INTERNET DRAFT                                             A. C. Snoeren
Expires May 2001                                                 MIT LCS
                                                         H. Balakrishnan
                                                                 MIT LCS
                                                           November 2000

                        TCP Connection Migration

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-

   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

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   Distribution of this memo is unlimited.


   This document describes a set of TCP options to support the migration
   of an active TCP connection across IP addresses and TCP port numbers.
   Using this option, a TCP peer can open a migrateable connection,
   transfer one or more bytes on it, and continue the connection from
   another IP address/TCP port pair in an application-transparent
   fashion.  The set of addresses or ports from where a connection might
   be continued need not be known in advance.  Security against
   connection hijacking is achieved using a secret cryptographic cookie
   negotiated through an Elliptic Curve Diffie-Hellman [ANSI-X962]
   exchange during connection establishment.  The initiation of
   migration can be done either by one of the communicating peers, or by
   a trusted third-party that presents the negotiated cryptographic

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   TCP connection migration has several applications, including
   preserving TCP communication across host mobility in the Internet,
   and inter-host migration of connections for fault-tolerance or load-
   balancing.  Furthermore, unlike previous approaches such as ETCP
   [Huitema95], the Migrate options require no modification to the TCP
   header, packet format, or semantics, and can be initiated by a
   trusted node different from the communicating peers.


   A TCP connection [RFC-793] is associated with a particular IP address
   and TCP port pair.  When one of the communicating peers changes its
   IP address, all TCP on-going connections are terminated.  This
   document describes a set of TCP options to migrate connections from
   one IP address to another, thereby permitting on-the-fly re-
   addressing of end-hosts.  In-progress connections may also change
   port numbers, enabling them to function correctly across legacy
   Network Address Translators (NATs) and Port Address Translators
   (PATs) [RFC-2663].

   Although TCP connections today are bound to particular IP addresses,
   most TCP applications communicate with end-hosts or end-services.  An
   end-host (e.g., a desktop or mobile computer) may have multiple
   network interfaces on it, or have one interface whose address changes
   due to mobility, while an end-service (e.g., a Web server) may be
   replicated on several different computers on the Internet.  Most TCP
   applications open a connection to a name, which gets resolved by the
   Domain Name System (DNS) to an IP address.  If the IP address
   associated with the name were to change for any reason, connections
   in progress cannot continue.  The TCP migration options are motivated
   by the desire for a TCP that can work across changes in name-to-
   address mappings, e.g., due to host mobility, or server-to-server
   migration of one end-point of a connection for fail-over or load-
   balancing reasons.

   The Migrate options allow hosts to divert established TCP connections
   to a new IP address/TCP port pair by referencing them with a
   cryptographic cookie, which protects against connection hijacking by
   malicious parties.  This cookie is established using an Elliptic
   Curve Diffie-Hellman key exchange during the establishment of the
   initial connection [ANSI-X962].  The security provided by this
   cryptographic cookie also allows a trusted host other than the
   communicating peers to take over a connection.  Any host with
   knowledge of the cookie can request migration of the connection,
   allowing servers in a cluster to continue servicing a client of a
   failed server.

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Document Conventions

   The terms "migrating host" and "mobile host" are used to refer to the
   TCP peer that is being re-addressed.  The connection, however, is
   completely symmetric.  Either (or both) peer(s) may move after the
   connection is established, but not at the same time.  During a
   migration by the migrating host, the peer TCP host is referred to as
   the fixed host.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC-2119].


   Mobile IP [Perkins] supports re-addressing and could be used in this
   context, but has two major drawbacks.  First, all traffic to a mobile
   host must (at least initially) travel via the home agent.  The
   necessity of a home agent can be a significant burden, especially in
   ad-hoc networks of devices.  Additionally, depending on the network
   topology, packets routed through the home agent may travel a
   significantly longer path than necessary.

