Network Working Group Kireeti Kompella (Juniper)
Internet Draft Ping Pan (Juniper)
Expiration Date: September 2002 Nischal Sheth (Juniper)
Network Working Group Dave Cooper (Global Crossing)
George Swallow (Cisco)
Sanjay Wadhwa (Unisphere)
Ron Bonica (Worldcom)
Detecting Data Plane Liveliness in MPLS
draft-ietf-mpls-lsp-ping-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
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Abstract
This document describes a simple and efficient mechanism that can be
used to detect data plane failures in MPLS LSPs. There are two parts to
this document: information carried in an MPLS "echo request" and "echo
reply" for the purposes of fault detection and isolation; and mechanisms
for transporting the echo reply.
Sub-IP ID Summary
(See Abstract above.)
RELATED DOCUMENTS
May be found in the "references" section.
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WHERE DOES IT FIT IN THE PICTURE OF THE SUB-IP WORK
Fits in the MPLS box.
WHY IS IT TARGETED AT THIS WG
MPLS WG is currently looking at MPLS-specific error detection and
recovery mechanisms. The mechanisms proposed here are for packet-based
MPLS LSPs, which is why the MPLS WG is targeted.
JUSTIFICATION
The WG should consider this document, as it allows network operators to
detect MPLS LSP data plane failures in the network. This type of
failures have occurred, and are a source of concern to operators
implementing MPLS networks.
1. Introduction
This document describes a simple and efficient mechanism that can be
used to detect data plane failures in MPLS LSPs. There are two parts
to this document: information carried in an MPLS "echo request" and
"echo reply"; and mechanisms for transporting the echo reply. The
first part aims at providing enough information to check correct
operation of the data plane, as well as a mechanism to verify the
data plane against the control plane, and thereby localize faults.
The second part suggests two methods of reliable reply channels for
the echo request message, for more robust fault isolation.
An important consideration in this design is that MPLS echo requests
follow the same data path that normal MPLS packets would traverse.
MPLS echo requests are meant primarily to validate the data plane,
and secondarily to verify the data plane against the control plane.
Mechanisms to check the control plane are valuable, but are not
covered in this document.
To avoid potential Denial of Service attacks, it is recommended to
regulate the MPLS ping traffic going to the control plane. A rate
limiter should be applied to the well-known UDP port defined below.
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2. Motivation
When an LSP fails to deliver user traffic, the failure cannot always
be detected by the MPLS control plane. There is a need to provide a
tool that would enable users to detect such traffic "black holes" or
misrouting within a reasonable period of time; and a mechanism to
isolate faults.
In this document, we describe a mechanism, termed "MPLS ping", that
accomplishes these goals. This mechanism is modeled after the ICMP
echo request/reply, used by ping and traceroute to detect and
localize faults in IP networks. This document also offers some
alternative methods for replying to MPLS echo requests.
The basic idea is to test that packets that belong to a particular
Forwarding Equivalence Class (FEC) actually end their MPLS path on an
LSR that is an egress for that FEC. Therefore, an MPLS echo request
carries information about the FEC whose MPLS path is being verified.
This echo request is forwarded just like any other packet belonging
to that FEC. In "ping" mode (basic connectivity check), the packet
should reach the end of the path, at which point it is sent to the
control plane of the egress LSR, which then verifies that it is
indeed an egress for the FEC. In "traceroute" mode (fault
isolation), the packet is sent to the control plane of each transit
LSR, which performs various checks that it is indeed a transit LSR
for this path; this LSR also returns further information that helps
check the control plane against the data plane, i.e., that forwarding
matches what the routing protocols determined as the path.
One way these tools can be used is to periodically ping a FEC to
ensure connectivity. If the ping fails, one can then initiate a
traceroute to determine where the fault lies. One can also
periodically traceroute FECs to verify that forwarding matches the
control plane; however, this places a greater burden on transit LSRs
and thus should be used with caution.
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3. Packet Format
An MPLS ping packet is a (possibly labelled) UDP packet with the
following payload 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TimeStamp (seconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TimeStamp (microseconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reply mode | Reply flags | Return Code | Must be zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLVs ... |
. .
. .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Sequence Number is assigned by the sender of the MPLS echo
request, and can be used to detect missed replies (for example).
The TimeStamp is set to the time of day (in seconds and microseconds)
when the MPLS echo request or reply is sent, and may be used to
compute delay or round trip time (for example).
