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PPPEXT Working Group Ashwin Palekar
INTERNET-DRAFT Dan Simon
Category: Standards Track Microsoft Corporation
<draft-josefsson-pppext-eap-tls-eap-07.txt> Glen Zorn
26 October 2003 Joe Salowey
Hao Zhou
Cisco Systems
S. Josefsson
Exundo
Protected EAP Protocol (PEAP) Version 2
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC 2026.
Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups. Note that other groups
may also distribute working documents as Internet- Drafts.
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and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
The Extensible Authentication Protocol (EAP) provides for support of
multiple authentication methods. This document defines the Protected
Extensible Authentication Protocol (PEAP) Version 2, which provides
an encrypted and authenticated tunnel based on transport layer
security (TLS) that encapsulates EAP authentication mechanisms.
PEAPv2 uses TLS to protect against rogue authenticators, protect
against various attacks on the confidentiality and integrity of the
inner EAP method exchange and provide EAP peer identity privacy.
PEAPv2 also provides support for chaining multiple EAP mechanisms,
cryptographic binding between authentications performed by inner EAP
mechanisms and the tunnel, exchange of arbitrary parameters (TLVs),
optimized session resumption, and fragmentation and reassembly.
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Table of Contents
1. Introduction .......................................... 4
1.1 Requirements language ........................... 6
1.2 Terminology ..................................... 6
1.3 Operational model ............................... 8
2. Protocol overview ..................................... 11
2.1 PEAPv2 Part 1 .................................. 11
2.2 PEAPv2 Part 2 .................................. 16
2.3 Error handling ................................. 19
2.4 Fragmentation .................................. 21
2.5 Key derivation ................................. 22
2.6 Ciphersuite negotiation ........................ 24
3. Detailed description of the PEAPv2 protocol .......... 25
3.1 PEAPv2 Packet Format ........................... 25
3.2 PEAPv2 Request Packet .......................... 26
3.3 PEAPv2 Response Packet ......................... 28
3.4 PEAPv2 Part 2 Packet Format ................... 29
4. EAP TLV method ....................................... 30
4.1 EAP-TLV Request packet ......................... 30
4.2 EAP-TLV Response packet ......,,................ 31
4.3 TLV format ..................................... 32
4.4 Result TLV ..................................... 33
4.5 NAK TLV ........................................ 34
4.6 Crypto-Binding TLV ............................. 35
4.7 Connection-Binding TLV ......................... 37
4.8 Vendor-Specific TLV ............................ 38
4.9 URI TLV ........................................ 39
4.10 EAP Payload TLV ................................ 40
4.11 Intermediate Result TLV ........................ 41
4.12 TLV Rules ...................................... 42
5. Security considerations .............................. 43
5.1 Authentication and integrity protection ........ 43
5.2 Method negotiation ............................. 44
5.3 TLS session cache handling ..................... 44
5.4 Certificate revocation ......................... 45
5.5 Separation of EAP server and authenticator ..... 46
5.6 Separation of PEAPv2 Part 1 and Part 2 Servers . 47
5.7 Identity verification .......................... 48
5.8 Man-in-the-middle attack protection ............ 50
5.9 Cleartext forgeries ............................ 50
5.10 TLS Ciphersuites ............................... 51
5.11 Denial of service attacks ...................... 51
5.12 Security Claims ................................ 52
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6. IANA Considerations ................................. 53
6.1 Definition of Terms ............................ 53
6.2 Recommended Registration Policies .............. 53
7. References .......................................... 53
7.1 Normative references ........................... 53
7.2 Informative references ......................... 54
Appendix A - Examples ........................................ 56
Acknowledgments .............................................. 70
Author's Addresses ........................................... 70
Intellectual Property Statement .............................. 71
Full Copyright Statement ..................................... 72
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1. Introduction
The Extensible Authentication Protocol (EAP), defined in
[RFC2284bis], provides for support of multiple authentication
methods. EAP was developed for use on wired networks, where physical
security was presumed. EAP over PPP, defined in [RFC2284bis], is
typically deployed with leased lines or modem connections, requiring
an attacker to gain access to the telephone network in order to snoop
on the conversation or inject packets. [IEEE8021X] defines EAP over
IEEE 802 local area networks(EAPOL), presuming the existence of
switched media; in order to snoop or inject packets, an attacker
would need to gain administrative access to the switch. Due to the
presumption of physical security, facilities for protection of the
EAP conversation were not provided. Where an attacker can easily gain
access to the medium (such as on a wireless network or where EAP is
run over IP), the presumption of physical security is no longer
valid.
Since its deployment, a number of weaknesses in EAP framework have
become apparent. These include lack of:
identity protection
protected method negotiation
protected notification messages
protected termination messages
support for sequences of EAP methods
support for fragmentation and reassembly
support for a generic way to exchange arbitrary
parameters in a secure channel
support for generic optimized re-authentication
In addition many EAP methods lack the following features:
mutual authentication
resistance to dictionary attacks
adequate key generation
By wrapping the EAP protocol within TLS, Protected EAP (PEAP) Version
2 addresses these deficiencies in EAP or EAP methods. TLS provides
per-packet encryption, authentication, integrity and replay
protection of the EAP conversation. Benefits of PEAP Version 2
include:
Identity protection
By encrypting the identity exchange, and allowing client
certificates to be provided after negotiation of the TLS channel,
PEAPv2 provides for identity protection.
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Dictionary attack resistance
By conducting the EAP conversation within a TLS channel, PEAPv2
protects EAP methods that might be subject to an offline dictionary
attack were they to be conducted in the clear.
Protected negotiation
Since within PEAPv2, the EAP conversation is authenticated,
integrity and replay protected on a per-packet basis, the EAP
method negotiation that occurs within PEAPv2 is protected, as are
error messages sent within the TLS channel (TLS alerts or EAP
Notification packets). EAP negotiation outside of PEAPv2 is not
protected.
Header protection
Within PEAPv2, the EAP conversation is conducted within a TLS
channel. As a result, the Type-Data field within PEAPv2 (including
the EAP header of the EAP method within PEAPv2) is protected
against modification. However, the EAP header of PEAPv2 itself is
not protected against modification, including the Code, Identifier
and Type fields.
Protected termination
By sending success/failure indications within the TLS channel,
PEAPv2 provides support for protected termination of the EAP
conversation. This prevents an attacker from carrying out denial
of service attacks by spoofing EAP Failure messages, or fooling the
EAP peer into accepting a rogue NAS, by spoofing EAP Success
messages.
Fragmentation and Reassembly
Since EAP does not include support for fragmentation and
reassembly, individual methods need to include this capability. By
including support for fragmentation and reassembly within PEAPv2,
methods leveraging PEAPv2 do not need to support this on their own.
Fast reconnect
Where EAP is used for authentication in wireless networks, the
authentication latency is a concern. As a result, it is valuable
to be able to do a quick re-authentication on roaming between
access points. PEAPv2 supports this capability by leveraging the
TLS session resumption facility, and any EAP method running under
PEAPv2 can take advantage of it.
Standard key establishment
In order to provide keying material for a wide range of link layer
ciphersuites, EAP methods need to provide keying material. Key
derivation is complex. PEAPv2 provides for key establishment by
relying on the widely implemented and well-reviewed TLS [RFC2246]
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key derivation mechanism. PEAPv2 provides keying material for any
EAP method running within it. If EAP methods will also be deployed
without external protection (e.g PEAPv2 or IPSec), then the EAP
methods should follow the guidelines in section 6.8 to prevent the
man-in-the-middle attacks.
Sequencing of multiple EAP methods
In order to enhance security, PEAPv2 implementations may choose to
provide multi-factor authentication that validates different
identities (for example user and machine identities) and/or uses
different credentials of the same or different identities of the
peer (e.g. user password and machine cert). PEAPv2 provides a
standard way to chain different types of authentication mechanisms
supporting different types of credentials.
Protected exchange of arbitrary parameters (TLVs)
Type-Length-Value (TLV) tuples provide a way to exchange arbitrary
information between peer and EAP server within a secure channel.
This information can include signaling parameters for EAP protocol,
provisioning parameters, media specific and environment specific
data, and authorization parameters. The advantage of using PEAP
TLVs is that every EAP method does not have to be modified.
1.1. Requirements language
In this document, the key words "MAY", "MUST, "MUST NOT",
"OPTIONAL", "RECOMMENDED", "SHOULD", and "SHOULD NOT", are to be
interpreted as described in [RFC2119].
1.2. Terminology
This document frequently uses the following terms:
Access Point
A Network Access Server implementing 802.11.
Authenticator
The end of the link initiating EAP authentication. This term is
also used in [IEEE8021X] and has the same meaning in this document.
Backend Authentication Server
A backend authentication server is an entity that provides an
authentication service to an Authenticator. When used, this server
typically executes EAP methods for the Authenticator. This
terminology is also used in [IEEE8021X].
EAP server
The entity that terminates the EAP authentication method with the
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peer. In the case where no backend authentication server is used,
the EAP server is part of the Authenticator. In the case where the
authenticator operates in pass-through mode, the EAP server is
located on the backend authentication server.
Link layer ciphersuite
The ciphersuite negotiated for use at the link layer.
NAS Short for "Network Access Server".
Peer The end of the link that responds to the authenticator. In
[IEEE8021X], this end is known as the Supplicant.
TLS Ciphersuite
The ciphersuite negotiated for protection of the PEAPv2 Part 2
conversation.
EAP Master key (MK)
A key derived between the PEAPv2 client and server during the
authentication conversation, and that is kept local to PEAPv2 and
not exported or made available to a third party.
Master Session Key (MSK)
Keying material (64 octets) that is derived between the PEAPv2
client and server and exported by the PEAPv2 implementation.
AAA-Key
Where a backend authentication server is present, acting as an EAP
server, keying material known as the AAA-Key is transported from
the authentication server to the authenticator. The AAA-Key is
used by the EAP peer and authenticator in the derivation of
Transient Session Keys (TSKs) for the ciphersuite negotiated
between the EAP peer and authenticator. As a result, the AAA-Key
is typically known by all parties in the EAP exchange: the peer,
authenticator and the authentication server (if present).
Extended Master Session Key (EMSK)
Additional keying material (64 octets) derived between the EAP
client and server that is exported by the EAP method. The EMSK is
known only to the EAP peer and server and is not provided to a
third party.
Initialization Vector (IV)
A 64 octet quantity, suitable for use in an initialization vector
field, that is derived between the EAP client and server. Since
the IV is a known value in PEAPv2, it cannot be used by itself for
computation of any quantity that needs to remain secret. As a
result, PEAPv2 implementations are not required to generate it.
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Pairwise Master Key (PMK)
The AAA-Key is divided into two halves, the "Peer to Authenticator
Encryption Key" (Enc-RECV-Key) and "Authenticator to Peer
Encryption Key" (Enc-SEND-Key) (reception is defined from the point
of view of the authenticator). Within [IEEE80211i] Octets 0-31 of
the AAA-Key (Enc-RECV-Key) are known as the Pairwise Master Key
(PMK).
Transient EAP Keys (TEKs)
Session keys which are used to establish a protected channel
between the EAP peer and server during the EAP authentication
exchange. The TEKs are appropriate for use with the ciphersuite
negotiated between EAP peer and server for use in protecting the
EAP conversation. Note that the ciphersuite used to set up the
protected channel between the EAP peer and server during EAP
authentication is unrelated to the ciphersuite used to subsequently
protect data sent between the EAP peer and Authenticator.
TLV TLV standards for objects of format Type-Length-Value. The TLV
format is defined in Section 4 of this document.
1.3. Operational model
In EAP, the EAP server may be implemented either within a Network
Access Server (NAS) or on a backend authentication server. Where the
EAP server resides on a NAS, the NAS is required to implement the
desired EAP methods, and therefore needs to be upgraded to support
each new EAP method.
One of the goals of EAP is to enable development of new
authentication methods without requiring deployment of new code on
the Network Access Server (NAS). Where a backend authentication
server is deployed, the NAS acts as a "passthrough" and need not
understand specific EAP methods.
This allows new EAP methods to be deployed on the EAP peer and
backend authentication server, without the need to upgrade code
residing on the NAS.
Figure 1 describes the relationship between the EAP peer, NAS and EAP
server. As described in the figure, the EAP conversation occurs
between the EAP peer and EAP server, "passing through" the NAS. In
order for the conversation to proceed in the case where the NAS and
EAP server reside on separate machines, the NAS and EAP server need
to establish trust beforehand.
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+-+-+-+-+-+ +-+-+-+-+-+
| | | |
| Link | | Link |
| Layer | | Layer |
| Cipher- | | Cipher- |
| Suite | | Suite |
| | | |
+-+-+-+-+-+ +-+-+-+-+-+
^ ^
| |
| |
| |
V V
+-+-+-+-+-+ +-+-+-+-+-+ Trust +-+-+-+-+-+
| | EAP | |<======>| |
| | Conversation | | | |
| EAP |<================================>| EAP |
| Peer | (over PPP, | NAS | | Server |
| | 802.11,etc.) | |<=======| |
| | | | Keys | |
| | | | | |
+-+-+-+-+-+ +-+-+-+-+-+ +-+-+-+-+-+
^ ^
| |
| EAP API | EAP API
| |
V V
+-+-+-+-+-+ +-+-+-+-+-+
| | | |
| | | |
| EAP | | EAP |
| Method | | Method |
| | | |
+-+-+-+-+-+ +-+-+-+-+-+
Figure 1 - Relationship between EAP client, backend authentication
server and NAS.
