One document matched: draft-ietf-pana-pana-00.txt
PANA Working Group
Internet Draft D. Forsberg
Nokia
Y. Ohba
Toshiba
B. Patil
Nokia
H. Tschofenig
Siemens
A. Yegin
DoCoMo USA Labs
Document: draft-ietf-pana-pana-00.txt
Expires: September 2003 March 2003
Protocol for Carrying Authentication for Network Access (PANA)
<draft-ietf-pana-pana-00.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.
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.
Internet-Drafts are draft documents valid for a maximum of six
months 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.
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Abstract
This document defines the Protocol for Carrying Authentication for
Network Access (PANA), a link-layer agnostic transport for
Extensible Authentication Protocol (EAP) to enable network access
authentication between clients and access networks. PANA can carry
any authentication method that can be specified as an EAP method,
and can be used on any link that can carry IP. PANA covers the
client-to-network access authentication part of an overall secure
network access framework, which additionally includes other
protocols and mechanisms for service provisioning, access control as
a result of initial authentication, and accounting.
Table of Contents
1 Introduction...................................................2
2 Terminology....................................................3
3 Protocol Overview..............................................4
4 Protocol Details...............................................5
5 PANA Security Association Establishment.......................15
6 Authentication Method Choice..................................16
7 Filter Rule Installation......................................16
8 Data Traffic Protection.......................................17
9 Message Formats...............................................18
10 Open Issues...................................................18
11 Security Considerations.......................................18
12 References....................................................23
13 Acknowledgments...............................................25
14 Author's Addresses............................................25
15 Full Copyright Statement......................................35
1 Introduction
Providing secure network access service requires access control
based on the authentication and authorization of the clients and the
access networks. Initial and subsequent client-to-network
authentication provides parameters that are needed to police the
traffic flow through the enforcement points. A protocol is needed to
carry authentication methods between the client and the access
network. IETF PANA Working Group has been chartered with the goal
of designing a network-layer access authentication protocol.
Link-layer authentication mechanisms are used as enablers of secure
network access. A higher-layer authentication is deemed necessary
when link-layer authentication mechanisms are either not available
for lack of technology or deployment difficulties, or not able to
meet the overall requirements, or when multi-layer (e.g., link-layer
and network-layer) authentication is needed. Currently there is no
standard network-layer solution for authenticating clients for
network access. In the absence of such a solution, some inadequate
standards-based solutions are deployed or non-standard ad-hoc
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solutions are invented. [USAGE] Internet-Draft describes the problem
statement in detail.
Scope of this working group is identified as designing a link-layer
agnostic transport for network access authentication methods. PANA
Working Group has identified EAP [RFC2284] as the payload for this
protocol and carrier for authentication methods. In other words,
PANA will carry EAP which can carry various authentication methods.
By the virtue of enabling transport of EAP above IP, any
authentication method that can be carried as an EAP method is
made available to PANA and hence to any link-layer technology. There
is a clear division of labor between PANA, EAP and EAP methods.
Defining new authentication methods, or deriving/distributing keys
is outside the scope of PANA. Providing a secure channel that
protects EAP and EAP methods against eavesdropping and spoofing is
not an objective of the PANA design.
While PANA is a fundamental part of a complete secure network access
solution, its responsibility is limited to authentication and
authorization of the client and the network. Providing access
control is outside the scope of PANA. A separate provisioning
protocol is needed for passing filtering
information to access control nodes in the network. Additionally,
mechanisms to provide data traffic protection in terms of
authentication, integrity and replay protection, and encryption are
outside the scope as well.
Various environments and usage models for PANA are identified in the
[USAGE] Internet-Draft. Potential security threats for network-layer
access authentication protocol is discussed in [THREATS] draft.
These two drafts have been essential in defining the requirements
[PY+02] on the PANA protocol. Note that some of these requirements
are imposed by the chosen payload, EAP [RFC2284].
This Internet-Draft makes an attempt for defining the PANA protocol
based on the other drafts discussed above. Special care has been
given to ensure the currently stated scope is observed and to keep
the protocol as simple as possible. The current state of this draft
is not complete, but it should be regarded as a work in progress.
The authors made effort to capture the common understanding
developed within the working group as much as possible. The design
choices being made in this draft should not be considered as cast in
stone.
2 Terminology
This section describes some terms introduced in this document:
PANA Session:
PANA session is defined as the exchange of messages between the
PaC and the PAA to authenticate a user(PaC) for network access.
If the authentication is unsuccessful, the session is
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terminated. The session is considered as active until there is a
disconnect indication by the PaC or the PAA terminates it.
Session Identifier:
The device identifier is also used as the session identifier.
This is used for indicating a disconnect or session revocation
or for charging purposes.
PANA Disconnect Indication:
PANA session termination with explicit notification from a PaC
sent to the PAA. The PDI also includes the session identifier.
PANA Session Revocation:
PANA session termination with explicit notification sent from
the PAA to the PaC. The PSR includes the session identifier.
PANA Security Association:
The representation of the trust relation between the PaC and the
PAA that is created at the end of the authentication phase
(PH2). This security association includes the device identifier
of the peer, and a shared key when available.
The terms PaC, PAA, EP and Device Identifier can be found in
[PY+02].
3 Protocol Overview
The PANA protocol involves two functional entities namely the PaC
and the PAA. The EP, mentioned in the context with PANA, is a
logical entity. There is, however, the option that the EP is not
physically co-located with the PAA. In case that the PAA and the EP
are co-located only an API is required instead of a separate
protocol. In the case where the PAA is separated from the EP, a
separate protocol will be used between the PAA and the EP for
managing access control. The protocol and messaging between the PAA
and EP for access authorization is outside the scope of this draft
and will be dealt separately.
The PANA protocol (PaC<->PAA) resides above the transport layer and
the details are explained in Section 4.2. Although this document
describes the interaction with a number of entities and with other
protocol which enable network access authentication; the PANA
protocol itself is executed between the PaC and the PAA.
The protocol has three primary functions:
1. The PaC discovering the address of the PAA
2. The transport of EAP payloads between the PaC and the PAA
3. Access authorization by the PAA to the EP [Note that this aspect
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is outside the scope of the PANA protocol.]
The placement of the entities used in PANA largely depend on a
certain architecture. The PAA may optionally interact with a AAA
backend to authenticate the user (PaC). And in the case where the
PAA and EP are co-located, step 3 mentioned above may not require a
separate protocol. The figure below illustrates the interactions in
a simplified manner:
PaC EP PAA AAA
--- --- --- ---
PAA Discovery
<---------------------o-----------------> (1)
| PANA_REQUEST
| ---------------------------------------->
| AAA interaction
|(2) ----------->
| <-----------
| PANA_RESPONSE
| <---------------------------------------
|
Authorization
<----------------- (3)
Figure 1: PANA Protocol
The details of each of these aspects of the protocol are described
in section 4 of this document. PANA supports authentication of a PaC
using various EAP methods. The EAP method used depends on the level
of security required for the EAP messaging itself. PANA does not
secure the data traffic itself. However EAP methods that enable key
exchange may allow other protocols to be bootstrapped for securing
the data traffic.
From a state machine aspect, PANA protocol consists of three phases
1. Discovery and initial handshake phase
2. Authentication phase
3. Termination phase
In the first phase, an IP address of PAA is discovered and a PANA
session is established between PaC and PAA. EAP messages are
exchanged and a PANA SA is established in the second phase. The
established PANA session as well as a PANA SA is deleted in the
third phase.
4 Protocol Details
Throughout the section, we use a notation "MESSAGENAME_ack" to
represent a message which is used as an acknowledgment to a message
"MESSAGENAME". In actual message formats, the two messages have the
same message type and are distinguished by an acknowledgment flag
(i.e., A-flag) in PANA header.
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4.1. Common Processing Rules
4.1.1. Payload Encoding
The payload of any PANA message consists of zero or more AVPs
(Attribute Value Pairs). Brief description on the AVPs defined in
this document is listed below.
- Cookie AVP: contains a random value that is used for making
initial handshake robust against blind resource consumption DoS
attacks.
- Data-Protection AVP: contains a flag which indicates if link-layer
or network-layer ciphering should be initiated after PANA.
- Device-Id AVP: contains a device identifier of the sender of the
message. A device identifier is represented as a pair of device
identifier type and device identifier value. Either a layer-2
address or an IP address is used for the device identifier value.
- EAP AVP: contains an EAP PDU.
