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NSIS                                                       A. Pashalidis
Internet-Draft                                                       NEC
Intended status: Informational                             H. Tschofenig
Expires: January 9, 2008                                         Siemens
                                                            July 8, 2007


                           GIST NAT Traversal
            draft-pashalidis-nsis-gimps-nattraversal-05.txt

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   Copyright (C) The IETF Trust (2007).













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Abstract

   This document describes a number of mechanisms for the implementation
   of the General Internet Signalling Transport (GIST) protocol [1] on
   different types of Network Address Translator (NAT).  The focus of
   these mechanisms is the interaction of GIST with the address
   translation function of the NAT, and their purpose is to enable GIST
   hosts that are located on either side of the NAT to correctly
   interpret signalling messages with respect to the data traffic they
   refer to.  The purpose of this document is to provide guidance to
   people that implement GIST and NSLPs on both NAT and non-NAT nodes.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Assumptions  . . . . . . . . . . . . . . . . . . . . . . . . . 11
   5.  Transparent NAT traversal for GIST . . . . . . . . . . . . . . 13
     5.1.  NI-side NSLP-unaware GaNATs  . . . . . . . . . . . . . . . 13
     5.2.  NR-side NSLP-unaware GaNATs  . . . . . . . . . . . . . . . 19
     5.3.  NSLP-aware GaNATs  . . . . . . . . . . . . . . . . . . . . 21
     5.4.  Combination of NSLP-aware and NSLP-unaware GaNATs  . . . . 25
   6.  Non-transparent NAT traversal for GIST . . . . . . . . . . . . 27
     6.1.  NI-side NSLP-unaware GaNATs  . . . . . . . . . . . . . . . 27
     6.2.  NR-side NSLP-unaware GaNATs  . . . . . . . . . . . . . . . 32
     6.3.  GIST peer processing . . . . . . . . . . . . . . . . . . . 38
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 41
     7.1.  Service Denial Attacks . . . . . . . . . . . . . . . . . . 41
     7.2.  Network Intrusions . . . . . . . . . . . . . . . . . . . . 42
   8.  IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 44
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 45
   10. Normative References . . . . . . . . . . . . . . . . . . . . . 46
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 47
   Intellectual Property and Copyright Statements . . . . . . . . . . 48















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1.  Introduction

   Network Address Translators (NATs) modify certain fields in the IP
   and transport layer header of the packets that traverse them.  In the
   context of signalling as specified by the General Internet Signalling
   Transport (GIST) protocol [1], this behaviour may lead to the
   installation of state at network nodes that may be inconsistent and
   meaningless with respect to the data traffic that traverses these
   nodes.

   This document describes mechanisms that can be used in order for GIST
   signalling messages to traverse NATs in a way that preserves the
   consistency of state that is installed in the network with respect to
   the data flows to which the signalling messages refer.  As the
   mechanisms that are described in this document exclusively operate at
   the GIST layer, they are transparent to signalling applications.  The
   document is organised as follows.  The next section introduces the
   terminology that is used throughout this document.  Section 3
   provides a detailed discussion of the NAT traversal problem and
   highlights certain design decisions that have to be taken when
   addressing the problem.  Section 4 lists the assumptions on which the
   subsequently proposed mechanisms are based.  The mechanisms are
   described in Section 5 and Section 6.  Finally, Section 7 presents
   some security issues that arise in conjunction with the mechanisms
   described in this document.


























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2.  Terminology

   The terminology, abbreviations and notational conventions that are
   used throughout the document are as follows.

   o  DR: Data Receiver, same as Flow Receiver as defined in [1]

   o  DS: Data Sender, same as Flow Sender as defined in [1]

   o  GaNAT: GIST-aware NAT - a GaNAT MAY implement a number of NSLPs.

   o  GIST: General Internet Messaging Protocol for Signalling [1]

   o  NAT: Network Address Translator

   o  NI: NSIS Initiator; this is the GIST node (as defined in [1]) that
      initiates a signalling session for a given NSLP.  The NI may or
      may not be identical to the DS or the DR.

   o  NR: NSIS Responder; this is the GIST node (as defined in [1]) that
      acts as the last in a sequence of nodes that participate in a
      given signalling session.  The NR may or may not be identical to
      the DR or the DS.

   o  NSIS: Next Steps in Signalling: The name of the IETF working group
      that specified the family of signalling protocols of which this
      document is also a member.  The term NSIS is also used to refer to
      this family of signalling protocols as a whole.

   o  GIST-aware: Implements GIST and MAY also implement a number of
      NSLPs.

   o  GIST-unaware: GIST-unaware, does not implement any NSLP.  The term
      is synonymous to NSIS-unaware.

   o  NSLP: NSIS Signalling Layer Protocol, as defined in [1]

   o  downstream: as defined in [1]

   o  upstream: as defined in [1]

   o  MRI: Message Routing Information, as defined in [1]

   o  NLI.IA: Interface Address field of the Network Layer Information
      object, as defined in [1]

   o  NSLP: Network Signalling Layer Protocol




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   o  <- : Assignment operator.  The quantity to the right of the
      operator is assigned to the variable to its left.

   o  A.B: Element B of structure A. Example: [IP
      header].SourceIPAddress denotes the source IP address of an IP
      header.

   o  [data item]: This notation indicates that "data item" is a single
      identifier of a data structure.  (Square brackets do not denote
      optional arguments in this document.)









































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3.  Problem Statement

   According to [1], all GIST messages between two peers carry IP
   addresses in order to define the data flow to which the signalling
   refers.  Moreover, certain GIST messages also carry the IP address of
   the sending peer, in order to enable the receiving peer to address
   subsequent traffic to the sender.  Packets that cross an addressing
   boundary, say from addressing space S1 to S2, have the IP addresses
   in the IP header translated from space S1 to S2 by the NAT; if GIST
   payloads are not translated in a consistent manner, the MRI in a GIST
   packet that crosses the boundary, e.g. from address space S1 to S2,
   refers to a flow that does not exist in S2.  In fact, the flow may be
   invalid in S2 because at the IP address that belongs to S1 may not be
   routable or invalid in S2.  Moreover, the IP address of the sending
   peer may also be not routable or invalid in the addressing space of
   the receiving peer.  The purpose of this document is to describe a
   way for GIST messages to be translated in a way that is consistent
   with the translation that NATs apply to the IP headers of the data
   traffic.

   A NAT may either be GIST-unaware or GIST-aware.  We refer to a GIST-
   aware NAT as a "GaNAT" in the sequel.  A GaNAT MAY also support at
   least one NSLP.  Note that there exists an NSLP, namely the NATFW
   NSLP [2], that specifically addresses NAT traversal for data flows.
   Inevitably, the NATFW NSLP also provides the necessary mechanisms for
   the related signalling to traverse the involved NATs.  Consider a
   GaNAT that supports both the NATFW NSLP, and the NAT traversal
   mechanism that is described in this document (which operates at the
   GIST layer).  Suppose now that a GIST QUERY message arrives at this
   GaNAT that contains the NSLP identifier (NSLPID) of the NATFW NSLP.
   A question that arises is whether the GaNAT should use the GIST-layer
   NAT traversal mechanism (described in this document), or the NATFW
   NSLP mechanism, in order to provide "NAT traversal" for both the
   signalling message and the data flow to which it refers.  The answer
   to this question is that a GaNAT should implement a policy according
   to which one method is used in preference to the other.  Note that,
   however, if the GaNAT prefers GIST-layer NAT traversal, then it may
   happen, if no on-path GaNATs exist that prefer the NATFW NSLP, that
   no downstream NATFW NSLP peers are discovered.  This may make the
   entire NATFW session obsolete.  It is therefore anticipated that the
   NATFW NSLP will be the preferred NAT traversal mechanism in most
   circumstances.

   However, in certain cicumstances it may be desirable for GIST
   signalling messages to traverse a NAT, and not desirable or possible
   to use the NATFW NSLP for this purpose.  Examples of such
   circumstances are the following.




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   o  GaNATs that do not implement the NATFW NSLP are on the path taken
      by GIST signalling messages.  This situation may arise during
      incremental deployment of the signalling protocols that are
      developed by the NSIS working group.

   o  GaNATs that implement the NATFW NSLP are on the path taken by GIST
      signalling messages that refer to a given data flow.  However, the
      NSLP that is being signalled is *not* the NATFW NSLP and there
      exists no NATFW signalling session for the data flow in question.

   Describing NAT traversal for GIST signalling messages in the above
   circumstances is the subject matter of this document.

   In general, a given data flow between a data sender (DS) and a data
   receiver (DR) may have to traverse a number of NATs, some of which
   may be GIST-and-NATFW-aware, some may be GIST-aware, and some may be
   GIST-unaware.  Additionally, NSLP signalling for such a data flow may
   be required to traverse through a subset of those NATs.  Whether or
   not the routing infrastructure and state of the network causes the
   signalling for such a data flow to traverse the same NATs as the flow
   depends, among other things, on which NSLP is being signalled.  While
   signalling of the QoS NSLP, for example, might not traverse any of
   the NATs that are traversed by the data flow, the signalling of the
   NATFW NSLP traverses at least those NATs that implement the NATFW
   NSLP (otherwise the signalling path would no longer be coupled to the
   data path, as this coupling is defined by the GIST QUERY/RESPONSE
   discovery mechanism for the "path coupled" Message Routing Method).
   It is desirable that the GIST-layer NAT traversal provides NAT
   traversal for every possible combination of NATs, either on the data
   or the signalling path, in a secure manner.