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In PEAPv2, the conversation between the EAP peer and the EAP server
is encrypted, authenticated, integrity and replay protected within a
TLS channel.
As a result, where the NAS acts as a "passthrough" it does not have
knowledge of the TLS master secret derived between the peer and the
EAP server. In order to provide keying material for link-layer
ciphersuites, the NAS obtains the master session key, which is
derived from a one-way function of the TLS master secret as well as
keying material provided by EAP methods protected within a TLS
channel. This enables the NAS and EAP peer to subsequently derive
transient session keys suitable for encrypting, authenticating and
integrity protecting session data. However, the NAS cannot decrypt
the PEAPv2 conversation or spoof session resumption, since this
requires knowledge of the TLS master secret.
1.3.1. Sequences
EAP [RFC2284bis] prohibits use of multiple authentication methods
within a single EAP conversation, except when tunneled methods such
as PEAPv2 are used. This restriction was imposed in order to limit
vulnerabilities to man-in-the-middle attacks as well as to ensure
compatibility with existing EAP implementations.
Within PEAP these concerns are addressed since PEAPv2 includes
support for cryptographic binding to address man-in-the-middle
attacks, as well as version negotiation so as to enable backward
compatibility with prior versions of PEAP.
Within this document, the term "sequence" refers to a series of EAP
authentication methods run in sequence. The methods need not be
distinct - for example, EAP-TLS could be run initially with machine
credentials followed by the same protocol authenticating with user
credentials.
PEAPv2 supports two types of sequences:
[1] Serial authentication. Initiating additional EAP method(s) after a
first successful authentication. In this case the sequence is
successful if each of the EAP authentication methods completes
successfully. For example, successful authentication might require
a successful machine authentication followed by a successful user
authentication.
[2] Parallel authentication. Initiating an alternative EAP method after
failure of one or more initial methods. In this case the overall
authentication is successful if any of the methods is successful.
For example, if machine authentication fails, then user
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authentication can be attempted.
2. Protocol overview
Protected EAP (PEAP) Version 2 is comprised of a two-part
conversation:
[1] In Part 1, a TLS session is negotiated, with server authenticating
to the client and optionally the client to the server. The
negotiated key is then used to encrypt the rest of the
conversation.
[2] In Part 2, within the TLS session, zero or more EAP methods are
carried out. Part 2 completes with a success/failure indication
protected by the TLS session or a protected error (TLS alert).
In the next two sections, we provide an overview of each of the parts
of the PEAPv2 conversation.
2.1. PEAPv2 Part 1
2.1.1. Initial identity exchange
The PEAP conversation typically begins with an optional identity
exchange. The authenticator will typically send an EAP-
Request/Identity packet to the peer, and the peer will respond with
an EAP-Response/Identity packet to the authenticator.
The initial identity exchange is used primarily to route the EAP
conversation to the EAP server. Since the initial identity exchange
is in the clear, the peer MAY decide to place a routing realm instead
of its real name in the EAP-Response/Identity. The real identity of
the peer can be established later in PEAPv2 part 2.
If the EAP server is known in advance (such as when all users
authenticate against the same backend server infrastructure and
roaming is not supported), or if the identity is otherwise determined
(such as from the dialing phone number or client MAC address), then
the EAP-Request/Response-identity exchange MAY be omitted.
Once the optional initial Identity Request/Response exchange is
completed, while nominally the EAP conversation occurs between the
authenticator and the peer, the authenticator MAY act as a
passthrough device, with the EAP packets received from the peer being
encapsulated for transmission to a backend authentication server.
However, PEAP does not require a backend authentication server; if
the authenticator implements PEAP, then it can authenticate local
users.
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In the discussion that follows, we will use the term "EAP server" to
denote the ultimate endpoint conversing with the peer.
2.1.2. TLS Session Establishment
In this section, the protocol is described assuming a certificate
based ciphersuite is negotiated in TLS. The conversation may
slightly differ if other TLS ciphersuites are used.
Once having received the peer's Identity, and determined that PEAP
authentication is to occur, the EAP server MUST respond with a
PEAP/Start packet, which is an EAP-Request packet with EAP-Type=PEAP,
the Start (S) bit set, the PEAP version as specified in Section
2.1.5, and no data. Assuming that the peer supports PEAP, the PEAP
conversation will then begin, with the peer sending an EAP-Response
packet with EAP-Type=PEAP.
The data field of the EAP-Response packet will encapsulate one or
more TLS records in TLS record layer format, containing a TLS
client_hello handshake message. The current cipher spec for the TLS
records will be TLS_NULL_WITH_NULL_NULL and null compression. This
current cipher spec remains the same until the change_cipher_spec
message signals that subsequent records will have the negotiated
attributes for the remainder of the handshake.
The client_hello message contains the client's TLS version number, a
sessionId, a random number, and a set of TLS ciphersuites supported
by the client. The version offered by the client MUST correspond to
TLS v1.0 or later.
The EAP server will then respond with an EAP-Request packet with EAP-
Type=PEAP. The data field of this packet will encapsulate one or
more TLS records. These will contain a TLS server_hello handshake
message, possibly followed by TLS certificate, server_key_exchange,
certificate_request, server_hello_done and/or finished handshake
messages, and/or a TLS change_cipher_spec message.
Since after the TLS session is established, another complete EAP
negotiation will occur and the peer will authenticate using a
secondary mechanism, with PEAPv2 the client need not authenticate as
part of TLS session establishment. As a result, although the EAP-
Request packet sent by the EAP Server MAY contain a
certificate_request message, this is not required.
The certificate_request message indicates that the server desires the
client to authenticate itself via public key. It is valid for the
server to request a certificate in the server_hello and for the
client refuse to provide one.
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Note that since TLS client certificates are sent in the clear, if
identity protection is required, then it is possible for the TLS
authentication to be re-negotiated after the first server
authentication. To accomplish this, the server will typically not
request a certificate in the server_hello, then after the
server_finished message is sent, and before PEAP part 2, the server
MAY send a TLS hello_request. This allows the client to perform
client authentication by sending a client_hello if it wants to, or
send a no_renegotiation alert to the server indicating that it wants
to continue with PEAP part 2 instead. Assuming that the client
permits renegotiation by sending a client_hello, then the server will
respond with server_hello, a certificate and certificate_request
messages. The client replies with certificate, client_key_exchange
and certificate_verify messages. Since this re-negotiation occurs
within the encrypted TLS channel, it does not reveal client
certificate details.
The server_hello handshake message contains a TLS version number,
another random number, a sessionId, and a TLS ciphersuite. The
version offered by the server MUST correspond to TLS v1.0 or later.
If the client's sessionId is null or unrecognized by the server, the
server MUST choose the sessionId to establish a new session;
otherwise, the sessionId will match that offered by the client,
indicating a resumption of the previously established session with
that sessionId. The server will also choose a TLS ciphersuite from
those offered by the client; if the session matches the client's,
then the TLS ciphersuite MUST match the one negotiated during the
handshake protocol execution that established the session.
PEAP implementations need not necessarily support all TLS
ciphersuites listed in [RFC2246]. Not all TLS ciphersuites are
supported by available TLS tool kits and licenses may be required to
support some TLS ciphersuites (e.g. TLS ciphersuites utilizing the
IDEA encryption algorithm).
To ensure interoperability, PEAP peers and EAP servers MUST support
and be able to negotiate the following TLS ciphersuites:
TLS_RSA_WITH_3DES_EDE_CBC_SHA
In addition, PEAP peers and EAP servers SHOULD support and be able to
negotiate the following TLS ciphersuites.
TLS_RSA_WITH_AES_128_CBC_SHA
TLS_RSA_WITH_RC4_128_MD5
TLS_RSA_WITH_RC4_128_SHA
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TLS as described in [RFC2246] supports compression as well as
ciphersuite negotiation. Therefore during the PEAPv2 Part 1
conversation the EAP endpoints MAY request or negotiate TLS
compression.
2.1.3. Full handshake
If the EAP server is not resuming a previously established session,
and if a ciphersuite based on certificates is used, then it MUST
include a TLS server_certificate handshake message, and a
server_hello_done handshake message MUST be the last handshake
message encapsulated in this EAP-Request packet.
The certificate message contains a public key certificate chain for
either a key exchange public key (such as an RSA or Diffie-Hellman
key exchange public key) or a signature public key (such as an RSA or
DSS signature public key). In the latter case, a TLS
server_key_exchange handshake message MUST also be included to allow
the key exchange to take place.
If the preceding server_hello message sent by the EAP server in the
preceding EAP-Request packet did not indicate the resumption of a
previous session, then the peer MUST respond to the EAP-Request with
an EAP-Response packet of EAP-Type=PEAP. The data field of this
packet will encapsulate one or more TLS records containing a TLS
change_cipher_spec message and finished handshake message, and
possibly certificate, certificate_verify and/or client_key_exchange
handshake messages.
The EAP server MUST then respond with an EAP-Request packet with EAP-
Type=PEAP, which includes, in the case of a new TLS session, one or
more TLS records containing TLS change_cipher_spec, finished
handshake messages; and the first payload of PEAPv2 part 2. The
first payload of PEAPv2 part 2 is sent along with finished handshake
message to reduce number of round trips.
2.1.4. Session resumption
The purpose of the sessionId within the TLS protocol is to allow for
improved efficiency in the case where a client repeatedly attempts to
authenticate to an EAP server within a short period of time. This
capability is particularly useful for support of wireless roaming.
It is left up to the peer whether to attempt to continue a previous
session, thus shortening the PEAP Part 1 conversation. Typically the
peer's decision will be made based on the time elapsed since the
previous authentication attempt to that EAP server.
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Based on the sessionId chosen by the peer, and the time elapsed since
the previous authentication, the EAP server will decide whether to
allow the continuation, or whether to choose a new session.
In the case where the EAP server and the authenticator reside on the
same device, then the client will only be able to continue sessions
when connecting to the same NAS or channel server. Should these
devices be set up in a rotary or round-robin then it may not be
possible for the peer to know in advance the authenticator it will be
connecting to, and therefore which sessionId to attempt to reuse. As
a result, it is likely that the continuation attempt will fail.
In the case where the EAP authentication is remoted then continuation
is much more likely to be successful, since multiple NAS devices and
channel servers will remote their EAP authentications to the same
backend authentication server.
If the EAP server is resuming a previously established session, then
it MUST include only a TLS change_cipher_spec message and a TLS
finished handshake message after the server_hello message. The
finished message contains the EAP server's authentication response to
the peer.
If the preceding server_hello message sent by the EAP server in the
preceding EAP-Request packet indicated the resumption of a previous
session, then the peer MUST send only the change_cipher_spec and
finished handshake messages. The finished message contains the
peer's authentication response to the EAP server. The latter contains
the EAP server's authentication response to the peer. The peer will
verify the hash in order to authenticate the EAP server.
If authentication fails, then the peer and EAP-server MUST follow the
error handling behavior specified in section 2.3.
Even if the session is successfully resumed with the same EAP server,
the peer and EAP server MUST not assume that either will skip inner
EAP methods. The peer may have roamed to a network which may use the
same EAP server, but may require conformance with a different
authentication policy.
2.1.5. Version negotiation
PEAP packets contain a three bit version field, which enables PEAP
implementations to be backward compatible with previous versions of
the protocol. This specification documents the PEAP version 2
protocol; implementations of this specification MUST use a version
field set to 2. Version negotiation proceeds as follows:
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[1] In the first EAP-Request sent with EAP type=PEAP, the EAP server
MUST set the version field to the highest supported version number.
[2] If the EAP peer supports this version of the protocol, it MUST
respond with an EAP-Response of EAP type=PEAP, and the version
number proposed by the EAP server.
[3] If the EAP peer does not support this version, it responds with an
EAP-Response of EAP type=PEAP and the highest supported version
number.
[4] If the PEAP server does not support the version number proposed by
the PEAP peer, it terminates the conversation, as described in
Section 2.2.2.
The version negotiation procedure guarantees that the EAP peer and
server will agree to the latest version supported by both parties.
If version negotiation fails, then use of PEAP will not be possible,
and another mutually acceptable EAP method will need to be negotiated
if authentication is to proceed.
The PEAP version field is not protected by TLS and therefore can be
modified in transit. In order to detect modification of the PEAP
version which could occur as part of a "downgrade" attack, the
PEAPv2 peer and server MUST exchange information on the highest
supported versions proposed by the peers using the crypto-binding-
TLV.
2.2. PEAPv2 Part 2
The second portion of the PEAPv2 conversation typically consists of a
complete EAP conversation occurring within the TLS session negotiated
in PEAPv2 Part 1; ending with protected termination using the Result-
TLV. PEAPv2 part 2 will occur only if establishment of a new TLS
session in Part 1 is successful or a TLS session is successfully
resumed in Part 1. In cases where a new TLS session is established
in PEAPv2 part 1, the first payload of the part 2 conversation is
sent by the EAP server along with the finished message to save a
round-trip.
Part 2 MUST NOT occur if the EAP Server authenticates unsuccessfully
or if an EAP-Failure has been sent by the EAP server to the peer,
terminating the conversation. Since all packets sent within the
PEAPv2 Part 2 conversation occur after TLS session establishment,
they are protected using the negotiated TLS ciphersuite. All EAP
packets of the EAP conversation in part 2 including the EAP header of
the inner EAP method are protected using the negotiated TLS
ciphersuite.