- MAC AVP: contains a Message Authentication Code that protects a
PANA message PDU.
- Revocation-Status AVP: contains the reason of session revocation.
4.1.2. Transport Layer Protocol
PANA uses UDP as its transport layer protocol. The UDP port number
is TBD. All messages except for PANA_discovery are always unicast.
PaC MAY use unspecified IP address for communicating with PAA.
4.1.3. Fragmentation
PANA does not provide fragmentation of PANA messages. Instead, it
relies on fragmentation provided by EAP methods and IP layer when
needed.
4.1.4. Sequence Number and Retransmission
PANA uses sequence numbers to provide ordered delivery of EAP
messages. The design involves use of two sequence numbers to prevent
some of the DoS attacks on the sequencing scheme. Every PANA packet
include one transmitted sequence number (tseq) and one received
sequence number (rseq) in the PANA header. See Appendix for
detailed explanation on why two sequence numbers are needed.
The two sequence number fields have the same length of N (TBD:
possibly 32) bits and appear in PANA header. tseq starts from
initial sequence number (ISN) and is monotonically increased by 1.
The serial number arithmetic defined in [RFC1982] is used for
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sequence number operation. The ISNs are exchanged between PaC and
PAA during the discovery and initial handshake phase (see section
"Discovery and Initial Handshake Phase"). The rules that govern the
sequence numbers in other phases are described as follows.
o When a message is sent, a new sequence number is placed on the
tseq field of message regardless of whether it is sent as a result
of retransmission or not. When a message is sent, rseq is copied
from the tseq field of the last accepted message.
o When a message is received, it is considered as valid in terms of
sequence numbers if and only if (i) its tseq is greater than the
tseq of the last accepted message and (ii) its rseq falls in the
range between the tseq of the last acknowledged message + 1 and the
tseq of the last transmitted message.
PANA relies on EAP-layer retransmission for retransmitting EAP
Request based on timer. Other PANA layer messages that require a
response from the communicating peer are retransmitted based on
timer at PANA-layer until a response is received (in which case the
retransmission timer is stopped) or the number of retransmission
reaches the maximum value (in which case the PANA session MUST be
deleted immediately). For PANA-layer retransmission, the
retransmission timer SHOULD be calculated as described in [RFC2988]
to provide congestion control (TBD: default timer and maximum
retransmission count suggestions).
4.1.5. Message Authentication Code
A PANA message can contain a MAC (Message Authentication Code) AVP
for cryptographically protecting the message.
When a MAC AVP is included in a PANA message, the value field of the
MAC AVP is calculated in the following way:
MAC AVP value = PRF(PANA_MAC_Key, PANA_PDU)
where PANA_PDU is the PANA message including the PANA header, with
the MAC AVP value field first initialized to 0. The default
algorithm used for the PRF function is TBD (possibly HMAC-SHA1).
PANA_MAC_Key MUST be derived from the Master Session Key (MSK) and
thus MUST be a part of the EAP key hierarchy [Ab02]. Detailed
derivation algorithm is TBD.
4.1.6. Message Validity Check
When a PANA message is received, the message is considered to be
invalid at least when one of the following conditions are not met:
o Each field in the message header contains a valid value including
sequence number, message length, message type, version number,
flags,
etc.
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o When a device identifier of the communication peer is bound to the
PANA session, it matches the device identifier carried in MAC and/or
IP header(s).
o The message type is one of the expected types in the current
state.
o The message payload contains a valid set of AVPs allowed for the
message type and there is no missing AVP that needs to be included
in
the payload.
o Each AVP is decoded correctly.
o When a MAC AVP is included, the AVP value matches the MAC value
computed
against the received message.
o When a Device-Id AVP is included, the AVP is valid if the device
identifier type contained in the AVP matches the expected one (this
check is for PAA only) and the device identifier value contained in
the AVP matches the value extracted from the lower-layer
encapsulation
header corresponding to the device identifier type contained in the
AVP.
Invalid messages MUST be discarded in order to provide robustness
against DoS attacks and an unprotected. (TBD: in addition, a
non-acknowledged error notification message MAY be returned to the
sender.)
4.2. Discovery and Initial Handshake Phase
When a PaC attaches to a network, and knows that it has to discover
PAA for PANA, it can send a PANA_discovery message to a well-known
link local multicast address (TBD) over UDP. The source address
may be unspecified IP address if the PaC has not configured an
address yet. In all PAA_discovery messages, both tseq and rseq
fields of the header are set to zero (0). PANA PAA discovery
assumes that PaC and PAA are one hop away from each other. If PaC
knows the IP address of the PAA (some pre-configuration), it can
unicast the PANA_discovery message to that address. PAA answers to
the PANA_discovery message with a PANA_start message.
When the PAA receives such a request, or upon receiving some lower
layer indications of a new PaC, PAA can unicast a PANA_start
message. The destination address may be unspecified IP address,
but the L2 destination would be a unicast address (something for
the implementations to deal with). This message announces the PAA
to the PaC.
There can be multiple PAAs on the link. The result does not depend
on which PAA PaC chooses. By default PaC chooses the PAA that sent
the first response.
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PaC may also choose to start sending packets before getting
authenticated. In that case, the network should detect this and
send an unsolicited PANA_start message to PaC. EP is the node that
can detect such activity. If EP and PAA are co-located, then an
internal mechanism (e.g. API) between the EP module and the PAA
module on the same host can prompt PAA to start PANA. In case they
are separate, there needs to an explicit message to prompt PAA.
Upon detecting the need to authenticate a client, EP can send a
PAC_discovery message to the PAA on behalf of the PaC. This
message carries a device identifier of the PaC in a Device-Id AVP.
So that, PAA can send the unsolicited PANA_start message directly
to the PaC. If the link between the EP and PAA is not secure, the
PAC_discover message sent from EP to PAA MUST be protected by
using, e.g., IPsec.
PANA_start message contains a cookie carried in a Cookie AVP in the
payload, respectively. The rseq field of the header is set to zero
(0). The tseq field of the header contains the initial sequence
number. The cookie is used for preventing the PAA from resource
consumption DoS attacks by blind attackers. The cookie is computed
in such a way as not to require any saved per-session state to
recognize its valid cookie when a particular message sent by the
PaC in response to the PANA_start message arrives. The exact
algorithms and syntax used for generating cookies does not affect
interoperability and hence is not specified here. An example
algorithm is described below.
Cookie =
<secret-version> | HMAC_SHA1( <Device-Id of PaC> | <secret> )
where <secret> is a randomly generated secret known only to the
PAA, <secret-version> is an index used for choosing the secret for
generating the cookie and '|' indicates concatenation. The
secret-version should be changed frequently enough to prevent
replay attacks.
When a PaC receives the PANA_start message, it responds with a
PANA_start message. The PANA_start message sent from the PaC
contains the initial sequence numbers in the tseq and rseq fields
of the PANA header, a copy of the received Cookie as the PANA
payload.
When the PAA receives the PANA_start message from the PaC, it
verifies the cookie. The cookie is considered as valid if the
received cookie has the expected value. If the computed cookie is
valid, the protocol enters the authentication phase. Otherwise, it
MUST silently discard the received message.
PANA_start exchange is needed before entering authentication phase
even when the PaC is pre-configured with PAAs IP address and the
PANA_discover is a unicast message.
PANA_start message sent from PAA is never retransmitted.
PANA_start message sent from PaC is retransmitted based on timer in
the same manner as other messages retransmitted at PANA-layer.
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PaC PAA Message(tseq,rseq)[AVPs]
------------------------------------------------------
-----> PANA_discover(0,0)
<----- PANA_start(x,0)[Cookie]
-----> PANA_start(y,x)[Cookie]
(continued to authentication phase)
(PANA_discover sent by PaC)
Figure 2: Example Sequence for Discovery and Initial Handshake Phase
PaC EP PAA Message(tseq,rseq)[AVPs]
------------------------------------------------------
---->o (Data packet arrival or L2 trigger)
------> PANA_discover(0,0)[Device-Id]
<------------ PANA_start(x,0)[Cookie]
------------> PANA_start(y,x)[Cookie]
(continued to authentication phase)
(PANA_discover sent by EP)
Figure 3: Example Sequence for Discovery and Initial Handshake Phase
4.3. Authentication Phase
The main task in authentication phase is to carry EAP messages
between PaC and PAA. All EAP messages except for EAP
Success/Failure messages are carried in PANA_auth messages. When an
EAP Success/Failure message is sent from a PAA, the message is
carried in PANA_success or PANA_failure messages. PANA_success and
PANA_failure messages are acknowledged with PANA_success_ack and
PANA_failure_ack messages, respectively. It is possible to carry
multiple EAP sequences in a single PANA sequence, with using a
PANA_success or a PANA_failure message as a delimiter of each EAP
sequence. An EAP Success or an EAP Failure message is carried in a
PANA_success or a PANA_failure message, respectively.