   Due to the GIST QUERY/RESPONSE discovery mechanism (according to
   which QUERY messages are simply forwarded if the current node does
   not support the required NSLP), two GIST nodes typically identify
   themselves as NSLP peers only if they both implement the same NSLP.
   If one or more NATs that are unaware of this NSLP are between them,
   then the two NSLP peers are not able to discover each other at all.
   This is because, even in the unlikely event that the NAT bindings
   that are necessary for the GIST traffic to traverse the in-between
   NAT(s) exist, the NLI.IA field included in the RESPONSE message sent
   by the downstream peer is invalid (or the IP address is unreachable)
   in the address space of the upstream peer.  In order to overcome this
   limitation, either the two peers need to cope with the in-between
   NAT(s), or, if the NAT(s) are GaNATs, they (the GaNATs) need to apply
   additional processing in order to transparently create and maintain
   consistency between the information in the header of GIST signalling
   messages and the information in the IP header of the data traffic.
   Additionally, if NSLP-aware NATs are on the data path, then these



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   NATs should process NSLP traffic in a way the preserves consistency
   after address translation.  This processing deviates from the
   processing of NSLP-aware non-NAT nodes.  The following sections
   describe how to overcome the limitation of two adjacent NSLP peers
   not being able to execute the NSLP in the presence of in-between
   NAT(s).

   A number of different variations are possible, depending on the level
   of NSIS support by the in-between NAT(s).  The following combinations
   of NATs that are located between two adjacent NSLP peers are
   considered.

   o  all NAT(s) are NSLP-unaware GaNAT(s)

   o  all NAT(s) are NSLP-aware

   The approach taken in this document is to propose separate mechanisms
   for the traversal of each of the above type of NAT.  If NATs that
   belong to multiple types exist on the path between two adjacent NSLP
   peers, the proposed mechanisms should work in combination.  Thus,
   traversal of multiple NATs of different types should not require
   further specification from a functional perspective.  However,
   security issues that arise due to the combination of NAT types may
   have to be considered.

   A GIST-unaware NAT cannot tell data and signalling traffic apart.
   The installation of the NAT binding for the signalling traffic in
   such a NAT occurs typically independently from the installation of
   the NAT binding for the data traffic.  Furthermore, as the NAT cannot
   associate the signalling and the data traffic, it cannot indicate
   that an association exists between the two NAT bindings.  Therefore,
   in the presence of such a NAT, non-NAT GIST nodes that are located on
   either side of the NAT have to cope with the NAT without assistance
   from the NAT.  This would typically require initially discovering the
   NAT and subsequently establishing an association between between the
   MRI in the signalling messages and the translated IP header in the
   data traffic.  Due to the variety of behaviours that a GIST-unaware
   NAT may exhibit, establishing this association is a non-trivial task.
   Therefore, traversal of such (i.e.  GIST-unaware) NATs is considered
   a special case and is outside the scope of this version of this
   document.

   Traversal of GaNAT(s) is comparatively more straightforward.  This is
   because, based on the MRI in a given incoming GIST message, a GaNAT
   can identify the data flow to which the message refers.  It can then
   check its NAT binding cache and determine the translation that is
   (or, if no NAT binding for the flow exists yet, will be) applied to
   the IP header of the data flow.  The GaNAT can then include



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   sufficient information about this translation into the signalling
   message, such that its receiver (i.e. the GIST peer that receives the
   data traffic after network address translation has been applied) can
   map the signalling message to the data flow.

   There exist a variety of ways for a GaNAT to encode the above-
   mentioned information into signalling messages.  In this document the
   following two ways are considered.

   1.  Non-transparent approach: The GaNAT includes an additional "NAT
       Traversal" payload (see section A.3.8 of [1]) into the GIST
       header of the GIST QUERY message.  This "NAT Traversal" payload
       is echoed by the GIST responder on the other side of the NAT.
       The responder (which is assumed to be located on the "other side"
       of the NAT) uses the information in this payload in order to map
       subsequent signalling messages to the data flows they refer to.

   2.  Transparent approach: The GaNAT replaces GIST header fields in a
       way that is consistent with the translation it applies to the
       data traffic, as necessary.  The GaNAT does this for GIST QUERY
       and RESPONSE messages, for D-mode as well as for C-mode messages
       throughout the duration of the signalling session.

   The second approach being "transparent" means that a GaNAT that
   follows this approach remains completely transparent to the GIST
   peers that are located either side of it.  Thus, this approach works
   even if these GIST peers do not support the NAT traversal object for
   GIST (as described in [1]).  Unfortunately though, the transparent
   approach does not work if the signalling traffic is to be
   cryptographically protected between the two GIST peers that are
   located either side of the GaNAT, and the GaNAT is NSLP-unaware.  If,
   however, the GaNAT is NSLP-aware, then cryptographic protection is
   terminated at the GaNAT (i.e. the GaNAT is a GIST peer itself).  In
   this scenario, it is clearly preferable for the GaNAT to follow the
   transparent approach, rather than to include a NAT Traversal object.
   Thus, if a GaNAT acts as a GIST peer for a signalling session, it
   MUST follow the transparent approach, as described in Section 5.3.
   However, due to the fact that the transparent approach does not work
   if signalling is to be cryptographically protected, a GaNAT MUST also
   implement the non-transparent approach (for the case where an NSLP is
   signalled that the GaNAT does not support), unless the GaNAT is going
   to be used only in deployments where cryptographic protection of
   signalling traffic is not a requirement.

   Note that a GaNAT MAY implement both approaches.  If such a GaNAT is
   NSLP-unaware, it can then adopt the desired behaviour, based on
   whether or not cryptographic protection is required for the
   signalling traffic between two GIST peers.  If such protection is



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   required, the GaNAT MUST adopt the mechanisms that follow the non-
   transparent approach; if it is not, it MAY follow the mechanisms
   implementing the transparent approach.  The GaNAT can tell whether or
   not cryptographic protection is required from the stack proposal in
   the GIST QUERY and RESPONSE messages; inclusion of IPsec or TLS
   proposals amounts to cryptographic protection being required.













































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4.  Assumptions

   The discussion in this document is based on the following
   assumptions.

   1.  No IP addresses and port numbers are carried in the payloads of
       an NSLP.  If this is not the case, then the NSLP has to provide
       additional mechanisms for the traversal of (Ga)NATs.  These
       mechanisms must be compatible the mechanisms described in this
       document.  Note that the NATFW NSLP is an exception to this rule
       in that it does not need to be compatible with the mechanisms
       described in this document.  This is because the GIST-layer NAT
       traversal mechanisms described in this document and the NATFW
       NSLP are mutually exclusive (i.e. it is not permissible that a
       given (Ga)NAT applies both GIST-layer NAT traversal and NATFW
       NSLP processing to the messages that belong to the same
       signalling session).

   2.  The path taken by the signalling traffic between those GIST peers
       that have GaNATs in between is such that the responses to packets
       that a GaNAT sends on a given interface arrive on the same
       interface (if such responses are sent at all).

   3.  The path taken by signalling traffic remains fixed between the
       two GIST peers, as far as the in-between GaNATs are concerned.
       That is, we assume that signalling traffic traverses the same
       GaNAT(s) until at least one of the following conditions is met.

       *  The NSIS state that is installed at the two GIST peers
          expires.

       *  The NSIS state that is installed at the two GIST peers is
          refreshed using a GIST QUERY.

       *  A new GIST QUERY/RESPONSE exchange takes place due to other
          reasons, e.g. a detected route change.

       Note that this assumption is not necessarily met by "normal" data
       path coupled signalling.  This is because, under "normal" data
       path coupled signalling, the signalling traffic is "coupled" to
       the data traffic at nodes that decide to act as GIST peers.
       Thus, under "normal" path coupled signalling, it is not an error
       condition (e.g. a reason to trigger a "route change"), for
       example, if the set of on-path nodes, which do not act as GIST
       peers, changes, as long as adjacent GIST peers remain the same.

   4.  The data flow traverses the same set of GaNATs as the signalling
       traffic.  By assumption 3, this set of GaNATs is fixed until the



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       next GIST QUERY/RESPONSE procedure is executed.


                           +-----+
                      +----+GaNAT|-----+
                      |    |  A  |     |
                      |    +-----+     |
        +------+  +------+          +--+---+  +------+
   +--+ | GIST |  |  IP  |          |  IP  |  | GIST | +--+
   |DS+-+peer 1+--+router|          |router+--+peer 2+-+DR|
   +--+ +------+  +---+--+          +--+---+  +------+ +--+
                      |    +-----+     |
                      |    |GaNAT|     |
                      +----+  B  +-----+
                           +-----+

    Figure 1: Network with more than one NAT at an addressing boundary

   Figure 1 illustrates the importance of assumptions (3) and (4).  With
   regard to that figure, suppose that a (D-mode) signalling session has
   been setup between the two adjacent GIST peers 1 and 2 and that both
   signalling and data traffic follows the path GIST peer 1 -> IP router
   -> GaNAT A -> IP router -> GIST peer 2.  Suppose now that, after some
   time, GIST peer 1 decides to set up a C-mode connection with peer 2.
   Suppose moreover that the left IP router decides to forward the
   C-mode signalling traffic on the link towards GaNAT B. Thus,
   signalling traffic now follows the alternative path GIST peer 1 -> IP
   router -> GaNAT B -> IP router -> GIST peer 2.  Note that this change
   in forwarding between the two adjacent GIST peers does not trigger a
   "route change" at the GIST layer because (a) it does not necessarily
   destroy the adjacency of peer 1 and 2 and (b) it does not necessarily
   destroy the coupling of the path taken by signalling traffic to that
   taken by data traffic (at GIST nodes).  Nevertheless, assumptions (3)
   and (4) mandate that this situation does not occur.  However, even if
   such a situation occurs, the mechanisms described in this document
   may still work as state expires after a certain timeout period.

   Assumptions (2), (3) and (4) hold if, at an addressing boundary, only
   one NAT exists.  Due to security and management reasons, this is
   likely to be the case in many settings.











