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NSIS Working Group                                        M. Stiemerling
Internet-Draft                                                       NEC
Expires: April 17, 2004                                    H. Tschofenig
                                                                 Siemens
                                                               M. Martin
                                                                     NEC
                                                                 C. Aoun
                                                         Nortel Networks
                                                        October 18, 2003


          A NAT/Firewall NSIS Signaling Layer Protocol (NSLP)
                     draft-ietf-nsis-nslp-natfw-00

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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

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

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

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

   This Internet-Draft will expire on April 17, 2004.

Copyright Notice

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

Abstract

   This draft describes scenarios, problems and solutions for
   path-coupled  Network Address Translator and Firewall signaling.
   This is one of the two NSIS Signaling Layer Protocols (NSLPs) the
   working group will address during its work.





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Table of Contents

   1.      Introduction . . . . . . . . . . . . . . . . . . . . . . .  4

   2.      Terminology and Abbreviations  . . . . . . . . . . . . . .  5

   3.      Framework and Scenarios  . . . . . . . . . . . . . . . . .  7
   3.1     Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.2     What problem should be solved? . . . . . . . . . . . . . .  7
   3.3     Basic NSIS Usage for NAT/FW traversal  . . . . . . . . . .  8
   3.4     Scenarios for Protocol Functionality . . . . . . . . . . .  9
   3.4.1   Firewall traversal . . . . . . . . . . . . . . . . . . . .  9
   3.4.2   NAT with two private networks  . . . . . . . . . . . . . . 10
   3.4.3   NAT with private network on sender side  . . . . . . . . . 11
   3.4.4   NAT with private network on receiver side  . . . . . . . . 11
   3.4.5   Both end hosts in same private network behind NATs . . . . 12
   3.4.5.1 Both end hosts in same private network behind same NAT . . 13
   3.4.6   IPv4/v6 NAT with two private networks  . . . . . . . . . . 13
   3.5     Trust Relationship and Authorization . . . . . . . . . . . 14
   3.5.1   Peer-to-Peer Trust Relationship  . . . . . . . . . . . . . 14
   3.5.2   Intra-Domain Trust Relationship  . . . . . . . . . . . . . 15
   3.5.3   End-to-Middle Trust Relationship . . . . . . . . . . . . . 16

   4.      Problems and Challenges  . . . . . . . . . . . . . . . . . 18
   4.1     Missing Network-to-Network Trust Relationship  . . . . . . 18
   4.2     End-to-end significance  . . . . . . . . . . . . . . . . . 19
   4.3     Relationship with routing  . . . . . . . . . . . . . . . . 19
   4.4     Dynamic state installation and maintenance . . . . . . . . 20
   4.5     Affected Parts of the Network  . . . . . . . . . . . . . . 20
   4.6     NSIS backward compatibility with NSIS unaware NAT and
           Firewalls  . . . . . . . . . . . . . . . . . . . . . . . . 20
   4.7     Authentication and Authorization . . . . . . . . . . . . . 21
   4.8     Directional Properties . . . . . . . . . . . . . . . . . . 21
   4.9     Routing Asymmetry  . . . . . . . . . . . . . . . . . . . . 22
   4.10    Addressing . . . . . . . . . . . . . . . . . . . . . . . . 22
   4.11    NTLP/NSLP NAT Support  . . . . . . . . . . . . . . . . . . 23
   4.12    Route changes  . . . . . . . . . . . . . . . . . . . . . . 23
   4.13    Combining Middlebox and QoS signaling  . . . . . . . . . . 23
   4.14    Difference between sender- and receiver-initiated
           signaling  . . . . . . . . . . . . . . . . . . . . . . . . 24
   4.15    Inability to know the scenario . . . . . . . . . . . . . . 24

   5.      NSIS NAT Handling Solution . . . . . . . . . . . . . . . . 25
   5.1     Problem Description  . . . . . . . . . . . . . . . . . . . 25
   5.2     Solution Overview  . . . . . . . . . . . . . . . . . . . . 28
   5.2.1   Destination IP address Selection . . . . . . . . . . . . . 30

   6.      Protocol Description . . . . . . . . . . . . . . . . . . . 32



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   6.1     Basic protocol overview  . . . . . . . . . . . . . . . . . 32
   6.2     Message format and types . . . . . . . . . . . . . . . . . 32
   6.3     Message flow . . . . . . . . . . . . . . . . . . . . . . . 35
   6.4     Middle box configuration . . . . . . . . . . . . . . . . . 36
   6.5     Message handling . . . . . . . . . . . . . . . . . . . . . 37
   6.5.1   Detailed behaviour . . . . . . . . . . . . . . . . . . . . 38
   6.5.1.1 Reserve external address message . . . . . . . . . . . . . 38
   6.5.1.2 Return external address message  . . . . . . . . . . . . . 39
   6.5.1.3 Create message . . . . . . . . . . . . . . . . . . . . . . 39
   6.5.1.4 Delete session message . . . . . . . . . . . . . . . . . . 41
   6.5.1.5 Prolong session message  . . . . . . . . . . . . . . . . . 41
   6.5.1.6 Path Succeded message  . . . . . . . . . . . . . . . . . . 41

   7.      Solution examples  . . . . . . . . . . . . . . . . . . . . 43
   7.1     Firewall traversal . . . . . . . . . . . . . . . . . . . . 43
   7.2     NAT with private network on sender side  . . . . . . . . . 43
   7.3     NAT with private network on receiver side  . . . . . . . . 45
   7.4     Both end hosts are in same private network behind NATs . . 49
   7.5     IPv4/v6 NAT with two private networks  . . . . . . . . . . 52
   7.6     Full example for NAT/FW with two private networks  . . . . 52

   8.      NSIS NAT and Firewall transitions issues . . . . . . . . . 59

   9.      Security Considerations  . . . . . . . . . . . . . . . . . 60

   10.     Open Issues  . . . . . . . . . . . . . . . . . . . . . . . 62

   11.     Contributors . . . . . . . . . . . . . . . . . . . . . . . 63

           Normative References . . . . . . . . . . . . . . . . . . . 64

           Informative References . . . . . . . . . . . . . . . . . . 65

           Authors' Addresses . . . . . . . . . . . . . . . . . . . . 66

   A.      Interworking of SIP with NSIS NATFW NSLP . . . . . . . . . 68
   A.1     The Session Initiation Protocol  . . . . . . . . . . . . . 68
   A.2     Conclusions  . . . . . . . . . . . . . . . . . . . . . . . 73

   B.      Ad-Hoc networks  . . . . . . . . . . . . . . . . . . . . . 74

   C.      Interworking of Security Mechanisms and NSIS NATFW NSLP  . 75

   D.      Solution approaches in case of missing authorization . . . 76
   D.1     Solution Approach: Local authorization from both end
           points . . . . . . . . . . . . . . . . . . . . . . . . . . 76
   D.2     Solution Approach: Access Network-Only Signaling . . . . . 77
   D.3     Solution Approach: Authorization Tokens  . . . . . . . . . 77



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   E.      Acknowledgements . . . . . . . . . . . . . . . . . . . . . 80

           Intellectual Property and Copyright Statements . . . . . . 81
















































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1. Introduction

   Even though the NSIS WG (Next Steps in Signaling) has QoS signaling
   as a primary application, other types of applications should be
   possible.

   In this draft, we address the scenario, framework, problems, and
   issues when signaling Network Address Translators (NAT) and/or
   Firewalls to allow packet traversal.

   One of the requirements in NSIS [1] is that the NTLP signaling
   protocol must be independent of the service requested.   In some
   cases the NSIS protocol suite could be used to request end-to-end or
   edge-to-edge QoS from IP networks; in other cases the service might
   be allow packet traversal from one host to the other through all the
   NATs and firewalls deployed on the data path.

   See also [17] and [18] for proposals to use RSVP or CASP for NAT and
   Firewall traversal.
































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2. Terminology and Abbreviations

   This document uses terms defined in [22].  In addition the following
   terms are used:

   o  NSIS NAT Forwarding State: The term "NSIS NAT Forwarding State" in
      this context refers to a state used to forward the NSIS signaling
      message beyond the targeted destination address; that state  is
      typically used when the NSIS Responder address is not known

   o  Sender-/Receiver Initiated Signaling

         Sender-initiated: NAT bindings and firewall rules are created
         immediately when the "path" message hits the NSIS nodes.  With
         "path" message we refer to the signaling message traveling from
         the data sender towards the data receiver.

         Receiver-initiated: NAT bindings and firewall rules are created
         when the "resv" message returns from the other end.  With
         "resv" message we refer to a signaling message on the reverse
         path, this means from the receiver to the sender (i.e.
         backwards routed).

         Note that these definitions have nothing to do with number of
         roundtrips, who performs authorization etc.

   o  Firewalls vs.  Security Gateway: As discussed in  Section 3.1
      different types of firewalls exist.  This document focuses on
      firewalls, which perform packet filtering, and possibly
      application level filtering and does not address IPsec based
      security gateways.

   o  NSIS Initiator (NI): the signaling entity which makes the resource
      request, usually as a result of user application request.

   o  NSIS Responder (NR): the signaling entity that acts as the
      endpoint for the signaling and can optionally interact with
      applications as well.

   o  NSIS Forwarder (NF): the signaling entity between an NI and NR
      which propagates NSIS signaling further through the network.

   o  Receiver (DR or R): the node in the network which is receiving the
      data packets in a flow.

   o  Sender (DS or S): the node in the network which is sending the
      data packets in a flow.




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   o  NATFW NSLP session: an artifact that groups the installed states
      for a given data flow.

   o  Middlebox: from [21]: "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".  The term middlebox in context of
      this document and in NSIS refers to firewalls and NATs only.
      Other types of middlebox are currently outside the scope.










































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3. Framework and Scenarios

3.1 Scope

   The term firewall and middlebox in general raises different
   expectations about the functionality provided by such a device.
   Different groups have worked on the problem of securing access to a
   network by using different procedures and protocols.  From an
   abstract point of view two different mechanisms for restricting
   access to a network can be differentiated:

   o  Packet Filters

   o  Cryptographically protected data traffic

   Within this document we assume that packet filters are installed at
   devices along the path.  These packet filters may consist of a 5
   tuple (src/dst ip address, transport protocol, src/dst port).  Some
   devices entitled as firewalls only accept traffic after cryptographic
   verification (i.e.  IPsec protected data traffic).  Particularly for
   network access scenarios either link layer or network layer data
   protection is common.  Hence we do not address these types of devices
   (referred as security gateways) since per-flow signaling is rather
   uncommon in this environment.  For a discussion of network access
   authentication and associated scenarios the reader is referred to the
   PANA working group (see [16]).

   In mobility scenarios an often experienced problem is the traversal
   of a security gateway at the edge of the corporate network.  Network
   administrators often rely on the policy that only authenticated data
   traffic is allowed to enter the network.  A problem statement for the
   traversal of these security gateways in the context of Mobile IP can
   be found at [15]).

3.2 What problem should be solved?

   The goal of NSIS FW/NAT signaling therefore focuses on packet filter
   installation combined  with associated actions to be enforced on
   packets matching the filter expression.  In the case of NATs and
   Firewalls the associated actions are packet forwarding and address/
   port translation in packet headers.

   Discovering security gateways, which was also mentioned as an
   application for NSIS signaling, for the purpose of executing an IKE
   to create an IPsec SA, is already solved without requiring NSIS.

