Network Working Group M. Parthasarathy
Request for Comments: 4016 Nokia
Category: Informational March 2005
Protocol for Carrying Authentication and Network Access (PANA)
Threat Analysis and Security Requirements
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document discusses the threats to protocols used to carry
authentication for network access. The security requirements arising
from these threats will be used as additional input to the Protocol
for Carrying Authentication for Network Access (PANA) Working Group
for designing the IP based network access authentication protocol.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Keywords . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Terminology and Definitions. . . . . . . . . . . . . . . . . . 2
4. Usage Scenarios. . . . . . . . . . . . . . . . . . . . . . . . 3
5. Trust Relationships. . . . . . . . . . . . . . . . . . . . . . 4
6. Threat Scenarios . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. PAA Discovery. . . . . . . . . . . . . . . . . . . . . . 6
6.2. Authentication . . . . . . . . . . . . . . . . . . . . . 6
6.3. PaC Leaving the Network. . . . . . . . . . . . . . . . . 9
6.4. Service Theft. . . . . . . . . . . . . . . . . . . . . . 10
6.5. PAA-EP Communication . . . . . . . . . . . . . . . . . . 11
6.6. Miscellaneous Attacks. . . . . . . . . . . . . . . . . . 12
7. Summary of Requirements. . . . . . . . . . . . . . . . . . . . 13
8. Security Considerations. . . . . . . . . . . . . . . . . . . . 13
9. Normative References . . . . . . . . . . . . . . . . . . . . . 14
10. Informative References . . . . . . . . . . . . . . . . . . . . 14
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
The Protocol for Carrying Authentication for Network Access (PANA)
Working Group is developing methods for authenticating clients to the
access network using IP based protocols. This document discusses the
threats to such IP based protocols.
A client wishing to get access to the network must carry on multiple
steps. First, it needs to discover the IP address of the PANA
authentication agent (PAA) and then execute an authentication
protocol to authenticate itself to the network. Once the client is
authenticated, there might be other messages exchanged during the
lifetime of the network access. This document discusses the threats
in these steps without discussing any solutions. The requirements
arising out of these threats will be used as input to the PANA
Working Group. The use of word co-located in this document means
that the referred entities are present on the same node.
2. Keywords
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 [KEYWORDS].
3. Terminology and Definitions
Client Access Device
A network element (e.g., notebook computer, PDA) that requires
access to a provider's network.
Network Access Server (NAS)
Network device that provides access to the network.
PANA Client (PaC)
An entity in the edge subnet that seeks to obtain network access
from a PANA authentication agent within a network. A PANA client
is associated with a device and a set of credentials to prove its
identity within the scope of PANA.
PANA Authentication Agent (PAA)
An entity whose responsibility is to authenticate the PANA client
and to grant network access service to the client's device.
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Authentication Server (AS)
An entity that authenticates the PANA client. It may be
co-located with the PANA authentication agent or part of the
back-end infrastructure.
Device Identifier (DI)
The identifier used by the network to control and police the
network access of a client. Depending on the access technology,
the identifier might contain the IP address, link-layer address,
switch port number, etc., of a device. The PANA authentication
agent keeps a table for binding device identifiers to the PANA
clients. At most one PANA client should be associated with a DI
on a PANA authentication agent.
Enforcement Point (EP)
A node capable of filtering packets sent by the PANA client by
using the DI information authorized by PANA authentication agent.
Compound methods
Authentication protocol in which methods are used in a sequence
one after another or in which methods are tunneled inside another
independently established tunnel between the client and server
[TUN-EAP].
4. Usage Scenarios
PANA is intended to be used in an environment where there is no a
priori trust relationship or security association between the PaC
and other nodes, such as the PAA and EP. In these environments,
one may observe the following:
o The link between PaC and PAA may be a shared medium (e.g.,
Ethernet) or may not be a shared medium (e.g., DSL network).
o All the PaCs may be authenticated to the access network at
layer 2 (e.g., 3GPP2 CDMA network) and share a security
association with a layer 2 authentication agent (e.g., BTS).