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5.  Transparent NAT traversal for GIST

   This section describes the operation of GaNATs that implement the
   transparent approach listed in Section 3.  An NSLP-aware GaNAT MUST
   follow this approach, as described in Section 5.3.  An NSLP-unaware
   GaNAT MAY follow this approach, as described in Section 5.1 and
   Section 5.2, only if no cryptographic protection of signalling data
   is requested by the two NSLP peers.

   Note that two types of NSLP-unaware GaNAT have to be dealt with,
   namely those that are located at the NSIS initiator (NI-side), and
   those that are located at the NSIS responder (NR-side).  This
   distinction arises due to the fact that NI-side and NR-side GaNATs
   obtain the destination IP address of the downstream GIST peer in
   different ways.

5.1.  NI-side NSLP-unaware GaNATs

   This section describes the "transparent" operation of an NI-side,
   NSLP-unaware GaNAT.

   For every arriving IP packet P, an NSLP-unaware, NI-side GaNAT
   executes the following algorithm.

   1.  If P has a RAO followed by the GIST header with an NSLP ID that
       is not supported, and if P is identified as a GIST QUERY, the
       GaNAT performs the following.

       1.  We denote P by GQ.  The GaNAT looks at the stack proposal in
           GQ.  If it includes a proposal with cryptographic protection,
           the mechanism that is applied is the one described
           Section 6.1.

       2.  The GaNAT remembers GQ along with the interface on which it
           arrived.  We call this interface the "upstream link".

       3.  It searches its table of existing NAT bindings against
           entries that match the GQ.MRI.  A matching entry means that
           the data flow, to which the signalling refers, already
           exists.

           +  If a matching entry is found, the GaNAT looks at which
              link the packets of the data flow are forwarded; we call
              this link the "downstream" link.  Further, the GaNAT
              checks how the headers of the data flow (IP addresses and
              port numbers) are translated according to this NAT
              binding.  We denote the source IP address of translated
              data packets by IPds, and their [Transport layer



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              header].SourcePort by SPDTds.

           +  If no matching entry is found, the GaNAT determines, based
              on its routing table, the link on which packets that match
              GQ.MRI (excluding GQ.MRI.SourceIPAddress) would be
              forwarded.  We call this link the "downstream" link.
              Then, the GaNAT acquires an IP address and source port for
              itself on the downstream link, denoted by IPds and SPDTds
              respectively.  This address and port could be dynamic or
              static, and will be used (among other things) for the
              installation of a NAT binding for the data traffic in the
              future.

       4.  The GaNAT aquires a source port number for the version of the
           GIST QUERY that will be forwarded over the downstream link.
           We denote this port by SPSTds.  (There is no requirement that
           SPSTds must be different from GQ.[UDP Header].SourcePort.)

           Issues: The reason why the GaNAT may also assign a different
           source port number to the signalling traffic, is to enable
           the GaNAT to demultiplex (i.e. forward to the correct
           internal address) the signalling responses that arrive from
           the downstream direction.  Of course, a GaNAT does not need
           to actually change the source port of signalling traffic; it
           can always use SPSTds the same port as in the incoming
           packet.  Such a GaNAT may use the GIST session ID in order to
           demultiplex (i.e. forward to the correct internal address)
           the traffic that arrives from the downstream direction.  It
           is unclear which of the two approaches is preferable.

       5.  It creates a new GIST QUERY packet GQ', as follows.

           1.  GQ' <- GQ

           2.  GQ'.MRI.SourceIPAddress <- IPds

           3.  GQ'.MRI.SourcePortNumber <- SPDTds

           4.  GQ'.[IP header].SourceIPAddress <- IPds

           5.  GQ'.[UDP header].SourcePort <- SPSTds

           6.  GQ'.NLI.IA <- IPds

           7.  GQ'.S <- true






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       6.  It remembers GQ and GQ', the fact that they are associated,
           and the associated upstream and downstream links.  (Note: The
           GaNAT does not have to remember the entire packets; for
           simplicity of exposition, however, we assume it does.  An
           implementation SHOULD discard at this point all information
           that is not used later.)

       7.  It forwards GQ' on the downstream link.

   2.  Otherwise, if P carries an [IP header].DestinationIPAddress that
       belongs to the GaNAT, and if it is identified as a GIST RESPONSE
       in D-mode with an NSLP ID that is not supported, the GaNAT does
       the following (P is denoted by GR).

       1.  It searches for a matching GQ' in its buffer.  A GQ' is said
           to match a GR if they carry the same cookie value.  If none
           is found, GR is discarded.  Otherwise, the GaNAT may also
           perform further consistency checks on a matching GR/GQ' pair,
           such as checking that they contain the same session IDs,
           MRIs, NSLP IDs.  If consistency checks fail, GR is discarded.
           Otherwise, the GaNAT constructs a new GIST RESPONSE GR', as
           follows.

           1.  GR' <- GR

           2.  GR'.MRI <- GQ.MRI, where GQ is the packet associated with
               GQ' (as remembered previously), and GQ' is the packet
               that matches the received GR.

           3.  GR'.[IP header].SourceIPAddress <- IPus, where IPus is an
               IP address that is bound to the upstream link.

           4.  GR'.[IP header].DestinationIPAddress <- GQ.NLI.IA

           5.  GR'.[UDP header].DestinationPort <- GQ.[UDP
               header].SourcePort

           6.  GR'.NLI.IA <- IPus

           7.  GR'.S <- true

           8.  The GaNAT inspects the Stack-Configuration-Data object in
               GR' and the corresponding GQ' in order to check whether
               or not the upstream NSLP peer can select one of multiple
               transport layer protocol/destination port number
               combinations for the establishment of a messaging
               association.  If multiple choices exist, the GaNAT
               invalidates as many transport layer protocol/port number



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               combination proposals from GR' as necessary, until the
               upstream NSLP peer can only initiate the establishment of
               a messaging association with the downstream NSLP peer
               using a single transport layer protocol/destination port
               number combination.  This invalidation is done by setting
               the D-flag in those MA-Protocol-Options fields that carry
               the port number proposals that are to be invalidated.
               Note that, by setting the D-flag in a particular MA-
               Protocol-Option field, the GaNAT may also invalidate the
               associated transport layer protocol and security (e.g.
               TLS) proposal.  The actions of the GaNAT MUST NOT result
               in the strongest, in terms of security, proposal to be
               invalidated.  In the end, the NAT will expect the
               upstream NSLP peer to use a particular combination of
               transport layer protocol and destination port (and
               possibly other details that are associated with the valid
               proposal) for the establishment of the messaging
               association.  We call this combination the "stack
               proposal expected by the NAT" and denote it by ST.  The
               GaNAT remembers this ST, its association with GQ, GQ',
               GR, GR', and the upstream and downstream links.  By doing
               so, the GaNAT is said to "install" the ST.

       2.  It forwards GR' on the upstream link.

       3.  If no NAT binding for the data traffic was found in step
           1.3.2, the GaNAT now installs a NAT binding (for the
           unidirectional data traffic) which says that "a packet K that
           arrives on the upstream link and for which it holds that

           +  K.[IP
              header].DestinationIPAddress=GQ.MRI.DestinationIPAddress,

           +  K.[IP header].Protocol=GQ.MRI.Protocol, and

           +  K.[Transport layer header].PortNumbers=GQ.MRI.PortNumbers

           should be forwarded on the downstream link, with [IP
           header].SourceIPAddress = IPds and [Transport layer
           header].SourcePort=SPDTds".

           Issues: there is a question of whether this NAT binding
           should also enable data traffic in the opposite direction to
           traverse the NAT; in order to be able to demultiplex upstream
           traffic that carries data that belongs to different flows,
           the GaNAT should keep the necessary per-flow state.  From a
           signalling point of view, however, upstream data traffic that
           corresponds (on the application level) to the downstream flow



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           to which this GIST session refers, is a separate flow for
           which, depending on the application, there may or there may
           not exist a signalling session.  If such a signalling session
           exists, then the GaNAT acts as an NR-side GaNAT for this
           session.  Thus, during the processing of this signalling care
           has to be taken not to establish a NAT binding for a flow for
           which a NAT binding already exists.  Moreover, security
           issues may arise when traffic, for which no signalling
           exists, is allowed to traverse a GaNAT.

           Another issue is about refreshing the NAT binding.  A NAT
           binding that was established as a result of GIST signalling
           should remain in place for as long as the associated GIST
           state in the GaNAT remains valid.  If GIST signalling refers
           to a NAT binding that already exists, then the timeout of the
           NAT binding should occur according to the NAT policy, in a
           manner independent from GIST processing.  (If signalling
           persists after the deletion of a NAT binding, then the NAT
           binding may be re-installed and then timed out together with
           GIST state).

   3.  Otherwise, if P.[IP header].DestinationIPAddress belongs to the
       GaNAT, and if P carries the transport protocol and destination
       port number indicated by some stack ST that has previously been
       installed by the GaNAT, and if P has arrived on either the
       upstream or the downstream interface that is associated with ST,
       then P is said to "match" ST.  For such a packet, the GaNAT does
       the following.  If P is expected to contain a GIST header, then
       the GaNAT checks whether or not the bits where the GIST header is
       expected, constitute a valid GIST header.  If they do not, P is
       silently discarded.  If all is in order, the GaNAT constructs an
       outgoing packet P' as follows (the variables used below refer to
       those stored in association with ST).

