP is designed to request the dynamic configuration of
   NATs and/or Firewalls along the data path to enable data flows to
   traverse these devices without being obstructed.  Here is a
   simplified example: 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.
   Afterwards, the actual data flow can traverse every configured
   Firewall/NAT.

   NATFW NSLP runs in two different modes, one is the CREATE mode in
   which state at firewalls and NATs is created.  In the above example,
   this takes place in the direction from the data sender to the data
   receiver.  The other mode is the RESERVE mode.  In this mode, NATs
   are discovered by the NSLP/NTLP signaling messages, and a publicly
   reachable IP address and a port number are reserved at each NAT.
   This mode enables hosts located in a private addressing realm
   delimited by a NAT to subsequently receive data originated in the
   public network.  Both modes create NATFW NSLP and NTLP state in
   network entities.  NTLP state allows signaling messages to travel in
   the forward (downstream ) and the reverse (upstream) direction along
   the path between an 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 every now and then by a CREATE
   message.  If state is to be deleted explicitly before it
   automatically expires, another message can be used for that.  To find
   out what state is currently installed in NSIS NAT/Firewall nodes, a
   QUERY message can be used at any 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 5 defines
   the messages and and message components.  In the remaining parts of
   the main body of the document, Section 6 covers transition issues,
   while Section 7 addresses security considerations, with more
   extensive discussions of security issues currently being contained in
   [20].  Currently unsolved problems and challenges are listed and
   discussed in Appendix A.  Please note that readers familiar with
   Firewalls and NATs and their possible location within networks can
   safely skip Section 2.





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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 RFC 2119.

   This document uses a number of terms defined in [2].  The following
   additional terms are used:
   o  NSIS NAT Forwarding State: This term refers to a state used to
      forward the NSIS signaling message towards the targeted
      destination address.
   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" [38].  In the context
      of NSIS 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 "reserved", "deny" or action "allow".
   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 realm to another,
      in an attempt to provide transparent routing between hosts (see
      [8]).  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" [12].  In the context of this document, the term
      middlebox refers to Firewalls and NATs only.  Other types of
      middlebox are currently outside 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: An application layer flow of information for
      which some network control state information is to be manipulated
      or monitored (as defined in [1]).  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 [3].
   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" [8].
   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" [8].  IP address space allocation for
      private networks is recommended in [36]
   o  Public/Global IP address: An IP address located in the public
      network according to Section 2.7 of [8].
   o  Private/Local IP address: An IP address located in the private
      network according to Section 2.8 of [8].
   o  Initial CREATE: A CREATE message creating a new session.

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
   o  Twice-NAT
   o  Multihomed NAT
   For definitions and a detailed discussion about the characteristics
   of each NAT type please see [8].

   Both 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:



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   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 5.1 of [3].

   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 IPSec security gateway, are out of
      scope.
      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 devices categorized as firewalls only accept traffic after
      cryptographic verification (i.e., IPsec protected data traffic).
      Particularly for network access scenarios, either link layer or
      network layer data protection is common.  We do not address these
      types of devices (referred to as security gateways) since per-flow
      signaling is typically not used in this environment.
      Another application, for which NSIS signaling has been proposed
      but which is out of scope for this document, is discovering
      security gateways, for the purpose of executing IKE to create an
      IPsec SA.
      In mobility scenarios, a common problem is the traversal of a
      security gateway at the edge of a corporate network.  Network
      administrators allow only authenticated data to enter the network.
      A problem statement for the traversal of these security gateways
      in the context of Mobile IP can be found in [28]).  This topic is
      not within the scope of the present document.

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



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   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 traverse them.  NSIS NATFW NSLP
   signaling is used to dynamically install additional policy rules in
   all NATFW middleboxes along the data path.  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.

   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 (NAT/FW 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 provide 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.

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

   +----+                        +----+                        +----+
   |    +------------------------+    +------------------------+    |
   +-+--+                        +----+                        +-+--+
     |                                                           |
     |         NSIS Entities                      NSIS Entities  |
   +-+--+        +----+                            +-----+     +-+--+
   |    +--------+    +----------------------------+     +-----+    |
   +-+--+        +-+--+                            +--+--+     +-+--+
     |             |               ------             |          |
     |             |           ////      \\\\\        |          |
   +-+--+        +-+--+      |/               |     +-+--+     +-+--+
   |    |        |    |     |   Internet       |    |    |     |    |
   |    +--------+    +-----+                  +----+    +-----+    |
   +----+        +----+      |\               |     +----+     +----+
                               \\\\      /////
   sender    NAT/FW (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 implement 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.  To reduce the number of network
   elements needed, combined Firewall and NATs have been made available.

   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 peers 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.

2.2  NAT with two private Networks

   Figure 3 shows a scenario with NATs at both ends of the network.
   Therefore, each application instance, NSIS initiator and NSIS
   responder, are behind NATs.  The outermost NAT at each side is
   connected to the 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
   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



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   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 3.3.2 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).

               //----\\    +----+     +----+
        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



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   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 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 [8] 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.

                                   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, at private and public
   interfaces.

   This scenario requires the assistance of application level entities,
   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



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   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.

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
   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

   The NSIS NATFW signaling protocol should support mechanisms to detect
   such a scenario.  The signaling should be exchanged directly between
   NI and NR without involving the middlebox.

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



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   translation, i.e., using Network Address and Protocol Translators
   (NAT-PT, [11]).

   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

   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.

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.




















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             +----+
   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 3.3.2 for an expanded discussion of this topic with respect
   to NATs.




























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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 3.3.  Section 5 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 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 [3].  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 when
   they have no interest.

   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 10).  Although it is
   expected that the DS and the NATFW NSLP NI will normally reside on
   the same host, this specification does not rule out scenarios where
   the DS and NI reside on different hosts.





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


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

                  --->  : 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 10: General NSIS signaling

   The sequence of NSLP events is as follows:
   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).
   o  The response message is processed at each NF implementing the
      NATFW NSLP.
   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,
   this immediately enables communication between both hosts for
   scenarios with only Firewalls on the data path or NATs on 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 behind a NAT, the NI cannot signal to the NR 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



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   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 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 mode of operation is detailed in Section 3.3.2

   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/ forgotten after a certain period of time.  To prevent
   this, 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
   appropriate actions can be taken; this ability would allow debugging
   and error recovery.  Error messages could be sent upstream (for
   errors related to received messages as well as asynchronous error
   notification messages) towards the NI as well as downstream towards
   the NR (in the case of asynchronous error notification messages).

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

3.3  Protocol Operations

   This section defines the protocol operations, how to create sessions,
   maintain them, and how to reserve addresses.  All the NATFW NSLP
   protocol messages require C-mode handling by the NTLP and cannot 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 more details in Section 5.

