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RFC 5944

IP Mobility Support for IPv4, Revised

Pages: 100
Proposed Standard
Errata
Obsoletes:  3344
Part 4 of 4 – Pages 77 to 100
First   Prev   None

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5. Security Considerations

The mobile computing environment is potentially very different from the ordinary computing environment. In many cases, mobile computers will be connected to the network via wireless links. Such links are particularly vulnerable to passive eavesdropping, active replay attacks, and other active attacks.

5.1. Message Authentication Codes

Home agents and mobile nodes MUST be able to perform authentication. The default algorithm is HMAC-MD5 [10], with a key size of 128 bits. The foreign agent MUST also support authentication using HMAC-MD5 and key sizes of 128 bits or greater, with manual key distribution. Keys with arbitrary binary values MUST be supported. The "prefix+suffix" use of MD5 to protect data and a shared secret is considered vulnerable to attack by the cryptographic community. Where backward compatibility with existing Mobile IP implementations
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   that use this mode is needed, new implementations SHOULD include
   keyed MD5 [19] as one of the additional authentication algorithms for
   use when producing and verifying the authentication data that is
   supplied with Mobile IP registration messages, for instance, in the
   extensions specified in Sections 3.5.2, 3.5.3, and 3.5.4.

   More authentication algorithms, algorithm modes, key distribution
   methods, and key sizes MAY also be supported for all of these
   extensions.

5.2. Areas of Security Concern in This Protocol

The registration protocol described in this document will result in a mobile node's traffic being tunneled to its care-of address. This tunneling feature could be a significant vulnerability if the registration were not authenticated. Such remote redirection, for instance, as performed by the mobile registration protocol, is widely understood to be a security problem in the current Internet if not authenticated [30]. Moreover, the Address Resolution Protocol (ARP) is not authenticated, and can potentially be used to steal another host's traffic. The use of gratuitous ARP (Section 4.6) brings with it all of the risks associated with the use of ARP.

5.3. Key Management

This specification requires a strong authentication mechanism (keyed MD5) that precludes many potential attacks based on the Mobile IP registration protocol. However, because key distribution is difficult in the absence of a network key management protocol, messages with the foreign agent are not all required to be authenticated. In a commercial environment it might be important to authenticate all messages between the foreign agent and the home agent, so that billing is possible and service providers do not provide service to users that are not legitimate customers of that service provider.

5.4. Picking Good Random Numbers

The strength of any authentication mechanism depends on several factors, including the innate strength of the authentication algorithm, the secrecy of the key used, the strength of the key used, and the quality of the particular implementation. This specification requires implementation of keyed MD5 for authentication, but does not preclude the use of other authentication algorithms and modes. For keyed MD5 authentication to be useful, the 128-bit key must be both secret (that is, known only to authorized parties) and pseudo-random.
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   If nonces are used in connection with replay protection, they must
   also be selected carefully.  RFC 4086 [8] written by Eastlake, et al.
   provides more information on generating pseudo-random numbers.

5.5. Privacy

Users who have sensitive data that they do not wish others to see should use mechanisms outside the scope of this document (such as encryption) to provide appropriate protection. Users concerned about traffic analysis should consider appropriate use of link encryption. If absolute location privacy is desired, the mobile node can create a tunnel to its home agent. Then, datagrams destined for correspondent nodes will appear to emanate from the home network, and it may be more difficult to pinpoint the location of the mobile node. Such mechanisms are all beyond the scope of this document.

5.6. Ingress Filtering

Many routers implement security policies such as "ingress filtering" [35] that do not allow forwarding of packets that have a Source Address that appears topologically incorrect. In environments where this is a problem, mobile nodes may use reverse tunneling [12] with the foreign agent supplied care-of address as the Source Address. Reverse-tunneled packets will be able to pass normally through such routers, while ingress filtering rules will still be able to locate the true topological source of the packet in the same way as packets from non-mobile nodes.