       1.  P' <- P

       2.  If P has arrived on the upstream link, then

           1.  P'.[IP header].SourceIPAddress <- IPds

           2.  P'.[IP header].DestinationIPAddress <- GR.NLI.IA

           3.  P'.MRI <- GQ'.MRI

           4.  P'.NLI.IA <- IPds

           5.  The GaNAT forwards P' on the downstream link.




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       3.  else (if P has arrived on the downstream link)

           1.  P'.[IP header].SourceIPAddress <- IPus

           2.  P'.[IP header].DestinationIPAddress <- GQ.NLI.IA

           3.  P'.MRI <- GQ.MRI

           4.  P'.NLI.IA <- IPus

           5.  The GaNAT forwards P' on the upstream link.


           Note that the GaNAT can determine the location in a packet
           where a GIST header is expected.  If, for example, the packet
           is a UDP packet, then the GIST header should follow
           immediately after the UDP header.  If the packet is a TCP
           packet, then the GaNAT can determine the location where the
           GIST header should start by counting the number of NSLP
           payload bits that followed the end of the previous GIST
           header.  The start of the next GIST header is expected at the
           position where the previous GIST message, including NSLP
           payload, ends.  The GaNAT can tell where this message ends
           from the LENGTH field inside the previous GIST header.  It
           should be noted here that, in order to correctly count the
           bits, the GaNAT may have to keep track of TCP sequence
           numbers, and thereby be aware of the correct ordering of
           packets.  However, the GaNAT only has to keep buffers that
           are as long as the LENGTH field inside the previous GIST
           header (and possibly up to one MTU size more than that).

           Also note that some TCP packets P may not be expected to
           contain any GIST header (this happens when the NSLP payload
           from a previous packet stretches over several packets).  For
           those packets, the GaNAT only applies the transformation in
           the IP header.  Finally, note that a GIST header may start a
           packet but finish in another.  If such a packet is received,
           the GaNAT MUST buffer that packet, until the packet is
           received where the GIST header completes.  It can then apply
           the required processing and forward both packets.

   4.  Otherwise, if P matches a (data) NAT binding, the GaNAT applies
       normal NAT processing and forwards the packet on the
       corresponding link.

   5.  Otherwise, P is subjected to normal NAT processing.  That is, P
       is either silently discarded or it causes the installation of a
       (data) NAT binding.



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   Brief discussion of the algorithm: The fact that the GaNAT replaces
   the NSLP peers' NLI.IA with its own IP address (in both directions),
   causes the GIST peers to send subsequent signalling messages to the
   GaNAT, in the belief that they talk to their adjacent NSLP peer.  The
   GaNAT transparently forwards the signalling traffic and appropriately
   translates the fields in the GIST header, in a way that is consistent
   with the translation it applies to the data traffic.

   Note that, according to this mechanism, the size of outgoing GIST
   messages is always the same as the size of corresponding incoming
   GIST messages.  Also note that the MRI that the NR sees indicates as
   destination address the IP address of the DR (as expected), but as
   source address it sees indicates the IPds of the GaNAT that is
   closest to the NR.

5.2.  NR-side NSLP-unaware GaNATs

   The case of NR-side GaNATs is more subtle, since, in this setting,
   the DS does not learn the IP address of the DR (which is assumed to
   be on the same side of the GaNATs as the NR) and the NI does not
   learn the address of the NR.  In this setting we assume that each NR-
   side GaNAT that is in between two GIST peers, a priori knows a
   routable IP address of the next downstream GaNAT.  The last GaNAT of
   this chain is assumed to know the IP address of the DR.  In order to
   clarify this assumption, see, for example, Figure 2.  In this figure,
   GaNAT A is assumed to know the IP address of GaNAT B, GaNAT B is
   assumed to know the IP address of GaNAT C, and GaNAT C is assumed to
   know the IP address of the DR.  A given GaNAT that knows such an
   address, in effect anticipates to receive a signalling message from
   the upstream direction that refers to a data flow that terminates in
   a downstream node.  In other words, such a GaNAT may typically have
   already a NAT binding in place for the data traffic.  We call the IP
   address of the next downstream GaNAT (or, if the GaNAT is the last in
   the chain, the address of the DR) the "pending" IP address and denote
   it by IPNext.  The GaNAT may also have a destination port associated
   with IPNext.  If IPNext is derived from an existing data traffic NAT
   binding, then this port is typically the destination port after
   translation from that binding.  This port, if known, is denoted
   PortNext.  How IPNext and PortNext are made known to each GaNAT (e.g.
   how the NAT binding for the data traffic is installed in the GaNAT)
   is outside the scope of this document.










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   +--+  +------+   +-----+   +-----+   +-----+   +------+  +--+  +--+
   +NI+--+ NSLP +---+GaNAT+---+GaNAT+---+GaNAT+---+ NSLP +--+NR+--+DR|
   +--+  |peer 1|   |  A  |   |  B  |   |  C  |   |peer 2|  +--+  +--+
         +------+   +-----+   +-----+   +-----+   +------+

   Figure 2: Network with NR-side GaNATs (the public Internet is assumed
                    to be  between NI and NSLP peer 1)

   For every arriving IP packet P, an NSLP-unaware, NR-side GaNAT
   executes the following algorithm.

   1.  If P has a RAO followed by the GIST header with the NSLP ID
       indicates an unsupported NSLP, and if it is identified as a GIST
       QUERY, the GaNAT does the following.

       1.  We denote P by GQ.  The GaNAT looks at the stack proposal in
           GQ.  If it indicates that cryptographic protection is
           required, the algorithm that is executed is the one described
           in section Section 6 below.

       2.  The GaNAT remembers GQ along with the link on which it
           arrived.  We call this link the "upstream" link.

       3.  The GaNAT determines whether or not this GIST QUERY is
           anticipated, i.e. if a pending IPNext (and possibly PortNext)
           exists that matches this GIST QUERY.  A pending IPNext is
           said to "match" a GIST QUERY, if [this condition is an open
           issue!]  If no pending IPNext is matching, P is discarded (it
           is a question whether or not an error message should be
           sent).  Otherwise, additional checks may be performed (e.g.
           something like a DSInfo object may have to be checked against
           the GQ).  If these checks fail, P is discarded.  Otherwise,
           the GaNAT performs the following.

       4.  It searches its table of existing NAT bindings against
           entries that match the GQ.MRI.  A matching entry means that
           the data flow, to which the signalling refers, already
           exists.

           +  If a matching entry is found, the GaNAT looks at which
              link the packets of the data flow are forwarded; we call
              this link the "downstream" link.  Further, the GaNAT
              checks how the IP and transport layer headers of the data
              flow are translated according to this NAT binding.  Note
              that the [IP header].DestinationIPAddress and [Transport
              layer header].DestinationPort of this NAT binding should
              be equal to IPNext and PortNext respectively.  If they are
              not, this should be handled as an auditive error



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              condition.

           +  If no matching entry is found, the GaNAT determines, based
              on its routing table, the link on which packets that match
              GQ.MRI (excluding GQ.MRI.SourceIPAddress and where
              GQ.MRI.DestinationIPAddress is replaced with IPNext) would
              be forwarded.  We call this link the "downstream" link.

       5.  The GaNAT acquires an IP address for itself on the downstream
           link.  (This address could be dynamic or static.)  Depending
           on its type, the GaNAT may also acquire a UDP source port
           number for the version of the GIST QUERY that will be
           forwarded to the downstream direction.  We denote the
           acquired IP address and source port number by IPds SPSTds
           respectively.  The GaNAT then constructs a new GIST QUERY
           packet GQ', as follows.

           1.  GQ' <- GQ

           2.  GQ'.MRI.DestinationIPAddress <- IPNext.

           3.  GQ'.MRI.DestinationPort <- PortNext.

           4.  GQ'.NLI.IA <- IPds.

           5.  GQ'.[IP header].SourceIPAddress <- IPds.

           6.  GQ'.[IP header].DestinationIPAddress <- IPNext.

           7.  GQ'.[UDP header].SourcePort <- SPSTds.

           8.  GQ'.S <- true

       6.  It remembers GQ, GQ', the fact that they are associated, and
           the associated upstream and downstream links (interfaces).

       7.  It forwards GQ' on the downstream link.

   The remaining steps of the algorithm are analogous to the
   corresponding steps of the algorithm executed by NSLP-unaware, NI-
   side GaNATs, which was described in Section 5.1.

5.3.  NSLP-aware GaNATs

   The difference of NSLP-aware GaNATs and NSLP-unaware GaNATs is that
   the former perform NSLP processing in addition to the processing of
   the NSLP-unaware GaNATs.  Another way to see this is by observing
   that NSLP-aware GaNATs should provide an "MRI translation service"



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   (MRITS) in addition to normal GIST and NSLP processing.  The MRITS
   operates at the GIST layer.  The motivation behind this is to hide
   from the NSLP that signalling messages traverse an addressing
   boundary.  In other words, the purpose of the MRITS is to make the
   NSLP believe that it is operating in a single IP addressing space.
   When and how the MRITS is invoked for a particular packet depends on
   (i) the direction of an incoming message (i.e. downstream or
   upstream) and (ii) the location of the GaNAT (i.e.  NI-side or NR-
   side).  It should also be noted that certain NSLP layer tasks must be
   carried out in consistency with the placement of the MRITS.  This is
   to prevent events triggered by the NSLP to cause installation of
   inconsistent state.  In order to clarify this, consider the scenario
   of the QoS NSLP running in a GaNAT that operates according to the
   mechanisms described in this section.  Since the GaNAT only presents
   a single addressing space to the NSLP (say, the internal addressing
   space), the packet classifier of the GaNAT's QoS provisioning
   subsystem should classify data packets based on internal addresses
   only (i.e. it should first translate packets that carry external
   addresses and then classify them).  Whether the MRITS presents
   internal-only or external-only addresses to the NSLP is not
   significant, as long as NSLP layer operations are carried out
   consistently.  In the remainder of this section we present the case
   where internal addresses are presented to the NSLP.

   The MRITS is obviously invoked only on GIST packets that carry an
   NSLP identifier that corresponds to an NSLP that the GaNAT
   implements.  For non-GIST packets, normal NAT behaviour applies.
   Although the MRITS is part of GIST processing, in order to clarify
   the exposition, we view it as a somewhat separate processing step
   (i.e. like a subroutine) that is executed in addition to GIST, as
   this is specified in [1].  For NI-side, NSLP-aware GaNATs, it holds
   that

   o  for a GIST/NSLP packet that is to be forwarded on the downstream
      link of an NI-side GaNAT, the MRITS is invoked after the packet
      has been processed by the NSLP and before it is given to GIST, and

   o  for a GIST/NSLP packet that is received on the downstream link,
      the MRITS is invoked after GIST processing and before the packet
      is given to the NSLP.

