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Versions: 00 01 02 03 04 05 06 RFC 3103

INTERNET DRAFT                                       Michael Borella
Expires February 2000                                David Grabelsky
                                                     3Com Corp.

                                                     Jeffrey Lo
                                                     Kunihiro Tuniguchi
                                                     NEC USA

                                                     August 1999




               Realm Specific IP: Protocol Specification
                 <draft-ietf-nat-rsip-protocol-02.txt>

Status of this Memo

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

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

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

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

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

Abstract

   This draft presents a protocol that enables an alternative to network
   address translation (NAT).  Realm-Specific IP (RSIP) defines an
   architecture in which an RSIP server is a multi-homed host connecting
   two routing realms.  An RSIP client in the first routing realm may be
   assigned a plurality of routing parameters from the second routing
   realm.  The RSIP client will be allowed to create packets using these
   parameters, and tunnel them across the first routing realm.  For
   example, more than one host from a private address space using RSIP
   may share one or more public address.  Unlike NAT, RSIP does not



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   break the end-to-end integrity of protocols.  We present a general,
   extensible negotiation protocol to be operated between RSIP clients
   and servers which facilitates the assignment of parameters and
   resources from the server to the client.

1.  Introduction

   Network Address Translation (NAT) has gained popularity as a method
   of separating public and private address spaces, and alleviating
   network address shortages.  A NAT translates the addresses of packets
   leaving a first routing realm to an address from a second routing
   realm, and performs the reverse function for packets entering the
   first routing realm from the second routing realm.  This translation
   is performed transparently to the hosts in either space, and may
   include modification of TCP/UDP port numbers as well as IP addresses.

   While a NAT does not require hosts to be aware of the translation, it
   will require an application layer gateway (ALG) for any protocol that
   transmits IP addresses or port numbers in packet payloads (such as
   FTP).  Additionally, a NAT will not work with protocols that require
   IP addresses and ports to remain unmodified between the source and
   destination hosts, or protocols that prevent such modifications to
   occur (such as some IPSEC modes, or application-layer end-to-end
   encryption).

   An alternative to a transparent NAT is an architecture that allows
   the clients within the first (e.g., private) routing realm to
   directly use addresses and other routing parameters from the second
   (e.g., public) routing realm.  This form of Realm-Specific IP (RSIP)
   requires that an RSIP server (a router or gateway between the two
   realms) assign at least one address from the second routing realm,
   and perhaps some other resources, to each RSIP client host in the
   first routing realm that needs to establish end-to-end connectivity
   to a host, entity or device in the second routing realm. Thus, the
   second routing realm is not transparent to RSIP client, but this
   system allows packets to maintain their integrity from RSIP client to
   destination.  In order to resolve addressing, routing, and/or
   firewall policy ambiguities in the first routing realm, all publicly
   routed packets are tunneled between RSIP client and the RSIP server,
   or tunneled such that the RSIP server can perform a NAT function on
   the outer IP header only.  ALGs are not required in the RSIP server.

   A disadvantage to RSIP is that it requires that hosts be modified so
   that they tunnel externally-bound packets and place some number of
   layer three, layer four or other values from those assigned by the
   RSIP server in each packet bound for the second routing realm.

   This draft discusses a method for assigning parameters to an RSIP



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   client from an RSIP server.  The requirements, scope and
   applicability of RSIP are discussed in a companion framework draft
   [RSIP-FRAME].

   1.1.  Specification of Requirements

      The keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD
      NOT", "SHALL", "SHALL NOT", "MAY" and "MAY NOT" that appear in
      this document are to be interpreted as described in [RFC2119].

2.  Architecture

   For simplicity, for the remainder of this document we will assume
   that the RSIP clients in the first routing realm (network) use
   private (e.g. see [RFC1918]) IP addresses, and that the second
   routing realm (network) uses public IP addresses.  The RSIP server
   connects the public and private realms and contains interfaces to
   both.  Other NAT terminology found in this document is defined in
   [RFC2663].

   The diagram below describes an exemplary reference architecture for
   RSIP. Some number of RSIP clients are attached via a private network
   to an RSIP server, which also acts as a router or gateway between the
   private and public networks. This router has been assigned some
   number of public addresses that it may use or allocate for use on the
   public network.

   +-------------+
   | RSIP client |
   |       1     +--+
   |   10.0.0.2  |  |                 +-------------+
   +-------------+  |        10.0.0.1 |             |   149.112.240.0/24
                    +-----------------+ RSIP server +-------------------
   +-------------+  |                 |             |
   | RSIP client |  |                 +-------------+
   |       2     +--+     private                     public
   |   10.0.0.3  |  |     network                     network
   +-------------+  |
                    |
                    |
                   ...

   RSIP MAY be based on either the Basic NAT or the NAPT model.  In the
   Basic NAT model, a unique public IP address is assigned to each
   private NAT client that is actively communicating with the public
   network.  Only one NAT client uses a given public address.  In the
   NAPT model, one public address is shared by one or more NAT clients.
   One or more NAT clients may use the same public address by limiting



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   the ports that they use to disjoint subsets of the port space.  The
   NAT server assigns these disjoint port sets to each host, along with
   the public IP address.  For the remainder of this document we will
   assume the NAPT model unless otherwise stated.  With respect to RSIP,
   we refer to these models as the Realm Specific Address IP (RSA-IP)
   method (NAT) and Realm Specific Address and Port IP (RSAP-IP) method
   (NAPT).  The RSIP method determines the demultiplexing field(s) of
   the packets coming from the public network to the private network.
   The RSIP method is extensible to other yet-to-be-defined
   demultiplexing fields.  A demultiplexing field must be able to
   uniquely identify an RSIP client.

   RSIP may be deployed incrementally.  An RSIP server may be co-located
   with a NAT router such that RSIP clients can use the RSIP server, but
   non-RSIP clients can use the NAT.

