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NSIS Working Group                                        M. Stiemerling
Internet-Draft                                                       NEC
Expires: January 19, 2006                                  H. Tschofenig
                                                                 Siemens
                                                                 C. Aoun
                                                                    ENST
                                                           July 18, 2005


           NAT/Firewall NSIS Signaling Layer Protocol (NSLP)
                     draft-ietf-nsis-nslp-natfw-07

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 19, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

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



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

Table of Contents

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

   2.   Network Deployment Scenarios using NATFW NSLP  . . . . . . .  13
     2.1  Firewall Traversal . . . . . . . . . . . . . . . . . . . .  13
     2.2  NAT with two private Networks  . . . . . . . . . . . . . .  14
     2.3  NAT with Private Network on Sender Side  . . . . . . . . .  15
     2.4  NAT with Private Network on Receiver Side Scenario . . . .  15
     2.5  Both End Hosts behind twice-NATs . . . . . . . . . . . . .  16
     2.6  Both End Hosts Behind Same NAT . . . . . . . . . . . . . .  17
     2.7  IPv4/v6 NAT with two Private Networks  . . . . . . . . . .  18
     2.8  Multihomed Network with NAT  . . . . . . . . . . . . . . .  19
     2.9  Multihomed Network with Firewall . . . . . . . . . . . . .  20

   3.   Protocol Description . . . . . . . . . . . . . . . . . . . .  21
     3.1  Policy Rules . . . . . . . . . . . . . . . . . . . . . . .  21
     3.2  Basic protocol overview  . . . . . . . . . . . . . . . . .  21
     3.3  Protocol Operations  . . . . . . . . . . . . . . . . . . .  25
       3.3.1  Creating Sessions  . . . . . . . . . . . . . . . . . .  25
       3.3.2  Reserving External Addresses . . . . . . . . . . . . .  28
       3.3.3  NATFW Session refresh  . . . . . . . . . . . . . . . .  32
       3.3.4  Deleting Sessions  . . . . . . . . . . . . . . . . . .  34
       3.3.5  Reporting Asynchronous Events  . . . . . . . . . . . .  35
       3.3.6  Query and diagnosis capabilities within the NATFW
              NSLP protocol  . . . . . . . . . . . . . . . . . . . .  36
       3.3.7  Proxy Mode for Data Receiver behind NAT  . . . . . . .  39
       3.3.8  Proxy Mode for Data Sender behind Middleboxes  . . . .  42
       3.3.9  Proxy Mode for Data Receiver behind Firewall . . . . .  43
     3.4  Calculation of Session Lifetime  . . . . . . . . . . . . .  45
     3.5  Message Sequencing . . . . . . . . . . . . . . . . . . . .  47
     3.6  De-Multiplexing at NATs  . . . . . . . . . . . . . . . . .  48
     3.7  Selecting Opportunistic Addresses for REA  . . . . . . . .  49
     3.8  Session Ownership  . . . . . . . . . . . . . . . . . . . .  50
     3.9  Authentication and Authorization . . . . . . . . . . . . .  53
     3.10   Reacting to Route Changes  . . . . . . . . . . . . . . .  54

   4.   NATFW NSLP Message Components  . . . . . . . . . . . . . . .  55
     4.1  NSLP Header  . . . . . . . . . . . . . . . . . . . . . . .  55
     4.2  NSLP message types . . . . . . . . . . . . . . . . . . . .  55



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     4.3  NSLP Objects . . . . . . . . . . . . . . . . . . . . . . .  56
       4.3.1  Session Lifetime Object  . . . . . . . . . . . . . . .  57
       4.3.2  PBK Public Key . . . . . . . . . . . . . . . . . . . .  57
       4.3.3  External Address Object  . . . . . . . . . . . . . . .  58
       4.3.4  Extended Flow Information Object . . . . . . . . . . .  59
       4.3.5  Response Code Object . . . . . . . . . . . . . . . . .  59
       4.3.6  Proxy Support Object . . . . . . . . . . . . . . . . .  60
       4.3.7  Nonce Object . . . . . . . . . . . . . . . . . . . . .  60
       4.3.8  Message Sequence Number Object . . . . . . . . . . . .  61
       4.3.9  Bound Session ID Object  . . . . . . . . . . . . . . .  61
       4.3.10   Data Sender Information Object . . . . . . . . . . .  62
       4.3.11   NATFW NF Hop Count Object  . . . . . . . . . . . . .  62
       4.3.12   Maximum Hops Object  . . . . . . . . . . . . . . . .  63
       4.3.13   Session Status object  . . . . . . . . . . . . . . .  63
       4.3.14   QDRQ type  . . . . . . . . . . . . . . . . . . . . .  64
       4.3.15   QDRQ Response object . . . . . . . . . . . . . . . .  64
     4.4  Message Formats  . . . . . . . . . . . . . . . . . . . . .  64
       4.4.1  CREATE . . . . . . . . . . . . . . . . . . . . . . . .  65
       4.4.2  RESERVE-EXTERNAL-ADDRESS (REA) . . . . . . . . . . . .  65
       4.4.3  RESPONSE . . . . . . . . . . . . . . . . . . . . . . .  66
       4.4.4  QDRQ . . . . . . . . . . . . . . . . . . . . . . . . .  66
       4.4.5  NOTIFY . . . . . . . . . . . . . . . . . . . . . . . .  67
       4.4.6  UCREATE  . . . . . . . . . . . . . . . . . . . . . . .  67

   5.   NATFW NSLP NTLP Requirements . . . . . . . . . . . . . . . .  68

   6.   NSIS NAT and Firewall Transition Issues  . . . . . . . . . .  69

   7.   Security Considerations  . . . . . . . . . . . . . . . . . .  70
     7.1  Trust Relationship and Authorization . . . . . . . . . . .  70
       7.1.1  Peer-to-Peer Trust Relationship  . . . . . . . . . . .  71
       7.1.2  Intra-Domain Trust Relationship  . . . . . . . . . . .  71
       7.1.3  End-to-Middle Trust Relationship . . . . . . . . . . .  72
     7.2  Security Threats and Requirements  . . . . . . . . . . . .  73
       7.2.1  Attacks related to authentication and authorization  .  73
       7.2.2  Denial-of-Service Attacks  . . . . . . . . . . . . . .  80
       7.2.3  Man-in-the-Middle Attacks  . . . . . . . . . . . . . .  81
       7.2.4  Message Modification by non-NSIS on-path node  . . . .  82
       7.2.5  Message Modification by malicious NSIS node  . . . . .  82
       7.2.6  Session Modification/Deletion  . . . . . . . . . . . .  83
       7.2.7  Misuse of unreleased sessions  . . . . . . . . . . . .  86
       7.2.8  Data traffic injection . . . . . . . . . . . . . . . .  87
       7.2.9  Eavesdropping and traffic analysis . . . . . . . . . .  89
     7.3  Security Framework for the NAT/Firewall NSLP . . . . . . .  90
       7.3.1  Security Protection between neighboring NATFW NSLP
              Nodes  . . . . . . . . . . . . . . . . . . . . . . . .  90
       7.3.2  Security Protection between non-neighboring NATFW
              NSLP Nodes . . . . . . . . . . . . . . . . . . . . . .  90



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       7.3.3  End-to-End Security  . . . . . . . . . . . . . . . . .  92

   8.   Open Issues  . . . . . . . . . . . . . . . . . . . . . . . .  93

   9.   Contributors . . . . . . . . . . . . . . . . . . . . . . . .  94

   10.  References . . . . . . . . . . . . . . . . . . . . . . . . .  95
     10.1   Normative References . . . . . . . . . . . . . . . . . .  95
     10.2   Informative References . . . . . . . . . . . . . . . . .  95

        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  98

   A.   Firewall and NAT Resources . . . . . . . . . . . . . . . . .  99
     A.1  Wildcarding of Policy Rules  . . . . . . . . . . . . . . .  99
     A.2  Mapping to Firewall Rules  . . . . . . . . . . . . . . . . 100
     A.3  Mapping to NAT Bindings  . . . . . . . . . . . . . . . . . 100
     A.4  Mapping for combined NAT and Firewall  . . . . . . . . . . 100
     A.5  NSLP Handling of Twice-NAT . . . . . . . . . . . . . . . . 100

   B.   Problems and Challenges  . . . . . . . . . . . . . . . . . . 101
     B.1  Missing Network-to-Network Trust Relationship  . . . . . . 101
     B.2  Relationship with routing  . . . . . . . . . . . . . . . . 102
     B.3  Affected Parts of the Network  . . . . . . . . . . . . . . 102
     B.4  NSIS backward compatibility with NSIS unaware NAT and
          Firewalls  . . . . . . . . . . . . . . . . . . . . . . . . 102
     B.5  Authentication and Authorization . . . . . . . . . . . . . 103
     B.6  Directional Properties . . . . . . . . . . . . . . . . . . 103
     B.7  Addressing . . . . . . . . . . . . . . . . . . . . . . . . 104
     B.8  NTLP/NSLP NAT Support  . . . . . . . . . . . . . . . . . . 104
     B.9  Combining Middlebox and QoS signaling  . . . . . . . . . . 104
     B.10   Inability to know the scenario . . . . . . . . . . . . . 105

   C.   Object ID allocation for testing . . . . . . . . . . . . . . 106

   D.   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 107

        Intellectual Property and Copyright Statements . . . . . . . 108














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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 some 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 [6].
   Several other work-around solutions are available, including STUN
   [25] and TURN [28].  However, all of these approaches introduce other
   problems that are generally hard to solve, such as dependencies on
   the type of NAT implementation (full-cone, symmetric, ...), or
   dependencies on certain network topologies.