The PaCs still don't trust each other; any PaC can pretend to
be a PAA, spoof IP addresses, and launch various other attacks.
The scenarios mentioned above affect the threat model of PANA. This
document discusses the various threats in the context of the above
network access scenarios for a better understanding of the threats.
In the following discussion, any reference to a link that is not
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shared (or non-shared) is assumed to be physically secure. If such
an assumption cannot be made about the link, then the case becomes
the same as that for a link being shared by more than one node.
5. Trust Relationships
PANA authentication involves a client (PaC), a PANA agent (PAA), an
Authentication server (AS), and an Enforcement point (EP). The AS
here refers to the AAA server that resides in the home realm of the
PaC.
The entities that have a priori trust relationships before PANA
begins are as follows:
1) PAA and AS: The PaC belonging to the same administrative domain
that the AS does often has to use resources provided by a PAA
that belongs to another administrative domain. A PAA
authenticates the PaC before providing local network access.
The credentials provided by the PaC for authentication may or
may not be understood by the PAA. If the PAA does not
understand the credentials, it needs to communicate with the AS
in a different domain to verify the credentials. The threats
in the communication path between the PAA and AS are already
covered in [RAD-EAP]. To counter these threats, the
communication between the PAA and AS is secured by using a
static or dynamic security association.
2) PAA and EP: The PAA and EP belong to the same administrative
domain. Hence, the network operator can set up a security
association to protect the traffic exchanged between them.
This document discusses the threats in this path.
3) PaC and AS: The PaC and AS belong to the same administrative
domain and share a trust relationship. When the PaC uses a
different domain than its home for network access, it provides
its credentials to the PAA in the visited network for
authentication. The information provided by the PaC traverses
the PaC-PAA and PAA-AS paths. The threats in the PAA-AS path
are already discussed in [RAD-EAP]. This document discusses
the threats in the PaC-PAA path.
It is possible that some of the entities such as the PAA, AS, and EP
are co-located. In those cases, it can be safely assumed that there
are no significant external threats in their communication.
The entities that do not have any trust relationship before PANA
begins are as follows:
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1) PaC and PAA: The PaC and PAA normally belong to two different
administrative domains. They do not necessarily share a trust
relationship initially. They establish a security association
in the process of authentication. All messages exchanged
between the PaC and PAA are subject to various threats, which
are discussed in this document.
2) PaC and EP: The EP belongs to the same administrative domain as
the PAA. Hence, the PaC and EP do not necessarily share a
trust relationship initially. When the PaC is successfully
authenticated, it may result in key establishment between the
PaC and PAA, which can be further used to secure the link
between the PaC and EP. For example, the EAP keying framework,
[EAP-KEY], defines a three party EAP exchange in which the
clients derive the transient sessions keys to secure the link
between the peer and NAS in their final step. Similarly, PANA
will provide the ability to establish keys between the PaC and
EP that can be used to secure the link further. This is
discussed further in Section 6.4 below.
6. Threat Scenarios
First, the PaC needs to discover the PAA. This involves either
sending solicitations or waiting for advertisements. Once it has
discovered the PAA, the two will enter authentication exchange. Once
the access is granted, the PaC will most likely exchange data with
other nodes in the Internet. These steps are vulnerable to man-in-
the-middle (MITM), denial of service (DoS), and service theft
attacks, which are discussed below.
The threats are grouped by the various stages the client goes through
to gain access to the network. Section 6.1 discusses the threats
related to PAA discovery. Section 6.2 discusses the threats related
to authentication itself. Section 6.3 discusses the threats involved
when leaving the network. Section 6.4 discusses service theft.
Section 6.5 discusses the threats in the PAA-EP path. Section 6.6
discusses the miscellaneous threats.
