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NSIS Working Group                                        M. Stiemerling
Internet-Draft                                                       NEC
Intended status: Standards Track                           H. Tschofenig
Expires: April 26, 2007                                          Siemens
                                                                 C. Aoun
                                                                    ENST
                                                               E. Davies
                                                        Folly Consulting
                                                        October 23, 2006


           NAT/Firewall NSIS Signaling Layer Protocol (NSLP)
                   draft-ietf-nsis-nslp-natfw-13.txt

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 April 26, 2007.

Copyright Notice

   Copyright (C) The Internet Society (2006).









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Abstract

   This memo defines the NSIS Signaling Layer Protocol (NSLP) for
   Network Address Translators (NATs) and firewalls.  This NSLP allows
   hosts to signal on the data path for NATs and firewalls to be
   configured according to the needs of the application data flows.  It
   enables hosts behind NATs to obtain a public reachable address and
   hosts behind firewalls to receive data traffic.  The overall
   architecture is given by the framework and requirements defined by
   the Next Steps in Signaling (NSIS) working group.  The network
   scenarios, the protocol itself, and examples for path-coupled
   signaling are given in this memo.


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.1.  Terminology and Abbreviations . . . . . . . . . . . . . .   7
     1.2.  Middleboxes . . . . . . . . . . . . . . . . . . . . . . .  10
     1.3.  General Scenario for NATFW Traversal  . . . . . . . . . .  11

   2.  Network Deployment Scenarios using the 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.  Multihomed Network with NAT . . . . . . . . . . . . . . .  18
     2.8.  Multihomed Network with Firewall  . . . . . . . . . . . .  19

   3.  Protocol Description  . . . . . . . . . . . . . . . . . . . .  20
     3.1.  Policy Rules  . . . . . . . . . . . . . . . . . . . . . .  20
     3.2.  Basic Protocol Overview . . . . . . . . . . . . . . . . .  21
       3.2.1.  Message Types . . . . . . . . . . . . . . . . . . . .  25
       3.2.2.  Classification of RESPONSE Messages . . . . . . . . .  25
       3.2.3.  NATFW NSLP Signaling Sessions . . . . . . . . . . . .  26
     3.3.  Basic Message Processing  . . . . . . . . . . . . . . . .  27
     3.4.  Calculation of Signaling Session Lifetime . . . . . . . .  27
     3.5.  Message Sequencing  . . . . . . . . . . . . . . . . . . .  30
     3.6.  Authentication, Authorization, and Policy Decisions . . .  31
     3.7.  Protocol Operations . . . . . . . . . . . . . . . . . . .  31
       3.7.1.  Creating Sessions . . . . . . . . . . . . . . . . . .  31
       3.7.2.  Reserving External Addresses  . . . . . . . . . . . .  34
       3.7.3.  NATFW Session Refresh . . . . . . . . . . . . . . . .  41
       3.7.4.  Deleting Sessions . . . . . . . . . . . . . . . . . .  42
       3.7.5.  Reporting Asynchronous Events . . . . . . . . . . . .  44
       3.7.6.  Proxy Mode of Operation . . . . . . . . . . . . . . .  45



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     3.8.  De-Multiplexing at NATs . . . . . . . . . . . . . . . . .  48
     3.9.  Reacting to Route Changes . . . . . . . . . . . . . . . .  50
     3.10. Updating Policy Rules . . . . . . . . . . . . . . . . . .  51

   4.  NATFW NSLP Message Components . . . . . . . . . . . . . . . .  52
     4.1.  NSLP Header . . . . . . . . . . . . . . . . . . . . . . .  52
     4.2.  NSLP Objects  . . . . . . . . . . . . . . . . . . . . . .  53
       4.2.1.  Signaling Session Lifetime Object . . . . . . . . . .  54
       4.2.2.  External Address Object . . . . . . . . . . . . . . .  54
       4.2.3.  Extended Flow Information Object  . . . . . . . . . .  55
       4.2.4.  Information Code Object . . . . . . . . . . . . . . .  56
       4.2.5.  Nonce Object  . . . . . . . . . . . . . . . . . . . .  59
       4.2.6.  Message Sequence Number Object  . . . . . . . . . . .  59
       4.2.7.  Data Terminal Information Object  . . . . . . . . . .  60
       4.2.8.  ICMP Types Object . . . . . . . . . . . . . . . . . .  61
     4.3.  Message Formats . . . . . . . . . . . . . . . . . . . . .  62
       4.3.1.  CREATE  . . . . . . . . . . . . . . . . . . . . . . .  63
       4.3.2.  EXTERNAL (EXT)  . . . . . . . . . . . . . . . . . . .  63
       4.3.3.  RESPONSE  . . . . . . . . . . . . . . . . . . . . . .  64
       4.3.4.  NOTIFY  . . . . . . . . . . . . . . . . . . . . . . .  64

   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  66
     5.1.  Authorization Framework . . . . . . . . . . . . . . . . .  66
       5.1.1.  Peer-to-Peer Relationship . . . . . . . . . . . . . .  66
       5.1.2.  Intra-Domain Relationship . . . . . . . . . . . . . .  67
       5.1.3.  End-to-Middle Relationship  . . . . . . . . . . . . .  68
     5.2.  Security Threats and Requirements . . . . . . . . . . . .  69
       5.2.1.  Data Sender (DS) behind a firewall  . . . . . . . . .  69
       5.2.2.  Data Sender (DS) behind a NAT . . . . . . . . . . . .  70
       5.2.3.  Data Receiver (DR) behind a firewall  . . . . . . . .  70
       5.2.4.  Data Receiver (DR) behind a NAT . . . . . . . . . . .  72
       5.2.5.  NSLP Message Injection  . . . . . . . . . . . . . . .  73
     5.3.  Denial-of-Service Attacks . . . . . . . . . . . . . . . .  74
       5.3.1.  Flooding with CREATE messages from outside  . . . . .  74
       5.3.2.  Flooding with EXT messages from inside  . . . . . . .  75
     5.4.  Man-in-the-Middle Attacks . . . . . . . . . . . . . . . .  75
     5.5.  Message Modification by non-NSIS on-path node . . . . . .  76
     5.6.  Message Modification by malicious NSIS node . . . . . . .  76
     5.7.  Session Ownership . . . . . . . . . . . . . . . . . . . .  77
       5.7.1.  Misuse of Mobility in a NAT Handling Scenario . . . .  77
     5.8.  Misuse of unreleased sessions . . . . . . . . . . . . . .  78
     5.9.  Data Traffic Injection  . . . . . . . . . . . . . . . . .  80
     5.10. Eavesdropping and Traffic Analysis  . . . . . . . . . . .  80
     5.11. Security Framework for the NAT/Firewall NSLP  . . . . . .  80
       5.11.1. Security Protection between neighboring NATFW NSLP
               Nodes . . . . . . . . . . . . . . . . . . . . . . . .  81
       5.11.2. Security Protection between non-neighboring NATFW
               NSLP Nodes  . . . . . . . . . . . . . . . . . . . . .  81



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   6.  IAB Considerations on UNSAF . . . . . . . . . . . . . . . . .  84

   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  85

   8.  Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .  87

   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  88

   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  89
     10.1. Normative References  . . . . . . . . . . . . . . . . . .  89
     10.2. Informative References  . . . . . . . . . . . . . . . . .  89

   Appendix A.  Selecting Signaling Destination Addresses for EXT  .  92

   Appendix B.  Applicability Statement on Data Receivers behind
                Firewalls  . . . . . . . . . . . . . . . . . . . . .  94

   Appendix C.  Firewall and NAT Resources . . . . . . . . . . . . .  96
     C.1.  Wildcarding of Policy Rules . . . . . . . . . . . . . . .  96
     C.2.  Mapping to Firewall Rules . . . . . . . . . . . . . . . .  96
     C.3.  Mapping to NAT Bindings . . . . . . . . . . . . . . . . .  97
     C.4.  NSLP Handling of Twice-NAT  . . . . . . . . . . . . . . .  97

   Appendix D.  Assigned Numbers for Testing . . . . . . . . . . . .  99

   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 100
   Intellectual Property and Copyright Statements  . . . . . . . . . 101
























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1.  Introduction

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

   Several solutions to enable applications to traverse such entities
   have been proposed and are currently in use.  Typically, application
   level gateways (ALG) have been integrated with the firewall or NAT to
   configure the firewall or NAT dynamically.  Another approach is
   middlebox communication (MIDCOM).  In this approach, ALGs external to
   the firewall or NAT configure the corresponding entity via the MIDCOM
   protocol [7].  Several other work-around solutions are available,
   such as STUN [19].  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, etc), 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, respectively, NATFW or QoS
   treatment of data packets.  This means, that for both, NATFW and QoS,
   it is convenient if signaling travels path-coupled, meaning that the
   signaling messages follow exactly the same path that the data packets
   take.  RSVP [13] is an example of a current QoS signaling protocol
   that is path-coupled. [26] proposes the use of RSVP as firewall
   signaling protocol but does not include NATs.

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



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   Internet Signaling Transport (GIST) [2] being the implementation of
   the abstract NTLP).  The signaling logic for QoS and NATFW signaling
   is implemented in the different NSLPs.  The QoS NSLP is defined in
   [6].

   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
   rules with an action that allows the data flow packets to be
   forwarded and creating NAT bindings.  Blocking of data flows requires
   the loading of firewalls rules with an action that will deny
   forwarding of the data flow packets.  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-enabled NAT/firewall along the data path
   intercepts these messages, processes them, and configures itself
   accordingly.  Thereafter, the actual data flow can traverse all these
   configured firewalls/NATs.

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

   1.  Both, data sender and data receiver, are NSIS NATFW NSLP aware.
       This includes the cases where the data sender is logically
       decomposed from the NSIS initiator or the data receiver logically
       decomposed from the NSIS receiver, but both sides support NSIS.
       This scenario assumes deployment of NSIS all over the Internet,
       or at least at all NATs and firewalls.  This scenario is used as
       base assumption, if not otherwise noted.

   2.  Only one end host or region of the network is NSIS NATFW NSLP
       aware, either data receiver or data sender.  This scenario is
       referred to as proxy mode.

   The NATFW NSLP has two basic signaling messages which are sufficient
   to cope with the various possible scenarios likely to be encountered
   before and after widespread deployment of NSIS:

      CREATE message: The basic mode for configuring a path downstream
      from a data sender to a data receiver.

      EXTERNAL (EXT) message: Used to locate upstream NATs/firewalls and
      prime them to expect downstream signaling and at NATs to pre-
      allocate a public address.  This is used for data receivers behind
      these devices to enable their reachability.



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   CREATE and EXT messages are sent by the NSIS initiator (NI) towards
   the NSIS responder (NR).  Both type of messages are acknowledged by a
   subsequent RESPONSE message.  This RESPONSE message is generated by
   the NR if the requested configuration can be established, otherwise
   the NR or any of the NSIS forwarders (NFs) can also generate such a
   message if an error occurs.  NFs and the NR can also generate
   asynchronous messages to notify the NI, the so called NOTIFY
   messages.

   If the data receiver resides in a private addressing realm or
   firewall, and needs to preconfigure the edge-NAT/edge-firewall to
   provide a (publicly) reachable address for use by the data sender, a
   combination of EXTERNAL and CREATE messages is used.

   During the introduction of NSIS, it is likely that one or other of
   the data sender and receiver will not be NSIS aware.  In these cases,
   the NATFW NSLP can utilize NSIS aware middleboxes on the path between
   the data sender and data receiver to provide proxy NATFW NSLP
   services (i.e., the proxy mode operation).  Typically, these boxes
   will be at the boundaries of the realms in which the end hosts are
   located.

   The CREATE and EXT messages 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.
   This state is managed using a soft-state mechanism, i.e., it expires
   unless it is refreshed from time to time.  The NAT bindings and
   firewall rules being installed during the state setup are bound to
   the particular signaling session.  However, the exact local
   implementation of the NAT bindings and firewall rules are NAT/
   firewall specific.

   This memo is structured as follows.  Section 2 describes the network
   environment for NATFW NSLP signaling.  Section 3 defines the NATFW
   signaling protocol and Section 4 defines the message components and
   the overall messages used in the protocol.  The remaining parts of
   the main body of the document, covers security considerations
   Section 5, IAB considerations on UNilateral Self-Address Fixing
   (UNSAF) [15] in Section 6 and IANA considerations in Section 7.
   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 [1].



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   This document uses a number of terms defined in [5] and [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" [20].  In the context of NSIS
      NATFW NSLP, the conditions are the specification of a set of
      packets to which the rule is applied.  The set of actions always
      contains just a single element per rule, and is limited to either
      action "deny" or action "allow".

   o  Reserved policy rule: A policy rule stored at NATs or firewalls
      for activation by a later, different signaling exchange.  This
      type of policy rule is kept in the NATFW NSLP and is not loaded
      into the firewall or NAT engine, i.e., it does not affect the data
      flow handling.

   o  Installed policy rule: A policy rule in operation at NATs or
      firewalls.  This type of rule is kept in the NATFW NSLP and is
      loaded into the firewall or NAT engine, i.e., it is affecting the
      data flow.

   o  Remembered policy rule: A policy rule stored at NATs and firewalls
      for immediate use, as soon as the signaling exchange is
      successfully completed.

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

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

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

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





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

   o  NATFW NSLP peer or peer: An NSIS NATFW NSLP node with which an
      NSIS adjacency has been created as defined in [2].

   o  NATFW NSLP signaling session or signaling session: A signaling
      session defines an association between the NI, NFs, and the NR
      related to a data flow.  All the NATFW NSLP peers on the path,
      including the NI and the NR, use the same identifier to refer to
      the state stored for the association.  The same NI and NR may have
      more than one signaling session active at any time.  The state for
      NATFW NSLP consists of NSLP state and associated policy rules at a
      middlebox.

