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A Traffic-Based Method of Detecting Dead Internet Key Exchange (IKE) Peers

The information below is for an old version of the document that is already published as an RFC.
Document Type This is an older version of an Internet-Draft that was ultimately published as an RFC.
Authors Stephane Beaulieu , Geoffrey Huang , Dany Rochefort
Last updated 2015-10-14 (Latest revision 2003-10-23)
Stream Internet Engineering Task Force (IETF)
Stream WG state (None)
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IESG IESG state RFC 3706 (Informational)
Consensus boilerplate Unknown
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Responsible AD Russ Housley
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IPSec Working Group 
                                                               G. Huang 
                                                            S. Beaulieu 
   Internet Draft                                          D. Rochefort 
   Document: draft-ietf-ipsec-dpd-04.txt            Cisco Systems, Inc. 
   Expires: April 2004                                     October 2003 
            A Traffic-Based Method of Detecting Dead IKE Peers 
 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 
   The list of Internet-Draft Shadow Directories can be accessed at 
   This document describes the method detecting a dead IKE peer that is
   presently in use by a number of vendors.  The method, called Dead
   Peer Detection (DPD) uses IPSec traffic patterns to minimize the
   number of IKE messages that are needed to confirm liveness.  DPD,
   like other keepalive mechanisms, is needed to determine when to
   perform IKE peer failover, and to reclaim lost resources.

 Table of Contents 
   Status of this Memo................................................1 
   Table of Contents..................................................1 
   1. Introduction....................................................2 
   2. Conventions used in this document...............................3 
   3. Document Roadmap................................................3 
   4. Rationale for Periodic Message Exchange for Proof of Liveliness.3 
   5. Keepalives vs. Heartbeats.......................................3 
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     5.1 Keepalives:..................................................3 
     5.2 Heartbeats:..................................................5 
   6. DPD Protocol....................................................6 
     6.1 DPD Vendor ID................................................6 
     6.2 Message Exchanges............................................7 
     6.3 NOTIFY(R-U-THERE/R-U-THERE-ACK) Message Format...............7 
     6.4 Impetus for DPD Exchange.....................................8 
     6.5 Implementation Suggestion....................................8 
     6.6 Comparisons..................................................9 
   7. Resistance to Replay Attack and False Proof of Liveliness.......9 
     7.1 Sequence Number in DPD Messages..............................9 
     7.2 Selection and Maintenance of Sequence Numbers...............10 
   8. References.....................................................10 
   9. Editors' Addresses.............................................11 
 1. Introduction 
   When two peers communicate with IKE [1] and IPSec [2], the situation 
   may arise in which connectivity between the two goes down 
   unexpectedly.  This situation can arise because of routing problems, 
   one host rebooting, etc., and in such cases, there is often no way 
   for IKE and IPSec to identify the loss of peer connectivity.  As 
   such, the SAs can remain until their lifetimes naturally expire, 
   resulting in a "black hole" situation where packets are tunneled to 
   oblivion.   It is often desirable to recognize black holes as soon 
   as possible so that an entity can failover to a different peer 
   quickly.  Likewise, it is sometimes necessary to detect black holes 
   to recover lost resources. 
   This problem of detecting a dead IKE peer has been addressed by 
   proposals that require sending periodic HELLO/ACK messages to prove 
   liveliness.  These schemes tend to be unidirectional (a HELLO only) 
   or bidirectional (a HELLO/ACK pair).  For the purpose of this draft, 
   the term "heartbeat" will refer to a unidirectional message to prove 
   liveliness.  Likewise, the term "keepalive" will refer to a 
   bidirectional message. 
   The problem with current heartbeat and keepalive proposals is their 
   reliance upon their messages to be sent at regular intervals.  In 
   the implementation, this translates into managing some timer to 
   service these message intervals.  Similarly, because rapid detection 
   of the dead peer is often desired, these messages must be sent with 
   some frequency, again translating into considerable overhead for 
   message processing.  In implementations and installations where 
   managing large numbers of simultaneous IKE sessions is of concern, 
   these regular heartbeats/keepalives prove to be infeasible. 