   Secondly, packets sent by the mobile host use the mobile's home
   address as the source address.  It is often the case that this source
   address does not topologically belong to the subnet that the mobile
   is currently attached to.  Current firewalls and secure routers often
   block such traffic to prevent IP spoofing.  As an alternative,
   packets may be "reverse tunnelled" back to the home agent before
   being sent to the destination using the host's home IP address.
   Unfortunately, this imposes the "trangle routing" penalty described
   above on packets leaving the mobile host as well.

   More importantly, however, Mobile IP requires that the original
   (home) address continue to be allocated to the mobile host.  In cases
   when the available address space is small, such a restriction may
   make re-addressing prohibitive.  One would like the mobile node's
   previous address to be immediately reusable by another host.

   The widespread adoption of the incrementally-deployable migration
   options described in this document will obviate the need for network-
   layer approaches like Mobile IP to handle host mobility for TCP
   applications.  Furthermore, many mobile computers use multiple
   network interfaces; selecting between them based on the available
   connectivity and network performance on a per-connection basis is
   supported by connection migration.

   The TCP migrate option avoids these drawbacks, but functions only for

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   TCP.  UDP-based protocols typically fall into two classes: short
   transactions and longer streams.  Transactional protocols, such as
   DNS and Remote Procedure Call (RPC) protocols are typically mostly
   stateless and involve at most a small number of packets.  Although
   the server in such protocols might move, locating it is a matter of
   determining the most current name-to-address binding, which can be
   done using dynamic DNS [RFC-2136].  It is, of course, possible for
   the requesting host to have moved concurrently, but in a network
   where the probability of this occurring is significantly smaller than
   the probability of multiple packet losses (as is typically expected
   in today's Internet), an application-level retry (consisting of a DNS
   lookup) of the form used for loss recovery should suffice.

   UDP-based unicast streaming applications, on the other hand, tend to
   make use of an application-level protocol like RTP [RFC-1889] to
   implement a connection-like abstraction.  Although this document does
   not describe any details, we expect that a connection migration
   scheme for such protocols to be easy to develop, similar to the one
   for TCP proposed herein.  Alternatively, the Transport Area Working
   Group has recently proposed the Stream Control Transport Protocol
   (SCTP) [SCTP], which provides similar "multi-homing" functionality,
   although the semantics provided by the current draft are not as
   amenable to address changes.

   Extending the lives of TCP connections across address changes is not
   a new idea.  A proposal based on a "context identifier" (essentially
   equivalent to our cryptographic cookie) was proposed by Huitema
   [Huitema95] and Steve Deering.  However, these identifiers were
   passed in the clear, so any eavesdropper could hijack the connection
   quite easily.

   Huitema's "Multi-homed TCP" allowed multiple concurrent addresses on
   both ends of the connection, but required a new protocol, extended
   TCP (ETCP) to be deployed on the end hosts [Huitema95].  While our
   proposal limits the connection to one active address per peer at a
   time, it requires significantly less modification to the TCP stack,
   and is compatible with many protocol-based firewalls, filters, and
   such "middle boxes" that are a reality of today's Internet

   Alternative techniques, aimed at separating routing identifiers
   (e.g., IP addresses) from endpoint identification (e.g., permanent
   endpoint identifiers, often called EIDs), have been proposed.  We
   believe that such proposals are worthy of research, but that they are
   likely to incur greater deployment costs.  The approach proposed in
   this document is rather different, since it does not require
   additional level of global addressing or indirection, but only a
   (normally pre-existing) DNS entry for endpoint identification.

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   Furthermore, any technique based upon "permanent identifiers" only
   pushes the problem one level higher, and awaits the time when those
   too need to be re-assigned.

Basic Approach

   The Migrate Options facilitate rebinding TCP connections to a new
   address/port pair that need not be known a priori.  This differs for
   the "multi-homing" capability found in SCTP [SCTP], which requires
   the set of approved addresses for the connection to be negotiated in
   advance.  Further, standard TCP connection semantics are preserved
   for transparent operation in the presence of legacy network
   infrastructure such as NATs or PEPs.  Finally, this is achieved with
   signalling sent only on connection establishment--individual data
   segments have no additional overhead.

