Relayed Echo Reply mechanism for LSP Ping
draft-ietf-mpls-lsp-ping-relay-reply-02

Network Working Group                                        J. Luo, Ed.
Internet-Draft                                                       ZTE
Updates: 4379 (if approved)                                  L. Jin, Ed.
Intended status: Standards Track
Expires: August 18, 2014                                  T. Nadeau, Ed.
                                                             Lucidvision
                                                         G. Swallow, Ed.
                                                                   Cisco
                                                       February 14, 2014

               Relayed Echo Reply mechanism for LSP Ping
                draft-ietf-mpls-lsp-ping-relay-reply-02

Abstract

   In some inter autonomous system (AS) and inter-area deployment
   scenarios for Label Switched Path (LSP) Ping and Traceroute, a
   replying LSR may not have the available route to the initiator, and
   the Echo Reply message sent to the initiator would be discarded
   resulting in false negatives or complete failure of operation of LSP
   Ping and Traceroute.  This document describes extensions to LSP Ping
   mechanism to enable the replying Label Switching Router (LSR) to have
   the capability to relay the Echo Response by a set of routable
   intermediate nodes to the initiator.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on August 18, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   (http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Conventions Used in This Document  . . . . . . . . . . . .  3
   2.  Motivation . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Extensions . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Relayed Echo Reply message . . . . . . . . . . . . . . . .  5
     3.2.  Relay Node Address Stack . . . . . . . . . . . . . . . . .  5
     3.3.  New Return Code  . . . . . . . . . . . . . . . . . . . . .  7
   4.  Procedures . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Sending an Echo Request  . . . . . . . . . . . . . . . . .  7
     4.2.  Receiving an Echo Request  . . . . . . . . . . . . . . . .  7
     4.3.  Originating an Relayed Echo Reply  . . . . . . . . . . . .  8
     4.4.  Relaying an Relayed Echo Reply . . . . . . . . . . . . . .  9
     4.5.  Sending an Echo Reply  . . . . . . . . . . . . . . . . . .  9
     4.6.  Receiving an Echo Reply  . . . . . . . . . . . . . . . . .  9
   5.  LSP Ping Relayed Echo Reply Example  . . . . . . . . . . . . . 10
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   7.  Backward Compatibility . . . . . . . . . . . . . . . . . . . . 12
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
     8.1.  New Message Type . . . . . . . . . . . . . . . . . . . . . 12
     8.2.  New TLV  . . . . . . . . . . . . . . . . . . . . . . . . . 12
     8.3.  New Return Code  . . . . . . . . . . . . . . . . . . . . . 12
   9.  Acknowledgement  . . . . . . . . . . . . . . . . . . . . . . . 13
   10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 13
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 13
     11.2. Informative References . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13

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1.  Introduction

   This document describes the extensions to the Label Switched Path
   (LSP) Ping as specified in [RFC4379], by adding a relayed echo reply
   mechanism which could be used to detect data plane failures in inter
   autonomous system (AS) and inter-area LSPs.  Without this extension,
   the ping functionality provided by [RFC4379] would fail in many
   deployed inter-AS scenarios, since the replying LSR in one AS may not
   have the available route to the initiator in the other AS.  The
   mechanism in this draft defines a new message type referred as
   "Relayed Echo Reply message", and a new TLV referred as "Relay Node
   Address Stack TLV".

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  Motivation

   LSP Ping [RFC4379] defines a mechanism to detect data plane failures
   and localize faults.  The mechanism specifies that the Echo Reply
   should be sent back to the initiator usig an UDP packet with the
   IPv4/ IPv6 address of the originating LSR.  This works in
   administrative domains allowing IP address reachability and routing
   back to the originating LSR.  However, in practice, this is often not
   the case due to intra-provider routing policy, route hiding, network
   address translation at autonomous system border routers (ASBR), and
   etc.  In fact, it is almost uniformly the case that in inter-AS
   scenarios, it is not allowed the distribution or direct routing to
   the IP addresses of any of the nodes other than the ASBR.

   Figure 1 demonstrates a case where one LSP is set up between PE1 and
   PE2.  If private addresses were in use within AS2, a traceroute from
   PE1 directed to PE2 could fail if the fault exists somewhere between
   ASBR2 and PE2.  Because P2 cannot forward packets back to PE1 given
   that it is a private address within AS1.  In this case, PE1 would
   detect a path break, as the Echo Request messages would not be sent
   back; however, localization of the actual fault would not be
   possible.