The Reply Mode can take one of the following values:
Value Meaning
----- -------
1 Reply via an IPv4 UDP packet
2 Reply via an IPv4 UDP packet with Router Alert
3 Reply via the control plane
Reply Flags are a bit vector with bit 0x1 being the Least Significant
Bit and bit 0x80 being the Most Significant Bit; the following bits
are defined:
Bit Meaning when set
--- ----------------
0x1 Downstream Mappings desired
0x2 Upstream direction pinged
Bit 0x2 is set when the reverse (upstream) direction of a bidirection
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LSP is being tested. The details of the procedures in this case will
be given in a later version.
The rest of the bits are reserved and must be zero.
The Return code can take one of the following values:
Value Meaning
----- -------
1 Replying router is an egress for the FEC
2 Replying router has no mapping for the FEC
3 Replying router is not one of the "Downstream Routers".
4 Replying router is one of the "Downstream Routers",
and its mapping for this FEC on the received interface
is the given label
5 Replying router is one of the "Downstream Routers",
but its mapping for this FEC is not the given label
TLVs (Type-Length-Value tuples) have the following 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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value |
. .
. .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Types are defined below; Length is the length of the Value field in
octets. The Value field depends on the Type; it is zero padded to
align to a four-octet boundary.
Type # Value Field
------ -----------
1 Target FEC Stack
2 Downstream Mapping
3.1. Target FEC Stack
A Target FEC Stack is a list of sub-TLVs. The number of elements is
determined by the looking at the sub-TLV length fields.
Type # Length Value Field
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------ ------ -----------
1 5 IPv4 prefix
2 17 IPv6 prefix
3 16 RSVP IPv4 Session
4 52 RSVP IPv6 Session
5 6 CR-LDP LSP ID
6 13 VPN IPv4 prefix
7 25 VPN IPv4 prefix
8 ?? L2 VPN "prefix"
Other FEC Stack Types will be defined as needed.
Note that this TLV defines a stack of FECs, the first FEC element
corresponding to the top of the label stack, etc. However, we will
assume for now that the stack consists of just one element. Also,
only the formats for FEC Types 1-5 will be described in this version.
3.1.1. IPv4 Prefix
The value consists of four octets of an IPv4 prefix followed by one
octet of prefix length in bits. The IPv4 prefix is in network byte
order.
3.1.2. IPv6 Prefix
The value consists of sixteen octets of an IPv6 prefix followed by
one octet of prefix length in bits. The IPv6 prefix is in network
byte order.
3.1.3. RSVP IPv4 Session
The value has the format below. The value fields are taken from
[RFC3209, sections 4.6.1.1 and 4.6.2.1]
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 tunnel end point address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LSP ID | Tunnel ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Tunnel ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 tunnel sender address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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3.1.4. RSVP IPv6 Session
The value has the format below. The value fields are taken from
[RFC3209, sections 4.6.1.2 and 4.6.2.2]
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 tunnel end point address |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LSP ID | Tunnel ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Tunnel ID |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 tunnel sender address |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.1.5. CR-LDP LSP ID
The value consists of the LSPID of the LSP being pinged. An LSPID is
a four octet IPv4 address (a local address on the head end LSR) plus
a two octet identifier that is unique for each LSP that starts on an
LSR.
3.2. Downstream Mapping
The Downstream Mapping is an optional TLV in an echo request. The
Length is 4 + 4*N, where N is the number of Downstream Labels. The
Value of a Downstream Mapping has the following 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream IPv4 Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream Label | Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream Label | Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
'Protocol' is taken from the following table:
Protocol # Signaling Protocol
---------- ------------------
0 Unknown
1 Static
2 BGP
3 LDP
4 RSVP-TE
5 CR-LDP
The notion of "downstream router" should be explained. Consider an
LSR X. If a packet with outermost label L and TTL n>1 arrived at X
on interface I, X must be able to compute which LSRs could receive
the packet with TTL=n, and what label they would see. (It is outside
the scope of this document to specify how this computation may be
done.) The set of these LSRs are the downstream routers (and their
corresponding labels) for X with respect to L.
The case where X is the LSR originating the echo request is a special
case. X needs to figure out what LSRs would receive a labelled
packet with TTL=1 when X tries to send a packet to the FEC Stack that
is being pinged.
4. Theory of Operation
4.1. MPLS Echo Request
An MPLS echo request is a labeled UDP packet sent to the well-known
port for MPLS ping [UDP port # 3503 assigned by IANA], with
destination IP address set to the ALL-ROUTERS multicast address
(224.0.0.2). The source IP address is set to a routable address of
the sender; the source port identifies the sending process. The IP
TTL in the UDP packet is set to 1.