Some of the threats discussed in the following sections may be
specific to shared links. The threat may be absent on non-shared
links. Hence, it is only required to prevent the threat on shared
links. Instead of specifying a separate set of requirements for
shared links and non-shared links, this document specifies one set of
requirements with the following wording: "PANA MUST be able to
prevent threat X". This means that the PANA protocol should be
capable of preventing threat X. The feature that prevents threat X
may or may not be used depending on the deployment.
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6.1. PAA Discovery
The PAA is discovered by sending solicitations or receiving
advertisements. The following are possible threats.
T6.1.1: A malicious node can pretend to be a PAA by sending a spoofed
advertisement.
In existing dial-up networks, the clients authenticate to the network
but generally do not verify the authenticity of the messages coming
from Network Access Server (NAS). This mostly works because the link
between the device and the NAS is not shared with other nodes
(assuming that nobody tampers with the physical link), and clients
trust the NAS and the phone network to provide the service. Spoofing
attacks are not present in this environment, as the PaC may assume
that the other end of the link is the PAA.
In environments where the link is shared, this threat is present, as
any node can pretend to be a PAA. Even if the nodes are
authenticated at layer 2, the threat remains present. It is
difficult to protect the discovery process, as there is no a priori
trust relationship between the PAA and PaC. In deployments where EP
can police the packets that are sent among the PaCs, it is possible
to filter out the unauthorized PANA packets (e.g., PAA advertisements
sent by PaC) to prevent this threat.
The advertisement may be used to include information (such as
supported authentication methods) other than the discovery of the PAA
itself. This can lead to a bidding down attack, as a malicious node
can send a spoofed advertisement with capabilities that indicate
authentication methods less secure than those that the real PAA
supports, thereby fooling the PaC into negotiating an authentication
method less secure than would otherwise be available.
Requirement 1
PANA MUST not assume that the discovery process is protected.
6.2. Authentication
This section discusses the threats specific to the authentication
protocol. Section 6.2.1 discusses the possible threat associated
with success/failure indications that are transmitted to PaC at the
end of the authentication. Section 6.2.2 discusses the man-in-the-
middle attack when compound methods are used. Section 6.2.3
discusses the replay attack, and Section 6.2.4 discusses the device
identifier attack.
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6.2.1. Success or Failure Indications
Some authentication protocols (e.g., EAP) have a special message to
indicate success or failure. An attacker can send a false
authentication success or failure message to the PaC. By sending a
false failure message, the attacker can prevent the client from
accessing the network. By sending a false success message, the
attacker can prematurely end the authentication exchange, effectively
denying service for the PaC.
If the link is not shared, then this threat is absent, as ingress
filtering can prevent the attacker from impersonating the PAA.
If the link is shared, it is easy to spoof these packets. If layer 2
provides per-packet encryption with pair-wise keys, it might make it
hard for the attacker to guess the success or failure packet that the
client would accept. Even if the node is already authenticated at
layer 2, it can still pretend to be a PAA and spoof the success or
failure.
This attack is possible if the success or failure indication is not
protected by using a security association between the PaC and the
PAA. In order to avoid this attack, the PaC and PAA should mutually
authenticate each other. In this process, they should be able to
establish keys to protect the success or failure indications. It may
not always be possible to protect the indication, as the keys may not
be established prior to transmitting the success or failure packet.
If the client is re-authenticating to the network, it can use the
previously established security association to protect the success or
failure indications. Similarly, all PANA messages exchanged during
the authentication prior to key establishment may not be protected.
Requirement 2
PANA MUST be able to mutually authenticate the PaC and PAA. PANA
MUST be able to establish keys between the PaC and PAA to protect the
PANA messages.
6.2.2. MITM Attack
A malicious node can claim to be the PAA to the real PaC and claim to
be the PaC to the real PAA. This is a man-in-the-middle (MITM)
attack, whereby the PaC is fooled to think that it is communicating
with the real PAA and the PAA is fooled to think that it is
communicating with the real PaC.
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If the link is not shared, this threat is absent, as ingress
filtering can prevent the attacker from acting as a man-in-the-
middle.