   For purposes of illustration, we will provide a brief example of the
   transport operation of RSIP with respect to the above architecture.
   We assume that RSIP client 10.0.0.2 has been assigned public address
   149.112.240.10 and port range 10000-10015 (the actual mechanism for
   this assignment is discussed below).  Packets transmitted from this
   client to a WWW server at the external address of 128.153.4.3 will
   appear as follows on the private network:

            Outer IP        Inner IP      TU Ports
        +--------------+----------------+----------+
   Src: |   10.0.0.2   | 149.112.240.10 |  10000   |
        +--------------+----------------+----------+
   Dst: |   10.0.0.1   |   128.153.4.3  |    80    |
        +--------------+----------------+----------+

   Note that 10000 was chosen arbitrarily from the port set by the
   kernel port selection code of the RSIP client.  Upon reaching the
   RSIP server, the outer IP header is stripped off, and the remaining
   packet is transmitted on the public network.  Incoming packets to the
   RSIP client are demultiplexed based on the RSIP method and tunneled
   from the RSIP server to the RSIP client.  Note that the tunneling
   need not be IP-IP and other tunneling mechansims may be used.

   Note that the architecture and protocols for RSIP may allow for
   cascading of RSIP servers.  For example, RSIP server A may assign
   some number of public IP addresses and port sets to RSIP server B,
   which is entirely inside of the private network.  RSIP server B will
   then assign some of these addresses and port sets to private hosts.
   This architecture conforms to the model in which a corporation (in
   charge of RSIP server B) buys public network access from an ISP (in
   charge of RSIP server A). In this scenario, RSIP server B and end
   hosts within the corporation could be considered the RSIP client of



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   RSIP server A and RSIP server B respectively.

   2.1.  RSIP Parameter Negotiation

      The goal of the RSIP protocol is to specify the basic requirements
      of RSIP client / server communication in a series of simple
      request / response messages.  All request messages from an RSIP
      client are replied to by the RSIP server with either the
      appropriate response message or an ERROR_RESPONSE. The
      ERROR_RESPONSE message may be used at any time by the RSIP server
      to indicate that an RSIP client's request was denied or malformed.

      An RSIP client initiates a binding request with an RSIP server by
      transmitting a REGISTER_REQUEST message.  This allows the RSIP
      server to become aware of the RSIP client.  The RSIP server will
      assign a unique identifier to the RSIP client, and respond with a
      REGISTER_RESPONSE message.

      An RSIP client must then use one of the ASSIGN_REQUEST messages to
      request an IP address, port set and/or other parameters from the
      public realm.  Negotiation of tunnel type and RSIP method may also
      occur. The RSIP server will respond with an ASSIGN_RESPONSE
      message if it approves the request.  All parameters will be
      assigned with a lease time, after which their assignment must be
      renewed.  After parameters have been assigned, the RSIP client may
      use the parameters to transmit packets to the public network.

      If an RSIP client no longer needs some parameters that it has been
      assigned, it may release them by transmitting a FREE_REQUEST
      message to the RSIP server, and specifying the parameters to
      release within that message.  The RSIP server will respond with a
      FREE_RESPONSE.

      If an RSIP client no longer needs to communicate with the public
      network, it may use the DE-REGISTER_REQUEST message to notify the
      RSIP server of such.  Use of the DE-REGISTER_REQUEST message
      effectively releases all of the RSIP client's parameters.  The
      RSIP server will respond with a DE-REGISTER_RESPONSE.  Once an
      RSIP client has de-registered, it must register and be assigned
      parameters once more before it transmits packets to the public
      network again.

      An RSIP server may deallocate the parameters of a given client
      with the DEALLOCATE message.  An RSIP client MUST acknowledge a
      DEALLOCATE message with an OK message.

   2.2.  Operation




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      In the case of RSA-IP, an RSIP client MUST request an IP address,
      but not any ports.  With the RSAP-IP method, an RSIP client MUST
      request an IP address and a port range in the same ASSIGN_REQUEST
      message.

      If the resources requested are temporarily unavailable, an RSIP
      server MAY NOT commit a subset of the resources requested; it MUST
      return an ERROR_RESPONSE message indicating that the requested
      resources were temporarily unavailable.  An RSIP client is free to
      attempt another ASSIGN_REQUEST for the same or other resources
      after an interval of time.  An RSIP server MUST NOT allocate ports
      without an associated IP address.

      If an RSIP server receives a FREE_REQUEST message that refers to
      parameters not assigned to the originating RSIP client, an
      ERROR_RESPONSE message indicating that the parameter range was
      invalid MUST be returned.  If an RSIP server receives a
      FREE_REQUEST message referring to an unknown binding, an
      ERROR_RESPONSE message indicating that the binding is unknown MUST
      be returned.

      If an RSIP client wishes to extend the duration of an existing
      binding, an ASSIGN_REQUEST with the same Bind ID and the desired
      extension duration MAY be sent. The RSIP server MUST either grant
      the request, grant a smaller duration than that requested or deny
      the request. If a smaller duration is granted, this duration MUST
      be included in the response message to the ASSIGN_REQUEST. In the
      case when the request is denied, the appropriate ERROR_RESPONSE
      MUST be sent.

      RSIP servers SHOULD NOT forward packets from a RSIP client without
      checking the validity of the packets' demultiplexing parameters.
      Other necessary policy based routing checks SHOULD also be made.
      Improper use of demultiplexing parameters or other parameters, or
      any RSIP client using a resource or parameter assigned to a
      different RSIP client are auditable events.

      If RSAP-IP is used, an RSIP client can request that the RSIP
      server pass to it all packets sent to a particular IP address and
      port number.  The RSIP client must specify the IP address and
      port(s) to listen on with the LISTEN_REQUEST message.  A
      successful request will be followed by a LISTEN_RESPONSE message
      from the RSIP server.  This would enable, for example, an RSIP
      client to bind a WWW server to a public address at port 80, and
      receive all traffic to that address/port pair.

      Note that the RSIP control protocol operates on a simple
      request/response basis, as described above.  However, an RSIP



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      client MAY transmit an OK message upon receiving a response from
      an RSIP server. Such OK messages serve to complete a three-way
      handshake and are useful for verifying the authenticity of a
      request.  However, use of a three-way handshake over an unreliable
      transport mechanism leads to more complicated client and server
      state maintenance.

      If an RSIP server must deallocate the resources associated with an
      RSIP client's Bind ID, it may do so with a DEALLOCATE message.
      This message must be followed by an OK message from the RSIP
      client.