   The converse holds for NR-side NSLP-aware GaNATs.  In particular,

   o  for a GIST/NSLP packet that is to be forwarded on the upstream
      link of an NI-side GaNAT, the MRITS is invoked after the packet
      has been processed by the NSLP and before it is given to GIST, and





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   o  for a GIST/NSLP packet that is received on the upstream link, the
      MRITS is invoked after GIST processing and before NSLP processing.

   Figure 3 illustrates this idea.


        +----------------+               +----------------+
        |  +------+      |               |     +------+   |
        |  | NSLP |      |               |     | NSLP |   |
        |  +-+---++      |               |     +-+--+-+   |
        |    |   |       |               |       |  |     |
        |    | +-+---+   |               |  +----++ |     |
        |    | |MRITS|   |               |  |MRITS| |     |
        |    | +---+-+   |               |  ++----+ |     |
        |    |     |     |               |   |      |     |
        |  +-+-----+-+   |               |  ++------+-+   |
        |  |  GIST  |   |               |  |   GIST |   |
    u/s |  +-+-----+-+   | d/s       u/s |  ++------+-+   | d/s
   -----+----+     +-----+-----     -----+---+      +-----+-----
   link +----------------+ link     link +----------------+ link
              NI-side                          NR-side
             NSLP-aware                       NSLP-aware
               GaNAT                            GaNAT


            Figure 3: Operation of the MRI Translation Service

   The reason for this construction is to give the NSLP the impression
   that it works only with flows that originate and terminate in the
   internal address space.  We now describe the operation of the MRITS
   and GIST in NSLP-aware GaNATs.  An NI-side NSLP-aware GaNAT operates
   according to the following rules.

   1.  When the NSLP asks for a message to be sent towards the
       downstream GIST peer, the MRITS does the following (IPds and
       SPDTds are obtained similarly to the case of an NSLP-unaware
       GaNAT).

       1.  MRI.SourceIPAddress <- IPds

       2.  MRI.SourcePort <- SPDTds

   2.  Additionally, GIST performs the following on the resulting packet
       before it is forwarded on the downstream link (SPSTds is obtained
       similarly to the case of an NSLP-unaware GaNAT).

       1.  [IP header].SourceIPAddress <- IPds




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       2.  [UDP/TCP header].SourcePort <- SPSTds

       3.  NLI.IA <- IPds

       4.  S <- true

   3.  If a message is received on the downstream link, the MRITS does
       the following before the NSLP is invoked.

       1.  MRI.SourceIPAddress <- IPflow

       2.  MRI.SourcePort <- SPDTus, where IPflow is the IP address of
           the DS (as seen by the GaNAT) and SPDTus is the destination
           port number used in the original MRI.

   4.  If, after NSLP processing, a message is to be forwarded on the
       upstream link, GIST performs the following processing (note that
       no MRITS processing takes place in this case).

       1.  [IP header].SourceIPAddress <- IPus

       2.  [IP header].DestinationIPAddress <- IPpeer

       3.  NLI.IA <- IPus

       4.  S <- true, where IPus is the GaNATs IP address for the
           upstream link, IPpeer is the IP address of the NI (or the
           next GaNAT in the upstream direction), and IPflow is the IP
           address of the DS (as seen by the GaNAT).  The GaNAT is
           assumed to determine the correct IPus and IPpeer from
           previous communications and in cooperation with GIST.
           [Issue: how exactly should IPus, IPpeer and IPflow be
           resolved; i.e. what exactly should the GaNAT remember?]

   An NR-side NSLP-aware GaNAT operates according to the following
   rules.

   1.  If the packet is received on the upstream link, the MRITS does
       the following, before the NSLP is notified.

       1.  P.MRI.SourceIPAddress <- IPds

       2.  P.MRI.DestinationIPAddress <- IPNext, where IPds is the
           GaNAT's IP address for the downstream link and IPNext is the
           address of the DR.  IPNext is obtained in a way similar to
           the case of an NSLP-unaware GaNAT.





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   2.  If, after NSLP processing, a message is to be forwarded on the
       downstream link, GIST performs the following processing (note
       that no MRITS processing takes place in this case).

       1.  [IP header].SourceIPAddress <- IPds

       2.  [IP header].DestinationIPAddress <- IPNext

       3.  NLI.IA <- IPds

       4.  S <- true, where IPds is the GaNATs IP address for the
           downstream link, IPNext is the IP address of the DR (or the
           next GaNAT in the downstream direction).  The GaNAT is
           assumed to determine the correct IPNext in a way similar to
           the case of an NSLP-unaware GaNAT.

   3.  When the NSLP asks for a message to be sent towards the upstream
       peer, the MRITS does the following.

       1.  MRI.SourceIPAddress <- IPflow

       2.  MRI.Destination_IP_Address <- IPus

   4.  Additionally, GIST performs the following on the resulting packet
       before it is forwarded on the downstream link.

       1.  [IP header].SourceIPAddress <- IPus

       2.  [IP header].DestinationIPAddress <- IPpeer

       3.  NLI.IA <- IPus

       4.  S <- true, where IPus is the GaNATs IP address for the
           upstream link, IPpeer is the IP address of the NI (or the
           next GaNAT in the upstream direction), and IPflow is the IP
           address of the DS.  The GaNAT is assumed to determine the
           correct IPus and IPpeer fields from previous communications
           and in cooperation with GIST. [question: how exactly should
           IPus and IPpeer be resolved; i.e. what exactly should the
           GaNAT remember]?

5.4.  Combination of NSLP-aware and NSLP-unaware GaNATs

   In the absence of an adversary, a combination of NSLP-aware and NSLP-
   unaware GaNATs should work without further specification.  However,
   in the presence of an adversary, additional security issues may arise
   from the combination.  These issues may introduce opportunities for
   attack that do not exist in setting where the on-path GaNATs are



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   either all NSLP-aware or all NSLP-unaware.


















































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6.  Non-transparent NAT traversal for GIST

   This section discusses the "non-transparent" operation for GaNAT
   traversal at the GIST layer, i.e. the first approach listed in
   Section 3.  For this approach the behaviour of both the GaNAT and the
   GIST peers is defined.  As with the transparent approach, the case of
   the in-between GaNAT(s) being located at the NI-side is different
   from that of NR-side GaNATs.  Note that the mechanisms in this
   section apply only to NSLP-unware GaNATs.

   The GaNAT informs the NSLP peers about its presence during the GIST
   discovery process.  This information enables the NSLP peers to map
   the translated data flow to the signalling messages, and to
   consistently translate the MRI, so that the NSLP only "sees" the
   correct MRI.  Cryptographic protection of signalling messages can be
   supported with this approach because the GaNAT only modifies the GIST
   QUERY and RESPONSE messages, which are never cryptographically
   protected in their entirety.

   In this approach, the GaNAT embeds a "NAT Traversal Object" (NTO)
   payload type into the GIST QUERY.  The NTO encodes the aforementioned
   information and is an optional payload in the GIST header of a GIST
   QUERY.  It is added, and processed, by the GaNAT(s) through which the
   QUERY traverses.  The information in the NTO enables the two NSLP
   peers to locally translate the MRI in the same way as if it were
   consistently and transparently translated by the in-between GaNAT(s).
   Note that there may be more than one GaNAT between the two NSLP
   peers.  The format of the NTO follows the format of the object in the
   GIST common header.  In particular, the NTO is preceded by a TLV
   common header, as defined in [1].  The A and B flags are both set to
   0 in this header, indicating that support for the NTO is mandatory.
   The type value is TBD.  The NTO is defined as in section A.3.8 of
   [1].

6.1.  NI-side NSLP-unaware GaNATs

   For every arriving IP packet P, an NSLP-unaware, NI-side GaNAT
   executes an algorithm that is equivalent to the following.

   1.  If P has a RAO followed by the GIST header with an NSLP ID that
       is not supported, and if it is identified as a GIST QUERY, the
       GaNAT does the following.

       1.  We denote P by GQ.  The GaNAT looks at the stack proposal in
           GQ.  If it does not include any proposal with cryptographic
           protection, the GaNAT MAY choose to follow the approach
           described in Section 5.1 above.




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       2.  The GaNAT remembers GQ along with the link on which it
           arrived.  We call this link the "upstream" link.

       3.  The GaNAT searches its table of existing NAT bindings against
           entries that match the GQ.MRI.  A matching entry means that
           the data flow, to which the signalling refers, already
           exists.

           +  If a matching entry is found, the GaNAT looks at which
              link the packets of the data flow are forwarded; we call
              this link the "downstream" link.  Further, the GaNAT
              checks how the headers of the data flow (IP addresses and
              port numbers) are translated according to this NAT
              binding.  We denote the source IP address of translated
              data packets by IPds, and their [Transport layer
              header].SourcePort by SPDTds.

           +  If no matching entry is found, the GaNAT determines, based
              on its routing table, the link on which packets that match
              GQ.MRI (excluding GQ.MRI.SourceIPAddress) would be
              forwarded.  We call this link the "downstream" link.
              Then, the GaNAT acquires an IP address and source port for
              itself on the downstream link, denoted by IPds and SPDTds
              respectively.  This address and port could be dynamic or
              static, and will be used (among other things) for the
              installation of a NAT binding for the data traffic in the
              future.

       4.  The GaNAT aquires a source port number for the version of the
           GIST QUERY that will be forwarded over the downstream link.
           We denote this port by SPSTds.  (There is no requirement that
           SPSTds must be different from GQ.[UDP Header].SourcePort.)