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

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

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

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

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

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

   o  Signaling Destination Address (SDA): An IP address generally taken
      from the public/global IP address range, although, the SDA may in
      certain circumstances be part of the private/local IP address
      range.  This address is used in EXT signaling message exchanges,
      if the data receiver's IP address is unknown.







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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, are out of scope of this
   memo.

   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 [9].

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

   o  Source and destination IP addresses

   o  Transport protocol number

   o  Transport source and destination port numbers

   Actions for firewalls are usually one or more of:

   o  Allow: forward data packet

   o  Deny: block data packet and discard it





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   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.

   It should be noted that a middlebox may contain two logical
   representations of the policy rule.  The policy rule has a
   representation within the NATFW NSLP, comprising the message routing
   information (MRI) of the NTLP and NSLP information (such as the rule
   action).  The other representation is the implementation of the NATFW
   NSLP policy rule within the NAT and firewall engine of the particular
   device.  Refer to Appendix C for further details.

1.3.  General Scenario for NATFW Traversal

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

   The basic high-level picture of NSIS usage is that end hosts are
   located behind middleboxes, meaning that there is a middlebox on the
   data path from the end host in a private network and the external
   network (NATFW in Figure 1).  Applications located at these end hosts
   try to establish communication with corresponding applications on
   other such end hosts.  They trigger the NSIS entity at the local host
   to control provisioning for middlebox traversal along the prospective



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   data path (e.g., via an API call).  The NSIS entity in turn uses NSIS
   NATFW NSLP signaling to establish policy rules along the data path,
   allowing the data to travel from the sender to the receiver
   unobstructed.


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

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

   Note that 1+ refers to one or more NATFW nodes.

         Figure 1: Generic View of NSIS with NATs and/or Firewalls

   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 the NATFW NSLP

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

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

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

2.1.  Firewall Traversal

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


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

                 private     public        private

             FW: Firewall
             NI: NSIS Initiator
             NR: NSIS Responder

                   Figure 2: Firewall Traversal Scenario

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



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

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

2.2.  NAT with two private Networks

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



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   number of ways to learn its public IP address and port number
   (including the NATFW NSLP) 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.

   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.

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 therefore establish the path via the middleboxes
   in the receiver side private network.

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

2.5.  Both End Hosts behind twice-NATs

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

   Sender and receiver are both within a single private address realm
   but the two partitions potentially have overlapping IP address
   ranges.  Figure 6 shows the arrangement of NATs.

















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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 entities must handle
   requests that are based on symbolic names, and configure the
   middleboxes so that data packets are correctly forwarded from NI to
   NR.  The configuration of those middleboxes may require other
   middlebox communication protocols, such as MIDCOM [7].  NSIS
   signaling is not required in the twice-NAT only case, since
   middleboxes of the twice-NAT type are normally configured by other
   means.  Nevertheless, NSIS signaling might be useful when there are
   also firewalls on the 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, [2]) within the NSLP
   and NTLP accordingly.  For more information see Appendix C.4

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



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   NSIS initiator.  Afterwards, the NSIS initiator can start sending the
   signaling messages to the responder's public IP address.  During this
   process, a public IP address will be allocated for the NSIS initiator
   at the same middlebox as for the responder.  Now, the NSIS signaling
   and the subsequent data packets will traverse the NAT twice: from
   initiator to public IP address of responder (first time) and from
   public IP address of responder to responder (second time).


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


             MB: Middlebox
             NI: NSIS Initiator
             NR: NSIS Responder

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

2.7.  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          public

             MB: Middlebox
             NI: NSIS Initiator
             NR: NSIS Responder

                Figure 8: Multihomed Network with Two NATs




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   Depending on the destination, 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 3.7.2 for an extended discussion of
   this topic with respect to NATs.

2.8.  Multihomed Network with Firewall

   This section describes a multihomed scenario with two firewalls
   placed on alternative paths to the public network (Figure 9).  The
   routing in the private and public network decides 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 the EXT mode of the NATFW NSLP where the NSIS
   responder is located behind these firewalls within the private
   network.  The EXT mode is used to block a particular data flow on an
   upstream firewall.  NSIS must route the EXT mode message upstream
   from NR to NI probably without knowing which path the data traffic
   will take from NI to NR (see also Appendix B).


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

        private          public

             MB: Middlebox
             NI: NSIS Initiator
             NR: NSIS Responder

              Figure 9: Multihomed Network with two Firewalls













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3.  Protocol Description

   This section defines messages, objects, and protocol semantics for
   the NATFW NSLP.

3.1.  Policy Rules

   Policy rules, bound to a session, are the building blocks 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 the action 'translate this address' and further mapping
   information, that might be, in the simplest case, internal IP address
   and external IP address.

   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 in use.  For
   pure firewalls one or more filter rules must be created and for pure
   NATs one or more NAT bindings must be created.  In mixed firewall and
   NAT boxes, the policy rule must be mapped to 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 public or private IP addresses.  However,
   the exact mapping depends on the implementation of the firewall or
   NAT which is different for each vendor.

   The policy rule at the NATFW NSLP level comprises the message routing
   information (MRI) part, carried in the NTLP, and the information
   available in the NATFW NSLP.  The information provided by the NSLP is
   stored in the 'extend flow information' (NATFW_EFI) and 'data
   terminal information' (NATFW_DTINFO) objects, and the message type.
   Additional information, such as the external IP address and port
   number, stored in the NAT or firewall, will be used as well.  The MRI
   carries the filter part of the NAT/firewall-level policy rule that is
   to be installed.

   The NATFW NSLP specifies two actions for the policy rules: deny and
   allow.  A policy rule with action set to deny will result in all
   packets matching this rule to be dropped.  A policy rule with action
   set to allow will result in all packets matching this rule to be
   forwarded.





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3.2.  Basic Protocol Overview

   The NSIS NATFW NSLP is carried over the General Internet Signaling
   Transport (GIST, the implementation of the NTLP) defined in [2].
   NATFW NSLP messages are initiated by the NSIS initiator (NI), handled
   by NSIS forwarders (NF) and received by the NSIS responder (NR).  It
   is required that at least NI and NR implement this NSLP, intermediate
   NFs only implement this NSLP when they provide relevant middlebox
   functions.  NSIS forwarders that do not have any NATFW NSLP functions
   just forward these packets as they have no interest in them.

   A Data Sender (DS), intending to send data to a Data Receiver (DR)
   has to start NATFW NSLP signaling.  This causes the NI associated
   with the data sender (DS) to launch NSLP signaling towards the
   address of data receiver (DR) (see Figure 10).  Although it is
   expected that the DS and the NATFW NSLP NI will 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 3.7.6.)


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

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


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

                     Figure 10: General NSIS signaling

   The following list shows the normal sequence of NSLP events without
   detailing the interaction with the NTLP and the interactions on the
   the NTLP level.

   o  NSIS initiators generate NATFW NSLP CREATE/EXT messages and send
      these towards the NSIS responder.  This CREATE/EXT message is the
      initial message which creates a new signaling session.  The NI and
      the NR will most likely already share an application session
      before they start the NATFW NSLP signaling session.  Note the



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      difference between both sessions.

   o  NSLP CREATE/EXT messages are processed each time a NF with NATFW
      NSLP support is traversed.  Each NF that is intercepting a CREATE/
      EXT message and is accepting it for further treatment is joining
      the particular signaling session.  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).  The NR is
      also joining the signaling session if the CREATE/EXT message is
      accepted.

   o  The RESPONSE message is processed at each NF that has been
      included in the prior signaling session setup.

   o  If the NI has received a successful RESPONSE message and if the
      signaling session started with a CREATE message, the data sender
      can start sending its data flow to the data receiver.  If the Ni
      has received a successful RESPONSE message and if the signaling
      session started with a EXT message, the data receiver is ready to
      receive further CREATE messages.

   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 the sender
   side.  For scenarios with NATs on the receiver side certain problems
   arise, as described in Section 2.4.

   When the NR and the NI are located in different address realms and
   the NR is located behind a NAT, the NI cannot signal to the NR
   address directly.  The DR/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.  This application
   layer signaling, which is out of scope of the NATFW NSLP, may involve
   third parties that assist in exchanging these messages.

   The same holds partially true for NRs located behind firewalls that
   block all traffic by default.  In this case, NR must tell its



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   upstream firewalls of inbound NATFW NSLP signaling and corresponding
   data traffic.  Once the NR has informed the upstream firewalls, it
   can start its application level signaling to initiate communication
   with the NI.  This application layer signaling, which is out of scope
   of the NATFW NSLP, may involve third parties that assist in
   exchanging these messages.  This mechanism can be used by machines
   hosting services behind firewalls as well.  In this case, the NR
   informs the upstream firewalls as described, but does not need to
   communicate this to the NIs.

   NATFW NSLP signaling supports this scenario by using the EXT mode of
   operation

   1.  The DR acquires a public address by signaling on the reverse path
       (DR towards DS) and thus making itself available to other hosts.
       This process of acquiring public addresses is called reservation.
       During this process the DR reserves publicly reachable addresses
       and ports suitable for further usage in application level
       signaling and the publicly reachable address for further NATFW
       NSLP signaling.  However, the data traffic will not be allowed to
       use this address/port initially (see next point).  In the process
       of reservation the DR becomes the NI for the messages necessary
       to obtain the publicly reachable IP address, i.e., the NI for
       this specific signaling session.

   2.  Now on the side of DS, the NI creates a new signaling session and
       signals directly to the public IP address of DR.  This public IP
       address is used as NR's address., 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 forwarding of
       signaling messages, but not data flow packets.  Policy rules
       allowing forwarding of data flow packets set up by the prior EXT
       mode signaling will be activated when the signaling from NI
       towards NR is confirmed with a positive RESPONSE message.  The
       EXTERNAL (EXT) mode of operation is detailed inSection 3.7.2.
















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

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


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

                  Figure 11: A NSIS proxy mode signaling

   The above usage assumes that both ends of a communication support
   NSIS, but fails when NSIS is only deployed at one end of the path.
   In this case only one of the receiving or sending side is NSIS aware
   and not both at the same time.  NATFW NSLP supports this scenario
   (i.e., the DR does not support NSIS) by using a proxy mode, as
   described in Section 3.7.6; the proxy mode operation also supports
   scenarios with a data sender that does not support NSIS, i.e. the
   data receiver must act to enable data flows towards itself.

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

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




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   If the actions requested by a NATFW NSLP message cannot be carried
   out, NFs and the NR must return a failure, such that appropriate
   actions can be taken.  They can do this either during a the request
   message handling (synchronously) by sending an error RESPONSE
   message, or at any time (asynchronously) by sending a notification
   message.

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

3.2.1.  Message Types

   The protocol uses four messages types:

   o  CREATE: a request message used for creating, changing, refreshing,
      and deleting NATFW NSLP signaling sessions, i.e., open the data
      path from DS to DR.

   o  EXTERNAL (EXT): a request message used for reserving, changing,
      refreshing, and deleting EXT NATFW NSLP sessions.  EXT messages
      are forwarded to the edge-NAT or edge-firewall and allow inbound
      CREATE messages to be forwarded to the NR.  Additionally, EXT
      messages reserve an external address and, if applicable, port
      number at an edge-NAT.

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

   o  RESPONSE: used as a response to CREATE and EXT request messages.

3.2.2.  Classification of RESPONSE Messages

   RESPONSE messages will be generated synchronously to CREATE and EXT
   messages by NSIS Forwarders and Responders to report success or
   failure of operations or some information relating to the session or
   a node.  RESPONSE messages MUST NOT be generated for any other
   message, such as NOTIFY and RESPONSE.

   All RESPONSE messages MUST carry a NATFW_INFO object which contains a
   severity class code and a response code (see Section 4.2.4).  This
   section defines terms for groups of RESPONSE messages depending on
   the severity class.

   o  Successful RESPONSE: Messages carrying NATFW_INFO with severity
      class 'Success' (0x2).

   o  Informational RESPONSE: Messages carrying NATFW_INFO with severity
      class 'Informational' (0x1) (only used with NOTIFY messages).



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   o  Error RESPONSE: Messages carrying NATFW_INFO with severity class
      other than 'Success' or 'Informational'.

3.2.3.  NATFW NSLP Signaling Sessions

   A signaling session defines an association between the NI, NFs, and
   the NR related to a data flow.  This association is created when the
   initial CREATE or EXT message is successfully received at the NFs or
   the NR.  There is signaling session state stored at the NTLP layer
   and at the NATFW NSLP level.  The signaling session state for the
   NATFW NSLP comprises NSLP state and the associated policy rules at a
   middlebox.

   The signaling session is identified by the session ID (plus other
   information at the NTLP level).  The session ID is generated by the
   NI before the initial CREATE or EXT message is sent.  The value of
   the session ID MUST generated in a random way, i.e., the output MUST
   NOT be easily guessable by third parties.  The session ID is not
   stored in any NATFW NSLP message but passed on to the NTLP.