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Part 2 MAY NOT always include a EAP conversation within the TLS
session, referred to in this document as inner EAP methods. However,
Part 2 MUST always end with either protected termination or protected
error termination.
Within Part 2, protected EAP conversation and protected termination
packets are always carried within an EAP-TLV packet. The EAP-TLV
packet does not include an EAP header. There are TLVs defined for
specific purposes such as carrying EAP-authentication messages and
carrying cryptographic binding. New TLVs may be developed for other
purposes.
2.2.1. Protected conversation
Part 2 of the PEAP conversation typically begins with the EAP server
sending an optional EAP-Request/Identity packet to the peer,
protected by the TLS ciphersuite negotiated in PEAP Part 1. The peer
responds with an EAP-Response/Identity packet to the EAP server,
containing the peer's userId. Since this Identity Request/Response
exchange is protected by the ciphersuite negotiated in TLS, it is not
vulnerable to snooping or packet modification attacks.
After the TLS session-protected Identity exchange, the EAP server
will then select authentication method(s) for the peer, and will send
an EAP-Request with the Type field set to the initial method. As
described in [RFC2284BIS], the peer can NAK the suggested EAP method,
suggesting an alternative. Since the NAK will be sent within the TLS
channel, it is protected from snooping or packet modification. As a
result, an attacker snooping on the exchange will be unable to inject
NAKs in order to "negotiate down" the authentication method. An
attacker will also not be able to determine which EAP method was
negotiated.
The EAP conversation within the TLS protected session may involve a
sequence of zero or more EAP authentication methods; it completes
with the protected termination described in section 2.2.2. Several
EAP-TLVs may be included in each Request and Response. EAP-methods
(except if the type is EAP-TLV) are always encapsulated within EAP
Payload-TLV.
In a typical EAP conversation, the result of the conversation is
communicated by sending EAP Success or EAP Failure packets after the
EAP method is complete. The EAP Success or Failure packet is
considered the last packet of the EAP conversation; and therefore
cannot be used when sequences need to be supported. Hence, instead
of using the EAP-success or EAP-failure packet, both peer and EAP
server MUST use the Intermediate Result TLV to communicate the
result.
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In a typical EAP conversation, the EAP Success or EAP Failure is
considered the last packet of the EAP conversation. Within PEAP, the
EAP server can start another EAP method after success or failure of
the previous EAP method inside the protected session.
In a sequence of more than one EAP authentication method, to make
sure the same parties are involved in tunnel establishment and
successful completion of previous inner EAP methods, before
completing negotiation of the next EAP method, both peer and EAP
server MUST use crypto binding (Crypto-Binding TLV). If no EAP
methods have been negotiated inside the tunnel or no EAP methods have
been successfully completed inside the tunnel, the crypto-binding TLV
MUST NOT be used.
The Crypto-Binding TLV and the Intermediate-Result TLV MUST be sent
after each successful EAP method (except for type EAP-TLV). If these
TLVs are not sent, it should be considered as tunnel compromise error
by peer and EAP server.
Both TLVs can be sent in an EAP-TLV packet. Alternatively, if a
subsequent EAP conversation is being attempted, then in order to
reduce round trips, both TLVs SHOULD be sent in the EAP-Payload of
the first EAP packet of the next EAP conversation (for example, EAP-
Identity or EAP-packet of the EAP method). Alternatively, if the
next packet is the PEAP termination packet, then in order to reduce
round trips, both TLVs SHOULD be sent with the termination packet.
If the EAP server sends a valid Crypto-Binding-TLV to the peer, the
peer MUST respond with a Crypto-Binding TLV. If the Crypto-Binding-
TLV is invalid, it should be considered a tunnel compromise error by
the peer. If the peer does not respond with a EAP-TLV packet
containing the crypto-binding TLV, it should be considered a tunnel
compromise error by the EAP server.
2.2.2. Protected Termination
The PEAPv2 part 2 conversation is completed by an exchange of
success/failure indications (Result-TLV) within a EAP-TLV packet
protected by the TLS session.
If the Crypto-Binding TLV and the Intermediate-Result TLV have NOT
been exchanged in the previous conversation, then they MUST be
present in the protected indication packet. Otherwise, it should be
considered a tunnel compromise error by the peer and EAP server.
The Result TLV is sent within the TLS channel. The PEAP client then
replies with a Result-TLV. The conversation concludes with the PEAP
server sending a cleartext success/failure indication.
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The only outcome which should be considered as successful
authentication is when an EAP Request of Type=EAP-TLVs with Result
TLV of Status=Success, is answered by an EAP Response of Type=EAP-
TLVs with Result TLV of Status=Success.
All other combinations (EAP-TLVs Failure, EAP-TLVs Success), (EAP-
TLVs Failure, EAP-TLVs Failure), (no EAP-TLVs exchange or no
protected EAP Success or Failure) should be considered failed
authentications, both by the PEAP peer and EAP server. Once the
PEAPv2 peer and PEAPv2 server considers them as failed
authentications, they are the last packets inside the protected
tunnel. These are considered failed authentications regardless of
whether a cleartext EAP Success or EAP Failure packet is subsequently
sent.
After the PEAPv2 server has sent the success indication, the peer is
allowed to refuse to accept a Success message from the PEAPv2 server
since the client's policy may require completion of certain EAP
methods. If the peer wants the server to negotiate EAP methods, it
MUST send the EAP-TLV packet with Result-TLV with Status=Failure. If
the success indication from the EAP server contains the crypto-
binding TLV, then the peer MUST include a crypto-binding TLV in the
EAP-TLV response. If the peer does not respond with a EAP-TLV packet
containing the crypto-binding TLV or if the crypto-binding TLV is
invalid, it should be considered as a tunnel compromise error by the
EAP server.
If the EAP server has set Result-TLV with Status=Success; and the
response from the peer is Status=Failure, then the EAP server MUST
either start another EAP conversation inside the protected channel or
return Result=TLV with Status=Failure without a crypto-binding TLV
and Intermediate Result TLV.
A PEAPv2 tunnel may be nested inside another tunnel, for example,
PEAPv2 may be negotiated as a EAP method inside a PEAPv2 tunnel. In
this case, each tunnel MUST use protected termination.
2.3. Error handling
Once the peer responds with the first PEAP packet and the EAP server
receives the first PEAP packet from the peer, both MUST silently
discard all clear text EAP messages unless both the PEAP peer and
server have indicated success or failure or an error using a
protected error or protected termination mechanism. If the PEAPv2
tunnel is nested inside another tunnel, then the clear text EAP
messages should be accepted after protected termination of all the
tunnels. After a Fatal alert is received or after protected
termination is complete, the peer or EAP server should accept clear
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text EAP messages.
Other than supporting TLS alert messages, PEAPv2 does not have its
own error message capabilities. This is unnecessary since errors in
the PEAPv2 Part 1 conversation are communicated via TLS alert
messages, and errors in the PEAPv2 Part 2 conversation are expected
to be handled by individual EAP methods.
If an error occurs at any point in the TLS layer, the EAP server
SHOULD send a TLS alert message instead of the next EAP-request
packet to the peer. The EAP server SHOULD send an EAP-Request packet
with EAP-Type=PEAP, encapsulating a TLS record containing the
appropriate TLS alert message. The EAP server SHOULD send a TLS
alert message rather than immediately terminating the conversation so
as to allow the peer to inform the user of the cause of the failure
and possibly allow for a restart of the conversation. To ensure that
the peer receives the TLS alert message, the EAP server MUST wait for
the peer to reply with an EAP-Response packet.
The EAP-Response packet sent by the peer MAY encapsulate a TLS
client_hello handshake message, in which case the EAP server MAY
allow the PEAPv2 conversation to be restarted, or it MAY contain an
EAP-Response packet with EAP-Type=PEAP and no data, in which case the
PEAPv2 server MUST send an EAP-Failure packet, and terminate the
conversation.
It is up to the EAP server whether to allow restarts, and if so, how
many times the conversation can be restarted. An EAP server
implementing restart capability SHOULD impose a limit on the number
of restarts, so as to protect against denial of service attacks.
If an error occurs at any point in the TLS layer, the peer SHOULD
send a TLS alert message instead of the next EAP-response packet to
the EAP server. The peer SHOULD send an EAP-Response packet with
EAP-Type=PEAP, encapsulating a TLS record containing the appropriate
TLS alert message. The EAP server may restart the conversation by
sending a EAP-Request packet encapsulating the TLS
hello_request_handshake message, in which case the peer MAY allow the
PEAPv2 conversation to be restarted; or the EAP server can response
with EAP Failure.
Any time the peer or the EAP server finds an error when processing
the sequence of exchanges, such a violation of TLV rules, it should
send a Result TLV of failure and terminate the tunnel. This is
usually due to an implementation problem and is considered an fatal
error.
If a tunnel compromise error (see Section 2.2) is detected by the
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peer, the peer SHOULD send a TLS Internal Error alert (a Fatal error)
message instead of the next EAP-response packet to the EAP server.
Similarly, if a tunnel compromise error is detected by the EAP
server, the EAP server SHOULD send a TLS Internal error alert (a
Fatal error) message instead of the next EAP-response packet to the
peer.
2.4. Fragmentation
A single TLS record may be up to 16384 octets in length, but a TLS
message may span multiple TLS records, and a TLS certificate message
may in principle be as long as 16MB. The group of PEAP messages sent
in a single round may thus be larger than the PPP MTU size, the
maximum RADIUS packet size of 4096 octets, or even the Multilink
Maximum Received Reconstructed Unit (MRRU). As described in
[RFC1990], the multilink MRRU is negotiated via the Multilink MRRU
LCP option, which includes an MRRU length field of two octets, and
thus can support MRRUs as large as 64 KB.
However, note that in order to protect against reassembly lockup and
denial of service attacks, it may be desirable for an implementation
to set a maximum size for one such group of TLS messages. Since a
typical certificate chain is rarely longer than a few thousand
octets, and no other field is likely to be anywhere near as long, a
reasonable choice of maximum acceptable message length might be 64
KB.
If this value is chosen, then fragmentation can be handled via the
multilink PPP fragmentation mechanisms described in [RFC1990]. While
this is desirable, EAP methods are used in other applications such as
[IEEE80211] and there may be cases in which multilink or the MRRU LCP
option cannot be negotiated. As a result, a PEAPv2 implementation
MUST provide its own support for fragmentation and reassembly.
Since EAP is an ACK-NAK protocol, fragmentation support can be added
in a simple manner. In EAP, fragments that are lost or damaged in
transit will be retransmitted, and since sequencing information is
provided by the Identifier field in EAP, there is no need for a
fragment offset field as is provided in IPv4.
PEAPv2 fragmentation support is provided through addition of flag
bits within the EAP-Response and EAP-Request packets, as well as a
TLS Message Length field of four octets. Flags include the Length
included (L), More fragments (M), and PEAP Start (S) bits. The L
flag is set to indicate the presence of the four octet TLS Message
Length field, and MUST be set only for the first fragment of a
fragmented TLS message or set of messages.
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The TLS Message Length field in the PEAPv2 header is not protected;
and hence can be modified by a attacker. The TLS record length in
the TLS data is protected. Hence, if TLS Message length received in
the first packet (with L bit set) is greater or less than the total
size of TLS data received including multiple fragments, then the TLS
message length should be ignored.
A single TLS record may be up to 16384 octets in length, but a TLS
message may span multiple TLS records, and a TLS certificate message
may in principle be as long as 16MB. However, note that in order to
protect against reassembly lockup and denial of service attacks, it
may be desirable for an implementation to set a maximum size for one
such group of TLS messages. Since a typical certificate chain is
rarely longer than a few thousand octets, and no other field is
likely to be anywhere near as long, a reasonable choice of maximum
acceptable message length might be 64 KB.
The M flag is set on all but the last fragment. The S flag is set
only within the PEAP start message sent from the EAP server to the
peer. The TLS Message Length field is four octets, and provides the
total length of the TLS message or set of messages that is being
fragmented; this simplifies buffer allocation.
When a PEAPv2 peer receives an EAP-Request packet with the M bit set,
it MUST respond with an EAP-Response with EAP-Type=PEAP and no data.
This serves as a fragment ACK. The EAP server MUST wait until it
receives the EAP-Response before sending another fragment. In order
to prevent errors in processing of fragments, the EAP server MUST
increment the Identifier field for each fragment contained within an
EAP-Request, and the peer MUST include this Identifier value in the
fragment ACK contained within the EAP-Response. Retransmitted
fragments will contain the same Identifier value.
Similarly, when the EAP server receives an EAP-Response with the M
bit set, it MUST respond with an EAP-Request with EAP-Type=PEAP and
no data. This serves as a fragment ACK. The EAP peer MUST wait until
it receives the EAP-Request before sending another fragment. In
order to prevent errors in the processing of fragments, the EAP
server MUST increment the Identifier value for each fragment ACK
contained within an EAP-Request, and the peer MUST include this
Identifier value in the subsequent fragment contained within an EAP-
Response.
2.5. Key derivation
Since the normal TLS keys are used in the handshake, and therefore
should not be used in a different context, new keys must be derived
from the TLS master secret for use with the selected link layer
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ciphersuites.