   Installing packet filters provides some security but has some
   weaknesses, which heavily depend on the type of packet filter



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

3.3 Basic NSIS Usage for NAT/FW traversal

   The basic high-level picture of NSIS usage is that of the endhosts
   signaling to establish packet filters and NAT bindings on a data
   path, which allows the said data to travel from the endhost to the
   endpoint unobstructed.

   Therefore it is necessary that each firewall and each NAT involved in
   the signaling communication runs an NSIS daemon.  There might be
   several NATs and FWs in various possible combinations on a path
   between two hosts.  The reader is referred to  Section 3.4 where
   different scenarios are presented.




























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

   +----+                        +----+                        +----+
   |    +------------------------+    +------------------------+    |
   +-+--+                        +----+                        +-+--+
     |                                                           |
     |                                            NSIS Agents    |
   +-+--+        +----+                            +-----+     +-+--+
   |    +--------+    +----------------------------+     +-----+    |
   +-+--+        +-+--+                            +--+--+     +-+--+
     |             |               ------             |          |
     |             |           ////      \\\\\        |          |
   +-+--+        +-+--+      |/               |     +-+--+     +-+--+
   |    |        |    |     |   Internet       |    |    |     |    |
   |    +--------+    +-----+                  +----+    +-----+    |
   +----+        +----+      |\               |     +----+     +----+
                               \\\\      /////
   sender    NAT/FW (1+)           ------          NATFW (1+) receiver


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


3.4 Scenarios for Protocol Functionality

   This section introduces several scenarios for middleboxes in the
   Internet.  These middleboxes are firewalls or different flavours of
   NATs [5], like NAPT.  Combinations of Firewalls and NATs in the same
   device are possible as well.

   Each section introduces a different scenario for a different set of
   middleboxes and their ordering within the topology.

3.4.1 Firewall traversal

   The following scenario shows two end hosts behind a firewall but
   connected via the public Internet.  The application can somehow
   trigger firewall traversal (e.g.  via an API call) at the NSIS agent
   at the local host.  The NSIS agent then signals this request to the
   next NSIS aware node and therefore to the receiver.  Each firewall in
   the middle must provide traversal service in order to permit the NSIS
   message to reach the other end host.

   The difference between this scenario and the following is that
   firewalls are on the path, but no NATs.  This has specific
   implication concerning the used destination address for path-coupled
   signaling message sent by the NSIS Initiator to an NR hosted behind a
   NAT.



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            +----+    //----\\       +----+
     S -----| FW |---|        |------| FW |--- R
            +----+    \\----//       +----+

            private          public          private


       FW: Firewall
       S: Data Sender
       R: Data Receiver


                 Figure 2: Firewall Traversal Scenario


3.4.2 NAT with two private networks

   This scenario deals with NATs on both ends of the network.
   Therefore, each application instance is behind a NAT and is connected
   to the public Internet (see Figure 3 ).

   The case where more than one MB on each side ("only" two are shown in
   the figure) is present must be taken into account.  This aspect
   introduces more topology problems.


       +----+     +----+    //----\\    +----+     +----+
   S --| MB |-----| MB |---|        |---| MB |-----| MB |--- R
       +----+     +----+    \\----//    +----+     +----+

            private          public          private

       MB: Middlebox
       S: Data Sender
       R: Data Receiver


            Figure 3: NAT with two private networks Scenario

   Data traffic from the sender to the receiver has to traverse all four
   middleboxes on the path and all four middleboxes must be configured
   properly to allow subsequent data packets to flow.  The sender has to
   know the IP address of the receiver in advance, i.e.  before any NSIS
   message can be sent.  Or more general the NSIS Initiator must know
   the IP addresses of the NSIS Responder, otherwise he cannot send a
   single NSIS signaling message 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 obtained from



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   the NAT which subsequently allows packets hitting the NAT to be
   forwarded to the receiver within the private address realm.  This in
   general requires further support from an application layer protocol
   for the purpose of discovering and exchanging information.  The
   receiver might have a number of ways to learn its public IP address
   and port number and might need to signal this information to the
   sender using the application level signaling protocol.

3.4.3 NAT with private network on sender side

   This scenario shows an application instance at the sending node which
   is behind one ore more FW/NATs.  The receiver is located in the
   public internet.


       +----+     +----+    //----\\
   S --| MB |-----| MB |---|        |--- R
       +----+     +----+    \\----//

            private          public

       MB: Middlebox
       S: Data Sender
       R: Data Receiver


         Figure 4: NAT with private network on sender scenario

   The traffic from the sender to the receiver has to traverse only
   middleboxes on the sender's side.  The receiver has a public IP
   address and therefore the procedure is simple.  The sender sends its
   signaling message directly to the receiver whereby it is intercepted
   by the middleboxes along the path.

   Note that the data sender does not necessarily know whether the
   receiver is behind a NAT or not, hence, it is the receiving side that
   has to detect the whether it is behind a NAT or not.  As described in
   Section 5 NSIS can also provide help for this procedure.

3.4.4 NAT with private network on receiver side

   The application instance receiving data is behind one or more NATs.









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         //----\\    +----+     +----+
   S ---|        |---| MB |-----| MB |--- R
         \\----//    +----+     +----+

          public          private


       MB: Middlebox
       S: Data Sender
       R: Data Receiver


        Figure 5: NAT with private network on receiver Scenario

   First, the sender must determine the public IP address of the
   receiver.

   One possibility is that an application level protocol is used.  In
   this case, the receiver must first find out its public IP addresses
   at the middlebox on its side.  This information about IP address and
   port numbers could be signalled somehow to the actual sender directly
   or indirectly via a third party.  In the scenario, this means the
   receiver has to determine its public IP address (NAT binding) and
   register this address with the third party.

   The sender can start NSIS signaling after he has received information
   about the receiver's address and port number.

   Note: The solution design will determine where to discontinue
   forwarding the signaling messages.

3.4.5 Both end hosts in same private network behind NATs

   This is a special case, where the main problem is to detect that both
   nodes are within the same network behind a NAT.  This scenario
   primarily addresses performance aspects.

   Sender and receiver are both within a private address realm and
   potentially have overlapped addresses.  Figure 6 shows the ordering
   of NATs.  This is a common configuration in several networks,
   particularly after the merging of companies that have overlapped
   addresses.









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                             public
       +----+     +----+    //----\\
   S --| MB |--+--| MB |---|        |
       +----+  |  +----+    \\----//
               |
               |  +----+
               +--| MB |------------ R
                  +----+

            private

       MB: Middlebox
       S: Data Sender
       R: Data Receiver


   Figure 6: NAT to public, receiver in same private network Scenario

   The middleboxes are twice-NATs, i.e.  they map the IP addresses and
   port numbers on both sides, private and public interface.

   From a protocol point of view, this means that the protocol must be
   robust enough to at least not break with this scenario.

   In the worst case, both sender and receiver obtain a public IP
   address at the NAT and the communication path is not optimal anymore.

3.4.5.1 Both end hosts in same private network behind same NAT

   A slight variation of the previous scenario consists in having both
   the DS and the DR behind the same NAT.  In case the data sender and
   data receiver can not communicate which address is the most suitable
   among the several available (local scoped or globally scoped)
   addresses to be used for the NSIS messages, the protocol should be
   robust enough to handle the worst case, where the messages are sent
   through the NAT.

3.4.6 IPv4/v6 NAT with two private networks

   This scenario combines the usage case mentioned in Section 3.4.2 with
   the IPv4 to IPv6 transition scenario, i.e.  using Network Address and
   Protocol Translators (NAT-PT).

   The difference to the other scenarios lies in the use of IPv6 - IPv4
   address translation, which happens in both directions.  Additionally,
   the base NTLP must take care of this case for its own functionality
   of forwarding messages between NSIS peers.




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       +----+  +----+   //----\\   +----+  //----\\   +----+  +----+
   S --| MB |--| MB |--|        |--| MB |-|        |--| MB |--| MB |-- R
       +----+  +----+   \\----//   +----+  \\----//   +----+  +----+

            private      public             public       private
                          IPv4               IPv6

       MB: Middlebox
       S: Data Sender
       R: Data Receiver


            Figure 7: IPv4/v6 NAT with two private networks


3.5 Trust Relationship and Authorization

   Trust relationships and authorization are very important for the
   protocol machinery.  Trust and authorization closely related to each
   other in the sense that a certain degree of trust is required to
   authorize a particular action.  If the action is "create/delete
   packet filters" then authorization is very important due to the
   nature of a firewall.

   It is not particularly surprising that differences exist between
   authorization in a QoS signaling environment and firewall signaling.
   As elaborated in [13] the establishment of a financial relationship
   is very important for QoS, signaling whereas for firewall signaling
   is not directly of interest.

   In the subsequent sections different trust relationships will be
   described which appear in firewall signaling environments.
   Peer-to-peer trust relationships are those, which are used in QoS
   signaling today and seem to be the simplest.  However, there are
   reasons to believe that this is not the only type of trust
   relationship found in today's networks.

3.5.1 Peer-to-Peer Trust Relationship

   Starting with the simplest scenario it is assumed that neighboring
   nodes trust each other.  The required security association to
   authenticate and to protect a signaling message is either available
   (manual configuration) or dynamically established with the help of an
   authentication and key exchange protocol.  If nodes are located
   closely together it is assumed that security association
   establishment is easier than establishing it between far distant
   node.  It is, however, difficult to describe this relationship
   generally due to the different usage scenarios and environments.



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   Authorization heavily depends on the participating entities but for
   this scenario it is assumed that neighboring entities trust each
   other (at least for the purpose of packet filter creation and
   deletion).  Note that Figure 8 does not illustrate the trust
   relationship between the end host and the access network, which is
   dynamically established as part of the network access authentication
   procedure as stated in Section 1.


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


               Figure 8: Peer-to-Peer Trust Relationship


3.5.2 Intra-Domain Trust Relationship

   In larger corporations often more than one firewall is used to
   protect different departments.  In many cases the entire enterprise
   is controlled by a security department, which gives instructions to
   the department administrators.  In such a scenario a peer-to-peer
   trust-relationship might be prevalent.  Sometimes however 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 firewalls could
   exchange the authorization decision to another firewalls within the



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   same trust domain.  Individual middleboxes within an administrative
   domain should exploit their trust relationship instead of requesting
   authentication and authorization of the signaling initiator again and
   again.  Thereby complex protocol interaction is avoided.  This
   provides both a performance improvement without a security
   disadvantage since a single administrative domain can be seen as a
   single entity.  Figure 9 illustrates a network structure which uses a
   centralized entity.


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


               Figure 9: Intra-domain Trust Relationship


3.5.3 End-to-Middle Trust Relationship

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



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

   Figure 10 shows the slightly more complex trust relationships in this
   scenario.


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


              Figure 10: End-to-Middle Trust Relationship














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4. Problems and Challenges

   This section describes a number of problems which have to be
   addressed for NSIS NAT/Firewall.  These are also of relevance for
   other NSLP protocols.

4.1 Missing Network-to-Network Trust Relationship

   Peer-to-peer trust relationship, as shown in Figure 8, is a very
   convenient assumption that allows simplified signaling message
   processing.  However, it might not always be applicable, especially
   between two arbitrary access networks (over a core network where
   signaling messages are not interpreted) does possibly not exist
   because of the large number of networks and the unwillingness of
   administrators to have other network operators to create holes in
   their firewalls without proper authorization.  Hence in the following
   scenario we assume a somewhat different message processing and show
   three possible approaches to tackle the problem.  None of these three
   approaches is without drawbacks or constraining assumptions.
