   A NATFW NSLP signaling session can conceptually be in different
   states, implementations may use other or even more states.  The
   signaling session can have these states at a node:

   o  Pending: The signaling session has been created and the node is
      waiting for a RESPONSE message to the CREATE or EXT message.  A
      signaling session in state 'Pending' MUST be marked as 'Dead' if
      no corresponding RESPONSE message has been received within the
      time of the locally granted session lifetime of the forwarded
      CREATE or EXT message (as described in Section 3.4).

   o  Established: The signaling session is established, i.e, the
      signaling has been successfully performed.  A signaling session in
      state 'Established' MUST be marked as 'Dead' if no refresh message
      has been received within the time of the locally granted session
      lifetime of the RESPONSE message (as described in Section 3.4).

   o  Dead: Either the signaling session is timed out or the node has
      received an error RESPONSE message for the signaling session and
      the signaling session can be deleted.

   o  Transit: The node has received an asynchronous message, i.e., a
      NOTIFY, and can delete the signaling session if needed.  When a
      node has received a NOTIFY message (for instance, indicating a
      route change) it marks it as 'Transit' and deletes this session if
      it is unused for some time specific to the local node.  This idle
      time does not need to be fixed, since it can depend on the node
      local maintenance cycle, i.e., the session could be deleted if the



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      node runs it garbage collection cycle.

3.3.  Basic Message Processing

   All NATFW messages are subject to some basic message processing when
   received at a node, independent of message type.  Initially, the
   syntax of the NSLP message is checked and a RESPONSE message with an
   appropriate error of class 'Protocol error' (0x1) code is generated
   if any problem is detected.  If a message is delivered to the NATFW
   NSLP, this implies that the NTLP layer has been able to correlate it
   with the SID and MRI entries in its database.  There is therefore
   enough information to identify the source of the message and routing
   information to route the message back to the NI through an
   established chain of NTLP messaging associations.  The message is not
   further forwarded if any error in the syntax is detected.  The
   specific response codes stemming from the processing of objects are
   described in the respective object definition section (see
   Section 4).  After passing this check, the NATFW NSLP node performs
   authentication/authorization related checks described in Section 3.6.
   Further processing is executed only if these tests have been
   successfully passed, otherwise the processing stops and an error
   RESPONSE is returned.

   Further message processing stops whenever an error RESPONSE message
   is generated, and the EXT or CREATE message is discarded.

3.4.  Calculation of Signaling Session Lifetime

   NATFW NSLP signaling sessions, and the corresponding policy rules
   which may have been installed, are maintained via a soft-state
   mechanism.  Each signaling session is assigned a signaling session
   lifetime and the signaling session is kept alive as long as the
   lifetime is valid.  After the expiration of the signaling session
   lifetime, signaling sessions and policy rules MUST be removed
   automatically and resources bound to them MUST be freed as well.
   Signaling session lifetime is handled at every NATFW NSLP node.  The
   NSLP forwarders and NSLP responder MUST NOT trigger signaling session
   lifetime extension refresh messages (see Section 3.7.3): this is the
   task of the NSIS initiator.

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

   o  the number of lost refresh messages that NFs should cope with;





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

   o  network vulnerability due to session hijacking ([8]), session
      hijacking is made easier when the NI does not explicitly remove
      the session);

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

   o  the load on the signalling plane.  Short lifetimes imply more
      frequent signaling messages.

   o  the acceptable time for a signaling session to be present after it
      is no longer actually needed.  For example, if the existence of
      the signaling session implies a monetary cost and teardown cannot
      be guaranteed, shorter lifetimes would be preferable.

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

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

   2.  To avoid premature loss of state, lt (with lt being the signaling
       session lifetime) must satisfy lt >= (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 signaling
       session lifetime and the refresh message period; the algorithm
       provided is only given as an example.

   This requested signaling session lifetime value lt is stored in the
   NATFW_LT object of the NSLP message.

   NSLP forwarders can execute the following behavior with respect to
   the lifetime handling:

   Requested signaling session lifetime acceptable:

      No changes to the signaling session lifetime values are needed.
      The CREATE or EXT message is forwarded.





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   Signaling session lifetime can be lowered:

      The NSLP responder MAY also lower the requested signaling session
      lifetime to an acceptable value (based on its local policies).  If
      an NF changes the signaling session lifetime value, it MUST store
      the new value in the NATFW_LT object.  The CREATE or EXT message
      is forwarded.


   Requested signaling session lifetime is too big:

      The NSLP responder MAY reject the requested signaling session
      lifetime value as being too big and MUST generate an error
      RESPONSE message of class 'Signaling session failures' (0x6) with
      response code 'Requested lifetime is too big' (0x02) upon
      rejection.  Lowering the lifetime is preferred instead of
      generating an error message.


   Requested signaling session lifetime is too small:

      The NSLP responder MAY reject the requested signaling session
      lifetime value as being to small and MUST generate an error
      RESPONSE message of class 'Signaling session failures' (0x6) with
      response code 'Requested lifetime is too small' (0x10) upon
      rejection.

   NFs MUST NOT increase the signaling session lifetime value.  Messages
   can be rejected on the basis of the signaling session lifetime being
   too long when a session is first created and also on refreshes.

   The NSLP responder generates a successful RESPONSE for the received
   CREATE or EXT message, sets the signaling session lifetime value in
   the NATFW_LT object to the above granted lifetime and sends the
   message back towards NSLP initiator.

   Each NSLP forwarder processes the RESPONSE message, reads and stores
   the granted signaling session lifetime value.  The forwarders MUST
   accept the granted signaling session lifetime, as long as this value
   is less than or equal to the acceptable value.  The acceptable value
   refers to the value accepted by the NSLP forwarder when processing
   the CREATE or EXT message.  For received values greater than the
   acceptable value, NSLP forwarders MUST generate a RESPONSE message of
   class 'Signaling session failures' (0x6) with response code
   'Requested lifetime is too big' (0x02).  For received values lower
   than the values acceptable by the node local policy, NSLP forwarders
   MUST generate a RESPONSE message of class 'Signaling session
   failures' (0x6) with response code 'Requested lifetime is too small'



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   (0x10).  Figure 12 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.  When the NSLP forwarder receives
   the RESPONSE message with a signaling session lifetime value of 15
   seconds it checks whether this value is lower or equal to the
   acceptable value.


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

      lt  = lifetime

           Figure 12: Signaling Session Lifetime Setting 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 in
   time.  Messages can be lost along the path or duplicated.  So all
   NATFW NSLP nodes should be able to identify either old messages that
   have been received before (duplicated), or the case that messages
   have been lost before (loss).  For message replay protection it is
   necessary to keep information about messages that have already been
   received and requires every NATFW NSLP message to carry a message
   sequence number (MSN), see also Section 4.2.6.

   The MSN MUST be set by the NI and MUST NOT be set or modified by any
   other node.  The initial value for the MSN MUST be generated randomly
   and MUST be unique only within the session for which it is used.  The
   NI MUST increment the MSN by one for every message sent.  Once the
   MSN has reached the maximum value, the next value it takes is zero.
   All NATFW NSLP nodes MUST use the algorithm defined in [3] to detect
   MSN wrap-arounds.

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

   When receiving a CREATE or EXT message, a NATFW NSLP node uses the
   MSN given in the message to determine whether the state being
   requested is different to the state already installed.  The message
   MUST be discarded if the received MSN value is equal to or lower than



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   the stored MSN value.  Such a received MSN value can indicate a
   duplicated and delayed message or replayed message.  If the received
   MSN value is greater than the already stored MSN value, the NATFW
   NSLP MUST update its stored state accordingly, if permitted by all
   security checks (see Section 3.6), and stores the updated MSN value
   accordingly.

3.6.  Authentication, Authorization, and Policy Decisions

   NATFW NSLP nodes receiving signaling messages MUST first check
   whether this message is authenticated and authorized to perform the
   requested action.  NATFW NSLP nodes requiring more information than
   provided MUST generate an error RESPONSE of class 'Permanent failure'
   (0x5) with response code 'Authentication failed' (0x01) or with
   response code 'Authorization failed' (0x02).

   The NATFW NSLP is expected to run in various environments, such as
   IP-based 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 more strongly
   authenticated/authorized signaling.  This enterprise scenario may
   require the use of an infrastructure and administratively assigned
   identities to operate the NATFW NSLP.

   Once the NI is authenticated and authorized, another step is
   performed.  The requested policy rule for the session is checked
   against a set of policy rules, i.e., whether the requesting NI is
   allowed to request the policy rule to be loaded in the device.  If
   this fails the NF or NR must send an error RESPONSE of class
   'Permanent failure' (0x5) and with response code 'Authorization
   failed' (0x02).

3.7.  Protocol Operations

   This section defines the protocol operations including, how to create
   sessions, maintain them, and how to reserve addresses.

3.7.1.  Creating Sessions

   Allowing two hosts to exchange data even in the presence of
   middleboxes is realized in the NATFW NSLP by use of the CREATE
   message.  The NI (either the data sender or a proxy) generates a
   CREATE message as defined in Section 4.3.1 and hands it to the NTLP.
   The NTLP forwards the whole message on the basis of the message
   routing information (MRI) towards the NR.  Each NSIS forwarder along



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   the path that implements NATFW NSLP, processes the NSLP message.
   Forwarding is thus managed NSLP hop-by-hop but may pass transparently
   through NSIS forwarders which do not contain NATFW NSLP functionality
   and non-NSIS aware routers between NSLP hop way points.  When the
   message reaches the NR, the NR can accept the request or reject it.
   The NR generates a response to CREATE 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 in this example), a NF (e.g., NAT), and NR (DR in this
   example).


       NI      Private Network        NF    Public Internet        NR
       |                              |                            |
       | CREATE                       |                            |
       |----------------------------->|                            |
       |                              |                            |
       |                              |                            |
       |                              | CREATE                     |
       |                              |--------------------------->|
       |                              |                            |
       |                              | RESPONSE                   |
       |    RESPONSE                  |<---------------------------|
       |<-----------------------------|                            |
       |                              |                            |
       |                              |                            |

           Figure 13: CREATE message flow with success RESPONSE

   There are several processing rules for a NATFW peer when generating
   and receiving CREATE messages, since this message type is used for
   creating new signaling sessions, updating existing, extending the
   lifetime and deleting signaling session.  The three latter functions
   operate in the same way for all kinds of CREATE message, and are
   therefore described in separate sections:

   o  Extending the lifetime of signaling sessions is described in
      Section 3.7.3.

   o  Deleting signaling sessions is described in Section 3.7.4.

   o  Updating policy rules is described in Section 3.10.

   For an initial CREATE message creating a new NATFW NSLP session, the
   processing of CREATE messages is different for every NATFW node type:

   o  NSLP initiator: An NI only generates CREATE messages and hands
      them over to the NTLP.  The NI should never receive CREATE



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      messages and MUST discard it.

   o  NATFW NSLP forwarder: NFs that are unable to forward the CREATE
      message to the next hop MUST generate an error RESPONSE of class
      'Permanent failure' (0x6) with response code 'Did not reach the
      NR' (0x07).  This case may occur if the NTLP layer cannot find an
      NATFW NSLP peer, either another NF or the NR, and returns an error
      via the GIST API.  The NSLP message processing at the NFs 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 EXT message (see Section 3.7.2).  The matching process
         considers the received MRI information and the stored MRI
         information, as described in Section 3.8.  If no matching
         reservation can be found, i.e. no reservation has been made in
         advance, the NSLP MUST return an error RESPONSE of class
         'Signaling session failure' (0x6) with response code 'No
         reservation found matching the MRI of the CREATE request'
         (0x03) MUST be generated.  If there is a matching reservation,
         the NSLP stores the data sender's address (and if applicable
         port number) as part of the source address of the policy rule
         ('the remembered policy rule') to be loaded and forwards the
         message with the destination address set to the internal
         (private in most cases) address of NR.  When the initial CREATE
         message is received at the private side, the NAT binding is
         allocated, but not activated (see also Appendix C.3).  The MRI
         information is updated to reflect the address, and if
         applicable port, translation.  The NSLP message is forwarded
         towards the NR with source address set to the NAT's external
         address from the newly remembered 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 towards
         the NR.

      *  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.






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   o  NSLP receiver: NRs receiving initial CREATE messages MUST reply
      with a success RESPONSE of class 'Success' (0x2) with response
      code set to 'All successfully processed' (0x01), if they accept
      the CREATE message.  Otherwise they MUST generate a RESPONSE
      message with a suitable response code.  RESPONSE messages are sent
      back NSLP hop-by-hop towards the NI, irrespective of the response
      codes, either success or error.

   Remembered policy rules at middleboxes MUST be only installed upon
   receiving a corresponding successful RESPONSE message with the same
   SID and MSN as the CREATE message that caused them to be remembered.
   This is a countermeasure to several problems, for example, wastage of
   resources due to loading policy rules at intermediate NFs when the
   CREATE message does not reach the final NR for some reason.

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

   o  NSLP initiator: 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  NSLP forwarder: NFs install the remembered policy rules, if a
      successful RESPONSE message with matching SID and MSN is received.
      If an ERROR RESPONSE message with matching SID and MSN is
      received, the session is marked as dead, no policy rule is
      installed and the remembered rule is discarded.

   o  NSIS responder: The NR should never receive RESPONSE messages and
      MUST silently drop any such messages received.

3.7.2.  Reserving External Addresses

   NSIS signaling is intended to travel end-to-end, even in the presence
   of NATs and firewalls on-path.  This works well in cases where the
   data sender is itself behind a NAT or a firewall as described in
   Section 3.7.1.  For scenarios where the data receiver is located
   behind a NAT or a firewall and it needs to receive data flows from
   outside its own network (usually referred to as inbound flows, see
   Figure 5) the problem is more troublesome.