   To this end, a number of vendors have implemented their own
   approach to detect peer liveliness without needing to send messages
   at regular intervals.  This informational document describes the
   current practice of those implementations.  This scheme, called Dead
   Peer Detection (DPD), relies on IKE Notify messages to query the
   liveliness of an IKE peer.
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 2. Conventions used in this document 
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 
   this document are to be interpreted as described in RFC-2119 [3]. 
 3. Document Roadmap 
   As mentioned above, there are already proposed solutions to the 
   problem of detecting dead peers.  Section 4 elaborates the rationale 
   for using an IKE message exchange to query a peer's liveliness.  
   Section 5 examines a keepalives-based approach as well as a 
   heartbeats-based approach.  Section 6 presents the DPD proposal 
   fully, highlighting differences between DPD and the schemes 
   presented in Section 5 and emphasizing scalability issues.  Section 
   7 examines security issues surrounding replayed messages and false 
 4. Rationale for Periodic Message Exchange for Proof of Liveliness 
   As the introduction mentioned, it is often necessary to detect a 
   peer is unreachable as soon as possible.  IKE provides no way for 
   this to occur -- aside from waiting until the rekey period, then 
   attempting (and failing the rekey).  This would result in a period 
   of loss connectivity lasting the remainder of the lifetime of the 
   security association (SA), and in most deployments, this is 
   unacceptable.  As such, a method is needed for checking up on a 
   peer's state at will.  Different methods have arisen, usually using 
   an IKE Notify to query the peer's liveliness.  These methods rely on 
   either a bidirectional "keepalive" message exchange (a HELLO 
   followed by an ACK), or a unidirectional "heartbeat" message 
   exchange (a HELLO only).  The next section considers both of these 
 5. Keepalives vs. Heartbeats 
 5.1 Keepalives:  
   Consider a keepalives scheme in which peer A and peer B require 
   regular acknowledgements of each other's liveliness.  The messages 
   are exchanged by means of an authenticated notify payload.  The two 
   peers must agree upon the interval at which keepalives are sent, 
   meaning that some negotiation is required during Phase 1.  For any 
   prompt failover to be possible, the keepalives must also be sent at 
   rather frequent intervals -- around 10 seconds or so.  In this 
   hypothetical keepalives scenario, peers A and B agree to exchange 
   keepalives every 10 seconds.  Essentially, every 10 seconds, one 
   peer must send a HELLO to the other.  This HELLO serves as proof of 
   liveliness for the sending entity.  In turn, the other peer must 
   acknowledge each keepalive HELLO.  If the 10 seconds elapse, and one 
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   side has not received a HELLO, it will send the HELLO message 
   itself, using the peer's ACK as proof of liveliness.  Receipt of 
   either a HELLO or ACK causes an entity's keepalive timer to reset.  
   Failure to receive an ACK in a certain period of time signals an 
   error.  A clarification is presented below: 
   Scenario 1:  
   Peer A's 10-second timer elapses first, and it sends a HELLO to B.   
   B responds with an ACK. 
   Peer A:                              Peer B:  
   10 second timer fires;  ------>  
   wants to know that B is alive;  
   sends HELLO. 
                                         Receives HELLO; acknowledges  
                                         A's liveliness; 
                               <------   resets keepalive timer, sends  
   Receives ACK as proof of  
   B's liveliness; resets timer. 
   Scenario 2:  
   Peer A's 10-second timer elapses first, and it sends a HELLO to B.  
   B fails to respond.  A can retransmit, in case its initial HELLO is 
   lost.  This situation describes how peer A detects its peer is dead. 
   Peer A:                              Peer B (dead): 
   10 second timer fires;  ------X  
   wants to know that B is  
   alive; sends HELLO. 
   Retransmission timer    ------X  
   expires; initial message  
   could have been lost in  
   transit; A increments  
   error counter and  
   sends another HELLO. 
   . . . 