   The protocol uses six messages:



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   o  CREATE: a request message used for creating, changing, refreshing
      and deleting NATFW NSLP sessions.
   o  RESERVE-EXTERNAL-ADDRESS (REA): a request message used for
      reserving an external address
   o  RESPONSE: used as a response to CREATE, REA and QUERY messages
      with Success or Error information
   o  QUERY: a request message used by authorized NATFW NEs for querying
      installed NATFW states
   o  NOTIFY: an asynchronous message used by NATFW NEs to alert
      upstream and/or downstream NATFW NEs about specific events
      (especially failures).
   o  TRIGGER: a message sent upstream to trigger the sending of CREATE
      messages
   The following sections will present the semantics of these messages
   by exhibiting their impact on the protocol state machine.

3.3.1  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 5.4.1 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 waypoints.  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
   11 sketches the message flow between NI (DS), a NF (NAT), and NR
   (DR).


















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


                    Figure 11: Creation message flow

   Since the CREATE message is used for several purposes within the
   lifetime of a session, there are several processing rules for NATFW
   NEs 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 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 check authentication and authorization 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) [3]
      (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 presence
      of existing NSLP session state related to the same session ID or
      the same MRI.  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 Section 3.3.2, that matches the destination
         address/port of the MRI provided by the NTLP.  If no



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         reservation had 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 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, for a new session, is received
         at the private side the NAT binding is reserved, but not
         activated.  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
         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 3.3.8 and
         [17]).
      *  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.
      Otherwise they SHOULD generate a RESPONSE message with an error
      code.  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.

3.3.2  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 as described in Section 3.3.1.
   For scenarios where the data receiver is located behind a NAT and



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   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.



                      +-------------+   AS-Data Receiver Communication
            +-------->| Application |<-----------------------------+
            |         | Server      |                              |
            |         +-------------+                              |
            |                                          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|
        +--------+                                          +---------+



            Figure 12: The Data Receiver behind NAT problem

   Figure 12 describes a typical message communication 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.

   Some sort of communication between the data sender/data receiver and
   a third party is typically necessary (independently of whether NSIS
   is in use).  NSIS signaling messages cannot be used to communicate



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   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).  The primary goal of path-coupled
   middlebox communication was not to avoid end hosts learning and
   preserving this type of topology knowledge.



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

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


                  Figure 13: Reservation message flow

   Figure 13 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



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      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) an arbitrary address, that would force the message to get
      intercepted by the far outmost NAT in the network.

   The NI+ agent (could be on the data receiver DR or on any other host
   within the private network) sends a the REA message targeted to the
   Opportunistic Address (OA).  The OA selection for this message is
   discussed in Section 3.7.  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 NSIS NAT Forwarding State) the 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 REA signaling message creates NSIS NAT Forwarding State at any
   intermediate NSIS NAT node(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 network addressing realm.
   Forwarding of the REA message beyond this entity is not necessary,
   and should 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.

   Processing of REA messages is specific to the NSIS node type:
   o  NSLP initiator: NI+ only generate REA messages and should never
      receive them.
   o  NSLP forwarder: NSLP forwarders receiving REA messages MUST first
      check authentication and authorization 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,



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         an IP address/port is reserved.  In the case it is an edge-NAT,
         the NSLP message is not forwarded any further and a RESPONSE
         message with the external address and port information is
         generated.  If it is not an edge-NAT, the NSLP message is
         forwarded further with the translated IP address/port (if
         required by the NI+).
      *  Firewall:  Firewalls MUST not change their configuration upon a
         REA message.  They simply MUST forward the message and MUST
         keep NTLP state.  Firewalls that are configured as
         edge-Firewalls MAY 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
      an NR and it SHOULD be discarded silently.
   o  Edge-NATs: This type of message should never be received by any
      Edge-NAT and it SHOULD be discarded silently.

3.3.3  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.
   The protocol uses a CREATE message to refresh sessions.  Even if used
   for refresh purposes the CREATE message requires that a response
   message is 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
   CREATE 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 3.4  defines the process of
   calculating lifetimes in detail.







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   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 14: State Refresh Message Flow

   Processing of session refresh CREATE messages is different for every
   NSIS node type:
   o  NSLP initiator: The NI can generate session refresh CREATE
      messages before the session times out.  The rate at which the
      refresh CREATE messages are sent and their relation to the session
      state lifetime are further discussed in Section 3.4.  The message
      routing information and the extended flow information object MUST
      be set equal to the values of the initial CREATE request message.
   o  NSLP forwarder: NSLP forwarders receiving session refresh messages
      MUST first check authentication and authorization 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 message
      generate a RESPONSE message with response object set to success.

3.3.4  Deleting Sessions

   NATFW NSLP sessions may be deleted at any time.  NSLP initiators can
   trigger this deletion by using a CREATE messages with a lifetime
   value set to 0, as shown in Figure 15.










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


                     Figure 15: Delete message flow

   NSLP nodes receiving this message 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 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.

3.3.5  Reporting Asynchronous Events

   NATFW NSLP forwarders and NATFW NSLP responders must have the ability
   to report asynchronous events to other NATFW NSLP nodes, especially
   to allow reporting back to the NATFW NSLP initiator.  Such
   asynchronous events may be premature session termination, changes in
   local policies, routing 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 are the only events that are defined, but other events may
   be defined in later versions of this memo.  One or several events
   could be reported within the NOTIFY message.

   NFs and NRs may generate NOTIFY messages upon asynchronous events,
   with a response object indicating the reason of the event.  There are
   two suggested mode of operations:
   1.  NOTIFY messages are sent hop-by-hop upstream towards NI.  Those
       NOTIFY messages may be sent downstream towards NR, if generated
       by a NF, if needed.  Indication to send NOTIFY messages
       downstream to NR is indicated within the CREATE message.
   2.  During session creation, via CREATE or REA, NIs may insert a
       special 'notify address' object into the NSLP message, indicating
       a node's address that should be notified about this event.  When
       this address is used, NOTIFY messages are not sent to the NR.
       Note: Should this mode include the first one?, in that case NR
       could be reached by providing its address as the 'notify address'
       this would use 32 or 128 bits instead of a flag bit as done in
       the first mode.