   NSIS signaling, as well as subsequent data flows, are directed to a
   particular destination IP address that must be known in advance and
   reachable.  Data receivers must tell the local NSIS infrastructure
   (i.e., the upstream firewalls/NATs) about incoming NATFW NSLP
   signaling and data flows before they can receive these flows.  It is



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   necessary to differentiate between data receivers behind NATs and
   behind firewalls for understanding the further NATFW procedures.
   Data receivers that are only behind firewalls already have a public
   IP address and they need only to be reachable for NATFW signaling.
   Unlike data receivers behind just firewalls, data receivers behind
   NATs do not have public IP addresses; consequently they are not
   reachable for NATFW signaling by entities outside their addressing
   realm.

   The preceding discussion addresses the situation where a DR node that
   wants to be reachable is unreachable because the NAT lacks a suitable
   rule with the 'allow' action which would forward inbound data.
   However, in certain scenarios, a node situated behind upstream
   firewalls that do not block inbound data traffic (firewalls with
   "default to allow") unless requested might wish to prevent traffic
   being sent to it from specified addresses.  In this case, NSIS NATFW
   signaling can be used to achieve this by installing a policy rule
   with its action set to 'deny' using the same mechanisms as for
   'allow' rules.

   The required result is obtained by sending a EXTERNAL (EXT) message
   in the upstream direction of the intended data flow.  When using this
   functionality the NSIS initiator for the 'Reserve External Address'
   signaling is typically the node that will become the DR for the
   eventual data flow.  To distinguish this initiator from the usual
   case where the NI is associated with the DS, the NI is denoted by NI+
   and the NSIS responder is similarly denoted by NR+.
























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       Public Internet                Private Address
                                           Space

                    Edge
    NI(DS)         NAT/FW                  NAT                   NR(DR)
    NR+                                                          NI+

    |               |                       |                       |
    |               |                       |                       |
    |               |                       |                       |
    |               |    EXT[(DTInfo)]      |    EXT[(DTInfo)]      |
    |               |<----------------------|<----------------------|
    |               |                       |                       |
    |               |RESPONSE[Success/Error]|RESPONSE[Success/Error]|
    |               |---------------------->|---------------------->|
    |               |                       |                       |
    |               |                       |                       |

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

     Figure 14: Reservation message flow for DR behind NAT or firewall

   Figure 14 shows the EXT message flow for enabling inbound NATFW NSLP
   signaling messages.  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 EXTERNAL (EXT) 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  It may be necessary to use a signaling destination address (SDA)
      as the actual target of the EXT message (NR+) if the DR is located
      behind a NAT and the address of the DS is unknown.  The SDA is an
      arbitrary address in the outermost address realm on the other side
      of the NAT from the DR.  Typically this will be a suitable public
      IP address when the 'outside' realm is the public Internet.  This
      choice of address causes the EXT message to be routed through the
      NATs towards the outermost realm and would force interception of
      the message by the outermost NAT in the network at the boundary
      between the private address and the public address realm (the
      edge-NAT).  It may also be intercepted by other NATs and firewalls
      on the path to the edge-NAT.




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   Basically, there are two different signaling scenarios.  Either

   1.  the DR behind the NAT/firewall knows the IP address of the DS in
       advance,

   2.  or the address of DS is not known in advance.

   Case 1 requires the NATFW NSLP to request the path-coupled message
   routing method (PC-MRM) from the NTLP.  The EXT message MUST be sent
   with PC-MRM (see Section 5.8.1 in [2]) with the direction set to
   'upstream'.  The handling of case 2 depends on the situation of DR:
   If DR is solely located behind a firewall, the EXT message MUST be
   sent with the PC-MRM, direction 'upstream', and data flow source IP
   address set to wildcard.  If DR is located behind a NAT, the EXT
   message MUST be sent with the loose-end message routing method (LE-
   MRM, see Section 5.8.2 in [2]), the destination-address set to the
   signaling destination address (SDA, see also Appendix A).  For
   scenarios with DR being behind a firewall, special conditions apply
   (applicability statement, Appendix B).  The data receiver is
   challenged to determine whether it is solely located behind firewalls
   or NATs, for choosing the right message routing method.  This
   decision can depend on a local configuration parameter, possibly
   given through DHCP, or it could be discovered through other non-NSLP
   related testing of the network configuration.

   For case 2 with NAT, the NI+ (which could be on the data receiver DR
   or on any other host within the private network) sends the EXT
   message targeted to the signaling destination address.  The message
   routing for the EXT 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 an NSIS session) the signaling
   direction does not matter since the data path is modified through
   route pinning due to the external IP address at the NAT.  Subsequent
   NSIS messages (and also data traffic) will travel through the same
   NAT boxes.  However, this is only valid for the NAT boxes, but not
   for any intermediate firewall.  That is the reason for having a
   separate CREATE message enabling the reservations made with EXT at
   the NATs and either enabling prior reservations or creating new
   pinholes at the firewalls which are encountered on the downstream
   path depending on whether the upstream and downstream routes
   coincide.

   The EXT signaling message creates an NSIS NATFW session at any
   intermediate NSIS NATFW peer(s) encountered, independent of the
   message routing method used.  Furthermore, it has to be ensured that
   the edge-NAT or edge-firewall device is discovered as part of this
   process.  The end host cannot be assumed to know this device -



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   instead the NAT or firewall box itself is assumed to know that it is
   located at the outer perimeter of the network.  Forwarding of the EXT
   message beyond this entity is not necessary, and MUST be prohibited
   as it may provide information on the capabilities of internal hosts.
   It should be noted, that it is the outermost NAT or firewall that is
   the edge-device that must be found during this discovery process.
   For instance, when there are a NAT and afterwards a firewall on the
   outbound path at the network border, the firewall is the edge-
   firewall.  All messages must be forwarded to the topology-wise
   outermost edge-device, to ensure that this devices knows about the
   signaling sessions for incoming CREATE messages.  However, the NAT is
   still the edge-NAT because it has a public globally routable IP
   address on its public side: this is not affected by any firewall
   between the edge-NAT and the public network.


   Possible edge arrangements are:

          Public Net   -----------------  Private net  --------------

        | Public Net|--|Edge-FW|--|FW|...|FW|--|DR|

        | Public Net|--|Edge-FW|--|Edge-NAT|...|NAT or FW|--|DR|

        | Public Net|--|Edge-NAT|--|NAT or FW|...|NAT or FW|--|DR|

   The edge-NAT or edge-firewall device closest to the public realm
   responds to the EXT message with a successful RESPONSE message.  An
   edge-NAT includes an NATFW_EXT_IP object (see Section 4.2.2),
   carrying the public reachable IP address, and if applicable port
   number.

   There are several processing rules for a NATFW peer when generating
   and receiving EXT messages, since this message type is used for
   creating new reserve signaling sessions, updating existing, extending
   the lifetime and deleting signaling session.  The three latter
   functions operate in the same way for all kinds of CREATE and EXT
   messages, and are therefore described in separate sections:

   o  Extending the lifetime of signaling sessions is described in
      Section 3.7.3.

   o  Deleting signaling sessions is described in Section 3.7.4.

   o  Updating policy rules is described in Section 3.10.

   The NI+ MAY include a NATFW_DTINFO object in the EXT message when
   using the LE-MRM.  The LE-MRM does not include enough information for



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   some types of NATs (basically, those NATs which also translate port
   numbers) to perform the address translation.  This information is
   provided in the NATFW_DTINFO (see Section 4.2.7).  This information
   SHOULD include at least the 'dst port number' and 'protocol' fields,
   in the DTInfo object as these may be required by en-route NATs,
   depending on the type of the NAT.  All other fields MAY be set by the
   NI+ to restrict the set of possible NIs.  An edge-NAT will use the
   information provided in the NATFW_DTINFO object to allow only NATFW
   CREATE message with the MRI matching ('src IPv4/v6 address', 'src
   port number', 'protocol') to be forwarded.  A NAT requiring
   information carried in the NATFW_DTINFO can generate a number of
   error RESPONSE messages of class 'Signaling session failures' (0x6):

   o  'Requested policy rule denied due to policy conflict' (0x04)

   o  'DTINFO object is required' (0x07)

   o  'Requested value in sub_ports field in NATFW_EFI not permitted'
      (0x08)

   o  'Requested IP protocol not supported' (0x09)

   o  'Plain IP policy rules not permitted -- need transport layer
      information' (0x0A)

   o  'source IP address range is too large' (0x0C)

   o  'destination IP address range is too large' (0x0D)

   o  'source L4-port range is too large' (0x0E)

   o  'destination L4-port range is too large' (0x0F)

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

   o  NSLP initiator: NI+ only generate EXT messages.  When the data
      sender's address information is known in advance the NI+ MAY
      include a NATFW_DTINFO object in the EXT message (as described
      above).  When the data sender's IP address is not known, the NI+
      MUST NOT include a NATFW_DTINFO object.  The NI should never
      receive EXT messages and MUST silently discard it.

   o  NSLP forwarder: The NSLP message processing at NFs 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 MUST



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         generate an error RESPONSE of class 'Protocol error' (0x3) with
         response code 'Received EXT request message on external side'
         (0x0B) MUST be generated.  If received at the internal (private
         address) and the NATFW_EFI object contains the action 'deny',
         an error RESPONSE of class 'Protocol error' (0x3) with response
         code 'Requested rule action not applicable' (0x06) MUST be
         generated.  If received at the internal address, an IP address,
         and if applicable a port, is reserved.  If it is an edge-NAT
         and there is no edge-firewall beyond, the NSLP message is not
         forwarded any further and a successful RESPONSE message is
         generated containing an NATFW_EXT_IP object holding the
         translated address, and if applicable port, information in the
         binding reserved as a result of the EXT message.  The RESPONSE
         message is sent back towards the NI+.  If it is not an edge-
         NAT, the NSLP message is forwarded further using the translated
         IP address as signaling source address and including the
         translated IP address/port in the MRI.  The edge-NAT or any
         other NAT MAY reject EXT messages not carrying a NATFW_DTINFO
         object or if the address information within this object is
         invalid or is not compliant with local policies (e.g., the
         information provided relates to a range of addresses
         ('wildcarded') but the edge-NAT requires exact information
         about DS' IP address and port) with the above mentioned
         response codes.

      *  Firewall: Non edge-firewalls remember the requested policy
         rule, keep session state, and forward the message.  Edge-
         firewalls stop forwarding the EXT message.  The policy rule is
         immediately loaded if the action in the NATFW_EFI object is set
         to 'deny' and the node is an edge-firewall.  The policy rule is
         remembered, but not activated, if the action in the NATFW_EFI
         object is set to 'allow'.  In both cases, a successful RESPONSE
         message is generated.

      *  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 MUST generate an error RESPONSE message of class
      'Permanent failure' (0x5) with response code 'No edge-device here'
      (0x06).

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

   o  NSLP initiator: Upon receiving a successful RESPONSE message, the
      NI+ can rely on the requested configuration for future inbound
      sessions.  If the response contains an NATFW_EXT_IP object, the NI



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      can use IP address and port pairs carried for further application
      signaling.  After receiving a error RESPONSE message, the NI+ MAY
      try to generate the EXT message again or give up and report the
      failure to the application, depending on the error condition.

   o  NSLP forwarder: NFs simply forward this message as long as they
      keep state for the requested reservation, if the RESPONSE message
      contains NATFW_INFO object with class set to 'Success' (0x2).  If
      the RESPONSE message contains NATFW_INFO object with class set not
      to 'Success' (0x2), the session is marked as dead.

   o  NSIS responder: This type of message should never be received by
      any NR+.  The NF should never receive response messages and MUST
      silently discard it.

   Reservations with action 'allow' made with EXT MUST be enabled by a
   subsequent CREATE message.  A reservation made with EXT (independent
   of selected action) 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 3.7.1 or EXT in proxy mode Section 3.7.6 no data
   traffic will be forwarded to DR beyond the edge-NAT or edge-firewall.
   The only function of EXT is to ensure that subsequent CREATE messages
   traveling towards the NR will be forwarded across the public-private
   boundary towards the DR.  Correlation of incoming CREATE messages to
   EXT reservation states is described in Section 3.8.

3.7.3.  NATFW Session Refresh

   NATFW NSLP sessions are maintained on a soft-state basis.  After a
   specified timeout, sessions and corresponding policy rules are
   removed automatically by the middlebox, if they are not refreshed.
   Soft-state is created by CREATE and EXT and the maintenance of this
   state must be done by these messages.  State created by CREATE must
   be maintained by CREATE, state created by EXT must be maintained by
   EXT.  Refresh messages, are messages carrying the same session ID as
   the initial message and a NATFW_LT lifetime object with a lifetime
   greater than zero.  Messages with the same SID but carrying a
   different MRI are treated as updates of the policy rules and are
   processed as defined in Section 3.10.  Every refresh CREATE or EXT
   message MUST be acknowledged by an appropriate response message
   generated by 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



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   detail.


   NI      Public Internet        NAT    Private address       NR

      |                              |          space             |
      | CREATE[lifetime > 0]         |                            |

      |----------------------------->|                            |
      |                              |                            |
      |                              |                            |
      |                              |  CREATE[lifetime > 0]      |
      |                              |--------------------------->|
      |                              |                            |
      |                              |   RESPONSE[Success/Error]  |
      |   RESPONSE[Success/Error]    |<---------------------------|
      |<-----------------------------|                            |
      |                              |                            |
      |                              |                            |

       Figure 16: Successful Refresh Message Flow, CREATE as example

   Processing of session refresh CREATE and EXT messages is different
   for every NSIS node type:

   o  NSLP initiator: The NI/NI+ can generate session refresh CREATE/EXT
      messages before the session times out.  The rate at which the
      refresh CREATE/EXT messages are sent and their relation to the
      session state lifetime is discussed further in Section 3.4.

   o  NSLP forwarder: Processing of this message is independent of the
      middlebox type and is as described in Section 3.4.

   o  NSLP responder: NRs accepting a session refresh CREATE/EXT message
      generate a successful RESPONSE message, including the granted
      lifetime value of Section 3.4 in a NATFW_LT object.