   NAT and Firewall (NATFW) signaling shares a property with Quality of
   Service (QoS) signaling.  The signaling of both must reach any device
   on the data path that is involved in QoS or NATFW treatment of data
   packets.  This means, that for both, NATFW and QoS, it is convenient
   if signaling travels path-coupled, meaning that the signaling
   messages follow exactly the same path that the data packets take.
   RSVP [12] is an example of a current QoS signaling protocol that is
   path-coupled. [35] proposes the use of RSVP as firewall signaling
   protocol but does not include NATs.

   This memo defines a path-coupled signaling protocol for NAT and
   Firewall configuration within the framework of NSIS, called the NATFW
   NSIS Signaling Layer Protocol (NSLP).  The general requirements for
   NSIS are defined in [4].  The general framework of NSIS is outlined
   in [3].  It introduces the split between an NSIS transport layer and
   an NSIS signaling layer.  The transport of NSLP messages is handled



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   by an NSIS Network Transport Layer Protocol (NTLP, with GIMPS [1]
   being the implementation of the abstract NTLP).  The signaling logic
   for QoS and NATFW signaling is implemented in the different NSLPs.
   The QoS NSLP is defined in [5], while the NATFW NSLP is defined in
   this memo.

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

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

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

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

   NATFW NSLP provides three modes to cope with various possible
   scenarios likely to be encountered before and after widespread
   deployment of NSIS.  Once there is full deployment of NSIS (in the
   sense that both end hosts support NATFW NSLP signaling), the
   requisite NAT and firewall state can be created using either just
   CREATE mode if the data receiver resides in a public addressing
   realm, or a combination of RESERVE-EXTERNAL-ADDRESS and CREATE modes
   if the data receiver resides in a private addressing realm and needs
   to preconfigure the boundary NAT to provide a publicly reachable
   address for use by the data sender.  During the introduction of NSIS,
   it is likely that one or other of the data sender and receiver will
   not be NSIS capable.  In these cases the NATFW NSLP can utilize NSIS
   aware middleboxes on the path between the sender and receiver to



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   provide proxy NATFW NSLP services.  Typically these boxes will be at
   the boundaries of the realms in which the end hosts are located.  If
   the data receiver is NSIS unaware, the normal modes can be employed
   but the NSIS signaling terminates at the NSIS aware node
   topologically closest to the receiver which then acts as a proxy for
   the receiver.  If the data sender is unaware a variant of the
   RESERVE-EXTERNAL-ADDRESS mode can be used by a data receiver behind a
   NAT and the specialized UCREATE mode can be used by a data receiver
   behind a firewall.

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

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

1.1  Terminology and Abbreviations

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.

   This document uses a number of terms defined in [4].  The following
   additional terms are used:

   o  Policy rule: A policy rule is "a basic building block of a policy-
      based system.  It is the binding of a set of actions to a set of
      conditions - where the conditions are evaluated to determine
      whether the actions are performed" [27].  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



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      NSIS NATFW NSLP we refer to this device as a Firewall.

   o  Network Address Translator: Network Address Translation is a
      method by which IP addresses are mapped from one IP address realm
      to another, in an attempt to provide transparent routing between
      hosts (see [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" [10].  In the context of this document, the term
      middlebox refers to Firewalls and NATs only.  Other types of
      middlebox are currently outside of the scope of this document.

   o  Security Gateway: IPsec based gateways.

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

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

   o  NATFW NSLP session: An application layer flow of information for
      which some network control state information is to be manipulated
      or monitored (as defined in [3]).  The control state for NATFW
      NSLP consists of NSLP state and associated policy rules at a
      middlebox.

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

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

   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



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      facilitated by a NAT router" [8].  IP address space allocation for
      private networks is recommended in [26]

   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 5.2.2 of [1].

   Actions for Firewalls are usually one or more of:

   o  Allow: forward data packet

   o  Deny: block data packet and discard it

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

   Actions for NATs include (amongst many others):

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

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


1.3  Non-Goals

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

      Only Firewalls and NATs are considered in this document, any other
      types of devices, for instance 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



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      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 [23]).  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
   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.  However, there is an
   exception to the primary goal of NSIS NATFW signaling, NSIS NATFW
   nodes can request blocking of particular data flows instead of
   enabling these flows at upstream firewalls.

   The basic high-level picture of NSIS usage is that end hosts are
   located behind middleboxes, meaning that there is a middlebox on the
   data path from the end host in a private network and the external
   network (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.














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   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 implements the NSIS NATFW NSLP.  There might be several NATs
   and FWs in various possible combinations on a path between two hosts.
   Section 2 presents a number of likely scenarios with different
   combinations of NATs and firewalls.
























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2.  Network Deployment Scenarios using NATFW NSLP

   This section introduces several scenarios for middlebox placement
   within IP networks.  Middleboxes are typically found at various
   different locations, including at Enterprise network borders, within
   enterprise networks, as mobile phone network gateways, etc.  Usually,
   middleboxes are placed more towards the edge of networks than in
   network cores.  Firewalls and NATs may be found at these locations
   either alone, or they may be combined; other categories of
   middleboxes may also be found at such locations, possibly combined
   with the NATs and/or Firewalls.  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 nodes that
   implement the NATFW NSLP.  Each NATFW NSLP node processes the
   signaling messages according to Section 3 and, if necessary, installs
   policy rules for subsequent data packets.

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

2.1  Firewall Traversal

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

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

                 private     public        private


             FW: Firewall
             NI: NSIS Initiator
             NR: NSIS Responder

                   Figure 2: Firewall Traversal Scenario

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



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

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

2.2  NAT with two private Networks

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



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

2.3  NAT with Private Network on Sender Side

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

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

                  private          public

             MB: Middlebox
             NI: NSIS Initiator
             NR: NSIS Responder

        Figure 4: NAT with Private Network on Sender Side Scenario

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

   Note that the data sender does not necessarily know whether the
   receiver is behind a NAT or not, hence, it is the receiving side that
   has to detect whether itself is behind a NAT or not.  As described in
   Section 3.3.2 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).














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

                public          private


             MB: Middlebox
             NI: NSIS Initiator
             NR: NSIS Responder

          Figure 5: NAT with Private Network on Receiver Scenario

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

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

2.5  Both End Hosts behind twice-NATs

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













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

                   private

             MB: Middlebox
             NI: NSIS Initiator
             NR: NSIS Responder

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

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

   This scenario requires the assistance of application level 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
   NR.  The configuration of those middleboxes may require other
   middlebox communication protocols, such as MIDCOM [6].  NSIS
   signaling is not required in the twice-NAT only case, since
   middleboxes of the twice-NAT type are normally configured by other
   means.  Nevertheless, NSIS signaling might by useful when there are
   also Firewalls on path.  In this case NSIS will not configure any
   policy rule at twice-NATs, but will configure policy rules at the
   Firewalls on the path.  The NSIS signaling protocol must be at least
   robust enough to survive this scenario.  This requires that twice-
   NATs must implement the NATFW NSLP also and participate in NATFW
   sessions but they do not change the configuration of the NAT, i.e.,
   they only read the address mapping information out of the NAT and
   translate the Message Routing Information (MRI, [1])within the NSLP
   and NTLP accordingly.  For more information see Appendix A.5

2.6  Both End Hosts Behind Same NAT

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



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

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

             MB: Middlebox
             NI: NSIS Initiator
             NR: NSIS Responder

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

   The NSIS NATFW signaling protocol should support mechanisms to detect
   such a scenario.

2.7  IPv4/v6 NAT with two Private Networks

   This scenario combines the use case described in Section 2.2 with the
   IPv4 to IPv6 transition scenario involving address and protocol
   translation, i.e., using Network Address and Protocol Translators
   (NAT-PT, [9]).

   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.














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

             +----+
   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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2.9  Multihomed Network with Firewall

   This section describes a multihomed scenario with two firewalls
   placed on alternative paths to the public network (Figure 10).  The
   routing in the private and public network decided which firewall is
   being taken for data flows.  Depending on the data flow's direction,
   either outbound or inbound, a different firewall could be traversed.
   This is a challenge for a certain mode of the NATFW NSLP where the
   NSIS responder is located behind these firewalls within the private
   network: the UCREATE mode.  The UCREATE mode is used to block a
   particular data flow on an upstream firewall.  NSIS must route the
   UCREATE mode message upstream from NR to NI without probably knowing
   the data traffic's subsequent path will take from NI to NR.

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

             MB: Middlebox
             NI: NSIS Initiator
             NR: NSIS Responder

             Figure 10: Multihomed Network with Two Firewalls





















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3.  Protocol Description

   This section defines messages, objects, and protocol semantics for
   the NATFW NSLP.  Section 3.1 introduces the base element of a NSLP
   session , the policy rule.  Section 3.2 introduces the protocol and
   the protocol behavior is defined in Section 3.3.  Section 4 defines
   the syntax of the messages and objects.

3.1  Policy Rules

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

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

3.2  Basic protocol overview

   The NSIS NATFW NSLP is carried over the NSIS Transport Layer Protocol
   (NTLP) defined in [1].  The interworking with the NTLP and other
   components is shown in Figure 60.  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 11).  Although it is
   expected that the DS and the NATFW NSLP NI will usually reside on the
   same host, this specification does not rule out scenarios where the
   DS and NI reside on different hosts, the so-called proxy mode (see
   Section 1.)



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


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

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


                     Figure 11: General NSIS signaling





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


                 ========================================>
                         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 12: A NSIS proxy mode signaling

   The sequence of NSLP events is as follows:

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



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      message.