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   Processing is different for every NATFW NSLP node type and depends on
   the notified events :
   o  NSLP initiator: NIs receiving NOTIFY messages MUST first check for
      authentication and authorization.  After successfully doing so,
      NIs analyze the notified event(s) and behave appropriately based
      on the event type.  Section 5.3.4 discusses the required behavior
      for each notified event.  NIs MUST NOT generate NOTIFY messages.
   o  NSLP forwarder: NFs receiving NOTIFY messages MUST first check for
      authentication and authorization.  After successfully doing so,
      NFs analyze the notified event(s) and behave based on the notified
      events defined in Section 5.3.4.  NFs occurring an asynchronous
      event generate NOTIFY messages and set the response object(s) code
      based on the reported event(s).  NOTIFY messages are sent
      hop-by-hop upstream towards NI (This depends on the design choice
      mentioned above).
   o  NSLP responder: NRs may generate NOTIFY messages.  NRs receiving
      NOTIFY messages MUST first check for authentication and
      authorization.  After successfully doing so, NRs analyze the
      notified event(s) and behave appropriately based on the event
      type.  Section 5.3.4 discusses the required behavior for each
      notified event.  NRs occurring an asynchronous event generate
      NOTIFY messages and set the response object(s) code based on the
      reported event(s).  NOTIFY messages are sent hop-by-hop upstream
      towards NI (This depends on the design choice mentioned above).

3.3.6  QUERY capabilities within the NATFW NSLP protocol

   The NATFW NSLP provides query capabilities that could be used by a
   session owner to track the session state.  This would be used for
   diagnostic purposes when no data packets were received and the policy
   rule was supposed to have been created on the NATFW NFs.

   The QUERY message could be used to query the following session
   information: session id, flow source, destination and status of the
   state listed in best status to worst status: up, high traffic (used
   to detect DOS attack or unexpected traffic rate), pending, down.  The
   status of the policy rule will probably provide sufficient diagnostic
   information; in case more diagnostic information is required it could
   be provided by the NATFW NF logs.  Session status is only provided by
   an NF if no session status was provided in the QUERY message or the
   NF's session status is worse than the one provided by the queried
   upstream NEs.  The Session information could be retrieved by sending
   a QUERY against a specific session id, a flow source and destination
   or user identifier with session id or flow source and destination.

   QUERY message processing is different for every NATFW NSLP node type:
   o  NSLP initiator: NIs only generate QUERY messages, but never with
      session status information, so that received QUERY messages MUST



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      be discarded.
   o  NSLP forwarder: NFs receiving QUERY messages MUST first check for
      authentication and authorization.  After successfully doing so,
      NFs will behave differently depending on the QUERY.  If the QUERY
      is about a specific session: if it contains a session status the
      NF compares it to  the current local session status; if no session
      status is provided in the QUERY message the NF will insert its own
      session status in the QUERY message.  If the current local session
      status is worse, it will incorporate its own session status field
      in the QUERY message.  Every NF will provide the flow description
      in case it was not inside the QUERY.  Once the message processing
      is done, if the message was not scoped then NF will forward the
      QUERY message to the next downstream node.
   o  NSLP responder: NRs (any node being the destination of the
      message) receiving QUERY messages MUST first check for
      authentication and authorization.  After successfully doing so,
      NRs must process the message as the NFs and respond with a
      RESPONSE message to the NI.  The RESPONSE message will travel
      along the established reverse path given by the message routing
      state.

   Responses to QUERY messages are processed differently for every NATFW
   NSLP node type:
   o  NSLP initiator: NIs receiving RESPONSEs to QUERY messages MUST
      first check for authentication and authorization.  After
      successfully doing so, the objects within the RESPONSE messages
      are provided up to the application layers and the session state
      remains as it was unless the application triggers NATFW NSLP state
      changes.
   o  NSLP forwarder: NFs receiving RESPONSEs to QUERY messages MUST
      first check for authentication and authorization.  After
      successfully doing so, NFs forward the message upstream without
      any interpretation.
   o  NSLP responder: if an NR receives a RESPONSE to QUERY message it
      MUST discard it.

3.3.7  QUERY Message semantics

   From a semantics perspective, the QUERY messages may require the
   following information incorporated within the messages:
   o  Session ID
   o  Flow source (address and port) and destination (address and port),
      in case the flow doesn't use a transport protocol a protocol
      number would be used with another identifier (SPI for IPsec)
   QUERY responses should provide the following information:
   o  List of active sessions
   o  Editor's note: next version will discuss in which form the list
      publishes the active sessions (by session id or session ID and



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      flow description or other formats)
   o  Information related to a session (when the query is specific to
      one session): session ID, flow description and policy rule state
      information

3.3.8  Reserving External Addresses and triggering Create messages

   Some migration scenarios need specialized support to cope with cases
   where only 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 sessions.  The goal of the described method is to trigger the
   network to generate a CREATE message at the edge NAT.  In this case,
   a NR can signal towards the Opportunistic Address as is done in the
   standard REA message handling scenario Section 3.3.2.  The message is
   forwarded until it reaches the edge-NAT and retrieves a public IP
   address and port number.  As shown in Figure 16, unlike the standard
   REA message handling case, the edge-NAT is triggered to send a CREATE
   message on a new reverse path to handle asymmetric routes, which
   could go through internal firewalls or NATs.  This behavior requires
   within the REA message an indication to the edge NAT if either the
   standard behavior is required or a CREATE message is required to be
   sent by the edge-NAT.  In addition when a CREATE message need to be
   sent by the edge-NAT, the REA message includes the data sender
   address if available to the NR.





      DS       Public Internet       NAT     Private address      NR
     No NI                            |          space
      |                               |   REA[CREATE]             |
      |                               |<------------------------- |
      |                               |  RESPONSE[Error/Success]  |
      |                               | ---------------------- >  |
      |                               |   CREATE                  |
      |                               | ------------------------> |
      |                               |  RESPONSE[Error/Success]  |
      |                               | <----------------------   |
      |                               |                           |
      |                               |                           |



    Figure 16: REA Triggering Sending of CREATE Message on Separate
                              Reverse Path




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   If a CREATE message is received from the far end NI and relates to an
   installed session (correlation being made with the message routing
   information, MRI), that CREATE message would have precedence over the
   previous CREATE.  The CREATE sent by the NI could specify a more
   granular policy rule as only the data sender (the triggered CREATE
   may not have that information) could send data whereas in the REA
   triggered CREATE message any data source can send packets to the data
   receiver.  The edge NAT is not aware of the applications context for
   which the CREATE messages were required.  Hence it is up to the NR to
   inform the Edge NAT if there was a possibility to reduce the number
   of accepted data sources to the real data sender, as well as to
   inform the Edge NAT to refresh the established session.