       5.  It creates a new GIST QUERY packet GQ', as follows.

           1.   GQ' <- GQ

           2.   GQ'.MRI.SourceIPAddress <- IPds

           3.   GQ'.MRI.SourcePortNumber <- SPDTds

           4.   GQ'.NLI.IA.<- IPds.

           5.   GQ'.[IP header].SourceIPAddress <- IPds.

           6.   GQ'.[UDP header].SourcePort <- SPSTds.





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           7.   GQ'.S <- true.

           8.   It checks whether or not an NTO was included in GQ.

                -  If none was included, it creates a new NTO as follows
                   and adds it to GQ'.  Note that the MRI field of the
                   NTO is taken from GQ.

                   o  NTO.[NAT Count] <- 1.

                   o  NTO.MRI <- GQ.MRI.

                   o  NTO.[List of translated objects] <- [type of NLI]

                   o  NTO.opaque information replaced by NAT 1 <-
                      GQ.NLI.IA, GQ.[UDP header].SourcePort, LinkID,
                      where LinkID represents the upstream link.

                -  If one was included, it replaces certain fields and
                   appends new fields into the NTO, as follows, and adds
                   the resulting object to GQ'.  Note that the MRI field
                   of the NTO is not modified.

                   o  NTO.[NAT Count] <- i, where i is the current [NAT
                      count] value increased by one.

                   o  NTO.[List of translated objects] <- [type of NLI]

                   o  NTO.opaque information replaced by NAT i <-
                      GQ.NLI.IA, GQ.[UDP header].SourcePort, LinkID,
                      where LinkID represents the upstream link.

           9.   It remembers GQ, GQ', the fact that they are associated,
                and the associated upstream and downstream links.

           10.  It forwards GQ' on the downstream link.

   2.  Otherwise, if P carries an [IP header].DestinationIPAddress that
       belongs to the GaNAT, and if it is identified as a GIST RESPONSE
       with an NSLP ID that is not supported, the GaNAT does the
       following (P is denoted by GR).

       1.  If P does not contain an NTO, the GaNAT discards it without
           further processing.  Otherwise, it searches for a matching
           GQ' in its buffer.  A GQ' is said to be matching if it
           carries the same cookie value.  If none is found, GR is
           discarded.  Otherwise, the GaNAT should also make sure that
           the session ID in GR is the same as in GQ', that the NSLP IDs



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           match, and that GR arrived on the downstream link.  If these
           consistency checks fail, GR should be discarded.  Otherwise,
           the GaNAT constructs a new GIST RESPONSE GR', as follows
           (note that no changes are made to the MRI).

           1.  GR' <- GR

           2.  The GaNAT selects the information that it encoded in the
               [opaque information replaced by NAT i] field of the
               embedded NTO, denoted by IPAddressToSend,
               PortAddressToSend and LinkID, where i is the current
               value of [NAT Count] as indicated in the NTO.

           3.  GR'.[IP header].DestinationIPAddress <- IPAddressToSend.

           4.  GR'.[UDP header].DestinationPort=PortAddressToSend.

           5.  GR'.NTO.[NAT Count] <- reduce by one.

           6.  GR'.S <- true.

       2.  The GaNAT inspects the Stack-Configuration-Data object in GR
           and the corresponding GQ' in order to check whether or not
           the upstream NSLP peer can select one of multiple transport
           layer protocol/destination port number combinations for the
           establishment of a messaging association.  If multiple
           choices exist, the GaNAT invalidates as many transport layer
           protocol/port number combination proposals from GR' as
           necessary, until the upstream NSLP peer can only initiate the
           establishment of a messaging association with the downstream
           NSLP peer using a single transport layer protocol/destination
           port number combination.  This invalidation is done by
           setting the D-flag in those MA-Protocol-Options fields that
           carry the port number proposals that are to be invalidated.
           Note that, by setting the D-flag in a particular MA-Protocol-
           Option field, the GaNAT may also invalidate the associated
           transport layer and security protocol (e.g.  TCP/TLS)
           proposal.  The actions of the GaNAT MUST NOT result in the
           strongest, in terms of security, proposal to be invalidated.
           In the end, the NAT will expect the upstream NSLP peer to use
           a particular combination of transport layer protocol and
           destination port (and possibly other details that are
           associated with the valid proposal) for the establishment of
           the messaging association.  We call this combination the
           "stack proposal expected by the NAT" and denote it by ST.
           The GaNAT remembers this ST, its association with GQ, GQ',
           GR, GR', and the upstream and downstream links.  By doing so,
           the GaNAT is said to "install" ST.



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       3.  It forwards GR' on the link identified by LinkID.

       4.  The GaNAT now installs a NAT binding for the signalling
           traffic that is exchanged over a messaging association which
           says that "a packet K that arrives on the upstream link and
           for which it holds that

           +  K.[IP header].DestinationIPAddress=GR.NLI.IA,,

           +  K.[IP header].Protocol=ST.Protocol, and

           +  K.[Transport layer
              header].DestinationPort=ST.DestinationPort

           should be forwarded on the downstream link, with [IP
           header].SourceIPAddress = IPds and [UDP/TCP
           header].DestinationPort=SIGPort, where SIGPort is a port that
           the GaNAT allocates for use as a source port for signalling
           traffic.

       5.  The GaNAT now installs a NAT binding for the UDP-encapsulated
           signalling traffic which says that "a packet M that arrives
           on the upstream link and for which it holds that

           +  M.[IP header].DestinationIPAddress=GR.NLI.IA,

           +  M.[IP header].Protocol=UDP, and

           +  M.[UDP header].DestinationPort=GIST well-known port

           should be forwarded on the downstream link, with [IP
           header].SourceIPAddress = IPds.  Note that this is a special
           type of NAT binding, in that the source port in M may vary
           from one incoming message to another.  This is why each
           packet M may be mapped by the GaNAT to a different source
           port.  Translation in the upstream direction must be applied
           consistently, and timeouts must also be selected
           appropriately.  That is, the overall binding must be timed
           out together with the GIST state that is associated with this
           session.  However, each incoming packet M that matches this
           binding causes the installation of a "sub"-binding (in the
           sense that a new port mapping may occur) that will typically
           time out faster.

       6.  If no NAT binding for the data traffic was found in step
           1.3.2, the GaNAT now installs a NAT binding (for the
           unidirectional data traffic) which says that "a packet L that
           arrives on the upstream link and for which it holds that



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           +  L.[IP
              header].DestinationIPAddress=GQ.MRI.DestinationIPAddress,

           +  L.[IP header].Protocol=GQ.MRI.Protocol, and

           +  L.[Transport layer header].PortNumbers=GQ.MRI.PortNumbers

           should be forwarded on the downstream link, with [IP
           header].SourceIPAddress = IPds and [UDP/TCP
           header].SourcePort=SPDTds.

           Issues: there is a question of whether this NAT binding
           should also enable data traffic in the opposite direction to
           traverse the NAT; in order to be able to demultiplex upstream
           traffic that carries data that belongs to different flows,
           the GaNAT should keep the necessary per-flow state.  From a
           signalling point of view, however, upstream data traffic that
           corresponds (on the application level) to the downstream flow
           to which this GIST session refers, is a separate flow for
           which, dependent on the application, there may or there may
           not exist a signalling session.  If such a signalling session
           exists, then the GaNAT acts as an NR-side GaNAT for this
           session.  Thus, during the processing of this signalling care
           has to be taken not to establish a NAT binding for a flow for
           which a NAT binding already exists.  Finally, security issues
           arise when traffic, for which no signalling exists, is
           allowed to traverse a GaNAT.

   3.  Otherwise, if P matches an existing NAT binding, normal NAT
       processing is applied.

   4.  Otherwise, P is subjected to normal NAT processing.  That is, P
       is either silently discarded or it causes the installation of a
       (data) NAT binding.

6.2.  NR-side NSLP-unaware GaNATs

   As is the case with NR-side NSLP-unaware GaNATs that follow the
   "transparent" approach, an NR-side NSLP-unaware GaNAT that follows
   the "non-transparent" approach must know a "pending" IP address and
   optionally destination port number, as described in Section 5.2.
   This IP address and destination port number are denoted by IPNext and
   PortNext respectively.  How they are made known to the GaNAT is
   outside the scope of this document.  Note, however, that a typical
   scenario would be that the GaNAT has an existing NAT binding in place
   from where this information can be derived.

   For every incoming IP packet P, an NSLP-unaware, NR-side GaNAT



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   executes the following algorithm.

   1.  If P carries an [IP header].DestinationIPAddress that belongs to
       the GaNAT, if it has a RAO followed by the GIST header with an
       unsupported NSLPID, and if it is identified as a GIST QUERY, the
       GaNAT does the following.

       1.  We denote P by GQ.  The GaNAT looks at the stack proposal in
           GQ.  If it does not include any proposal with cryptographic
           protection, the GaNAT MAY choose to follow the "transparent"
           approach as described in Section 5.2 above.

       2.  If GQ.[IP header].DestinationIPAddress, denoted by IPus in
           the sequel, is not bound to the link on which GQ arrived, the
           GaNAT silently discards the packet.  Otherwise, it remembers
           GQ along with the link on which it arrived.  We call this
           link the "upstream" link.

       3.  The GaNAT determines whether or not this GIST QUERY is
           anticipated, i.e. if a pending IPNext and PortNext exists.
           One way of determining whether or not a pending IPNext and
           PortNext exists is checking whether or not a NAT binding for
           the data traffic, as this is defined by the MRI in the GIST
           QUERY, exists in the NAT binding cache.  If one exists, then
           IPNext and PortNext is the address and destination port
           number on which this traffic is forwarded.  If no pending
           IPNext is found, then GQ is discarded (it is a question
           whether or not an error message should be sent).  Otherwise,
           additional checks may be performed (e.g. a DSInfo object may
           have to be checked against the GQ).  If these checks fail, GQ
           is discarded.  Otherwise, the GaNAT performs the following.