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

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

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

   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 located 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 acquired a public IP address, it forwards this
   information to the DS via a separate protocol (such as SDP within
   SIP).  This application layer signaling, which is out of scope of the
   NATFW NSLP, may involve third parties that assist in exchanging these
   messages.

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

   1.  The NR acquires a public address by signaling on the reverse path
       (NR towards NI) and thus making itself available to other hosts.



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       This process of acquiring a public addresses is called
       reservation.  During this process the DR reserves publicly
       reachable addresses and ports suitable for NATFW NSLP signaling,
       but data traffic will not be allowed to use this address/port
       initially.

   2.  The NI signals directly to the NR as the NI would do if there is
       no NAT in between, and creates  policy rules at middleboxes.
       Note, that the reservation  mode will only allow  the forwarding
       of signaling messages but not data flow packets.  Data flow
       packets will be 'activated' by the signaling from NI towards NR.
       The RESERVE-EXTERNAL-ADDRESS mode of operation is detailed in
       Section 3.3.2

   The above usage assumes that both ends of a communication support
   NSIS but fail when NSIS is only deployed at one end of the network.
   In this case only the receiving or sending side are NSIS aware and
   not both at the same time (see also Section 1).  NATFW NSLP supports
   this scenario by using a proxy mode, as described in Section 3.3.7
   and Section 3.3.8.

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

   The protocol works on a soft-state basis, meaning that whatever state
   is installed or reserved on a middlebox will expire, and thus be de-
   installed/ 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).




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   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 including, 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 detail in Section 4.

   The protocol uses six messages:

   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 and probably port number, depending
      on the type of NAT.

   o  Query and Diagnosis ReQuest (QDRQ): a request message used by
      authorized NATFW NEs for querying and diagnosing 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  UCREATE: a request message used by data receivers to instruct
      upstream firewalls to block data traffic.

   o  RESPONSE: used as a response to CREATE, REA, UCREATE and QDRQ
      messages with Success or Error information


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 4.4.1 and hands it to the NTLP.  The NTLP forwards the whole
   message on the basis of the message routing information towards the
   NR.  Each NSIS forwarder along the path that implements NATFW NSLP,
   processes the NSLP message.  Forwarding is thus managed NSLP hop-by-
   hop but may pass transparently through NSIS forwarders which do not
   contain NATFW NSLP functionality and non-NSIS aware routers between
   NSLP hop way points.  When the message reaches the NR, the NR can



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   accept the request or reject it.  The NR generates a response to the
   request and this response is transported hop-by-hop towards the NI.
   NATFW NSLP forwarders may reject requests at any time.  Figure 13
   sketches the message flow between NI (DS), a NF (NAT), and NR (DR).



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


                     Figure 13: Creation message flow

   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 CREATE message creating a new NSIS session, the
   processing of CREATE messages is different for every NSIS node type:

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

   o  NATFW NSLP forwarder:  NFs receiving an initial CREATE message
      MUST first perform the checks defined in Section 3.8 and
      Section 3.9, if applicable, before any further processing is
      executed.  The NF SHOULD check with its local policies if it can
      accept the desired policy rule given the combination of the NTLP's
      'Message-Routing-Information' (MRI) [1] (the flow description



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      information) and the CREATE payload (behavior to be enforced on
      the packet stream).  An initial CREATE is distinguished from
      subsequent CREATE messages by the absence of existing NSLP session
      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
         reservation had been made in advance the NSLP MAY 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 3.3.7 and
         [14]).

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

   o  NSLP receiver: NRs receiving initial CREATE messages MUST reply
      with a 'success' (response object has success information)
      RESPONSE message if they accept the CREATE request message and the
      defined in Section 3.8 and Section 3.9, if applicable, have been
      successful executed.  Otherwise they SHOULD generate a RESPONSE
      message with an error code.  RESPONSE messages are sent back NSLP



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      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
   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|
        +--------+                                          +---------+





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              Figure 14: The Data Receiver behind NAT problem

   Figure 14 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 used).  NSIS signaling messages cannot be used to communicate the
   relevant application level end point identifiers (in the generic case
   at least) as a replacement for communication with the application
   server.

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


















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       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 15: Reservation message flow

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

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

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

   o  The actual target of the REA message, the Opportunistic Address
      (OA) is an arbitrary address, that would force the message to get
      intercepted by the far outmost NAT in the network.  The
      Opportunistic Address is shown as NR+.

   The NI+ (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 defined earlier).  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 session state) the direction does not matter



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   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.

   NI+ may include a data sender's address information object (DSInfo)
   if they are aware about the data sender.  The DSInfo object is used
   by the edge-NAT to limit the possible NI addresses to one address.  A
   NI+ can specify a specific IP address and port from where the
   subsequent NSIS signaling must be originated.

   The REA signaling message creates NSIS NAT session state at any
   intermediate NSIS NAT peer(s) encountered.  Furthermore it has to be
   ensured that the edge-NAT device is discovered as part of this
   process.  The end host cannot be assumed to know this device -
   instead the NAT box itself is assumed to know that it is located at
   the outer perimeter of the private 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.  When the data sender's address information is known
      in advance the NI+ MAY include a DSInfo object in the REA message.
      When the data sender's IP address is not known, NI+s MUST NOT
      include a DSInfo object.

   o  NSLP forwarder: NSLP forwarders receiving REA messages MUST first
      perform the checks defined in Section 3.8 and Section 3.9, if
      applicable, before any further processing is executed.  The NF
      SHOULD check with its local policies if it can accept the desired
      policy rule given by NTLP's message routing information (MRI).
      Further processing depends on the middlebox type:

      *  NAT:  NATs check whether the message is received at the
         external (public in most cases) address or at the internal
         (private) address.  If received at the external address a NF
         MAY generate a RESPONSE message with an  error of type 'REA
         received from outside'.  If received at the internal address,
         an IP address/port is reserved.  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



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         forwarded further with the translated IP address/port.  The
         edge-NAT MAY reject REA messages not carrying a DSInfo object
         or if the address information within this object is invalid or
         too much wildcarded.

      *  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.

   Reservations made with REA MUST be enabled by a subsequent CREATE
   message.  A reservation made with REA is kept alive as long as the
   NI+ refreshes the particular signaling session and it can be reused
   for multiple, different CREATE messages.  An NI+ may decide to
   teardown a reservation immediately after receiving a CREATE message.
   Without using CREATE (Section 3.3.1 or REA in proxy mode
   Section 3.3.7 no data traffic will be forwarded to DR beyond the
   edge-NAT.  REA is just taking care about enabling the forwarding of
   subsequent CREATE messages traveling towards the NR.  Correlation of
   incoming CREATE messages to REA reservation states is described in
   Section 3.6

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



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   removed automatically by the middlebox, if they are not refreshed.
   Soft-state is created by CREATE, REA, and UCREATE and the maintenance
   of this state must be done by these messages.  State created by
   CREATE must be maintained by CREATE, state created by REA must be
   maintained by REA, and state created by UCREATE must be maintained by
   UCREATE.  Refresh messages, either CREATE/REA/UCREATE, are messages
   carrying the exact MRI and session ID as the initial message and a
   lifetime object with a lifetime greater than zero.  Every refresh
   request message MUST be acknowledged by an appropriate response
   message generated by the NR.  This response message is routed back
   towards the NI, to allow the intermediate NFs to propose a refresh
   period that would align with their local policies.  The NI sends
   refresh messages destined for the NR.  Upon reception by each NSIS
   forwarder, the state for the given session ID is extended by the
   session refresh period, a period of time calculated based on a
   proposed refresh message period.  The lifetime extension of a session
   is calculated as current local time plus proposed lifetime value
   (session refresh period).  Section 3.4  defines the process of
   calculating lifetimes in detail.



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



         Figure 16: State Refresh Message Flow, CREATE as example

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

   o  NSLP initiator: The NI can generate session refresh CREATE/REA/
      UCREATE messages before the session times out.  The rate at which
      the refresh CREATE/REA/UCREATE messages are sent and their
      relation to the session state lifetime are further discussed in



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      Section 3.4.  The message routing information and the extended
      flow information object MUST be set equal to the values of the
      initial request message.

   o  NSLP forwarder: NSLP forwarders receiving session refresh messages
      MUST first perform the checks defined in Section 3.8 and
      Section 3.9, if applicable, before any further processing is
      executed.  The NF SHOULD check with its local policies if it can
      accept the desired lifetime extension for the session referred by
      the session ID.  Processing of this message is independent of the
      middlebox type.

   o  NSLP responder: NRs accepting a session refresh CREATE/REA/UCREATE
      message generate a RESPONSE message with response object set to
      success.  NRs MUST perform the checks defined in Section 3.8 and
      Section 3.9, if applicable.


3.3.4  Deleting Sessions

   NATFW NSLP sessions may be deleted at any time.  NSLP initiators can
   trigger this deletion by using a CREATE, REA, or UCREATE messages
   with a lifetime value set to 0, as shown in Figure 17.




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


             Figure 17: Delete message flow, CREATE as example

   NSLP nodes receiving this message MUST first perform the checks
   defined in Section 3.8 and Section 3.9, if applicable, and afterwards
   MUST delete the session immediately.  Policy rules associated with
   this particular session MUST be deleted immediately.  This message is
   forwarded until it reaches the final NR.  The CREATE/REA/UCREATE
   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.




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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, route change or any other reason that indicates
   change of the NATFW NSLP session state.  Currently, asynchronous
   session termination, re-authorization required and route change
   detected (see Section 3.10) 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 and a
   corresponding session ID.  NOTIFY messages are sent hop-by-hop
   upstream towards NI until they reach NI.