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


        Figure 11: Missing Network-to-Network Trust Relationship

   Figure 11 illustrates a problem whereby an external node is not
   allowed to manipulate (create, delete, query, etc.) packet filters at
   a firewall.  Opening pinholes is only allowed for internal nodes or
   with a certain authorization permission.  Hence the solution
   alternatives in Section 5 focus on establishing the necessary trust
   with cooperation of internal nodes.

4.2 End-to-end significance

   In the case of NAT/firewalls traversal, the NSIS signaling messages
   need to be sent all they way from the DS and DR or viceversa.  This
   is so because a middle box does not know wether the remaining path to
   the destination is clear of pottentially obstructing middleboxes or
   not.

4.3 Relationship with routing

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



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   flow will go through.

   Creating NAT bindings modifies routing of data packets between end
   points.  This is unlike other NSIS NSLPs, which do not interfere with
   routing - instead they only follow the path of the data packets.

4.4 Dynamic state installation and maintenance

   For NAT/Firewall traversal, the lifetime of a NAT binding or a packet
   filter must be determined is often mantained through periodic
   refresh.For short-lived flows, having unpredictable filtersi,
   signaling for pinholes and NAT bindings is preferable as opposed to
   statically configured NAT bindings and pinholes requested for long
   duration in time.  The capability to specify a lifetime for a NAT
   binding provides some advantages to what exists today where unknown
   NAT binding lifetimes can lead to unexpected protocol actions.

   For static state other mechanisms than an NSIS signaling protocol
   might be preferable; such mechanisms would include a management
   protocols such as SNMP or CLI.

4.5 Affected Parts of the Network

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

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

4.6 NSIS backward compatibility with NSIS unaware NAT and Firewalls

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

   NSIS NATFW NSLP's backward compatibility issues is different than the
   NSIS QoS NSLP backward compatibility issues, where an NSIS unaware
   Qos gate will simply forward the Qos NSLP message.  An NSIS unaware
   firewall  rejects NSIS messages, since firewalls typically implement



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   the policy "default = deny".

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

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

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

4.7 Authentication and Authorization

   Since a firewall has security functionality, strong authentication
   and authorization means MUST be provided.

   For NATs security is not a major concern, but might play a role in
   the perceived security measure of some administrators.  For NAT
   sometimes address depletion is considered as a threat.

4.8 Directional Properties

   There two directional properties that need to be addressed by the
   NATFW NSLP:

   o  Directionality of the data

   o  Directionality of NSLP signaling

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

   With regards to NSLP signaling directionality: As stated in the
   previous sections, the authentication and authorization of NSLP
   signaling messages received from hosts within the same trust domain
   (typically from hosts located within the security perimeter delimited



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   by firewalls) is normally simpler than received messages sent by
   hosts located in different trust domains.

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

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

   As opposed to traditional NAT [6] where the NAT bind was implicitly
   created when an outbound packet was sent; the NSIS NATFW NSLP aware
   NATs will be explicitly requested to create NAT binds.  Hence data
   packet directionality is no longer significant for the bind creation,
   in the case of signaled NAT bind creations on NSIS aware NATs.
   However depending on the used NAT implementation, data directionality
   may still apply to implicitly maintain the NAT bind (since NAT binds
   consume memory and addressing resources); as such some NAT
   implementation might still require that outbound packet ONLY maintain
   (implicitly and not explicitly) binds.

4.9 Routing Asymmetry

   Routing asymmetry [14] is a general problem for path-coupled
   signaling, especially when installed states on NSIS forwarders are
   related to bi-directional flows.

   Path state, on an NSIS forwarder, including the next NSIS hop (for
   packets sent from the NR to NI), is used to handle routing asymmetry
   for NSIS messages but not for data flows (i.e.  no route pinning for
   data flows).

   Similarly to path-coupled QoS signaling, firewall signaling also has
   to be aware of the routing asymmetry when bi-directional flows
   relevant states need to be installed on NSIS aware nodes, although
   the routing asymmetry might not be significant within the local
   networks where firewalls are typically located.  For signaling NAT
   bindings this issue comes with a different flavor since an
   established NAT binding changes the path of the data packets.  Hence
   a data receiver might still be able to send NSIS signaling messages
   to create a NAT binding, although they travel the previously "wrong"
   path.

4.10 Addressing




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

4.11 NTLP/NSLP NAT Support

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

4.12 Route changes

   The effect of route changes are more severe than in other signaling
   applications since a firewall pinhole and NAT binding is needed
   before further communication can takes place.  This is true for both
   NSIS signaling and for subsequent data traffic.  If a route changes
   and NSIS signaling messages do not configure NSIS NATs and firewalls
   along the new path then the communication is temporarily interrupted.
   This is naturally a big problem for networks where routes frequently
   change e.g.  ad-hoc networks or in case of fast mobility.  In these
   cases state refresh messages have to provide a mechanism for fast
   reaction.

4.13 Combining Middlebox and QoS signaling

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

   - NAT/FW NSLP signaling affects a much small number of NSIS nodes
   along the path (for example compared to the QoS signaling).

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



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   - The refresh interval is likely to be different.

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

4.14 Difference between sender- and receiver-initiated signaling

   For NAT/FW signaling there seems to be little difference between
   sender- and receiver- initiated signaling messages.  Some other
   characteristics of QoS signaling protocols are not applicable (e.g.
   the adspec object) to the NAT/FW context.  It seems that a full
   roundtrip is always required if the protocol aims to be generic
   enough.

4.15 Inability to know the scenario

   In  Section 3.4 a number of different scenarios are presented.  In
   some scenario NSIS signaling is fairly easy whereas in others it is
   quite complex.  Additionally different trust relationships exist
   between networks along the path, which might require interaction with
   the end host or a different signaling behavior.  However, the user
   (or the NSIS agent initially) typically does not know which scenario
   is currently applicable.  To make things worse, the scenario might
   actually change with moving networks, adhoc networks or with mobility
   in general.  Hence NSIS signaling must assume the worst case and
   cannot put responsibility to the user to know which scenario is
   currently applicable.  As a result, it might be necessary to perform
   a "discovery" periodically such that the NSIS agent at the end host
   has enough information to decide which scenario is currently
   applicable.  This additional messaging, which might not be necessary
   in all cases, eats performance, bandwidth and adds complexity.
   Additional information by the user can provide information to assist
   this "discovery" process but cannot replace it.

   Some protocols already aim to provide a solution for an end host to
   learn something about the topology such as STUN [11].  To some extend
   these protocols can help NSIS NAT/FW signaling.













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5. NSIS NAT Handling Solution

   This section describes a mechanism for allowing NSIS signaling
   messages to travel end-to-end in the presence of NATs at the
   receiving side.  This requires to establish state information at the
   NSIS-aware NAT device.

   Note: The discussed mechanism only creates state relevant for NSIS
   message handling.  It does not create NAT bindings for data traffic.

5.1 Problem Description

   NSIS signaling messages follow the data path from the data sender to
   the data receiver.  To provide this property of being path-coupled
   the discovery process sends signaling messages along the same route
   as taken by subsequent data packets.  The NSIS messages are directed
   to a particular destination IP address and hence the destination
   address needs to be known in advance before NSIS signaling can
   start.
































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



            Figure 12: The Data Receiver behind NAT problem

   Figure 12 describes a typical message communication in a peer-to-peer
   networking environment whereby the two end points learn of each
   others existence with the help of a third party (referred as
   Application Server).  The communication with the application server
   and the two end points (data sender and data receivers) serves a
   number of functions.  As one of the most important functions it
   enables the two end hosts to learn the IP address of each other.

   Without the proposed mechanism it would not be possible to establish
   a NAT binding end-to-end in all scenarios.

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

   If the data receiver is behind a NAT then an NSIS signaling message
   will be addressed to the IP address allocated at the NAT (if there



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   was one allocated).  If no corresponding NSIS NAT Forwarding State at
   NAT/NAPT B exists (binding IP(R-NAT B) <-> IP(R)) then the signaling
   message will terminate at the NAT device (most likely without proper
   response message).  The signaling message transmitted by the data
   sender cannot install the NAT binding or NSIS NAT Forwarding State
   "on-the-fly" since this would assume that the data sender knows the
   topology at the data receiver side (i.e.  the number and the
   arrangement of the NAT and the private IP address(es) of the data
   receiver).  The primary goal of path-coupled middlebox communication
   was not to force end hosts to have this type of topology knowledge.

   A number of solutions exist to allow nodes behind a NAT to establish
   a NAT binding to allow the receiver to receive IP packets.  These
   solutions can at best be labeled as hacks (see [NATP2P]) and they
   have their drawbacks:

   o  They assume a certain behavior of NAT boxes.

   o  They work in some environments whereas in others they do not
      properly function.

   o  They only allow NAT bindings for UDP traffic to be established.

   o  They often fail.

   Some other solutions assume that both nodes are registered in the DNS
   directory (see [19]).

   The requirements for an NSIS solution are two-fold:

   1.  NSIS signaling messages must be able to travel end-to-end
       (between data sender and data receiver) - if desired.  This is
       important for a number of NSIS NSLPs

   2.  NSIS relies on a generic solution which works in all scenarios
       (see section 5 of [22]).

   Since the NSIS signaling messages are intercepted at each NSIS device
   the NAT solution depends on the properties of the NTLP.  In
   particular, multiplexing capability is important.  Two possible
   options are feasible:

   1.  Multiplexing with the help of transport layer information (i.e.
       port information)

   2.  Multiplexing at the NSIS application layer (e.g.  based on
       session identifier)




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   We describe the second approach although we believe that alternatives
   are possible.

   Enough information has to be available to convert IP address
   information of an incoming signaling message to different IP
   addresses of an outgoing NSIS message.  Finally the signaling message
   must reach the data receiver.

   It seems that the session identifier can be used to associate state
   information of the two independent signaling exchanges.  The two
   exchanges (as described in Section 5.2) are:

   1.  Signaling exchange from the data receiver (NR) to the NAT(s)

   2.  End-to-end NSIS signaling message exchange from the NI to the NR.

   If the session identifier is used for this purpose then it is
   necessary to communicate the session id from the data receiver (NR)
   to the NI.  Communicating the IP address information instead (as an
   alternative solution approach) is easier since this functionality is
   already provided by SIP whereas securely exchanging (e.g.
   confidentiality protected) the Session Identifier is not available.

5.2 Solution Overview

   The data receiver starts to signal an NSIS Create-NAT-Binding message
   into the "wrong direction".  By "wrong" we refer to the usual
   behavior of path-coupled signaling where the data sender starts
   signaling in order to tackle with routing asymmetry.  The data
   receiver would typically return signaling messages to the data sender
   in the reverse direction by utilizing state created at nodes along
   the path (i.e.  to reverse route signaling messages).  The concept of
   path-coupled or path-decoupled signaling is, however, no relevant for
   this special type of signaling communication.  In case of
   establishing NAT bindings (and NSIS NAT Forwarding State) the
   direction does not matter since routing is modified.  Subsequent NSIS
   messages (and also data traffic) will travel through the same NAT
   boxes.