If the link is shared, this threat is present. Even if the layer 2
provides per-packet protection, the attacker can act as a man-in-
the-middle and launch this attack. An instance of MITM attack, in
which compound authentication methods are used is described in
[TUN-EAP]. In these attacks, the server first authenticates to the
client. As the client has not proven its identity yet, the server
acts as the man-in-the-middle, tunneling the identity of the
legitimate client to gain access to the network. The attack is
possible because there is no verification that the same entities
participated among the compound methods. It is not possible to do
such verification if compound methods are used without being able to
create a cryptographic binding among them. This implies that PANA
will be vulnerable to such attacks if compound methods are used
without being able to cryptographically bind them. Note that the
attack does not exist if the keys derived during the tunnel
establishment are not used to authenticate the client (e.g., tunnel
keys are used for just protecting the identity of the client).
Requirement 3
When compound authentication methods are used in PANA, the methods
MUST be cryptographically bound.
6.2.3. Replay Attack
A malicious node can replay the messages that caused authentication
failure or success at a later time to create false failures or
success. The attacker can also potentially replay other messages of
the PANA protocol to deny service to the PaC.
If the link is not shared, this threat is absent, as ingress
filtering can prevent the attacker from impersonating the PAA to
replay the packets.
If the link is shared, this threat is present. If the packets are
encrypted at layer 2 by using pair-wise keys, it will make it hard
for the attacker to learn the unencrypted (i.e., original) packet
that needs to be replayed. Even if layer 2 provides replay
protection, the attacker can still replay the PANA messages (layer 3)
for denying service to the client.
Requirement 4
PANA MUST be able to protect itself against replay attacks.
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6.2.4. Device Identifier Attack
When the client is successfully authenticated, the PAA sends access
control information to the EP for granting access to the network.
The access control information typically contains the device
identifier of the PaC, which is either obtained from the IP headers
and MAC headers of the packets exchanged during the authentication
process or carried explicitly in the PANA protocol field. The
attacker can gain unauthorized access into the network by taking the
following steps.
o An attacker pretends to be a PAA and sends advertisements. The
PaC is fooled and starts exchanging packets with the attacker.
o The attacker modifies the IP source address on the packet,
adjusts the UDP/TCP checksum, and forwards the packet to the
real PAA. It also does the same on return packets.
o When the real PaC is successfully authenticated, the attacker
gains access to the network, as the packets contained the IP
address (and potentially the MAC address also) of the attacker.
If the link is not shared, this threat is absent, as the attacker
cannot impersonate the PAA and intercept the packets from the PaC.
If the link is shared, this threat is present. If the layer 2
provides per-packet protection, it is not possible to change the MAC
address, and hence this threat may be absent in such cases if EP
filters on both the IP and MAC address.
Requirement 5
PANA MUST be able to protect the device identifier against spoofing
when it is exchanged between the PaC and PAA.
6.3. PaC Leaving the Network
When the PaC leaves the network, it can inform the PAA before
disconnecting from the network so that the resources used by PaC can
be accounted properly. The PAA may also choose to revoke the access
anytime it deems necessary. The following are possible threats:
T6.3.1: A malicious node can pretend to be a PAA and revoke the
access to PaC.
T6.3.2: A malicious node can pretend to be a real PaC and transmit a
disconnect message.
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T6.3.3: The PaC can leave the network without notifying the PAA or EP
(e.g., the Ethernet cable is unplugged, system crash). An
attacker can pretend to be the PaC and start using the
network.
If the link is not shared, threats T6.3.1 and T6.3.2 are absent.
Threat T6.3.3 may still be present. If there is no layer 2
indication, or if the layer 2 indication cannot be relied upon, then
threat T6.3.3 is still present on non-shared links.
If the link is shared, all of the above threats are present, as any
node on the link can spoof the disconnect message. Even if layer 2
has per-packet authentication, the attacker can pretend to be a PaC
(e.g., by spoofing the IP address) and disconnect from the network.