       4.  It searches its table of existing NAT bindings against
           entries that match GQ.MRI.  A matching entry means that the
           data flow, to which the signalling refers, already exists.

           +  If a matching entry is found, the GaNAT looks at which
              link the packets of the data flow are forwarded; we call
              this link the "downstream" link.  Further, the GaNAT
              checks how the headers of the data flow (IP addresses,
              port numbers) are translated according to this NAT
              binding.  Note that the [IP header].DestinationIPAddress
              and DestinationPort in this NAT binding should be equal to
              IPNext and PortNext respectively.  If they are not, this
              should be handled as an auditive error condition.  (This
              check is done as a consistency check.)





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           +  If no matching entry is found, the GaNAT determines, based
              on its routing table, the link on which packets that match
              GQ.MRI (where GQ.MRI.DestinationIPAddress is replaced with
              IPNext) would be forwarded.  We call this link the
              "downstream" link.

       5.  It creates a new GIST QUERY packet GQ', as follows.

           1.  GQ' <- GQ

           2.  GQ'.MRI.DestinationIPAddress <- IPnext

           3.  GQ'.MRI.DestinationPortNumber <- PortNext

           4.  GQ'.[IP header].DestinationIPAddress <- IPnext

           5.  GQ'.[UDP header].DestinationPort <- GIST well-known port
               (TBD)

           6.  It checks whether or not an NTO was included in GQ.

               -  If none was included, it creates a new NTO as follows
                  and adds it to GQ'.  Note that the MRI field of the
                  NTO is taken from GQ.

                  o  NTO.[NAT Count] <- 1.

                  o  NTO.MRI <- GQ.MRI.

                  o  NTO.opaque information for NAT 1 <- LinkID of
                     upstream link.

               -  If one was included, it replaces certain fields and
                  appends new fields into the NTO, as follows, and adds
                  the resulting object to GQ'.  Note that the MRI field
                  of the NTO is not modified.

                  o  NTO.[NAT Count] <- i, where i is the current [NAT
                     count] value increased by one.

                  o  NTO.opaque information replaced by NAT i <- LinkID
                     of upstream link.

           7.  It remembers GQ, GQ', the fact that they are associated,
               and the associated upstream and downstream links.

           8.  It forwards GQ' on the downstream link.




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   2.  Otherwise, if P is identified as a GIST RESPONSE packet with an
       NSLP ID that is not supported, the GaNAT does the following (P is
       denoted by GR).

       1.  It searches for a matching GQ' in its buffer.  A GQ' is said
           to be matching if it carries the same cookie value.  If none
           is found, GR is discarded.  Otherwise, the GaNAT should also
           make sure that the session ID in GR is the same as in GQ',
           that the NSLP IDs match, and that GR arrived on the
           downstream link.  If these consistency checks fail, GR should
           be discarded.  Otherwise, the GaNAT constructs a new GIST
           RESPONSE GR', as follows.

       2.  If P does not contain an NTO, the GaNAT discards it without
           further processing.  Otherwise, the GaNAT constructs a new
           GIST RESPONSE GR', as follows (note that no changes are made
           to the MRI).

           1.  GR' <- GR.

           2.  The GaNAT selects the information that it encoded in the
               [opaque information replaced by NAT i] field of the
               embedded NTO, denoted by LinkID, where i is the current
               value of [NAT Count] as indicated in the NTO.

           3.  GR'.NLI.IA <- IPus

           4.  GR'.NTO.[List of translated objects by NAT i] <- [type of
               NLI], where i is the current value of [NAT Count] as
               indicated in the NTO.

           5.  GR'.NTO.[NAT Count] <- reduce by one.

           6.  GR'.[IP header].SourceIPAddress <- IPus (this is the IP
               address that is bound to the link identified by LinkID
               and must be equal to GQ.[IP header].DestinationIPAddress,
               where GQ is the GIST QUERY associated with GQ').

           7.  GR'.[UDP header].DestinationPort <- GQ.[UDP
               header].SourcePort, where GQ is the GIST QUERY associated
               with GQ'.

           8.  GR'.S <- true.

       3.  The GaNAT inspects the Stack-Configuration-Data object in GR
           and the corresponding GQ' in order to check whether or not
           the upstream NSLP peer can select one of multiple transport
           layer protocol/destination port number combinations for the



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           establishment of a messaging association.  If multiple
           choices exist, the GaNAT invalidates as many transport layer
           protocol/port number combination proposals from GR' as
           necessary, until the upstream NSLP peer can only initiate the
           establishment of a messaging association with the downstream
           NSLP peer using a single transport layer protocol/destination
           port number combination.  This invalidation is done by
           setting the D-flag in those MA-Protocol-Options fields that
           carry the port number proposals that are to be invalidated.
           Note that, by setting the D-flag in a particular MA-Protocol-
           Option field, the GaNAT may also invalidate the associated
           transport layer and security protocol (e.g.  TCP/TLS)
           proposal.  The actions of the GaNAT MUST NOT result in the
           strongest, in terms of security, proposal to be invalidated.
           In the end, the NAT will expect the upstream NSLP peer to use
           a particular combination of transport layer protocol and
           destination port (and possibly other details that are
           associated with the valid proposal) for the establishment of
           the messaging association.  We call this combination the
           "stack proposal expected by the NAT" and denote it by ST.
           The GaNAT remembers this ST, its association with GQ, GQ',
           GR, GR', and the upstream and downstream links.  By doing so,
           the GaNAT is said to "install" ST.  If ST.DestinationPort is
           already used by the GaNAT as a destination port in order to
           demultiplex an existing flow, the GaNAT reserves a
           destination port SIGPORT and modifies the valid port proposal
           in GR' such that SIGPORT will be used by the upstream GIST
           peer.  Otherwise it sets SIGPORT=ST.DestinationPort.

       4.  It forwards GR' on the link identified by LinkID (i.e. the
           upstream link).

       5.  The GaNAT now installs a NAT binding for the signalling
           traffic that is exchanged over a messaging association which
           says that "a packet K that arrives on the upstream link and
           for which it holds that

           +  K.[IP header].DestinationIPAddress=IPus (which is equal to
              GQ.MRI.DestinationIPAddress and GQ.[IP
              header].DestinationIPAddress),

           +  K.[IP header].Protocol=ST.Protocol, and

           +  K.[Transport layer header].DestinationPort=SIGPORT

           should be forwarded on the downstream link, with [IP
           header].DestinationIPAddress = GR.NLI.IA and [Transport layer
           header].DestinationPort=ST.DestinationPort.



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       6.  The GaNAT now installs a NAT binding for the UDP-encapsulated
           signalling traffic which says that "a packet M that arrives
           on the upstream link and for which it holds that

           +  M.[IP header].DestinationIPAddress=IPus,

           +  M.[IP header].Protocol=UDP, and

           +  M.[UDP header].DestinationPort=GIST well-known port

           should be forwarded on the downstream link, with [IP
           header].SourceIPAddress = GR.NLI.IA".  Note that this is a
           special type of NAT binding, in that the source port in M may
           vary from one incoming message to another.  This is why each
           packet M may be mapped by the GaNAT to a different source
           port.  Translation in the upstream direction must be applied
           consistently, and timeouts must also be selected
           appropriately.  That is, the overall binding must be timed
           out together with the GIST state that is associated with this
           session.  However, each incoming packet M that matches this
           binding causes the installation of a "sub"-binding (in the
           sense that a new port mapping may occur) that will typically
           time out faster.

       7.  If no NAT binding for the data traffic was found in step
           1.3.2, the GaNAT now installs a NAT binding (for the
           unidirectional data traffic) which says that "a packet L that
           arrives on the upstream link and for which it holds that

           +  L.[IP header].DestinationIPAddress=IPus (which is equal to
              GQ.MRI.DestinationIPAddress and GQ.[IP
              header].DestinationIPAddress),

           +  L.[IP header].Protocol=GQ.MRI.Protocol, and

           +  L.[Transport layer header].PortNumbers=GQ.MRI.PortNumbers

           should be forwarded on the downstream link, with [IP
           header].DestinationIPAddress = IPNext and [Transport layer
           header].DestinationPort=PortNext.

           Note: If the GaNAT also allows data traffic to traverse in
           the other direction (i.e. in the upstream direction), then
           the IP packets of this data traffic MUST have
           SourceIPAddress=IPus, SourcePort=GQ.MRI.DestinationPort,
           DestinationPort=GQ.MRI.SourcePort, and must be forwarded on
           the upstream link.  (This applies anyway for GaNATs with only
           two links and where each link is bound to a single IP



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           address.  However, for other types of GaNAT care has to be
           taken that this restriction is enforced.)

           Issues: there is a question of whether this NAT binding
           should also enable data traffic in the opposite direction to
           traverse the NAT; in order to be able to demultiplex upstream
           traffic that carries data that belongs to different flows,
           the GaNAT should keep the necessary per-flow state.  From a
           signalling point of view, however, upstream data traffic that
           corresponds (on the application level) to the downstream flow
           to which this GIST session refers, is a separate flow for
           which, dependent on the application, there may or there may
           not exist a signalling session.  If such a signalling session
           exists, then the GaNAT acts as an NR-side GaNAT for this
           session.  Thus, during the processing of this signalling care
           has to be taken not to establish a NAT binding for a flow for
           which a NAT binding already exists.  Finally, security issues
           arise when traffic, for which no signalling exists, is
           allowed to traverse a GaNAT.

   3.  Otherwise, if P matches an existing NAT binding, normal NAT
       processing is applied.

   4.  Otherwise, P is subjected to normal NAT processing.  That is, P
       is either silently discarded or it causes the installation of a
       (data) NAT binding.

   The remaining steps of the algorithm are analogous to the algorithm
   of NSLP-unaware, NR-side GaNATs, which was described in the previous
   section.