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   +-------+   +-------+   +------+   +------+   +------+   +------+
   |       |   |       |   |      |   |      |   |      |   |      |
   |  PE1  +---+   P1  +---+ ASBR1+---+ ASBR2+---+  P2  +---+  PE2 |
   |       |   |       |   |      |   |      |   |      |   |      |
   +-------+   +-------+   +------+   +------+   +------+   +------+
   <---------------AS1-------------><---------------AS2------------>
   <---------------------------- LSP ------------------------------>

                Figure 1: Simple Inter-AS LSP Configuration

   A second example that illustrates how [RFC4379] would be insufficient
   would be the inter-area situation in a Seamless MPLS architecture
   [I-D.ietf-mpls-seamless-mpls] as shown below in Figure 2.  In this
   example P nodes the in core network would not have IP reachable route
   to any of the ANs.  When tracing an LSP from AN to remote AN, the
   LSR1/LSR2 node could not make a response to the Echo Request either,
   like P2 node in the inter-AS scenario in Figure 1.

              +-------+   +-------+   +------+   +------+
              |       |   |       |   |      |   |      |
           +--+ AGN11 +---+ AGN21 +---+ ABR1 +---+ LSR1 +--> to AGN
          /   |       |  /|       |   |      |   |      |
   +----+/    +-------+\/ +-------+   +------+  /+------+
   | AN |              /\                     \/
   +----+\    +-------+  \+-------+   +------+/\ +------+
          \   |       |   |       |   |      |  \|      |
           +--+ AGN12 +---+ AGN22 +---+ ABR2 +---+ LSR2 +--> to AGN
              |       |   |       |   |      |   |      |
              +-------+   +-------+   +------+   +------+
   static route     ISIS L1 LDP             ISIS L2 LDP
   <-Access-><--Aggregation Domain--><---------Core--------->

                   Figure 2: Seamless MPLS Architecture

   This document describes extensions to the LSP Ping mechanism to
   facilitate a response from the replying LSR, by defining a simple
   mechanism that uses the relay node (e.g, ASBR) to relay the message
   back to the initiator.  This approach will work because every
   designated or learned relay node must have an IP route back to the
   initiator.  Using a recursive approach, relay node could relay the
   message to the next relay node until the initiator is reached.

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3.  Extensions

   [RFC4379] describes the basic MPLS LSP Ping mechanism, which defines
   two message types, Echo Request and Echo Reply message.  This draft
   defines a new message, Relayed Echo Reply message.  This new message
   is used to replace Echo Reply message which is sent from the replying
   LSR to a relay node or from a relay node to another relay node.

   A new TLV named Relay Node Address Stack TLV is defined in this
   draft, to carry the IP addresses of the possible relay nodes for the
   replying LSR.

   In addition, a new Return Code is defined to notify the initiator
   that the packet length was exceeded unexpected by the Relay Node
   Address Stack TLV.

   It should be noted that this document focuses only on detecting the
   LSP which is set up using a uniform type of IP address.  That is, all
   hops between the source and destination use one address type of their
   control planes.  This does not preclude nodes that support both IPv6
   and IPv4 addresses simultaneously, but the entire path must be
   addressable using only one address family type.  Supporting for mixed
   IPv4-only and IPv6-only is beyond the scope of this document.

3.1.  Relayed Echo Reply message

   The Relayed Echo Reply message is a UDP packet, and the UDP payload
   has the same format with Echo Request/Reply message.  A new message
   type is requested from IANA.

   New Message Type:
       Value    Meaning
       -----    -------
       TBD      MPLS Relayed Echo Reply

   The TCP and UDP port number 3503 has been allocated in [RFC4379] by
   IANA for LSP Ping messages.  The Relayed Echo Reply message will use
   the same port number.

3.2.  Relay Node Address Stack

   The Relay Node Address Stack TLV is an optional TLV.  It MUST be
   carried in the Echo Request, Echo Reply and Relayed Echo Reply
   messages if the echo reply relayed mechanism described in this draft
   is required.  Figure 3 illustrates the TLV format.

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     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           |               Length          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Initiator Source Port       |   Number of Relayed Addresses |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                Stack of Relayed Addresses                     ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 3: Relay Node Address Stack TLV

   -  Type: to be assigned by IANA.  A suggested value is assigned from
      32768-49161 as suggested by RFC4379 Section 3.

   -  Length: The Length of the Value field in octets.

   -  Initiator Source Port: The port that the initiator sends the Echo
      Request message, and also the port that expected to receive the
      Echo Reply message.