Instead of deriving keys specific to link layer ciphersuites EAP
methods provides a Master Session Key (MSK) used to derive keys in a
link layer specific manner. The method used to extract ciphering
keys from the MSK is beyond the scope of this document.
PEAPv2 also derives an Extended Master Session Key (EMSK) which is
reserved for use in deriving keys in other ciphering applications.
This draft also does not discuss the format of the attributes used to
communicate the master session keys from the backend authentication
server to the NAS; examples of such attributes are provided in
[RFC2548].
PEAPv2 combines key material from the TLS exchange with key material
from inner key generating EAP methods to provide stronger keys and to
bind inner authentication mechanisms to the TLS tunnel. Both the
peer and EAP server MUST derive compound MAC and compound session
keys using the procedure described below.
The input for the cryptographic binding includes the following:
[a] The PEAPv2 tunnel key (TK) is calculated using the first 64 octets
of the (secret) key material generated as described in the EAP-TLS
algorithm (RFC2716 section 3.5)
[b] The MSK provided by each successful inner EAP method (should not
include the 64 octets of EMSK); for each successful EAP method
completed within the tunnel.
ISK1..ISKn are the MSK portion of the EAP keying material obtained
from methods 1 to n. In some cases (except in the case of the EAP-
TLV method), where the inner EAP method does not provide keys: ISKi,
for some i, may be the null string ("").
The algorithm uses P_SHA-1 PRF specified in the TLS specification
[RFC2246] ("|" denotes concatenation).
The intermediate combined key is generated after each successful EAP
method inside the tunnel.
Generating the intermediate combined key:
Take the second 32 octets of TK
IPMK0 = TK
for j = 1 to k do
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IPMKj = P_SHA1(IPMK(j-1),"Intermediate PEAP MAC key" | ISKj);
k = the last successful EAP method inside the tunnel at the point
where the combined MAC key is derived.
Each IPMKj output is 32 octets. IPMKn is the intermediate combined
key used to derive combined session and combined MAC keys.
Compound MAC Key derivation:
The Compound MAC Key for the server (the B1_MAC) is derived CMK_B1
CMK_B1 = P_SHA1(IPMKn,"PEAP Server B1 MAC key" | S_NONCE)
The Compound MAC Key for the client (the B2_MAC) is derived from MAC
key called CMK B2.
CMK_B2 = P_SHA1(IPMKn,"PEAP Client B2 MAC key" | C_NONCE | S_NONCE)
The compound MAC keys (CMK_B1 and CMK_B2) are each 20 octets long.
Compound Session Key derivation:
The compound session key (CSK) is derived on both the peer and EAP
server after successful completion of protected termination.
CSK = P_SHA1 (IPMKn, "PEAP compound session key" | C_NONCE | S_NONCE
| OutputLength)
The output length of the CSK must be at least 128 bytes. The first
64 octets are taken and the MSK and the second 64 octets are taken as
the EMSK. The MSK and EMSK are described in [RFC2284bis].
2.6. Ciphersuite negotiation
Since TLS supports TLS ciphersuite negotiation, peers completing the
TLS negotiation will also have selected a TLS ciphersuite, which
includes key strength, encryption and hashing methods. However,
unlike in [RFC2716], within PEAPv2, the negotiated TLS ciphersuite
relates only to the mechanism by which the PEAPv2 Part 2 conversation
will be protected, and has no relationship to link layer security
mechanisms negotiated within the PPP Encryption Control Protocol
(ECP) [RFC1968] or within IEEE 802.11 [IEEE80211].
As a result, this specification currently does not support secure
negotiation of link layer ciphersuites, although this capability may
be added in future by addition of TLVs to the EAP TLV method defined
in Section 4.
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3. Detailed description of the PEAPv2 protocol
3.1. PEAPv2 Packet Format
A summary of the PEAPv2 Request/Response packet format is shown
below. The fields are transmitted from left to right.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Flags | Ver | Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1 - Request
2 - Response
Identifier
The Identifier field is one octet and aids in matching responses
with requests.
Length
The Length field is two octets and indicates the length of the EAP
packet including the Code, Identifier, Length, Type, and Data
fields. Octets outside the range of the Length field should be
treated as Data Link Layer padding and should be ignored on
reception.
Type
25 - PEAP
Flags
0 1 2 3 4
+-+-+-+-+-+
|L M S R R|
+-+-+-+-+-+
L = Length included
M = More fragments
S = PEAP start
R = Reserved (must be zero)
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The L bit (length included) is set to indicate the presence of the
four octet TLS Message Length field, and MUST be set for the first
fragment of a fragmented TLS message or set of messages. The L
bit MUST NOT be set for other fragments of the same set of
messages. The M bit(more fragments) is set on all but the last
fragment. The S bit (PEAP start) is set in a PEAP Start message.
This differentiates the PEAP Start message from a fragment
acknowledgment.
Version
0 1 2
+-+-+-+
|R|1|0|
+-+-+-+
R = Reserved (must be zero)
Data
The format of the Data field is determined by the Code field.
3.2. PEAPv2 Request Packet
A summary of the PEAPv2 Request packet format is shown below. The
fields are transmitted from left to right.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Flags | Ver | TLS Message Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLS Message Length | TLS Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1
Identifier
The Identifier field is one octet and aids in matching responses
with requests. The Identifier field MUST be changed on each
Request packet.
Length
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The Length field is two octets and indicates the length of the EAP
packet including the Code, Identifier, Length, Type, Flags, TLS
message length and TLS data fields.
Type
25 - PEAP
Flags
0 1 2 3 4
+-+-+-+-+-+
|L M S R R|
+-+-+-+-+-+
L = Length included
M = More fragments
S = PEAP start
R = Reserved (must be zero)
The L bit (length included) is set to indicate the presence of the
four octet TLS Message Length field, and MUST be set for the first
fragment of a fragmented TLS message or set of messages. The M
bit(more fragments) is set on all but the last fragment. The S
bit (PEAP start) is set in a PEAP Start message. This
differentiates the PEAP Start message from a fragment
acknowledgment.
Version
0 1 2
+-+-+-+
|R|1|0|
+-+-+-+
R = Reserved (must be zero)
TLS Message Length
The TLS Message Length field is four octets, and is present only
if the L bit is set. This field provides the total length of the
TLS message or set of messages that is being fragmented.
TLS data
The TLS data consists of the encapsulated packet in TLS record
format.
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3.3. PEAPv2 Response Packet
A summary of the PEAPv2 Response packet format is shown below. The
fields are transmitted from left to right.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Flags |Ver| TLS Message Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLS Message Length | TLS Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
2
Identifier
The Identifier field is one octet and MUST match the Identifier
field from the corresponding request.
Length
The Length field is two octets and indicates the length of the EAP
packet including the Code, Identifier, Length, Type, Flags, Ver,
TLS message length, and TLS data fields.
Type
25 - PEAP
Flags
0 1 2 3 4
+-+-+-+-+-+
|L M S R R|
+-+-+-+-+-+
L = Length included
M = More fragments
S = PEAP start
R = Reserved (must be zero)
The L bit (length included) is set to indicate the presence of the
four octet TLS Message Length field, and MUST be set for the first
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fragment of a fragmented TLS message or set of messages. The M
bit (more fragments) is set on all but the last fragment. The S
bit (PEAP start) is set in a PEAP Start message. This
differentiates the PEAP Start message from a fragment
acknowledgment.
Version
0 1 2
+-+-+-+
|R|1|0|
+-+-+-+
R = Reserved (must be zero)
TLS Message Length
The TLS Message Length field is four octets, and is present only
if the L bit is set. This field provides the total length of the
TLS message or set of messages that is being fragmented.
TLS data
The TLS data consists of the encapsulated TLS packet in TLS record
format.
3.4. PEAPv2 Part 2 Packet Format
The PEAPv2 Part 2 packet format is the same as the PEAPv2 Request and
Response packet formats described in Sections 3.1 and 3.2, except
that the TLS Data field encapsulates TLS packets in TLS record
format, representing encrypted EAP-TLVs.
Although the EAP-TLV method has been allocated an EAP Type, use of
this method is prohibited outside of a tunnel by [RFC2284bis]. Since
EAP-TLVs are self-describing, when transmitted within PEAPv2, the EAP
header portion of the EAP-TLV packet is absent (including the Code,
Identifier, Length and Type fields), leaving only a list of TLVs as
the payload.
Within PEAPv2, all inner EAP method packets are encapsulated in EAP-
TLV format. The EAP-Payload is an (optional) TLV which encapsulates
EAP packets (including all EAP header fields) and in addition carries
TLVs associated with them.
PEAP Part 2 packet format = EAP-TLV [EAP-Payload TLV [[EAP packet],
TLVs, ...], TLVs, ...] OR TLS Alert
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The EAP-TLV packet is included without the EAP header fields (Code,
Identifier, Length, Type)
4. EAP-TLV method
The EAP-TLV method is a payload with standard Type-Length-Value (TLV)
objects. The TLV objects could be used to carry arbitrary parameters
between EAP peer and EAP server. Possible uses for TLV objects
include: language and character set for Notification messages;
cryptographic binding; MIPv6 Binding Update.
The EAP peer may not necessarily implement all the TLVs supported by
the EAP server; and hence to allow for interoperability, the TLV
method allows a EAP server to discover if a TLV is supported by the
EAP peer, using the NAK TLV.
The mandatory bit in a TLV indicates that if the peer does not
support the TLV, it MUST send a NAK TLV in response; and all the
other TLVs in the message MUST be ignored. If an EAP peer finds an
unsupported TLV which is marked as optional, it MUST NOT send an NAK
TLV.
The mandatory bit does not imply that the peer is required to
understand the contents of the TLV. The appropriate response to a
supported TLV with content that is not understood is defined by the
TLV specification.
If the EAP peer finds that the packet has no TLVs, then it MUST send
a response with EAP-TLV Response Packet.
The mandatory bit in a TLV indicates that if the EAP server does not
support the TLV, it MUST send a NAK TLV in response; otherwise it
MUST send a protected termination message. If an EAP server finds an
unsupported TLV which is marked as mandatory; the other TLVs in the
message MUST be ignored.
An EAP-TLV packet is a EAP method and within a PEAPv2 tunnel, it can
be sequenced before or after any other EAP method. An EAP-TLV packet
does not have to contain any TLVs nor need it contain any mandatory
TLVs.
PEAPv2 implementations MUST support the EAP TLV method, as well as
processing of mandatory/optional settings on the TLV.
4.1. EAP-TLV Request Packet
A summary of the EAP-TLV Request packet format is shown below. The
fields are transmitted from left to right.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Data....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1
Identifier
The Identifier field is one octet and aids in matching responses
with requests. The Identifier field MUST be changed on each
Request packet.
Length
The Length field is two octets and indicates the length of the EAP
packet including the Code, Identifier, Length, Type, and Data
fields.
Type
33 - EAP-TLV
Data
The Data field is of variable length, and contains Type-Length-
Value tuples (TLVs).
4.2. EAP-TLV Response Packet
A summary of the Extension Response packet format is shown below.
The fields are transmitted from left to right.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Data....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
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2
Identifier
The Identifier field is one octet and aids in matching responses
with requests. The Identifier field MUST be changed on each
Request packet.
Length
The Length field is two octets and indicates the length of the EAP
packet including the Code, Identifier, Length, Type, and Data
fields.
Type
33 - EAP-TLV
Data
The Data field is of variable length, and contains Type-Length-
Value tuples (TLVs).
4.3. TLV format
EAP-TLV TLVs are defined as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
0 - Non-mandatory TLV
1 - Mandatory TLV
R
Reserved, set to zero (0)
TLV Type
A 14-bit field, denoting the TLV type. Allocated Types include:
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0 - Reserved
1 - Reserved
2 - Reserved
3 - RESULT_TLV - Acknowledged Result
4 - NAK_TLV
5 - Crypto-Binding TLV
6 - Connection-Binding TLV
7 - Vendor-Specific TLV
8 - URI TLV
9 - EAP Payload TLV
10 - Intermediate Result TLV
Length
The length of the Value field in octets.
Value
The value of the TLV.
4.4. Result TLV
The Result TLV provides support for acknowledged success and failure
messages within PEAPv2. PEAPv2 implementations MUST support this
TLV, which cannot be responded to with a NAK TLV. If the Status
field does not contain one of the known values, then the peer or EAP
server MUST drop the connection. The Result TLV is defined as
follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved, set to zero (0)
TLV Type
3
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Length
2
Status
The Status field is two octets. Values include:
1 - Success
2 - Failure
4.5. NAK TLV
The NAK TLV allows a peer to detect TLVs that are not supported by
the other peer. An EAP-TLV packet can contain 0 or more NAK TLVs.
PEAPv2 implementations MUST support this TLV and this TLV cannot be
responded to with a NAK TLV. The NAK TLV is defined as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor-Id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NAK-Type | TLVs....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved, set to zero (0)
TLV Type
4
Length
>=6
Vendor-Id
The Vendor-Id field is four octets, and contains the Vendor-Id of
the TLV that was not supported. The high-order octet is 0 and the
Palekar et al. Standards Track [Page 34]
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low-order 3 octets are the SMI Network Management Private
Enterprise Code of the Vendor in network byte order. The Vendor-
Id field MUST be zero for TLVs that are not Vendor-Specific TLVs.