Similarly, any node can pretend to be a PAA and revoke the access to
the PaC. Therefore, T6.3.1 and T6.3.2 are possible even on links
where layer 2 is secured. Threat T6.3.3 can be prevented if layer 2
provides per-packet authentication. The attacker cannot subsume the
PaC that left the network without knowing the keys that protect the
packet at layer 2.
Requirement 6
PANA MUST be able to protect disconnect and revocation messages.
PANA MUST NOT depend on the PaC sending a disconnect message.
6.4. Service Theft
An attacker can gain unauthorized access into the network by stealing
the service from another client. Once the real PaC is successfully
authenticated, the EP will have filters in place to prevent
unauthorized access into the network. The filters will be based on
something that will be carried on every packet. For example, the
filter could be based on the IP and MAC addresses, where the packets
will be dropped unless the packets coming with certain IP addresses
also match the MAC addresses. The following are possible threats:
T6.4.1: An attacker can spoof both the IP and MAC addresses of an
authorized client to gain unauthorized access. The attacker
can launch this attack easily by just sniffing the wire for
IP and MAC addresses. This lets the attacker use the network
without any authorization, getting a free service.
T6.4.2: The PaC can leave the network without notifying the PAA or EP
(e.g., the Ethernet cable is unplugged, system crash). An
attacker can pretend to be the PaC and start using the
network.
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Service theft allows the possibility of exploiting the weakness in
other authentication protocols that use IP address for
authentication. It also allows the interception of traffic destined
for other nodes by spoofing the IP address.
If the link is not shared, T6.4.1 is absent, as there is only one
client on the link, and ingress filtering can prevent the use of the
authorized IP and MAC addresses by the attacker on another link.
Threat T6.4.2 exists, as the attacker can use the IP or MAC address
of the real PaC to gain access to the network.
If the link is shared, both the threats are present. If layer 2
provides per-packet protection using pair-wise keys, both the threats
can be prevented.
Requirement 7
PANA MUST securely bind the authenticated session to the device
identifier of the client, to prevent service theft. PANA MUST be
able to bootstrap a shared secret between the PaC and PAA that can be
further used to set up a security association between the PaC and EP
to provide cryptographic protection against service theft.
6.5. PAA-EP Communication
After a successful authentication, the PAA needs to communicate the
access control information of the PaC to the EP so that the PaC will
be allowed to access the network. The information communicated would
contain at least the device identifier of the PaC. If strong
security is needed, the PAA will communicate a shared secret known
only to the PaC and PAA, for setting up a security association
between the PaC and EP. The following are possible threats:
T6.5.1: An attacker can eavesdrop to learn the information
communicated between the PAA and EP. The attacker can
further use this information to spoof the real PaC and also
to set up security association for gaining access to the
network. This threat is absent if the attacker cannot
eavesdrop on the link; e.g., the PAA and EP communicate on a
link separate from that of visiting PaCs.
T6.5.2: An attacker can pretend to be a PAA and send false
information to an EP to gain access to the network. In the
case of stronger security, the attacker has to send its own
device identifier and also a shared secret, so that the EP
will let the attacker access the network.
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If the communication between the PAA and EP is protected, these
threats are absent.
Requirement 8
The communication between the PAA and EP MUST be protected against
eavesdropping and spoofing attacks.
6.6. Miscellaneous Attacks
T6.6.1: There are various forms of DoS attacks that can be launched
on the PAA or AS. A few are mentioned below. As it is hard
to defend against some of the DoS attacks, the protocol
should be designed carefully to mitigate or prevent such
attacks.
o An attacker can bombard the PAA with lots of
authentication requests. If the PAA and AS are not co-
located, the PAA may have to allocate resources to store
some state about the PaC locally before it receives the
response from the back-end AS. This can deplete memory
resources on the PAA.
o With minimal effort, an attacker can force the PAA or AS
to make computationally intensive operations with minimal
effort, that can deplete the CPU resources of the PAA or
AS.
