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rfc:rfc4016

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

Parthasarathy Informational [Page 1] RFC 4016 PANA Threat Analysis March 2005

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.

Parthasarathy Informational [Page 2] RFC 4016 PANA Threat Analysis March 2005

 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

Parthasarathy Informational [Page 3] RFC 4016 PANA Threat Analysis March 2005

 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:

Parthasarathy Informational [Page 4] RFC 4016 PANA Threat Analysis March 2005

    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.

Parthasarathy Informational [Page 5] RFC 4016 PANA Threat Analysis March 2005

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.

Parthasarathy Informational [Page 6] RFC 4016 PANA Threat Analysis March 2005

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.

Parthasarathy Informational [Page 7] RFC 4016 PANA Threat Analysis March 2005

 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.

Parthasarathy Informational [Page 8] RFC 4016 PANA Threat Analysis March 2005

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.

Parthasarathy Informational [Page 9] RFC 4016 PANA Threat Analysis March 2005

 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.

Parthasarathy Informational [Page 10] RFC 4016 PANA Threat Analysis March 2005

 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.

Parthasarathy Informational [Page 11] RFC 4016 PANA Threat Analysis March 2005

 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,

Parthasarathy Informational [Page 12] RFC 4016 PANA Threat Analysis March 2005

         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.

Parthasarathy Informational [Page 13] RFC 4016 PANA Threat Analysis March 2005

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

Parthasarathy Informational [Page 14] RFC 4016 PANA Threat Analysis March 2005

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Parthasarathy Informational [Page 15]

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