A single PANA session can enable more than one EAP authentication.
This is used to satisfy the separate NAP and ISP authentications
scenario. Each EAP authentication is delineated from the subsequent
one with a PANA_success or PANA_failure message. F-flag in the PANA
header indicates if this was the final authentication from sender's
perspective. If PAA enables two separate authentication, it should
not set F-flag in the PANA_success or PANA_failure message after the
first EAP method. This indicates PAA's willingness to offer another
authentication method for NAP-ISP separation. PaC can respond with
a F-flag unset, indicating PaC's willingness to go through a second
authentication method. PaC can optionally decline by setting the
F-flag, and this concludes the PANA authentication. If PAA does not
offer two levels of authentication, then it sets the F-flag even at
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the end of first EAP method. In that case PaC has no other option
but to set the F-flag to mark the end of PANA authentication.
Currently, use of multiple EAP methods in PANA is designed only for
NAP-ISP authentication separation. It is not for arbitrary EAP
method sequencing, or giving PaC another chance when an
authentication method fails. NAP and ISP authentication are
considered completely independent. Presence or success of one
should not effect the other. Making a decision based on the success
or failure of each authentication is a network policy issue. A
PANA_success or PANA_failure message is only qualified to signal the
result of immediately preceding authentication method.
When an EAP method that is capable of deriving keys is used during
the authentication phase and the keys are successfully derived,
PANA_success, PANA_success_ack, PANA_failure, PANA_failure_ack
messages MUST contain a MAC AVP. The PANA_success and
PANA_success_ack message exchange also is used for binding device
identifiers of the PaC and PAA to the PANA SA. To achieve this,
PANA_success and PANA_success_ack messages SHOULD contain a device
identifier of the PAA and PaC, respectively, in a Device-Id AVP.
The PaC MUST use the same type of device identifier as contained in
the PANA_success message. In this case, the device identifier type
contained in the PANA_success message indicates the device
identifier type that the PaC needs to use. Validity check on the
device identifier MUST be performed for these messages (see section
"Message Validity Check").
PANA_success message MAY also contain a Data-Protection AVP to
indicate if link-layer or network-layer ciphering should be
initiated after PANA. A bit flag is the only information carried in
the AVP and it states whether PANA SA will be used with link-layer
ciphers (e.g., WEP) or network-layer ciphers (e.g, IKE and IPsec).
It does not carry any other information specific to ciphering
methods at all. When the information is preconfigured on PaC and
PAA this AVP can be omitted. It is assumed that at least PAA is
aware of the security capabilities of the access network. PANA
protocol does not specify how the PANA SA and the Data-Protection
AVP will be used to provide per-packet protection for data traffic.
PANA_success and PANA_failure messages MUST be retransmitted based
on the retransmission rule described in section "Sequence Number and
Retransmission".
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PaC PAA Message(tseq,rseq)[AVPs]
-------------------------------------------------
(continued from discovery and initial handshake phase)
<----- PANA_auth(x+1,y)[EAP{Request}]
-----> PANA_auth(y+1,x+1)[EAP{Response}]
.
.
<----- PANA_auth(x+2,y+1)[EAP{Request}]
-----> PANA_auth(y+2,x+2)[EAP{Response}]
<----- PANA_success(x+3,y+2) // F-flag set
[EAP{Success}, Device-Id, Data-Protection, MAC]
-----> PANA_success_ack(y+3,x+3)
[Device-Id, MAC] // F-flag set
Figure 4: Example Sequence in Authentication Phase
4.4. Re-authentication
There are two types of re-authentication supported by PANA.
The first type of re-authentication is based on EAP by entering the
authentication phase. In this case, the discovery and initial
handshake phase MAY be omitted. If there is an established PANA SA,
PANA_auth messages MAY be protected by adding a MAC AVP to each
message.
The second type of re-authentication is based on a single protected
message exchange without entering the authentication phase.
PANA_reauth/PANA_reauth_ack messages are used for this purpose. If
there is an established PANA SA, both PaC and PAA can send a
PANA_reauth message to the communicating peer whenever it needs to
make sure the availability of the PANA SA on the peer and expect the
peer to return a PANA_reauth_ack message. Both PANA_reauth /
PANA_reauth_ack messages MUST be protected with a MAC AVP.
Implementations MUST limit the rate of performing re-authentication
for both types of re-authentication.
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PaC PAA Message(tseq,rseq)[AVPs]
------------------------------------------------------
-----> PANA_reauth(q,p)[MAC]
<----- PANA_reauth_ack(p+1,q)[MAC]
Figure 5: Example Sequence for PaC-initiated Re-authentication
PaC PAA Message(tseq,rseq)[AVPs]
------------------------------------------------------
<----- PANA_reauth(p,q)[MAC]
-----> PANA_reauth_ack(q+1,p)[MAC]
Figure 6: Example Sequence for PAA-initiated Re-authentication
4.5. Termination Phase
A procedure for explicitly terminating a PANA session can be
initiated either from PaC (i.e., disconnect indication) or from PAA
(i.e., session revocation). PANA_disconnect/PANA_disconnect_ack and
PANA_revocation/PANA_revocation_ack message exchanges are used for
disconnect indication and session revocation procedures,
respectively.
A PANA_revocation message contains the reason of revocation in
Revocation-Status AVP. When there is an established PANA SA
established between the PaC and PAA, all messages exchanged during
the termination phase MUST be protected with a MAC AVP. When the
sender of PANA_disconnect or PANA_revocation message receives a
valid acknowledgment, all states maintained for the PANA session
MUST be deleted immediately.
PaC PAA Message(tseq,rseq)[AVPs]
------------------------------------------------------
-----> PANA_disconnect(q,p)[MAC]
<----- PANA_disconnect_ack(p+1,q)[MAC]
Figure 7: Example Sequence for Disconnect Indication
PaC PAA Message(tseq,rseq)[AVPs]
------------------------------------------------------
<----- PANA_revocation(p,q)[Revocation-Status,MAC]
-----> PANA_revocation_ack(q+1,p)[MAC]
Figure 8: Example Sequence for Session Revocation
4.6. Illustration of a Complete Message Sequence
A complete PANA message sequence is illustrated in Figure 5.6. The
example assumes the following scenario.
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- PaC multicasts PANA_discover message
- ISNs used by PAA and PaC are x and y, respectively.
- A single EAP sequence is used in authentication phase.
- A single EAP authentication method is used in the EAP sequence.
- The EAP authentication method derives keys. PANA SA is
established based on the keys.
- After PANA SA is established, all messages are integrity protected
with MAC AVP.
- Re-authentication based on PANA_reauth/PANA_reauth_ack exchange is
performed.
- PANA session is terminated as a result of disconnect indication
from PaC.
PaC PAA Message(tseq,rseq)[AVPs]
-----------------------------------------------------
// Discovery and initial handshake phase
-----> PANA_discover(0,0)
<----- PANA_start(x,0)[Cookie]
-----> PANA_start(y,x)[Cookie]
// Authentication phase
<----- PANA_auth(x+1,y)[EAP]
-----> PANA_auth(y+1,x+1)[EAP]
<----- PANA_auth(x+2,y+1)[EAP]
-----> PANA_auth(y+2,x+2)[EAP]
<----- PANA_success(x+3,y+2) // F-flag set
[EAP, Device-Id, Data-Protection, MAC]
-----> PANA_success_ack(y+3,x+3) // F-flag set
[Device-Id, MAC]
// Re-authentication
<----- PANA_reauth(x+4,y+3)[MAC]
-----> PANA_reauth_ack(y+4,x+4)[MAC]
// Termination phase
-----> PANA_disconnect(y+5,x+4)[MAC]
<----- PANA_disconnect_ack(x+5,y+5)[MAC]
Figure 9: A Complete Message
4.7. Device ID choice
PaC has to pick a device identifier to provide for PANA exchanges.
In this version of the specification, device ID is considered to be
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fixed. Future versions might enable changing it during a PANA
session.