   2.3.  RSIP Server State

      An RSIP server must maintain state for all RSIP clients with
      assigned resources.  This state may be maintained on the basis of
      individual RSIP clients, or on the basis of their active flows.
      The required RSIP server state will vary based on the RSIP method,
      but will always include the chosen methods demultiplexing
      parameters.

      2.3.1.  RSA-IP State

         An RSIP server serving an RSIP client using the RSA-IP method
         MUST maintain the following state:

         - Client's private address
         - Client's assigned public address(es)

         For flow-based state, the additional parameters that MAY be
         maintained are:

         - Source port number
         - Destination port number
         - Peer's public IP address
         - ToS / DS byte

         For a packet destined to an RSIP client, an RSIP server must
         examine, at a minimum, the destination IP address.  From this
         address, and possibly some number of the fields listed above,
         the RSIP server will be able to determine the RSIP client to
         which the packet should be tunneled.  It is highly recommended
         that all RSIP servers use flow-based state in order to minimize
         the impact of denial of service flooding attacks.

      2.3.2.  RSAP-IP State

         An RSIP server serving an RSIP client using the RSAP-IP method



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         MUST maintain the following state:

         - Client's private address
         - Client's assigned public address(es)
         - Client's assigned port range(s)

         For flow-based state, the additional parameters that MAY be
         maintained are:

         - Source port number
         - Destination port number
         - Peer's public IP address
         - ToS / DS byte

         For a packet destined to an RSIP client, the RSIP server must
         examine, at a minimum, the destination IP address and
         destination port number.  From these fields, and possibly some
         number of the fields listed above, the RSIP server will be able
         to determine the RSIP client to which the packet should be
         tunneled. It is highly recommended that all RSIP servers use
         flow-based state in order to minimize the impact of denial of
         service flooding attacks

      Policy may be applied such that public hosts are allowed to
      initiate contact to private RSIP clients through their assigned
      address(es) and port(s).  Thus, after a successful assignment, an
      RSIP client will be able to start a service on one of its public
      ports, and this service may be used by hosts on the public network
      (how the public hosts are notified of this service's existence is
      an open issue).  Alternatively, the RSIP server may restrict a
      public host from initiating contact with an RSIP client.  In this
      case, packets destinated to an RSIP client are forwarded by the
      RSIP server only if the RSIP client has already transmitted a
      packet to the sender.

      Note that if an RSIP client is issued a binding to an address/port
      pair with the LISTEN_RESPONSE message, then the RSIP server MUST
      forward all packets destinated to that address/port on to the RSIP
      client.

   2.4.  Subnet Query

      In some cases it is not possible for an RSIP client to know
      whether a packet should be tunneled to the RSIP server or
      transmitted using the local (private) routing realm only. The RSIP
      protocol provides a subnet query mechanism through which an RSIP
      client may query an RSIP server to ask whether a particular subnet
      falls within the private domain.  An RSIP client queries the



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      server using the QUERY_REQUEST message with the subnet included in
      the IP address field of the message. The RSIP server uses the
      QUERY_RESPONSE to reply to this message.  The RSIP server MAY
      confirm the subnet queried or MAY return the whole range of
      subnets supported so as to enable the RSIP client to cache the
      entries.  The RSIP server may be manually configured to know the
      topology of the private domain.

   2.5.  Unreliable Transport and Message IDs

      A message ID field SHOULD be included in all messages if UDP, or
      some other unreliable transport mechanism, is used. The message ID
      starts with zero in the client's REGISTER_REQUEST message and end
      with a maximum value in the server's DE-REGISTER_RESPONSE message.
      This field SHOULD be incremented by one for every request issued.
      Responses MUST include the same message ID as that of the request
      which it acknowledges.

      If an RSIP client does not receive a response from the RSIP server
      for a request, a new request with the same message ID MAY be
      issued after an interval of time.  An RSIP server receiving a
      request with a message ID smaller than what it previously received
      (within a REGISTERed session) SHOULD ignore the request.
      Pipelining of requests and aggregation of responses MUST NOT be
      allowed.

      Message ID wraparound is highly unlikely, however it must be
      considered in both the RSIP clients and servers.

      RSIP servers SHOULD implement message IDs, and SHOULD respond with
      message IDS to all clients that offer message IDs in their
      requests. An RSIP client MAY accept messages without message IDs
      from servers even if the client has offered message IDs.

   2.6.  Parameter Assignment Transport and Mechanisms

      In order to assign parameters to the RSIP client from the RSIP
      server, a transport protocol and an assignment mechanism must be
      used. Design of the RSIP protocol aims to be transport
      independent, so that existing transport protocols such as UDP or
      TCP can be used.  The advantages of using either UDP or TCP in
      different situations is discussed in [RSIP-FRAME].

      RSIP has been designed to be protocol agnostic, in that RSIP
      messages can ride on top of other parameter assignment and
      negotiation protocols such as DHCP or SOCKS.  However, it is
      likely that initial RSIP implementations will use a dedicated port
      number and implement the RSIP protocol in the raw form specified



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      in this draft.  Current RSIP implementations have been using UDP
      and TCP port 4455 for testing.  Note that this port number has not
      been officially assigned to RSIP by IANA and therefore may change
      at any time.

   2.7.  ICMP Handling

      Like NAT, RSIP server are required to remember recent ICMP packets
      for which responses cannot be demultiplexed by port number (i.e.,
      echo request packets).  ICMP request packets originating on the
      private network consist of echo request, timestamp request and
      address mask request.  These packets and their responses can be
      identified by the tuple of source IP address, ICMP identifier,
      ICMP sequence number, and target IP address.  An RSIP client
      sending an ICMP request packet tunnels it to the RSIP server, just
      as it does TCP and UDP packets.  The RSIP server must use this
      tuple to map incoming ICMP responses to the private address of the
      appropriate RSIP client.  Once it has done so, it will tunnel the
      ICMP response to the client.  Note that it is possible for two
      RSIP clients to use the same values for the tuples listed above,
      and thus create an ambiguity.  However, this occurance is likely
      to be quite rare, and is not addressed further in this draft.