   For that purpose the NR will send TRIGGER messages, to the edge NAT
   that responded to the REA message.  These messages are sent upon
   reception, from the user application, of further information on the
   Data Sender (either explicit information or implied information such
   as far end-host data sender address is equivalent to data reception
   address and same for the transport port).  The TRIGGER messages would
   be sent periodically to the Edge NAT that responded to the REA.  The
   TRIGGER messages would be sent until either a CREATE message is
   received from the far-end or when the user application no longer
   needs the NSIS session.  Figure 17 shows how TRIGGER messages would
   be used after the message sequences of Figure 16.  Triggered CREATE
   message follow the path pinned down by the earlier CREATE messages
   triggered by the REA message in Figure 16.  If a CREATE message is
   received from the far end NI and relates to an installed session,
   that CREATE message would have precedence over the triggered CREATE
   messages.  TRIGGER messages do not require to be responded back with
   a RESPONSE message on the existing established reverse path.  In case
   normal CREATE messages (i.e.  not sent by the NI) where not received
   by the NR and the session is no longer needed, the NR could use the
   TRIGGER message indicating that the session should be removed (CREATE
   message will be sent by edge NAT with lifetime=0).  The benefits of
   using REA triggering a CREATE and then using the TRIGGER messages are
   that an end-host does not need to know if the far-end host support
   the NSIS protocol.














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   Foo.com       Public Internet      Bar.com
    DS                                NAT       Firewall           NR
    No NI                              |            | TRIGGER[DSinfo]
                                       TRIGGER[DSinfo]<-------------|
                                       <-------------|              |
                                       |CREATE                      |
                                       |----------->|CREATE         |
                                       |            |-------------->|
                                       |            | RESPONSE[SUCCESS]
                                       |            | <-------------|
                                      RESPONSE[SUCCESS]             |
                                       |<-----------|               |
                                    Refresh period expiry           |
                                   or updates to Data Sender information
                                       |                            |
                                       |            | TRIGGER[DSinfo]
                                       TRIGGER[DSinfo]<-------------|
                                       <-------------|              |
                                       |CREATE                      |
                                       |----------->|CREATE         |
                                       |            |-------------->|
                                       |            | RESPONSE[SUCCESS]
                                       |            | <-------------|
                                      RESPONSE[SUCCESS]             |
                                       |<-----------|               |



                    Figure 17: TRIGGER message usage


3.3.9  Using CREATE messages to Trigger Reverse Path CREATE Messages

   In certain network deployments, where the NATFW NSLP might not be
   available on the end-host (Figure 18) or the NSIS messages are scoped
   (Figure 19) implicitly or explicitly with a scoping object, a CREATE
   message could be used to trigger another CREATE message sent by the
   last NF terminating the CREATE message.  To handle routing asymmetry
   issues, the triggered CREATE messages will be sent on a newly pinned
   down reverse path.

   In Figure 18, the first CREATE indicates that if the message can not
   reach its destination (NoNR indication), a CREATE message should be
   sent back to the NI by the last reached NATFW NE.  As in Section
   3.3.8 this mode of operation requires that the CREATE message
   indicate the type of required response behavior which in this case is
   a triggered CREATE message.  However this response behavior is
   subject to a condition: it should be sent only if the NR can not



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   respond.  This conditional behavior requires a specific flag to
   indicate it (NoNR indication in Figure 18).





                    Foo.com     Public Internet    Bar.com
                             2-RESPONSE1
               /-------------|---------------------
              / --> FW1-NF  ---------------------  \
             V /    1-CREATE1[CREATE,NoNR]     |  \ \
      Host Foo/              |                 |   NF3-NF      Host Bar
       NI/NR ^               |                 |    |^
            \ \              | 3-CREATE2       |    ||
             \ \--- FW2-NF --------------------------|
              \----/       \--------------------------
                             | 4-RESPONSE2     |


  Figure 18: CREATE Triggering Sending of CREATE Message at last NSLP
                    and Using Separate Reverse path

   To minimize the asymmetric route problem, the node responding with a
   CREATE message would request the NTLP to rediscover the reverse path.
   A RESPONSE message would be sent on the existing pinned down reverse
   path (Step 2 in Figure 18), and a CREATE would be sent on a newly
   discovered reverse path (Step 3 in Figure 18).  Upon reception of the
   latter message, the initiating NI will respond with a RESPONSE
   message (Step 4 in Figure 18) as is done for the normal CREATE
   message operations (Section 3.3.1).  The CREATE message would need to
   indicate to the last NATFW NF that a CREATE must be sent on a
   separately discovered path and that a RESPONSE message needs to be
   sent on the established pinned down reverse path.  The new CREATE
   message need to indicate to the NI that this session is bound to the
   previous session.  In addition the first message should indicate that
   the last available NATFW NF will need to terminate the message and
   start the above procedures.













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                    Foo.com     Public Internet    Bar.com
                             2-RESPONSE1
               /-------------|---------------------
              / --> FW1-NF  ---------------------  \
             V /    1-CREATE1[CREATE,scope]    |  \ \
      Host Foo/              |                 |   NF3-NF      Host Bar
       NI/NR ^               |                 |    |^
            \ \              | 3-CREATE2       |    ||
             \ \--- FW2-NF --------------------------|
              \----/       \--------------------------
                             | 4-RESPONSE2     |


  Figure 19: CREATE Triggering Sending of CREATE Message with Scoping
                    and Using Separate Reverse path

   In Figure 19, the first CREATE indicates that once the end of the
   scope is reached, the last NATFW NSLP will send a CREATE message (if
   the first CREATE request was successful).  As in Section 3.3.8, this
   mode of operation requires that the CREATE message indicate the
   required type of response behavior which in this case is a CREATE
   message.  As the CREATE needs to terminate at a scope end, the scope
   need to be provided within the CREATE message.

3.4  Calculation of Session Lifetime

   NATFW NSLP sessions, and the corresponding policy rules which may
   have been installed, are maintained via soft-state mechanism.  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 kept at
   every NATFW NSLP node.  The NSLP forwarders and NSLP responder
   (except when TRIGGER is used) are not responsible for triggering
   lifetime extension refresh messages (see Section 3.3.3): this is the
   task of the NSIS initiator.

   NSIS initiator MUST choose a session lifetime (expressed in seconds)
   value before sending any message (except 'delete session' messages)
   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 ([21]).  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
      seconds, minutes or hours and networking needs.  This duration is



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      modeled as M x R, with R the message refresh period (in seconds)
      and M a multiple of R.

   As opposed to the NTLP Message Routing state [3] lifetime, the NSLP
   session lifetime is not required to have a small value since the NSLP
   state refresh is not handling routing changes but security related
   concerns.  [14] provides a good algorithm to calculate the session
   lifetime as well as how to avoid refresh message synchronization
   within the network.  [14] recommends:
   1.  The refresh message timer to be randomly set to a value in the
       range [0.5R, 1.5R].
   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
   unless the lifetime value was below their acceptable range; 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
   20 shows the procedure with an example, where an initiator requests



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   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.




   +-------+ 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 20: Lifetime Calculation Example


3.5  Firewall and NAT Resources

   TBD: This section needs to be done and will describe how to map flow
   routing information to middlebox policy rules.  Further, this section
   should clarify wildcarding.