3.7.4.  Deleting Sessions

   NATFW NSLP sessions can be deleted at any time.  NSLP initiators can
   trigger this deletion by using a CREATE or EXT messages with a
   lifetime value set to 0, as shown in Figure 17.  Whether a CREATE or
   EXT message type is used, depends on how the session was created.








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      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 delete the session immediately.
   Policy rules associated with this particular session MUST be also
   deleted immediately.  This message is forwarded until it reaches the
   final NR.  The CREATE/EXT 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.

   NSIS initiators can use CREATE/EXT message with lifetime set to zero
   in an aggregated way, such that a single CREATE or EXT message is
   terminating multiple signaling sessions.  NIs can follow this
   procedure if the like to aggregate session deletion requests: The NI
   uses the CREATE or EXT message with the session ID set to zero and
   the MRI's source-address set to its used IP address.  All other
   fields of the respective sessions to be terminated are set as well,
   otherwise these fields are completely wildcarded.  The NSLP message
   is transferred to the NTLP requesting 'explicit routing' as described
   in Sections 5.2.1 and 7.1.4. in [2].

   The downstream NF receiving such an aggregated CREATE or EXT message
   MUST reject it with an error RESPONSE of class 'Permanent failure'
   (0x5) with response code 'Authentication failed' (0x01) if the
   authentication fails and with an error RESPONSE of class 'Permanent
   failure' (0x5) with response code 'Authorization failed' (0x02) if
   the authorization fails.  Per session proof of ownership, as it is
   defined in this memo, is not possible anymore when using this
   aggregated mode.  However, the downstream NF can use the relationship
   between the information of the received CREATE or EXT message and the
   GIST messaging association where the request has been received.  The
   downstream NF MUST only accept this aggregated CREATE or EXT message
   through already established GIST messaging associations with the NI.
   The downstream NF MUST NOT propagate this aggregated CREATE or EXT
   message but it MAY generate and forward per session CREATE or EXT
   messages.






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3.7.5.  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 3.9) are the only events that are defined, but
   other events may be defined in later revisions of this memo.

   NFs and NRs may generate NOTIFY messages upon asynchronous events,
   with a NATFW_INFO object indicating the reason for event.  These
   reasons can be carried in the NATFW_INFO object (class MUST be set to
   'Informational' (0x1)) within the NOTIFY message.  This list shows
   the response codes and the associated actions to take at NFs and the
   NI:

   o  'Route change: possible route change on the downstream path'
      (0x01): Follow instructions in Section 3.9.

   o  'Re-authentication required' (0x02): The NI should re-send the
      authentication.

   o  'NATFW node is going down soon' (0x03): The NI and other NFs
      should be prepared for a service interruption at any time.

   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 analyze the notified event and behave
      appropriately based on the event type.  NIs MUST NOT generate
      NOTIFY messages.

   o  NSLP forwarder: NFs analyze the notified event and behave based on
      the above description per response code.  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 including a response code based on the



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      reported event.  The NR should never receive NOTIFY messages and
      MUST silently discard it.

   NATFW NSLP forwarders, keeping multiple signaling sessions at the
   same time, can experience problems when shutting down service
   suddenly.  This sudden shutdown can be result of node local failure,
   for instance, due to a hardware failure.  This NF generates NOTIFY
   messages for each of the signaling sessions and tries to send them
   upstream.  Due to the number of NOTIFY messages to be sent, the
   shutdown of the node may be unnecessarily prolonged, since not all
   messages can be sent at the same time.  This case can be described as
   a NOTIFY storm, if a multitude of signaling sessions is involved.

   To avoid the need of generating per signaling session NOTIFY messages
   in such a scenario described or similar cases, NFs SHOULD follow this
   procedure: The NF uses the NOTIFY message with the session ID in the
   NTLP set to zero, with the MRI completely wildcarded, using the
   'explicit routing' as described in Sections 5.2.1 and 7.1.4. in [2].
   The upstream NF receiving this type of NOTIFY immediately regards all
   signaling sessions from that peer matching the MRI as void.  This
   message will typically result in multiple NOTIFY messages at the
   upstream NF, i.e., the NF can generate per terminated session a
   NOTIFY message.  However, a NF MAY aggregate again the NOTIFY
   messages as described here.

3.7.6.  Proxy Mode of Operation

   Some migration scenarios need specialized support to cope with cases
   where NSIS is only deployed in same areas of the Internet.  End-to-
   end signaling is going to fail without NSIS support at or near both
   data sender and data receiver terminals.  A proxy mode of operation
   is needed.  This proxy mode of operation must terminate the NATFW
   NSLP signaling as topologically close to the terminal for which it is
   proxying and proxy all messages.  This NATFW NSLP node doing the
   proxying of the signaling messages becomes either the NI or the NR
   for the particular signaling session, depending on whether it is the
   DS or DR that does not support NSIS.  Typically, the edge-NAT or the
   edge-firewall would be used to proxy NATFW NSLP messages.

   This proxy mode operation does not require any new CREATE or EXT
   message type, but relies on extended CREATE and EXT message types.
   They are called respectively CREATE-PROXY and EXT-PROXY and are
   distinguished by setting the P flag in the NSLP header to P=1.  This
   flag instructs edge-NATs and edge-firewalls receiving them to operate
   in proxy mode for the session in question.  The semantics of the
   CREATE and EXT message types are not changed and the behavior of the
   various node types is as defined in Section 3.7.1 and Section 3.7.2,
   except for the proxying node.  The following paragraphs describe the



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   proxy mode operation for data receivers behind middleboxes and data
   senders behind middleboxes.

3.7.6.1.  Proxying for a Data Sender

   The NATFW NSLP gives the NR the ability to install state on the
   upstream path towards the data sender for downstream data packets,
   even when only the receiving side is running NSIS (as shown in
   Figure 18).  The goal of the method described is to trigger the edge-
   NAT/edge-firewall to generate a CREATE message on behalf of the data
   receiver.  In this case, an NR can signal towards the network border
   as it is performed in the standard EXT message handling scenario as
   in Section 3.7.2.  The message is forwarded until the edge-NAT/
   edge-firewall is reached.  A public IP address and port number is
   reserved at an edge-NAT/edge-firewall.  As shown in Figure 18, unlike
   the standard EXT message handling case, the edge-NAT/edge-firewall is
   triggered to send a CREATE message on a new reverse path which
   traverse several firewalls or NATs.  The new reverse path for CREATE
   is necessary to handle routing asymmetries between the edge-NAT/
   edge-firewall and DR.  It must be stressed that the semantics of the
   CREATE and EXT messages is not changed, i.e., each is processed as
   described earlier.


      DS       Public Internet     NAT/FW    Private address      NR
     No NI                            NF         space            NI+
      NR+

      |                               |   EXT-PROXY[(DTInfo)]     |
      |                               |<------------------------- |
      |                               |  RESPONSE[Error/Success]  |
      |                               | ---------------------- >  |
      |                               |   CREATE                  |
      |                               | ------------------------> |
      |                               |  RESPONSE[Error/Success]  |
      |                               | <----------------------   |
      |                               |                           |

            Figure 18: EXT Triggering Sending of CREATE Message

   A NATFW_NONCE object, carried in the EXT and CREATE message, is used
   to build the relationship between received CREATEs at the message
   initiator.  An NI+ uses the presence of the NATFW_NONCE object to
   correlate it to the particular EXT-PROXY.  The absence of a NONCE
   object indicates a CREATE initiated by the DS and not by the edge-
   NAT.  Therefore, these processing rules of EXT-PROXY messages are
   added to the regular EXT processing:




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   o  NSLP initiator (NI+): The NI+ MUST choose a random value and place
      it in the NATFW_NONCE object.

   o  NSLP forwarder being either edge-NAT or edge-firewall: When the NF
      accepts a EXT_PROXY message, it generates a successful RESPONSE
      message as if it were the NR and additionally, it generates a
      CREATE message as defined in Section 3.7.1 and includes a
      NATFW_NONCE object having the same value as of the received
      NATFW_NONCE object.  The NF MUST not generate a CREATE-PROXY
      message.  The NF MUST refresh the CREATE message session only if a
      EXT-PROXY refresh message has been received first.

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

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

      b) the DS is NSIS aware but the DR assumes the 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
   or edge-firewall.  The edge-NAT/edge-firewall will send CREATE
   messages and DS will send CREATE messages as well.  Both CREATE
   messages are handled as separated sessions and therefore the common
   rules per session apply; the NATFW_NONCE object is used to
   differentiate CREATE messages generated by the edge-NAT/edge-firewall
   from NI initiated CREATE messages.  It is the NR's responsibility to
   decide whether to teardown the EXT-PROXY sessions in the case where
   the data sender's side is NSIS aware, but was incorrectly assumed not
   to be so by the DR.  It is RECOMMENDED that a DR behind NATs uses the
   proxy mode of operation by default, unless the DR knows that the DS
   is NSIS aware.  The DR MAY cache information about data senders which
   it has found to be NSIS aware in past sessions.

   There is a possible race condition between the RESPONSE message to
   the EXT-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 EXT-PROXY was not received.







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3.7.6.2.  Proxying for a Data Receiver

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


      DS       Private Address      NAT/FW   Public Internet      NR
      NI           Space              NF                         no NR

      |                               |                           |
      |         CREATE-PROXY          |                           |
      |------------------------------>|                           |
      |                               |                           |
      |   RESPONSE[SUCCESS/ERROR]     |                           |
      |<------------------------------|                           |
      |                               |                           |

                 Figure 19: Proxy Mode CREATE Message Flow

   The processing of CREATE-PROXY messages and RESPONSE messages is
   similar to Section 3.7.1, 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.

   The NI can choose the proxy mode of operation although the DR is NSIS
   aware.  The CREATE-PROXY mode would not configure all NATs and
   firewalls along the data path, since it is terminated at the edge-
   device.  Any device beyond this point will never receive any NATFW
   NSLP signaling for this flow.

3.8.  De-Multiplexing at NATs

   Section 3.7.2 describes how NSIS nodes behind NATs can obtain a
   public reachable IP address and port number at a NAT and and how the
   resulting mapping rule can be activated by using CREATE messages (see
   Section 3.7.1).  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



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   flows are sent towards the address of the NAT at which this
   particular IP address and port number is allocated and not directly
   to the allocated IP address and port number.  The NATFW NSLP
   forwarder at this NAT needs to know how the incoming NSLP CREATE
   messages are related to reserved addresses, meaning how to de-
   multiplex incoming NSIS CREATE messages.

   The de-multiplexing method uses information stored at the local NATFW
   NSLP node and the of the policy rule.  The policy rule uses the LE-
   MRM MRI source-address (see [2]) as the flow destination IP address
   and the network-layer-version as IP version.  The external IP address
   at the NAT is stored as the external flow destination IP address.
   All other parameters of the policy rule other than the flow
   destination IP address are wildcarded if no NATFW_DTINFO object is
   included in the EXT message.  The LE-MRM MRI destination-address MUST
   NOT be used in the policy rule, since it is solely a signaling
   destination address.

   If the NATFW_DTINFO object is included in the EXT message, the policy
   rule is filled with further information.  The 'dst port number' field
   of the NATFW_DTINFO is stored as the flow destination port number.
   The 'protocol' field is stored as the flow protocol.  The 'src port
   number' field is stored as the flow source port number.  The 'data
   sender's IPv4 address' is stored as the flow source IP address.  Note
   that some of these field can contain wildcards.

   When receiving a CREATE message at the NATFW NSLP it uses the flow
   information stored in the MRI to do the matching process.  This table
   shows the parameters to be compared against each others.  Note that
   not all parameters can be present in a MRI at the same time.





















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    +-------------------------------+--------------------------------+
    |  Flow parameter (Policy Rule) | MRI parameter (CREATE message) |
    +-------------------------------+--------------------------------+
    |           IP version          |      network-layer-version     |
    |                               |                                |
    |            Protocol           |           IP-protocol          |
    |                               |                                |
    |     source IP address (w)     |       source-address (w)       |
    |                               |                                |
    |      external IP address      |       destination-address      |
    |                               |                                |
    |  destination IP address (n/u) |               N/A              |
    |                               |                                |
    |     source port number (w)    |       L4-source-port (w)       |
    |                               |                                |
    |    external port number (w)   |     L4-destination-port (w)    |
    |                               |                                |
    | destination port number (n/u) |               N/A              |
    |                               |                                |
    |           IPsec SPI           |            ipsec-SPI           |
    +-------------------------------+--------------------------------+

    Table entries marked with (w) can be wildcarded and entries marked
                 with (n/u) are not used for the matching.

                                  Table 1

3.9.  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 [2] and the NATFW NSLP relies on
   notifications about route changes by the NTLP.  This notification
   will be conveyed by the API between NTLP and NSLP, which is out of
   scope of this memo.

   A NATFW NSLP node other than the NI or NI+ detecting a route change,
   by means described in the NTLP specification or others, generates a
   NOTIFY message indicating this change and sends this upstream towards
   NI.  Intermediate NFs on the way to the NI can use this information
   to decide later if their session can be deleted locally, if they do
   not receive an update within a certain time period, as described in
   Section 3.2.3.  It is important to consider the transport limitations
   of NOTIFY messages as mandated in Section 3.7.5.