5.7. Replay Protection for Registration Requests

The Identification field is used to let the home agent verify that a registration message has been freshly generated by the mobile node, not replayed by an attacker from some previous registration. Two methods are described in this section: timestamps (mandatory) and "nonces" (optional). All mobile nodes and home agents MUST implement timestamp-based replay protection. These nodes MAY also implement nonce-based replay protection. The style of replay protection in effect between a mobile node and its home agent is part of the Mobility Security Association. A mobile node and its home agent MUST agree on which method of replay protection will be used. The interpretation of the Identification field depends on the method of replay protection as described in the subsequent subsections. Whatever method is used, the low-order 32 bits of the Identification field MUST be copied unchanged from the Registration Request to the Reply. The foreign agent uses those bits (and the mobile node's home
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   address) to match Registration Requests with corresponding replies.
   The mobile node MUST verify that the low-order 32 bits of any
   Registration Reply are identical to the bits it sent in the
   Registration Request.

   The Identification field in a new Registration Request MUST NOT be
   the same as in an immediately preceding Request, and SHOULD NOT
   repeat while the same security context is being used between the
   mobile node and the home agent.  Retransmission as in Section 3.6.3
   is allowed.

5.7.1. Replay Protection Using Timestamps

The basic principle of timestamp replay protection is that the node generating a message inserts the current time of day, and the node receiving the message checks that this timestamp is sufficiently close to its own time of day. Unless specified differently in the security association between the nodes, a default value of 7 seconds MAY be used to limit the time difference. This value SHOULD be greater than 3 seconds. Obviously the two nodes must have adequately synchronized time-of-day clocks. As with any messages, time synchronization messages may be protected against tampering by an authentication mechanism determined by the security context between the two nodes. If timestamps are used, the mobile node MUST set the Identification field to a 64-bit value formatted as specified by the Network Time Protocol [11]. The low-order 32 bits of the NTP format represent fractional seconds, and those bits that are not available from a time source SHOULD be generated from a good source of randomness. Note, however, that when using timestamps, the 64-bit Identification used in a Registration Request from the mobile node MUST be greater than that used in any previous Registration Request, as the home agent uses this value as a sequence number. Without such a sequence number, it would be possible for a delayed duplicate of an earlier Registration Request to arrive at the home agent (within the clock synchronization required by the home agent), and thus be applied out of order, mistakenly altering the mobile node's current registered care-of address. Upon receipt of a Registration Request with an authorization-enabling extension, the home agent MUST check the Identification field for validity. In order to be valid, the timestamp contained in the Identification field MUST be close enough to the home agent's time- of-day clock, and the timestamp MUST be greater than all previously accepted timestamps for the requesting mobile node. Time tolerances and resynchronization details are specific to a particular Mobility Security Association.
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   If the timestamp is valid, the home agent copies the entire
   Identification field into the Registration Reply it returns to the
   mobile node.  If the timestamp is not valid, the home agent copies
   only the low-order 32 bits into the Registration Reply, and supplies
   the high-order 32 bits from its own time of day.  In this latter
   case, the home agent MUST reject the registration by returning Code
   133 (registration Identification mismatch) in the Registration Reply.

   As described in Section 3.6.2.1, the mobile node MUST verify that the
   low-order 32 bits of the Identification field in the Registration
   Reply are identical to those in the rejected registration attempt,
   before using the high-order bits for clock resynchronization.

5.7.2. Replay Protection Using Nonces

The basic principle of nonce replay protection is that node A includes a new random number in every message to node B, and checks that node B returns that same number in its next message to node A. Both messages use an authentication code to protect against alteration by an attacker. At the same time, node B can send its own nonces in all messages to node A (to be echoed by node A), so that it too can verify that it is receiving fresh messages. The home agent may be expected to have resources for computing pseudo-random numbers useful as nonces [8]. It inserts a new nonce as the high-order 32 bits of the Identification field of every Registration Reply. The home agent copies the low-order 32 bits of the Identification field from the Registration Request message into the low-order 32 bits of the Identification field in the Registration Reply. When the mobile node receives an authenticated Registration Reply from the home agent, it saves the high-order 32 bits of the Identification field for use as the high-order 32 bits of its next Registration Request. The mobile node is responsible for generating the low-order 32 bits of the Identification field in each Registration Request. Ideally, it should generate its own random nonces. However, it may use any expedient method, including duplication of the random value sent by the home agent. The method chosen is of concern only to the mobile node, because it is the node that checks for valid values in the Registration Reply. The high-order and low-order 32 bit values of the identification chosen SHOULD both differ from their previous values. The home agent uses a new high-order value, and the mobile node uses a new low-order value for each registration message. The foreign agent uses the low-order value (and the mobile host's home address) to correctly match registration replies with pending Requests (Section 3.7.1).
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   If a registration message is rejected because of an invalid nonce,
   the Reply always provides the mobile node with a new nonce to be used
   in the next registration.  Thus, the nonce protocol is self-
   synchronizing.