      Incoming ICMP error response messages can be forwarded to the
      appropriate RSIP client by examining the IP header and port
      numbers embedding within the packet.  In the case of RSA-IP, only
      the source IP address is necessary to determine the RSIP client.
      In the case of RSAP-IP, the source IP address and source port
      number is necessary to determine the RSIP client.

      Occasionally, an RSIP client will have to send an ICMP response
      (e.g., port unreachable).  These responses are tunneled to the
      RSIP server, as is done for TCP and UDP packets.  All ICMP
      requests (e.g., echo request) arriving at the RSIP server MUST be
      processed by the RSIP server and MUST NOT be forwarded to an RSIP
      client.

3.  General Message and Parameter Formats

   In this section we define the general message and parameter format.
   Codes for each parameter and message types will be discussed the
   following sections.  Within an RSIP message, all required parameters
   SHOULD appear before all optional parameters.

   The general message format is shown below.






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       1 byte      Variable        Variable
   +-----------+---------------+-------------------
   |    Type   |  Parameter 1  |  Parameter 2 ...
   +-----------+---------------+-------------------

   The type field indicates how the parameters are to be interpreted
   (e.g., request, response, error, etc.).

   The general format of all parameters is shown below.

    1 byte   2 bytes  'Length' bytes
   +------+----------+----------------------
   | Code |  Length  | Parameter value ...
   +------+----------+----------------------

   All parameters consist of a fixed portion and a variable portion.
   The fixed portion is a 1 byte code value and a 2 byte length. The
   remaining portion of the parameter is the parameter value, the length
   which is the number of bytes indicated by the length field.

   Note that the code value of 0 is reserved.

4.  Parameter Types and Formats

   4.1.  IP Address Request

        Code   Length
      +------+--------+
      |  1   |    0   |
      +------+--------+

      Used in ASSIGN_REQUEST messages to request an IP address.

   4.2.  Number of Ports

        Code   Length    Number of Ports
      +------+--------+-------------------+
      |  2   |    1   |      (1 byte)     |
      +------+--------+-------------------+

      Used in ASSIGN_REQUEST messages to request a particular number of
      ports to be assigned.

   4.3.  IP Address

        Code   Length        IP Address
      +------+--------+-----------------------+
      |  3   |    4   |      (4 bytes)        |



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      +------+--------+-----------------------+

      This field represents the IPv4 address negotiated. Used in
      ASSIGN_RESPONSE, QUERY_REQUEST and QUERY_RESPONSE messages.

   4.4.  Port Range

        Code   Length    Low Port    High Port
      +------+--------+-----------+-----------+
      |  4   |    4   | (2 bytes) | (2 bytes) |
      +------+--------+-----------+-----------+

      The range of ports allocated to an RSIP client.  The port range
      MUST be contiguous and is inclusive.

   4.5.  Lease Time

        Code   Length         Lease Time
      +------+--------+-----------------------+
      |  5   |    4   |       (4 bytes)       |
      +------+--------+-----------------------+

      Number of seconds that an RSIP client may retain the parameters
      assigned by an RSIP server.

   4.6.  Error

        Code   Length     Error
      +------+--------+-----------+
      |  6   |    2   | (2 bytes) |
      +------+--------+-----------+

      These error codes allow an RSIP server to inform an RSIP client
      why a particular request has failed.

      1  UNKNOWNERROR - An error that cannot be identified has occurred.
      2  BADBINDID - The request refers to an invalid Bind ID.
      3  BADCLIENTID - The request refers to an invalid Client ID.
      4  MISSINGPARAM - The request does not contain a required parameter.
      5  DUPLICATEPARAM - The request contains an illegal duplicate parameter.
      6  ILLEGALPARAM - The request contains a parameter that it should not.
      7  ILLEGALMESSAGE - The RSIP server does not understand the message type.
      8  REGISTERFIRST - The RSIP client has attempted to request or use
              resources without registering.
      9  BADMESSAGEID - The request contains an unexpected message ID.
      10 ALREADYREGISTERED - The RSIP client has attempted to register again
              without first de-registering.
      11 ALREADYUNREGISTERED - The RSIP client has attempted to de-register



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              but it is already in the unregistered state.
      12 BADTUNNELTYPE - The RSIP client has tried to use a tunnel type that
              the RSIP server does not support.
      13 ADDRUNAVAILABLE - The RSIP server was not able to allocate an IP address.
      14 PORTUNAVAILABLE - The RSIP server was not able to allocate port(s).

   4.7.  Client ID

        Code   Length        Client ID
      +------+--------+-------------------+
      |  7   |    4   |      (4 bytes)    |
      +------+--------+-------------------+

      A unique number assigned by an RSIP server when an RSIP client
      registers with the server. It is used to identify the RSIP client.

   4.8.  Bind ID

        Code   Length        Bind ID
      +------+--------+-------------------+
      |  8   |    4   |      (4 bytes)    |
      +------+--------+-------------------+

      The Bind ID is a unique number allocated for each new assignment.
      It is returned as part of an ASSIGN_RESPONSE to a successful
      ASSIGN_REQUEST.  Subsequent message exchanges pertaining a bind
      MUST include its Bind ID.  When parameters are assigned to an RSIP
      client, these parameters are said to be 'bound' to that client.
      More than one bind, each with different Bind IDs, may be
      established between an RSIP client and RSIP server pair. A binding
      will expire when its lease time runs out or when the RSIP client
      de-registers itself with the RSIP server.

   4.9.  Message ID

      Code    Length       Message ID
      +------+--------+-------------------+
      |  9   |    1   |      (1 byte)     |
      +------+--------+-------------------+

      If the transport protocol is connectionless, such as UDP, the
      message ID field MUST be included as a means to order the messages
      and/or match requests and responses.

   4.10.  Tunnel Type






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        Code   Length   Tunnel Type
      +------+--------+------------+
      |  10  |    1   |  (1 byte)  |
      +------+--------+------------+

      The type of tunnel used between an RSIP client and an RSIP server.
      Values are assigned as follows:

      0  Reserved
      1  IP-IP
      2  GRE
      3  L2TP

   4.11.  RSIP Method

        Code   Length    RSIP Method
      +------+--------+-------------+
      |  11  |    1   |  (1 byte)   |
      +------+--------+-------------+

      The RSIP method that the server will support.