   Fundamentally, the Migrate options allows corresponding hosts to
   synchronize two separate TCP connections such that the context is
   identical.  In particular, establishment of the second connection
   terminates the first, and transfers the original TCB to the second
   connection, except for the IP address/port binding, and congestion
   control state.  This allows TCP (and any of its attending options) to
   function properly across both connections, regardless of its current

   By establishing a completely separate connection to continue
   communication, the correspondent hosts may conspire to support
   connection migration without the network's knowledge or consent.
   Both connections appear to be standard TCP connections (except for
   the new options passed in the SYN packets), so stateful firewalls,
   NATs, PATs, and the like will not interfere with proper operation.

   The migration process consists of two parts: an initial key exchange
   conducted during the three-way handshake, and a migration request.
   In order for a connection to by migrateable, the correspondent hosts
   must first exchange Migrate-Permitted Options during the SYN
   handshake.  This negotiates a shared connection token, which is then
   used to identify the connection in later Migrate Requests (as opposed
   to the IP address/port pair, which may change).

   At any point after the initial SYN handshake (while the connection is
   in either ESTABLISHED or FIN-WAIT) either host may attempt to
   establish a second connection (from a new address/port pair)
   containing a Migrate Request option.  This Migrate Request option
   contains the token referencing the previously established connection.
   If the request meets various security requirements described below,
   the correspondent host terminates the first connection (without

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   emitting any further packets) and instantiates the second connection
   with a TCB identical to the first.  The new connection is associated
   with the same socket (application) as the first.  In practice, this
   is often implemented by simply modifying the TCB of the first
   connection rather than allocating an entirely new one.

Migrate-Permitted Option

   This variable-length option SHOULD be sent in SYN segments by a TCP
   that has been extended to support the Migrate options.  It MUST NOT
   be sent on non-SYN segments.

    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
   |    Kind: #    | Length = 3/20 |    Curve ID   |    Key Data   |
   |                        Key Data (cont.)                       |
   |                        Key Data (cont.)                       |
   |                        Key Data (cont.)                       |
   |                        Key Data (cont.)                       |

                    TCP Migrate-Permitted Option Format

   The Migrate-Permitted option has two varieties.  It may either
   specify the non-secured variant of the Migrate Options (NOT
   RECOMMENDED) or the secure version.  The non-secure version has
   length 3, and contains a zero in the Curve ID field.

   The secured version has length 20 and contains two fields, a one byte
   Curve ID field and 17 bytes of Key data.  The Curve ID contains one
   of a predefined set of values to select a particular set of Elliptic
   Curve parameters.  The Key Data contains the most significant 17
   bytes of the 25 bytes of Key Data used to compute the secret
   connection token.

   In order to support key lengths up to 200 bits, the remaining 8 bytes
   of Key Data are transmitted in the Timestamp option [RFC-1323], which
   MUST be included on any SYN packet containing a non-secure variant of
   the Migrate-Permitted Option.  The least significant 4 bytes are
   transmitted in the Time Stamp Echo Reply (TSecr) field, and the
   remaining 4 bytes are transmitted in the Time Stamp Value (TSval)

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Migrate Option

   The Migrate option is used to request the migration of a currently
   open TCP connection to a new address.  A variable-length option, it
   may only be sent in a SYN segment to a host with which a previously-
   established connection already exists (in the ESTABLISHED or FIN-WAIT
   states), over which the Migrate-Permitted option has been negotiated.
   It MUST NOT be sent in non-SYN segments.

    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
   |///////////////|    Kind: #    |  Length = 19  |P|    ReqNo    |
   |                             Token                             |
   |                          Token (cont.)                        |
   |                            Request                            |
   |                         Request (cont.)                       |

                         TCP Migrate Option Format

   A 19-byte option, the Migrate Option includes four fields.  There are
   two 64-bit fields in a Migrate option: a Token, and a Request.  In
   addition, there is a 7-bit sequence number field, ReqNo, which must
   be monotonically increasing with each new migrate request issued by
   an end host for a connection, and a 1 bit Preload flag field.  The
   Preload flag SHOULD be zero, unless the Migrate Option is sent by a
   host other than one of the original two corresponding hosts.