6. IANA Considerations

Mobile IP specifies several new number spaces for values to be used in various message fields. These number spaces include the following: o Mobile IP message types sent to UDP port 434, as defined in Section 1.8. o types of extensions to Registration Request and Registration Reply messages (see Sections 3.3 and 3.4, and also consult [12], [43], [2], [3], and [7]). o values for the code in the Registration Reply message (see Section 3.4, and also consult [12], [43], [2], [3], and [7]). o Mobile IP defines so-called Agent Solicitation and Agent Advertisement messages. These messages are in fact Router Discovery messages [5] augmented with Mobile-IP-specific extensions. Thus, they do not define a new name space, but do define additional Router Discovery extensions as described below in Section 6.2. Also see Section 2.1, and consult [3] and [7]. There are additional Mobile IP numbering spaces specified in [3]. Information about assignment of Mobile IP numbers derived from specifications external to this document is given by IANA at http://www.iana.org/protocols. From that URL, see the "Mobile Internet Protocol (IP) Numbers" section. In this revised specification, a new code value (for the field in the Registration Reply message) is needed within the range typically used for foreign agent messages. This error code is needed to indicate the status "Invalid Home Agent Address". See Section 3.7.2 for details.

6.1. Mobile IP Message Types

Mobile IP messages are defined to be those that are sent to a message recipient at port 434 (UDP or TCP). The number space for Mobile IP messages is specified in Section 1.8. Approval of new extension
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   numbers is subject to Expert Review, and a specification is required
   [22].  The currently standardized message types have the following
   numbers, and are specified in the indicated sections.

     Type  Name                                             Section
     ----  --------------------------------------------     ---------
     1     Registration Request                             3.3
     3     Registration Reply                               3.4

6.2. Extensions to RFC 1256 Router Advertisement

RFC 1256 defines two ICMP message types, Router Advertisement and Router Solicitation. Mobile IP defines a number space for extensions to Router Advertisement, which could be used by protocols other than Mobile IP. The extension types currently standardized for use with Mobile IP have the following numbers. Type Name Section ---- -------------------------------------------- --------- 0 One-byte Padding 2.1.3 16 Mobility Agent Advertisement 2.1.1 19 Prefix-Lengths 2.1.2 Approval of new extension numbers for use with Mobile IP is subject to Expert Review, and a specification is required [22].

6.3. Extensions to Mobile IP Registration Messages

The Mobile IP messages specified within this document and listed in Sections 1.8 and 6.1 may have extensions. Mobile IP message extensions all share the same number space, even if they are to be applied to different Mobile IP messages. The number space for Mobile IP message extensions is specified within this document. Approval of new extension numbers is subject to Expert Review, and a specification is required [22]. Type Name Section ---- -------------------------------------------- --------- 0 One-byte Padding 32 Mobile-Home Authentication 3.5.2 33 Mobile-Foreign Authentication 3.5.3 34 Foreign-Home Authentication 3.5.4
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6.4. Code Values for Mobile IP Registration Reply Messages

The Mobile IP Registration Reply message, specified in Section 3.4, has a Code field. The number space for the Code field values is also specified in Section 3.4. The Code number space is structured according to whether the registration was successful, the foreign agent denied the Registration Request, or the home agent denied the Registration Request, as follows: +---------+------------------------------------------------------+ | Code #s | Guideline | +---------+------------------------------------------------------+ | 0-8 | Success Codes | | | | | 9-63 | Allocation guidelines not specified in this document | | | | | 64-127 | Error Codes from the Foreign Agent | | | | | 128-192 | Error Codes from the Home Agent | | | | | 193-200 | Error Codes from the Gateway Foreign Agent [29] | | | | | 201-255 | Allocation guidelines not specified in this document | +---------+------------------------------------------------------+ Approval of new code values requires Expert Review [22]. Table 1: Guidelines for Allocation of Code Values