T6.6.2: PaC acquires an IP address by using stateful or stateless
mechanisms before PANA authentication begins [PANAREQ]. When
the IP addresses are assigned before the client
authentication, it opens up the possibility of DoS attacks in
which unauthenticated malicious nodes can deplete the IP
address space by acquiring multiple IP addresses or deny
allocation to others by responding to every duplicate address
detection (DAD) query.
Depleting a /64 IPv6 link-local address space or a /8 RFC1918
private address space requires a brute-force attack. Such an
attack is part of a DoS class that can equally target the
link capacity or the CPU cycles on the target system by
bombarding arbitrary packets. Therefore, solely handling the
IP address depletion attack is not going to improve the
security, as a more general solution is needed to tackle the
whole class of brute-force attacks.
The DAD attack can be prevented by deploying secure address
resolution that does not depend on the client authentication,
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such as [SEND]. The attack may also be prevented if the EP
is placed between the PaCs to monitor the ND/ARP activity and
to detect DAD attacks (excessive NA/ARP replies). If none of
these solutions are applicable to a deployment, the PaCs can
send arbitrary packets to each other without going through
the EP, which enables a class of attacks that are based on
interfering with the PANA messaging (See T6.1.1). Since
there will always be a threat in this class (e.g., insecure
discovery), it is not going to improve the overall security
by addressing DAD.
7. Summary of Requirements
1. PANA MUST not assume that the discovery process is protected.
2. PANA MUST be able to mutually authenticate the PaC and PAA. PANA
MUST be able to establish keys between the PaC and PAA to protect
the PANA messages.
3. When compound authentication methods are used in PANA, the methods
MUST be cryptographically bound.
4. PANA MUST be able to protect itself against replay attacks.
5. PANA MUST be able to protect the device identifier against
spoofing when it is exchanged between the PaC and PAA.
6. PANA MUST be able to protect disconnect and revocation messages.
PANA MUST NOT depend on whether the PaC sends a disconnect
message.
7. PANA MUST securely bind the authenticated session to the device
identifier of the client, to prevent service theft. PANA MUST be
able to bootstrap a shared secret between the PaC and PAA that can
be further used to set up a security association between the PaC
and EP to provide cryptographic protection against service theft.
8. The communication between the PAA and EP MUST be protected against
eavesdropping and spoofing attacks.
8. Security Considerations
This document discusses various threats with IP based network access
authentication protocol. Though this document discusses the threats
for shared and unshared links separately, it may be difficult to make
such a distinction in practice (e.g., a dial-up link may be a point-
to-point IP tunnel). Hence, the link should be assumed to be a
shared link for most of the threats in this document.
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9. Normative References
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
10. Informative References
[PANAREQ] Yegin, A., Ed., Ohba, Y., Penno, R., Tsirtsis, G., and
C. Wang, "Protocol for Carrying Authentication for
Network Access (PANA) Requirements and Terminology",
Work in Progress, August 2004.
[EAP-KEY] Aboba, B., et al., "EAP keying framework", Work in
Progress.
[RAD-EAP] Aboba, B. and P. Calhoun, "RADIUS (Remote
Authentication Dial In User Service) Support For
Extensible Authentication Protocol (EAP)", RFC 3579,
September 2003.
[TUN-EAP] Puthenkulam, J., et al., "The compound authentication
binding problem", Work in Progress.
[SEND] Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
"SEcure Neighbor Discovery (SEND)", RFC 3971, March
2005.
11. Acknowledgements
The author would like to thank the following people (in no specific
order) for providing valuable comments: Alper Yegin, Basavaraj Patil,
Pekka Nikander, Bernard Aboba, Francis Dupont, Michael Thomas,
Yoshihiro Ohba, Gabriel Montenegro, Tschofenig Hannes, Bill
Sommerfeld, N. Asokan, Pete McCan, Derek Atkins, and Thomas Narten.
Author's Address
Mohan Parthasarathy
Nokia
313 Fairchild Drive
Mountain View, CA-94303
EMail: mohanp@sbcglobal.net
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