      1 Realm Specific Address IP (RSA-IP) method.
      2 Realm Specific Address and Port IP (RSAP-IP) method.

   4.12.  Vendor Specific Parameter

        Code    Length    Vendor ID     Subcode    Parameter
      +------+----------+------------+-----------+-----------+
      |  12  |   n+4    | (2 bytes)  | (2 bytes) | (n bytes) |
      +------+----------+------------+-----------+-----------+

      This parameter allows vendors to specify vendor specific
      information. The Vendor ID field the vendor-specific ID assigned
      by IANA.  Subcodes are defined and used by each vendor.


5.  Message Types

   Apart from the message type field, which MUST appear at the beginning
   of each message, other parameters MAY appear in any order.  Note that
   message sequencing MAY need to be introduced depending on the
   transport.  The following message types are defined in simple BNF.
   Required parameters are enclosed in <> and MUST appear.  Optional
   parameters are enclosed in [] and MAY appear.  Message type numbers
   are defined below.  Not all message types need to be implemented in
   order to be RSIP compliant.  For example, an RSIP client and/or
   server may not support LISTEN_REQUEST and LISTEN_RESPONSE, but will



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   still be able to perform RSIP.

   Note that for all RSIP messages, each parameter MUST NOT appear more
   than once, with the exception of parameters such as tunnel type and
   RSIP method, which MAY appear more than once in an ASSIGN_REQUEST
   message.

5.1.  Message Type Numbers

   Numbers are assigned to message types as follows:

    1 ERROR_RESPONSE
    2 REGISTER_REQUEST
    3 REGISTER_RESPONSE
    4 DE-REGISTER_REQUEST
    5 DE-REGISTER_RESPONSE
    6 ASSIGN_REQUEST_ADDR
    7 ASSIGN_REQUEST_PORT
    8 ASSIGN_REQUEST_EXT
    9 ASSIGN_RESPONSE_ADDR
   10 ASSIGN_RESPONSE_PORT
   11 ASSIGN_RESPONSE_EXT
   12 FREE_REQUEST
   13 FREE_RESPONSE
   14 QUERY_REQUEST
   15 QUERY_RESPONSE
   16 DEALLOCATE
   17 OK
   18 LISTEN_REQUEST
   19 LISTEN_RESPONSE


   5.2.  ERROR_RESPONSE

      An ERROR_RESPONSE is used to provide error messages to an RSIP
      client message.  If the error is related to a particular Client ID
      or Bind ID, these parameters MUST be included.  Multiple errors
      MAY NOT be reported in the same ERROR_RESPONSE.

         <ERROR_RESPONSE>::= <Message Type> <Error Parameter>
                             [Message ID][Client ID]
                             [Bind ID]

   5.3.  REGISTER_REQUEST

      The REGISTER_REQUEST message is used by an RSIP client to register
      with an RSIP server.  An RSIP client MUST register before it
      requests parameters from the RSIP server.



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         <REGISTER_REQUEST> ::= <Message Type> [Message ID]

   5.4.  REGISTER_RESPONSE

      The REGISTER_RESPONSE message is used by an RSIP server to confirm
      the registration of an RSIP client, and to provide a Client ID.

         <REGISTER_RESPONSE> ::= <Message Type> <Client ID> [Message ID]

   5.5.  DE-REGISTER_REQUEST

      The DE-REGISTER_REQUEST message is used by an RSIP client to de-
      register with an RSIP server.

         <DE-REGISTER_REQUEST> ::= <Message Type> <Client ID> [Message ID]


   5.6.  DE-REGISTER_RESPONSE

      The DE-REGISTER_RESPONSE message is used by an RSIP server to
      confirm the de-registration of an RSIP client.

         <DE-REGISTER_RESPONSE> ::= <Message Type> <Client ID> [Message ID]


   5.7.  ASSIGN_REQUEST

      The ASSIGN_REQUEST message is used by an RSIP client to request
      parameter assignments. In RSA-IP method, ASSIGN_REQUEST_ADDR
      format MUST be used and an IP address MUST be requested. For RSAP-
      IP, ASSIGN_REQUEST_PORT format MUST be used, and a port range MUST
      be requested for every IP address requested.  When requesting a
      renewal of a previously-assigned, but not yet expired lease,
      ASSIGN_REQUEST_EXT format MUST be used, and a valid Bind ID MUST
      be specified.  All types of ASSIGN_REQUEST message share the same
      message type. The RSIP client MAY list all of the tunnel types
      supported.  The default tunnel type is IP-IP; thus, if no tunnel
      type is specified, the RSIP server MUST assume that IP-IP
      tunneling will be used, unless a different type of tunnel is
      implied by the RSIP method.  All RSIP implementations MUST support
      IP-IP tunneling.










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         <ASSIGN_REQUEST_ADDR> ::= <Message Type> <Client ID>
                                   <RSIP method = RSA-IP> <IP Addr Req>
                                   [Lease Time] [Tunnel Type...]
                                   [Message ID]

         <ASSIGN_REQUEST_PORT> ::= <Message Type> <Client ID>
                                   <RSIP method = RSAP-IP> <IP Addr Req>
                                   <Number of Ports> [Lease Time]
                                   [Tunnel Type...] [Message ID]

         <ASSIGN_REQUEST_EXT> ::= <Message Type> <Client ID>
                                  <Bind ID> [Lease Time] [Message ID]

   5.8.  ASSIGN_RESPONSE

      The ASSIGN_RESPONSE message is used by an RSIP server to deliver
      parameter assignments to an RSIP client.  A client-wise unique
      Bind ID must be provided for every assignment.  If a lease time is
      not requested by an RSIP client, an RSIP server MUST assign a
      default lease time.  If an ASSIGN_REQUEST is not granted, the
      appropriate ERROR_RESPONSE message(s) MUST be generated,
      specifying all reasons why the ASSIGN_REQUEST failed.  If the
      requested RSIP method is not supported, the RSIP server MUST NOT
      allocate any resources.  For RSA-IP, ASSIGN_RESPONSE_ADDR is used.
      For RSAP-IP, ASSIGN_RESPONSE_PORT is used.  When a lease extension
      is granted, ASSIGN_RESPONSE_EXT is used.  All types of
      ASSIGN_RESPONSE message share the same message type.  Note that
      the chosen tunnel type MUST be included in the message, except for
      extension requests and when the RSIP method implies that a
      particular type of tunnel must be used.