   The proposed solution requires two NSIS signaling messages:

   1.  Create-NAT-Binding Request

   2.  Create-NAT-Binding Acknowledgement

   The semantic of the two messages will be described in detail in this
   section.




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   The data receiver sends a Create-NAT-Binding NSIS signaling message
   into the local network (before the data sender starts NSIS
   signaling).  In Section Section 5.2.1 we will discuss where to
   address this signaling message (i.e.  which destination IP address to
   use).

   The signaling message creates NSIS NAT Forwarding State at
   intermediate NSIS NAT node(s).  Furthermore it has to be ensured that
   the edge NAT device is discovered as part of this process.  By edge
   NAT we refer to the NAT device which is reachable from outside and
   has a globally routable IP address.  The end host cannot be assumed
   to know this device - instead the NAT box itself is assumed to know
   that it has such a capability.  Forwarding of the Create-NAT-Binding
   NSIS message beyond this entity is not necessary, and should be
   prohibited as it provides information on internal hosts capabilities.



                                                 Create-NAT-Binding Request
             +-------+    +-------+    +-------+    +---------+
             | NAT X |<---| NAT Y |<---| NAT Z |<---| Data    |
             |       |--->|       |--->|       |--->| Receiver|
             +-------+    +-------+    +-------+    +---------+
                            Create-NAT-Binding Response

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



          Figure 13: Create-NAT-Binding NSIS Signaling Message

   The goal of this signaling message exchange is:

   o  to create one (or more) NAT binding(s)

   o  to allow the data receiver to learn its global routable IP address
      (for communication with NSIS)

   o  not to require the data receiver to learn topology information.

   Figure 13 shows a number of NAT devices at the data receivers network
   side.  NSIS NAT Forwarding State is established at these network
   elements.

   The Create-NAT-Binding Request message triggers the state creation
   and the discovery.  The message carries information where the sender
   expects incoming NSIS signaling messages.



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   The Create-NAT-Binding Response message confirms the state creation
   and allows to return information about the NATs and the topology to
   the end host (for informational purposes).  As a result the end host
   will learn the public IP address which can be used by the data sender
   to address NSIS signaling messages.

5.2.1 Destination IP address Selection

   The Create-NAT-Binding Request message has to be addressed to a
   specific destination IP address.  Since there is no natural candidate
   a few alternatives might be considered.  The discussed options refer
   to entities of Figure 12

   Possible options are:

   1.  Public IP address of the data sender

   2.  Public IP address of the data receiver (allocated at NAT B)

   3.  IP address at the Application Server

   Actually, there is no "correct" answer to this question and from a
   theoretical point of view it does not really matter as long as Host A
   learns an IP address where he has to send the NSIS signaling message.
   >From a performance point of view there is, however, a difference
   since it would be desirable to create an "optimal" routing path.

   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 with SIP after using NSIS to
             establish a NAT binding.





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   2.  Public IP address of the data receiver (allocated at NAT B):

       *  Assumption:

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

       *  Problems:

          +  Communication with a third party is required.

   3.  IP address at the Application Server:

       *  Assumption:

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

       *  Problems:

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

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
























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6. Protocol Description

6.1 Basic protocol overview

   In the NSIS protocol, hosts try to communicate with each other in
   spite of firewalls or NAT's by opening pinholes or making NAT
   bindings in them.  For things to work, we need to signal the proper
   boxes, and that means we will have to send the signals on the data
   path, and in the proper direction.  So, if the Data Sender (DS) wants
   to send data to the Data Receiver (DR), it is DS that will have to
   signal towards DR.

   Problems arise when DR has a private IP address and is not publicly
   reachable.  In that case, DR will have to make due for it, and get a
   public IP address it can use by using specific NSIS signaling.  DS
   will then signal towards the public address DR got.

   Given this fact, we have two modes of operation:

   o  First, DR making itself available by signaling on the reverse path
      (DR towards DS).  This way, it reserves publicly reachable
      addresses and ports, which it communicates to DR through means out
      of the scope of this document, but probably involving a third
      party and an application level protocol.

   o  And second, DS signaling directly to DR, and creating NAT bindings
      and installing FW rules.  Nothe that the first mode will usually
      make reservations only, which will be "activated" by the signaling
      from DS towards DR.  The first mode is detailed in the Section 5

   The protocol is meant to work on a soft-state basis.  This means,
   that whatever state is installed or reserved on a middle box, will
   expire, and thus be uninstalled/forgotten after a certain timeout.
   To prevent this the involved boxes will have to specifically request
   a session prolongation.  An explicit NATFW NSLP state deletion
   message is also provided by the protocol.

   Middle boxes should report back in case of error, so that appropriate
   measures and debugging can be performed.

6.2 Message format and types

   At the moment of writing this document, no decision has been reached
   on the details of the NTLP, (NSIS Transport Layer Protocol), hence
   the current NATFW NSLP protocol specification uses raw IP transport
   with an arbitrary protocol number.  The NSIS Protocol Data Unit is
   constituted of a header, and a series of objects.




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   The NSIS header contains:

   o  header_len: length of the nsisp header in bytes

   o  version: protocol version number.

   o  obj_count: number of objects that follow after the nsis header.

   It is followed by a series of objects.  Each object is composed of an
   object header and the object data.  The object header format is
   common to all the object types, and has the following format:

   o  obj_len: total length of the object

   o  obj_h_len: length of the object header

   o  obj_type: type of NATFW NSLP object.  Identifies the data that
      follows.

   The object data is attached next to the object header, and varies
   depending on the required action to be  taken.  For the moment four
   object types are defined as follows, if required others could be
   defined later on.

   Create session (a temporary pinhole or a nat-binding):

   o  sid: Session ID.  Randomly generated by the endhosts.

   o  src_addr: where the data will come from.  If it is DS sending data
      to DR, the source address is either DS or the closest NAT in the
      route from DS to the middlebox that gets the packet; That is, the
      address where each middlebox will see the packet come from.

   o  dst_addr: whre the data is headed.  If it is DS sending data to
      DR, the destination address is either DR or the public address DR
      reserved itself.

   o  src_port: the transport port the data will come from

   o  dst_port: the transport port the data will go to.

   o  proto: protocol number.

   Note: you might want to leave the source address or port set to ANY,
   to accept any source address port.  This makes the pinhole not so pin
   like, but might be necessary at the integration with certain NAT/FW
   types.  Whether this loose pinhole is authorized or not by the middle
   box, is a policy decision based on the middle box configuration.



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   Prolong session:

   o  sid: the session we intend to prolong.

   Delete session:

   o  sid: the session we want to delete.

   Reserve external addr:

   o  sid: session for the reserve external address process.

   o  tgt_addr: address we want our external address to point to.  If A
      reserves an external address in a NAT, it will want that external
      address to point to A.

   o  tgt_port: port the external addres:port pair should point to.

   o  src_addr: The address the data will come from.  We might want to
      set this to 0.0.0.0, to make a loose pinhole reservation, if we
      don't the src_addr yet.

   o  src_port: The port the data will come from.  Can also be set to
      zero if unknown.

   Note that no state, be it a firewall rule or a nat binding, is
   installed as a result of this message.  The state is only remembered,
   and might be later installed by a create message.

   Return  external addr:

   o  sid: the session this packet is replyng about.

   o  ext_addr: the external address that the nat has just created for
      the endhost.

   o  ext_port: the external port.

   Path succeeded:

   o  sid: the session ID for which a path was succesfully installed

   Error:

   o  sid: the session id of the object that generated the error

   o  error_code: what the error was.




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6.3 Message flow

   Of the shown message types, 4 of them are sent by the end hosts to
   achieve some service (create, reserve, prolong, and delete) and 3 are
   sent back as a response (return external address, path succeded and
   error).

   The usual message flow for the reservation mode of operation, to
   achieve an external address, is:


    DS      Public Internet        NAT    Private addres        DR
    |                              |          space             |
    |                              |                            |
    |                              |                    Reserve |
    |                              |<---------------------------|
    |                              |                            |
    |                              | Return ext addr / Error    |
    |                              |--------------------------->|
    |                              |                            |
    |                              |                            |


   In the case of the creation mode, when rules are actually installed,
   the message flow is as follows:


























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    DS      Public Internet        NAT    Private addres        DR
    |                              |          space             |
    | Create                       |                            |
    |----------------------------->|                            |
    |                              |                            |
    | Error (if necessary)         |                            |
    |<-----------------------------| Create                     |
    |                              |--------------------------->|
    |                              |                            |
    |                              |       Path Succeeded/Error |
    |         Path Succeeded/Error |<---------------------------|
    |<-----------------------------|                            |
    |                              |                            |
    |                              |                            |


   When it comes to prolonging or deleting sessions:



    DS      Public Internet        NAT    Private addres        DR
    |                              |          space             |
    | Prolong/Delete               |                            |
    |----------------------------->|                            |
    |                              |                            |
    | Error (if necessary)         |                            |
    |<-----------------------------| Prolong/Delete             |
    |                              |--------------------------->|
    |                              |                            |
    |                              |       Error (if necessary) |
    |         Error (if necessary) |<---------------------------|
    |<-----------------------------|                            |
    |                              |                            |
    |                              |                            |




6.4 Middle box configuration

   The way a message is handled in each box will depend on its
   configuration.

   Currently defined ocal NSIS NAT/FW configuration parameters are:

   o  nat_capabilities: am I a NAT?

   o  nat_external_addr: if I'm a nat, what is the address closest to



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      the public internet?

   o  edge_nat: true, false or auto.  Tells us how much of an edge_nat
      we are.  See explanation below.

   The edge_nat parameter requires some more explanation, though.  When
   a host reserves an external address, as seen in Section 5, it makes
   itself reachable by the other end.  This does not always mean a
   public internet address.  In Section 7.4 we see that a private
   address can also be reachable by another box in the same address
   space.  Still, things work well if we get a public ip address, only
   the route is unnecesarily long (see an example in Section 7.4).

   For this reason, we can set up the boxes to always or never behave as
   an edge nat, or else set it to auto.  The auto setting will compare
   the destination ip address of the reserve address objects, and see if
   they belong to the same address space as the nat external address.
   If it is so, the nat behaves as an edge_nat for this connection,
   otherwise, it doesn't.

   Note that the address spaces are set in RFC3330 [2].

6.5 Message handling

   When a box receives an NSIS message, it might take an action based on
   the message type, the nature of the box, its configuration and its
   security policies.

   As a summary, here's the behaviour of the boxes, depending on message
   type and config parameters:

                     NAT     FW     NAT+FW    DS    DR
      reserve        5       -      5         +     +
      ret_ext_addr   -       -      -         +     8
      create         1       2      3         +     4
      prolong        6       6      6         +     4
      delete         7       7      7         +     4
      path_succeed   -       -      -         8     +

      1: install the rule, rewriting either the source or destination
      address depending on whether the packet comes from the
      external_address or not.  Always forward.

      2: Install the rule, always forward.

      3: 1+2.  The order depends on whether it comes from the outside
      address (NAT, then FW) or the inside one (FW, then NAT)




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      4: If it fits one of its requests, send a path_succeeded packet
      back.  Drop the packet.

      5: Make a reservation.  If edge_nat is set, send back the
      external_addr and don't forward.  Otherwise, forward and don't
      send anything back.

      6: Prolong the session.  Always forward.