3.6  De-Multiplexing at NATs

   Section 3.3.2 describes how NSIS nodes behind NATs can obtain a
   publicly reachable IP address and port number at a NAT.  The
   information IP address/port number can then be transmitted via a
   signaling protocol and/or third party to the communication partner
   that would like to send data towards hosts 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.  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 requests.

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

3.7  Selecting Opportunistic Addresses for REA

   REA do need, as with all other message types, a final destination IP



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   address to reach.  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'.  [19] shows
   choices for SIP and this section provides some hints about choosing a
   good destination IP address.

   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
             12.  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 (allocated at NAT B):
       *  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
             receiver and the data sender might be different.








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4.  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 3.3.8 and [17].
   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 [21]



































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5.  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.

5.1  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 21.



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



                     Figure 21: 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.

5.2  NSLP message types

   The message types identify requests and responses.  Defined messages
   types for requests are:
   o  0x0101 : CREATE
   o  0x0102 : RESERVE-EXTERNAL-ADDRESS(REA)
   o  0x0103 : QUERY
   o  0x0104 : NOTIFY
   o  0x0105 : RESPONSE
   o  0x0106 : TRIGGER
   Defined message types for responses are (TBD):




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   o  TBD

5.3  NSLP Objects

   NATFW NSLP objects use a common header format defined by Figure 22.
   Objects are Type-Length-Value (TLV) encoded using big endian (network
   ordered) binary data representations.  The object header contains two
   fields, the NSLP object type and the object length.  Its total length
   is 32 bits.

   Note that all objects MUST be padded to a length which is a multiple
   of 32 bits.



      0                               16                            31
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   NSLP object type            |       NSLP object length      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                  Figure 22: 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.

   TBD: Processing of unknown options is currently subject to
   discussions within the working group.  It is proposed to extend the
   NSLP object header with some bits that indicate treatment of unknown
   options.  The compatibility bits (CP) are coded into two 2 bits and
   determine the action to take upon receiving an unknown option.  The
   applied behavior based on the CP bits is:
      00 - Abort processing and report error
      01 - Ignore object and do not forward
      10 - Ignore object and do forward
   All other combinations MUST NOT be set and objects carrying these
   other CP bit combinations MUST discarded.

5.3.1  Session Lifetime Object

   The session lifetime object carries the requested or granted lifetime
   of a NATFW NSLP session measured in seconds.  The object consists
   only of the 4 bytes lifetime field.





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      0                               16                            31
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          OID_NATFW_LT         |            0x0001             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  NATFW NSLP session lifetime                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                       Figure 23: Lifetime object


5.3.2  External Address Object

   The external address objects can be included in RESPONSE messages
   (Section 5.4.4) only.  It contains the external IP address and port
   number allocated at the edge-NAT.  Two fields are defined, the
   external IP address, and the external port number.  For IPv4 the
   object with value OID_NATFW_IPv4 is defined.  It has a length of 8
   bytes and is shown in Figure 24.



      0                               16                            31
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            OID_NATFW_IPv4     |            0x0002             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         port number           |           reserved            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           IPv4 address                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



         Figure 24: External Address Object for IPv4 addresses

   For IPv6 the object with value OID_NATFW_IPv6 is defined.  It has a
   length of 20 bytes and is shown in Figure 25.













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      0                               16                            31
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           OID_NATFW_IPv6      |           0x0005              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         port number           |          reserved             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                          IPv6 address                         +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



         Figure 25: External Address Object for IPv6 addresses


5.3.3  Extended Flow Information Object

   In general, flow information is kept at the NTLP level during
   signaling.  The message routing information of the NTLP carries all
   necessary information.  Nevertheless, some additional information may
   be required for NSLP operations.  The 'extended flow information'
   object carries this additional information about action to be taken
   on the installed policy rules and subsequent numbers of policy rules.

   These fields are defined for the policy rule object:
   o  Rule action: This field indicates the action for the policy rule
      to be activated.  Allow values are 'allow' (0x01) and 'deny'
      (0x02)
   o  Number of ports: This field gives the number of ports that should
      be allocated beginning at the port given in NTLP's message routing
      information.















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      0                               16                            31
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          OID_NATFW_FLOW       |           0x0001              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           rule action         |       number of ports         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                  Figure 26: Extended Flow Information


5.3.4  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.



      0                               16                            31
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        OID_NATFW_RESPONSE     |            0x0001             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         response code                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                    Figure 27: Response Code Object

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

5.3.5  Response Type Object

   The response type object indicates that a specific response is needed



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   to the NSLP responder.



      0                               16                            31
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       OID_NATFW_RESP_TYPE     |            0x0001             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |C|S|L|                      reserved                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Source IP address                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                    Figure 28: Response Type Object

   If the C bit is set to 1 the required response is a CREATE request
   message, otherwise a RESPONSE message.  If the S bit is set to 1 the
   scoping object MUST be used.  If the L bit is set to 1 the CREATE
   request message is ONLY sent if the message does not reach its
   target, even though the if the C bit is set.

   The source IP address is optional and may be set to a zero IP address
   or to a real IP address.  If set to a real address, NATFW NSLP uses
   this address as assumed data sender's address.

5.3.6  Message Sequence Number Object

   TBD.



      0                               16                            31
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          OID_NATFW_MSN        |            0x0001             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    message sequence number                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



               Figure 29: Message Sequence Number Object


5.3.7  Scoping Object

   The scoping object determines the allowed scope for the particular



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   message.



      0                               16                            31
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        OID_NATFW_SCOPE        |            0x0001             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         message scope                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                       Figure 30: Scoping Object

   These possible message scope values are: region, single hop.

5.3.8  Bound Session ID Object

   This object carries a session ID and is used for QUERY messages only.



      0                               16                            31
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        OID_NATFW_BID          |            0x0001             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       bound session ID                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                   Figure 31: Bound Session ID Object


5.3.9  Notify Target Object

   This object carries the IP address of the notify target node.  TBD:
   Details on this, like IPv6 version etc.












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      0                               16                            31
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        OID_NATFW_NOTIFY_TGT   |            0x0001             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        notify nodes' IPv4 address             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                    Figure 32: Notify Target Object


5.4  Message Formats

   This section defines the content of each NATFW NSLP message type.
   The message types are defined in Section 5.2.  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.

5.4.1  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  Respose type object [O]
   o  Scoping object[O]
   o  Notify target [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.  When the CREATE



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   messages is received by a node operating in proxy mode Section 3.3.8
   the NI address is the NR address from the message that triggered the
   CREATE to be sent, if that address is not valid (wildcarded) the
   proxy node address is used instead.  The NR address would be the NI's
   address provided by the message routing information of the message
   that triggered the CREATE.