A PaC will configure an IP address before PANA if it can. It might
either have a pre-configured IP address, or have to obtain one via
dynamic methods such as DHCP or stateless address autoconfiguration.
Dynamic methods may or may not succeed depending on the local
security policy. In networks where the PaCs need to use PANA prior
to address configuration, EPs will detect the PaCs attempt to get IP
address and help PAA to initiate authentication.
Either an IP address or link-layer address should be used as device
DI at any time. The only case an IP address should be used as
device ID is when IPsec will be used for protecting data traffic
after initial authentication. Any other time a link-layer address
can be used by both PAA and PaC as device ID. It is assumed that PAA
knows the security mechanisms being provided or required on the
access network (e.g., physical security, link-layer ciphers prior to
PANA, link-layer ciphers enabled after PANA, IPsec). When IPsec is
the choice of data ciphering, PAA should provide its IP address as
device ID, and expect the PaC to provide its IP address if it has
one. In all other cases, link-layer addresses can be provided from
both sides. [TBD: can we allow IP address allocation after PANA and
still be able to use IPsec?]
4.8. PaC Implications
- PaC state machine. [TBD]
4.9. PAA Implications
- PAA state machine. [TBD]
5 PANA Security Association Establishment
When PANA is used over an already established secure channel, such
as physically secured wires or ciphered link-layers, we can
reasonably assume that man-in-the-middle attack or service theft is
not possible [THREATS].
Anywhere else where there is no secure channel prior to PANA, the
protocol needs to protect itself against such attacks. The device
identifier that is used during the authentication needs to be
verified at the end of the authentication to prevent service theft
and DoS attacks. Additionally, a free loader should be prevented
from spoofing data packets by using the device identifier of an
already authorized legitimate client. Both of these requirements
necessitate generation of a security association between the
PaC and the PAA at the end of the authentication. This can only be
done when the authentication method used can generate cryptographic
keys. Use of secret keys can prevent attacks which would otherwise
be very easy to launch by eavesdropping on and spoofing traffic over
a insecure links.
Tschofenig (ed.) Expires September 2003 15
PANA March 2003
PANA relies on EAP and the EAP methods to provide a session key in
order to establish a PANA security association. An example of such a
method is EAP-TLS [EAPTLS], whereas EAP-MD5 [RFC2284] is an example
of a method that cannot create such keying material. The choice of
EAP method becomes important, as already discussed in the next
section.
This keying material is already used within PANA during the final
handshake. This handshake ensures that the device identifier that is
bound to the PaC at the end of the authentication process is not
coming from a man-in-the-middle, but from the legitimate PaC.
Knowledge of the same keying material on both PaC and the PAA helps
prove this. The other use of the keying material will be discussed
in sections 7 and 8.
6 Authentication Method Choice
Authentication methods' capabilities and therefore applicability to
various environments differ among them. Not all methods provide
support for mutual authentication, key derivation or distribution,
and DoS attack resiliency that are necessary for operating in
insecure networks. Such networks might be susceptible to
eavesdropping and spoofing, therefore a stronger authentication
method needs to be used to prevent attacks on the client and
the network.
The authentication method choice is a function of the underlying
security of the network (e.g., physically secured, shared link,
etc.). It is the responsibility of the user and the network operator
to pick the right method for authentication. PANA carries EAP
regardless of the EAP method used. It is outside the scope of PANA
to mandate, recommend, or limit use of any authentication methods.
PANA cannot increase the strength of a weak authentication method to
make it suitable for an insecure environment. There are some EAP-
based approaches to achieve this goal [PEAP][TTLS]. PANA
can carry these EAP encapsulating methods but it does not concern
itself with how they achieve protection for the weak methods (i.e.,
their EAP method payloads).
7 Filter Rule Installation
PANA protocol provides client authentication and authorization
functionality for securing network access. The other component of a
complete solution is the access control which ensures that only
authenticated and authorized clients can gain access to the network.
PANA enables access control by identifying legitimate clients and
generating filtering information for access control mechanisms.
Getting this filtering information to the EPs (enforcement points)
and performing filtering are outside the scope of PANA.
Access control can be achieved by placing EPs in the network for
policing the traffic flow. EPs should prevent data traffic from and
to any unauthorized client unless it's PANA traffic. When a client
is authenticated and authorized, PAA should notify EP(s) and ask for
Tschofenig (ed.) Expires September 2003 16
PANA March 2003
changing filtering rules to allow traffic for a recently authorized
client. There needs to be a protocol between PAA and EP(s) when
these entities are not co-located. PANA Working Group will not be
defining a new protocol for this interaction. Instead, it will
(preferably) identify one of the existing protocols that can fit the
requirements. Possible candidates include but not limited to COPS,
SNMP, DIAMETER. This task is similar to what MIDCOM Working Group is
trying to achieve, therefore some of the MIDCOM's output might be
useful here.
EPs location in the network topology should be appropriate for
performing access control functionality. The closest IP-capable
access device to the client devices is the logical choice. PAA and
EPs on an access network should be aware of each other as this is
necessary for access control. Generally this can be achieved by
manual configuration. Dynamic discovery is another possibility, but
this is clearly outside the scope of PANA.
Filtering rules generally include device identifiers for a client,
and also cryptographic keying material when needed. Such keys are
needed when attackers can eavesdrop and spoof on the device
identifiers easily. They are used with link-layer or network-layer
ciphering to provide additional protection. For issues regarding
data-origin authentication see Section 8.
8 Data Traffic Protection
Protecting data traffic of authenticated and authorized clients from
others is another component of providing a complete secure network
access solution. Authentication, integrity and replay protection of
data packets are needed to prevent spoofing when the underlying
network is not physically secured. Encryption is needed when
eavesdropping is a concern in the network.
When the network is physically secured, or the link-layer ciphering
is already enabled prior to PANA, data traffic protection is already
in place. In other cases, enabling link-layer ciphering or network-
layer ciphering might rely on PANA authentication. The user and
network have to make sure an appropriate EAP method that can
generate required keying materials is used. Once the keying material
is available, it needs to be provided to the EP(s) for use with
ciphering.
Network-layer ciphering, i.e., IPsec, can be used when data traffic
protection is required but link-layer ciphering capability is not
available. Note that a simple shared secret generated by an EAP
method is not readily usable by IPsec for authentication and
encryption of IP packets. Fresh and unique session key derived from
the EAP method is still insufficient to produce an IPsec SA since
both traffic selectors and other IPsec SA parameters are missing.
The shared secret can be used in conjunction with a key management
protocol like IKE [RFC2409] to turn a simple shared secret into the
required IPsec SA. The details of this mechanism is outside the
scope of PANA protocol, and it can be outlined in a separate
Tschofenig (ed.) Expires September 2003 17
PANA March 2003
Internet-Draft. PANA provides bootstrapping functionality for such a
mechanism by carrying EAP methods that can generate initial keying
material.
Using network-layer ciphers should be regarded as a substitute for
link-layer ciphers when the latter is not available. IKE involves
several message exchanges which can incur additional delay in
getting basic IP connectivity for a mobile device. Such a latency is
inevitable when there is no other alternative and this level of
protection is required. Network-layer ciphering can also be used in
addition to link-layer ciphering if the added benefits outweigh its
cost to the user and the network.
9 Message Formats
Bits and bytes on the wire...
10 Open Issues
The following list describes some open issues for PANA:
- Should the PANA protocol provide downgrade protection?
- How extensible or flexible should the device identifier be?
- Should the PANA protocol support a modify message to be able to
alter state? This would, for example, be useful in case of IP
address change without mobility (e.g. in IPv6 for privacy reasons).
- The PANA SA needs a session key and either this session key is
derived from the EAP method as part of the EAP key derivation
framework or within PANA.
11 Security Considerations
The PANA protocol provides ordered delivery for EAP messages. If an
EAP method that provides session keys is used, a PANA SA is created.
The EAP Success/Failure message is one of the signaling messages
which is integrity protected with this PANA SA. The PANA protocol
does not provide security protection for the initial EAP message
exchange. Integrity protection can only be provided after the PANA
SA has been established. Thus, PANA re-authentication, revocation
and disconnect notifications can be authenticated, integrity and
replay protected. In certain environments (e.g. on a shared link)
the EAP method selection is an important issue.
The PANA framework described in this document covers the discussion
of different protocols which are of interest for a protocol between
the PaC and the PAA (typically referred as the PANA protocol).