Migrate-Permitted Processing

   A host wishing to establish a migrateable connection MUST include a
   Migrate-Permitted option in the initial SYN segment.  For the
   insecure variant, the length MUST be 3, and the Curve ID field set to
   zero.  In the secure variant, the Curve ID must contain a defined
   value specifying a particular set of Elliptic Curve domain
   parameters.  In particular, each set of domain parameters contains a
   finite field, F, a generating point P, and the order of P, n.  The
   Key Data field MUST contain a randomly generated public key, k,
   generated over the field F by selecting a random number X in the
   range [1,n-1] and computing

                                k = X * P,

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   where the * operation is the scalar multiplication operation over the
   field F.  The securing of the connection hinges on the secrecy of the
   negotiated key, hence X should be randomly generated and stored in
   the control block for each new connection.  Any retransmissions of
   the SYN packet MUST contain the same values in the Migrate-Permitted

   Upon receipt of an initial SYN (without an ACK) with a Migrate-
   Permitted Option, a compliant TCP MUST include a Migrate-Permitted
   Option in its SYN/ACK segment or send a RST.  If a SYN/ACK is sent,
   the length and Curve ID MUST be identical to those in the received
   SYN segment.  If the secure variant is transmitted, it must be
   transmitted along with a Timestamp option, and contain a random
   public key selected as described above.

   If the implementation does not support the domain parameters
   corresponding to the Curve ID in the received option, it MUST NOT
   include a secure Migrate-Permitted option in its SYN/ACK.

   Any necessary retransmissions of SYN or SYN/ACKs MUST include
   identical values for the Migrate-Permitted option, if present.  A
   compliant host receiving a SYN/ACK with the Migrate-Permitted option
   set MUST compare the Curve ID.  If they differ, a RST MUST be sent
   and the connection aborted as in [RFC-793].

   If both acceptable, secure-variant Migrate-Permitted Options are
   received by both hosts, they MUST compute a shared secret key, K,
   using their random value, X, and received public key, k,

                                K = k * X,

   where * is the same operation as before.  If the indicated domain
   parameters result in a key of length less than 200 bits, the value
   MUST be left padded with zeros to 200 bits.  If the unsecured variant
   is negotiated, K MUST be set to all zeros.  The secret key is then
   used to compute a connection validation token T, by computing the
   SHA-1 hash of the initial sequence numbers of the SYN (Ni) and
   SYN/ACK (Na) packets as:
                           T = SHA-1(Ni, Na, K)

   SHA-1 produces a 160-bit hash, of which only the 64 most significant
   bits are retained, and stored in the connections TCB.

Timestamp Option Processing

   As stated above, the TSecr and TSval fields of a Timestamp option
   sent in the same SYN segment as a Migrate-Permitted option MUST
   contain the least significant 8 bytes of the Migrate-Permitted Key

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   Data field.  Hence these values MUST NOT be used in either of the
   algorithms specified in [RFC-1323].  We note that both Protection
   Against Wrapped Sequence Numbers (PAWS) and the Round-Trip Time
   Measurement (RTTM) are performed only on Synchronized connections,
   hence a compliant TCP stack should not operate on these values in a
   SYN segment in any event.

   The ramifications of the Migrate-Permitted option's interaction with
   the Timestamp option are two-fold.  First, a Migrate-compliant host
   MOST NOT echo the value contained in the TSval from the SYN segment
   in the Timestamp option of its SYN/ACK.  Instead, it should fill both
   the TSval and TSecr fields of the Timestamp option as described
   above.  Secondly, a host receiving a SYN/ACK with the Migrate-
   Permitted option MUST NOT perform either a PAWS check or a RTTM
   calculation on the values in the Timestamp option.  It should,
   however, echo the value in the TSval back in the TSecr field of its
   next segment, as specified in [RFC-1323].  After the three-way
   handshake, timestamp processing SHOULD proceed as specified in

Before Migration

   If a connection becomes aware that its address/port pair is about to
   change, the migrating host SHOULD pause transmission of all buffered
   TCP traffic, and MAY generate a duplicate ACK advertising a receive
   window size of zero to suppress further data transmissions from the
   fixed host.