An MPLS echo request MUST have a FEC Stack TLV. Also, the Reply Mode
must be set to the desired reply mode; the Return Code is set to zero
and ignored on receipt.
In "ping" mode (end-to-end connectivity check), the TTL in the
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outermost label is set to 255.
In "traceroute" mode (fault isolation mode), the TTL is set
successively to 1, 2, ....
In the "traceroute" mode, the echo request SHOULD contain one or more
Downstream Mapping TLVs. For TTL=1, all the downstream routers (and
corresponding labels) for the sender with respect to the FEC Stack
being pinged SHOULD be sent in the echo request. For n>1, the
Downstream Mapping TLVs from the echo reply for TTL=(n-1) are copied
to the echo request with TTL=n.
4.2. MPLS Echo Reply
An MPLS echo reply is a UDP packet. It MUST ONLY be sent in response
to an MPLS echo request. The source IP address is the Router ID of
the replier; the source port is the well-known UDP port for MPLS
ping. The destination IP address and UDP port are copied from the
source IP address and UDP port of the echo request. The IP TTL is
set to 255. If the Reply Mode in the echo request is "Reply via an
IPv4 UDP packet with Router Alert", then the IP header MUST contain
the Router Alert IP option.
The format of the echo reply is the same as the echo request. The
Sequence Number is copied from the echo request; the TimeStamp is set
to the time-of-day that the echo request is received (note that this
information is most useful if the time-of-day clocks on the requestor
and the replier are synchronized). The FEC Stack TLV from the echo
request is copied to the reply.
The replier MUST fill in the Return Code. This is set based on
whether the replier has a mapping for the FEC, and whether it is an
egress for that FEC. Note that 'having a mapping' for an RSVP FEC
means that the replier is a transit LSR for the RSVP LSP defined by
the FEC.
If the echo request contains a Downstream Mapping TLV, the replier
MUST further check whether its Router ID matches one of the
Downstream IPv4 Router IDs; and if so, whether the given Downstream
Label is in fact the label that the replier sent as its mapping for
the FEC. For an RSVP FEC, the downstream label is the label that the
replier sent in its Resv message. The result of these checks are
captured in the Return Code.
If the flag requesting Downstream Mapping TLVs is set in the Reply
Flags, the replier SHOULD compute its downstream routers and
corresponding labels for the incoming label, and add Downstream
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Mapping TLVs for each one to the echo reply it sends back.
4.3. Non-compliant Routers
If the egress for the FEC Stack being pinged does not support MPLS
ping, then no reply will be sent, resulting in possible "false
negatives". If in "traceroute" mode, a transit LSR does not support
MPLS ping, then no reply will be forthcoming from that LSR for some
TTL, say n. The LSR originating the echo request SHOULD try sending
the echo request with TTL=n+1, n+2, ..., n+k in the hope that some
transit LSR further downstream may support MPLS echo requests and
reply. In such a case, the echo request for TTL>n MUST NOT have
Downstream Mapping TLVs, until a reply is received with a Downstream
Mapping.
5. Reliable Reply Path
One of the issues that are faced with MPLS ping is to distinguish
between a failure in the forward path (the MPLS path being 'pinged')
and a failure in the return path. Note that this problem exists with
vanilla IP ping as well. In the case of MPLS ping, it is assumed
that the IP control and data planes are reliable. However, it could
be that the forwarding in the return path is via an MPLS LSP.
In this specification, we give two solutions for this problem. One
is to set the Router Alert option in the MPLS echo reply. When a
router sees this option, it MUST forward the packet as an IP packet.
Note that this may not work if some transit LSR does not support MPLS
ping.
Another option is to send the echo reply via the control plane. At
present, this is defined only for RSVP-TE LSPs, and described below.
These options are controlled by the ingress LSR, using the Reply Mode
in the MPLS echo request packet.
5.1. RSVP-TE Extension
To test an LSP's liveliness, an ingress LSR sends MPLS echo requests
over the LSP being tested. When an egress LSR receives the message,
it needs to acknowledge the ingress LSR by sending an LSP_ECHO object
in a RSVP Resv message. The object has the following format:
Class = LSP_ECHO (use form 11bbbbbb for compatibility)
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C-Type = 1
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TimeStamp (seconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TimeStamp (microseconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP Source Port | Return Code | Must be zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Sequence Number is copied from the Sequence Number of the echo
request. The TimeStamp is set to the time the echo request is
received. The UDP Source Port is copied from the UDP source port of
the MPLS echo request. The FEC is implied by the Session and the
Sender Template Objects.