      7: Terminate the session.  Always forward.

      8: hand it over to superior layers, and drop it.

      -: ignore and forward.

      +: ignore and drop.

   Note that the order is important, when it comes to NAT+FW boxes,
   because the filter rules have to be set up according to the packet
   they will see.  Source NAT is done at the end so it doesn't disturb
   routing decisons, this means the filter sees the original packets.
   Destination NAT, on the other hand, is done at the beginning, so it
   can be routed properly, and so the filter sees the modified packets.

   Note also that for each action, the host might demand a certain
   degree of authorization, and thus refuse to take the action, sending
   an error message back instead.

   The details of the message handling in each of the boxes follows.

6.5.1 Detailed behaviour

6.5.1.1 Reserve external address message

   o  NAT Box:

      When a NAT box gets a Reserve external addressm message, it checks
      whether it arrived on the public address, or the private one.  If
      it arrived in the public one, an error message of the type:
      "Requested an external address from the outside" is sent back.

      If it arrived on the private side, an entry is made in the
      internal reservation list with the packet information.  If the box
      is an edge nat (either by configuring it to true, or just for that
      connection if it is set to auto), it drops the message, and
      replies with a return external address message containing the
      allocated address port pair.  If it is not an edge NAT, it
      forwards the packet on.



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   o  Firewall Box:

      Reserve messages are silently ignored in Firewall boxes.  They are
      simply forwarded on.

   o  NAT+FW Box:

      When a box that integrates both a NAT and a Firewall gets a
      reserve message, it will hand it to its NAT part.  Its firewall
      part will simple ignore it.

   o  Data Sender:

      The message should never get here.  It should be ignored and
      dropped.

   o  Data Receiver:

      The message should never get here.  It should be ignored and
      dropped.


6.5.1.2 Return external address message

   o  NAT Box, Firewall Box and NAT+Firewall Box:

      When one of these boxes gets a Return external address message, it
      must simply ignore it and let it traverse.

   o  Data Sender:

      The message should never get here.  It should be ignored and
      dropped.

   o  Data Receiver:

      A return external address message in the Data receiver, has
      reached its destination.  It must be dropped, and it's information
      handed to superior layers.


6.5.1.3 Create message

   o  NAT Box:

      When a NAT box gets a create message, it first checks if it
      arrived on the public address or not.




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      If it came from the public side, it means an external box will try
      to send data.  It then looks for a reservation in its reservation
      list, that matches the dst_addr and dst_port of the packet.  If it
      doesn't find it, it returns an error message of the type "No
      reservation found".  If it finds it, it fills in the reservation
      with the data from the packet, and installs the given rule.  It
      then changes the dst_addr and dst_port fields of the create packet
      and forwards it to the tgt_addr of the reservation.

      If it came from the private side, it installs the nat rule with
      the information in the packet.  It then changes the src_addr and
      src_port of the create message to its own external address and
      port.

   o  Firwall Box:

      When a firewall box gets a create message, it simply installs the
      rule specified in the message and forwards the packet.

   o  NAT+FW Box:

      When a box that integrates both a NAT and a Firewall gets a create
      message, it first checks whether it arrived on the public address
      or not.

      If it arrived on the public side, the NAT part of the box takes
      care of the packet first, as said in the NAT Box case.
      Afterwards, the modified packet is handed to the firewall part,
      where it is handled as in the Firewall Box case.

      If it arrived on the private side, the message is handed to the
      firewall part first, and then to the NAT one.

   o  Data Sender:

      The message should never get here.  It should be ignored and
      dropped.

   o  Data Receiver:

      If the data receiver gets a create message, it means all the boxes
      on the way accepted it, and so the signaling succeded.  All it
      does is drop the packet, and send back a Path Succeded message to
      the ip packet source address.







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6.5.1.4 Delete session message

   o  NAT Box, Firewall Box and NAT+Firewall Box:

      When one of these boxes gets a Delete session message, it should
      erase the session refered in the message.

   o  Data Sender:

      As in the create session message, this packet should never get to
      the DS, so it is to be ignored and dropped.

   o  Data Receiver:

      As in the create session message, this is the final destination of
      the message.  If it got here, everything is fine.  No further
      action should be taken on the packet, which means it is dropped at
      the endhost.


6.5.1.5 Prolong session message

   o  NAT Box, Firewall Box and NAT+Firewall Box:

      When one of these boxes gets a Prolong session message, the
      expiration time of the session should be changed to the time of
      reception plus the configured session lifetime.

   o  Data Sender:

      As in the create session message, this packet is sent from the DS
      to the DR, and should never arrive at the DS.  Again, it should be
      ignored and dropped.

   o  Data Receiver:

      The same behaviour as in the case of a Delete session message on
      the DR should be applied.


6.5.1.6 Path Succeded message

   o  NAT Box, Firewall Box and NAT+Firewall Box:

      When one of these boxes gets a Path succeded message, it should
      simply ignore it and allow it to traverse.

   o  Data Sender:



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      The message ends here.  It must be dropped, and its contents sent
      to upper software layers.

   o  Data Receiver:

      The message should never get here.  It should be ignored and
      dropped.














































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7. Solution examples

7.1 Firewall traversal

   DS wants to send ata traffic to DR through tight firewalls, as seen
   in Figure 18.  To do that, it will have to signal using NSIS, on the
   data path.


         +-----+     +-----+    //----\\    +-----+     +-----+
    DS --| FW1 |-----| FW2 |---|        |---| FW3 |-----| FW4 |--- DR
         +-----+     +-----+    \\----//    +-----+     +-----+

            private          public          private

                           Data Flow
                         ===============================>



                 Figure 18: Firewall Traversal Scenario

   Therefore, DS initiates signaling to DR by sending a create object to
   the ip address od DR.  Note that DS already knows its source address
   and port (say, 1111), and the destination address of DR.  The
   destination port (let's say 9999) has been send to DS by DR via
   application layer messages, possibily, but not necessarily involving
   a third party.  The message looks like:

   o  dst_addr = DR

   o  dst_port = 9999

   o  src_addr = DS

   o  src_port = 1111

   This message is received by FW1, which installs the state that reads:
   "Any packet coming from DS:1111 headed for DR:9999 will be allowed
   traversal"

   FW2, FW3 and FW4 do exactly the same, and forward the packet to each
   other, until it finally reaches DR.  At this point, the data path is
   open, and DR sends back a Path succeeded message to DS, which can now
   start sending traffic.

7.2 NAT with private network on sender side




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   In the example in Figure 19, DS is in a private network  and wants to
   send data to DR, out in the public internet.  To do so, DS will have
   to initiate NSIS signaling towards DR


        +------+     +------+    //----\\
   DS --| NAT1 |-----| NAT2 |---|        |--- DR
        +------+     +------+    \\----//

            private          public



         Figure 19: NAT with private network on sender scenario

   Now, this is a deceivingly easy example.  Apparently, the normal NAT
   functionality will take care of sending the data from DS out into the
   public internet, and route back the replies from DR.  This is indeed
   true, but doesn't give NSIS control on what the source address or
   port is, as it is usually asigned dinamycally by the NAT.  Moreover,
   the NSLP would have no information on this hops, and could not
   install proper pinholes, as it would set DS as the source address,
   and not that of the last NAT.

   DS builds a create packet with the information he has, which is the
   same as that in Section 7.1.  It looks like this:

   o  dst_addr = DR

   o  dst_port = 9999

   o  src_addr = DS

   o  src_port = 1111

   NAT1 is the first to get the packet; It is not coming from its
   configured "nat external address", and so, it knows it will have to
   rewrite the information on the source, and not that of the
   destination.  NAT1 then picks a free port (incidentally 1011) and
   installs a nat rule that reads:  "Whatever packet comes from DS:111,
   heading for DR:9999 will be rewriten so that the source address looks
   like NAT1:1011".

   It then rewrites the packet it received as follows:

   o  dst_addr = DR

   o  dst_port = 9999



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   o  src_addr = NAT1

   o  src_port = 1011

   And forwards the packet.

   NAT2 gets it now, and does exactly the same.  Port 2022 is chosen,
   and the rule:  "Whatever packet comes from NAT1:1011, heading for
   DR:9999 will be rewriten so that the source address looks like
   NAT2:2022" is installed.  The packet gets modified as follows:

   o  dst_addr = DR

   o  dst_port = 9999

   o  src_addr = NAT2

   o  src_port = 2022

   And is forwarded.  It eventually reaches DR, who sends back a path
   succeded message.  Data flow from DS:1111 to DR:9999 is now possible.

7.3 NAT with private network on receiver side

   In this example, DS wants to send data to DR over the network in
   Figure 20:


          //----\\    +------+     +------+
   DS ---|        |---| NAT1 |-----| NAT2 |--- DR
          \\----//    +------+     +------+

          public          private



        Figure 20: NAT with private network on receiver Scenario

   The problem, of course, is that DR is not publicly reachable.
   Because of that, DR will have to signal on the data path, in the
   opposite direction (DR->DS) to get itself a public address it can
   use.  This method is described in Section 5

   To get an external address, DR sends a packet to DS.  It could
   actually send it to anything in the public internet, as it would
   force it to traverse what NATs are on its way.  In the case of
   multihomed environments, though, more than one NAT to the outside is
   possible, so the better we "aim" the more the chances we go out the



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   right NATs and get more optimal routes.

   The said packet is an NSIS reserve_addr object which looks like this:

   o  tgt_addr = DR

   o  tgt_port = 9999

   o  src_addr = 0.0.0.0

   o  src_port = 0

   Notice that this is a really loose pinhole, since any src_addr and
   port is allowed.

   NAT2 gets the packet and looks for a free port (say, 2022, for
   clarity's sake).  It then adds an entry to its reservation list.  The
   entry looks like this:

   o  src_addr = 0.0.0.0

   o  src_port = 0

   o  dst_addr = NAT2

   o  dst_port = 2022

   o  tgt_addr = DR

   o  tgt_port = 9999

   This means simply that packets coming from any source, destinated to
   the public address we just reserved, shoud be targeted to the
   internal box DR, on port 9999

   It then rewrites the packet so that it looks like:

   o  tgt_addr = NAT2

   o  tgt_port = 2022

   o  src_addr = 0.0.0.0

   o  src_port = 0

   Because it is not an edge nat, it forwards the modified packet and
   does not sent a return_external_addr object to DR.  Note that no nat
   binding is installed so far in NAT2, although the state is now



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   reserved.

   NAT1 now gets the packet, picks free port 1011 and adds the following
   entry to its reservation list:

   o  src_addr = 0.0.0.0

   o  src_port = 0

   o  dst_addr = NAT1

   o  dst_port = 1011

   o  tgt_addr = NAT2

   o  tgt_port = 2022

   As it turns out, NAT1 IS an edge_nat, so it doesn't forward the
   packet.  Instead, it replies to DR sending back a return external
   address packet on the same connection, so it finds its way back
   through the NATs:

   o  ext_addr = NAT2

   o  ext_port = 2022

   By using some application layer protocol, and possibly, although not
   necesarily, using a third party box, DR sends it's freshly allocated
   external address and port to DS.