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

   o  NSLP initiator: NIs receiving NOTIFY messages MUST first perform
      the checks defined in Section 3.8 and Section 3.9, if applicable.
      After successfully doing so, NIs analyze the notified event(s) and
      behave appropriately based on the event type.  Section 4.3.5
      discusses the required behavior for each notified event.  NIs MUST
      NOT generate NOTIFY messages.

   o  NSLP forwarder: NFs receiving NOTIFY messages MUST first perform
      the checks defined in Section 3.8 and Section 3.9, if applicable,
      and MUST only accept NOTIFY messages from downstream peers.  After
      successfully doing so, NFs analyze the notified event(s) and
      behave based on the notified events defined in Section 4.3.5.  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 further hop-by-hop upstream towards the
      NI.  NFs SHOULD generate NOTIFY messages upon asynchronous events
      and forward them upstream towards the NI.

   o  NSLP responder: NRs SHOULD generate NOTIFY messages upon
      asynchronous events.  NRs receiving NOTIFY messages MUST ignore
      this message and discard it.  NOTIFY messages are sent hop-by-hop
      upstream towards NI





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3.3.6  Query and diagnosis capabilities within the NATFW NSLP protocol

   The NATFW NSLP provides query and diagnosis capabilities that could
   be used by a session(s) owner to monitor the state of those sessions.
   This would be used for:

   o  Diagnostic purposes when no data packets were received (or the
      packet stream is subject to significant packet loss) and NATFW
      NSLP signaling was supposed to have created appropriate policy
      rules on the NATFW NFs along the data path.

   o  Discover the number of NATFW NSLP Hops between the NI and the NR
      (or the last NATFW NE responding to the QDRQ)

   o  Collecting session states owned by a specific NI, this is required
      in case the NI loses its sessions' information (mainly due to node
      system issues).

   The QDRQ message can be used to query and diagnose the following
   session information: session id, the number of NE hops (between the
   NI and the last NE responding to the QDRQ) and the following
   session's status ordered from best to worst: 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 most cases;if more diagnostic information is required
   it could be provided by the NATFW NF logs.  QDRQ messages may include
   an optional maximum hop count number value provided by the NI, when
   the hop count value reaches the maximum hop count the receiving NF
   should stop propagating the QDRQ and generate a response message to
   be sent back upstream to the NI.  A QDRQ message usage is shown in
   Figure 18 where downstream NF increments the hop counter (except when
   they are the responding NF) and when the session's state is not "UP"
   (or "UP" but a QDRQ of type LIST is sent) they insert a session
   status value and their IP address.  The Session information could be
   retrieved by sending a QDRQ against a specific session id or a QDRQ
   type equal to LIST (this is only applicable when the NI's identity is
   available and identical to the one used during the session's
   establishment process).  In the message sequences shown in Figure 18,
   the QDRQ message (which a QDRQ type value of SINGLE) is sent for a
   single session ID (provided through the NTLP API), the traversed NAT
   didn't have any issues to report for the session however the
   Firewall's (FW) traffic meters reported that the flow has exceeded
   the maximum number of packets provisioned against the flow, hence in
   addition to the session status the firewall provides its address.  As
   the firewall is the last hop (it is configured to proxy and respond
   to QDRQ messages) it does not increment the hop counter and responds
   hop by hop back to the NI.




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      NI  Private address  NAT                     FW
      |                     |                       |
      |QDRQ(SINGLE,HOPCNT=0)|                       |
      |-------------------->|                       |
      |                     |QDRQ(SINGLE,HOPCNT=1)  |
      |                     |---------------------> |
      |                     |                       |
      |                     |RESPONSE(SINGLE,       |
      |                     |HOPCNT=1,              |
      |                     | [HIGH_PPS,FW@]        |
      |                     |<--------------------- |
      |RESPONSE(SINGLE,     |                       |
      |HOPCNT=1,[HIGH_PPS,  |                       |
      | FW@])               |                       |
      |<--------------------|                       |


                 Figure 18: Query and Diagnosis operation

   QDRQ message processing is dependent on the NATFW NSLP node type:

   o  NSLP initiator: NIs only generate QDRQ messages, while inserting:

      *  a HOPCNT object with a zero value

      *  a QDRQ type to indicate if the QDRQ is for a single session
         (QDRQ type would be SINGLE) or to gather information on all the
         sessions initiated by the NI (QDRQ type would be LIST)

      *  When required (i.e. this is optional) a maximum hop count value

      *  A SID (embedded in the Bound SIP object) when the QDRQ is not
         related to the session handled within the Message Routing State
         used to route the QDRQ.

      An NI MUST discard received QDRQ messages.

   o  NSLP forwarder: NFs receiving QDRQ messages MUST first
      authenticate and authorize the message source.  After successfully
      doing so, NFs will behave differently depending if the QDRQ is
      specific to one session and whether the NF co-hosts a NAT engine
      or not.

      *  If the QDRQ is about a single session:

         1.  the NF checks first if the QDRQ includes a maximum hop
             count, if the current hop counter is smaller (else the
             procedures continues as defined in 2) and the NF was



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             previously able to forward NSLP messages downstream for the
             same session (else the procedure continues as defined in
             2), the NF increments the hop counter.  Furthermore if the
             NF's session status is not "UP", the NF will insert a
             session status object, which includes the session's status
             and the node's IP address, as defined in Section 4.3.13.
             In case the NF was co-hosting a NAT engine, the NF needs to
             ensure the validity of the session status object's embedded
             IP address and modify the address based on the local NAT
             bind entry.  After completing these operations the NF
             forwards the message downstream.

         2.  In addition to the conditions discussed above, this
             procedure is applied when the QDRQ message is scoped by the
             receiving NF.  The NF responds, with a RESPONSE message,
             hop by hop back to the NI while copying the hop counter,
             the bound session ID if it was present and a series of
             session status objects.

      *  If the QDRQ is of type LIST then the following procedures are
         applied:

         1.  the NF checks first if the QDRQ includes a maximum hop
             counter, if the current hop counter is smaller (else the
             procedures continues as defined in 2), the NF creates one
             or several QDRQ response objects which include a bound
             session ID (session ID created by the NI before it lost all
             its session states), a flow descriptor and the session
             status object (session state and NF's IP address).  This is
             only performed if the NF is able to get the NI's proof of
             ownership on stored sessions within the node.  In case the
             NF was co-hosting a NAT engine, the NF needs to ensure the
             validity of all embedded IP addresses includes in QDRQ
             objects and modify the addresses based on the local NAT
             bind entry.  After completing these operations the NF
             increments the hop counter and forwards the message
             downstream, if there were no downstream nodes then the hop
             counter is decremented and the procedure continues as
             described below in step 2.

         2.  In addition to the conditions discussed above, this
             procedure is applied when the QDRQ message is scoped by the
             receiving NF.  The NF responds, with a RESPONSE message,
             hop by hop back to the NI while copying the hop counter and
             the series of QDRQ response objects which would include in
             addition to the session status objects, bound session ID as
             discussed in Section 4.3.15.




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   o  NSLP responder: NRs (any node being the destination of the
      message) receiving QDRQ messages MUST first perform the checks
      defined in Section 3.8 and Section 3.9, if applicable.  After
      successfully doing so, NRs must process the message as the NFs
      when responding with a RESPONSE message to the NI.  The RESPONSE
      message would include a copy of all the received objects within
      the QDRQ message.  The RESPONSE message will travel along the
      established reverse path given by the message routing state.

   Responses to QDRQ messages are processed differently depending on
   theNATFW NSLP node type:

   o  NSLP initiator: NIs receiving RESPONSEs to QDRQ messages MUST
      first perform the checks defined in Section 3.8 and Section 3.9,
      if applicable.  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 QDRQ messages MUST
      first perform the checks defined in Section 3.8 and Section 3.9,
      if applicable.  After successfully doing so, NFs forward the
      message upstream without any interpretation.

   o  NSLP responder: if an NR receives a RESPONSE to QDRQ message it
      MUST discard it.

   QDRQ messages are mainly sent for debugging and outage recovery and
   hence should be sent within a trusted network infrastructure, this
   could either be achieved by implicitly scoping QDRQ messages at the
   edge of the trusted network infrastructure or using the maximum hop
   count counter.

3.3.7  Proxy Mode for Data Receiver behind NAT

   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 state on the upstream path towards the data sender for
   downstream data packets.  The goal of the described method is to
   trigger the network to generate a CREATE message at the edge-NAT on
   behalf of the data receiver.  In this case, a NR can signal towards
   the Opportunistic Address as is performed in the standard REA message
   handling scenario for NATs as in Section 3.3.2.  The message is
   forwarded until the edge-NAT is reached.  A public IP address and
   port number is reserved at an edge-NAT.  As shown in Figure 19,
   unlike the standard REA message handling case, the edge-NAT is



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   triggered to send a CREATE message on a new reverse path which
   traverse several firewalls or NATs.  The new reverse path for CREATE
   is necessary to handle routing asymmetries between the edge-NAT and
   DR.  This behavior requires an indication to the edge-NAT within the
   REA message if either the standard behavior (as defined in
   Section 3.3.2) is required or a CREATE message is required to be sent
   by the edge-NAT.  In addition when a CREATE message needs to be sent
   by the edge-NAT, the REA message may include the data sender's
   address (DSInfo), if available to the data receiver.  Figure 19 shows
   this proxy mode REA as REA-PROXY.