   After some number of errors, A assumes B is dead; deletes SAs and 
   possibly initiates failover. 
   An advantage of this scheme is that the party interested in the 
   other peer's liveliness begins the message exchange.  In Scenario 1, 
   peer A is interested in peer B's liveliness, and peer A consequently 
   sends the HELLO.  It is conceivable in such a scheme that peer B 
   would never be interested in peer A's liveliness.  In such a case, 
   the onus would always lie on peer A to initiate the exchange. 
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 5.2 Heartbeats:  
   By contrast, consider a proof-of-liveliness scheme involving 
   unidirectional (unacknowledged) messages.  An entity interested in 
   its peer's liveliness would rely on the peer itself to send periodic 
   messages demonstrating liveliness.  In such a scheme, the message 
   exchange might look like this: 
   Scenario 3:  
   Peer A and Peer B are interested in each other's liveliness.  Each 
   peer depends on the other to send periodic HELLOs. 
   Peer A:                              Peer B:  
   10 second timer fires;  ------>  
   sends HELLO.  Timer also  
   signals expectation of  
   B's HELLO. 
                                         Receives HELLO as proof of A's 
                               <------   10 second timer fires; sends 
   Receives HELLO as proof  
   of B's liveliness. 
   Scenario 4:  
   Peer A fails to receive HELLO from B and marks the peer dead.  This 
   is how an entity detects its peer is dead. 
   Peer A:                              Peer B (dead):  
   10 second timer fires;  ------X  
   sends HELLO.  Timer also  
   signals expectation of 
   B's HELLO. 
   . . . 
   Some time passes and A assumes B is dead. 
   The disadvantage of this scheme is the reliance upon the peer to 
   demonstrate liveliness.  To this end, peer B might never be   
   interested in peer A's liveliness.  Nonetheless, if A is interested 
   B's liveliness, B must be aware of this, and maintain the necessary 
   state information to send periodic HELLOs to A.  The disadvantage of 
   such a scheme becomes clear in the remote-access scenario.  Consider 
   a VPN aggregator that terminates a large number of sessions (on the 
   order of 50,000 peers or so).  Each peer requires fairly rapid 
   failover, therefore requiring the aggregator to send HELLO packets 
   every 10 seconds or so.  Such a scheme simply lacks scalability, as 
   the aggregator must send 50,000 messages every few seconds. 
   In both of these schemes (keepalives and heartbeats), some 
   negotiation of message interval must occur, so that each entity can 
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   know how often its peer expects a HELLO.  This immediately adds a 
   degree of complexity.  Similarly, the need to send periodic messages  
   (regardless of other IPSec/IKE activity), also increases 
   computational overhead to the system. 
 6. DPD Protocol 
   DPD addresses the shortcomings of IKE keepalives- and heartbeats-
   schemes by introducing a more reasonable logic governing message 
   exchange.  Essentially, keepalives and heartbeats mandate exchange 
   of HELLOs at regular intervals.  By contrast, with DPD, each peer's 
   DPD state is largely independent of the other's.  A peer is free to 
   request proof of liveliness when it needs it -- not at mandated 
   intervals.  This asynchronous property of DPD exchanges allows fewer 
   messages to be sent, and this is how DPD achieves greater 
   As an elaboration, consider two DPD peers A and B.  If there is 
   ongoing valid IPSec traffic between the two, there is little need 
   for proof of liveliness.  The IPSec traffic itself serves as the 
   proof of liveliness.  If, on the other hand, a period of time lapses 
   during which no packet exchange occurs, the liveliness of each peer 
   is questionable.  Knowledge of the peer's liveliness, however, is 
   only urgently necessary if there is traffic to be sent.  For 
   example, if peer A has some IPSec packets to send after the period 
   of idleness, it will need to know if peer B is still alive.  At this 
   point, peer A can initiate the DPD exchange. 