For Vendor-Specific TLVs, the Vendor-ID MUST be set to the SMI
code.
NAK-Type
The NAK-Type field is two octets. The field contains the Type of
the TLV that was not supported. A TLV of this Type MUST have been
included in the previous packet.
TLVs
This field contains a list of TLVs, each of which MUST NOT have
the mandatory bit set. These optional TLVs can be used in the
future to communicate why the offending TLV was determined to be
unsupported.
4.6. Crypto-binding TLV
The Crypto-Binding TLV is used prove that both peers participated in
the sequence of authentications (specifically the TLS session and
inner EAP methods that generate keys).
Both the Binding Request (B1) and Binding Response (B2) use the same
packet format. However the Sub-Type indicates whether it is B1 or
B2.
The Crypto-Binding TLV MUST be used to perform Cryptographic Binding
after each successful EAP method (except EAP-TLV) in a sequence of
EAP methods is complete in PEAPv2 part 2. The Crypto-Binding TLV can
also be used during Protected Termination.
The crypto-binding TLV must have the version number received during
the PEAP version negotiation. The receiver of the crypto binding TLV
must verify that the version in the crypto binding TLV matches the
version it sent during the PEAP version negotiation. If this check
fails then the TLV is invalid.
The receiver of the crypto binding TLV must verify that the subtype
is not set to any value other than the ones allowed. If this check
fails then the TLV is invalid.
This message format is used for the Binding Request (B1) and also the
Binding Response. This uses TLV type CRYPTO_BINDING_TLV. PEAPv2
implementations MUST support this TLV and this TLV cannot be
responded to with a NAK TLV. The format is as given below.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version |Received Ver. | Sub-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Nonce ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Compound MAC ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved, set to zero (0)
TLV Type
5
Length
52
Version
The Version field is a single octet, which is set to the version
of crypto binding TLV. For the crypto-binding TLV defined in this
specification, It is set to zero (0).
Received Version
The Received Version field is a single octet and MUST be set to
the PEAP version number received during version negotiation. Note
that this field only provides protection against downgrade attacks
where a version of PEAP requiring support for this TLV is required
on both sides (such as PEAPv2 or a more recent version).
Sub-Type
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The Sub-Type field is two octets. Possible values include:
0 - Binding Request
1 - Binding Response
Nonce
The Nonce field is 32 octets. It contains a 256 bit nonce that is
temporally unique, used for compound MAC key derivation at each
end. This is the S_NONCE for the B1 message and a C_NONCE for the
B2 message.
Compound MAC
The Compound MAC field is 16 octets. This can be the Server MAC
(B1_MAC) or the Client MAC (B2_MAC). It is computed over the
entire Crypto-Binding TLV attribute using the HMAC-SHA1-128 that
provides 128 bits of output using the CMK_B1 or CMK_B2 with the
MAC field zeroed out.
4.7. Connection-Binding TLV
The Connection-Binding TLV allows for connection specific information
to be sent by the peer to the AAA server. This TLV should be logged
by the EAP or AAA server. The AAA or EAP server should not deny
access if there i s a mismatch between the value sent through the AAA
protocol and this TLV.
The format of this TLV is defined for the layer that defines the
parameters. The format of the value sent by the peer to the EAP
server may be different from the format of the corresponding value
sent through the AAA protocol. For example, the connection binding
TLV may contain 802.11 MAC Address and SSID.
PEAP implementations MAY support this TLV and this TLV MUST NOT be
responded to with a NAK TLV. It is defined as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLVs...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
0 - Optional TLV
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R
Reserved, set to zero (0)
TLV Type
6
Length
>=0
TLVs...
The field contains a list of TLVs, each in the same format defined
in Section 4.3, with the optional bit set. These TLVs contain
information on the identity of the peer and authenticator (layer 2
or IP addresses); the media used to connect the peer and
authenticator (NAS-Port-Type); and/or the service the client is
trying to access on the gateway (SSID). The format of these TLVs
will be defined in a separate draft.
4.8. Vendor-Specific TLV
The Vendor-Specific TLV is available to allow vendors to support
their own extended attributes not suitable for general usage.
A Vendor-Specific-TLV attribute can contain one or more TLVs,
referred to as Vendor-TLVs. The TLV-type of the Vendor-TLV will be
defined by the vendor. All the Vendor-TLVs inside a single Vendor-
Specific TLV belong to the same vendor.
PEAPv2 implementations MUST support the Vendor-Specific TLV; and this
TLV cannot be responded to with a NAK TLV. PEAPv2 implementations
MAY NOT support the Vendor-TLVs included in the Vendor-Specific TLV
and can respond with a NAK TLV.
Vendor-TLVs may be optional or mandatory. Vendor-TLVs sent in the
protected success and failure packets MUST be marked as optional. If
Vendor-TLVs sent in protected success/failure packets are marked as
Mandatory, then the peer or EAP server MUST drop the connection.
A summary of the Vendor-Specific Attribute format is shown below.
The fields are transmitted from left to right.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor-Id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor-TLVs....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved, set to zero (0)
TLV Type
7
Length
>=4
Vendor-Id
The Vendor-Id field is four octets. The high-order octet is 0 and
the low-order 3 octets are the SMI Network Management Private
Enterprise Code of the Vendor in network byte order. The Vendor-
Id MUST be zero for TLVs that are not Vendor-Specific TLVs. For
Vendor-Specific TLVs, the Vendor-ID MUST be set to the SMI code.
Vendor-TLVs
This field is of indefinite length. It contains vendor-specific
TLVs, in a format defined by the vendor.
4.9. URI TLV
The URI TLV allows a server to send a URI to the client to refer it
to a resource. The TLV contains a URI in the format specified in
RFC2396 with UTF-8 encoding. If a packet contains multiple URI TLVs,
then the client SHOULD select the first TLV it can implement, and
ignore the others. If the client is unable to implement any of the
URI TLVs, then it MAY ignore the error. PEAP implementations MAY
support this TLV; and this TLV cannot be responded to with a NAK TLV.
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A summary of this field is shown 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| URI...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
0 - Optional TLV
R
Reserved, set to zero (0)
TLV Type
8
Length
>=0
URI
This field is of indefinite length, and conforms to the format
specified in [RFC2396].
4.10. EAP-Payload TLV
To allow piggybacking EAP request and response with other TLVs, the
EAP Payload TLV is defined, which includes an encapsulated EAP packet
and 0 or more TLVs. PEAPv2 implementations MUST support this TLV,
which cannot be responded to with a NAK TLV. The EAP-Payload TLV is
defined as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EAP packet...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLVs...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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M
1 - Mandatory TLV
R
Reserved, set to zero (0)
TLV Type
8
Length
>=0
EAP packet
This field contains a complete EAP packet, including the EAP
header (Code, Identifier, Length, Type) fields. The length of
this field is determined by the Length field of the encapsulated
EAP packet.
TLVs...
This (optional) field contains a list of TLVs associated with the
EAP packet field. The TLVs utilize the same format described
Section 4.3, and MUST NOT have the mandatory bit set. The total
length of this field is equal to the Length field of the EAP-
Payload-TLV, minus the Length field in the EAP header of the EAP
packet field.
4.11. Intermediate Result TLV
The Intermediate Result TLV provides support for acknowledged
intermediate Success and Failure messages within EAP. PEAPv2
implementations MUST support this TLV, which cannot be responded to
with a NAK TLV. This TLV is defined as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status | TLVs...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
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1 - Mandatory TLV
R
Reserved, set to zero (0)
TLV Type
10
Length
>=2
Status
The Status field is two octets. Values include:
1 - Success
2 - Failure
TLVs
This (optional) field is of indeterminate length, and contains the
TLVs associated with the Intermediate Result TLV, in the same
format as described in Section 4.3. The TLVs in this field MUST
NOT have the mandatory bit set.
4.12. TLV Rules
To save round trips, multiple TLVs can be sent in the single PEAPv2
packet. However, multiple EAP Payload TLVs within one single PEAPv2
packet is not supported in this version and MUST NOT be sent. If the
peer or EAP server receives multiple EAP Payload TLVs, then it MUST
drop the connection.
The following table provides a guide to which TLVs may be found in
which kind of packets, and in which quantity:
Request Response Success Failure TLV
0-1 0-1 0-1 0-1 Intermediate Result TLV
0-1 0-1 0 0 EAP Payload TLV
0-1 0-1 1 1 Result TLV
0-1 0-1 0-1 0-1 Crypto-Binding TLV
0+ 0+ 0 0 NAK TLV
0-1 0-1 0-1 0-1 Connection-Binding TLV
0+ 0+ 0+ 0+ Vendor-Specific TLV
0+ 0 0+ 0-1 URI TLV
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The following table defines the meaning of the above table entries.
0 This TLV MUST NOT be present in the packet.
0+ Zero or more instances of this TLV MAY be present in packet.
0-1 Zero or one instance of this TLV MAY be present in packet.
1 Exactly one instance of this TLV MUST be present in packet.
Packet type Description
----------------------------
Request - EAP-TLV request packet sent by EAP server to peer.
Response - EAP-TLV response packet sent by peer to EAP server.
Success - EAP-TLV packet sent by Peer or EAP server as
protected success
Failure - EAP-TLV packet sent by Peer or EAP server as
protected failure.
The EAP-Payload TLV can contain other TLVs. The table below
defines which TLVs can be contained inside the EAP-Payload TLV
and how many such TLVs can be included.
EAP-Payload-TLV TLV
0 Intermediate Result TLV
0 EAP Payload TLV
0 Result TLV
0 Crypto-Binding TLV
0+ NAK TLV
0 Connection-Binding TLV
0+ Vendor-Specific TLV
0 URI TLV
5. Security Considerations
5.1. Authentication and integrity protection
PEAPv2 provides a server authenticated, encrypted and integrity
protected tunnel. All data within the tunnel has these properties.
Data outside the tunnel such as EAP Success and Failure,
authentication methods negotiated outside of PEAPv2 and the PEAPv2
headers themselves are not protected by this tunnel.
In addition, the crypto-binding TLV can reveal man-in-the-middle
attack described in section 6.8. Hence, the server should not reveal
any sensitive data to the client until after the Crypto-Binding TLV
has been properly verified.
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5.2. Method negotiation
If the peer does not support PEAPv2, or does not wish to utilize
PEAPv2 authentication, it MUST respond to the initial EAP-
Request/PEAP-Start with a NAK, suggesting an alternate authentication
method. Since the NAK is sent in cleartext with no integrity
protection or authentication, it is subject to spoofing. Inauthentic
NAK packets can be used to trick the peer and authenticator into
"negotiating down" to a weaker form of authentication, such as EAP-
MD5 (which only provides one way authentication and does not derive a
key).
Since a subsequent protected EAP conversation can take place within
the TLS session, selection of PEAPv2 as an authentication method does
not limit the potential secondary authentication methods. As a
result, the only legitimate reason for a peer to NAK PEAPv2 as an
authentication method is that it does not support it. Where the
additional security of PEAPv2 is required, server implementations
SHOULD respond to a NAK with an EAP-Failure, terminating the
authentication conversation.
Since method negotiation outside of PEAP is not protected, if the
peer is configured to allow PEAP and other EAP methods at the same
time, the negotiation is subject to downgrade attacks. Since method
negotiation outside of PEAP is not protected, if the peer is
configured to allow PEAP version 2; and previous PEAP versions at the
same time, the negotiation is subject to negotiation downgrade
attacks. However, peers configured to allow PEAPv2 and later PEAP
versions may not be subject to downgrade negotiation attack since the
highest version supported by both peers is checked within the
protected tunnel.
If peer implementations select incorrect methods or credentials with
EAP servers, then attacks are possible on the credentials. Hence, a
PEAPv2 peer implementation should preferably be configured with a set
of credentials and methods that may be used with a specific PEAPv2
Server. The peer implementation may be configured to use different
methods and/or credentials based on the PEAPv2 server.
5.3. TLS session cache handling
In cases where a TLS session has been successfully resumed, in some
circumstances, it is possible for the EAP server to skip the PEAPv2
Part 2 conversation, and successfully conclude the conversation with
a protected termination.
PEAPv2 "fast reconnect" is desirable in applications such as wireless
roaming, since it minimizes interruptions in connectivity. It is
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also desirable when the "inner" EAP mechanism used is such that it
requires user interaction. The user should not be required to re-
authenticate herself, using biometrics, token cards or similar, every
time the radio connectivity is handed over between access points in
wireless environments.
However, there are issues that need to be understood in order to
avoid introducing security vulnerabilities.
Since PEAPv2 Part 1 may not provide client authentication,
establishment of a TLS session (and an entry in the TLS session
cache) does not by itself provide an indication of the peer's
authenticity.
Some PEAPv2 implementations may not be capable of removing TLS
session cache entries established in PEAPv2 Part 1 after an
unsuccessful PEAPv2 Part 2 authentication. In such implementations,
the existence of a TLS session cache entry provides no indication
that the peer has previously been authenticated. As a result,
implementations that do not remove TLS session cache entries after a
failed PEAPv2 Part 2 authentication or failed protected termination
MUST use other means than successful TLS resumption as the indicator
of whether the client is authenticated or not. The implementation
MUST determine that the client is authenticated only after the
completion of protected termination. Failing to do this would enable
a peer to gain access by completing PEAPv2 Part 1, tearing down the
connection, re-connecting and resuming PEAPv2 Part 1, thereby proving
herself authenticated. Thus, TLS resumption MUST only be enabled if
the implementation supports TLS session cache removal. If an EAP
server implementing PEAPv2 removes TLS session cache entries of peers
failing PEAPv2 Part 2 authentication, then it MAY skip the PEAPv2
Part 2 conversation entirely after a successful session resumption,
successfully terminating the PEAPv2 conversation as described in
Section 2.4.