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



      Figure 19: REA Triggering Sending of CREATE Message on Separate
                               Reverse Path

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

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

   o  NSLP forwarder: NSLP forwarders receiving REA-PROXY messages MUST
      first perform the checks defined in Section 3.8 and Section 3.9,
      if applicable, before any further processing is executed.  The NF
      SHOULD check with its local policies if it can accept the desired



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

      *  NAT:  NATs check whether the message is received at the
         external (public in most cases) address or at the internal
         (private) address.  If received at the external address a NF
         MAY generate a RESPONSE message with an  error of type 'REA
         received from outside' and stop forwarding.  If received at the
         internal address, an IP address/port is reserved.  If it is not
         an edge-NAT, the NSLP message is forwarded further with the
         translated IP address/port.  In the case it is an edge-NAT, the
         NSLP message is not forwarded any further.  The edge-NAT checks
         whether it is willing to send CREATE messages on behalf on NI+
         and if so it checks the DSInfo object.  The edge-NAT MAY reject
         the REA-PROXY request if there is no DSInfo object or if the
         address information within DSInfo is not valid or too much
         wildcarded.  If accepted a RESPONSE message with the external
         address and port information is generated.  When the edge-NAT
         accepts it generates a CREATE message as defined in
         Section 3.3.1 and includes a NONCE object having the same value
         as of the received NONCE object.  The edge-NAT MUST not
         generate a CREATE-PROXY message.  The edge-NAT MUST refresh the
         CREATE message session only if a REA-PROXY refresh message has
         been received first.

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

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

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

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

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

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

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



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

   The scenario described in this chapter challenges the data receiver
   in a way that it must make a correct assumption about the data
   sender's ability to use NSIS NATFW NSLP signaling.  There are two
   cases a) DS is NSIS unaware and DR assumes DS to be NSIS aware and b)
   DS is NSIS aware but DR assumes DS to be NSIS unaware.  Case a) will
   result in middleboxes blocking the data traffic, since DS will never
   send the expected CREATE message.  Case b) will result in the DR
   successfully requesting proxy mode support by the edge-NAT.  The
   edge-NAT will send CREATE messages and DS will send CREATE messages
   too.  Both CREATE messages are handled as separated sessions and
   therefore the common rules per session apply.  It is up to the NR's
   responsibility to decide whether to teardown the REA-PROXY sessions
   in the case of the data sender's side being NSIS aware.  It is
   RECOMMENDED that a DR behind NATs uses the proxy mode of operation by
   default, unless the DR knows that the DS is NSIS aware.

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

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

3.3.8  Proxy Mode for Data Sender behind Middleboxes

   As with data receivers behind middleboxes in Section 3.3.7 also data
   senders behind middleboxes require proxy mode support as well.  The
   problem here is that there is no NSIS support at the data receiver's
   side and, by default, there will be no response to CREATE request
   messages.  This scenario requires the last NSIS NATFW NSLP aware node
   to terminate the forwarding and to proxy the response to the CREATE
   message, meaning that this node is generating RESPONSE messages.
   This last node may be an edge-NAT/edge-Firewall, or any other NATFW
   NSLP peer, that detects that there is no NR available (probably
   through GIMPS timeouts but not limited to).  This proxy mode handles
   data senders behind a middlebox only; for receivers behind a NAT see
   Section 3.3.7.

   NIs being aware about a NSIS unaware DR, send a CREATE message



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   towards DR with a proxy support object.  Intermediate NFs can use
   this additional information to decide whether to terminate the
   message forwarding or not.  This proxy support object is an implicit
   scoping of the CREATE message.  Termination of CREATE-PROXY request
   messages with proxy support object included MUST only be done by
   egde-NATs/edge-Firewalls; future revisions of this document may
   change this behavior.





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


                 Figure 20: Proxy Mode Create Message Flow

   The processing of CREATE-PROXY messages and RESPONSE messages is
   similar to Section 3.3.1, except that forwarding is stopped at the
   edge-NAT/edge-Firewall.  The edge-NAT/edge-Firewall responds back to
   NI according the situation (error/success) and will be the NR for
   future NATFW NSLP communication.

3.3.9  Proxy Mode for Data Receiver behind Firewall

   Data receivers behind firewalls would like to use a similar sort of
   proxy mode operation to those behind NATs.  While finding the
   upstream edge-NAT is quite easy, it is only required to find an edge-
   NAT but not a very specific one and then the data traffic is route
   pinned to the NAT, the location of the appropriate edge-Firewall is
   more difficult.  Data receivers that are located behind several
   firewalls that are placed topology-wise in parallel (multi-homed
   network), must find out the one firewall the data traffic will
   traverse.  This feature of locating the right firewall can be used
   for proxy mode support and for blocking certain incoming data
   traffic.  Proxy mode support is similar to Section 3.3.7 where the DR
   is behind one or more NATs and installs "allow" policy rules.
   Blocking incoming data traffic requires that the NATFW NSLP locates
   the appropriate firewall in order to install a deny policy rule.




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   The upstream CREATE (UCREATE) message is used to locate upstream
   firewalls and to request installation of deny policy rules.  The goal
   of the method described is to trigger the network to generate a
   CREATE message at the edge-Firewall on behalf of the data receiver.
   In this case, a NR can signal towards the data sender's address as in
   the standard REA message handling scenario for NATs Section 3.3.2.
   The message is forwarded until it reaches the edge-Firewall.  As
   shown in Figure 21, the edge-Firewall is triggered to send a CREATE
   message on a new reverse path which could go through internal
   firewalls or NATs.  The new reverse path for CREATE is necessary to
   handle routing asymmetries between the edge-Firewall and DR.  UCREATE
   does not install any policy rule but the subsequent CREATE message
   initiated by the edge-Firewall does.





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



    Figure 21: UCREATE Triggering Sending of CREATE Message on Separate
                               Reverse Path

   The processing of UCREATE messages is different for every NSIS
   entity:

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

   o  NSLP forwarder: NSLP forwarders receiving UCREATE messages MUST
      first perform the checks defined in Section 3.8 and Section 3.9,
      if applicable, before any further processing is executed.  The NF
      SHOULD check with its local policies if it can accept the desired



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

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

      *  Firewall: Non edge-Firewalls simply forward the message.  Edge-
         Firewalls stop forwarding the check for performing the checks
         defined in Section 3.8 and Section 3.9, if applicable.  If the
         message is accepted, load the specified policy rule and
         generate CREATE messages back towards the DR as defined in
         Section 3.3.1.

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

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

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

   o  NSLP initiator (NI+):  Upon receiving a RESPONSE message NI+
      should await incoming corresponding CREATE messages.

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

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

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

   EDITOR's NOTE:  The protocol behavior of UCREATE needs a refinement,
   see also issue no. 38.

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,



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   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 are
   not responsible for triggering lifetime extension refresh messages
   (see Section 3.3.3): this is the task of the NSIS initiator.

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

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

   o  The end to end delay between the NI and NR

   o  Network vulnerability due to session hijacking ([7]).  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
      modeled as M x R, with R the message refresh period (in seconds)
      and M a multiplier for R.

   As opposed to the NTLP Message Routing state [1] 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. [12] provides a good algorithm to calculate the session
   lifetime as well as how to avoid refresh message synchronization
   within the network. [12] 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.




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

   Each NSLP forwarder processes the response message, reads and stores
   the granted lifetime value.  The forwarders SHOULD accept the granted
   lifetime, as long as the value is within the tolerable lifetime range
   defined in their local policies.  They MAY reject the lifetime and
   generate a 'lifetime not acceptable' error response message.
   Figure 22 shows the procedure with an example, where an initiator
   requests 60 seconds lifetime in the CREATE message and the lifetime
   is shortened along the path by the forwarder to 20 seconds and by the
   responder to 15 seconds.




   +-------+ 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 22: Lifetime Calculation Example


3.5  Message Sequencing

   NATFW NSLP messages need to carry an identifier so that all nodes
   along the path can distinguish messages sent at different points of



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   time.  Messages can be lost along the path, delayed, or duplicated.
   So all NATFW NSLP nodes should be able to identify either old
   messages that have been received before (duplicated), or the case
   that messages have been lost before (loss).  For message replay
   protection it is necessary to keep information about already received
   messages and requires every NATFW NSLP message to carry a message
   sequence number (MSN), see also Section 4.3.8.

   The MSN MUST be set by the NI and MUST no be set or modified by any
   other node.  The initial value for the MSN MUST be generated randomly
   and MUST be only unique within the used session.  The NI MUST
   increment the MSN for every message sent.  Once the MSN has reached
   the maximum value, the next value it takes is zero.

   NSIS forwarders and the responder store the with the initial packet
   received MSN as start value.  NFs and NRs include the received MSN
   value in their response messages.

   When receiving a request message, a NATFW NSLP node uses the MSN
   given in the message to determine whether the state being requested
   is different to the state already installed.  The message MUST be
   discarded if the received MSN value is lower or equal than the stored
   MSN value.  This received MSN value can indicate a duplicated and
   delayed message or replayed message.  If the received MSN value is
   greater than the already stored MSN value, the NATFW NSLP MUST update
   its stored state accordingly, if permitted by all security checks
   (see Section 3.8 and Section 3.9, and stores the updated MSN value
   accordingly.

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

3.6  De-Multiplexing at NATs

   Section 3.3.2 describes how NSIS nodes behind NATs can obtain a
   public reachable IP address and port number at a NAT and how it can
   be activated by using CREATE messages (see Section 3.3.1)".  The
   information about the public IP address/port number can be
   transmitted via an application level signaling protocol and/or third
   party to the communication partner that would like to send data
   toward the host behind the NAT.  However, NSIS signaling flows are
   sent towards the address of the NAT at which this particular IP



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   address and port number is allocated and not directly to the
   allocated IP address and port number.  The NATFW NSLP forwarder at
   this NAT needs to know how the incoming NSLP requests are related to
   reserved addresses, meaning how to de-multiplex incoming NSIS
   requests.