   -  Number of Relayed Addresses: An integer indicating the number of
      relayed addresses in the stack.

   -  Stack of Relayed Addresses: A list of relay node addresses.

   The format of each relay node address is as 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Address  Type          | Address Length|  Reserved   |K|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~           Relayed Address (0, 4, or 16 octects)               ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type#   Address Type   Address Length
   ----    ------------   ------------
   0       Unspecified    0
   1       IPv4           4
   2       IPv6           16

   Reserved: This field is reserved for future use and MUST be set to
   zero.

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   K bit: If the K bit is set to 1, then this sub-TLV SHOULD be kept in
   Relay Node Address Stack, SHOULD not be deleted in compress process
   of section 4.2.  The K bit may be set by ASBRs which address would be
   kept in the stack if necessary.

   If the K bit is set to 0, then this sub-TLV SHOULD be processed
   normally according to section 4.2.

   Relayed Address: This field specifies the node address, either IPv4
   or IPv6.

3.3.  New Return Code

   A new Return Code is used by the replying LSR to notify the initiator
   that the packet length was exceeded unexpected by the Relay Node
   Address Stack TLV.

   New Return Code:
       Value    Meaning
       -----    -------
       TBD      Response Packet length was exceeded by the Relay Node
                Address Stack TLV unexpected

4.  Procedures

4.1.  Sending an Echo Request

   In addition to the procedures described in Section 4.3 of [RFC4379],
   a Relay Node Address Stack TLV MUST be carried in the Echo Request
   message for facilitate the relay functionality.

   When the Echo Request is first sent by initiator supporting these
   extensions, a Relay Node Address Stack TLV with the initiator address
   in the stack and its source port MUST be included.  That will ensure
   that the first relay node address in the stack will always be the
   initiator address.

   For the subsequent Echo Request messages, the initiator would copy
   the Relay Node Address Stack TLV from the received Echo Reply
   message.

4.2.  Receiving an Echo Request

   In addition to the processes in Section 4.4 of [RFC4379], the
   procedures of the Relay Node Address Stack TLV are defined here.

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   Upon receiving a Relay Node Address Stack TLV of the Echo Request
   message, the receiver MUST check the addresses of the stack in
   sequence from top to bottom (the first address in the stack ==== will
   be first one to be checked), to find out the first public routable IP
   address.  Those address entries behind of the first routable IP
   address in the address list with K bit set to 0 MUST be deleted, and
   the address entry of the replying LSR MUST be added at the bottom of
   the stack.  Those address entries with K bit set to 1 MUST be kept in
   the stack.  The updated Relay Node Address Stack TLV MUST be carried
   in the response message.

   If the replying LSR wishes to hide its routable address information,
   the address entry added in the stack SHOULD be a blank entry with
   Address Type set to unspecified.  The blank address entry in the
   receiving Echo Request SHOULD be treated as an unroutable address
   entry.

   If the packet length was exceeded unexpectedly by the Relay Node
   Address Stack TLV, the TLV SHOULD be returned back unchanged in the
   echo response message.  And the new return code SHOULD help to notify
   the initiator of the situation.

   If the first routable IP address is the first address in the stack,
   the replying LSR SHOULD respond an Echo Reply message to the
   initiator.

   If the first routable IP address is of an intermediate node, other
   than the first address in the stack, the replying LSR SHOULD send an
   Relayed Echo Reply instead of an Echo Reply in response.

   An LSR not recognize the Relay Node Address Stack TLV, SHOULD ignore
   it according to section 3 of RFC4379.

4.3.  Originating an Relayed Echo Reply

   When the replying LSR received an Echo Request with the initiator IP
   address in the Relay Node Address Stack TLV is IP unroutable, the
   replying LSR SHOULD send an Relayed Echo Reply message to the first
   routable intermediate node.  The processing of Relayed Echo Reply is
   the same with the procedure of the Echo Reply described in Section
   4.5 of RFC4379, except the destination IP address and the destination
   UDP port of the message part.  The destination IP address of the
   Relayed Echo Reply is set to the first routable IP address from the
   Relay Node Address Stack TLV, and both the source and destination UDP
   port is set to 3503.

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4.4.  Relaying an Relayed Echo Reply

   Upon receiving an Relayed Echo Reply message with its address as the
   destination address in the IP header, the relay node should check the
   address items in Relay Node Address Stack TLV in sequence from top to
   down, and find the first routable node address.