5.4. Certificate revocation
Since the EAP server is on the Internet during the EAP conversation,
the server is capable of following a certificate chain or verifying
whether the peer's certificate has been revoked. In contrast, the
peer may or may not have Internet connectivity, and thus while it can
validate the EAP server's certificate based on a pre-configured set
of CAs, it may not be able to follow a certificate chain or verify
whether the EAP server's certificate has been revoked.
In the case where the peer is initiating a voluntary Layer 2 channel
using PPTP or L2TP, the peer will typically already have Internet
connectivity established at the time of channel initiation. As a
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result, during the EAP conversation it is capable of checking for
certificate revocation.
As part of the TLS negotiation, the server presents a certificate to
the peer. The peer SHOULD verify the validity of the EAP server
certificate, and SHOULD also examine the EAP server name presented in
the certificate, in order to determine whether the EAP server can be
trusted. Please note that in the case where the EAP authentication is
remoted, the EAP server will not reside on the same machine as the
authenticator, and therefore the name in the EAP server's certificate
cannot be expected to match that of the intended destination. In
this case, a more appropriate test might be whether the EAP server's
certificate is signed by a CA controlling the intended destination
and whether the EAP server exists within a target sub-domain.
In the case where the peer is attempting to obtain network access, it
will not have Internet connectivity. The TLS Extensions [RFC3546]
support piggybacking of an Online Certificate Status Protocol (OCSP)
response within TLS, therefore can be utilized by the peer in order
to verify the validity of server certificate. However, since not all
TLS implementations implement the TLS extensions, it may be necessary
for the peer to wait to check for certificate revocation until after
Internet access has been obtained. In this case, the peer SHOULD
conduct the certificate status check immediately upon going online
and SHOULD NOT send data until it has received a positive response to
the status request. If the server certificate is found to be invalid
as per client policy, then the peer SHOULD disconnect.
If the client has a policy to require checking certificate revocation
and it cannot obtain revocation information then it may need to
disallow the use of all or some of the inner methods since some
methods may reveal some sensitive information.
5.5. Separation of the EAP server and the authenticator
As a result of a complete PEAPv2 Part 1 and Part 2 conversation, the
EAP endpoints will mutually authenticate, and derive a session key
for subsequent use in link layer security. Since the peer and EAP
client reside on the same machine, it is necessary for the EAP client
module to pass the session key to the link layer encryption module.
The situation may be more complex on the Authenticator, which may or
may not reside on the same machine as the EAP server. In the case
where the EAP server and the Authenticator reside on different
machines, there are several implications for security. Firstly, the
mutual authentication defined in PEAP will occur between the peer and
the EAP server, not between the peer and the authenticator. This
means that as a result of the PEAP conversation, it is not possible
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for the peer to validate the identity of the NAS or channel server
that it is speaking to.
The second issue is that the session key negotiated between the peer
and EAP server will need to be transmitted to the authenticator.
Therefore a secure mechanism needs to be provided to transmit the
session key from the EAP server to the authenticator or channel
server that needs to use the key. The specification of this transit
mechanism is outside the scope of this document.
5.6. Separation of PEAPv2 Part 1 and Part 2 Servers
The EAP server involved in PEAPv2 Part 2 need not necessarily be the
same as the EAP server involved in PEAPv2 Part 1. For example, a
local authentication server or proxy might serve as the endpoint for
the Part 1 conversation, establishing the TLS channel. Subsequently,
once the EAP-Response/Identity has been received within the TLS
channel, it can be decrypted and forwarded in cleartext to the
destination realm EAP server. The rest of the conversation will
therefore occur between the destination realm EAP server and the
peer, with the local authentication server or proxy acting as an
encrypting/decrypting gateway. This permits a non-TLS capable EAP
server to participate in the PEAPv2 conversation.
Note however that such an approach introduces security
vulnerabilities. Since the EAP Response/Identity is sent in the
clear between the proxy and the EAP server, this enables an attacker
to snoop the user's identity. It also enables a remote environments,
which may be public hot spots or Internet coffee shops, to gain
knowledge of the identity of their users. Since one of the potential
benefits of PEAP is identity protection, this is undesirable.
If the EAP method negotiated during PEAPv2 Part 2 does not support
mutual authentication, then if the Part 2 conversation is proxied to
another destination, the PEAP peer will not have the opportunity to
verify the secondary EAP server's identity. Only the initial EAP
server's identity will have been verified as part of TLS session
establishment.
Similarly, if the EAP method negotiated during PEAPv2 Part 2 is
vulnerable to dictionary attack, then an attacker capturing the
cleartext exchange will be able to mount an offline dictionary attack
on the password.
Finally, when a Part 2 conversation is terminated at a different
location than the Part 1 conversation, the Part 2 destination is
unaware that the EAP client has negotiated PEAPv2. As a result, it is
unable to enforce policies requiring PEAP. Since some EAP methods
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require PEAPv2 in order to generate keys or lessen security
vulnerabilities, where such methods are in use, such a configuration
may be unacceptable.
In summary, PEAPv2 encrypting/decrypting gateway configurations are
vulnerable to attack and SHOULD NOT be used. Instead, the entire
PEAPv2 connection SHOULD be proxied to the final destination, and the
subsequently derived master session keys need to be transmitted back.
T his provides end to end protection of PEAPv2. The specification of
this transit mechanism is outside the scope of this document, but
mechanisms similar to [RFC2548] can be used. These steps protect the
client from revealing her identity to the remote environment.
In order to find the proper PEAP destination, the EAP client SHOULD
place a Network Access Identifier (NAI) conforming to [RFC2486] in
the Identity Response.
There may be cases where a natural trust relationship exists between
the (foreign) authentication server and final EAP server, such as on
a campus or between two offices within the same company, where there
is no danger in revealing the identity of the station to the
authentication server. In these cases, a proxy solution without end
to end protection of PEAPv2 MAY be used. If RADIUS is used to
communicate between gateway and EAP server, then the PEAPv2
encrypting/decrypting gateway SHOULD provide support for IPsec
protection of RADIUS in order to provide confidentiality for the
portion of the conversation between the gateway and the EAP server,
as described in [RFC3579].
5.7. Identity verification
Since the TLS session has not yet been negotiated, the initial
Identity request/response occurs in the clear without integrity
protection or authentication. It is therefore subject to snooping and
packet modification.
In configurations where all users are required to authenticate with
PEAPv2 and the first portion of the PEAPv2 conversation is terminated
at a local backend authentication server, without routing by proxies,
the initial cleartext Identity Request/Response exchange is not
needed in order to determine the required authentication method(s) or
route the authentication conversation to its destination. As a
result, the initial Identity and Request/Response exchange MAY NOT
be present, and a subsequent Identity Request/Response exchange MAY
occur after the TLS session is established.
If the initial cleartext Identity Request/Response has been tampered
with, after the TLS session is established, it is conceivable that
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the EAP Server will discover that it cannot verify the peer's claim
of identity. For example, the peer's userID may not be valid or may
not be within a realm handled by the EAP server. Rather than
attempting to proxy the authentication to the server within the
correct realm, the EAP server SHOULD terminate the conversation.
The PEAPv2 peer can present the server with multiple identities. This
includes the claim of identity within the initial EAP-
Response/Identity (MyID) packet, which is typically used to route the
EAP conversation to the appropriate home backend authentication
server. There may also be subsequent EAP-Response/Identity packets
sent by the peer once the TLS channel has been established.
Note that since the PEAPv2 peer may not present a certificate, it is
not always possible to check the initial EAP-Response/Identity
against the identity presented in the certificate, as is done in
[RFC2716].
Moreover, it cannot be assumed that the peer identities presented
within multiple EAP-Response/Identity packets will be the same. For
example, the initial EAP-Response/Identity might correspond to a
machine identity, while subsequent identities might be those of the
user. Thus, PEAPv2 implementations SHOULD NOT abort the
authentication just because the identities do not match. However,
since the initial EAP-Response/Identity will determine the EAP server
handling the authentication, if this or any other identity is
inappropriate for use with the destination EAP server, there is no
alternative but to terminate the PEAPv2 conversation.
The protected identity or identities presented by the peer within
PEAPv2 Part 2 may not be identical to the cleartext identity
presented in PEAPv2 Part 1, for legitimate reasons. In order to
shield the userID from snooping, the cleartext Identity may only
provide enough information to enable routing of the authentication
request to the correct realm. For example, the peer may initially
claim the identity of "nouser@bigco.com" in order to route the
authentication request to the bigco.com EAP server. Subsequently,
once the TLS session has been negotiated, in PEAPv2 Part 2, the peer
may claim the identity of "fred@bigco.com". Thus, PEAPv2 can provide
protection for the user's identity, though not necessarily the
destination realm, unless the PEAPv2 Part 1 conversation terminates
at the local authentication server.
As a result, PEAPv2 implementations SHOULD NOT attempt to compare the
Identities claimed with Parts 1 and 2 of the PEAPv2 conversation.
Similarly, if multiple Identities are claimed within PEAPv2 Part 2,
these SHOULD NOT be compared. An EAP conversation may involve more
than one EAP authentication method, and the identities claimed for
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each of these authentications could be different (e.g. a machine
authentication, followed by a user authentication).
5.8. Man-in-the-middle attack protection
TLS protection can address a number of weaknesses in the EAP method;
as well as EAP protocol weaknesses listed in the abstract and
introduction sections in this document.
Hence, the recommended solution is to always deploy authentication
methods with protection of PEAPv2.
if a deployment chooses to allow a EAP method protected by PEAP
without protection of PEAP or IPsec at the same time, then this opens
up a possibility of a man-in-the-middle attack.
A man-in-the-middle can spoof the client to authenticate to it
instead of the real EAP server; and forward the authentication to the
real server over a protected tunnel. Since the attacker has access to
the keys derived from the tunnel, it can gain access to the network.
PEAP version 2 prevents this attack by using the keys generated by
the inner EAP method in the crypto-binding exchange described in
protected termination section. This attack is not prevented if the
inner EAP method does not generate keys or if the keys generated by
the inner EAP method can be compromised.
Alternatively, the attack can also be thwarted if the inner EAP
method can signal to the peer that the packets are being sent within
the tunnel. In most cases this may require modification to the inner
EAP method. In order to allow for these implementations, PEAPv2
implementations should inform inner EAP methods that the EAP method
is being protected by a PEAPv2 tunnel.
Since all sequence negotiations and exchanges are protected by TLS
channel, they are immune to snooping and MITM attacks with the use of
Crypto-Binding TLV. To make sure the same parties are involved tunnel
establishment and previous inner method, before engaging the next
method to sent more sensitive information, both peer and server MUST
use the Crypto-Binding TLV between methods to check the tunnel
integrity. If the Crypto-Binding TLV failed validation, they SHOULD
stop the sequence and terminate the tunnel connection, to prevent
more sensitive information being sent in subsequent methods.
5.9. Cleartext forgeries
As described in [RFC2284bis], EAP Success and Failure packets are not
authenticated, so that they may be forged by an attacker without fear
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of detection. Forged EAP Failure packets can be used to convince an
EAP peer to disconnect. Forged EAP Success and Failure packets may be
used to convince a peer to disconnect; or convince a peer to access
the network even before authentication is complete, resulting in
denial of service for the peer.
By supporting encrypted, authenticated and integrity protected
success/failure indications, PEAPv2 provides protection against these
attacks.
When the peer responds with the first PEAP packet; and the EAP server
receives the first PEAPv2 packet from the peer, both MUST silently
discard all clear text EAP messages unless both the PEAPv2 peer and
server have indicated success or failure or error using a protected
error or protected termination mechanism. The success/failure
decisions sent by a protected mechanism indicate the final decision
of the EAP authentication conversation. After success/failure has
been indicated by a protected mechanism, the PEAPv2 client can
process unprotected EAP success and EAP failure message; however MUST
ignore any unprotected EAP success or failure messages where the
decision does not match the decision of the protected mechanism.
[RFC2284bis] states that an EAP Success or EAP Failure packet
terminates the EAP conversation, so that no response is possible.
Since EAP Success and EAP Failure packets are not retransmitted, if
the final packet is lost, then authentication will fail. As a
result, where packet loss is expected to be non-negligible,
unacknowledged success/failure indications lack robustness.
As a result, a EAP server SHOULD send a clear text EAP success or
EAP-failure packet after the protected success or failure packet or
TLS alert. The peer MUST NOT require the clear text EAP Success or
EAP Failure if it has received the protected success or failure or
TLS alert. For more details, refer to [RFC228bis], Section 4.2.
5.10. TLS Ciphersuites
Anonymous ciphersuites are vulnerable to man-in-the-middle attacks,
and SHOULD NOT be used with PEAPv2, unless the EAP methods inside
PEAPv2 can address the man-in-the-middle attack or unless the man-in-
the-middle attack can be addressed by mechanisms external to PEAPv2.