   To this end, each peer may have different requirements for detecting 
   proof of liveliness.  Peer A, for example, may require rapid 
   failover, whereas peer B's requirements for resource cleanup are 
   less urgent.  In DPD, each peer can define its own "worry metric" - 
   an interval that defines the urgency of the DPD exchange.  
   Continuing the example, peer A might define its DPD interval to be 
   10 seconds.  Then, if peer A sends outbound IPSec traffic, but fails 
   to receive any inbound traffic for 10 seconds, it can initiate a DPD 
   Peer B, on the other hand, defines its less urgent DPD interval to 
   be 5 minutes.  If the IPSec session is idle for 5 minutes, peer B 
   can initiate a DPD exchange the next time it sends IPSec packets to 
   It is important to note that the decision about when to initiate a 
   DPD exchange is implementation specific.  An implementation might 
   even define the DPD messages to be at regular intervals following 
   idle periods.  See section 6.5 for more implementation suggestions. 
 6.1 DPD Vendor ID 
   To demonstrate DPD capability, an entity must send the DPD vendor 
   ID.  Both peers of an IKE session MUST send the DPD vendor ID before 
   DPD exchanges can begin.  The format of the DPD Vendor ID is: 
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                0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 
                !                           !M!M! 
                !      HASHED_VENDOR_ID     !J!N! 
                !                           !R!R! 
   where HASHED_VENDOR_ID = {0xAF, 0xCA, 0xD7, 0x13, 0x68, 0xA1, 0xF1, 
   0xC9, 0x6B, 0x86, 0x96, 0xFC, 0x77, 0x57}, and MJR and MNR 
   correspond to the current major and minor version of this protocol 
   (1 and 0 respectively).  An IKE peer MUST send the Vendor ID if it 
   wishes to take part in DPD exchanges. 
 6.2 Message Exchanges 
   The DPD exchange is a bidirectional (HELLO/ACK) Notify message.  The 
   exchange is defined as: 
            Sender                                      Responder 
           --------                                    ----------- 
   HDR*, NOTIFY(R-U-THERE), HASH   ------> 
                                 <------    HDR*, NOTIFY(R-U-THERE- 
                                            ACK), HASH 
   The R-U-THERE message corresponds to a "HELLO" and the R-U-THERE-ACK 
   corresponds to an "ACK."  Both messages are simply ISAKMP Notify 
   payloads, and as such, this draft defines these two new ISAKMP 
   Notify message types: 
      Notify                      Message Value  
      R-U-THERE                   36136 
      R-U-THERE-ACK               36137 
   An entity that has sent the DPD Vendor ID MUST respond to an R-U-
   THERE query.  Furthermore, an entity MUST reject unencrypted R-U-
   THERE and R-U-THERE-ACK messages.  
 6.3 NOTIFY(R-U-THERE/R-U-THERE-ACK) Message Format 
   When sent, the R-U-THERE message MUST take the following form: 
                       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 
   ! Next Payload  !   RESERVED    !         Payload Length        ! 
   !              Domain of Interpretation  (DOI)                  ! 
   !  Protocol-ID  !    SPI Size   !      Notify Message Type      ! 
   !                                                               ! 
   ~                Security Parameter Index (SPI)                 ~ 
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   !                                                               ! 
   !                    Notification Data                          ! 
   As this message is an ISAKMP NOTIFY, the Next Payload, RESERVED, and 
   Payload Length fields should be set accordingly.  The remaining 
   fields are set as: 
   - Domain of Interpretation (4 octets) - SHOULD be set to IPSEC-DOI. 
   - Protocol ID (1 octet) - MUST be set to the protocol ID for ISAKMP. 
   - SPI Size (1 octet) - SHOULD be set to sixteen (16), the length of 
     two octet-sized ISAKMP cookies. 
   - Notify Message Type (2 octets) - MUST be set to R-U-THERE 
   - Security Parameter Index (16 octets) - SHOULD be set to the 
     cookies of the Initiator and Responder of the IKE SA (in that  
   - Notification Data (4 octets) - MUST be set to the sequence number 
     corresponding to this message 
   The format of the R-U-THERE-ACK message is the same, with the 
   exception that the Notify Message Type MUST be set to R-U-THERE-ACK.   