The PANA itself consists of a sequence of steps which are executed
to complete the network access authentication procedure. Some of
these steps are optional.
The following execution steps have been identified as being relevant
for PANA. They security considerations will be discussed in detail
subsequently.
Tschofenig (ed.) Expires September 2003 18
PANA March 2003
a) Discovery message exchange
In general it is difficult to prevent a vulnerabilities of the
discovery protocol since the initial discovery are unsecured. To
prevent very basic attacks an adversary should not be able to cause
state creation with discovery messages at the PAA. This is prevented
by re-using a cookie concept (see [RFC2522]) which allows the
responder to be stateless in the first message exchange. Because of
the architectural assumptions made in PANA (i.e. the PAA is the on
the same link as the PaC) the return-routability concept does not
provide additional protection. Hence it is difficult to prevent this
threat entirely. Furthermore it is not possible to shift heavy
cryptographic operations to the PaC at the first few messages since
the computational effort depends on the EAP method. The usage of
client-puzzles as introduced by P. Nikander et. al. in [AN+00] is
under investigation.
Resistance against blind DoS attacks (i.e. attacks by off-path
adversaries) is achieved with sequence numbers and cookies.
Since PAA and PaC are one IP hop away from each other, PANA messages
can be filtered whenever messages arrive at interfaces where they
are not expected.
b) EAP over PANA message exchange
The EAP derived session key is used to create a PANA security
association. Since the execution of an EAP method might require a
large number of roundtrips and no other session key is available it
is not possible to secure the EAP message exchange itself. Hence an
adversary can both eavesdrop the EAP messages and is also able to
inject arbitrary messages which might confuse both the PaC and the
PAA. The threats caused by this ability heavily depend on the EAP
state machine. Since especially the PAA is not allowed to discard
packets and packets have to be stored or forwarded to an AAA
infrastructure some risk of DoS attacks exists.
Eavesdropping EAP packets might cause problems when (a) the EAP
method is weak and enables dictionary or replay attacks or even
allows an adversary to learn the long-term password directly.
Furthermore, if the optional EAP Identity payload is used then it
allows the adversary to learn the identity of the PaC. In such a
case a privacy problem is prevalent.
To prevent these threats Section 6 suggests using proper EAP methods
for particular environments. Depending on the usage environment an
EAP authentication has to be used for example which supports user
identity confidentiality, protection against dictionary attacks and
session key establishment. It is therefore the responsibility of the
network operators and end users to choose the proper EAP method.
Tschofenig (ed.) Expires September 2003 19
PANA March 2003
PANA does not protect the EAP method exchange, but provides ordered
delivery with sequence numbers. Sequence numbers and cookies
provide resistance against blind DoS attacks.
c) PANA SA establishment
Once the EAP message authentication is finished a fresh and unique
session key is available to the PaC and the PAA. This assumes that
the EAP method allows session key derivation and that the generated
session key has a good quality. For further discussion about the
importance of the session key generation refer to the next
subsection (c) about compound authentication. The session key
available for the PaC is established as part of the authentication
and key exchange procedure of the selected EAP method. The PAA
obtains the session key via the AAA infrastructure (if used). Draft
[CFB02] describes how a session key is securely carried (i.e. CMS
protected) between AAA servers. Security issues raised with this
session key transport are described in [WHC02].
The establishment of a PANA SA is required in environments where no
physical or link layer security is available. The PANA SA allows
subsequently exchanged messages to experience cryptographic
protection. For the current version of the document an Integrity
object is defined which is based on Diameter objects. The Integrity
Object supports data-origin authentication, replay protection based
on sequence numbers and integrity protection based on a keyed
message digest. Confidentiality protection is not provided. The
session keys (one for each direction) used for this object has to be
provided by the EAP method. For this version of the document it is
assumed that no negotiation of algorithms and parameters takes
place. Instead HMAC-SHA1 is used per-default. A different algorithm
such as HMAC-MD5 might be used as an option. The used algorithm is
indicated in the header of the Integrity object. To select the
security association for signaling message protection the Session
ID. The keyed message digest included in the Integrity object will
include all fields of the PANA signaling message including the
sequence number field of the packet.
The protection of subsequent signaling messages prevents an
adversary from acting as a man-in-the-middle adversary, from
injecting packets, from replaying messages and from modifying the
content of the exchanged packets. This prevents subsequently
described threats.
If an entity (PAA or PaC) looses its state (especially the current
sequence number) then the entire PANA protocol has to be restarted.
No re-synchronization procedure is provided.
The lifetime of the PANA SA has to be bound to the refresh interval
with an additional tolerance period. To provide fast re-
authentication a separate security association (e.g. one stored at
the local AAA server) should be used. By fast re-authentication we
mean a new PANA protocol execution which does not involve the entire
AAA communication. The ability to trigger such a protocol execution
Tschofenig (ed.) Expires September 2003 20
PANA March 2003
depends on the given EAP method and on the policy of the local
network requesting authentication.
d) Enabling weak legacy authentication methods in insecure networks
Some of the authentication methods are not strong enough to be used
in insecure networks where attackers can easily eavesdrop and spoof
on the link. They may not be able to produce much needed keying
material either. An example would be using EAP-MD5 over wireless
links. Use of such legacy methods can be enabled by carrying them
over a secure channel. There are EAP methods which are specifically
designed for this purpose, such as EAP-TTLS [TTLS] and PEAP [PEAP].
PANA can carry these EAP tunneling methods which can carry the
legacy methods. PANA does not do anything special for this case. The
EAP tunneling method will have to produce keying material for PANA
SA when needed. There are certain MitM vulnerabilities with
tunneling EAP methods [MITM]. Solving these problems are outside
the scope of PANA.
e) Preventing downgrading attacks
EAP supports a number of different EAP methods for authentication
and therefore it might be required to agree on a specific mechanism.
An unprotected negotiation mechanism is supported in EAP and a
secure negotiation procedure for the GSS-API methods. The support of
the GSS-API as an EAP method is described in [AS02]. A protected
negotiation is supported by the GSS-API with RFC 2478 [RFC2478]. If
desired, such a protection can also be offered by PANA by repeating
the list of supported EAP methods protected with the PANA SA. This
type of protection is similar to the protected negotiation described
in [RFC3329].
This issue requires further investigation especially since the EAP
protocol runs in most cases different endpoints than the PANA
protocol.
f) Device Identifier exchange
As part of the authorization procedure a Device Identifier has to be
installed at the EP by the PAA. The PaC provides the Device
Identifier information to the PAA secured with the PANA SA. Section
6.2.4 of [THREATS] describes a threat where an adversary modifies
the Device Identifier to gain unauthorized access to the network.
The installation of the Device Identifier at the EP (independently
whether the EP is co-located with the PAA or not) has to be
accomplished in a secure manner. These threats are, however, not
part of the PANA protocol itself since the protocol is not PANA
specific.
g) Triggering a data protection protocol
Recent activities in the EAP working group try to create a common
framework for key derivation which is described in [Ab02]. This
Tschofenig (ed.) Expires September 2003 21
PANA March 2003
framework is also relevant for PANA in various ways. First, a PANA
security association needs to be created. Additionally it might be
necessary to trigger a protocol which allows link layer and network
layer data protection to be established. As an example see Section 1
of [Ab02] with [802.11i] and [802.11] as an example. Furthermore, a
derived session key might help to create the pre-requisites for
network layer protection (for example IPsec).
As motivated in Section 6.4 of [THREATS] it might be necessary to
establish either a link layer or a network layer protection to
prevent certain thefts in certain scenarios.
Threats specific to the establishment of a link layer or a network
layer security association are outside the scope of PANA. The
interested reader should refer to the relevant working groups such
as IPsec or Midcom.
h) Periodic refresh messages
Network access authentication is done for a very specific purpose
and often charging procedures are involved which allow restricting
network resource usage based on some policies. In mobility
environments it is always possible that an end host suddenly
disconnects without transmitting a disconnect message. If network
access authentication as part of PANA is executed only at the
beginning then an adversary can gain advantage of the installed
packet filters to submit and receive data packets.
Also for the network operator it might be desirable to enforce a
disconnect based on some external events (e.g. because of
insufficient funds, etc.).
An additional motivation for detecting a disconnected end host is
the ability to release resources (i.e. garbage collection). The PAA
can remove per-session state information including installed
security association, packet filters etc.