         <ASSIGN_RESPONSE_ADDR> ::= <Message Type> <Client ID> <Bind ID>
                                    <RSIP method = RSA-IP>  <IP Address>
                                    <Lease Time> <Tunnel Type> [Message ID]

         <ASSIGN_RESPONSE_PORT> ::= <Message Type> <Client ID> <Bind ID>
                                    <RSIP method = RSAP-IP> <IP Address>
                                    <Port Range> <Lease Time> <Tunnel Type>
                                    [Message ID]

         <ASSIGN_RESPONSE_EXT> ::= <Message Type> <Client ID>
                                   <Bind ID> <Lease Time>
                                   [Message ID]

   5.9.  FREE_REQUEST

      The FREE_REQUEST message is used by an RSIP client to free
      parameter assignments.  The given Bind ID identifies the IP



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      address and/or port range being freed.  Resources may only be
      freed using the granularity of a Bind ID.

         <FREE_REQUEST> ::= <Message Type> <Client ID> <Bind ID>
                            [Message ID]


   5.10.  FREE_RESPONSE

      The FREE_RESPONSE message is used by an RSIP server to acknowledge
      a FREE_REQUEST sent by an RSIP client.

         <FREE_RESPONSE> ::= <Message Type> <Client ID> <Bind ID>
                             [Message ID]


   5.11.  QUERY_REQUEST

      A QUERY_REQUEST message is used by an RSIP client to request if a
      subnet or IP address is supported by an RSIP server.  If a subnet
      is queried, it is placed in an IP address parameter.

         <QUERY_REQUEST> ::= <Message Type> <Client ID> <IP address>
                             [Message ID]

   5.12.  QUERY_RESPONSE

      A QUERY_RESPONSE message is used by an RSIP server to answer a
      QUERY_REQUEST from an RSIP client.  The RSIP server SHOULD respond
      with a list of all subnets and/or addresses that are on the
      private side of the network.

         <QUERY_RESPONSE> ::= <Message Type> <Client ID> <IP address>
                              [IP address...] [Message ID]

   5.13.  DEALLOCATE

      A DEALLOCATE message is used by an RSIP server to force an RSIP
      client to relinquish specified resources (e.g., in the case of a
      lease expiration).  The Bind ID of the resources to be
      relinquished MUST be included.  Upon receiving a DEALLOCATE
      message, an RSIP client MUST stop all use of the said resources
      and immediately send an OK response to the server.

         <DEALLOCATE> ::= <Message Type> <Client ID> <Bind ID>
                          [Message ID]

   5.14.  OK



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      An OK message MAY be used by an RSIP client to positively
      acknowledge the receipt of a message from the RSIP server.  The OK
      message MUST be used to respond to a DEALLOCATE message from an
      RSIP server.

         <OK> ::= <Message Type> <Client ID> [Message ID]

   5.15.  LISTEN_REQUEST

      A LISTEN_REQUEST message is sent by an RSIP client that wants to
      register a service on a particular IP address and port number.
      The required parameters are and IP address and a port range.
      Optional parameters are RSIP method and tunnel type.  The RSIP
      client may request a range of listen ports, or just one (a single
      port is specified by placing the same port number in the "low" and
      "high" fields of the Port Range parameter).  LISTEN_REQUEST is not
      necessary for RSA-IP.

         <LISTEN_REQUEST> ::= <Message Type> <Client ID> <IP Address>
                              <Port range> [Lease Time] [RSIP method]
                              [Tunnel Type] [Message ID]

   5.16.  LISTEN_RESPONSE

      A LISTEN_RESPONSE message is used by an RSIP server to respond to
      a LISTEN_REQUEST message from an RSIP client.  The RSIP server
      MUST issue a Bind ID, and specify the IP address, port range, RSIP
      method, and tunnel type to be used.

         <LISTEN_RESPONSE> ::= <Message Type> <Client ID> <Bind ID>
                               <IP Address> <Port range> <RSIP method>
                               <Tunnel Type> <Lease Time> [Message ID]

6.  Miscellaneous Issues

   TCP TIME_WAIT at external servers: When a TCP server disconnects a
      socket, it enters the TCP TIME_WAIT state for a period of time.
      While it is in this state it will refuse to accept new connections
      using the same socket (i.e., the same source address/port and
      destination address/port).  Consider the circumstance in which an
      RSIP client terminates a connection with an external server, and
      immediately frees the port that it was using to source the
      connection (alternatively, the port may be deallocated by the RSIP
      server).  If the RSIP server immediately allocates this port to
      another RSIP client, and this client uses the same port to contact
      the same external server while that server is still in TIME_WAIT,
      then the client's connection may be rejected by the server.  In
      order to mitigate this problem, it is recommended that RSIP



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      servers hold recently deallocated ports for at least two minutes,
      which is the greatest duration of TIME_WAIT that is commonly
      implemented [STEV94].  In situations where ports are scarce, the
      RSIP server MAY choose to allocate ports in a FIFO fashion from
      the pool of recently deallocated ports.

   Split DNS: RSIP requires that DNS traffic from the private network
      not be propagation on the public network.  This issue is not
      specific to RSIP - it also occurs in NAT.

7.  Security Considerations

   RSIP, in and of itself, does not provide security.  It may provide
   the illusion of security or privacy by hiding a private address
   space, but security can only be ensured by the proper use of security
   protocols and cryptographic techniques.