   Again, the Notification Data MUST be sent to the sequence number 
   corresponding to the received R-U-THERE message. 
 6.4 Impetus for DPD Exchange 
   Again, rather than relying on some negotiated time interval to force 
   the exchange of messages, DPD does not mandate the exchange of R-U- 
   THERE messages at any time.  Instead, an IKE peer SHOULD send an R-
   U-THERE query to its peer only if it is interested in the liveliness 
   of this peer.  To this end, if traffic is regularly exchanged 
   between two peers, either peer SHOULD use this traffic as proof of 
   liveliness, and both peers SHOULD NOT initiate a DPD exchange. 
   A peer MUST keep track of the state of a given DPD exchange.  That 
   is, once it has sent an R-U-THERE query, it expects an ACK in 
   response within some implementation-defined period of time.  An 
   implementation SHOULD retransmit R-U-THERE queries when it fails to 
   receive an ACK.  After some number of retransmitted messages, an 
   implementation SHOULD assume its peer to be unreachable and delete 
   IPSec and IKE SAs to the peer. 
 6.5 Implementation Suggestion 
   Since the liveliness of a peer is only questionable when no traffic 
   is exchanged, a viable implementation might begin by monitoring 
   idleness.  Along these lines, a peer's liveliness is only important 
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   when there is outbound traffic to be sent.  To this end, an 
   implementation can initiate a DPD exchange (i.e., send an R-U-THERE 
   message) when there has been some period of idleness, followed by 
   the desire to send outbound traffic.  Likewise, an entity can 
   initiate a DPD exchange if it has sent outbound IPSec traffic, but 
   not received any inbound IPSec packets in response.  A complete DPD 
   exchange (i.e., transmission of R-U-THERE and receipt of 
   corresponding R-U-THERE-ACK) will serve as proof of liveliness until 
   the next idle period.  
   Again, since DPD does not mandate any interval, this "idle period" 
   (or "worry metric") is left as an implementation decision.  It is 
   not a negotiated value. 
 6.6 Comparisons 
   The performance benefit that DPD offers over traditional keepalives- 
   and heartbeats-schemes comes from the fact that regular messages do 
   not need to be sent.  Returning to the examples presented in section 
   5.1, a keepalive implementation such as the one presented would 
   require one timer to signal when to send a HELLO message and another 
   timer to "timeout" the ACK from the peer (this could also be the 
   retransmit timer).  Similarly, a heartbeats scheme such as the one 
   presented in section 5.2 would need to keep one timer to signal when 
   to send a HELLO, as well as another timer to signal the expectation 
   of a HELLO from the peer.  By contrast a DPD scheme needs to keep a 
   timestamp to keep track of the last received traffic from the peer 
   (thus marking beginning of the "idle period").  Once a DPD R-U-THERE 
   message has been sent, an implementation need only maintain a timer 
   to signal retransmission.  Thus, the need to maintain active timer 
   state is reduced, resulting in a scalability improvement (assuming 
   maintaining a timestamp is less costly than an active timer).  
   Furthermore, since a DPD exchange only occurs if an entity has not 
   received traffic recently from its peer, the number of IKE messages 
   to be sent and processed is also reduced.  As a consequence, the 
   scalability of DPD is much better than keepalives and heartbeats. 
   DPD maintains the HELLO/ACK model presented by keepalives, as it 
   follows that an exchange is initiated only by an entity interested 
   in the liveliness of its peer. 
 7. Resistance to Replay Attack and False Proof of Liveliness  
 7.1 Sequence Number in DPD Messages  
   To guard against message replay attacks and false proof of 
   liveliness, a 32-bit sequence number MUST be presented with each R-
   U-THERE message.  A responder to an R-U-THERE message MUST send an 
   R-U-THERE-ACK with the same sequence number.  Upon receipt of the R-
   U-THERE-ACK message, the initial sender SHOULD check the validity of 
   the sequence number.  The initial sender SHOULD reject the R-U-
   THERE-ACK if the sequence number fails to match the one sent with 
   the R-U-THERE message. 