5.4.2  RESERVE-EXTERNAL-ADDRESS (REA)

   The RESERVE-EXTERNAL-ADDRESS (REA) request message is used to target
   a NAT and to allocated an external IP address and possibly port
   number, so that the initiator of the REA request has a public
   reachable IP address/port number.

   The REA request message carries these objects:
   o  Lifetime object [M]
   o  Message sequence number object [M]
   o  Response type object [M]
   o  Scoping object [M]
   o  Extended flow information [O]

   The REA message needs special NTLP treatment.  First of all, REA
   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 3.7).  Therefore, the NTLP flow routing information is set to
   DR as initiator and DS as responders, a special field is given in the
   NTLP: The signaling destination.

5.4.3  TRIGGER

   The TRIGGER request message is used in proxy mode operation.

   The TRIGGER request message carries these objects:
   o  Lifetime object [M]
   o  Message sequence number object [M]
   o  Response type object [M]
   o  Scoping object [M]
   o  Extended flow information [O]

   NTLP settings to be defined.

5.4.4  RESPONSE

   RESPONSE messages are responses to CREATE, REA, and QUERY messages.

   The RESPONSE message carries these objects:
   o  Lifetime object [M]




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

   This message is routed upstream.

5.4.5  QUERY

   QUERY messages are used for diagnosis purposes.

   The QUERY message carries these objects:
   o  Response object [M]
   o  Message sequence number object [M]
   o  Scoping object [M]
   o  Bound session ID [O]

   This message is routed downstream.

5.4.6  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 message routing information in the NTLP MUST be set to DS as
   source address and DR as destination address, forwarding direction is
   upstream (Note that Section 5.4.6 discusses some design options
   regarding the message transport).  The session id object must be set
   to the corresponding session that is effected by this asynchronous
   event.




















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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 [17]).  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.  Security threats for NSIS signaling in general have been
   described in [6] and they are applicable to this document.  [21]
   extends this threat investigtion by considering NATFW NSLP specific
   threats.  Based on the identified threats a list of security
   requirements have been defined.  As an important requirement for
   security protection it is necessary to provide
   o  data origin authentication
   o  replay protection
   o  integrity protection and
   o  optionally confidentiality protection
   between neighboring NATFW NSLP nodes.

   To consider the aspect of authentication and key exchange we aim to
   reuse the mechanisms provided in [3] between neighboring nodes.

   Some scenarios also demand security between non-neighboring nodes but
   this aspect is still in discussions.  A possible commonality with the
   QoS NSLP has been identified and CMS [24] has been investigated as a
   possible candidate for security protection between non-neighboring
   entities.  Note that this aspect also includes some more
   sophisticated user authentication mechanisms, as described in [23].
   With regard to end-to-end security the need for a binding between an
   NSIS signaling session and application layer session has been
   described in Section 3.3 of [6].

   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.

   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.  This section lists a few scenarios relevant for security
   and illustrates possible trust reationships which have an impact to
   authorization.  More problematic scenarios are described in Appendix
   A.

7.1  Trust Relationship and Authorization

   Trust relationships and authorization are very important for the
   protocol machinery.  Trust and authorization 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
   /prolong policy rules), authorization is very important due to the
   nature of middleboxes.



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   Different types of trust relationships may affect different
   categories of middleboxes.  As explained in [26], 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.  For NATFW middlebox signaling, a
   stronger or weaker degree of authorization might be needed.
   Typically NATFW signaling requires authorization to configure and
   traverse particular nodes or networks which may derive indirectly
   from a financial relationship.  This is a more 'absolute' situation
   either the usage is allowed or not, and the effect on both network
   owner and network user is 'binary'.

   Different trust relationships that appear in middlebox signaling
   environments are described in the subsequent sub-sections.  QoS
   signaling today uses peer-to-peer trust relationships.  They are
   simplest kind of trust relationships.  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 33 does not
   illustrate the trust relationship between the end host and the access
   network.












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

               Figure 33: 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
   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 34
   illustrates a network structure which uses a centralized entity.





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

               Figure 34: 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
   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 35 shows the slightly more complex trust relationships in this
   scenario.






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

              Figure 35: End-to-Middle Trust Relationship

   Finally it should be noted that installing packet filters provides
   some security, but also has some weaknesses, which heavily depend on
   the type of packet filter installed.  A packet filter cannot prevent
   an adversary to inject traffic (due to the IP spoofing capabilities).
   This type of attack might not be particular helpful if the packet
   filter is a standard 5 tuple which is very restrictive.  If packet
   filter installation, however, allows specifying a rule, which
   restricts only the source IP address, then IP spoofing allows
   transmitting traffic to an arbitrary address.  NSIS aims to provide
   path-coupled signaling and therefore an adversary is somewhat
   restricted in the location from which attacks can be performed.  Some
   trust is therefore assumed from nodes and networks along the path.












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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) [17],
   intra-realm signaling [18], and inter-working with SIP [19].
   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:
   http://nsis.srmr.co.uk/cgi-bin/roundup.cgi/nsis-natfw-issues/index








































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9.  Contributors

   A number of individuals have contributed to this draft.  Since it was
   not possible to list them all in the authors section, it was decided
   to split it and move Marcus Brunner and Henning Schulzrinne into the
   contributors section.  Separating into two groups was done without
   treating any one of them better (or worse) than others.












































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10.  References

10.1  Normative References

   [1]  Hancock et al, R., "Next Steps in Signaling: Framework", DRAFT
        draft-ietf-nsis-fw-05.txt, October 2003.

   [2]  Brunner et al., M., "Requirements for Signaling Protocols",
        DRAFT draft-ietf-nsis-req-09.txt, October 2003.

   [3]  Schulzrinne, H. and R. Hancock, "GIMPS: General Internet
        Messaging Protocol for Signaling", DRAFT
        draft-ietf-nsis-ntlp-02.txt, October 2003.

   [4]  Van den Bosch, S., Karagiannis, G. and A. McDonald, "NSLP for
        Quality-of-Service signaling", DRAFT
        draft-ietf-nsis-qos-nslp-03.txt, May 2004.

   [5]  IANA, "Special-Use IPv4 Addresses", RFC 3330, September 2002.

   [6]  Tschofenig, H. and D. Kroeselberg, "Security Threats for NSIS",
        DRAFT draft-ietf-nsis-threats-01.txt, January 2003.

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

10.2  Informative References

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

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

   [10]  Srisuresh, P. and E. Egevang, "Traditional IP Network Address
         Translator (Traditional NAT), RFC 3022", January 2001.

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

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

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




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   [14]  Braden, B., Zhang, L., Berson, S., Herzog, S. and S. Jamin,
         "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
         Specification", September 1997.

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

   [16]  Tschofenig, H., Schulzrinne, H., Hancock, R., McDonald, A. and
         X. Fu, "Security Implications of the Session Identifier", June
         2003.