   An RSIP implementation must be guarded against potential denial- of-
   service attacks.  A malicious RSIP client may be able to determine
   the parameters associated with another RSIP client via packet
   sniffing.  An attacker could use or free the resources allocated by
   the RSIP server by spoofing a data packet or a FREE request,
   respectively.  It is desirable to secure negotiation between an RSIP
   client and RSIP server with an appropriate authentication mechanism.
   This general problem is not specific to RSIP - DHCP suffers from the
   same lack of authentication.  However, this security hole is
   mitigated to some extent by the following: (1) It is much more
   difficult to sniff packets if the RSIP clients are on a switched LAN,
   and this is the direction that the industry is moving towards, (2)
   use of the RSIP Bind ID and message IDs prevents some simple attacks
   such as packet replay, and (3) all RSIP clients must answer to the
   authority to which they are borrowing resources from; therefore, once
   detected, an attacker can be dealt with administratively.  Note that
   although IPSEC could be used for securing the channel between the
   RSIP client and the RSIP server, it would require implementations on
   both the client and the server, and may prove to be too heavyweight
   for some systems.

8.  Changelog

   01 to 02:
         - Added section on server state.
         - Re-wrote section on parameter negotiation.
         - Added details to ICMP Handling section.
         - Added LISTEN_REQUEST and LISTEN_RESPONSE messages.
         - Added appendix with client state diagram.
         - Updated references with respect to RFC 2663.
         - Clarified use/non-use of message IDs between clients and servers.



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         - Added recommendation that RSIP use port 4455 for initial implementation
           and testing, until further notice.
         - Bumped code values up by 1, made code value of 0 reserved.
         - Expanded ASSIGN_REQUEST into ASSIGN_REQUEST_ADDR for RSA-IP,
           ASSIGN_REQUEST_PORT for RSAP-IP and ASSIGN_REQUEST_EXT for lease
           extensions.  The same expansion applies for ASSIGN_RESPONSE.
         - Indiciated that all RSIP parameters must not appear more than once
           except for tunnel type and RSIP method in ASSIGN_REQUEST messages.
         - Exactly one error is now reported in each ERROR_RESPONSE message.

   00 to 01:
         - Eliminated number of IP addresses and IP address range
           parameters and fixed other parameters to reflect this change.
         - Added IP address request message.
         - Added discussion on authentication to Security Considerations
           section.
         - Added Miscellaneous Issues section.
         - Changed all mention of "sequence number" to "message ID".
         - Reformatted References section.
         - Added reference to RSIP framework draft.
         - Separated request and response messages, then renumbered them.
         - Required that all RSIP implementations support IP-IP tunneling
           and RSA-IP.
         - Modified message semantics slightly.
         - Added appendix with protocol example.
         - Added address and port resource error messages.
         - Specified that multiple error responses may be returned in the
           same ERROR_RESPONSE message.
         - RSIP method may now be specified per binding, so that different
           methods can be used when connecting to different external systems.
         - Synched up terminology with the latest NAT terminology draft.
         - Added mention of RSIP servers also implementing a NAT as a
           fallback.
         - Added DEALLOCATE and OK messages.
         - Tunneling now negotiated per bind rather than per-registration.

9.  Acknowledgements

   The authors would like to thank Gabriel Montenegro, Pyda Srisuresh,
   Dan Nessett, Gary Jaszewski, and Rick Cobb for their input.


10.  Appendix: Example RSIP client/server transactions

   In this appendix, we present an exemplary series of transactions
   between an RSIP client and an RSIP server.  All client to server
   traffic is denote by `C --> S' and all server to client traffic is
   denoted by `S --> C'.  Message types and message ID's are not



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   included in order to save space.

   C --> S: REGISTER_REQUEST ()

      The client attempts to register with the server.

   S --> C: REGISTER_RESPONSE (Client ID = 1)

      The server responds, assigning a Client ID of 1.

   C --> S: ASSIGN_REQUEST_PORT: (Client ID = 1, RSIP method = RSAP-IP,
      IP Address Request, Num Ports = 16, Lease Time = 3600 Tunnel type
      = IP-IP, Tunnel type = GRE)

      The client requests an IP address and 16 ports to use with it with
      RSAP-IP.  The client indicates that it would like to use these
      resources for 3600 seconds with either IP-IP and GRE tunneling.

   S --> C: ASSIGN_RESPONSE_PORT: (Client ID = 1, RSIP method = RSAP-IP,
      Bind ID = 0, IP address = 149.112.240.156, Port range = 9000-9015,
      Lease time = 1800, Tunnel type = IP-IP)

      The server responds by indicating that a Bind ID of 0 has been
      assigned to the resources of an IP address of 149.112.240.156 and
      a port range of 9000-9015 to be used for RSAP-IP.  The server also
      indicates that it will grant a lease time of only 1800 seconds,
      and indicates that IP-IP tunneling will be used.

      The client is now able to communicate with the public network
      using these resources.

   C --> S: QUERY_REQUEST: (Client ID = 1, IP address = 10.20.60.0)

      The client asks the server if the subnet 10.20.60.0 is local.

   S --> C: QUERY_RESPONSE: (Client ID = 1, IP address = 10.20.60.0, IP
      address = 10.20.66.0, IP address = 10.20.68.0)

      The server responds with a list of local subnets, implicitly
      informing the client that subnet 10.20.60.0 is local.

   C --> S: ASSIGN_REQUEST_PORT: (Client ID = 1, RSIP method = RSAP-IP,
      IP Address Request, Num Ports = 8, Lease Time = 1800)

      The client requests 8 more ports for use with RSAP-IP.  A lease of
      1800 seconds is requested.  IP-IP tunneling is implied by default.

   S --> C: ASSIGN_RESPONSE_PORT: (Client ID = 1, RSIP method = RSAP-IP,



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      Bind ID = 1, IP address = 149.112.240.156, Port range = 9074-9081,
      Lease time = 1800)

      The server grants the request, assigning ports 9074-9081 from IP
      address 149.112.240.156. IP-IP tunneling is implied by default.

   C --> S: FREE_REQUEST (Client ID = 1, Bind ID = 0)

      The client frees Bind ID 0; i.e., ports 9000-9015 from IP address
      149.112.240.156.  Note that the address itself is still assigned
      to the client because the client is still assigned ports
      9074-9081.

   S --> C: FREE_RESPONSE (Client ID = 1, Bind ID = 0)

      The server acknowledges that Bind ID 0 has been freed.

   C --> S: ASSIGN_REQUEST_EXT (Client ID = 1, Bind ID = 1, Lease Time =
      1800)

      The client request that the lease on Bind ID 1 be extended for
      1800 seconds.