Different procedures can be used for disconnect indication. PANA
cannot assume link layer disconnect indication. Hence this
functionality has to be provided at a higher layer. With this
version of the draft we suggest to apply the soft-state principle
found at other protocols (such as RSVP [RFC2205]). Soft-state means
that session state is kept alive as long as refresh messages refresh
the state. If no new refresh messages are provided then the state
automatically times out and resources are released. This process
includes stopping accounting procedures.
Based on the different environments where PANA could be used it is
difficult to fix a refresh interval. Hence a default refresh
interval of 30 seconds is suggested. Additionally there is the
possibility to negotiation this interval once the PANA security
association is established. A policy at the PAA and the PaC would
ensure that the refresh interval is selected with a value which is
either too high or too low. There is certainly a tradeoff between
Tschofenig (ed.) Expires September 2003 22
PANA March 2003
the refresh interval and the bandwidth consumption. To reduce the
bandwidth consumption a small PANA message consisting only of a
session identifier and the Integrity object is used. The session
identifier refers to the state that has to be refreshed. Some
environments do not need PANA refresh messages to detect orphan
states. For these environments the refresh interval should be set to
zero which effectively disables the usage of refresh messages. In
case of IPsec protection a dead-peer mechanism can be used to detect
inactivity (see [HBR03]).
Refresh messages are sent from the PaC to the PAA.
From a security point of view an adversary must not be able to
inject, modify or replay refresh messages nor must he be able to
change the refresh interval (e.g. setting it to zero) without
detection. Hence these messages experience cryptographic protection.
i) Tear-Down message
The PANA protocol supports the ability for both the PaC and the PAA
to transmit a tear-down message. This message causes state removal,
a stop of the accounting procedure and removes the installed packet
filters.
It is obvious that such a message must be protected to prevent an
adversary from deleting state information and thereby causing denial
of service attacks.
12 References
[802.11] I. S. 802.11-1997, "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," tech. rep., 1997.
[RFC2522] P. Karn and W. Simpson, "Photuris: Session-key management
protocol," RFC 2522, Internet Engineering Task Force, Mar. 1999.
[PEAP] H. Andersson, S. Josefsson, G. Zorn, et al. , "Protected
extensible authentication protocol (PEAP)," Internet Draft, Internet
Engineering Task Force, Feb. 2002. Work in progress.
[Ab02] B. Aboba, "The EAP session key problem," Internet Draft,
Internet Engineering Task Force, Feb. 2002. Work in progress.
[802.11i] I. D. 802.11i/D2, "Draft supplement to standard for
telecommunications and information exchange between systems -
lan/man specific requirements - part 11: Wireless medium access
control (mac) and physical layer (phy) specifications: Specification
for enhanced security," tech. rep., 2001.
[AS02] Aboba, B., Simon, D.: "EAP GSS Authentication Protocol",
<draft-aboba-pppext-eapgss-12.txt>, (work in progress), April, 2002.
Tschofenig (ed.) Expires September 2003 23
PANA March 2003
[CFB02] P. Calhoun, S. Farrell, and W. Bulley, "Diameter CMS
security application," Internet Draft, Internet Engineering Task
Force, Mar. 2002. Work in progress.
[RFC2284] Blunk, L. and J. Vollbrecht, "PPP Extensible
Authentication Protocol (EAP)", RFC 2284, March 1998.
[RFC2716] Aboba, B., and D. Simon, "PPP EAP TLS Authentication
Protocol", RFC 2716, October 1999.
[HBR03] G. Huang, S. Beaulieu, and D. Rochefort, "A traffic-based
method of detecting dead ike peers," internet draft, Internet
Engineering Task Force, 2003. Work in progress.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[IKEv2] Kaufman, C.: "Internet Key Exchange (IKEv2) Protocol",
<draft-ietf-ipsec-ikev2-05.txt>, (work in progress), February, 2003.
[MITM] N. Asokan, V. Niemi, and K. Nyberg, "Man-in-the-middle in
tunneled authentication," in http://eprint.iacr.org/2002/163/ ,
2002.
[PANATLS] Y. Ohba, S. Baba, and S. Das, "Pana over tls," Internet
Draft, Internet Engineering Task Force, 2002. Work in progress.
[PEAP] H. Andersson, S. Josefsson, G. Zorn, et al. , "Protected
[PIC] Y. Sheffer, H. Krawczyk, and B. Aboba, "PIC, a pre-IKE
credential provisioning protocol," Internet Draft, Internet
Engineering Task Force, Feb. 2002. Work in progress.
[PL+03] J. Puthenkulam, V. Lortz, A. Palekar, D. Simon, and B.
Aboba, "The compound authentication binding problem," internet
draft, Internet Engineering Task Force, 2003. Work in progress.
[AN+00] Aura, T., Nikander, P., Leiwo, J.: "DOS-resistant
Authentication with Client Puzzles", in "Proc. Security Protocols
Workshop 2000, Cambridge, UK", 2000.
[PY+02] Penno, R., Yegin, A., Ohba, Y., Tsirtsis, G., Wang, C.:
"Protocol for Carrying Authentication for Network Access (PANA)
Requirements and Terminology", Internet-Draft, <draft-ietf-pana-
requirements-04.txt>, (work in progress), October, 2002.
[RFC2284bis] Blunk, L., Vollbrecht, J., Aboba, B., Carlson, J.:
"Extensible Authentication Protocol (EAP)", < <draft-ietf-eap-
rfc2284bis-01.txt>, (work in progress), January, 2003.
[RFC1982] Elz, R., Bush, R.: "Serial Number Arithmetic", RFC 1982,
August 1996.
Tschofenig (ed.) Expires September 2003 24
PANA March 2003
[RFC2205] Braden, R., Zhang, L., Berson, S., Herzog, S., Jamin, S.:
ƒResource ReSerVation Protocol (RSVP) Ï Version 1 Functional
Specification", RFC 2205, September 1997.
[RFC2478] E. Baize and D. Pinkas, "The simple and protected GSS-API
negotiation mechanism," RFC 2478, Internet Engineering Task Force,
Dec. 1998.
[RFC2988] Paxson, V., Allman, M.: "Computing TCP's Retransmission
Timer", RFC 2988, November, 2000.
[RFC3329] Arkko, J., Torvinen, V., Camarillo, G., Niemi, A., Haukka,
T.: "Security Mechanism Agreement for the Session Initiation
Protocol (SIP)", RFC 3329, January, 2003.
[SENAA] D. Forsberg and J. Rajahalme, "Secure network access
authentication (senaa)," Internet Draft, Internet Engineering Task
Force, 2002. Work in progress.
[THREATS] Parthasarathy, M.: "PANA Threat Analysis and security
requirements", <draft-ietf-pana-threats-01.txt>, (work in progress),
January, 2003.
[TTLS] P. Funk and S. Blake-Wilson, "EAP tunneled TLS authentication
protocol (EAP-TTLS)," Internet Draft, Internet Engineering Task
Force, Mar. 2002. Work in progress.
[USAGE] Ohba, Y., Das, S., Patil, B., Soliman, H., Yegin, A.:
"Problem Statement and Usage Scenarios for PANA", <draft-ietf-pana-
usage-scenarios-04.txt>, (work in progress), February, 2003.
[WHC02] J. Walker, R. Housley, and N. Cam-Winget, "AAA key
distribution," Internet Draft, Internet Engineering Task Force, Apr.
2002. Work in progress.
13 Acknowledgments
Place your name here
14 Author's Addresses
Basavaraj Patil
Nokia
6000 Connection Dr.
Irving, TX. 75039
USA
Phone: +1 972-894-6709
Email: Basavaraj.Patil@nokia.com
Dan Forsberg
Nokia Research Center
P.O. Box 407
FIN-00045 NOKIA GROUP, Finland
Tschofenig (ed.) Expires September 2003 25
PANA March 2003
Phone: +358 50 4839470
EMail: dan.forsberg@nokia.com
Alper E. Yegin
DoCoMo USA Labs
181 Metro Drive, Suite 300
San Jose, CA, 95110
USA
Phone: +1 408 451 4743
Email: alper@docomolabs-usa.com
Yoshihiro Ohba
Toshiba America Research, Inc.
P.O. Box 136
Convent Station, NJ, 07961-0136
USA
Phone: +1 973 829 5174
Email: yohba@tari.toshiba.com
Hannes Tschofenig
Siemens Corporate Technology
Otto-Hahn-Ring 6
81739 Munich
Germany
Email: Hannes.Tschofenig@siemens.com
Appendix A. Adding sequence number to PANA for carrying EAP
A.1. Why is sequence number needed for PANA to carry EAP?
