rfc2344.txt

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3.4. New Registration Reply Codes

   Foreign and home agent registration replies MUST convey if the
   reverse tunnel request failed.  These new reply codes are defined:

      Service denied by the foreign agent:

      74 requested reverse tunnel unavailable
      75 reverse tunnel is mandatory and 'T' bit not set
      76 mobile node too distant

   and

      Service denied by the home agent:

      137 requested reverse tunnel unavailable
      138 reverse tunnel is mandatory and 'T' bit not set
      139 requested encapsulation unavailable





Montenegro                  Standards Track                     [Page 7]

RFC 2344            Reverse Tunneling for Mobile IP             May 1998


   In response to a Registration Request with the 'T' bit set, mobile
   nodes may receive (and MUST accept) code 70 (poorly formed request)
   from foreign agents and code 134 (poorly formed request) from home
   agents. However, foreign and home agents that support reverse
   tunneling MUST use codes 74 and 137, respectively.

   Absence of the 'T' bit in a Registration Request MAY elicit denials
   with codes 75 and 138 at the foreign agent and the home agent,
   respectively.

   Forward and reverse tunnels are symmetric, that is, both are able to
   use the same tunneling options negotiated at registration.  This
   implies that the home agent MUST deny registrations if an unsupported
   form of tunneling is requested (code 139).  Notice that Mobile IP [1]
   already defines the analogous failure code 72 for use by the foreign
   agent.

4. Changes in Protocol Behavior

   Unless otherwise specified, behavior specified by Mobile IP [1] is
   assumed. In particular, if any two entities share a mobility security
   association, they MUST use the appropriate Authentication Extension
   (Mobile-Foreign, Foreign-Home or Mobile-Home Authentication
   Extension) when exchanging registration protocol datagrams. The
   Mobile-Home Authentication Extension MUST always be present.

   Reverse tunneling imposes additional protocol processing requirements
   on mobile entities.  Differences in protocol behavior with respect to
   Mobile IP [1] are specified in the subsequent sections.

4.1. Mobile Node Considerations

   This section describes how the mobile node handles registrations that
   request a reverse tunnel.

4.1.1. Sending Registration Requests to the Foreign Agent

   In addition to the considerations in [1], a mobile node sets the 'T'
   bit in its Registration Request to petition a reverse tunnel.

   The mobile node MUST set the TTL field of the IP header to 255. This
   is meant to limit the reverse tunnel hijacking attack (Section 6).

   The mobile node MAY optionally include an Encapsulating Delivery
   Style Extension.






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RFC 2344            Reverse Tunneling for Mobile IP             May 1998


4.1.2. Receiving Registration Replies from the Foreign Agent

   Possible valid responses are:

      - A registration denial issued by either the home agent or the
        foreign agent:

         a. The mobile node follows the error checking guidelines in
            [1], and depending on the reply code, MAY try modifying the
            registration request (for example, by eliminating the
            request for alternate forms of encapsulation), and issuing a
            new registration.

         b. Depending on the reply code, the mobile node MAY try
            zeroing the 'T' bit, eliminating the Encapsulating Delivery
            Style Extension (if one was present), and issuing a new
            registration. Notice that after doing so the registration
            may succeed, but due to the lack of a reverse tunnel data
            transfer may not be possible.

      - The home agent returns a Registration Reply indicating that the
        service will be provided.

   In this last case, the mobile node has succeeded in establishing a
   reverse tunnel between its care-of address and its home agent.  If
   the mobile node is operating with a co-located care-of address, it
   MAY encapsulate outgoing data such that the destination address of
   the outer header is the home agent. This ability to selectively
   reverse-tunnel packets is discussed further in section 5.4.

   If the care-of address belongs to a separate foreign agent, the
   mobile node MUST employ whatever delivery style was requested (Direct
   or Encapsulating) and proceed as specified in section 5.

   A successful registration reply is an assurance that both the foreign
   agent and the home agent support whatever alternate forms of
   encapsulation (other than IP in IP) were requested. Accordingly, the
   mobile node MAY use them at its discretion.

4.2. Foreign Agent Considerations

   This section describes how the foreign agent handles registrations
   that request a reverse tunnel.








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RFC 2344            Reverse Tunneling for Mobile IP             May 1998


4.2.1. Receiving Registration Requests from the Mobile Node

   A foreign agent that receives a Registration Request with the 'T' bit
   set processes the packet as specified in the Mobile IP specification
   [1], and determines whether it can accomodate the forward tunnel
   request. If it cannot, it returns an appropriate code. In particular,
   if the foreign agent is unable to support the requested form of
   encapsulation it MUST return code 72.

   The foreign agent MAY reject Registration Requests without the 'T'
   bit set by denying them with code 75 (reverse tunnel is mandatory and
   'T' bit not set).

   The foreign agent MUST verify that the TTL field of the IP header is
   set to 255. Otherwise, it MUST reject the registration with code 76
   (mobile node too distant). The foreign agent MUST limit the rate at
   which it sends these registration replies to a maximum of one per
   second.

   As a last check, the foreign agent verifies that it can support a
   reverse tunnel with the same configuration. If it cannot, it MUST
   return a Registration Reply denying the request with code 74
   (requested reverse tunnel unavailable).

4.2.2. Relaying Registration Requests to the Home Agent

   Otherwise, the foreign agent MUST relay the Registration Request to
   the home agent.