7. Acknowledgments

Special thanks to Steve Deering (Xerox PARC), along with Dan Duchamp and John Ioannidis (JI) (Columbia University), for forming the working group, chairing it, and putting so much effort into its early development. Columbia's early Mobile IP work can be found in [37], [38], [39]. Thanks also to Kannan Alaggapan, Greg Minshall, Tony Li, Jim Solomon, Erik Nordmark, Basavaraj Patil, and Phil Roberts for their contributions to the group while performing the duties of chairperson, as well as for their many useful comments. Thanks to the active members of the Mobile IP Working Group, particularly those who contributed text, including (in alphabetical order)
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      Ran Atkinson (Naval Research Lab)
      Samita Chakrabarti (Sun Microsystems)
      Ken Imboden (Candlestick Networks, Inc.)
      Dave Johnson (Carnegie Mellon University)
      Frank Kastenholz (FTP Software)
      Anders Klemets (KTH)
      Chip Maguire (KTH)
      Alison Mankin (ISI)
      Andrew Myles (Macquarie University)
      Thomas Narten (IBM)
      Al Quirt (Bell Northern Research)
      Yakov Rekhter (IBM)
      Fumio Teraoka (Sony)
      Alper Yegin (NTT DoCoMo)

   Thanks to Charlie Kunzinger and to Bill Simpson, the editors who
   produced the first drafts of this document, reflecting the
   discussions of the working group.  Much of the new text in the later
   revisions preceding RFC 2002 is due to Jim Solomon and Dave Johnson.

   Thanks to Greg Minshall (Novell), Phil Karn (Qualcomm), Frank
   Kastenholz (FTP Software), and Pat Calhoun (Sun Microsystems) for
   their generous support in hosting interim working group meetings.

   Sections 1.10 and 1.11, which specify new extension formats to be
   used with aggregatable extension types, were included from a
   specification document (entitled "Mobile IP Extensions
   Rationalization (MIER)", which was written by

      Mohamed Khalil (Nortel Networks)
      Raja Narayanan (nVisible Networks)
      Haseeb Akhtar (Nortel Networks)
      Emad Qaddoura (Nortel Networks)

   Thanks to these authors, and also for the additional work on MIER,
   which was contributed by Basavaraj Patil, Pat Calhoun, Neil
   Justusson, N. Asokan, and Jouni Malinen.

   Thanks to Vijay Devarapalli, who put in many hours to convert the
   source for this text document into XML format.
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8. References

8.1. Normative References

[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] Calhoun, P. and C. Perkins, "Mobile IP Network Access Identifier Extension for IPv4", RFC 2794, March 2000. [3] Perkins, C., Calhoun, P., and J. Bharatia, "Mobile IPv4 Challenge/Response Extensions (Revised)", RFC 4721, January 2007. [4] Cong, D., Hamlen, M., and C. Perkins, "The Definitions of Managed Objects for IP Mobility Support using SMIv2", RFC 2006, October 1996. [5] Deering, S., Ed., "ICMP Router Discovery Messages", RFC 1256, September 1991. [6] Deering, S., "Host extensions for IP multicasting", STD 5, RFC 1112, August 1989. [7] Dommety, G. and K. Leung, "Mobile IP Vendor/Organization- Specific Extensions", RFC 3115, April 2001. [8] Eastlake 3rd, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005. [9] Kent, S., "IP Authentication Header", RFC 4302, December 2005. [10] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997. [11] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, "Network Time Protocol Version 4: Protocol and Algorithms Specification", RFC 5905, June 2010. [12] Montenegro, G., Ed., "Reverse Tunneling for Mobile IP, revised", RFC 3024, January 2001. [13] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina, "Generic Routing Encapsulation (GRE)", RFC 2784, March 2000. [14] Perkins, C., "IP Encapsulation within IP", RFC 2003, October 1996.
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   [15]  Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
         October 1996.

   [16]  Plummer, D., "Ethernet Address Resolution Protocol: Or
         Converting Network Protocol Addresses to 48.bit Ethernet
         Address for Transmission on Ethernet Hardware", STD 37, RFC
         826, November 1982.

   [17]  Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
         1980.

   [18]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
         1981.

   [19]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April
         1992.

   [20]  Solomon, J., "Applicability Statement for IP Mobility Support",
         RFC 2005, October 1996.

   [21]  Perkins, C., Ed., "IP Mobility Support for IPv4", RFC 3344,
         August 2002.

   [22]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.