   The NI receiving this NOTIFY message MAY generate a new CREATE or EXT



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   message and sends it towards the session's NI as for the initial
   message using the same session ID.  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.
   Normal routing will guide the new CREATE or EXT message to the
   correct NFs along the changed route.  NFs that were on the original
   path receiving these new CREATE or EXT messages (see also
   Section 3.10), can use the session ID to update the existing session,
   whereas NFs that were not on the original path will create new state
   for this session.  The next section describes how policy rules are
   updated.

3.10.  Updating Policy Rules

   NSIS initiators can request an update of the installed/reserved
   policy rules at any time within a signaling session.  Updates to
   policy rules can be required due to node mobility (NI is moving from
   one IP address to another), route changes (this can result in a
   different NAT mapping at a different NAT device), or the wish of the
   NI to simply change the rule.  NIs can update policy rules in
   existing signaling sessions by sending an appropriate CREATE or EXT
   message (similar to Section 3.4) with modified message routing
   information (MRI) as compared with that installed previously, but
   using the existing session ID to identify the intended target of the
   update.  With respect to authorization and authentication, this
   update CREATE or EXT message is treated in exactly the same way as
   any initial message.  Therefore, any node along in the signaling
   session can reject the update with an error RESPONSE message, as
   defined in the previous sections.

   The message processing and forwarding is executed as defined in the
   particular sections.  A NF or the NR receiving an update, simply
   replaces the installed policy rules installed in the firewall/NAT.
   The local procedures on how to update the MRI in the firewall/NAT is
   out of scope of this memo
















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4.  NATFW NSLP Message Components

   A NATFW NSLP message consists of a NSLP header and one or more
   objects following the header.  The NSLP header is carried in all
   NATFW NSLP message 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

   All GIST NSLP-Data objects for the NATFW NSLP MUST contain this
   common header as the first 32 bits of the object (this is not the
   same as the GIST Common Header).  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 20.


      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Message type  |P| reserved    |       reserved                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 20: 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.

   The defined messages types are:

   o  IANA-TBD(1) : CREATE

   o  IANA-TBD(2) : EXTERNAL(EXT)

   o  IANA-TBD(3) : RESPONSE

   o  IANA-TBD(4) : NOTIFY

   If a message with another type is received, an error RESPONSE of
   class 'Protocol error' (0x3) with response code 'Illegal message
   type' (0x01) MUST be generated.




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   The P flag indicates the usage of proxy mode.  If proxy mode is used
   it MUST be set to 1.  Proxy mode usage is only allowed in combination
   with the message types CREATE and EXT, P=1 MUST NOT be set with
   message types other than CREATE and EXT.  The P flag MUST be ignored
   when processing messages with type RESPONSE.  An error RESPONSE
   message of class 'Protocol error' (0x3) and type 'Bad flags value'
   (0x03) MUST be generated, if the P flag is set in NOTIFY messages.

4.2.  NSLP Objects

   NATFW NSLP objects use a common header format defined by Figure 21.
   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 21: Common NSLP object header

   The object length field contains 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.  An error RESPONSE of class 'Protocol error' (0x3) with
   response code 'Wrong object length' (0x07) MUST be generated if the
   length given for the object in the object header did not match the
   length of the object data present.  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 [2], Section
   A.2.1), i.e., the Object Type has not been defined.  NATFW NSLP uses
   a subset of the categories defined in GIST:

   o  AB=00 ("Mandatory"): If the object is not understood, the entire
      message containing it MUST be rejected with an error RESPONSE of
      class 'Protocol error' (0x3) with response code 'Unknown object
      present' (0x06).

   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.




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   The combination AB=11 MUST NOT be used and an error RESPONSE of class
   'Protocol error' (0x3) with response code 'Invalid Flag-Field
   combination' (0x09) MUST be generated.

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

4.2.1.  Signaling Session Lifetime Object

   The signaling session lifetime object carries the requested or
   granted lifetime of a NATFW NSLP signaling session measured in
   seconds.

      Type: NATFW_LT (IANA-TBD)

      Length: 1


      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          NATFW NSLP signaling session lifetime                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 22: Signaling Session Lifetime object

4.2.2.  External Address Object

   The external address object can be included in RESPONSE messages
   (Section 4.3.3) only.  It carries the publicly reachable IP address,
   and if applicable port number, at an edge-NAT.

      Type: NATFW_EXT_IP (IANA-TBD)

      Length: 2


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

           Figure 23: External Address Object for IPv4 addresses

   Please note that the field 'port number' MUST be set to 0 if only an



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   IP address has been reserved, for instance, by a traditional NAT.  A
   port number of 0 MUST be ignored in processing this object.

4.2.3.  Extended Flow Information Object

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

      Type: NATFW_EFI (IANA-TBD)

      Length: 1


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

                   Figure 24: Extended Flow Information

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

   o  0x0001: Allow: A policy rule with this action allows data traffic
      to traverse the middlebox and the NATFW NSLP MUST allow NSLP
      signaling to be forwarded.

   o  0x0002: Deny: A policy rule with this action blocks data traffic
      from traversing the middlebox and the NATFW NSLP MUST NOT allow
      NSLP signaling to be forwarded.

   If the 'rule action' field contains neither 0x0001 nor 0x0002, an
   error RESPONSE of class 'Signaling session error' (0x6) with response
   code 'Unknown policy rule action' (0x05) MUST be generated.

   The 'sub_ports' field contains the number of contiguous transport
   layer ports to which this rule applies.  The default value of this
   field is 0, i.e., only the port specified in the NTLP's MRM is used
   for the policy rule.  A value of 1 indicates that additionally to the
   port specified in the NTLP's MRM (port1), a second port (port2) is
   used.  This value of port 2 is calculated as: port2 = port1 + 1.
   Other values than 0 or 1 MUST NOT be used in this field and an error
   RESPONSE of class 'Signaling session error' (0x6) with response code
   'Requested value in sub_ports field in NATFW_EFI not permitted'



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   (0x08) MUST be generated.  Further version of this memo may allow
   other values for the 'sub_ports' field.  This two contiguous port
   numbered ports, can be used by legacy voice over IP equipment.  This
   legacy equipment assumes that two adjacent port numbers for its RTP/
   RTCP flows respectively.

4.2.4.  Information Code Object

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

      Type: NATFW_INFO (IANA-TBD)

      Length: 1


      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Resv. | Class | Response Code |r|r|r|r|     Object Type       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 25: Information Code Object

   The field 'resv.' is reserved for future extensions and MUST be set
   to zero when generating such an object and MUST be ignored when
   receiving.  The 'Object Type' field contains the type of the object
   causing the error.  The value of 'Object Type' is set to 0, if no
   object is concerned and the leading fours bits marked with 'r' are
   always set to zero and ignored.  The 4 bit class field contains the
   severity class.  The following classes are defined:

   o  0x1: Informational (NOTIFY only)

   o  0x2: Success

   o  0x3: Protocol error

   o  0x4: Transient failure

   o  0x5: Permanent failure

   o  0x6: Signaling session failures

   Within each severity class a number of responses values are defined





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   o  Informational:

      *  0x01: Route change: possible route change on the downstream
         path.

      *  0x02: Re-authentication required.

      *  0x03: NATFW node is going down soon.

   o  Success:

      *  0x01: All successfully processed.

   o  Protocol error:

      *  0x01: Illegal message type: the type given in the Message Type
         field of the NSLP header is unknown.

      *  0x02: Wrong message length: the length given for the message in
         the NSLP header does not match the length of the message data.

      *  0x03: Bad flags value: an undefined flag or combination of
         flags was set in the NSLP header.

      *  0x04: Mandatory object missing: an object required in a message
         of this type was missing.

      *  0x05: Illegal object present: an object was present which must
         not be used in a message of this type.

      *  0x06: Unknown object present: an object of an unknown type was
         present in the message.

      *  0x07: Wrong object length: the length given for the object in
         the object header did not match the length of the object data
         present.

      *  0x08: Unknown object field value: a field in an object had an
         unknown value.

      *  0x09: Invalid Flag-Field combination: An object contains an
         invalid combination of flags and/or fields.

      *  0x0A: Duplicate object present.

      *  0x0B: Received EXT request message on external side.





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   o  Transient failure:

      *  0x01: Requested resources temporarily not available.

   o  Permanent failure:

      *  0x01: Authentication failed.

      *  0x02: Authorization failed.

      *  0x03: Unable to agree transport security with peer.

      *  0x04: Internal or system error.

      *  0x05: No NAT here.

      *  0x06: No edge-device here.

      *  0x07: Did not reach the NR.

   o  Signaling session failures:

      *  0x01: Session terminated asynchronously.

      *  0x02: Requested lifetime is too big.

      *  0x03: No reservation found matching the MRI of the CREATE
         request.

      *  0x04: Requested policy rule denied due to policy conflict.

      *  0x05: Unknown policy rule action.

      *  0x06: Requested rule action not applicable.

      *  0x07: DTINFO object is required.

      *  0x08: Requested value in sub_ports field in NATFW_EFI not
         permitted.

      *  0x09: Requested IP protocol not supported.

      *  0x0A: Plain IP policy rules not permitted -- need transport
         layer information.

      *  0x0B: ICMP type value not permitted.





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      *  0x0C: source IP address range is too large.

      *  0x0D: destination IP address range is too large.

      *  0x0E: source L4-port range is too large.

      *  0x0F: destination L4-port range is too large.

      *  0x10: Requested lifetime is too small.

4.2.5.  Nonce Object

   This object carries the nonce value as described in Section 3.7.6.

      Type: NATFW_NONCE (IANA-TBD)

      Length: 1


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

                          Figure 26: Nonce Object

4.2.6.  Message Sequence Number Object

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

      Type: NATFW_MSN (IANA-TBD)

      Length: 1


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

                 Figure 27: Message Sequence Number Object








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4.2.7.  Data Terminal Information Object

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

      Type: NATFW_DTINFO (IANA-TBD)

      Length: variable.  Maximum 3.


      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |I|P|S|    reserved             | sender prefix |    protocol   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :      DR port number           |       DS port number          :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                            IPsec SPI                          :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  data sender's IPv4 address                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 28: Data Terminal IPv4 Address Object

   The flags are:

   o  I: I=1 means that 'protocol' should be interpreted.

   o  P: P=1 means that 'dst port number' and 'src port number' are
      present and should be interpreted.

   o  S: S=1 means that SPI is present and should be interpreted.

   The SPI field is only present if S is set.  The port numbers are only
   present if P is set.  The flags P and S MUST NOT be set at the same
   time.  An error RESPONSE of class 'Protocol error' (0x3) with
   response code 'Invalid Flag-Field combination' (0x09) MUST be
   generated if they are both set.  If either P or S is set, I MUST be
   set as well and the protocol field MUST carry the particular
   protocol.  An error RESPONSE of class 'Protocol error' (0x3) with



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   response code 'Invalid Flag-Field combination' (0x09) MUST be
   generated if S or P is set but I is not set.

   The fields MUST be interpreted according these rules:

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

   o  protocol: The IPv4 protocol field.  This field MUST be interpreted
      if I=1, otherwise it MUST be set to 0 and MUST be ignored.

   o  DR port number: The port number at the data receiver (DR), i.e.,
      the destination port.  A value of 0 indicates a port wildcard,
      i.e., the destination port number is not known.  Any other value
      indicates the destination port number.

   o  DS port number: The port number at the data sender (DS), i.e., the
      source port.  A value of 0 indicates a port wildcard, i.e., the
      source port number is not known.  Any other value indicates the
      source port number.

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

4.2.8.  ICMP Types Object

   The 'ICMP types' object contains additional information needed to
   configure a NAT of firewall with rules to control ICMP traffic.  The
   object contains a number of values of the ICMP Type field for which a
   filter action should be set up:

      Type: NATFW_ICMP_TYPES (IANA-TBD)

      Length: Variable = ((Number of Types carried + 1) + 3) DIV 4

   Where DIV is an integer division.











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      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Count      |     Type      |      Type     |    ........   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       ................                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    ........   |     Type      |           (Padding)           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 29: ICMP Types Object

   The fields MUST be interpreted according these rules:

      count: 8 bit integer specifying the number of 'Type' entries in
      the object.

      type: 8 bit field specifying an ICMP Type value to which this rule
      applies.

      padding: Sufficient 0 bits to pad out the last word so that the
      total size of object is an even multiple of words.  Ignored on
      reception.

4.3.  Message Formats

   This section defines the content of each NATFW NSLP message type.
   The message types are defined in Section 4.1.

   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.
   Objects defined in this memo carry always the flag combination AB=00
   in the NSLP object header.  An error RESPONSE message of class
   'Protocol error' (0x3) with response code 'Mandatory object missing'
   (0x02) MUST be generated if a mandatory declared object is missing.
   An error RESPONSE message of class 'Protocol error' (0x3) with
   response code 'Illegal object present' (0x05) MUST be generated if an
   object was present which must not be used in a message of this type.
   An error RESPONSE message of class 'Protocol error' (0x3) with
   response code 'Duplicate object present' (0x0A) MUST be generated if
   an object appears more than once in a message.

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



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   information is set.

4.3.1.  CREATE

   The CREATE 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  Signaling Session Lifetime object [M]

   o  Extended flow information object [M]

   o  Message sequence number object [M]

   o  Nonce object [M] if P flag set to 1 in the NSLP header, otherwise
      [O]

   o  ICMP Types Object [O]

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

4.3.2.  EXTERNAL (EXT)

   The EXTERNAL (EXT) message is used to a) reserve an external IP
   address/port at NATs, b) to notify firewalls about NSIS capable DRs,
   or c) to block incoming data traffic at upstream firewalls.