5.11. Denial of service attacks
Denial of service attacks are possible if the attacker can insert or
modify packets in the authentication channel. The attacker can
modify unprotected fields in the PEAP packet such as the EAP protocol
or PEAP version number. This can result in a denial of service
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attack. It is also possible for the attacker to modify protected
fields in a packet to cause decode errors resulting in a denial of
service. In these ways the attacker can prevent access for peers
connecting to the network.
Denial of service attacks with multiplier impacts are more
interesting than the ones above. It is possible to multiply the
impact by creating a large number of TLS sessions with the EAP
server.
5.12. Security Claims
Intended use: Wireless or Wired networks, and over
the Internet, where physical security
cannot be assumed.
Auth. mechanism: Use arbitrary EAP and TLS authentication
mechanisms for authentication of the
client and server.
Ciphersuite negotiation: Yes.
Mutual authentication: Yes. Depends on the type of EAP method
used within the tunnel and the type of
authentication used within TLS.
Integrity protection: Yes
Replay protection: Yes
Confidentiality: Yes
Key derivation: Yes
Key strength: Variable
Dictionary attack prot: Not susceptible.
Fast reconnect: Yes
Crypt. binding: Yes.
Acknowledged S/F: Yes
Session independence: Yes.
Fragmentation: Yes
PEAPv2 derives keys by combining keys from TLS and the inner EAP
methods. It should be noted that the use of TLS ciphersuites with a
particular key lengths does not guarantee that the key strength of
the keys will be equivalent to the length. The key exchange
mechanisms (eg. RSA or Diffie-Hellman) used must provide sufficient
security or they will be the weakest link. For example RSA key sizes
with a modulus of 1024 bits provides less than 128 bits of security,
this may provide sufficient key strength for some applications and
not for others. See [PKLENGTH] for a detailed analysis of
determining the public key strengths used to exchange symmetric keys.
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6. IANA Considerations
This section provides guidance to the Internet Assigned Numbers
Authority (IANA) regarding registration of values related to the EAP
protocol, in accordance with BCP 26, [RFC2434].
There is one name space in EAP-TLV that requires registration: PEAPv2
TLV-Types.
6.1. Definition of Terms
The following terms are used here with the meanings defined in BCP
26: "name space", "assigned value", "registration".
The following policies are used here with the meanings defined in BCP
26: "Private Use", "First Come First Served", "Expert Review",
"Specification Required", "IETF Consensus", "Standards Action".
6.2. Recommended Registration Policies
For "Designated Expert with Specification Required", the request is
posted to the EAP WG mailing list (or, if it has been disbanded, a
successor designated by the Area Director) for comment and review,
and MUST include a pointer to a public specification. Before a period
of 30 days has passed, the Designated Expert will either approve or
deny the registration request and publish a notice of the decision to
the EAP WG mailing list or its successor. A denial notice must be
justified by an explanation and, in the cases where it is possible,
concrete suggestions on how the request can be modified so as to
become acceptable.
EAP-TLVs have a 14-bit field, of which 0-10 have been allocated.
Additional EAP-TLV type codes may be allocated following Designated
Expert with Specification Required [RFC2434].
7. References
7.1. Normative references
[RFC1321] Rivest, R. and S. Dusse, "The MD5 Message-Digest Algorithm",
RFC 1321, April 1992.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, November 1998.
Palekar et al. Standards Track [Page 53]
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[RFC2486] Aboba, B. and M. Beadles, "The Network Access Identifier", RFC
2486, January 1999.
[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396, August
1998.
[RFC3546] Blake-Wilson, S., et al. "TLS Extensions", RFC 3546, June
2003.
[RFC2284bis]
Blunk, L. et al., "Extensible Authentication Protocol (EAP)",
draft-ietf-eap-rfc2284bis-06.txt, Internet draft (work in
progress), October 2003.
7.2. Informative references
[RFC1968] Meyer, G., "The PPP Encryption Protocol (ECP)", RFC 1968, June
1996.
[RFC1990] Sklower, K., Lloyd, B., McGregor, G., Carr, D. and T.
Coradetti, "The PPP Multilink Protocol (MP)", RFC 1990, August
1996.
[RFC2419] Sklower, K. and G. Meyer, "The PPP DES Encryption Protocol,
Version 2 (DESE-bis)", RFC 2419, September 1998.
[RFC2420] Hummert, K., "The PPP Triple-DES Encryption Protocol (3DESE)",
RFC 2420, September 1998.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
[RFC2548] Zorn, G., "Microsoft Vendor-specific RADIUS Attributes",
RFC2548, March 1999.
[RFC2716] Aboba, B. and D. Simon, "PPP EAP TLS Authentication Protocol",
RFC 2716, October 1999.
[RFC3078] Pall, G. and G. Zorn, "Microsoft Point-to-Point Encryption
(MPPE) Protocol", RFC 3078, March 2001.
[RFC3079] Zorn, G., "Deriving Keys for use with Microsoft Point-to-Point
Encryption (MPPE)", RFC 3079, March 2001.
[FIPSDES] National Bureau of Standards, "Data Encryption Standard", FIPS
PUB 46 (January 1977).
Palekar et al. Standards Track [Page 54]
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[IEEE80211]
Information technology - Telecommunications and information
exchange between systems - Local and metropolitan area
networks - Specific Requirements Part 11: Wireless LAN Medium
Access Control (MAC) and Physical Layer (PHY) Specifications,
IEEE Std. 802.11-1999, 1999.
[MODES] National Bureau of Standards, "DES Modes of Operation", FIPS
PUB 81 (December 1980).
[PEAPv0] Kamath, V., Palekar, A. and M. Wodrich, "Microsoft's PEAP
version 0 (Implementation in Windows XP SP1)", draft-kamath-
pppext-peapv0-00.txt, Internet draft (work in progress), July
2002.
[PKLENGTH]
H. Orman and P. Hoffman, "Determining Strengths For Public
Keys Used For Exchanging Symmetric Keys", draft-orman-public-
key-lengths-05.txt, Internet Draft (work in progress),
December 2002.
[RFC3579] Aboba, B. and P. Calhoun, "RADIUS Support for EAP", RFC 3579,
September 2003.
[CompoundBinding]
Puthenkulam, J., Lortz, V., Palekar, A. and D. Simon, "The
Compound Authentication Binding Problem", draft-puthenkulam-
eap-binding-03.txt, Internet Draft (work in progress), May
2003.
[IEEE8021X]
IEEE Standards for Local and Metropolitan Area Networks: Port
based Network Access Control, IEEE Std 802.1X-2001, June 2001.
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Appendix A - Examples
A.1 Cleartext Identity Exchange
In the case where an identity exchange occurs within PEAPv2 Part 1,
the conversation will appear as follows:
Authenticating Peer Authenticator
------------------- -------------
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID1) ->
// Identity sent in the clear. May be a hint to help route the
authentication request to EAP server, instead of the full user
identity.
<- EAP-Request/
EAP-Type=PEAP, V=2
(PEAP Start, S bit set)
EAP-Response/
EAP-Type=PEAP, V=2
(TLS client_hello)->
<- EAP-Request/
EAP-Type=PEAP, V=2
(TLS server_hello,
TLS certificate,
[TLS server_key_exchange,]
[TLS certificate_request,]
TLS server_hello_done)
EAP-Response/
EAP-Type=PEAP, V=2
([TLS certificate,]
TLS client_key_exchange,
[TLS certificate_verify,]
TLS change_cipher_spec,
TLS finished) ->
<- EAP-Request/
EAP-Type=PEAP, V=2
(TLS change_cipher_spec,
TLS finished,
EAP-Request/EAP-Type=EAP-TLV
[EAP-Payload-TLV[EAP-Request/
Identity]])
// identity protected by TLS. EAP-TLV packet does not include an EAP-
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header.
TLS channel established (EAP messages sent within TLS channel
encapsulated in EAP-TLV packets without EAP header)
EAP-TLV [EAP-Payload-TLV
[EAP-Response/Identity (MyID2)]]]->
<- EAP-TLV [EAP-Payload-TLV
[EAP-Request/EAP-Type=X]]
EAP-TLV [EAP-Payload-TLV
[EAP-Response/EAP-Type=X]] ->
// Protected termination
<- EAP-TLV [Result TLV (Success),
Crypto-Binding-TLV (Version=0,
received-version=2, Nonce, B1_MAC),
Intermediate-Result-TLV (Success)]
EAP-TLV [Result-TLV (Success),
Intermediate-Result-TLV (Success),
Crypto-Binding-TLV (Version=0,
received-version=2,Nonce, B2_MAC)]->
TLS channel torn down
(messages sent in cleartext)
<- EAP-Success
A.2 No cleartext Identity Exchange
Where all peers are known to support PEAPv2, a non-certificate
authentication is desired for the client and the PEAP Part 1
conversation is carried out between the peer and a local EAP server,
the cleartext identity exchange may be omitted and the conversation
appears as follows:
Authenticating Peer Authenticator
------------------- -------------
<- EAP-Request/
EAP-Type=PEAP, V=2
(PEAP Start, S bit set)
EAP-Response/
EAP-Type=PEAP, V=2
(TLS client_hello)->
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<- EAP-Request/
EAP-Type=PEAP, V=2
(TLS server_hello,
TLS certificate,
[TLS server_key_exchange,]
[TLS certificate_request,]
TLS server_hello_done)
EAP-Response/
EAP-Type=PEAP, V=2
([TLS certificate,]
TLS client_key_exchange,
[TLS certificate_verify,]
TLS change_cipher_spec,
TLS finished) ->
<- EAP-Request/
EAP-Type=PEAP, V=2
(TLS change_cipher_spec,
TLS finished,
EAP-TLV [EAP-Payload-TLV
(EAP-Request/Identity)])
TLS channel established
(messages sent within the TLS channel)
EAP-TLV [EAP-Payload-TLV
[EAP-Response/Identity (MyID)]] ->
<- EAP-TLV [EAP-Payload-TLV
[EAP-Type=EAP-Request/
EAP-Type=X]]
EAP-TLV [EAP-Payload-TLV
[EAP-Response/EAP-Type=X
or NAK] ->
<- EAP-TLV [EAP-Payload-TLV
[EAP-Request/EAP-Type=X]]
EAP-TLV [EAP-Payload-TLV [EAP-Response/
EAP-Type=X]] ->
// Protected success
<- EAP-TLV [Crypto-Binding-TLV=
(Version=0, Received-version=2,
Nonce, B1_MAC),
Intermediate-Result-TLV(Success),
Result TLV (Success)]
EAP-TLV [Crypto-Binding-TLV=
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(Version=0,Received-version=2,
Nonce, B2_MAC),
Intermediate-Result-TLV (Success),
Result TLV (Success)]->
TLS channel torn down
(messages sent in cleartext)
<- EAP-Success
A.3 Client certificate authentication with identity privacy
Where all peers are known to support PEAPv2, where client certificate
authentication is desired and the PEAPv2 Part 1 conversation is
carried out between the peer and a local EAP server, the cleartext
identity exchange may be omitted and the conversation appears as
follows:
Authenticating Peer Authenticator
------------------- -------------
<- EAP-Request/
EAP-Type=PEAP, V=2
(PEAP Start, S bit set)
EAP-Response/
EAP-Type=PEAP, V=2
(TLS client_hello)->
<- EAP-Request/
EAP-Type=PEAP, V=2
(TLS server_hello,
TLS certificate,
[TLS server_key_exchange,]
TLS server_hello_done)
EAP-Response/
EAP-Type=PEAP, V=2
(TLS client_key_exchange,
TLS change_cipher_spec,
TLS finished) ->
<- EAP-Request/
EAP-Type=PEAP, V=2
(TLS change_cipher_spec,
TLS finished,TLS Hello-Request)
EAP-Response/
EAP-Type=PEAP, V=2
(TLS client_hello)->
TLS channel established
(messages sent within the TLS channel)
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<- EAP-Request/
EAP-Type=PEAP, V=2
(TLS server_hello,
TLS certificate,
[TLS server_key_exchange,]
[TLS certificate_request,]
TLS server_hello_done)
EAP-Response/
EAP-Type=PEAP, V=2
([TLS certificate,]
TLS client_key_exchange,
[TLS certificate_verify,]
TLS change_cipher_spec,
TLS finished) ->
<- EAP-Request/
EAP-Type=PEAP, V=2
(TLS change_cipher_spec,
TLS finished, EAP-TLV
[Crypto-binding-TLV (version=0,
Received-version=2, Nonce,
B1_MAC),
Result-TLV (Success)])
// packet format within the TLS channel
EAP-TLV [
Crypto-Binding-TLV=(Version=0,
Received-version=2,
Nonce, B2_MAC),
Result TLV (Success)]
TLS channel torn down
(messages sent in cleartext)
<- EAP-Success