Requesting Migration

   If the host requesting migration is not the original host, it MUST
   preload the TCB with the appropriate state extracted from the TCB at
   the previous host.  This data can be stale (the current sequence
   space information, for example), but must be from the original

   In any event, before resuming communication, the congestion-related
   state of the TCP connection must be reset to initial slow-start
   conditions in order to re-discover the properties of the new path in
   both directions.

   After the host has determined its new address, a SYN segment SHOULD
   be generated including a Migrate Option, using the sequence number of
   the most recently acknowledged byte.  The Token field should be
   filled with the Token previously computed and stored in the TCB.  The
   ReqNo field MUST contain a monotonically increasing number in the
   range [0-127], with standard sequence number wraparound.  The Request
   field MUST contain the the following computed value:

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                      R = SHA-1(Ni, Na, K, S, ReqNo)

   where Ni and Na are the initial sequence numbers for the connection
   as before, K is the secret key, ReqNo is the ReqNo field from this
   option, and S is the sequence number of this SYN segment.  If the
   connection had previously negotiated the non-secured variant, K
   should simply be all zeros.

   This SYN segment MUST NOT contain any options except the Migrate
   option.  TCP MUST then move the connection to the SYN-SENT state.

   Upon receipt of a SYN/ACK, the connection then resumes as before,
   with all previously negotiated options enabled.  The sequence number
   in the ACK field of the SYN/ACK should be treated as an
   acknowledgement of data transmitted with that sequence number, and
   not of the SYN segment (the SYN segment need not be explicitly
   acknowledged).  Further, if the Preload flag was set (meaning this
   host was not an original end point), it should examine the sequence
   number of the SYN/ACK and consider that the most recently
   acknowledged segment.

Migrate Option Processing

   Upon receipt of a SYN packet with the Migrate Option set, a TCP stack
   that supports migration MUST attempt to locate the connection
   associated with the provided Token.  If none is found, a RST MUST be
   generated and the SYN ignored.  Otherwise, the ReqNo should be
   compared to the most recently received ReqNo on this connection.  If
   it is not larger, this SYN packet MUST be treated as a duplicated and
   handled accordingly.  If it is greater, the host should generate a
   request as described above and compared it to the received packet.
   If they do not match, the host MUST generate a RST segment and

   Only if the Request matches should the host update its stored value
   of the ReqNo and change the address and port in the TCB to match that
   of the received IP datagram.  After altering the TCB, a SYN/ACK
   packet should be generated using the sequence number of the last
   successfully ACKNOWLEDGED byte of data, and the ACK field set to
   acknowledge that last data packet on the previous connection (the
   sequence number in the SYN packet is ignored), and the connection
   placed in the SYN-RCVD state.  Further, the congestion-related state
   of the connection MUST be reset to initial values.  Upon receipt of
   an ACK, the connection continues as before.

   If the received Migrate Option has the Preload flag set, the
   connection MUST flush any previously-received out-of-order packets
   and corresponding SACK blocks.

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SYN Processing

   SYN packets may now correctly arrive on a bound port not in the
   LISTEN state.  They should be processed only if they contain the
   Migrate Option as specified above.  Otherwise, they should be treated
   as specified in [RFC-793].

RST Processing

   Migration requires additional care when handling RST packets, as an
   unfortunate circumstance may arise when the address used to establish
   a migrateable connection has been reassigned, and transmitted packets
   reach a new host with the same address before the old host has had a
   chance to properly migrate the connection.