8.2. Informative References

[23] Solomon, J. and S. Glass, "Mobile-IPv4 Configuration Option for PPP IPCP", RFC 2290, February 1998. [24] Montenegro, G., Dawkins, S., Kojo, M., Magret, V., and N. Vaidya, "Long Thin Networks", RFC 2757, January 2000. [25] Allman, M., Glover, D., and L. Sanchez, "Enhancing TCP Over Satellite Channels using Standard Mechanisms", BCP 28, RFC 2488, January 1999. [26] Paxson, V. and M. Allman, "Computing TCP's Retransmission Timer", RFC 2988, November 2000. [27] Levkowetz, H. and S. Vaarala, "Mobile IP Traversal of Network Address Translation (NAT) Devices", RFC 3519, April 2003. [28] Glass, S. and M. Chandra, "Registration Revocation in Mobile IPv4", RFC 3543, August 2003.
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   [29]  Fogelstroem, E., Jonsson, A., and C. Perkins, "Mobile IPv4
         Regional Registration", RFC 4857, June 2007.

   [30]  Bellovin, S., "Security Problems in the TCP/IP Protocol Suite",
         ACM Computer Communications Review, 19(2), March 1989.

   [31]  Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
         Shelby, "Performance Enhancing Proxies Intended to Mitigate
         Link-Related Degradations", RFC 3135, June 2001.

   [32]  Caceres, R. and L. Iftode, "Improving the Performance of
         Reliable Transport Protocols in Mobile Computing Environments",
         IEEE Journal on Selected Areas in Communication, 13(5):850-857,
         June 1995.

   [33]  Dawkins, S., Montenegro, G., Kojo, M., Magret, V., and N.
         Vaidya, "End-to-end Performance Implications of Links with
         Errors", BCP 50, RFC 3155, August 2001.

   [34]  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
         March 1997.

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

   [36]  Jacobson, V., "Compressing TCP/IP Headers for Low-Speed Serial
         Links", RFC 1144, February 1990.

   [37]  Ioannidis, J., Duchamp, D., and G. Maguire, "IP-Based Protocols
         for Mobile Internetworking", In Proceedings of the SIGCOMM '01
         Conference: Communications Architectures and Protocols, pages
         235-245, September 1991.

   [38]  Ioannidis, J. and G. Maguire, "The Design and Implementation of
         a Mobile Internetworking Architecture", In Proceedings of the
         Winter USENIX Technical Conference, pages 489-500, January
         1993.

   [39]  Ioannidis, J., "Protocols for Mobile Internetworking", PhD
         Dissertation - Columbia University in the City of New York,
         July 1993.

   [40]  Jacobson, V., "Congestion Avoidance and Control", In
         Proceedings of the SIGCOMM '88 Workshop, ACM SIGCOMM, ACM
         Press, pages 314-329, August 1998.
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   [41]  McCloghrie, K. and F. Kastenholz, "The Interfaces Group MIB",
         RFC 2863, June 2000.

   [42]  McGregor, G., "The PPP Internet Protocol Control Protocol
         (IPCP)", RFC 1332, May 1992.

   [43]  Montenegro, G. and V. Gupta, "Sun's SKIP Firewall Traversal for
         Mobile IP", RFC 2356, June 1998.

   [44]  Perkins, C., Ed., "IP Mobility Support", RFC 2002, October
         1996.

   [45]  Stevens, R., "TCP/IP Illustrated, Volume 1: The Protocols",
         Addison-Wesley, Reading, Massachusetts, 1994.

   [46]  Perkins, C. and P. Calhoun, "Authentication, Authorization, and
         Accounting (AAA) Registration Keys for Mobile IPv4", RFC 3957,
         March 2005.

   [47]  Simpson, W., Ed., "The Point-to-Point Protocol (PPP)", STD 51,
         RFC 1661, July 1994.

   [48]  IANA, "Mobile IPv4 Numbers", http://www.iana.org.

   [49]  Postel, J., "Multi-LAN address resolution", RFC 925, October
         1984.