5.2. Operation
For the sake of brevity in the context of this document by "the
control plane" we mean "the RSVP-TE component of the control plane".
Consider an LSP between an ingress LSR and an egress LSR spanning
multiple LSR hops.
5.3. Procedures at the ingress LSR
One must ensure before setting the Reply Mode to "reply via the
control plane" that the egress LSR supports this feature.
The ingress LSR, say X, selects a unique UDP source port for its MPLS
ping. X also sets the FEC Stack TLV Type to RSVP IPv4 (IPv6), and
copies the SESSION and SENDER_TEMPLATE into the appropriate fields of
the value field. Finally, X sets the Reply Mode to "reply via the
control plane".
If X does not receive an Resv message from the egress LSR that
contains an LSP_ECHO object within some period of time, it declares
the LSP as "down". At this point, the ingress LSR may apply the
necessary procedures to fix the LSP. These may include generating a
message to network management, tearing-down and re-building the LSP,
and/or rerouting user traffic to a backup LSP.
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To test an LSP that carries non-IP traffic, before injecting ICMP and
MPLS ping messages into the LSP, the IPv4 Explicit NULL label should
be prepended to such messages. The ingress and egress LSR's must
follow the procedures defined in [LABEL-STACKING].
5.4. Procedures at the egress LSR
When the egress LSR receives an MPLS ping message, it follows the
procedures given above. If the Reply Mode is set to "Reply via the
control plane", the LSR can, based on the RSVP SESSION and
SENDER_TEMPLATE objects carried in the MPLS ping message, find the
corresponding LSP in its RSVP-TE database. The LSR then checks to
see if the Resv message for this LSP contains an LSP_ECHO object with
the same source UDP port value. If not, the LSR adds or updates the
LSP_ECHO object and refreshes the Resv message.
5.5. Procedures for the intermediate LSR's
At intermediate LSRs, normal RSVP processing procedures will cause
the LSP_ECHO object to be forwarded as RSVP messages are refreshed.
At the LSR's that support MPLS ping the Resv messages that carry the
LSP_ECHO object MUST be delivered upstream immediately.
Note that an intermediate LSR using RSVP refresh reduction [RSVP-
REFRESH], the new or changed LSP_ECHO object will cause the LSR to
classify the RSVP message as a trigger message.
6. Security Considerations
The security considerations pertaining to the original RSVP protocol
remain relevant.
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7. Intellectual Property Considerations
Juniper Networks, Inc. is seeking patent protection on technology
described in this Internet-Draft. If technology in this Internet-
Draft is adopted as a standard, Juniper Networks agrees to license,
on reasonable and non-discriminatory terms, any patent rights it
obtains covering such technology to the extent necessary to comply
with the standard.
8. Acknowledgments
This is the outcome of many discussions among many people, that also
include Manoj Leelanivas, Paul Traina, Yakov Rekhter, Der-Hwa Gan,
Brook Bailey and Eric Rosen.
9. References
[ICMP] J. Postel, "Internet Control Message Protocol", RFC792.
[RSVP] R. Braden, Ed., et al, "Resource ReSerVation protocol (RSVP)
-- version 1 functional specification," RFC2205.
[RSVP-TE] D. Awduche, et al, "RSVP-TE: Extensions to RSVP for LSP
tunnels" Internet Draft.
[LABEL-STACKING] E. Rosen, et al, "MPLS Label Stack Encoding",
RFC3032.
[RSVP-REFRESH] L. Berger, et al, "RSVP Refresh Overhead Reduction
Extensions", RFC2961.
[RFC-IANA] T. Narten and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 2434.
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10. Author Information
Kireeti Kompella
Ping Pan
Nischal Sheth
Juniper Networks
1194 N.Mathilda Ave
Sunnyvale, CA 94089
e-mail: kireeti@juniper.net
e-mail: pingpan@juniper.net
e-mail: nsheth@juniper.net
phone: 408.745.2000
Dave Cooper
Global Crossing
960 Hamlin Court
Sunnyvale, CA 94089
email: dcooper@gblx.net
phone: 916.415.0437
George Swallow
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA 01824
e-mail: swallow@cisco.com
phone: 978.244.8143
Sanjay Wadhwa
Unisphere Networks, Inc.
10 Technology Park Drive
Westford, MA 01886-3146
email: swadhwa@unispherenetworks.com
phone: 978.589.0697
Ronald P. Bonica
WorldCom
22001 Loudoun County Pkwy
Ashburn, Virginia, 20147
email: ronald.p.bonica@wcom.com
phone: 703.886.1681
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