   DS now knows who to signal, so it sends a create message:

   o  dst_addr = NAT1

   o  dst_port = 1011

   o  src_addr = DS

   o  src_port = 1111

   When it reaches NAT1, it does so through NAT1 external address.  It
   realizes it is being asked to forward the traffic from some outside
   box towards the inside.  It then looks up its reservation list,
   looking for a session that has the external address and port
   NAT1:1011 assigned.  It finds this:

   o  src_addr = 0.0.0.0




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   o  src_port = 0

   o  dst_addr = NAT1

   o  dst_port = 1011

   o  tgt_addr = NAT2

   o  tgt_port = 2022

   Using the information in the create object, it then fills in this
   structure to:

   o  src_addr = DS

   o  src_port = 1111

   o  dst_addr = NAT1

   o  dst_port = 1011

   o  tgt_addr = NAT2

   o  tgt_port = 2022

   This IS a tight pinhole.  NAT1 installs the rules now, which say:
   "Whatever packet comes from DS:1111 heading for NAT1:1011, should
   have its destination address changed to NAT2:2022, and be forwarded".
   The packet is also rewritten into this:

   o  src_addr = DS

   o  src_port = 1111

   o  dst_addr = NAT2

   o  dst_port = 2022

   And is forwarded to NAT2.  Upon arrival, a similar process issues.
   NAT2 finds its reservation entry:

   o  src_addr = 0.0.0.0

   o  src_port = 0

   o  dst_addr = NAT2

   o  dst_port = 2022



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   o  tgt_addr = DR

   o  tgt_port = 9999

   Fills it in accordingly:

   o  src_addr = DS

   o  src_port = 1111

   o  dst_addr = NAT2

   o  dst_port = 2022

   o  tgt_addr = DR

   o  tgt_port = 9999

   Rewrites the packet:

   o  src_addr = DS

   o  src_port = 1111

   o  dst_addr = DR

   o  dst_port = 2222

   And forwards it to DR.  Once there, DR acknowledges it by sending
   back a path succeded message in reply, back to DS.

   The path is now open and data transmission from DS:1111->DR:9999 can
   comence.

7.4 Both end hosts are in same private network behind NATs

   In this example (see Figure 21), DS, in a private address space,
   wants to send data to DR, in another private address space.  The
   point marked "%" is yet another private address space.  Notice that
   since NAT1 and NAT3 have addresses in the same address space, NAT3
   might want to consider itself an edge nat.  We will consider both
   situations.









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                             public
        +------+  %  +------+    //----\\
   DS --| NAT1 |--+--| NAT2 |---|        |
        +------+  |  +------+    \\----//
                  |
                  |  +------+
                  +--| NAT3 |------------ DR
                     +------+

            private


  Figure 21: NAT to public, receiver in same private network Scenario

   We will first assume that NAT3 has the edge_nat option set to false.
   In this case, the connection is a combination of Section 7.3 and
   Section 7.2.

   Firstly DR will signal against on the data path, against the data
   flow, with a reserve external address object.  NAT3 will reserve the
   address and forward the packet on to NAT2, who IS an edge nat in all
   cases.  NAT2 will reply with the external address, and the connection
   goes on just as in  Section 7.2, except for the fact the topology
   becomes:



























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                             public
        +------+     +------+
   DS --| NAT1 |-----o------o---+
        +------+     |      |   |
                     | NAT2 |---+
                     |      |   |
                  +--o------o---+
                  |  +------+
                  |
                  |  +------+
                  +--| NAT3 |------------ DR
                     +------+

            private


   Figure 22: New topology due to the non optimal edge nat parameter
                                decision

   This is not optimal, but the connection does succeed, and data flow
   can commence.

   Let us now solve the case in which NAT3 has edge_nat set to auto.
   Back in Figure 21, NAT3 will decide it IS an edge_nat if the
   destination we pick up for the reserve address packet is in the
   address space marked as "%", and will NOT consider itself an edge_nat
   if we point it anywhere else.  This is an optimality issue such as
   the one pointed out in Section 7.3.

   Well so, if it doesn't consider itself an edge nat, we already saw
   what the topological equivalent is, and how it proceeds.  If it IS an
   edge nat, the topological equivalent would be:



















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        +------+
   DS --| NAT1 |--+
        +------+  |
                  |
                  |  +------+
                  +--| NAT3 |------------ DR
                     +------+

            private


      Figure 23: A good edge nat decision brings an optimal route

   And we would proceed in the same way, only on a more optimal route.

7.5 IPv4/v6 NAT with two private networks

   TBD

7.6 Full example for NAT/FW with two private networks

   The NAT's have the nat_capabilities variable set to true.  NAT+FW3
   and NAT+FW5 have the edge_nat variable set to true.  The rest of
   boxses have it set to false.

   Let's now suppose that DR wants to get a data stream from DS in
   Figure 24.  For that, we need some way for B to get messages from A,
   be it through some third party application server or some publicly
   reachable proxy, perhaps made public through a nat binding.






















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                         +-----+
                +--------| FW4 |--------+
                |        +-----+        |
           +---------+             +---------+
           | NAT+FW3 |             | NAT+FW5 |
           +---------+             +---------+
                |                       |
           +---------+             +---------+
           |  NAT3   |             |  NAT6   |
           +---------+             +---------+
                |                       |
           +---------+             +---------+
           |   FW1   |             |   FW7   |
           +---------+             +---------+
                 |                       |
           +---------+             +---------+
           |   DS    |             |   DR    |
           +---------+             +---------+

                        Data Flow
               ==========================>


                  Figure 24: Example network topology

   DR wants a data stream from DS, which means that the direction of the
   data is DS->DR.  A will have to make itself publicly reachable by
   signaling its NATs and firewalls as necessary.  This is a step by
   step guide to the whole process.

   In steps 1 to 4, DR makes itself publicly reachable.  From 5 and on,
   DS is signaling on the data path towards DR.

   1.  DR wants to get data from DS, so it sends a reserve_addr object
   to a target in the public internet.  The closest this target is, the
   more the chances that the outcoming route is optimal, but any will
   work.  The reserve_addr obj looks like this:

   o  tgt_addr = DR

   o  tgt_port = 888

   o  src_addr = 0.0.0.0

   o  src_port = 0

   Notice that this is a really loose pinhole, since any src_addr and
   port is allowed.



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   2.  FW7 gets the packet, ignores its contents and forwards it.
   Firewalls always ignore reserve_addr objects.

   3.  NAT6 gets the packet, and looks for a free port (say, 666, for
   clarity's sake).  It then adds an entry to its reservation list.  The
   entry looks like this:

   o  src_addr = 0.0.0.0

   o  src_port = 0

   o  dst_addr = NAT6

   o  dst_port = 666

   o  tgt_addr = DR

   o  tgt_port = 888

   It then rewrites the packet so that it looks like:

   o  tgt_addr = NAT6

   o  tgt_port = 666

   o  src_addr = 0.0.0.0

   o  src_port = 0

   Because it is not an edge nat (edge_nat=false), it does not sent a
   return_external_addr object to DR, but rather forwards the modified
   packet.  Note that no nat binding is installed so far in NAT6,
   although the state is now reserved.

   4.  NAT+FW5 receives the packet.  The firewall part gets the object,
   but, being as it is an address reservation only object, it ignores
   it.  The NAT part gets it next.  Because it is a NAT, it binds a free
   port, which is thus reserved.  An entry to the reservation list is
   added:

   o  src_addr = 0.0.0.0

   o  src_port = 0

   o  dst_addr = NAT+FW5

   o  dst_port = 555




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   o  tgt_addr = NAT6

   o  tgt_port = 666

   Because it is an edge_nat, it sends a return_external_addr packet
   with address NAT+FW5 and port 555 back to DR.  It does so by simply
   sending it back to the source IP addr in the IP header of the packet.
   In this case, it is NAT6.  The standard capabilities of NAT6 will
   send it back to DR, since we are always working on the same
   connection.  Because it is an edge_nat and this is a
   reserve_external_addr packet, it does not forward the packet.

   At this stage, the end host DR has learned what its (reserved)
   external address is, even if it can not be used.  It is now publicly
   reachable, and path-coupled nsis signaling in direction DS->DR can
   start.

   5.  Firstly, DR tells DS about it's freshly reserved outside address
   through some higher layer protocol, using the third-party box.

   6.  DS now initiates signaling to DR by sending a create object to
   the brand new public address of DR.  It looks like:

   o  dst_addr = NAT+FW5

   o  dst_port = 555

   o  src_addr = DS

   o  src_port = 111

   7.  The firewall FW1 gets it, and installs the requested pinhole.
   (Note this IS a tight pinhole with well defined source and
   destination).  It then forwards the packet.

   8.  NAT2 gets the packet.  Because it is NOT coming from it's
   external address, it realizes it is being asked to forward DS's
   future data packets, and so, it will have to rewrite it's source
   address.  To do so, NAT2 picks a random free port (which turns out to
   be 222), and installs a nat rule that says: "Whatever packet comes
   from DS:111, heading for NAT+FW5:555 will be rewriten so that the
   source address looks like NAT2:222".  That is usually known as Source
   NAT.  The nsis create request is then rewriten to look like:

   o  dst_addr = NAT+FW5

   o  dst_port = 555




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   o  src_addr = NAT2

   o  src_port = 222

   Because it is not an edge nat, it simply forwards the modified
   packet.

   9.  NAT+FW3 gets the packet next.  Because it is NOT coming from the
   extarnal_addr of the NAT+FW, The firewall part gets it first, and
   installs the filter rule that says: "Allow traversal of packets going
   from NAT2:222 towards NAT+FW5:555".  It then hands it to the NAT
   part.

   The NAT part gets it then.  It is not coming from its external addr,
   and so, it does as NAT2, binding a port (333) and installing a rule
   that says: "Whatever packet comes from NAT2:222, heading for
   NAT+FW5:555, will be rewriten so that the source address looks like
   NAT+FW3:333".  It will then rewrite the create object to:

   o  dst_addr = NAT+FW5

   o  dst_port = 555

   o  src_addr = NAT+FW3

   o  src_port = 333

   Note that the box won't send a packet back to DS informing it of its
   external address, because DS will never need that.

   10.  FW4 gets the create object, and installs the rule "Allow
   traversal of packets going from NAT+FW3:333 towards NAT+FW5:555" It
   then forwards the object.

   11.  NAT+FW5 gets the create object.  It arrived at its external
   address, so it realizes it doesn't have to change the source address
   of the future data packets of DS, but rather its destination.  It
   also means that the NAT part will have to handle it first.  It then
   tries to find out where it has to re-destinate it to, by looking up
   its reservation tables.  It finds the previous reservation, by
   matching it with ther dst_addr and dst_port of the create object:

   o  src_addr = 0.0.0.0

   o  src_port = 0

   o  dst_addr = NAT+FW5




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   o  dst_port = 555

   o  tgt_addr = NAT6

   o  tgt_port = 666

   And proceeds to fill it in with the information of the create object
   (src_addr and src_port):

   o  src_addr = NAT+FW3

   o  src_port = 333

   o  dst_addr = NAT+FW5

   o  dst_port = 555

   o  tgt_addr = NAT6

   o  tgt_port = 666

   It then installs a NAT rule with that information.  It reads:
   "Whatever packet comes from NAT+FW3:333, heading for NAT+FW5:555 will
   be rewritten, so that its destination address looks like NAT6:666".
   The reservation is erased and the rule starts working.  The NAT
   binding becomes thus usable.