A.4 Fragmentation and Reassembly
In the case where the PEAP fragmentation is required, the
conversation will appear as follows:
Authenticating Peer Authenticator
------------------- -------------
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID) ->
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<- EAP-Request/
EAP-Type=PEAP, V=2
(PEAP Start, S bit set)
EAP-Response/
EAP-Type=PEAP, V=2
(TLS client_hello)->
<- EAP-Request/
EAP-Type=PEAP, V=2
(TLS server_hello,
TLS certificate,
[TLS server_key_exchange,]
[TLS certificate_request,]
TLS server_hello_done)
(Fragment 1: L, M bits set)
EAP-Response/
EAP-Type=PEAP, V=2 ->
<- EAP-Request/
EAP-Type=PEAP, V=2
(Fragment 2: M bit set)
EAP-Response/
EAP-Type=PEAP, V=2 ->
<- EAP-Request/
EAP-Type=PEAP, V=2
(Fragment 3)
EAP-Response/
EAP-Type=PEAP, V=2
([TLS certificate,]
TLS client_key_exchange,
[TLS certificate_verify,]
TLS change_cipher_spec,
TLS finished)
(Fragment 1: L, M bits set)->
<- EAP-Request/
EAP-Type=PEAP, V=2
EAP-Response/
EAP-Type=PEAP, V=2
(Fragment 2)->
<- EAP-Request/
EAP-Type=PEAP, V=2
(TLS change_cipher_spec,
TLS finished, EAP-TLV
[EAP-Payload-TLV[
EAP-Request/Identity]])
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TLS channel established
(messages sent within the TLS channel)
EAP-TLV
[EAP-Payload-TLV
[EAP-Response/Identity(myID)]] ->
<- EAP-TLV [ EAP-Payload-TLV
[EAP-Request/EAP-Type=X]]
EAP-TLV [EAP-Payload-TLV
[EAP-Response/EAP-Type=X or NAK]]->
<- EAP-TLV [ EAP-Payload-TLV
[EAP-Request/EAP-Type=X]]
EAP-TLV [EAP-Payload-TLV
[EAP-Response/EAP-Type=X] ->
<- EAP-TLV [Crypto-Binding-TLV
=(Version=0, Received-Version=2,
Nonce, B1_MAC),
Intermediate-Result-TLV(Success),
Result TLV (Success)]
EAP-TLV [
Crypto-Binding-TLV=(Version=0,
Received-Version=2,Nonce, B2_MAC),
Result TLV (Success),
Intermediate-Result-TLV (Success)] ->
TLS channel torn down
(messages sent in cleartext)
<- EAP-Success
A.5 Server authentication fails in Part 2
In the case where the server authenticates to the client successfully
in PEAPv2 Part 1, but the client fails to authenticate to the server
in PEAPv2 Part 2, the conversation will appear as follows:
Authenticating Peer Authenticator
------------------- -------------
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID) ->
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<- EAP-Request/
EAP-Type=PEAP, V=2
(PEAP Start, S bit set)
EAP-Response/
EAP-Type=PEAP, V=2
(TLS client_hello)->
<- EAP-Request/
EAP-Type=PEAP, V=2
(TLS server_hello,
TLS certificate,
[TLS server_key_exchange,]
[TLS certificate_request,]
TLS server_hello_done)
EAP-Response/
EAP-Type=PEAP, V=2
([TLS certificate,]
TLS client_key_exchange,
[TLS certificate_verify,]
TLS change_cipher_spec,
TLS finished) ->
<- EAP-Request/
EAP-Type=PEAP, V=2
(TLS change_cipher_spec,
TLS finished, EAP-TLV
[EAP-Payload-TLV
[EAP-Request/Identity]])
TLS channel established
(messages sent within the TLS channel)
EAP-TLV [EAP-Payload-TLV
[EAP-Response/Identity (MyID)]] ->
<- EAP-TLV [EAP-Payload-TLV
[EAP-Request/EAP-Type=X]]
EAP-TLV [EAP-Payload
[EAP-Response/EAP-Type=X
or NAK]] ->
<- EAP-TLV [EAP-Payload
[EAP-Request/EAP-Type=X]]
EAP-TLV [EAP-Payload
[EAP-Response/
EAP-Type=X]] ->
<- EAP-TLV [Crypto-Binding-TLV
(Version=0, Received-Version=2,
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Nonce, B1_MAC),
Intermediate-Result-TLV (Failure),
Result TLV (Failure)]
EAP-TLV [Crypto-Binding-TLV
(Version=0, Received-version=2,
Nonce, B2_MAC),
Result TLV (Failure),
Intermediate-Result-TLV (Failure)]
(TLS session cache entry flushed)
TLS channel torn down
(messages sent in cleartext)
<- EAP-Failure
A.6 Server authentication fails in Part 1
In the case where server authentication is unsuccessful in PEAP Part
1, the conversation will appear as follows:
Authenticating Peer Authenticator
------------------- -------------
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID) ->
<- EAP-Request/
EAP-Type=PEAP, V=2
(PEAP Start)
EAP-Response/
EAP-Type=PEAP, V=2
(TLS client_hello)->
<- EAP-Request/
EAP-Type=PEAP, V=2
(TLS server_hello,
TLS certificate,
[TLS server_key_exchange,]
TLS server_hello_done)
EAP-Response/
EAP-Type=PEAP, V=2
(TLS client_key_exchange,
[TLS certificate_verify,]
TLS change_cipher_spec,
TLS finished) ->
<- EAP-Request/
EAP-Type=PEAP, V=2
(TLS change_cipher_spec,
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TLS finished, EAP-TLV
[EAP-Payload-TLV [
EAP-Request/Identity]])
EAP-Response/
EAP-Type=PEAP, V=2
(TLS Alert message) ->
<- EAP-Failure
(TLS session cache entry flushed)
A.7 Session resume success
In the case where a previously established session is being resumed,
the EAP server supports TLS session cache flushing for unsuccessful
PEAPv2 Part 2 authentications and both sides authenticate
successfully, the conversation will appear as follows:
Authenticating Peer Authenticator
------------------- -------------
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID) ->
<- EAP-Request/
EAP-Type=PEAP,V=2
(PEAP Start)
EAP-Response/
EAP-Type=PEAP, V=2
(TLS client_hello)->
<- EAP-Request/
EAP-Type=PEAP, V=2
(TLS server_hello,
TLS change_cipher_spec
TLS finished)
EAP-Response/
EAP-Type=PEAP, V=2
(TLS change_cipher_spec,
TLS finished) ->
<- EAP-Request/
EAP-Type=EAP-TLV
Result TLV (Success)
// Compound MAC calculated using TLS keys since there were no inner
EAP methods.
EAP-Response/
EAP-Type=EAP-TLV
Crypto-Binding-TLV=(Version=0, Nonce, B2_MAC),
Result TLV (Success)->
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TLS channel torn down
(messages sent in cleartext)
<- EAP-Success
A.8 Session resume failure
In the case where a previously established session is being resumed,
and the server authenticates to the client successfully but the
client fails to authenticate to the server, the conversation will
appear as follows:
Authenticating Peer Authenticator
------------------- -------------
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID) ->
<- EAP-Request/
EAP-Request/
EAP-Type=PEAP, V=2
(PEAP Start)
EAP-Response/
EAP-Type=PEAP, V=2
(TLS client_hello) ->
<- EAP-Request/
EAP-Type=PEAP, V=2
(TLS server_hello,
TLS change_cipher_spec,
TLS finished)
EAP-Response/
EAP-Type=PEAP, V=2
(TLS change_cipher_spec,
TLS finished) ->
<- EAP-Request
EAP-Type=PEAP, V=2
(TLS Alert message)
EAP-Response
EAP-Type=PEAP, V=2 ->
<- EAP-Failure
(TLS session cache entry flushed)
A.9 Session resume failure
In the case where a previously established session is being resumed,
and the server authentication is unsuccessful, the conversation will
appear as follows:
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INTERNET-DRAFT PEAPv2 26 October 2003
Authenticating Peer Authenticator
------------------- -------------
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID) ->
<- EAP-Request/
EAP-Request/
EAP-Type=PEAP, V=2
(PEAP Start)
EAP-Response/
EAP-Type=PEAP, V=2
(TLS client_hello)->
<- EAP-Request/
EAP-Type=PEAP, V=2
(TLS server_hello,
TLS change_cipher_spec,
TLS finished)
EAP-Response/
EAP-Type=PEAP, V=2
(TLS change_cipher_spec,
TLS finished)
<- EAP-Request/
EAP-Type=PEAP, V=2
EAP-Response/
EAP-Type=PEAP, V=2
(TLS Alert message) ->
(TLS session cache entry flushed)
<- EAP-Failure
A.10 PEAP version negotiation
In the case where the peer and authenticator have mismatched PEAP
versions (e.g. the peer has a pre-standard implementation with
version 0, and the authenticator has an implementation compliant with
this specification), the conversation will occur as follows:
Authenticating Peer Authenticator
------------------- -------------
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID) ->
<- EAP-Request/
EAP-Request/
EAP-Type=PEAP, V=2
(PEAP Start)
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EAP-Response/
EAP-Type=PEAP, V=0
(TLS client_hello)->
<- EAP-Request/
EAP-Type=PEAP, V=0
(TLS server_hello,
TLS change_cipher_spec,
TLS finished)
//conversation continued using pre-standard PEAP version 0
A.11 Sequences of EAP methods
Where PEAPv2 is negotiated, with a sequence of EAP method X followed
by method Y, the conversation will occur as follows:
Authenticating Peer Authenticator
------------------- -------------
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID1) ->
<- EAP-Request/
EAP-Type=PEAP, V=2
(PEAP Start, S bit set)
EAP-Response/
EAP-Type=PEAP, V=2
(TLS client_hello)->
<- EAP-Request/
EAP-Type=PEAP, V=2
(TLS server_hello,
TLS certificate,
[TLS server_key_exchange,]
[TLS certificate_request,]
TLS server_hello_done)
EAP-Response/
EAP-Type=PEAP, V=2
([TLS certificate,]
TLS client_key_exchange,
[TLS certificate_verify,]
TLS change_cipher_spec,
TLS finished) ->
<- EAP-Request/
EAP-Type=PEAP, V=2
(TLS change_cipher_spec,
TLS finished, EAP-TLV
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[EAP-Payload-TLV[
EAP-Request/Identity]])
TLS channel established
(messages sent within the TLS channel)
EAP-TLV [EAP-Payload-TLV
[EAP-Response/Identity]] ->
<- EAP-TLV [EAP-Payload-TLV
[EAP-Request/EAP-Type=X]]]
EAP-TLV [EAP-Payload-TLV
[EAP-Response/EAP-Type=X]] ->
<- EAP-TLV [ EAP-Payload-TLV
[EAP-Request/EAP-Type=X]]
EAP-TLV [EAP-Payload-TLV
[EAP-Response/EAP-Type=X]]->
<- EAP-TLV [EAP Payload TLV [EAP-Type=Y],
(Intermediate Result TLV (Success),
Crypto-Binding-TLV
(Version=0, Received-version=2,
Nonce, B1_MAC))]
// Next EAP conversation started after successful completion of
previous method X. The intermediate-status and crypto-binding TLVs
are sent in next packet to minimize round-trips. In this example,
identity request is not sent before negotiating EAP-Type=Y.
EAP-TLV [EAP-Payload-TLV [EAP-Type=Y],
(Intermediate Result TLV (Success),
Crypto-Binding-TLV (Version=0,
Received-version=2, Nonce, B2_MAC))]->
// Compound MAC calculated using Keys generated from
EAP methods X and the TLS tunnel.
<- EAP-TLV [EAP Payload TLV [
EAP-Type=Y]]
EAP-TLV[EAP Payload TLV
[EAP-Type=Y]] ->
<- EAP-TLV [Result-TLV (Success),
Intermediate Result TLV (Success),
Crypto-Binding-TLV
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(Version=0, Received-version=2,
Nonce, B1_MAC))]
EAP-TLV [(Result-TLV (Success),
Intermediate Result TLV (Success),
Crypto-Binding-TLV (Version=0,
Received-version=2, Nonce, B2_MAC))]->
// Compound MAC calculated using Keys generated from EAP methods X
and Y and the TLS tunnel. // Compound Keys generated using Keys
generated from EAP methods X and Y; and the TLS tunnel.
TLS channel torn down (messages sent in cleartext)
<- EAP-Success
Acknowledgments
Thanks to Hakan Andersson, Jan-Ove Larsson and Magnus Nystrom of RSA
Security; Bernard Aboba, Vivek Kamath, Stephen Bensley and Narendra
Gidwani of Microsoft; Ilan Frenkel and Nancy Cam-Winget of Cisco;
Jose Puthenkulam of Intel for their contributions and critiques.
The compound binding exchange to address man-in-the-middle attack is
based on the draft "The Compound Authentication Binding
Problem"[CompoundBinding].
The vast majority of the work by Simon Josefsson and Hakan Andersson
was done while they were employed at RSA Laboratories.
Author Addresses
Ashwin Palekar
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: +1 425 882 8080
EMail: ashwinp@microsoft.com
Dan Simon
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: +1 425 706 6711
EMail: dansimon@microsoft.com
Palekar et al. Standards Track [Page 70]
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Glen Zorn
Cisco Systems
500 108th Avenue N.E.
Suite 500
Bellevue, Washington 98004
Phone: + 1 425 438 8210
Fax: + 1 425 438 1848
EMail: gwz@cisco.com
Simon Josefsson
Drottningholmsvagen 70
112 42 Stockholm
Sweden
Phone: +46 8 619 04 22
EMail: jas@extundo.com
Hao Zhou
Cisco Systems, Inc.
4125 Highlander Parkway
Richfield, OH 44286
Phone: +1 330 523 2132
Fax: +1 330 523 2239
EMail: hzhou@cisco.com
Joseph Salowey
Cisco Systems
2901 3rd Ave
Seattle, WA 98121
Phone: +1 206 256 3380
EMail: jsalowey@cisco.com
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