   If the first routable address is the top one of the address list,
   e.g, the initiator address, the relay node SHOULD send an Echo Reply
   message to the initiator containing the same payload with the Relayed
   Echo Reply message received.  See section 4.5 for detail.

   If the first routable address is not the top one of the address list,
   e.g, another intermediate relay node, the relay node SHOULD send an
   Relayed Echo Reply message to this relay node with the payload
   unchanged.

   Note, the replying LSR SHOULD send a Relayed Echo Reply message to
   the first relay node found in Relay Node Address Stack TLV that is
   routable by the router.  The routable address MUST be located before
   the source IP address of the received Relayed Echo Reply which must
   be also in the stack, otherwise the Relayed Echo Reply should not be
   sent, so as to avoid potential loop.

4.5.  Sending an Echo Reply

   The Echo Reply is sent in two cases:

   1.  When the replying LSR received an Echo Request with the initiator
   IP address in the Relay Node Address Stack TLV is IP routable, the
   replying LSR would send an Echo Reply to the initiator.  In addition
   to the procedure of the Echo Reply described in Section 4.5 of
   RFC4379, the Relay Node Address Stack TLV would be carried in the
   Echo Reply.

   2.  When the intermediate relay node received an Relayed Echo Reply
   with the initiator IP address in the Relay Node Address Stack TLV IP
   routable, the intermediate relay node would send the Echo Reply to
   the initiator with the payload unchanged other than the Message Type
   field.  The destination IP address of the Echo Reply is set to the
   initiator IP address, and the destination UDP port would be copied
   from the Initiator Source Port field of the Relay Node Address Stack
   TLV.  The source UDP port should be 3503.

4.6.  Receiving an Echo Reply

   In addition to the processes in Section 4.6 of [RFC4379], the
   initiator would copy the Relay Node Address Stack TLV received in the

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   Echo Reply to the next Echo Request.

5.  LSP Ping Relayed Echo Reply Example

   Considering the inter-AS scenario in Figure 4 below.

   +-------+   +-------+   +------+   +------+   +------+   +------+
   |       |   |       |   |      |   |      |   |      |   |      |
   |  PE1  +---+   P1  +---+ ASBR1+---+ ASBR2+---+  P2  +---+  PE2 |
   |       |   |       |   |      |   |      |   |      |   |      |
   +-------+   +-------+   +------+   +------+   +------+   +------+
   <---------------AS1-------------><---------------AS2------------>
   <--------------------------- LSP ------------------------------->

                      Figure 4: Example Inter-AS LSP

   In the example, an LSP has been created between PE1 to PE2.  When
   performing LSP traceroute on the LSP, the first Echo Request sent by
   PE1 with outter-most label TTL=1, contains the Relay Node Address
   Stack TLV with the only address of PE1.

   After processed by P1, P1's address will be added in the Relay Node
   Address Stack TLV address list following PE1's address in the Echo
   Reply.

   PE1 copies the Relay Node Address Stack TLV into the next Echo
   Request when receiving the Echo Reply.

   Upon receiving the Echo Request, ASBR1 checks the address list in the
   Relay Node Address Stack TLV in sequence, and finds out that PE1
   address is routable.  Then deletes P1 address, and adds its own
   address following PE1 address.  As a result, there would be PE1
   address followed by ASBR1 address in the Relay Node Address Stack TLV
   of the Echo Reply sent by ASBR1.

   PE1 then sends an Echo Request with outer-most label TTL=3,
   containing the Relay Node Address Stack TLV copied from the received
   Echo Reply message.  Upon receiving the Echo Request message, ASBR2
   checks the address list in the Relay Node Address Stack TLV in
   sequence, and finds out that PE1 address is IP route unreachable, and
   ASBR1 address is the first routable one in the Relay Node Address
   Stack TLV.  ASBR2 adds its address as the last address item following
   ASBR1 address in Relay Node Address Stack TLV, sets ASBR1 address as

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   the destination address of the Relayed Echo Reply, and sends the
   Relayed Echo Reply to ASBR1.

   Upon receiving the Relayed Echo Reply from ASBR2, ASBR1 checks the
   address list in the Relay Node Address Stack TLV in sequence, and
   finds out that PE1 address is first routable one in the address list.
   Then ASBR1 send an Echo Reply to PE1 with the payload of received
   Relayed Echo Reply no changes other than the Message Type field.