   [17]  Aoun, C., Brunner, M., Stiemerling, M., Martin, M. and H.
         Tschofenig, "NAT/Firewall NSLP Migration Considerations", DRAFT
         draft-aoun-nsis-nslp-natfw-migration-01.txt, Februar 2004.

   [18]  Aoun, C., Brunner, M., Stiemerling, M., Martin, M. and H.
         Tschofenig, "NATFirewall NSLP Intra-realm considerations",
         DRAFT draft-aoun-nsis-nslp-natfw-intrarealm-00.txt, Februar
         2004.

   [19]  Martin, M., Brunner, M. and M. Stiemerling, "SIP NSIS
         Interactions for NAT/Firewall Traversal", DRAFT
         draft-martin-nsis-nslp-natfw-sip-00.txt, Februar 2004.

   [20]  Martin, M., Brunner, M., Stiemerling, M., Girao, J. and C.
         Aoun, "A NSIS NAT/Firewall NSLP Security Infrastructure", DRAFT
         draft-martin-nsis-nslp-natfw-security-01.txt, February 2004.

   [21]  Fessi, A., Brunner, M., Stiemerling, M., Thiruvengadam, S.,
         Tschofenig, H. and C. Aoun, "Security Threats for the NAT/
         Firewall NSLP", DRAFT draft-fessi-nsis-natfw-threats-01.txt,
         July 2004.

   [22]  Tschofenig, H., "Path-coupled NAT/Firewall Signaling Security
         Problems", draft-tschofenig-nsis-natfw-security-problems-00.txt
         (work in progress), July 2004.

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

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

   [25]  Manner, J., Suihko, T., Kojo, M., Liljeberg, M. and K.
         Raatikainen, "Localized RSVP", DRAFT draft-manner-lrsvp-00.txt,
         November 2002.



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

   [27]  Amini, L. and H. Schulzrinne, "Observations from router-level
         internet traces", DIMACS Workshop on Internet and WWW
         Measurement, Mapping and Modelin Jersey) , Februar 2002.

   [28]  Adrangi, F. and H. Levkowetz, "Problem Statement: Mobile IPv4
         Traversal of VPN Gateways",
         draft-ietf-mobileip-vpn-problem-statement-req-02.txt (work in
         progress), April 2003.

   [29]  Ohba, Y., Das, S., Patil, P., Soliman, H. and A. Yegin,
         "Problem Space and Usage Scenarios for PANA",
         draft-ietf-pana-usage-scenarios-06 (work in progress), April
         2003.

   [30]  Shore, M., "The TIST (Topology-Insensitive Service Traversal)
         Protocol", DRAFT draft-shore-tist-prot-00.txt, May 2002.

   [31]  Tschofenig, H., Schulzrinne, H. and C. Aoun, "A Firewall/NAT
         Traversal Client for CASP", DRAFT
         draft-tschofenig-nsis-casp-midcom-01.txt, March 2003.

   [32]  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.

   [33]  Brunner, M., Stiemerling, M., Martin, M., Tschofenig, H. and H.
         Schulzrinne, "NSIS NAT/FW NSLP: Problem Statement and
         Framework", DRAFT draft-brunner-nsis-midcom-ps-00.txt, June
         2003.

   [34]  Ford, B., Srisuresh, P. and D. Kegel, "Peer-to-Peer(P2P)
         communication  Network Address Translators(NAT)", DRAFT
         draft-ford-midcom-p2p-02.txt, March 2004.

   [35]  Rosenberg et al, J., "STUN - Simple Traversal of User Datagram
         Protocol (UDP) Through Network Address Translators (NATs)", RFC
         3489, March 2003.

   [36]  Rekhter et al, Y., "Address Allocation for Private Internets",
         RFC 1918, February 1996.

   [37]  Rosenberg, J., "Traversal Using Relay NAT (TURN)",
         draft-rosenberg-midcom-turn-05 (work in progress), July 2004.




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   [38]  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.


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:


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

   Phone: +49 (0) 6221 905 11 16
   EMail: miquel.martin@netlab.nec.de
   URI:


   Cedric Aoun
   Nortel Networks/ENST Paris

   France

   EMail: cedric.aoun@nortelnetworks.com




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Appendix A.  Problems and Challenges

   This section describes a number of problems that have to be addressed
   for NSIS NAT/Firewall.  Issues presented here are subject to further
   discussions.  These issues might be also of relevance to other NSLP
   protocols.

A.1  Missing Network-to-Network Trust Relationship

   Peer-to-peer trust relationship, as shown in Figure 33, is a very
   convenient assumption that allows simplified signaling message
   processing.  However, it might not always be applicable, especially
   between two arbitrary access networks (over a core network where
   signaling messages are not interpreted).  Possibly peer-to-peer trust
   relationship does not exist because of the large number of networks
   and the unwillingness of administrators to have other network
   operators to create holes in their Firewalls without proper
   authorization.


   +----------------------+              +--------------------------+
   |                      |              |                          |
   |          Network A   |              |              Network B   |
   |                      |              |                          |
   |            +---------+   Missing    +---------+                |
   |      +-///-+ Middle- |    Trust     | Middle- +-///-+          |
   |      |     |  box 1  |   Relation-  |  box 2  |     |          |
   |      |     +---------+     ship     +---------+     |          |
   |      |               |     or       |               |          |
   |      |               | Authorization|               |          |
   |      |               |              |               |          |
   |      |   Trust       |              |      Trust    |          |
   |      | Relationship  |              |  Relationship |          |
   |      |               |              |               |          |
   |      |               |              |               |          |
   |      |               |              |               |          |
   |   +--+---+           |              |            +--+---+      |
   |   | Host |           |              |            | Host |      |
   |   |  A   |           |              |            |  B   |      |
   |   +------+           |              |            +------+      |
   +----------------------+              +--------------------------+


        Figure 36: Missing Network-to-Network Trust Relationship

   Figure 36 illustrates a problem whereby an external node is not
   allowed to manipulate (create, delete, query, etc.) packet filters at
   a Firewall.  Opening pinholes is only allowed for internal nodes or



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   with a certain authorization permission.  Hence the solution
   alternatives in Section 3.3.2 focus on establishing the necessary
   trust with cooperation of internal nodes.

A.2  Relationship with routing

   The data path is following the "normal" routes.  The NAT/FW devices
   along the data path are those providing the service.  In this case
   the service is something like "open a pinhole" or even more general
   "allow for connectivity between two communication partners".  The
   benefit of using path-coupled signaling is that the NSIS NATFW NSLP
   does not need to determine what middleboxes or in what order the data
   flow will go through.