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

3.7  Selecting Opportunistic Addresses for REA

   As with all other message types, REA messages need a reachable final
   destination IP address.  But as many applications do not provide a
   destination IP address in the first place, there is a need to choose
   a destination address for REA messages.  This destination address can
   be the final target, but for applications which do not provide an
   upfront address, the destination address has to be chosen
   independently.  Choosing the 'correct' destination IP address may be
   difficult and it is possible there is no 'right answer'. [16] 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 14.  As a consequence it might be necessary to
             advertise a new (and different) external IP address within
             the application (which may or may not allow that) after
             using NSIS to establish a NAT binding.






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   2.  Public IP address of the data receiver:

       *  Assumption:

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

       *  Problems:

          +  Communication with a third party is required.

   3.  IP address of the Application Server:

       *  Assumption:

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

       *  Problems:

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

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


3.8  Session Ownership

   Prove of session ownership is a fundamental part of the NATFW NSLP
   signaling protocol.  It is used to validate the origin of a request,
   i.e. invariance of the message sender.  Within the NATFW NSLP, the
   NSIS initiator (the NI and the  NI+) is the ultimate session owner
   for all request messages.  The request messages are CREATE, REA,
   QDRQ, and UCREATE.  A prove of ownership MUST be provided for any
   request message sent downstream.  All intermediate NATFW NSLP nodes
   MUST use this prove of ownership to validate the message's origin.

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



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   PKI.

   As a solution, the NATFW NSLP uses purpose-built keys (PBK [2]) to
   provide session ownership.  A Purpose-built key is an ephemeral
   public/private key pair aiming to ensure the sameness principle,
   i.e., once an initial exchange of keying material has taken place
   successive messages in the communication come from the same source.
   Note that the usage of Purpose-built keys does not replace the usage
   of hop-by-by security between two neighboring NATFW NSLP nodes.  The
   usage of PBKs only aims to provider sender invariance and cannot
   provide user authentication and the ability for a NATFW node to
   authorize the request based on the authenticated identity.  A number
   of security requirements discussed in this document require the usage
   of hop-by-hop security independently of the sender invariance
   property.  The NATFW NSLP uses purpose-built keys (PBK, [2]) to prove
   the session ownership.  A Purpose-built key is a public/private key
   pair generated per session.  For every new session, the NI generates
   a new public/private key pair and uses a collision-resistant hash
   function to compute the hash of the public key.  The hash value is
   used as session ID and may be truncated to fit the session ID
   object's size.  The public key is used to sign certain parts of the
   signaling message, including the message sequence number (MSN)
   [EDITOR's note: objects to be included in the signature need to be
   listed].  The combination of signature and MSN mitigates replay
   attacks (see also Section 3.5).  NATFW NSLP nodes receiving a request
   message can use the public key (if distributed along the path, see
   later) to verify the session ID and the signature.

   The public key must be distributed amongst participating NATFW NSLP
   nodes down the path.  The absence of a deployed key distribution
   system forces distribution of the public key in-band with the NATFW
   NSLP signaling.  The public key will be carried in a NATFW_PK object
   and needs to be included in the signaling message exchange at an
   early point in each session signaling message sequence.  Selecting
   the point in the sequence for distributing the public key depends on
   a design choice to be made here.  There are two design choices:

   1.  The first message exchange from NI towards NR (either CREATE,
       REA, or UCREATE) creating a new session does not carry a
       signature nor the public key.  The second message exchange from
       NI towards NR carries the signature and the public key.  The
       public key is distributed with the second message exchange to all
       participating nodes.  All further message exchanges must carry
       the signature but do not need to repeat the public key.
       Figure 23 shows an example with four NATFW NSLP nodes using the
       here proposed scheme.





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            +----+       +-----+       +-----+         +----+
            | NI |-------| NF1 |-------| NF2 |---------| NR |
            +----+       +-----+       +-----+         +----+
              |              |             |              |
              |  CREATE(sid) |             |              |
          t0  |------------->|------------>|------------->|
              |              |             |    OK(sid)   |
          t1  |<-------------|<------------|--------------|
              |              |             |              |
              |  CREATE(sid,sig,pubkey)    |              |
          t2  |------------->|------------>|------------->|
              |              |             |    OK(sid)   |
          t3  |<-------------|<------------|<-------------|
              |              |             |              |
              |  CREATE(sid) |             |              |
          t4  |------------->|------------>|------------->|
              |              |             |    OK(sid)   |
          t5  |<-------------|<------------|--------------|
              |              |             |              |


                     Figure 23: Key Distribution Scheme 1

       This scheme first establishes the complete signaling path
       ultimately using the NTLP C-MODE with TLS support to secure the
       links hop-by-hop (Points t0 to t1).  At point t2, the
       distribution of the public key along the path is started.

   2.  The signature and the public key are included in the first
       message exchange from NI towards NR (either CREATE, REA, or
       UCREATE) and distributed amongst all nodes.  All further message
       exchanges must carry the signature only but do not need to repeat
       the public key.  Figure 24 shows an example with four NATFW NSLP
       nodes using the here proposed scheme.

















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            +----+        +----+        +----+         +----+
            | NI |--------| NF |--------| NF |---------| NR |
            +----+        +----+        +----+         +----+
              |              |             |              |
              |  CREATE(sid,sig,pubkey)    |              |
          t0  |------------->|------------>|------------->|
              |              |             |    OK(sid)   |
          t1  |<-------------|<------------|--------------|
              |              |             |              |
              |  CREATE(sid) |             |              |
          t2  |------------->|------------>|------------->|
              |              |             |    OK(sid)   |
          t3  |<-------------|<------------|--------------|
              |              |             |              |
              |  CREATE(sid) |             |              |
          t4  |------------->|------------>|------------->|
              |              |             |    OK(sid)   |
          t5  |<-------------|<------------|--------------|
              |              |             |              |


                     Figure 24: Key Distribution Scheme 2

       In this scheme, the distribution of the public key is started
       with the initial CREATE (point t0) and completed after the first
       completed message exchanged (point t1).  The first CREATE (at
       point t0) is probably exchanged via the unsecured D-MODE.

   EDITOR's note: To be done: It is needed to define the hashing
   functions as well as the to be used public/private key method.

3.9  Authentication and Authorization

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

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




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

3.10  Reacting to Route Changes

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

   A NATFW NSLP node detecting a route change, by means described in the
   NTLP specification or others, generates a NOTIFY message indicating
   this change and sends this upstream towards NI.  Intermediate NFs on
   the way to the NI can use this information to decide later if their
   session can be deleted locally if they do not receive an update
   within a certain time period.

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






















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4.  NATFW NSLP Message Components

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

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

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

4.1  NSLP Header

   The NSLP header is common to all NSLPs and is the first part of all
   NSLP messages.  It contains two fields, the NSLP message type and a
   reserved field.  The total length is 32 bits.  The layout of the NSLP
   header is defined by Figure 25.



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



                       Figure 25: Common NSLP header

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

4.2  NSLP message types

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

   o  0x0101 : CREATE

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

   o  0x0104 : UCREATE





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   o  0x0108 : QDRQ

   o  0x0201 : RESPONSE

   o  0x0301 : NOTIFY


4.3  NSLP Objects

   NATFW NSLP objects use a common header format defined by Figure 26.
   The object header contains two fields, the NSLP object type and the
   object length.  Its total length is 32 bits.



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



                   Figure 26: Common NSLP object header

   The length is the total length of the object without the object
   header.  The unit is a word, consisting of 4 octets.  The particular
   values of type and length for each NSLP object are listed in the
   subsequent sections that define the NSLP objects.  The two leading
   bits of the NSLP object header are used to signal the desired
   treatment for objects whose treatment has not been defined in this
   memo (see [1], Section 3.2), i.e., the Object Type has not been
   defined.  NATFW NSLP uses a subset of the categories defined in
   GIMPS:

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

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

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

   The combination AB=11 ("Refresh") MUST NOT be used since the NATFW
   NSLP refreshes its state end-to-end and not locally.  Fields marked
   with 'r' are reserved for future use.



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

4.3.1  Session Lifetime Object

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

      Type:   NATFW_LT

      Length: 2



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



                        Figure 27: Lifetime object


4.3.2  PBK Public Key

   The PBK public key object carries the public key used for session
   ownership as described in Section 3.8.  EDITOR's note: The key length
   needs to be defined.

      Type:   NATFW_LT

      Length: tbd.



    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //                         Public Key                          //
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                     Figure 28: PBK public key object




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4.3.3  External Address Object

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

      Type:   NATFW_EXT_IPv4

      Length: 2



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



           Figure 29: External Address Object for IPv4 addresses

      Type:   NATFW_EXT_IPv6

      Length: 5



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



           Figure 30: External Address Object for IPv6 addresses

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





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4.3.4  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.

      Type:   NATFW_EXT_FLOW

      Length: 1



     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           rule action         |       reserved                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                   Figure 31: Extended Flow Information

   These fields are defined for the policy rule object:

   o  Rule action: This field indicates the action for the policy rule
      to be activated.  Allowed values are 'allow' (0x01) and 'deny'
      (0x02)


4.3.5  Response Code Object

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

      Type:   NATFW_RESPONSE

      Length: 1



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






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                      Figure 32: 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


4.3.6  Proxy Support Object

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

      Type:   NATFW_PROXY

      Length: 0


4.3.7  Nonce Object

      Type:   NATFW_NONCE

      Length: 1








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


                          Figure 33: Nonce Object


4.3.8  Message Sequence Number Object

   This object carries the MSN value as described in Section 3.5.