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   Additionally, both the receiver of the R-U-THERE and the R-U-THERE-
   ACK message SHOULD check the validity of the Initiator and Responder 
   cookies presented in the SPI field of the payload. 
 7.2 Selection and Maintenance of Sequence Numbers 
   As both DPD peers can initiate a DPD exchange (i.e., both peers can 
   send R-U-THERE messages), each peer MUST maintain its own sequence 
   number for R-U-THERE messages.  The first R-U-THERE message sent in 
   a session MUST be a randomly chosen number.  To prevent rolling past 
   overflowing the 32-bit boundary, the high-bit of the sequence number 
   initially SHOULD be set to zero.  Subsequent R-U-THERE messages MUST 
   increment the sequence number by one.  Sequence numbers MAY reset at 
   the expiry of the IKE SA, moving to a newly chosen random number. 
   Each entity SHOULD also maintain its peer's R-U-THERE sequence 
   number, and an entity SHOULD reject the R-U-THERE message if it 
   fails to match the expected sequence number. 
   Implementations MAY maintain a window of acceptable sequence 
   numbers, but this specification makes no assumptions about how this 
   is done.  Again, it is an implementation specific detail. 
 8. Security Considerations 
   As the previous section highlighted, DPD uses sequence numbers to 
   ensure liveliness.  This section describes the advantages of using 
   sequence numbers over random nonces to ensure liveliness. 
   While sequence numbers do require entities to keep per-peer state, 
   they also provide an added method of protection in certain replay 
   attacks.  Consider a case where peer A sends peer B a valid DPD R-U-
   THERE message.  An attacker C can intercept this message and flood B 
   with multiple copies of the messages.  B will have to decrypt and 
   process each packet (regardless of whether sequence numbers or 
   nonces are in use).  With sequence numbers B can detect that the 
   packets are replayed: the sequence numbers in these replayed packets 
   will not match the incremented sequence number that B expects to 
   receive from A.  This prevents B from needing to build, encrypt, and 
   send ACKs.  By contrast, if the DPD protocol used nonces, it would 
   provide no way for B to detect that the messages are replayed 
   (unless B maintained a list of recently received nonces). 
   Another benefit of sequence numbers is that it adds an extra 
   assurance of the peer's liveliness.  As long as a receiver verifies 
   the validity of a DPD R-U-THERE message (by verifying its 
   incremented sequence number), then the receiver can be assured of 
   the peer's liveliness by the very fact that the sender initiated the 
   query.  Nonces, by contrast, cannot provide this assurance.  
 9. IANA Considerations 
   There is no IANA action required for this draft.  DPD uses notify 
   numbers from the private range. 
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 10. References 
   1  RFC 2409 Harkins, D. and Carrel, D., "The Internet Key Exchange 
      (IKE)," November 1998. 
   2  RFC 2401 Kent, S. and Atkinson, R., "Security Architecture for 
      the Internet Protocol," November 1998. 
   3  RFC 2119 Bradner, S., "Key words for use in RFCs to Indicate 
      Requirement Levels," BCP 14, RFC 2119, March 1997. 

 10. Editors' Addresses 
   Geoffrey Huang 
   Cisco Systems, Inc. 
   170 West Tasman Drive 
   San Jose, CA 95134 
   Phone: (408) 525-5354 
   Stephane Beaulieu 
   Cisco Systems, Inc. 
   2000 Innovation Drive 
   Kanata, ON 
   Canada, K2K 3E8 
   Phone: (613) 271-3678 
   Dany Rochefort 
   Cisco Systems, Inc. 
   124 Grove Street, Suite 205 
   Franklin, MA 02038 
   Phone: (508) 553-6136 
   The IPsec working group can be contacted through the chairs: 
   Barbara Fraser 
   Cisco Systems, Inc. 
   Ted T'so 
   Massachusetts Institute of Technology 
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