   Creating NAT bindings modifies the path of data packets between two
   end points.  Without NATs involved, packets flow from endhost to
   endhost following the path given by the routing.  With NATs involved,
   this end-to-end flow is not directly possible, because of separated
   address realms.  Thus, data packets flow towards the external IP
   address at a NAT (external IP address may be a public IP address).
   Other NSIS NSLPs, for instance QoS NSLP, which do not interfere with
   routing - instead they only follow the path of the data  packets.

A.3  Affected Parts of the Network

   NATs and Firewalls are usually located at the edge of the network,
   whereby other signaling applications affect all nodes along the path.
   One typical example is QoS signaling where all networks along the
   path must provide QoS in order to achieve true end-to-end QoS.  In
   the NAT/Firewall case, only some of the domains/nodes are affected
   (typically access networks), whereas most parts of the networks and
   nodes are unaffected (e.g., the core network).

   This fact raises some questions.  Should an NSIS NTLP node intercept
   every signaling message independently of the upper layer signaling
   application or should it be possible to make the discovery procedure
   more intelligent to skip nodes.  These questions are also related to
   the question whether NSIS NAT/FW should be combined with other NSIS
   signaling applications.

A.4  NSIS backward compatibility with NSIS unaware NAT and Firewalls

   Backward compatibility is a key for NSIS deployments, as such the
   NSIS protocol suite should be sufficiently robust to allow traversal
   of none NSIS aware routers (QoS gates, Firewalls, NATs, etc ).

   NSIS NATFW NSLP's backward compatibility issues are different than
   the NSIS QoS NSLP backward compatibility issues, where an NSIS



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   unaware QoS gate will simply forward the QoS NSLP message.  An NSIS
   unaware Firewall rejects NSIS messages, since Firewalls typically
   implement the policy "default to deny".

   The NSIS backward compatibility support on none NSIS aware Firewall
   would typically consist of configuring a static policy rule that
   allows the forwarding of the NSIS protocol messages (either protocol
   type if raw transport mode is used or transport port number in case a
   transport protocol is used).

   For NATs backward compatibility is more problematic since signaling
   messages are forwarded (at least in one direction), but with a
   changed IP address and changed port numbers.  The content of the NSIS
   signaling message is, however, unchanged.  This can lead to
   unexpected results, both due to embedded unchanged local scoped
   addresses and none NSIS aware Firewalls configured with specific
   policy rules allowing forwarding of the NSIS protocol (case of
   transport protocols are used for the NTLP).  NSIS unaware NATs must
   be detected to maintain a well-known deterministic mode of operation
   for all the involved NSIS entities.  Such a "legacy" NAT detection
   procedure can be done during the NSIS discover procedure itself.

   Based on experience it was discovered that routers unaware of the
   Router Alert IP option [RFC 2113] discarded packets, this is
   certainly a problem for NSIS signaling.

A.5  Authentication and Authorization

   For both types of middleboxes, Firewall and NAT security is a strong
   requirement.  Authentication and authorization means must be
   provided.

   For NATFW signaling applications it is partially not possible to do
   authentication and authorization based on IP addresses.  Since NATs
   change IP addresses, such an address based authentication and
   authorization scheme would fail.

A.6  Directional Properties

   There two directional properties that need to be addressed by the
   NATFW NSLP:
   o  Directionality of the data
   o  Directionality of NSLP signaling

   Both properties are relevant to NATFW NSLP aware NATs and Firewalls.

   With regards to NSLP signaling directionality: As stated in the
   previous sections, the authentication and authorization of NSLP



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   signaling messages received from hosts within the same trust domain
   (typically from hosts located within the security perimeter delimited
   by Firewalls) is normally simpler than received messages sent by
   hosts located in different trust domains.

   The way NSIS signaling messages enters the NSIS entity of a Firewall
   (see Figure 2) might be important, because different policies might
   apply for authentication and admission control.

   Hosts deployed within the secured network perimeter delimited by a
   Firewall, are protected from hosts deployed outside the secured
   network perimeter, hence by nature the Firewall has more restrictions
   on flows triggered from hosts deployed outside the security
   perimeter.

A.7  Addressing

   A more general problem of NATs is the addressing of the end-point.
   NSIS signaling message have to be addressed to the other end host to
   follow data packets subsequently sent.  Therefore, a public IP
   address of the receiver has to be known prior to sending an NSIS
   message.  When NSIS signaling messages contain IP addresses of the
   sender and the receiver in the signaling message payloads, then an
   NSIS entity must modify them.  This is one of the cases, where a NSIS
   aware NATs is also helpful for other types of signaling applications
   e.g., QoS signaling.

A.8  NTLP/NSLP NAT Support

   It must be possible for NSIS NATs along the path to change NTLP and/
   or NSLP message payloads, which carry IP address and port
   information.  This functionality includes the support of providing
   mid-session and mid-path modification of these payloads.  As a
   consequence these payloads must not be reordered, integrity protected
   and/or encrypted in a non peer-to-peer fashion (e.g., end-to-middle,
   end-to-end protection).  Ideally these mutable payloads must be
   marked (e.g., a protected flag) to assist NATs in their effort of
   adjusting these payloads.

A.9  Combining Middlebox and QoS signaling

   In many cases, middlebox and QoS signaling has to be combined at
   least logically.  Hence, it was suggested to combine them into a
   single signaling message or to tie them together with the help of
   some sort of data connection identifier, later on referred as Session
   ID.  This, however, has some disadvantages such as:

   - NAT/FW NSLP signaling affects a much small number of NSIS nodes



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   along the path (for example compared to the QoS signaling).

   - NAT/FW signaling might show different signaling patterns (e.g.,
   required end-to-middle communication).

   - The refresh interval is likely to be different.

   - The number of error cases increase as different signaling
   applications are combined into a single message.  The combination of
   error cases has to be considered.

A.10  Inability to know the scenario

   In Section 2 a number of different scenarios are presented.  Data
   receiver and sender may be located behind zero, one, or more
   Firewalls and NATs.  Depending on the scenario, different signaling
   approaches have to be taken.  For instance,  data receiver with no
   NAT and Firewall can receive any sort of data and signaling without
   any further action.  Data receivers behind a NAT must first obtain a
   public IP address before any signaling can happen.  The scenario
   might even change over time with moving networks, ad-hoc networks or
   with mobility.

   NSIS signaling must assume the worst case and cannot put
   responsibility to the user to know which scenario is currently
   applicable.  As a result, it  might be necessary to perform a
   "discovery" periodically such that the NSIS entity at the end host
   has enough information to decide which scenario is currently
   applicable.  This additional messaging, which might not be necessary
   in all cases, requires additional performance, bandwidth and adds
   complexity.  Additional, information by the user can provide
   information to assist this "discovery" process, but cannot replace
   it.


















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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 thank
   Elwyn Davis for his valuable help and input.













































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