   [50]  Perkins, C., Ed., "IP Mobility Support for IPv4", RFC 3220,
         January 2002.
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Appendix A. Link-Layer Considerations

The mobile node MAY use link-layer mechanisms to decide that its point of attachment has changed. Such indications include the Down/ Testing/Up interface status [41], and changes in cell or administration. The mechanisms will be specific to the particular link-layer technology, and are outside the scope of this document. The Point-to-Point-Protocol (PPP) [47] and its Internet Protocol Control Protocol (IPCP) [42] negotiate the use of IP addresses. The mobile node SHOULD first attempt to specify its home address, so that if the mobile node is attaching to its home network, the unrouted link will function correctly. When the home address is not accepted by the peer, but a transient IP address is dynamically assigned to the mobile node, and the mobile node is capable of supporting a co-located care-of address, the mobile node MAY register that address as a co-located care-of address. When the peer specifies its own IP address, that address MUST NOT be assumed to be a foreign agent care-of address or the IP address of a home agent. PPP extensions for Mobile IP have been specified in RFC 2290 [23]. Please consult that document for additional details for how to handle care-of address assignment from PPP in a more efficient manner.

Appendix B. TCP Considerations

B.1. TCP Timers

When high-delay (e.g., SATCOM) or low-bandwidth (e.g., High-Frequency Radio) links are in use, some TCP stacks may have insufficiently adaptive (non-standard) retransmission timeouts. There may be spurious retransmission timeouts, even when the link and network are actually operating properly, but just with a high delay because of the medium in use. This can cause an inability to create or maintain TCP connections over such links, and can also cause unneeded retransmissions that consume already scarce bandwidth. Vendors are encouraged to follow the algorithms in RFC 2988 [26] when implementing TCP retransmission timers. Vendors of systems designed for low-bandwidth, high-delay links should consult RFCs 2757 and 2488 [24], [25]. Designers of applications targeted to operate on mobile nodes should be sensitive to the possibility of timer-related difficulties.
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B.2. TCP Congestion Management

Mobile nodes often use media that are more likely to introduce errors, effectively causing more packets to be dropped. This introduces a conflict with the mechanisms for congestion management found in modern versions of TCP [40]. Now, when a packet is dropped, the correspondent node's TCP implementation is likely to react as if there were a source of network congestion, and initiate the slow- start mechanisms [40] designed for controlling that problem. However, those mechanisms are inappropriate for overcoming errors introduced by the links themselves, and have the effect of magnifying the discontinuity introduced by the dropped packet. This problem has been analyzed by Caceres, et al. [32]. TCP approaches to the problem of handling errors that might interfere with congestion management are discussed in documents from the PILC working group [31] [33]. While such approaches are beyond the scope of this document, they illustrate that providing performance transparency to mobile nodes involves understanding mechanisms outside the network layer. Problems introduced by higher media error rates also indicate the need to avoid designs that systematically drop packets; such designs might otherwise be considered favorably when making engineering tradeoffs.
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Appendix C. Example Scenarios

This section shows example Registration Requests for several common scenarios.

C.1. Registering with a Foreign Agent Care-of Address

The mobile node receives an Agent Advertisement from a foreign agent and wishes to register with that agent using the advertised foreign agent care-of address. The mobile node wishes only IP-in-IP encapsulation, does not want broadcasts, and does not want simultaneous mobility bindings: IP fields: Source Address = mobile node's home address Destination Address = copied from the IP source address of the Agent Advertisement Time to Live = 1 UDP fields: Source Port = <any> Destination Port = 434 Registration Request fields: Type = 1 S=0,B=0,D=0,M=0,G=0 Lifetime = the Registration Lifetime copied from the Mobility Agent Advertisement Extension of the Router Advertisement message Home Address = the mobile node's home address Home Agent = IP address of mobile node's home agent Care-of Address = the Care-of Address copied from the Mobility Agent Advertisement Extension of the Router Advertisement message Identification = Network Time Protocol timestamp or Nonce Extensions: An authorization-enabling extension (e.g., the Mobile-Home Authentication Extension)
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C.2. Registering with a Co-Located Care-of Address

The mobile node enters a foreign network that contains no foreign agents. The mobile node obtains an address from a DHCP server [34] for use as a co-located care-of address. The mobile node supports all forms of encapsulation (IP-in-IP, minimal encapsulation, and GRE), desires a copy of broadcast datagrams on the home network, and does not want simultaneous mobility bindings: IP fields: Source Address = care-of address obtained from DHCP server Destination Address = IP address of home agent Time to Live = 64 UDP fields: Source Port = <any> Destination Port = 434 Registration Request fields: Type = 1 S=0,B=1,D=1,M=1,G=1 Lifetime = 1800 (seconds) Home Address = the mobile node's home address Home Agent = IP address of mobile node's home agent Care-of Address = care-of address obtained from DHCP server Identification = Network Time Protocol timestamp or Nonce Extensions: The Mobile-Home Authentication Extension
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C.3. Deregistration