EAP [RFC2284bis] requires underlying transports to provide
ordered-delivery of messages. If an underlying transport does not
satisfy the ordering requirement, the following situation could
happen:
EAP Peer EAP Authenticator
--------------------------------------------
1. (got req 1) <------- Request ID=1
2. Response ID=1 ---+
| (timeout)
3. | +-- Request ID=1
| |
+-|--> (got resp 1)
4. (got req 2) <----|-- Request ID=2
|
5. Response ID=2 -----|--> (got resp 2)
|
6. (got req 1) <----+
7. Response ID=1 --------> [discarded due to unexpected ID]
Figure A.1 Undesirable scenario
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PANA March 2003
In Figure A.1, the second EAP Request message with Identifier=1
arrives at the EAP peer after the third EAP Request message with
Identifier=2. As a result, the EAP peer accepts the second EAP
Request as a new EAP Request while it is just an old EAP Request
that was already responded and the authentication might be totally
messed up.
This problem occurs due to the fact that EAP doesn't recognize
duplicate packets in the scope of one EAP protocol run, but only in
the scope of current and previous packet (i.e., request and response
message matching). When EAP is running over PPP or IEEE 802 links,
this is not a problem, because those link-layers have the ordering
invariant characteristic.
On the other hand, the PANA design has chosen UDP as its transport.
Given that UDP does not provide ordered delivery of packets and PANA
does not assume any specific link-layer technology to carry EAP,
PANA messages need to have a sequence number.
In the following text we describe two possible approaches for
sequence number handling in PANA. The first one makes use of a
single sequence number whereas the latter utilizes two. Finally a
comparison between the two approaches is provided. The method
described in Section A.3.1. (i.e., the dual sequence number with
orderly-delivery method) is suggested as the preferred method for
PANA transport.
A.2. Single sequence number approach
This section discusses several methods based on using a single
sequence number for providing orderly message delivery. Sequence
number handling for all methods discussed in Section A.2 must comply
to the following rules:
Rule 1: The sequence number starts from initial sequence number
(ISN)
and is monotonically increased by 1. The arithmetic defined
in [RFC1982] is used for sequence number operation.
Rule 2: When a PAA sends an EAP message passed from EAP layer to a
PaC, a new sequence number is placed in the message,
regardless of whether it is sent as a result of a
retransmission at the EAP layer or not.
Note: It might be possible to define other mechanisms for sequence
number handling if it can be assumed that a PAA detects EAP
retransmissions. However, such an assumption heavily depends on EAP
implementation details in particular on EAP APIs, thus it was
decided not to use such an assumption.
A.2.1. Single sequence number with EAP retransmission method
Again, the following rules must hold:
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PANA March 2003
Rule 3: Use EAP layer retransmission for retransmitting EAP messages
(based on a timer expiration).
Rule 4: When the PaC receives a message from the PAA, it checks the
sequence number and discards the message if the sequence
number is not greater than that of the last accepted
message.
Rule 5: When the PAA receives a message from the PaC, it checks the
sequence number and discards the message if the sequence
number does not match a pending request message.
PaC PAA Seq# Message
--------------------------------------------
1. <------- (x) PANA_auth[EAP Req ID=1]
2. ---+ (x) PANA_auth[EAP Res ID=1]
| (retransmission timeout at EAP-layer)
3. | +-- (x+1) PANA_auth[EAP Req ID=1]
| |
+-|--> (discarded due to Rule 5)
| (retransmission timeout at EAP-layer)
4. <----|-- (x+2) PANA_auth[EAP Req ID=1]
|
5. -----|--> (x+2) PANA_auth[EAP Res ID=1]
|
6. <----+ (discarded due to Rule 4)
7. <------- (x+3) PANA_auth[EAP Req ID=2]
.
.
Figure 10: Example for Single sequence number with EAP
retransmission method
This method is vulnerable to a blind DoS attack on the sequence
number since the PaC will accept quite a wide range of sequence
numbers. For example, if an attacker blindly sends a bogus message
to a legitimate PaC with a randomly chosen sequence number, it will
be accepted by the PaC with 50% probability, and once this happens,
all messages sent from the communicating PAA will be discarded as
long as they have a sequence number smaller than the accepted value.
The problem of this method leads to a requirement for PaC to have a
narrow range of acceptable sequence numbers to make the blind DoS
attack difficult. Note that the DoS attack cannot be prevented if
the attacker is on the same IP link as PaC and able to eavesdrop the
PANA conversation. However, the attacker needs to put itself in
promiscuous mode and thus spend more resources to eavesdrop and
launch the attack (in other words, non-blind DoS attack is still
possible as long as sequence numbers are unprotected.)
A.2.2. Single sequence number with PANA-layer retransmission method
The next method is still based on using a single sequence number but
Tschofenig (ed.) Expires September 2003 28
PANA March 2003
the PANA-layer takes the responsibility of retransmission. The
method uses the following rules in addition to the common rules
described in section A.2.
Rule 3: Use PANA-layer retransmission for retransmitting both EAP
and
non-EAP messages (based on a timer expiration). EAP layer
retransmission is turned off. Retransmission based on timer
occurs both on PaC and PAA side, but not on both sides
simultaneously. PAA does retransmission at least for
PANA_revocation and PANA_reauth messages, otherwise PaC
takes care of retransmission.
Rule 4: When the PaC receives a message from the PAA, it accepts the
message if the sequence number is equal to that of the last
accepted message + 1. If the sequence number is equal to
that of the last accepted message, the PaC retransmits the
last transmitted message. Otherwise, it silently discards
the message.
Rule 5: When the PAA receives a message from the PaC, it accepts the
message if the sequence number is equal to that of the last
transmitted message. If the receiving sequence number is
equal to that of the last transmitted message - 1, the PAA
retransmits the last transmitted message and discard the
received message. Otherwise, it silently discards the
message.
Rule 6: The PaC retransmits the last transmitted EAP Response until
a new EAP Request message or an EAP Success/Failure message
is received and accepted.
Rule 7: PAA must keep the copy of the last transmitted message and
must be able to retransmit it until either a valid message
is received and accepted by the PAA or a timer expires. The
timer is used if no new message will be sent from the PaC.
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PANA March 2003
PaC PAA Seq# Message
--------------------------------------------
1. <-------- (x) PANA_auth[EAP Req ID=1]
2. ---+ (x) PANA_auth[EAP Resp ID=1]
| (retransmission timeout at PaC)
3. ---|----> (x) PANA_auth[EAP Resp ID=1]
4. | +--- (x+1) PANA_auth[EAP Req ID=2]
| |
+-|--> (duplicate detected)
5. <----|--- (x+1) PANA_auth[EAP Req ID=2]
|
6. -----|--> (x+1) PANA_auth[EAP Resp ID=2]
|
<----|--- (x+2) PANA_auth[EAP Req ID=3]
7. -----|--> (x+2) PANA_auth[EAP Resp ID=3]
<----+ (discarded by PaC)
(retransmission timeout at PaC)
8. --------> (x+2) PANA_auth[EAP Resp ID=3]
9. lost<---- (x+3) PANA_auth[EAP Succ ID=3]
(retransmission timeout at PaC)
10.---->lost (x+2) PANA_auth[EAP Resp ID=3]
(retransmission timeout at PaC)
11.--------> (x+2) PANA_auth[EAP Resp ID=3]
12.<-------- (x+3) PANA_succ[EAP Succ ID=3]
(retransmission timer stopped at PaC)
(deletion timeout at PAA)
(message (x+3) deleted at PAA)
13.lost<---- (x+4) PANA_revocation
(retransmission timeout at PAA)
14.<-------- (x+4) PANA_revocation
15.---->lost (x+4) PANA_revocation_ack
(retransmission timeout at PAA)
16.<-------- (x+4) PANA_revocation
17.--------> (x+4) PANA_revocation_ack
(retransmission timer stopped at PAA)
Figure 11: Example for Single sequence number with PANA-layer
retransmission method
This method has an advantage of eliminating EAP layer retransmission
by providing reliability at the PANA layer. Retransmission at the
EAP layer has a problem with determining an appropriate
retransmission timer value, which occurs when the lower-layer is
unreliable. In this case an EAP authenticator cannot distinguish
between (i) EAP Request or EAP Response message loss (in this case
the retransmission timer should be calculated based on network
characteristics) and (ii) long latency for EAP Response generation
due to e.g., user input etc. (in this case the retransmission timer
should be calculated based on user or application characteristics).