   For the Echo Request with outer-most label TTL=4, P2 checks the
   address list in the Relay Node Address Stack TLV in sequence, and
   finds out that both PE1 and ASBR1 addresses are not IP routable, and
   ASBR2 address is the first routable address.  And P2 would send an
   Relayed Echo Reply to ASBR2 with the Relay Node Address Stack TLV of
   four addresses, PE1, ASBR1, ASBR2 and P2 address in sequence.

   Then according to the process described in section 4.4, ASBR2 would
   send the Relayed Echo Reply to ASBR1.  Upon receiving the Relayed
   Echo Reply, ASBR1 would send an Echo Reply to PE1 as PE1 address is
   routable.  And as relayed by ASBR2 and ASBR1, the echo response would
   finally be sent to the initiator PE1.

   For the Echo Request with outer-most label TTL=5, the echo response
   would relayed to PE1 by ASBR2 and ASBR1, similar to the case of
   TTL=4.

   The Echo Reply from the replying node which has no reachable route to
   the initiator is finally transmitted to the initiator by multiple
   relay nodes.

6.  Security Considerations

   The Relayed Echo Reply mechanism for LSP Ping creates an increased
   risk of DoS by putting the IP address of a target router in the Relay
   Node Address Stack.  These messages then could be used to attack the
   control plane of an LSR by overwhelming it with these packets.  A
   rate limiter SHOULD be applied to the well-known UDP port on the
   relay node as suggested in RFC4379.  The node which acts as a relay
   node SHOULD validate the relay reply against a set of valid source
   addresses and discard packets from untrusted border router addresses.
   An implementation SHOULD provide such filtering capabilities.

   If an operator wants to obscure their nodes, it is RECOMMENDED that
   they may replace the replying node address that originated the Echo
   Reply with blank address in Relay Node Address Stack TLV.

   Other security considerations discussed in [RFC4379], are also

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   applicable to this document.

7.  Backward Compatibility

   When one of the nodes along the LSP does not support the mechanism
   specified in this draft, the node will ignore the Relay Node Address
   Stack TLV as described in section 4.2.  Then the initiator may not
   receive the Relay Node Address Stack TLV in Echo Reply message from
   that node.  In this case, an indication should be reported to the
   operator, and the Relay Node Address Stack TLV in the next Echo
   Request message should be copied from the previous Echo Request, and
   continue the ping process.  If the node described above is located
   between the initiator and the first relay node, the ping process
   could continue without interruption.

8.  IANA Considerations

   IANA is requested to assign one new Message Type, one new TLV and one
   new Return Code.

8.1.  New Message Type

   New Message Type:
        Value    Meaning
        -----    -------
        TBD      MPLS Relayed Echo Reply

8.2.  New TLV

   New TLV: Routable Relay Node Address TLV
        Type    Meaning
        ----    --------
        TBD     Relay Node Address Stack TLV

   A suggested value is assigned from 32768-49161 as suggested by
   RFC4379 Section 3.

8.3.  New Return Code

   New Return Code:
       Value    Meaning
       -----    -------
       TBD      Response Packet length was exceeded by the Relay Node
                Address Stack TLV unexpected

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9.  Acknowledgement

   The authors would like to thank Carlos Pignataro, Xinwen Jiao, Manuel
   Paul, Loa Andersson, Wim Henderickx, Mach Chen, Thomas Morin and
   Gregory Mirsky for their valuable comments and suggestions.

10.  Contributors

   Ryan Zheng
   JSPTPD
   371, Zhongshan South Road
   Nanjing, 210006, China
   Email: ryan.zhi.zheng@gmail.com

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
              Label Switched (MPLS) Data Plane Failures", RFC 4379,
              February 2006.

11.2.  Informative References

   [I-D.ietf-mpls-seamless-mpls]
              Leymann, N., Decraene, B., Filsfils, C., Konstantynowicz,
              M., and D. Steinberg, "Seamless MPLS Architecture",
              draft-ietf-mpls-seamless-mpls-05 (work in progress),
              January 2014.

Authors' Addresses

   Jian Luo (editor)
   ZTE
   50, Ruanjian Avenue
   Nanjing, 210012, China

   Email: luo.jian@zte.com.cn

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Internet-Draft      MPLS LSP Ping Relayed Echo Reply       February 2014

   Lizhong Jin (editor)
   Shanghai, China

   Email: lizho.jin@gmail.com

   Thomas Nadeau (editor)
   Lucidvision

   Email: tnadeau@lucidvision.com

   George Swallow (editor)
   Cisco
   300 Beaver Brook Road
   Boxborough , MASSACHUSETTS 01719, USA

   Email: swallow@cisco.com

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