The mobile node returns home and wishes to deregister all care-of addresses with its home agent: IP fields: Source Address = mobile node's home address Destination Address = IP address of home agent Time to Live = 1 UDP fields: Source Port = <any> Destination Port = 434 Registration Request fields: Type = 1 S=0,B=0,D=0,M=0,G=0 Lifetime = 0 Home Address = the mobile node's home address Home Agent = IP address of mobile node's home agent Care-of Address = the mobile node's home address Identification = Network Time Protocol timestamp or Nonce Extensions: An authorization-enabling extension (e.g., the Mobile-Home Authentication Extension)

Appendix D. Applicability of Prefix-Lengths Extension

Caution is indicated with the use of the Prefix-Lengths Extension over wireless links, due to the irregular coverage areas provided by wireless transmitters. As a result, it is possible that two foreign agents advertising the same prefix might indeed provide different connectivity to prospective mobile nodes. The Prefix-Lengths Extension SHOULD NOT be included in the advertisements sent by agents in such a configuration. Foreign agents using different wireless interfaces would have to cooperate using special protocols to provide identical coverage in space, and thus be able to claim to have wireless interfaces situated on the same subnetwork. In the case of wired interfaces, a mobile node disconnecting and subsequently connecting to a new point of attachment may well send in a new Registration Request no matter whether the new advertisement is on the same medium as the last recorded advertisement. And, finally, in areas with dense populations of foreign agents it would seem unwise to require the propagation via routing protocols of the subnet prefixes associated with each individual wireless foreign agent; such a strategy could lead to quick depletion of available space for routing tables,
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   unwarranted increases in the time required for processing routing
   updates, and longer decision times for route selection if routes
   (which are almost always unnecessary) are stored for wireless
   "subnets".

Appendix E. Interoperability Considerations

This document specifies revisions to RFC 2002 that are intended to improve interoperability by resolving ambiguities contained in the earlier text. Implementations that perform authentication according to the new more precisely specified algorithm would be interoperable with earlier implementations that did what was originally expected for producing authentication data. That was a major source of non- interoperability before. However, this specification does have new features that, if used, would cause interoperability problems with older implementations. All features specified in RFC 2002 will work with the new implementations, except for V-J compression [36]. The following list details some of the possible areas of compatibility problems that may be experienced by nodes conforming to this revised specification, when attempting to interoperate with nodes obeying RFC 2002. o A client that expects some of the newly mandatory features (like reverse tunneling) from a foreign agent (FA) would still be interoperable as long as it pays attention to the 'T' bit. o Mobile nodes (MNs) that use the NAI extension to identify themselves would not work with old mobility agents. o Mobile nodes that use a zero home address and expect to receive their home address in the Registration Reply would not work with old mobility agents. o Mobile nodes that attempt to authenticate themselves without using the Mobile-Home authentication extension will be unable to successfully register with their home agent. In all of these cases, a robust and well-configured mobile node is very likely to be able to recover if it takes reasonable actions upon receipt of a Registration Reply with an error code indicating the cause for rejection. For instance, if a mobile node sends a Registration Request that is rejected because it contains the wrong kind of authentication extension, then the mobile node could retry the registration with a mobile-home authentication extension, since the foreign agent and/or home agent in this case will not be configured to demand the alternative authentication data.
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Appendix F. Changes since RFC 3344