   The EXT message carries these objects:

   o  Signaling Session Lifetime object [M]

   o  Message sequence number object [M]

   o  Extended flow information object [M]

   o  Data terminal information object [M]

   o  Nonce object [M if P flag set to 1 in the NSLP header, otherwise
      [O]

   o  ICMP Types Object [O]




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   The selected message routing method of the EXT message depends on a
   number of considerations.  Section 3.7.2 describes it exhaustively
   how to select the correct method.  EXT messages can be transported
   via the path-coupled message routing method (PC-MRM) or via the
   loose-end message routing method (LE-MRM).  In the case of PC-MRM,
   the source-address is set to DS' address and the destination address
   is set to DR's address, the direction is set to upstream.  In the
   case of LE-MRM, the destination-address is set to DR's address or to
   the signaling destination address.  The source-address is set to DS's
   address.

4.3.3.  RESPONSE

   RESPONSE messages are responses to CREATE and EXT messages.  RESPONSE
   messages MUST NOT be generated for any other message, such as NOTIFY
   and RESPONSE.

   The RESPONSE message for the class 'Success' (0x2) carries these
   objects:

   o  Signaling Session Lifetime object [M]

   o  Message sequence number object [M]

   o  Information code object [M]

   o  External address object [O]

   The RESPONSE message for other classes than 'Success' (0x2) carries
   these objects:

   o  Message sequence number object [M]

   o  Information code object [M]

   This message is routed towards the NI hop-by-hop, using existing NTLP
   messaging associations.  The MRM used for this message MUST be the
   same as MRM used by the corresponding CREATE or EXT message.

4.3.4.  NOTIFY

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

   The NOTIFY message carries this object:

   o  Information code object [M].




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   The NOTIFY message is routed towards the NI hop-by-hop using the
   existing upstream node messaging association entry within the node's
   Message Routing State table.  The MRM used for this message MUST be
   the same as MRM used by the corresponding CREATE or EXT message.















































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5.  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.

   In Section 5.1 we describe the participating entities relate to each
   other from a security point of view.  This subsection also motivates
   a particular authorization model.

   Security threats that focus on NSIS in general are described in [8]
   and they are applicable to this document as well.  In Section 5.2 we
   extend this threat investigation by considering NATFW NSLP specific
   threats in more detail.  Based on the investigated security threats
   we derive security requirements.

   Finally, we illustrate how the security requirements that were
   created based on the security threats can be fulfilled by specific
   security mechanisms.  These aspects will be elaborated in
   Section 5.11.

5.1.  Authorization Framework

   The NATFW NSLP is a protocol which may involve a number of NSIS nodes
   and is, as such, not a two-party protocol.  Figure 1 and Figure 2 of
   [8] already depict the possible set of communication patterns.  In
   this section we will re-evaluate these communication patters with
   respect to the NATFW NSLP protocol interaction.

   The security solutions for providing authorization have a direct
   impact on the treatment of different NSLPs.  As it can be seen from
   the QoS NSLP [6] and the corresponding Diameter QoS work [24]
   accounting and charging seems to play an important role for QoS
   reservations, whereas monetary aspects might only indirectly effect
   authorization decisions for NAT and firewall signaling.  Hence, there
   are differences in the semantic of authorization handling between QoS
   and NATFW signaling.  A NATFW aware node will most likely want to
   authorize the entity (e.g., user or machine) requesting the
   establishment of pinholes or NAT bindings.  The outcome of the
   authorization decision is either allowed or disallowed whereas a QoS
   authorization decision might indicate that a different set of QoS
   parameters is authorization (see [24] as an example).

5.1.1.  Peer-to-Peer Relationship

   Starting with the simplest scenario, it is assumed that neighboring
   nodes are able to authenticate each other and to establish keying
   material to protect the signaling message communication.  The nodes



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   will have to authorize each other, additionally to the
   authentication.  We use the term 'Security Context' as a placeholder
   for referring to the entire security procedure, the necessary
   infrastructure that needs to be in place in order for this to work
   (e.g., key management) and the established security related state.
   The required long-term key (symmetric or asymmetric keys) used for
   authentication are either made available using an out-of-band
   mechanism between the two NSIS NATFW nodes or they are dynamically
   established using mechanisms not further specified in this document.
   Note that the deployment environment will most likely have an impact
   on the choice of credentials being used.  The choice of these
   credentials used is also outside the scope of this document.


   +------------------------+              +-------------------------+
   |Network A               |              |                Network B|
   |              +---------+              +---------+               |
   |        +-///-+ Middle- +---///////----+ Middle- +-///-+         |
   |        |     |  box 1  | Security     |  box 2  |     |         |
   |        |     +---------+ Context      +---------+     |         |
   |        | Security      |              |  Security     |         |
   |        | Context       |              |  Context      |         |
   |        |               |              |               |         |
   |     +--+---+           |              |            +--+---+     |
   |     | Host |           |              |            | Host |     |
   |     |  A   |           |              |            |  B   |     |
   |     +------+           |              |            +------+     |
   +------------------------+              +-------------------------+

                   Figure 30: Peer-to-Peer Relationship

   Figure 30 shows a possible relationship between participating NSIS
   aware nodes.  Host A might be, for example, a host in an enterprise
   network that has keying material established (e.g., a shared secret)
   with the company's firewall (Middlebox 1).  The network administrator
   of Network A (company network) has created access control lists for
   Host A (or whatever identifiers a particular company wants to use).
   Exactly the same procedure might also be used between Host B and
   Middlebox 2 in Network B. For the communication between Middlebox 1
   and Middlebox 2 a security context is also assumed in order to allow
   authentication, authorization and signaling message protection to be
   successful.

5.1.2.  Intra-Domain Relationship

   In larger corporations, for example, a middlebox is used to protect
   individual departments.  In many cases, the entire enterprise is
   controlled by a single (or a small number of) security department,



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   which gives instructions to the department administrators.  In such a
   scenario, the previously discussed peer-to-peer 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 exchange the authorization
   decision with another middlebox within the same trust domain.
   Individual middleboxes within an administrative domain may exploit
   their relationship instead of requesting authentication and
   authorization of the signaling initiator again and again.  Figure 31
   illustrates a network structure which uses a centralized entity.


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

                   Figure 31: Intra-domain Relationship

   The interaction between individual middleboxes and a policy decision
   point (or AAA server) is outside the scope of this document.

5.1.3.  End-to-Middle Relationship

   The peer-to-peer relationship between neighboring NSIS NATFW NSLP
   nodes might not always be sufficient.  Network B might require
   additional authorization of the signaling message initiator (in
   addition to the authorization of the neighboring node).  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



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   the signaling message initiator might already be aware of the
   additionally required authorization before the signaling message
   exchange is executed.

   Figure 32 shows this scenario.

       +--------------------+              +---------------------+
       |          Network A |              |Network B            |
       |                    |   Security   |                     |
       |          +---------+   Context    +---------+           |
       |    +-///-+ Middle- +---///////----+ Middle- +-///-+     |
       |    |     |  box 1  |      +-------+  box 2  |     |     |
       |    |     +---------+      |       +---------+     |     |
       |    |Security       |      |       | Security      |     |
       |    |Context        |      |       | Context       |
       |    |               |      |       |               |     |
       | +--+---+           |      |       |            +--+---+ |
       | | Host +----///----+------+       |            | Host | |
       | |  A   |           |   Security   |            |  B   | |
       | +------+           |   Context    |            +------+ |
       +--------------------+              +---------------------+

                   Figure 32: End-to-Middle Relationship

5.2.  Security Threats and Requirements

   This section describes NATFW specific security threats and
   requirements.

5.2.1.  Data Sender (DS) behind a firewall

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

                           DS behind a firewall

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

   The following attacks are possible:

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




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

   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 requesting an action.
      Authentication and authorization of the initiator SHOULD be
      provided to NATs and firewalls along the path.

5.2.2.  Data Sender (DS) behind a NAT

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

   Figure 34 illustrates such a scenario:

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

                    Figure 34: 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 non-neighboring middleboxes
      SHOULD be provided.

5.2.3.  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  |    |
       +-----+ <------------- +-----+ <------------- +-----+    |
            successful RESPONSE  |  successful RESPONSE         |
                                 |                              |
                                 +------------------------------+

                           DR behind a firewall

   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 successful RESPONSE message.

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

   At this point we differentiate two cases:

   1.  DR knows the (publicly reachable) IP address and port number 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
       (publicly reachable) IP address of the Data Sender DS.

   For the second case, Figure 36 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 36: 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 36.  Many operators might not allow NSIS
   signaling message to traverse the firewall in Figure 36 without
   having the DR to interact with the FW first.

   SECURITY REQUIREMENT:  No requirements are created by this scenario.

5.2.4.  Data Receiver (DR) behind a NAT

   When a data receiver DR behind a NAT sends a EXTERNAL (EXT) message
   to get a public reachable address, this address 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) (accept data from any IP address, i.e.,
   wildcard).

   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 receiver 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
   (i.e., external address and port) allocated at the NAT possibly by



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   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
              <------------------------------------------>
             /                                           \
            /                      +----------------------\-------+
           v                       |       EXT            v       |
       +-----+                  +-----+  <-----------  +-----+    |
       | DS  |              ->  | NAT |  ----------->  | DR  |    |
       +-----+             /    +-----+  DR_external   +-----+    |
                  CREATE  /       |                               |
       +-----+           /        +-------------------------------+
       | M   |----------
       +-----+

                     DR behind a NAT with an adversary

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

5.2.5.  NSLP Message Injection

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

   By injecting a bogus CREATE message with signaling session 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,
   session 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 NOTIFY, can be misused by
      malicious hosts, and therefore MUST be authorized by the
      respective NATFW NLSP entities.






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5.3.  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.

5.3.1.  Flooding with CREATE messages from outside

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

   SECURITY REQUIREMENT:  A NAT/FW NSLP node MUST authorize the
      establishment of state information.

5.3.1.2.  Attacks due to authentication complexity

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

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

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

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



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   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 node MUST be configurable to
      block unauthorized signaling messages.

5.3.2.  Flooding with EXT 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 EXTERNAL
   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 EXTERNAL 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.

5.4.  Man-in-the-Middle Attacks

   Figure 38 illustrates a possible man-in-the-middle attack using the
   EXTERNAL (EXT) 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.  Mallory 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.
   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.






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

             Figure 38: MITM attack using the EXTERNAL 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 38 the firewall FW must
      perform an authorization with the neighboring entities.

5.5.  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.

5.6.  Message Modification by malicious NSIS node

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

   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.








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   SECURITY REQUIREMENT:  Malicious NSIS NATs and firewalls will not be
      addressed by this specification.

5.7.  Session Ownership

   Section 4.10 in [8] describes a threat where an adversary is able to
   modify or delete previously installed state information at NATFW NSLP
   nodes along the path.  An adversary therefore needs to know session
   specific information, such as the session identifier and MRI
   information.

   SECURITY REQUIREMENT:  Off-path adversaries MUST NOT be able to
      modify or delete sessions without proper authorization.

5.7.1.  Misuse of Mobility in a NAT Handling Scenario

   Another kind of session modification is related to mobility
   scenarios.  The NSIS protocol suite offers interworking with mobility
   protocol and a mobile node might need to update state along NATFW
   NSLP nodes.

   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 end host.


     Public                       Private Address
     Internet                     space

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

               Figure 39: 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



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   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 39 where an the end host(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:

   o  Redirecting packets to the attacker as in Figure 40.

   o  Third party flooding by redirecting packets to arbitrary hosts

   o  Service disruption by redirecting to non-existing hosts


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

                      Figure 40: Connection Hijacking

   SECURITY REQUIREMENT:  A NAT/FW signaling message MUST be
      authenticated, integrity and replay protected between neighboring
      NAT/FW NSLP nodes.  The NSIS NATFW NSLP aware NAT MUST authorize
      the end host to insure that the messages are indeed belonging to
      the previously established session.

5.8.  Misuse of unreleased sessions

   Assume that DS (N1) initiates NSIS session with DR (N2) through a
   series of middleboxes as in Figure 41.  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



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   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 42.  Note that here we
   make the assumption that the data receiver has to pay for receiving
   data packets.


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

                        Figure 41: Before mobility


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

                         Figure 42: 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:  In order to allow firewalls to verify that a
      legitimate end host indeed transmitted data traffic it is
      necessary to provide data origin authentication.  This is,
      however, outside the scope of this document.  Hence, there are no
      security requirements imposed by this threat, which will be
      addressed by the NATFW NSLP.



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5.9.  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
   practice by itself represents a security weakness.  Using IP spoofing
   an adversary is able to reach the target machines if they match,
   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 countermeasures need to be
      taken by the NAT/FW NSLP.  Techniques such as ingress filtering
      (see [23]) and, if necessary, data origin authentication (such as
      provided with IPsec based VPNs) can help mitigate this threat.
      This aspect is, however, outside the scope of this document.

5.10.  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 information to inject its own data traffic due to the IP
   spoofing capability already mentioned in Section 5.9.  An on-path
   adversary could also observe the data traffic and he could conclude
   that it is possible to traverse a firewall.

   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.  This threat is discussed in
   the respective document suggesting the usage of authorization token
   in the NSIS protocol suite.

   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 MAY
      be necessary to offer confidentiality protection.  The latter
      aspect is outside the scope of this document.