      Type:   NATFW_RESP_MSN

      Length: 1



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


                 Figure 34: Message Sequence Number Object


4.3.9  Bound Session ID Object

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

      Type:   NATFW_BSID

      Length: 1



     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       bound session ID                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                    Figure 35: Bound Session ID Object

   This object is used when a session owner queries multiple session,
   every session would be indicated with the bound session ID object.





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4.3.10  Data Sender Information Object

      Type:   NATFW_DSINFO_IPv4

      Length: 2



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



               Figure 36: Data Sender's IPv4 Address Object

      Type:   NATFW_DSINFO_IPv6

      Length: 5



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



      Figure 37: Data Sender's IPv6 Address Object for IPv6 addresses


4.3.11  NATFW NF Hop Count Object

      Type:   NATFW_NF_HOPCNT

      Length: 1





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     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    NATFW NF HOP COUNT                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                   Figure 38: NATFW NF Hop Count Object

   Editor note next revision will include Hop count, maximum hops and
   QDRQ type in the same object to minimize the overhead since 4 bits
   would be sufficient for the counters.

4.3.12  Maximum Hops Object

      Type:   NATFW_NF_MAX_HOPCNT

      Length: 1



     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    NATFW NF MAX HOP COUNT                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



               Figure 39: NATFW NF Maximum Hop Count Object


4.3.13  Session Status object

   The session status object is inserted within the QDRQ message and
   copied in a response message.  It embeds the current local node's
   session status and the node's IP address

      Type:   SESSION_STS

      Length: 2 or 5

      SESSION STATUS:

      Length 1, Possible values: UP(0),HIGH_PPS(1), PENDING(2), DOWN(3)

      Reserved bits: RRR

      Length 3 bits





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      IP version: V bit

      Length 1 bit: IPv4(0), IPv6(1)

      NODE IP ADDRESS:

      Length 1 or 4



     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    SESSION STATUS                     |R|R|R|V|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    NODE IP ADDRESS (1 or 4 words)             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                     Figure 40: Session Status Object


4.3.14  QDRQ type

      Type:   QDRQ_TYPE

      Length: 1

      Possible values: SINGLE(0), LIST(1)



     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           QDRQ TYPE                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                        Figure 41: QDRQ TYPE object


4.3.15  QDRQ Response object

   TBC for next version: includes an optional bound session id, an
   optional flow descriptor (used when a LIST QDRQ type is used) and a
   mandatory session status

4.4  Message Formats

   This section defines the content of each NATFW NSLP message type.



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   The message types are defined in Section 4.2.  First, the request
   messages are defined with their respective objects to be included in
   the message.  Second, the response messages are defined with their
   respective objects to be included.

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

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

4.4.1  CREATE

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

   The CREATE message carries these objects:

   o  Lifetime object [M]

   o  Extended flow information object [M]

   o  Message sequence number object [M]

   o  Proxy support object [O]

   o  Nonce object [M if CREATE-PROXY message]

   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.

4.4.2  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:





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   o  Lifetime object [M]

   o  Message sequence number object [M]

   o  Extended flow information object [M]

   o  Proxy support object [O]

   o  Nonce object [M if proxy support object is included]

   o  Data sender information object [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.

4.4.3  RESPONSE

   RESPONSE messages are responses to CREATE, REA, UCREATE, and QDRQ
   messages.

   The RESPONSE message carries these objects:

   o  Lifetime object [M]

   o  Message sequence number object [M]

   o  Response code object [M]

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

   This message is routed upstream.

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

4.4.4  QDRQ

   QDRQ messages are used for query and diagnosis purposes.

   The QDRQ message carries these objects:

   o  Response object [M]





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   o  Message sequence number object [M]

   o  NATFW NSLP Hop Count [M]

   o  Maximum Hop Count value [O]

   o  Bound session ID [O]

   o  Session status [O]

   o  QDRQ type [O]

   This message is routed downstream.

4.4.5  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 with the NI's
   address being the destination address and the node's address as
   source address.  The message is forwarded upstream hop by hop using
   the existing upstream node address entry within the node's Message
   Routing State table.  The session id object must be set to the
   corresponding session that is effected by this asynchronous event.

4.4.6  UCREATE

   TBD: XYX.


















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5.  NATFW NSLP NTLP Requirements

   The NATFW NSLP requires the following capabilities from the NTLP:

   o  Ability to detect that the NSIS Responder does not support NATFW
      NSLP.  This capability is key to launching the proxy mode behavior
      as described in Section 3.3.7 and [14].

   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 [7]






























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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 [14]).  An update of this section
   will be based on consensus.














































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7.  Security Considerations

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

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

   Security threats that focus on NSIS in general are described in [7]
   and they are applicable to this document.  Within Section 7.2 we
   extend this threat investigation by considering NATFW NSLP specific
   threats.  Based on the security threats we list security
   requirements.

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

7.1  Trust Relationship and Authorization

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

   Different types of trust relationships may affect different
   categories of middleboxes.  As explained in [22], establishment of a
   financial relationship is typically very important for QoS signaling,
   whereas financial relationships are less directly of interest for
   NATFW middlebox signaling.  It is therefore not particularly
   surprising that there are differences in the nature and level of
   authorization likely to be required in a QoS signaling environment
   and in NATFW middlebox signaling.  Typically NATFW signaling requires
   authorization to configure firewalls or to modify NAT bindings.  The
   outcome of the authorization is either allowed or disallowed whereas
   QoS signaling might just indicate that a lower QoS reservation is
   allowed.

   Different trust relationships that appear in middlebox signaling
   environments are described in the subsequent sub-sections.  As a
   comparison with other NSIS signaling application it might be



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

7.1.1  Peer-to-Peer Trust Relationship

   Starting with the simplest scenario, it is assumed that neighboring
   nodes trust each other.  The required security association to
   authenticate and to protect a signaling message is either available
   (after manual configuration), or has been dynamically established
   with the help of an authentication and key exchange protocol.  If
   nodes are located closely together, it is assumed that security
   association establishment is easier than establishing it between
   distant nodes.  It is, however, difficult to describe this
   relationship generally due to the different usage scenarios and
   environments.  Authorization heavily depends on the participating
   entities, but for this scenario, it is assumed that neighboring
   entities trust each other (at least for the purpose of policy rule
   creation, maintenance, and deletion).  Note that Figure 42 does not
   illustrate the trust relationship between the end host and the access
   network.

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

                Figure 42: 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



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   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 43
   illustrates a network structure which uses a centralized entity.

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

                Figure 43: 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



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   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 44 shows the slightly more complex trust relationships in this
   scenario.

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


                Figure 44: End-to-Middle Trust Relationship


7.2  Security Threats and Requirements

   This section describes NATFW specific security threats and
   requirements.

7.2.1  Attacks related to authentication and authorization

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








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

                          Figure 45: CREATE Mode

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

   o  Data Sender (DS) behind a firewall

   o  Data Sender (DS) behind a NAT

   o  Data Receiver (DR) behind a firewall

   o  Data Receiver (DR) behind a NAT

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

   Figure 46 shows one possible scenario:




















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

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

                Figure 46: Several middleboxes per network


7.2.1.1  Data Sender (DS) behind a firewall

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

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

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

   The following attacks are possible:

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

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





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

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


7.2.1.2  Data Sender (DS) behind a NAT

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

   Figure 48 illustrates such a scenario:

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

                    Figure 48: Several NIs behind a NAT

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

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


7.2.1.3  Data Receiver (DR) behind a firewall

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






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

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

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

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

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

   At this point we differentiate two cases:

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

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

   For the second case, Figure 50 illustrates a possible attack: an
   adversary Mallory M could be sniffing the application layer signaling
   and thus knows the address and port number where DR is expecting the
   data flow.  Thus it could pretend to be DS and send a CREATE message
   towards DR with the data flow description (M -> DR).  Since DR does
   not know the IP address of DS, it is not able to recognize that the
   request is coming from the "wrong guy".  It will send a success
   RESPONSE message back and the middlebox will install policy rules
   that will allow Mallory M to inject its data into the network.








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

             Figure 50: DR behind a firewall with an adversary

   Network administrators will probably not rely on a DR to check the IP
   address of the DS.  Thus we have to assume the worst case with an
   attack such as in Figure 50.  Many operators might not allow NSIS
   signaling message to traverse the firewall in Figure 50 without
   proper authorization.  In this case the threat is not applicable.

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


7.2.1.4  Data Receiver (DR) behind a NAT

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

   (DR_ext <=> DR_int) (*).

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

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

   We assume that the adversary Mallory M obtains the contact address



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


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

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


7.2.1.5  NSLP Message Injection

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

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

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

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







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7.2.2  Denial-of-Service Attacks

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

7.2.2.1  Flooding with CREATE messages from outside

7.2.2.1.1  Attacks due to NSLP state

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

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

   An adversary from outside could launch a DoS attack with arbitrary
   CREATE messages.  For each of these messages the middlebox needs to
   store state information such as the policy rules to be loaded, i.e.,
   the middlebox could run out of memory.  This kind of attack is also
   mentioned in [7] Section 4.8.

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


7.2.2.1.2  Attacks due to authentication complexity

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

   For a more detailed treatment of this kind of attack, the reader is
   encouraged to see [7].