   Upon receipt of a Registration Reply that satisfies validity checks,
   the foreign agent MUST update its visitor list, including indication
   that this mobile node has been granted a reverse tunnel and the
   delivery style expected (section 5).

   While this visitor list entry is in effect, the foreign agent MUST
   process incoming traffic according to the delivery style, encapsulate
   it and tunnel it from the care-of address to the home agent's
   address.

4.3. Home Agent Considerations

   This section describes how the home agent handles registrations that
   request a reverse tunnel.








Montenegro                  Standards Track                    [Page 10]

RFC 2344            Reverse Tunneling for Mobile IP             May 1998


4.3.1. Receiving Registration Requests from the Foreign Agent

   A home agent that receives a Registration Request with the 'T' bit
   set processes the packet as specified in the Mobile IP specification
   [1] and determines whether it can accomodate the forward tunnel
   request.  If it cannot, it returns an appropriate code. In
   particular, if the home agent is unable to support the requested form
   of encapsulation it MUST return code 139 (requested encapsulation
   unavailable).

   The home agent MAY reject registration requests without the 'T' bit
   set by denying them with code 138 (reverse tunnel is mandatory and '
   T' bit not set).

   As a last check, the home agent determines whether it can support a
   reverse tunnel with the same configuration as the forward tunnel. If
   it cannot, it MUST send back a registration denial with code 137
   (requested reverse tunnel unavailable).

   Upon receipt of a Registration Reply that satisfies validity checks,
   the home agent MUST update its mobility bindings list to indicate
   that this mobile node has been granted a reverse tunnel and the type
   of encapsulation expected.

4.3.2. Sending Registration Replies to the Foreign Agent

   In response to a valid Registration Request, a home agent MUST issue
   a Registration Reply to the mobile node.

   After a successful registration, the home agent may receive
   encapsulated packets addressed to itself. Decapsulating such packets
   and blindly injecting them into the network is a potential security
   weakness (section 6.1). Accordingly, the home agent MUST implement,
   and, by default, SHOULD enable the following check for encapsulated
   packets addressed to itself:

      The home agent searches for a mobility binding whose care-of
      address is the source of the outer header, and whose mobile node
      address is the source of the inner header.

   If no such binding is found, or if the packet uses an encapsulation
   mechanism that was not negotiated at registration the home agent MUST
   silently discard the packet and SHOULD log the event as a security
   exception.

   Home agents that terminate tunnels unrelated to Mobile IP (for
   example, multicast tunnels) MAY turn off the above check, but this
   practice is discouraged for the aforementioned reasons.



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RFC 2344            Reverse Tunneling for Mobile IP             May 1998


   While the registration is in effect, a home agent MUST process each
   valid reverse tunneled packet (as determined by checks like the
   above) by decapsulating it, recovering the original packet, and then
   forwarding it on behalf of its sender (the mobile node) to the
   destination address (the correspondent host).

5. Mobile Node to Foreign Agent Delivery Styles

   This section specifies how the mobile node sends its data traffic via
   the foreign agent. In all cases, the mobile node learns the foreign
   agent's link-layer address from the link-layer header in the agent
   advertisement.

5.1. Direct Delivery Style

   This delivery mechanism is very simple to implement at the mobile
   node, and uses small (non-encapsulated) packets on the link between
   the mobile node and the foreign agent (potentially a very slow link).
   However, it only supports reverse-tunneling of unicast packets, and
   does not allow selective reverse tunneling (section 5.4).

5.1.1. Packet Processing

   The mobile node MUST designate the foreign agent as its default
   router. Not doing so will not guarantee encapsulation of all the
   mobile node's outgoing traffic, and defeats the purpose of the
   reverse tunnel. The foreign agent MUST:

      - detect packets sent by the mobile node, and

      - modify its forwarding function to encapsulate them before
        forwarding.

5.1.2. Packet Header Format and Fields

   This section shows the format of the packet headers used by the
   Direct Delivery style. The formats shown assume IP in IP
   encapsulation [2].

   Packet format received by the foreign agent (Direct Delivery Style):

       IP fields:
         Source Address = mobile node's home address Destination Address
         = correspondent host's address
       Upper Layer Protocol

   Packet format forwarded by the foreign agent (Direct Delivery Style):




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RFC 2344            Reverse Tunneling for Mobile IP             May 1998


       IP fields (encapsulating header):
         Source Address = foreign agent's care-of address
         Destination Address = home agent's address
         Protocol field: 4 (IP in IP)
       IP fields (original header):
         Source Address = mobile node's home address
         Destination Address = correspondent host's address
       Upper Layer Protocol

   These fields of the encapsulating header MUST be chosen as follows:

      IP Source Address

         Copied from the Care-of Address field within the Registration
         Request.

      IP Destination Address

         Copied from the Home Agent field within the Registration
         Request.

      IP Protocol Field

         Default is 4 (IP in IP [2]), but other methods of encapsulation
         MAY be used as negotiated at registration time.

5.2. Encapsulating Delivery Style

   This mechanism requires that the mobile node implement encapsulation,
   and explicitly directs packets at the foreign agent by designating it
   as the destination address in a new outermost header.  Mobile nodes
   that wish to send either broadcast or multicast packets MUST use the
   Encapsulating Delivery Style.