6.3.  GIST peer processing

   In the presence of GaNATs on the signalling path between two NSLP
   peers, and if the GaNATs follow the "non-transparent" approach (which
   they have to follow in the context of cryptographically protected
   signalling), the consistent translation of the GIST header fields
   must be carried out by the NSLP peers.  The GIST processing that
   performs this task, is described next.  Note that this processing is
   in addition to the processing described in [1].  Also note that the
   processing described in this section applies only to non-NAT nodes.

   A GIST peer that receives a GIST QUERY that carries an NSLP ID for a
   supported NSLP and an NTO, constructs a GIST RESPONSE according to
   [1].  This response is sent to the public address of the last in-
   between GaNAT.  This address appeared as NLI.NI in the GIST QUERY
   (and also as the source address in the IP header).




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   If local policy allows the installation of state without the
   reception of a GIST CONFIRM message, then the responder stores the
   NTO carried with the QUERY together with the routing state
   information about the querying GIST peer.  In particular, the MRI
   field of the NTO must be saved in order for the peer to be able to
   map subsequently received signalling messages to this signalling
   session.

   Note that it is not sufficient for the NSLP to exclusively rely on
   the NTO.MRI for this purpose.  In order to see this, consider two
   private addressing domains, A and B, each with a GaNAT at its border,
   and a node N in the public internet.  In domain A, node N1 has a
   communication session with N, and in domain B, node N2 also has a
   communication session with N. Suppose that the (private) IP addresses
   of N1 and N2 are equal (e.g. 192.168.0.3), and that they both
   communicate with N using the same source and destination ports.  If
   they both have an NSIS signalling session for this data traffic, the
   NTO.MRI field in the GIST QUERY of their respective signalling
   sessions are identical.  If these signalling sessions meet at an NSLP
   node that is located "after" the GaNATs, then this NSLP node sees the
   same MRI in signalling messages that are received over a messaging
   association.  In this case, the node must use other information in
   the signalling messages (e.g. session ID, source IP address) in order
   to map subsequently received signalling messages to existing
   sessions.

   If local policy demands that no session-specific state is installed
   before the reception of a GIST CONFIRM message, then the responder
   must encode the information in NTO.MRI and NLI.IA from the GIST QUERY
   (and possibly other values such as NSLP ID and an identifier of the
   link on which the GIST QUERY arrived) in the responder cookie.  Since
   this cookie is echoed in the GIST CONFIRM message, the responder can
   then delay the installation of the relevant state until it receives
   the GIST CONFIRM.  The construction of the responder cookie is
   implementation-specific, in the sense that it does not raise
   interoperability issues.  Nevertheless, the cookie must be generated
   in a way that meets the requirements listed in section 8.5 of [1],
   and in a way that does not introduce additional attacks against the
   system.

   Two responder cookie construction mechanisms are described in the
   sequel.  These methods are in addition to those described in section
   8.5 of [1], and meet the requirements listed in that section.
   Additionally, they enable the responder to authenticate the contents
   of the cookie, i.e. to ensure that the cookie was not tampered with
   while in transit.  This feature is not provided by the cookie
   construction mechanisms described in [1].




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   Responder cookie generation mechanism 1: Responder cookie = (gennum
   || cookie-left || cookie-right), where || denotes concatenation,
   cookie-left is computed as ENC (Q-Node NLI, MRI, NSLPID, reception
   interface, [timestamp]), and cookie-right is computed as MAC (cookie-
   left).  ENC denotes a semantically secure symmetric encryption
   scheme, and MAC denotes an unforgeable message authentication code
   scheme.  The responder must use a key with ENC that has been selected
   independently from the one used with MAC.  Whenever these keys are
   refreshed, they MUST be refreshed together.  Gennum is the generation
   number of the ENC and MAC keys.  The timestamp is an optional field.
   Policy dictates whether or not it is included in the construction of
   the cookie.  For example, responders that have a fast enough key
   rollover may omit the timestamp.  Example algorithms for ENC and MAC
   are AES-128 in CBC mode [3], and HMAC-SHA1 [4].

   Responder cookie generation mechanism 2: Responder cookie = (Gennum
   || AUTHENC (Q-Node NLI, MRI, NSLPID, reception interface,
   [timestamp])) AUTHENC denotes a symmetric authenticated encryption
   scheme.  Gennum is the generation number of the key used with
   AUTHENC.  The timestamp is an optional element for the same reason as
   above.  Example AUTHENC algorithms include the one specified in
   RFC3610.

   The version of the MRI that the NSLP peers pass to the NSLP is the
   one in the header of the GIST QUERY (not the one in the NTO, if one
   is present).  Whether or not this is a translated MRI depends on the
   location of the peer with respect to the in-between GaNAT(s).  Note
   that the same MRI is used by the responder in signalling messages
   that are sent towards the downstream direction.






















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7.  Security Considerations

   The mechanisms proposed in this document give rise to a number of
   threats that must be considered.  In the following, some of these
   threats is mentioned.

7.1.  Service Denial Attacks

   As described above, NSLP-unaware GaNATs create some state whenever
   they receive a GIST QUERY message.  This state is necessary in order
   for the GaNAT to be able to map a GIST RESPONSE that arrives from the
   downstream direction to the corresponding GIST QUERY and thereby to
   perform the required translation.

   The threat here is an attacker flooding the GaNAT with maliciously
   constructed GIST QUERIES with the aim of exhausting the GaNAT's
   memory.  The attacker might use a variety of methods to construct
   such GIST QUERIES, including the following.

   1.  Use as [IP header].SourceIPAddress the address of some other node
       or an unallocated IP address.  This method is also known as IP
       spoofing.

   2.  Use an invalid NSLPID, in order to make sure that all on-path
       GaNAT(s) will behave like NSLP-unaware GaNATs.

   3.  For each packet, use a different value for the cookie field.

   4.  For each packet, use a different value for the session ID field.

   5.  Combinations of the above.

   How vulnerable a GaNAT is to the above service denial attack depends
   on a variety of factors, including the following.

   o  The amount of state allocated at the receipt of a GIST QUERY.
      This amount may vary depending on whether or not the data flow to
      which the signalling refers, already exists (i.e. whether or not
      the GaNAT already maintains a NAT binding for it).

   o  The mechanism that the GaNAT uses to map RESPONSEs to QUERIEs.

   o  Whether or not the GaNAT acquires dynamic IP addresses and ports
      for the downstream link.

   In order to decrease the exposure of a GaNAT to service denial
   attacks, the following recommendations are made.




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   o  The GaNAT should perform ingress filtering.  This limits the
      amount of locations from which an attacker can perform IP spoofing
      without being detected.

   o  The GaNAT should allocate the minimum amount of state required at
      the reception of a GIST QUERY.

   o  All state allocated by the GaNAT should timeout according to a
      local policy.  If the GaNAT detects heavy loads (which may
      indicate a service denial attack in progress), the GaNAT should
      timeout the state allocated as a result of a received GIST QUERY
      quicker, proportionally to the experienced load.

   o  The installation of a NAT binding for the data traffic (if such a
      binding does not exist prior to signalling) should be postponed
      until the correct GIST RESPONSE traverses the NAT.

   The service denial threats mentioned in this section do not apply to
   an NSLP-aware GaNAT, as such a GaNAT is required, in accordance with
   its local policy, to verify the validity of the cookie(s) before
   allocating any state, including the state required by the mechanisms
   in this document.

7.2.  Network Intrusions

   Although the primary goal of a NAT is to perform address translation
   between two addressing spaces, NATs are sometimes also used to
   provide a security service similar to the security service provided
   by firewalls.  That is, a NAT can be configured so that it does not
   forward packets from the external into the internal network, unless
   it determines that the packets belong to a communication session that
   was originally initiated from an internal node and are, as such,
   solicited.

   If an NSLP-unaware GaNAT performs the above security-relevant
   function in addition to address translation, then the presence of
   GIST signalling and, in particular the mechanisms described in this
   document, might allow an adversary to cause the installation of NAT
   bindings in the GaNAT using these mechansisms.  These NAT bindings
   would then enable the adversary to inject unsolicited traffic into
   the internal network, a capability that it might not have in the
   absence of the mechanisms described in this document.

   The administrator of an NSLP-unaware GaNAT should therefore make
   security-conscious decisions regarding the operation of the GaNAT.
   An NSLP-aware GaNAT, on the other hand, follows an NSLP policy which
   indicates the required security mechanisms.  This policy should
   account for the fact that this NSLP-aware node performs also NAT and



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   the associated packet filtering.


















































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8.  IAB Considerations

   None.
















































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9.  Acknowledgements

   The authors would like to thank Cedric Aoun, Christian Dickmann,
   Robert Hancock, and Martin Stiemerling for their insightful comments.
   Furthermore, we would like to mention that this document builds on
   top of a previous document regarding migration scenarios.













































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10.  Normative References

   [1]  Schulzrinne, H. and R. Hancock, "GIST:  General Internet
        Signalling Transport", draft-ietf-nsis-ntlp-13 (work in
        progress), April 2007.

   [2]  Stiemerling, M., Tschofenig, H., and C. Aoun, "NAT/Firewall NSIS
        Signaling Layer Protocol (NSLP)", draft-ietf-nsis-nslp-natfw-14
        (work in progress), March 2007.

   [3]  "Advanced Encryption Standard (AES)", FIPS PUB 197,
        November 2001.

   [4]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
        for Message Authentication", RFC 2104, February 1997.




































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Authors' Addresses

   Andreas Pashalidis
   NEC
   Kurfuersten-Anlage 36
   Heidelberg  69115
   Germany

   Email: Andreas.Pashalidis@netlab.nec.de


   Hannes Tschofenig
   Siemens
   Otto-Hahn-Ring 6
   Munich, Bavaria  81739
   Germany

   Email: Hannes.Tschofenig@siemens.com

































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Full Copyright Statement

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