   S --> C: ASSIGN_RESPONSE_EXT (Client ID = 1, Bind ID = 1, Lease Time
      = 1800)

      The server confirms the request.

   S --> C: DEALLOCATE (Client ID = 1, Bind ID = 1)

      The server forces the client to deallocate the resources of Bind
      ID 1.

   C --> S: OK (Client ID = 1)

      The client acknowledges that the resources have been deallocated.

   C --> S: DE-REGISTER_REQUEST (Client ID = 1)

      The client de-registers with the sever.

   S --> C: REGISTER_RESPONSE (Client ID = 1)

      The server acknowledges that the client has de-registered.

11.  Appendix: Example RSIP client state diagram

   This appendix provides an exemplary diagram of RSIP client state.



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   The client begins in the Unregistered state.  We assume that if a
   message are lost, the client will timeout and retransmit another copy
   of it.  We recommend a 7-fold binary exponential backoff timer for
   retransmissions, with the first timeout occuring after 25 ms.  This
   diagram does not include transitions for the LISTEN_REQUEST message
   or the DEALLOCATE message.


                          send
     +------------+ REGISTER_REQUEST +------------+
     |            |----------------->|Registration|<-- timeout/send
+--->|Unregistered|<-----------------|  Pending   |--- REGISTER_REQUEST
|    |            | 7th timeout/recv +------------+
|    +------------+  ERROR_RESPONSE        |
|          ^                               |
|          |7th timeout/recv               |recv               timeout/send
|          |DE-REGISTER_RESPONSE           |REGISTER_RESPONSE  QUERY_REQUEST
|          |                               |                        ^  |
|          |            send DE-           v        send            |  |
| +----------------+ REGISTER_REQUEST+----------+QUERY_REQUEST  +----------+
| |   Registered   |<----------------|          |-------------->|Registered|
| | De-registration|                 |Registered|               |   Query  |
| |    Pending     |---------------->|          |<--------------|  Pending |
| +----------------+      recv       +----------+  7th timeout/ +----------+
|         | ^        ERROR_RESPONSE        ^  |        recv
|         | |                              |  |  QUERY_RESPONSE or
|    timeout/send                          |  |    ERROR_RESPONSE
| DE-REGISTER_REQUEST      7th timeout/recv|  |
|                           ERROR_RESPONSE |  |
| +----------------+                       |  |
| |Go to Registered|                       |  |send
| +----------------+                       |  |ASSIGN_REQUEST
|         ^                   timeout/send |  |
|         |Yes                FREE_REQUEST |  |
|         +                       |  |     |  |
|       +   +                     v  |     |  v
|     +       +   7th timeout/ +--------+ +----------+
|   +  Are all  +      recv    |  Free  | |Assignment|<--timeout/send
| +   resources   +<-----------|Pending | |  Pending |---ASSIGN_REQUEST
|   +   freed?  + FREE_RESPONSE+--------+ +----------+
|     +       +                    ^ |         |
|       +   +                      | |         |
|         +                        | |         |recv
|         |No                 send | |recv     |ASSIGN_RESPONSE
|         v           ERROR_REQUEST| |ERROR_   |
| +---------------+                | |RESPONSE |
| | Go to Assigned|                | |         |
| +---------------+                | |         | 7th timeout/recv



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|                       recv       | |         | QUERY_RESPONSE or
| +---------------+ERROR_RESPONSE  | v         v ERROR_RESPONSE+-------------+
| |    Assigned   |-------------->+-------------+------------->|   Assigned  |
+>|De-registration|               |   Assigned  |              |    Query    |
  |    Pending    |<--------------+-------------+<-------------|   Pending   |
  +---------------+      send            ^  |         send     +-------------+
        ^  |       DE-REGISTER_REQUEST   |  |     QUERY_REQUEST     ^  |
        |  |                             |  |                       |  |
    timeout/send        7th/timeout/recv |  |send                   |  |
    DE-REGISTER_         ASSIGN_RESPONSE |  |ASSIGN_REQUEST      timeout/send
      REQUEST           or ERROR_RESPONSE|  |                    QUERY_REQUEST
                                         |  |
                                         |  v
                                     +----------+
                                     | Assigned |
                                     |Assignment|
                                     | Pending  |
                                     +----------+
                                         ^  |
                                         |  |
                                     timeout/send
                                    ASSIGN_REQUEST

12.  References

   [RFC2663] P. Srisuresh and M. Holdrege, "IP Network Address
      Translator (NAT) Terminology and Considerations," RFC 2663, Aug.
      1999.

   [RFC1918] Y. Rekhter, B. Moskowitz, D. Karrenberg, G. J. de Groot,
      and E. Lear, "Address Allocation for Private Internets," RFC 1918,
      Feb. 1996.

   [RFC2119] S. Bradner, "Key words for use in RFCs to indicate
      requirement levels," RFC 2119, Mar. 1997.

   [RSIP-FRAME] J. Lo, M. Borella, and D. Grabelsky, "Realm Specific IP:
      A Framework," Internet Draft <draft-ietf-nat-rsip-
      framework-01.txt>, Apr. 1999 (work in progress).

   [STEV94] W. R. Stevens, "TCP/IP Illustrated, Vol. 1," Addison-Wesley,
      1994.



13.  Authors' Addresses

   Michael Borella



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   3Com Corp.
   1800 W. Central Rd.
   Mount Prospect IL 60056
   (847) 342-6093
   mike_borella@3com.com

   David Grabelsky
   3Com Corp.
   1800 W. Central Rd.
   Mount Prospect IL 60056
   (847) 222-2483
   david_grabelsky@3com.com

   Jeffrey Lo
   NEC USA
   C&C Research Labs.
   110 Rio Robles
   San Jose, CA 95134
   (408) 943-3033
   jlo@ccrl.sj.nec.com

   Kunihiro Taniguchi
   NEC USA
   C&C Research Labs.
   110 Rio Robles
   San Jose, CA 95134
   (408) 943-3031
   taniguti@ccrl.sj.nec.com


   Copyright (c) The Internet Society (1999). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.



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   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.













































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