   In this case, a RST packet will be generated by the new host.
   [RFC-793] specifies that connections should be immediately closed
   upon receipt of a RST when in the ESTABLISHED state.  If, however,
   the connection had negotiated the Migrate-Permitted option, it MUST
   NOT be immediately closed.  Instead, it should be placed in a new
   MIGRATE-WAIT state.


   TCP connections in the MIGRATE-WAIT state SHOULD NOT emit any
   segments, instead, they MUST wait until either a SYN packet with the
   Migrate option is received, which will resume the connection, or the
   appropriate timeout (identical to the TIME-WAIT state) has occurred,
   in which case the connection is closed.

   Any packets received while in the MIGRATE-WAIT state should be
   processed as in the ESTABLISHED state, except that no ACKs should be
   generated.  Further, any RST packets received in the MIGRATE-WAIT
   state MUST be ignored.

Security Considerations

   The ability to blindly hijack connections is dangerous.  It is
   therefore of paramount importance that the cookie is
   cryptographically secure, such that it is highly unlikely an attacker
   can guess the appropriate cookie to hijack a connection.  The
   cryptographic security of a cookie generated using the Elliptic Curve
   Diffie-Hellman technique described in this document is fundamentally
   based on the quality of the random number generator used.

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   Many currently-deployed firewalls enforce restrictions on the
   directions in which TCP connections can be established.  Most do so
   by monitoring network traffic for TCP SYN packets, and blocking those
   traveling in the wrong direction.  The Migrate Option requires
   sending a SYN packet from the mobile host to the fixed host.
   Depending on the direction the connection was originally established,
   this may or may not be permitted by the firewall.

IPsec Interactions

   Note that Security Associations are established on a address basis.
   When a TCP connection with an associated SA is migrated, a new SA
   MUST be established with the new destination address before
   communications are resumed.  If the establishment of a this new SA
   would conflict with existing policy, the connection MUST be closed.

Denial of Service

   SYN packets carrying invalid Tokens are rejected immediately, so no
   additional resources are wasted processing invalid packets.  Hence
   the TCP migrate option introduces no additional security concerns in
   addition to standard TCP, modulo the cryptographic security of the
   cookie used to migrate connections.


   We thank Allison Mankin for encouraging us to document the Migrate
   options in this form and Dave Andersen for his comments on earlier
   versions of this document.

Authors' Addresses:

   Alex C. Snoeren
   MIT Laboratory for Computer Science
   200 Technology Square
   Cambridge, MA 02139
   Phone: +1 617 452-2820

   Hari Balakrishnan
   MIT Laboratory for Computer Science
   200 Technology Square
   Cambridge, MA 02139

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   Phone: +1 617 253-8713


   [ANSI-X962] American National Standards Institute, "Public Key
               Cryptography for the financial security industry: The
               Elliptic Curve Digital Signature Algorithm.
               ANSI X9.62-1998, January 1999.

   [Huitema95] Huitema, C., "Multi-homed TCP", Internet Draft, May 1995.
               Now expired.

   [Perkins]   Perkins, C., "IP Mobility Support for IPv4, revised",
               Internet Draft, October 1999.

   [RFC-793]   Postel, J., "Transmission Control Protocol", STD 5,
               RFC 793, September 1981.

   [RFC-1122]  Braden, R., "Requirements for Internet Hosts -
               Communication Layers", STD 5, RFC 1122, August 1989.

   [RFC-1323]  Jacobson, V., Braden, R., and Borman, D., "TCP Extensions
               for High Performance", RFC 1323, May 1992.

   [RFC-2119]  Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC-2136]  Vixie, P., et al., "Dynamic Updates in the Domain Name
               System (DNS UPDATE)", RFC 2136, April 1997.

   [RFC-2401]  Kent, S., and Atkinson, R., "Security Architecture for
               the Internet Protocol", RFC 2401, November 1998.

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

   [SCTP]      Stewart, R., et al., "Stream Control Transmission
               Protocol", Internet Draft, draft-ietf-sigran-sctp-13.txt,
               July 2000.

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