5.11.  Security Framework for the NAT/Firewall NSLP

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






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5.11.1.  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:

   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 [2] between neighboring nodes MUST be
   enabled when sending NATFW signaling messages.  The proposed security
   mechanisms at GIST provide support for authentication and key
   exchange in addition to denial of service protection.  Depending on
   the chosen security protocol, support for multiple authentication
   protocols might be provided.  If security between neighboring nodes
   is desired then 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 is demanded.  Almost all security threats at the
   NATFW NSLP layer can be prevented by using a mutually authenticated
   Transport Layer secured connection and by relying on authorization by
   the neighboring NATFW NSLP entities.

   The NATFW NSLP relies an established security association between
   neighboring peers to prevent unauthorized nodes to modify or delete
   installed state.  Between non-neighboring nodes the session ID (SID)
   carried in the NTLP is used to show ownership of a session.  The
   session ID MUST be generated in a random way and thereby prevent an
   off-path adversary to mount targeted attacks.  Hence, an adversary
   would have to learn the randomly generated Session ID to perform an
   attack.  In a mobility environment a former on-path node that is now
   off-path can perform an attack.  The cost-benefit tradeoff to counter
   this attack does not seem to be high enough to provide a solution.
   Messages for a particular session are handled by the NTLP to the
   NATFW NSLP for further processing.  Messages carrying a different
   session ID not associated with any NATFW NSLP are subject to the
   regular processing for new NATFW NSLP sessions.

5.11.2.  Security Protection between non-neighboring NATFW NSLP Nodes

   Based on the security threats and the listed requirements it was
   noted that some threats also demand authentication and authorization
   of a NATFW signaling entity (including the initiator) towards a non-
   neighboring node.  This mechanism mainly demands entity



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   authentication.  Additionally, security protection of certain
   payloads may be required between non-neighboring signaling entities
   and the Cryptographic Message Syntax (CMS) [17] migh be a potential
   solution.  Payload protection using CMS is not described in this
   document.  The most important information exchanged at the NATFW NSLP
   is information related to the establishment for firewall pinholes and
   NAT bindings.  This information can, however, not be protected over
   multiple NSIS NATFW NSLP hops since this information might change
   depending on the capability of each individual NATFW NSLP node.

   Some scenarios might also benefit from the usage of authorization
   tokens.  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 can be
   found in RFC 3520 [21] and RFC 3521 [22].  Figure 43 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 successfully 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 43: Authorization Token Usage

   Threats against the usage of authorization tokens have been mentioned
   in [8] and also in Section 5.2.  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.

   To harmonize the usage of authorization tokens in NSLPs a separate
   document is available, see [25].

















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6.  IAB Considerations on UNSAF

   UNilateral Self-Address Fixing (UNSAF) is described in [15] as a
   process at originating endpoints that attempt to determine or fix the
   address (and port) by which they are known to another endpoint.
   UNSAF proposals, such as STUN [RFC3489] are considered as a general
   class of workarounds for NAT traversal and as solutions for scenarios
   with no middlebox communication.

   This memo specifies a path-coupled middlebox communication protocol,
   i.e., the NSIS NATFW NSLP.  NSIS in general and the NATFW NSLP are
   not intended as a short-term workaround, but more as a long-term
   solution for middlebox communication.  In NSIS, endpoints are
   involved in allocating, maintaining, and deleting addresses and ports
   at the middlebox.  However, the full control of addresses and ports
   at the middlebox is at the NATFW NSLP daemon located to the
   respective NAT.

   Therefore, this document addresses the UNSAF considerations in
   [RFC3424] by proposing a long-term alternative solution.































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7.  IANA Considerations

   This section provides guidance to the Internet Assigned Numbers
   Authority (IANA) regarding registration of values related to the
   NATFW NSLP, in accordance with BCP 26 RFC 2434 [16].

   The NATFW NSLP requires IANA to create a number of new registries.
   These registries may require further coordination with the registries
   of the NTLP [2] and the QoS NSLP [6].

   NATFW NSLP Message Type Registry

   The NATFW NSLP Message Type is a 8 bit value.  The allocation of
   values for new message types requires standards action.  Updates and
   deletion of values from the registry is not possible.  This
   specification defines four NATFW NSLP message types, which form the
   initial contents of this registry.  IANA is requested to add these
   four NATFW NSLP Message Types: CREATE, EXT, RESPONSE, and NOTIFY.

   NATFW NSLP Header Flag Registry

   NATFW NSLP messages have a messages-specific 8 bit flags/reserved
   field in their header.  The registration of flags is subject to IANA
   registration.  The allocation of values for flag types requires
   standards action.  Updates and deletion of values from the registry
   is not possible.  This specification defines only one flag, the P
   flag in Figure 20.

   NSLP Object Type Registry

   [Delete this part if already done by another NSLP:

   A new registry is to be created for NSLP Message Objects.  This is a
   12-bit field (giving values from 0 to 4095).  This registry is shared
   between a number of NSLPs.  Allocation policies are as follows:

      0-1023: Standards Action

      1024-1999: Specification Required

      2000-2047: Private/Experimental Use

      2048-4095: Reserved

   When a new object is defined, the extensbility bits (A/B) must also
   be defined.]

   This document defines 9 objects for the NATFW NSLP: NATFW_LT,



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   NATFW_EXT_IP, NATFW_EFI, NATFW_INFO, NATFW_NONCE, NATFW_MSN,
   NATFW_DTINFO, NATFW_CREDENTIAL, NATFW_ICMP_TYPES.  IANA is request to
   assigned values for them from NSLP Object Type registry and to
   replace the corresponding IANA-TBD tags with the numeric values.

   NSLP Response Code Registry

   In addition it defines a number of Response Codes for the NATFW NSLP.
   These can be found in Section 4.2.4 and are to be assigned values
   from NSLP Response Code registry.  The allocation of values for
   Response Codes Codes requires standards action.  IANA is request to
   assigned values for them from NSLP Response Code registry.

   Furthermore, IANA is requested to add a new value to the NSLP
   Identifiers (NSLPID) registry defined in [2] for the NATFW NSLP.




































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8.  Open Issues

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















































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9.  Acknowledgments

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

   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  Henning Peters for his comments and suggestions,

   o  and the NSIS working group.


































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10.  References

10.1.  Normative References

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

   [2]  Schulzrinne, H. and R. Hancock, "GIST: General Internet
        Signaling Transport", draft-ietf-nsis-ntlp-11 (work in
        progress), August 2006.

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

10.2.  Informative References

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

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

   [6]   Manner, J., "NSLP for Quality-of-Service Signaling",
         draft-ietf-nsis-qos-nslp-11 (work in progress), June 2006.

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

   [8]   Tschofenig, H. and D. Kroeselberg, "Security Threats for Next
         Steps in Signaling (NSIS)", RFC 4081, June 2005.

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

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

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

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

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



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         Specification", RFC 2205, September 1997.

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

   [15]  Daigle, L. and IAB, "IAB Considerations for UNilateral Self-
         Address Fixing (UNSAF) Across Network Address Translation",
         RFC 3424, November 2002.

   [16]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 2434,
         October 1998.

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

   [18]  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.

   [19]  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.

   [20]  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.

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

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

   [23]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
         Defeating Denial of Service Attacks which employ IP Source
         Address Spoofing", BCP 38, RFC 2827, May 2000.

   [24]  Alfano, F., "Diameter Quality of Service Application",
         draft-alfano-aaa-qosprot-05 (work in progress), October 2005.

   [25]  Manner, J., "Authorization for NSIS Signaling Layer Protocols",
         draft-manner-nsis-nslp-auth-01 (work in progress), June 2006.

   [26]  Roedig, U., Goertz, M., Karten, M., and R. Steinmetz, "RSVP as
         firewall Signalling Protocol", Proceedings of the 6th IEEE



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         Symposium on Computers and Communications, Hammamet,
         Tunisia pp. 57 to 62, IEEE Computer Society Press, July 2001.

















































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Appendix A.  Selecting Signaling Destination Addresses for EXT

   As with all other message types, EXT 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 EXT 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'.

   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.  As
             a consequence it might be necessary to advertise a new (and
             different) external IP address within the application
             (which may or may not allow that) after using NSIS to
             establish a NAT binding.

   2.  Public IP address of the data receiver:

       *  Assumption:

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

       *  Problems:

          +  Communication with a third party is required.

   3.  IP address of the Application Server:

       *  Assumption:





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

       *  Problems:

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

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







































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Appendix B.  Applicability Statement on Data Receivers behind Firewalls

   Section 3.7.2 describes how data receivers behind middleboxes can
   instruct upstream firewalls/NATs to forward NATFW NSLP signaling
   towards them.  Finding an upstream edge-NAT in address environment
   with NAT'ed addresses is quite easy.  It is only required to find
   some edge-NAT, as the data traffic will be route-pinned to the NAT,
   which is done with the LE-MRM.  Locating the appropriate edge-
   firewall with the PC-MRM, sent upstream is difficult.  For cases with
   a single, symmetric route from the Internet to the data receiver, it
   is quite easy; simply follow the default route in the upstream
   direction.


                             +------+                  Data Flow
                     +-------| EFW1 +----------+     <===========
                     |       +------+       ,--+--.
                  +--+--+                  /       \
          NI+-----| FW1 |                 (Internet )----NR+/NI/DS
          NR      +--+--+                  \       /
                     |       +------+       `--+--'
                     +-------| EFW2 +----------+
                             +------+

           ~~~~~~~~~~~~~~~~~~~~~>
             Signaling Flow



   Figure 44: Data receiver behind multiple, parallel located firewalls

   When a data receiver, and thus NR, is located in a network site that
   is multihomed with several independently firewalled connections to
   the public Internet (as shown in Figure 44), the specific firewall
   through which the data traffic will be routed has to be ascertained.
   NATFW NSLP signaling messages sent from the NI+/NR during the EXT
   message exchange towards the NR+ must be routed by the NTLP to the
   edge-firewall that will be passed by the data traffic as well.  The
   NTLP would need to be aware about the routing within the Internet to
   determine the path between DS and DR.  Out of this, the NTLP could
   determine which of the edge-firewalls, either EFW1 or EFW2, must be
   selected to forward the NATFW NSLP signaling.  Signaling to the wrong
   edge-firewall, as shown in Figure 44, would install the NATFW NSLP
   policy rules at the wrong device.  This causes either a blocked data
   flow (when the policy rule is 'allow') or an ongoing attack (when the
   policy rule is 'deny').  Requiring the NTLP to know all about the
   routing within the Internet is definitely a tough challenge and
   usually not possible.  In such described case, the NTLP must



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   basically give up and return an error to the NSLP level, indicating
   that the next hop discovery is not possible.

















































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Appendix C.  Firewall and NAT Resources

   This section gives some examples on how NATFW NSLP policy rules could
   be mapped to real firewall or NAT resources.  The firewall rules and
   NAT bindings are described in a natural way, i.e., in a way one will
   find it in common implementation.

C.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 5.8 of
   [2].

   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.

C.2.  Mapping to Firewall Rules

   This section describes how a NSLP policy rule signaled with a CREATE
   message is mapped to a firewall rule.  The MRI is set as follows:

   o  network-layer-version=IPv4

   o  source-address=192.0.2.100, prefix-length=32

   o  destination-address=192.0.50.5, prefix-length=32

   o  IP-protocol=UDP

   o  L4-source-port=34543, L4-destination-port=23198

   The NATFW_EFI object is set to action=allow and sub_ports=0.

   The resulting policy rule (firewall rule) to be installed might look
   like: allow udp from 192.0.2.100 port=34543 to 192.0.50.5 port=23198






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C.3.  Mapping to NAT Bindings

   This section describes how a NSLP policy rule signaled with a EXT
   message is mapped to a NAT binding.  It is assumed that the EXT
   message is sent by a NI+ being located behind a NAT and does contain
   a NATFW_DTINFO object.  The MRI is set following using the signaling
   destination address, since the IP address of the real data sender is
   not known:

   o  network-layer-version=IPv4

   o  source-address= 192.168.5.100

   o  destination-address=SDA

   o  IP-protocol=UDP

   The NATFW_EFI object is set to action=allow and sub_ports=0.  The
   NATFW_DTINFO object contains these parameters:

   o  P=1

   o  dest prefix=0

   o  protocol=UDP

   o  dst port number = 20230, src port number=0

   o  src IP=0.0.0.0

   The edge-NAT allocates the external IP 192.0.2.79 and port 45000.

   The resulting policy rule (NAT binding) to be installed could look
   like: translate from any to 192.0.2.79 port=45000 to 192.168.5.100
   port=20230

C.4.  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



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   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 D.  Assigned Numbers for Testing

   NOTE: This section MUST be removed before publication.

   This section defines temporarily used values of the NATFW NSLP for
   testing the different implementations.

   Values for the NATFW NSLP message types:

   o  CREATE: 0x01

   o  EXT: 0x02

   o  RESPONSE: 0x03

   o  NOTIFY: 0x04

   Values for the NSLP object types

   o  NATFW_LT: 0x00F1

   o  NATFW_EXT_IP: 0x00F2

   o  NATFW_EFI: 0x00F3

   o  NATFW_INFO: 0x00F4

   o  NATFW_NONCE: 0x00F5

   o  NATFW_MSN: 0x00F6

   o  NATFW_DTINFO: 0x00F7

   o  NATFW_CREDENTIAL: 0x00F8

   o  NATFW_ICMP_TYPES: 0x00F9

   1345













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Authors' Addresses

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

   Phone: +49 (0) 6221 4342 113
   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


   Elwyn Davies
   Folly Consulting
   Soham
   UK

   Phone: +44 7889 488 335
   Email: elwynd@dial.pipex.com












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Internet-Draft              NAT/FW NSIS NSLP                October 2006


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