The following revisions to details of the specification in this document were made after RFC 3344 was published. A list of changes from RFC 2002 made during the development of RFC 3344 [21] may be found in the latter document. For items marked with issue numbers, more information is available by consulting the MIP4 mailing list archives. o Showed more bit definitions in the Agent Advertisement message structure (see Section 2.1.1). New advertisement bits have been defined by other specification documents, but not reflected in previous publications of this specification; this has led to confusion. Citations for the other specification documents have also been included. o (Issue 6) The behavior of the home agent was changed to avoid mandating error replies to Registration Requests that were invalidated because the foreign agent failed authentication. The intention is to make the home agent more robust against Denial of Service attacks in which the malicious device has no intention of providing a valid Registration Request but only wants to congest traffic on the home network. See Section 3.8.2.1. o Due to non-unique assignment of IPv4 addresses in many domains, it is possible for different mobile nodes to have the same home address. If they use the NAI, the foreign agent can still distinguish them. Language was added to Section 3.7.1 and Section 3.7.3.1 to specify that the foreign agent MUST use the NAI to distinguish mobile nodes with the same home address. o (Issue 45) Specified that a foreign agent MUST NOT apply a Foreign-Home Authentication extension to a mobile node's deregistration request. Also, the foreign agent MUST NOT apply a Foreign-Home Authentication extension unless the Care-of Address in the Registration Request matches an address advertised by the foreign agent. o Specified that the Mobility Security Association to be used by the foreign agent and home agent depends upon values contained in the message data, not the IP headers. o (Issues 9, 18) Created a new error code for use by the foreign agent, for the case when the foreign agent does not serve the mobile node as a home agent. Formerly, the foreign agent could use an error Code of 136 for this case.
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   o  (Issue 17) Specified that, if the home agent cannot support the
      requested nonzero unicast address in the Home Address field of the
      Registration Request, then it MUST reject the registration with an
      error Code of 129.  See Section 3.8.3.2.

   o  (Issue 19) Specified that multiple authorization-enabling
      extensions may be present in the Registration Request message, but
      that the home agent has to (somehow) ensure that all have been
      checked (see Section 3.8.3.1).

   o  (Issue 20) Specified that the foreign agent SHOULD NOT modify any
      of the fields of the Registration Reply message that are covered
      by the Mobile-Home Authentication Extension, when it relays the
      packet to the mobile node.

   o  (Issue 21) Clarified that the foreign agent removes extensions
      that do not precede any authorization-enabling extension, not just
      the Mobile-Home Authentication extension (Section 3.7.3.2).

   o  (Issue 44) Specified that the address advertised by the foreign
      agent in Agent Advertisements is the care-of address offered on
      that network interface, not necessarily the address of the network
      interface (Section 3.7.2.2).

   o  (Issue 45) Clarification in Section 3.7.2.1 that Code 77 can only
      apply to a Registration Request with nonzero Lifetime.

   o  Created a new error code for use when a foreign agent can detect
      that the Home Agent address field is incorrect.

   o  Prohibited the use of the Foreign-Home Authorization Extension on
      deregistration messages.

   o  Cleaned up some more wording having to do with authorization-
      enabling extensions.

   o  For consistency, changed some wording about copying UDP ports.

   o  Added wording to clearly not disallow dynamically configuring
      netmask and security information at the mobile node.

   o  Revamped Changes section.

   o  Updated citations.
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Appendix G. Example Messages

G.1. Example ICMP Agent Advertisement Message Format

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Code | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Num Addrs |Addr Entry Size| Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Router Address[1] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Preference Level[1] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Router Address[2] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Preference Level[2] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | .... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 16 | Length | Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Registration Lifetime |R|B|H|F|M|G|r|T|U|X|I|reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Care-of Address[1] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Care-of Address[2] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | .... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : Optional Extensions : : .... ...... ...... : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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G.2. Example Registration Request Message Format

The UDP header is followed by the Mobile IP fields shown below: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 1 |S|B|D|M|G|r|T|x| Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Home Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Home Agent | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Care-of Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + Identification + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Optional Non-Auth Extensions for HA ... | | ( variable length ) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 32 | Length | SPI | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SPI (cont.) | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | : MN-HA Authenticator ( variable length ) : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : Optional Non-Auth Extensions for FA ......... : Optional MN-FA Authentication Extension... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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G.3. Example Registration Reply Message Format

The UDP header is followed by the Mobile IP fields shown below: 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 3 | Code | Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Home Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Home Agent | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + Identification + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Optional HA Non-Auth Extensions ... | | ( variable length ) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 32 | Length | SPI | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SPI (cont.) | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | : MN-HA Authenticator ( variable length ) : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : Optional Extensions used by FA......... : Optional MN-FA Authentication Extension... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Author's Address

Charles E. Perkins (editor) WiChorus Inc. 3590 N. 1st Street, Suite 300 San Jose, CA 95134 USA EMail: charliep@computer.org