In general, the retransmission timer for case (ii) is longer than
that for case (i). If case (i) happens while the retransmission
timer is calculated based on user or application characteristics,
then it might frustrate an end user since the completion of the
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PANA March 2003
authentication procedure takes unnecessarily long. If case (ii)
happens while the retransmission timer is calculated based on
network characteristics (i.e., RTT), then unnecessarily traffic is
generated by retransmission. Note that in this method a PaC still
cannot distinguish case (i) and case (iii) the EAP authenticator or
a backend authentication server is taking time to generate an EAP
Request.
A problem of this method is that it is based on the assumption that
EAP authenticator does not send a new EAP message until an EAP
Response to the outstanding EAP Request is received. However, this
assumption does not hold at least EAP Success/Failure message which
does not need the outstanding EAP Request to be responded before
sending the EAP Success/Failure message. This would require
timer-based retransmission not only at PaC side but also at PAA
side.
Another problem occurs when a new EAP message overrides the
outstanding EAP Request, the PaC cannot assume any more that the
sequence number of the next message to be accepted is the last
accepted message + 1. So the PaC needs to accept a range of
sequence numbers, instead of a single sequence number. These two
additional things would increase the complexity of this method.
A.3. Dual sequence number approach
Based on the analysis of previous schemes, it is recognized that two
sequence numbers are needed anyway, one for each direction. Two
different methods are proposed based on this approach. Both methods
have the following rules in common.
Rule 1: A PANA packet carries two sequence numbers: transmitted
sequence number (tseq) and received sequence number (rseq).
tseq starts from initial sequence number (ISN) and is
monotonically increased by 1. The arithmetic defined in
[RFC1982] is used for sequence number operation. It is
assumed that the two sequence numbers have the same length
for simplicity.
Rule 2: When PAA or PAC sends a new message, a new sequence number
is placed on the tseq field of message. Every transmitted
message is given a new sequence number.
Rule 3: When a message is sent from PaC or PAA, rseq is copied from
the tseq field of the last accepted message.
Rule 4: For messages which experience a PANA layer retransmission,
the retransmission timer is stopped when the message is
acknowledged.
It is possible to carry multiple EAP sequences in a single PANA
sequence, with using EAP Success/Failure message as a delimiter of
each EAP sequence. In this case, EAP Success/Failure message needs
to be reliably delivered.
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A.3.1. Dual sequence number with orderly-delivery method
This method relies on EAP layer retransmission for EAP messages.
This method is referred to as orderly-delivery method. The
following rules are used in addition to the common rules.
Rule 5: Use the EAP-layer retransmission for retransmitting EAP
Requests (based on a timer expiration). For other PANA
layer messages that require a response from the peer, PANA
layer has its own mechanism to retransmit the request until
it gets a response or gives up. A new tseq value is always
used when sending any message even when it is retransmitted
at PANA layer.
Rule 6: When a message is received, it is accepted if (i) the tseq
value is greater than the tseq of the last accepted message
and (ii) the rseq falls in the range between the tseq of the
last acknowledged message + 1 and the tseq of the last
transmitted message. Otherwise, the received message is
discarded.
PaC PAA (tseq,rseq) Message
--------------------------------------------------
1. <------- (x,y) PANA_auth[EAP Req, ID=1]
2. -------> (y+1,x) PANA_auth[EAP Resp, ID=1]
3. <------- (x+1,y+1) PANA_auth[EAP Req, ID=2]
4. --->lost (y+2,x+1) PANA_auth[EAP Resp, ID=2]
(retransmission timeout at EAP layer)
5. <------- (x+2,y+1) PANA_auth[EAP Req, ID=2]
6. -------> (y+3,x+2) PANA_auth[EAP Resp, ID=2]
7. lost<--- (x+3,y+3) PANA_auth[EAP Req, ID=3]
(retransmission timeout at EAP layer)
8. +---- (x+4,y+3) PANA_auth[EAP Req, ID=3]
| (retransmission timeout at EAP layer)
9. <--|---- (x+5,y+3) PANA_auth[EAP Req, ID=3]
10.---|---> (y+4,x+5) PANA_auth[EAP Resp, ID=3]
|
<--+ (out of order. discarded)
11.lost<--- (x+6,y+4) PANA_succ[EAP Succ, ID=3]
(retransmission timeout at PAA)
12.<------- (x+7,y+4) PANA_succ[EAP Succ, ID=3]
13.--->lost (y+5,x+7) PANA_succ_ack
(retransmission timeout at PAA)
14.<------- (x+8,y+4) PANA_succ[EAP Succ, ID=3]
(dupicate detected by PaC)
15.-------> (y+6,x+8) PANA_succ_ack
Figure 12: Example for Dual sequence number with orderly-delivery
method
A.3.2. Dual sequence number with reliable-delivery method
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This method relies solely on PANA layer retransmission for all
messages. This method is referred to as reliable-delivery method.
The following additional rules are applied in addition to the common
rules.
Rule 5: Use the PANA layer retransmission for retransmitting all
messages (based on a timer expiration). EAP retransmission
is turned off.
Rule 6: Either an ACK message is used for acknowledgment or an
acknowledgment can be piggybacked with data. ACK messages
are not retransmitted. An ACK message is sent if no the
acknowledgement cannot be piggybacked with a data within a
given time frame W.
Rule 7: When a message is received, it is accepted if (i) the tseq
value is greater than the tseq of the last accepted message
and (ii) the rseq falls in the range between the tseq of the
last acknowledged message and the tseq of the last
transmitted message. Otherwise, the received message is
discarded.
Rule 8: When a duplicate message is received, the last transmitted
message is retransmitted if the received message is not an
ACK. A message is considered as duplicate if its tseq value
is equal to the tseq of the last accepted message.
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PaC PAA (tseq,rseq) Message
--------------------------------------------------
1. <------- (x,y) PANA_auth[EAP Req, ID=1]
(user input ongoing)
2. -------> (y+1,x) PANA_ACK
(user input completed)
3. -------> (y+2,x) PANA_auth[EAP Resp, ID=1]
4. <------- (x+1,y+2) PANA_auth[EAP Req, ID=2]
5. --->lost (y+3,x+1) PANA_auth[EAP Resp, ID=2]
(retransmission timeout at PAA)
6. <------- (x+1,y+2) PANA_auth[EAP Req, ID=2]
(duplicate detected by PaC)
7. -------> (y+3,x+1) PANA_auth[EAP Resp, ID=2]
8. lost<--- (x+2,y+3) PANA_auth[EAP Req, ID=3]
(retransmission timeout at PaC)
9. -------> (y+3,x+1) PANA_auth[EAP Resp, ID=2]
(duplicate detected at PAA)
10.<------- (x+2,y+3) PANA_auth[EAP Req, ID=3]
11.---+ (y+4,x+2) PANA_auth[EAP Resp, ID=3]
| (retransmission timeout at PAA)
12.<--|---- (x+2,y+3) PANA_auth[EAP Req, ID=3]
| (duplicate detected at PaC)
13.---|---> (y+4,x+2) PANA_auth[EAP Resp, ID=3]
14.<--|---- (x+3,y+4) PANA_succ[EAP Succ, ID=3]
15.---|---> (y+5,x+3) PANA_ACK
+---> (out of order. discarded)
Figure 13: Example for Dual sequence number with reliable-delivery
method
A.3.3 Comparison of the dual sequence number methods
The orderly-delivery method is simpler than the reliable-delivery
method in that the former does not allow sending a separate ACK
while the latter does.
In terms of authentication performance, the reliable-delivery method
is better than the orderly-delivery method in that the former gives
more detailed status of the link than the latter, e.g., an entity
can know whether a request has reached the communicating peer
without before receiving a response. The reliable-delivery can
reduce retransmission traffic and communication delay that would
occur if there is no reliability, as described in section A.2.2.
A.4 Consensus
Although it is recognizable that the reliable-delivery method would
be important in terms of improvement of overall authentication
latency, we believe that this is a performance problem of EAP and
not a problem of PANA. It is agreed that solving the EAP problem is
not the scope of PANA and simplicity is more important factor in the
PANA design.
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PANA March 2003
As a consequence, the orderly-delivery method is chosen as the
message transport part of PANA.
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Tschofenig (ed.) Expires September 2003 35
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