   The object is modified, so that it now looks like:

   o  dst_addr = NAT+FW3

   o  dst_port = 333

   o  src_addr = NAT6

   o  src_port = 666

   The FW part now gets the object, and installs the rule: "Allow
   traversal of whatever packet that comes from NAT+FW3:333 heading for
   NAT6:666".  The packet is then forwarded.

   12.  NAT6 gets the packet.  As it comes from the external adddress,
   it does as NAT+FW5, looking up the reservation list and filling it in
   with:

   o  src_addr = NAT+FW3

   o  src_port = 333



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   o  dst_addr = NAT6

   o  dst_port = 666

   o  tgt_addr = DR

   o  tgt_port = 888

   It then installs the rule: "Whatever packet comes from NAT+FW3:333,
   heading for NAT6:666 will be rewritten, so that its destination
   address looks like DR:888".  The rule reservation is erased, and the
   NAT binding becomes active.  The object is rewritten as:

   o  src_addr = NAT+FW3

   o  src_port = 333

   o  dst_addr = DR

   o  dst_port = 888

   The object is thus forwarded.

   13.  FW7 gets the packet now, and installs the rule: "Allow traversal
   of whatever packet that comes from NAT+FW3:333 heading for DR:888".
   It forwards the packet.

   14: DR gets (finally) the packet.  It realizes it is a create object
   headed for him, to the port which he expected, and so it sees
   everything went well.  A reply to the packet is send, and the NAT's
   on the way, knowing the already established connection, will route it
   to DS.  The packet is a path_succesful message, which simply means
   "Everything's fine, send data whenever you want".


















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8. NSIS NAT and Firewall transitions issues

   NSIS NAT and Firewall transition issues are premature and will be
   addressed in a separate draft (see [8]).  An update of this section
   will be based on consensus.














































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9. Security Considerations

   Security is of major concern particularly in case of firewall
   traversal.  Generic threats for NSIS signaling have been discussed in
   [3] and are applicable here as well.  It is necessary to provide
   proper signaling message protection and proper authorization.  Note
   that the NAT is likely to be co-located with a firewall and might
   therefore require packet filters to be changed in order to allow the
   signaling message to process and to traverse.  This section aims to
   raise some items for further discussion and illustrates the problems
   the authors faced when creating a security solution for the NAT/
   Firewall NSLP.  Without doubts there is a dependency on the security
   provided by the NTLP.  At the time of writing this version of the
   document no NTLP draft is available (or accessible for the authors).
   Section Section 4 and  Section 3.5 motivates some trust relationship
   and authorization scenarios.  These scenarios deserve a discussion
   since some of them (particularly one with a missing
   network-to-network trust relationship) is different to what is know
   from QoS signaling.  To address some of these trust relationships and
   authorization issues requires security mechanisms between
   non-neighboring nodes at the NSLP layer.  For the group of authors it
   seems that peer-to-peer and end-to-middle security needs to be
   provided.  An NSLP security mechanism between neighboring NSLP peers
   might be necessary if security mechanisms at the NTLP do not provide
   adequate protection mechanisms.  This issue is, however, still in
   discussion.  As a design goal it seems to be favorable to reuse
   existing mechanisms to the best extend possible.  In most cases it is
   necessary to carry the objects for end-to-middle as NSLP payloads
   since the precesence of NATs might prevent direct communication.
   Three security mechanisms have to be considered in more detail in a
   future version of this document: CMS [9] and  Identity Representation
   for RSVP  [10].  The authors believe that CMS more suitable (since it
   provides much more functionality).  The details deserve further
   discussion and implementation experience.

   With regard to signal between two end hosts even though the receiver
   is behind a NAT this proposal suggests creating state by the data
   receiver first.  This allows NSIS signaling messages to traverse a
   NAT at the receiver side (due to the established state at this NAT
   box) and simplifies security handling.To achieve this behavior it is
   required to install NSIS NTLP and NSLP state.  Furthermore, it is
   envisioned to associate the two signaling parts (one part from the
   data sender to the NAT and the other part from the NAT to the data
   receiver) with the help of the Session Identifier.  As such, the
   discussion in [10] is relevant for this document.

   Another interesting property of this protocol proposal is to prevent
   Denial of Service attacks against NAT boxes whereby an adversary



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   allocates NAT bindings with the help of data packets.  Since these
   data packets do not provide any type of authentication and are not
   authorized any adversary is able to mount such an attack.  This
   attack has  been mentioned at several places in the literature
   already and is particularly harmful if no NAPT functionality is used
   (i.e.  if a new NAT binding consumes one IP address of a pool of IP
   addresses).  Using the protocol described in this document additional
   security can be achieved and more fairness can be provided.











































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10. Open Issues

   At least the following issues require further discussion:

   o  Message format: The exact message format is still to be
      determined, both in regards of bit level details and on
      parameters, such as the need for an object header length, since,
      until now, that is a constant.

   o  Error codes: error codes have to be defined still.  Among others,
      we will need:  missing authorization, out of resources, unable to
      understand the packet, or maximum resources for that individual
      already allocated.

   o  Middle box default policies: allow for the configuration of the
      default policies of the box.  For a NAT+Firewall box, for
      instance, the firewall default policy might be "accept", and so,
      no packet filters would have to be installed on that regard (we
      would still need the nat bindings, though).
































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11. Contributors

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












































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Normative References

   [1]  Brunner et al., M., "Requirements for Signaling Protocols",
        DRAFT draft-ietf-nsis-req-07.txt, March 2003.

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

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

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






































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Informative References

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

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

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

   [8]   Aoun and others...., C., "NAT/Firewall NSLP migration, routing,
         NTLP requirements and off-path Considerations", October 2003.

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

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

   [11]  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.

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

   [13]  Tschofenig, H., Buechli, M., Van den Bosch, S. and H.
         Schulzrinne, "NSIS Authentication, Authorization and Accounting
         Issues", draft-tschofenig-nsis-aaa-issues-01 (work in
         progress), March 2003.

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

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

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



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

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

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

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

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

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


Authors' Addresses

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

   Phone: +49 (0) 6221 905 11 13
   EMail: stiemerling@ccrle.nec.de
   URI:


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

   Phone:
   EMail: Hannes.Tschofenig@siemens.com
   URI:





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

   Phone: +49 (0) 6221 905 11 16
   EMail: lopez@ccrle.nec.de
   URI:


   Cedric Aoun
   Nortel Networks

   France

   EMail: cedric.aoun@nortelnetworks.com


































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Appendix A. Interworking of SIP with NSIS NATFW NSLP

   This document aims at pinpointing the problems of using SIP in
   nowadays networks, focusing on the problems derived of NAT's,
   Firewalls and multi-path communications.  It is intended to fit in a
   scenario description that shows the necessity of NSIS, as well as
   depicting it's requirements.  However, note that there are a number
   of other solutions available.  For example the IETF Midcom working
   group is working on [4].

A.1 The Session Initiation Protocol

   [20] describes the Session Initiation Protocol, an application-layer
   control protocol that can establish, modify, and terminate multimedia
   sessions.  This often involves several flows for video and voice,
   which are transported over new connections.  These use of dynamically
   allocated ports which results in protocol complexity which can not be
   handled by nowadays NAT's and Firewalls.

   Session initiation when one or both of the users is behind a NAT is
   also not possible, given the impossibility to address a private IP
   over the internet.  Moreover, network deployments often allow for
   different paths per connection and direction, making the setup of the
   middle boxes even more complicated.

   The following figure depicts a typical SIP connection:

























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   Ernie(192.0.2.1)                          Bert(192.0.2.2)
      |                                        |
      |     1# SIP INVITE                      |
      +--------------------------------------->|
      |
      |                       2# SIP Ringing   |
      |<---------------------------------------+
      |                                        |
      |                       3# SIP OK        | <-- Call accepted
      |<---------------------------------------+
      |                                        |
      |     4# SIP ACK                         |
      +--------------------------------------->|
      |                                        |
      |     5# DATA                            |
      |=======================================>|
      |<=======================================|
      |                                        |


      1# SIP Invite (192.0.2.1:? -> 192.0.2.2:SIP): I Listen on
      192.0.2.1:1000 Ernie invites Bert to the conference, and informs
      it's awaiting media data on port 1000.

      2# SIP Ringing (192.0.2.2:SIP -> 192.0.2.1:?): Ringing Bert's
      phone The ringing simply inplies that there's something sip aware
      on Berts side, and that it's ringing his phone

      3# SIP OK (192.0.2.2:SIP -> 192.0.2.1:?): Call accepted, I listen
      on 192.0.2.2:2000 This OK means that the Bert took the phone off
      hook, and thus accepted the call.  It also informs Ernie that Bert
      is awaiting his media data at port 2000

      4# SIP ACK (192.0.2.1:? -> 192.0.2.2:SIP): All is fine, start
      transmitting.  ACK means the ports are accepted and the call can
      start in the slected data ports on both sides.

      5# DATA (192.0.2.1:? -> 192.0.2.2:2000 and 192.0.2.2:? ->
      192.0.2.1:1000): Voice,image, video..  This is the actual data
      being transmited.

   In the above example, SIP is used successfully to establish a
   communication, which includes negotiating the data ports for the
   actual transmission.  Unfortunatelly, this scheme will not work for
   more complex setups.

   Let's now consider one firewall in the data path, be it on Ernie's or
   Bert's network, or elsewhere in the middle.  We assume that the



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   firewall is allowing traffic directed to the SIP port.  As to the
   rest of the ports, a typical setup involves outgoing connections
   being allowed, and incoming connections being dropped, except for
   those already established.  That is, we allow packets to go out and
   their replies to come in, but disable all other traffic.

   In this case, the connection is as follows, for the case of a
   firewall on Ernie's network:


     Ernie(192.0.2.1)    FW                    Bert(192.0.2.2)
      |               |                        |
      | 1# SIP INVITE |                        |
      +--------------------------------------->|
      |               |                        |
      |               |         2# SIP Ringing |
      |<---------------------------------------+
      |               |                        |
      |               |              3# SIP OK | <-- Call accepted
      |<---------------------------------------+
      |               |                        |
      | 4# SIP ACK    |                        |
      +--------------------------------------->|
      |               |                        |
      |     5# DATA   |                        |
      |=======================================>|
      |               |<=======================|
      |               |                        |



   Notice how the SIP messages #1 and #4 traverse the firewall, because
   they are outbound, and how 2# and 3# traverse it too, because they
   are replies to the connection established at 1#.

   Notice now how 5# can go outwards, but Bert can not go through the
   firewall to reach Ernie's port 1000.  The reason is the connection is
   a new one, and the firewall won't allow it through.

   Bert will now get media from Ernie, but Ernie is never going to get
   anything from Bert.  The call is thus considered unsuccessful.  The
   reason is that the application level port negotiation is never
   acknowledge by the network-transport layer firewall, which doesn't
   know what to expect.  We would still face the same problem if the
   connection used a SIP Proxy, for it would only translate names into
   IP addresses.