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


7.2.2.1.3  Attacks to the endpoints

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



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

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

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


7.2.2.2  Flooding with REA messages from inside

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

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

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


7.2.3  Man-in-the-Middle Attacks

   Figure 52 illustrates a possible man-in-the-middle attack using the
   RESERVE-EXTERNAL-ADDRESS (REA) message.  This message travels from DR
   towards the public Internet.  The message might not be intercepted
   because there are no NSIS aware middleboxes.

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

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



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


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

     Figure 52: MITM attack using the RESERVE-EXTERNAL-ADDRESS message

   SECURITY REQUIREMENT: Mutual authentication between neighboring NATFW
      NSLP MUST be provided.  To ensure that only legitimate nodes along
      the path act as NSIS entities the initiator MUST authorize the
      responder.  In the example in Figure 52 the firewall FW must
      perform an authorization with the neighboring entities.


7.2.4  Message Modification by non-NSIS on-path node

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

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


7.2.5  Message Modification by malicious NSIS node

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






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

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

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


7.2.6  Session Modification/Deletion

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

   Consider the scenario described in Figure 53.  Here an adversary
   pretends to be 'DS in mobility'.  The signaling messages start from
   the DS and go through a series of routers towards the DR.  We assume
   that an off-path adversary is connected to one of the routers along
   the old path (here Router 3).  We also assume that the adversary
   knows the Session ID of the NSIS session initiated by the DS.
   Knowing the Session ID, the adversary now sends signaling messages
   towards the DR.  When the signaling message reaches Router3 then
   existing state information can be modified or even deleted.  The
   adversary can modify or delete the established reservation causing
   unexpected behavior for the legitimate user.  The source of the
   problem is that the Router 3 (cross-over router) is unable to decide
   whether the new signaling message was initiated from the owner of the
   session.  In this scenario, the adversary need not even be located in
   the DS-DR path.  This problem and the solution approaches are
   described in more detail in [24].

















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

            Figure 53: State Modification by off-path adversary

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

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


7.2.6.1  Misuse of mobility in NAT handling

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









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     Public                       Private Address
     Internet                     space

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

               Figure 54: Misuse of mobility in NAT binding

   A NAT binding can be changed with the help of NSIS signaling.  When a
   DR moves to a new location and obtains a new IP address, it sends an
   NSIS signaling message to modify the NAT binding.  It would use the
   Session-ID and the new flow-id to update the state.  The NAT updates
   the binding and the DR continues to receive the data traffic.
   Consider the scenario in Figure 54 where an the endhost(DR) and the
   adversary are behind a NAT.  The adversary pretending that it is the
   end host could generate a spurious signaling message to update the
   state at the NAT.  This could be done for these purposes:

      Connection hijacking by redirecting packets to the attacker as in
      Figure 55

      Third party flooding by redirecting packets to arbitrary hosts

      Service disruption by redirecting to non-existing hosts


















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

                      Figure 55: Connection Hijacking

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


7.2.7  Misuse of unreleased sessions

   Assume that DS (N1) initiates NSIS session with DR (N2) through a
   series of middleboxes as in Figure 56.  When the DS is sending data
   to DR, it might happen that the DR disconnects from the network
   (crashes or moves out of the network in mobility scenarios).  In such
   cases, it is possible that another node N3 (which recently entered
   the network protected by the same firewall) is assigned the same IP
   address that was previously allocated to N2.  The DS could take
   advantage of the firewall policies installed already, if the refresh
   interval time is very high.  The DS can flood the node (N3), which
   will consume the access network resources of the victim forcing it to
   pay for unwanted traffic as shown in Figure 57.  Note that here we
   make the assumption that the data receiver has to pay for receiving
   data packets.














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

                        Figure 56: Before mobility


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

                         Figure 57: After mobility

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

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


7.2.8  Data traffic injection

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



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

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

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

   Ingress Filtering: Consider the scenario shown in Figure 58.  In this
   scenario the DS is behind a router (R1) and a malicious node (M) is
   behind another router (R2).  The DS communicates with the DR through
   a firewall (FW).  The DS initiates NSIS signaling and installs
   firewall policies at FW.  But the malicious node is also able to send
   data traffic using DS's source address.  If R2 implements ingress
   filtering, these spoofed packets will be blocked.  But this ingress
   filtering may not work in all scenarios.  If both the DS and the
   malicious node are behind the same router, then the ingress filter
   will not be able to detect the spoofed packets as both the DS and the
   malicious node are in the same address range.


























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

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

                       Figure 58: Ingress filtering


7.2.9  Eavesdropping and traffic analysis

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

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

   As shown in Section 4.3 of [24] one possible solution for the session
   ownership problem is confidentiality protection of signaling messages

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





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7.3  Security Framework for the NAT/Firewall NSLP

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

7.3.1  Security Protection between neighboring NATFW NSLP Nodes

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

   o  data origin authentication

   o  replay protection

   o  integrity protection and

   o  optionally confidentiality protection

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

7.3.2  Security Protection between non-neighboring NATFW NSLP Nodes

   Based on the security threats and the listed requirements it was
   noted that some scenarios also demand authentication and
   authorization of a NATFW signaling entity (including the initiator)
   towards a non-neighboring node.  This mechanism mainly demands entity
   authentication.  Additionally, security protection of certain
   payloads MAY be required between non-neighboring signaling entities
   and the Cryptographic Message Syntax (CMS) [18] SHOULD be used.  CMS
   can be used

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

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

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



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

   Examples of authorization tokens or assertions can be found in RFC
   3520 [30] and RFC 3521 [31].  More recent work on authorization token
   alike mechanisms is Security Assertion Markup Language (SAML).  For
   details about SAML see [32], [33] and [34].  Figure 59 shows an
   example of this protocol interaction.  An authorization token is
   provided by the SIP proxy, which acts as the assertion generating
   entity and gets delivered to the end host with proper authentication
   and authorization.  When the NATFW signaling message is transmitted
   towards the network, the authorization token is attached to the
   signaling messages to refer to the previous authorization decision.
   The assertion verifying entity needs to process the token or it might
   be necessary to interact with the assertion granting entity using
   HTTP (or other protocols).  As a result of a successful authorization
   by a NATFW NSLP node, the requested action is executed and later a
   RESPONSE message is generated.




























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

                   Figure 59: Authorization Token Usage

   Threats against the usage of authorization tokens have been mentioned
   in [7] and also in  Section 7.2.  Hence, it is required to provide
   confidentiality protection to avoid allowing an eavesdropper to learn
   the token and to use it in another session (replay attack).  The
   token itself also needs to be protected against tempering.

7.3.3  End-to-End Security

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

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

   In order to solicit feedback from the IETF community on some hard
   security problems for path-coupled NATFW signaling a more detailed
   description in [21] is available.




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8.  Open Issues

   The NATFW NSLP has a series of related documents discussing several
   other aspects of path-coupled NATFW signaling, including security
   [21], migration (i.e., traversal of NSIS unaware NATs) [14], intra-
   realm signaling [15], and inter-working with SIP [16].  Summaries of
   the outcomes from these documents may be added, depending on WG
   feedback, to a later version of this draft.

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

   It is intended to add an overview figure for all NATFW NSLP building
   blocks into the next version of this memo.  Figure 60 sketches the
   overview



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


                   Figure 60: NATFW NSLP Building Blocks



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9.  Contributors

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

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

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

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

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

   o  and the NSIS working group.


































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10.  References

10.1  Normative References

   [1]  Schulzrinne, H. and R. Hancock, "GIMPS: General Internet
        Messaging Protocol for Signaling", draft-ietf-nsis-ntlp-06 (work
        in progress), May 2005.

10.2  Informative References

   [2]   Bradner, S., Mankin, A., and J. Schiller, "A Framework for
         Purpose-Built Keys (PBK)", draft-bradner-pbk-frame-06 (work in
         progress), June 2003.

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

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

   [5]   Bosch, S., Karagiannis, G., and A. McDonald, "NSLP for Quality-
         of-Service signaling", draft-ietf-nsis-qos-nslp-06 (work in
         progress), February 2005.

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

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

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

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

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

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

   [12]  Braden, B., Zhang, L., Berson, S., Herzog, S., and S. Jamin,
         "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
         Specification", RFC 2205, September 1997.



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

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

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

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

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

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

   [19]  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.

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

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

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

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

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



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         June 2003.

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

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

   [27]  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.

   [28]  Rosenberg, J., "Traversal Using Relay NAT (TURN)",
         draft-rosenberg-midcom-turn-07 (work in progress),
         February 2005.

   [29]  Tschofenig, H., "Using SAML for SIP",
         draft-tschofenig-sip-saml-02 (work in progress), December 2004.

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

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

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

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

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

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








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Authors' Addresses

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

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


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

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


   Cedric Aoun
   Ecole Nationale Superieure des Telecommunications
   Paris
   France

   Email: cedric@caoun.net





















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Appendix A.  Firewall and NAT Resources

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

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

A.1  Wildcarding of Policy Rules

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

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






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A.2  Mapping to Firewall Rules

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

A.3  Mapping to NAT Bindings

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

A.4  Mapping for combined NAT and Firewall

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

A.5  NSLP Handling of Twice-NAT

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






















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Appendix B.  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.

B.1  Missing Network-to-Network Trust Relationship

   Peer-to-peer trust relationship, as shown in Figure 42, 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 61: Missing Network-to-Network Trust Relationship

   Figure 61 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.

B.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.

B.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.

B.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.

B.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.

B.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.




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

B.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.

B.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.

B.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:



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

B.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 C.  Object ID allocation for testing

   TBD.
















































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Appendix D.  Acknowledgments

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













































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