   Let us now assume that we indeed have an application layer firewall,



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   be it by design, or because we load some sort of SIP module to it.
   The previous case would now work, since the firewall can now
   understand the packets going through it and open the necessary ports.
   Still, we cannot assume that SIP signalization packets and the actual
   data follow the same path.  The following figure shows a likely
   setup.  FW+ stands for one or more firewalls:


                        SIP Signalization Path   +-----+
                    /---------------->-----------| FW+ |-------\
                    |                            +-----+       |
    +------+    +------+                                    +-----+
    |Ernie |----|Router|                                    |Bert |
    +------+    +------+                                    +-----+
                    |   Data Path                +-----+       |
                    \---------------->-----------| FW+ |-------/
                                                 +-----+



   The SIP packets with the information about the listening ports now
   travels on the SIP Signalization path, and so the firewalls on that
   path can read them.  The Data, though, is traveling through the Data
   path, and the firewalls in that path never get to see the Invite and
   Ok packets.  They are thus unable to open the ports.

   Two issues are arisen here: first, we need on-path signalization
   unless we already know the path our packets will take; a highly
   unlikely situation in today's internet.  Second, if we patch the
   firewalls to understand SIP, we will provide any caller with a
   hole-puncher for the firewall, since SIP is not provisioned with
   proper authentication mechanism.

   It is now clear that tight firewalls prevent SIP from successfully
   working.  There is still another obstacle: NATs.

   NATs provide for a link between two different address spaces,
   typically connecting a private range network to a public range one.
   As a consequence, connections going from the inside (usually the
   private range) are translated using the NAT's public interface
   address, and the replies are routed back.  The public side of the
   network can only see the NATs public interface, and know nothing of
   the private network inside.  This means computers outside the NAT
   won't be able to address computers inside the NAT.

   Let us analyze the SIP example when Ernie is behind a NAT.  The
   following figure depicts a typical session:




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    Ernie(10.0.0.2)   (10.0.0.1) NAT (192.0.2.1)    Bert(192.0.2.2)
       |                          |                  |
       | 1# SIP INVITE            |                  |
       +--------------------------\                  |
       |                          |----------------->|
       |                          |                  |
       |                          |   2# SIP Ringing |
       |                          /------------------+
       |<-------------------------|                  |
       |                          |                  |
       |                          |        3# SIP OK | <-- Call accepted
       |                          /------------------+
       |<-------------------------|                  |
       |                          |                  |
       | 4# SIP ACK               |                  |
       +--------------------------\                  |
       |                          |----------------->|
       |                          |                  |
       | 5# DATA                  |                  |
       |==========================\                  |
       |                          |=================>|
       |                          |   ?<=============|
       |                          |                  |


   The communication is analogous to the one in the previous examples,
   except for the fact the NAT is rewriting the source address of the
   packets as they traverse it.

   For instance, packet 1# is going from 10.0.0.2:? towards
   192.0.2.2:SIP.  The NAT box intercepts the message and puts
   192.0.2.1:? as the source address and port, with ? being a
   dynamically picked port, which might be different from the original
   one 1# used.

   On the way back, Bert is replyinc to the source of the IP packet,
   that is, 192.0.2.1, and so, when 2# reaches 192.0.2.1, the NAT know
   it is a reply from 1#, because it established a NAT binding, and this
   replaces the destination address, 192.0.2.1:? with 10.0.0.2:? and
   forwards the packet inside the NAT.

   As a result, Ernie never knows there is a NAT in his communication
   path, since he sends and receives packets from 192.0.2.2 normally.
   This means that the INVITE packet will tell Bert to send data back to
   10.0.0.2, a private IP.  Once the signalization is finished, and the
   actual DATA transmission starts, Bert tries to connect to 10.0.0.2, a
   private IP address, from the internet; The routers don't know how to
   route this, and the packet is eventually dropped.



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   One possible solution would be for Ernie to know the NAT exists, and
   already indicate that it listens on 192.0.2.1, and not 10.0.0.2.
   That, still would not work, since the NAT binding is not performed at
   the NAT box.

A.2 Conclusions

   The above examples display the inability to use standard SIP through
   tight firewalls or NATs, and points at the necessity of a secure
   on-path protocol to negotiate firewall pinholes and NAT bindings.









































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Appendix B. Ad-Hoc networks

   Some forms of ad-hoc networks exist where trust in the network is not
   justified.  Figure Figure 29 mainly illustrates the problems of
   malicious NSIS entities graphically:



   +------------------------------------------+        +--------//
   |                             Adhoc        |        | ISP
   |                             Network      |        |
   |      regular data                        |        |
   |      traffic by          +---------+     |        |
   |      node A              |Malicious|     |      +-+--------+
   |          +-------------->+  Node   +-----+///-->+ Firewall +-//
   |          ^               |   3     |===========>|    1     |
   |          |               +---------+ injected   +-+--------+
   |          |                           data traffic |
   |          |                               |        |
   |          |                               |        |
   |      +---+-----+        +---------+      |        |
   |      +  Node   |        |  Node   |      |        |
   |      |    1    |        |    2    |      |        |
   |      +---------+        +---------+      |        |
   |          ^                               |        +--------//
   |          |                               |
   +----------+-------------------------------+
              |
           +--+---+
           | Node |
           |  A   |
           +------+


              Figure 29: Limits of packet filter security

   An ad-hoc network consists of a number of nodes between the end host
   (Node A) and the ISP to which Node A wants to get access.  Although
   Node A uses an authentication and key exchange protocol to create a
   policy rule at the firewall 1 it is still possible for an untrusted
   node (in this case Node 3) to inject data traffic which will pass
   Firewall 1 since the data traffic is not authenticated.  To prevent
   this type of threat two approaches are possible.  First, a
   restrictive packet filter limits the capabilities of an adversary.
   Finally, there is always the option of using data traffic protection.






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Appendix C. Interworking of Security Mechanisms and NSIS NATFW NSLP

   TBD
















































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Appendix D. Solution approaches in case of missing authorization

D.1 Solution Approach: Local authorization from both end points

   The first approach makes use of local authorization from both end
   points.  If Host A sends a signaling message toward the destination
   to Middlebox 1 the message will perform the desired action in Network
   A.  Middlebox 1 establishes some state information and forwards the
   signaling message towards Host B.  Signaling message protection
   between the two access networks might be difficult.  A missing trust
   relationship does not necessarily mean that no security association
   establishment is possible.  The lacking trust disallows Middlebox 1
   (or indirectly Host A where the signaling message was initiated) to
   create packet filters at Middlebox 2.  We assume that the NSIS
   signaling message is allowed to pass the firewall then it finally
   reaches Host B.  Due to the missing authorization no packet filter
   specific state is created.  The filters will be installed later after
   receiving an authorization from Host B.  When Host B returns a
   confirmation or acknowledgement then Middlebox 2 treats it as an
   authorization and finally triggers filter creation.  The message is
   then forwarded to Middlebox 1, where filters are either already
   installed or require an additional confirmation.  Finally the
   signaling message is forwarded to Host A, which can be assured that
   subsequent data traffic can be transmitted end-to-end from Host A to
   Host B.  The same procedure has to be applied again to signal
   information for the other direction (Host B to Host A).

   The following behavior has to be assumed in order for this approach
   to be applicable:

   1.  Signaling messages must be allowed to pass firewalls along the
       path.

   2.  NSIS signaling must operate in the described manner which could
       be described as: Install where you have authorization - delay and
       forward where you have no authorization.

   This approach suffers from the following drawbacks:

   1.  Firewalls which block NSIS signaling from external networks or
       nodes prevent a successful operation.

   2.  A full roundtrip is required to signal packet filter information.
       The NSIS signaling message must therefore provide the capability
       to route signaling message in both direction which might either
       require state installation at nodes along the path (route
       pinning) or a stateless version via record-route.  Some risk of
       DoS protection might exist.



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D.2 Solution Approach: Access Network-Only Signaling

   The next approach is based on signaling packet filter information by
   both hosts into the local access network only.  An NSIS allows
   specifying such a behavior by indicating the signaling endpoint with
   the help of scoping (for example with domain name or a "local network
   only" flag).  Scoping means that the signaling message although
   addressed to a particular destination IP address terminates somewhere
   along the path.  If packet filters for both directions have to be
   installed then the signaling messages have to make packet filter
   installations up- and downstream along the data path.  Similar to
   proposals in the area of QoS signaling some problems are likely to
   occur.  One such problem is that downstream signaling in general
   causes problems because of asymmetric routes.  In particular it is
   difficult to determine the firewall where the downstream data traffic
   will enter a network.  The problem of triggering downstream
   reservations is for example described in [12] .  Another problem for
   example is the placement of a firewall or NAT along the path other
   than in the access network.  This would prevent a successful data
   exchange.

   The following behavior has to be assumed in order for this approach
   to be applicable:

   1.  It must be possible to trigger a signaling message exchange for a
       downstream signaling message exchange at the firewall where the
       data traffic enters the network.

   2.  No other firewalls or NATs are present along the path other than
       in the access network.

   This approach suffers from the following drawbacks:

   1.  To signal policy rules only within the access network (by both
       end-points) has a number of disadvantage and challenges (see for
       example [12] ).  The complex message processing caused by this
       approach strongly argues against it although it might sound
       simple (and even might be simple in restricted environments).

   2.  Complex topologies might lead to ineffective policy rules (i.e.
       data traffic hits firewalls hits wrong firewalls).


D.3 Solution Approach: Authorization Tokens

   The last approach is based on some exchanged authorization tokens
   which are created by an authorized entity (such as the PDP) or by a
   trusted third party.  Both end hosts need to exchange these tokens



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   with protocols such as SIP or HTTP since these protocols are likely
   to be allowed to bypass the firewall.  The basic idea of this
   approach is to provide an end host, which requests access to the
   network, with credentials (referred as authorization tokens).  These
   tokens have to possess some properties, namely:

   1.  They have to be restrictive by including lifetimes, source and
       destination identifiers, usage indication and more.

   2.  They have to provide basic replay protection to prevent
       unauthorized reuse.

   3.  The have be cryptographically protected to prevent manipulations.

   4.  There has to be a mechanism to dynamically create them for a
       specific reason and to distribute them to the end points.

   5.  It has to be possible to exchange tokens via a trusted third part
       in cases where no direct communication between the end hosts is
       possible (due to NAT).

   6.  The token can be created locally at the network or by a trusted
       third party.

   An example of a possible signaling communication could have the
   following structure: After exchanging the tokens between the two end
   hosts.  Host A would include the received authorization token to the
   signaling message for Network B.  When the signaling message arrives
   at Middlebox 2 then the token is verified by the token-creating
   entity.  In order to prevent parties from reusing the token
   timestamps (e.g.  token creation, token lifetime, etc.) have to be
   included.  Adding IP address information about Host A would create
   difficulties in relationship with NATs.  Information about Host B
   might be possible to include in order to limit attacks where a token
   is lost and reused by a different host for a different purpose.  The
   goal is to restrict the usage of the token for a specific session.
   The content of the token only needs to be verified by the originator
   of the token since it only has to be verified locally.  Since
   authorization needs to be linked to the authorized actions, which
   have to be performed on the packets matching the packet filter, the
   token may include the associated action or a reference to it.  The
   following behavior has to be assumed in order for this approach to be
   applicable:

   1.  The exchange of authorization tokens between end-systems must be
       possible.  These protocols must be allowed to pass the firewalls.

   2.  An end-system must be able to request such an authorization token



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       at some entity in the local network or at a trusted third party.

   This approach suffers from the following drawback:

   1.  Possibly an additional protocol is required for an end host to
       request an authorization token from an entity in the local
       network.












































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Appendix E. Acknowledgements

   We would like to acknowledge Vishal Sankhla and Joao Girao for their
   input to this draft.















































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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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