Internet DRAFT - draft-wasserman-pcp-authentication
draft-wasserman-pcp-authentication
Network Working Group M. Wasserman
Internet-Draft S. Hartman
Intended status: Experimental Painless Security
Expires: September 12, 2012 D. Zhang
Huawei
March 11, 2012
Port Control Protocol (PCP) Authentication Mechanism
draft-wasserman-pcp-authentication-02
Abstract
An IPv4 or IPv6 host can use the Port Control Protocol (PCP) to
flexibly manage the IP address and port mapping information on
Network Address Translators (NATs) or firewalls, to facilitate
communications with remote hosts. However, the un-controlled
generation or deletion of IP address mappings on such network devices
may cause security risks and should be avoided. In some cases the
client may need to prove that it is authorized to modify, create or
delete PCP mappings. This document proposes an in-band
authentication mechanism for PCP that can be used in those cases.
The Extensible Authentication Protocol (EAP) is used to perform
authentication between PCP devices.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on September 12, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Protocol Details . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Session Initiation . . . . . . . . . . . . . . . . . . . . 5
3.2. Session Termination . . . . . . . . . . . . . . . . . . . 7
3.3. Result Codes . . . . . . . . . . . . . . . . . . . . . . . 7
4. PA Security Association . . . . . . . . . . . . . . . . . . . 7
5. Packet Format . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Authentication OpCode Format . . . . . . . . . . . . . . . 8
5.2. Nonce Option . . . . . . . . . . . . . . . . . . . . . . . 9
5.3. Authentication Tag Option . . . . . . . . . . . . . . . . 10
5.4. EAP Payload Option . . . . . . . . . . . . . . . . . . . . 11
5.5. PRF Option . . . . . . . . . . . . . . . . . . . . . . . . 11
5.6. Hash Algorithm Option . . . . . . . . . . . . . . . . . . 11
5.7. Session Lifetime Option . . . . . . . . . . . . . . . . . 12
6. Processing Rules . . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Authentication Data Generation . . . . . . . . . . . . . . 12
6.2. Authentication Data Validation . . . . . . . . . . . . . . 12
6.3. Sequence Number . . . . . . . . . . . . . . . . . . . . . 13
6.4. Retransmission Policies . . . . . . . . . . . . . . . . . 13
6.5. MTU Considerations . . . . . . . . . . . . . . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
8. Security Considerations . . . . . . . . . . . . . . . . . . . 15
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
10. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 15
10.1. Changes from -00 to -01 . . . . . . . . . . . . . . . . . 16
10.2. Changes from -01 to -02 . . . . . . . . . . . . . . . . . 16
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
11.1. Normative References . . . . . . . . . . . . . . . . . . . 16
11.2. Informative References . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
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1. Introduction
Using the Port Control Protocol (PCP) [I-D.ietf-pcp-base], an IPv4 or
IPv6 host can flexibly manage the IP address mapping information on
its network address translators (NATs) and firewalls, and control
their policies in processing incoming and outgoing IP packets.
Because NATs and firewalls both play important roles in network
security architectures, there are many situations in which
authentication and access control are required to prevent un-
authorized users from accessing such devices. This document proposes
a PCP security extension which enables PCP servers to authenticate
the clients that they are communicating with using Extensible
Authentication Protocol (EAP). The following issues are considered
in the design of this extension:
o Loss of EAP messages during transportation
o Disordered delivery of EAP messages
o Generation of transport keys
o Integrity protection and data origin authentication for PCP
messages
o Algorithm agility
The mechanism described in this document meets the security
requirements to address the Advanced Threat Model described in the
base PCP specification [I-D.ietf-pcp-base]. This mechanism can be
used to secure PCP in the following situations::
o On security infrastructure equipment, such as corporate firewalls,
that does not create implicit mappings.
o On equipment (such as CGNs or service provider firewalls) that
serve multiple administrative domains and do not have a mechanism
to securely partition traffic from those domains.
o For any implementation that wants to be more permissive in
authorizing explicit mappings than it is in authorizing implicit
mappings.
o For implementations that support the THIRD_PARTY Option (unless
they can meet the constraints outlined in Section 14.1.2.2).
o For implementations that wish to support any deployment scenario
that does not meet the constraints described in Section 14.1.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Most of the terms used in this document are introduced in
[I-D.ietf-pcp-base].
PCP Client (PCC): A PCP device (e.g., a host) which is responsible
for issuing PCP requests to a PCP server. In this document, a PCC is
also a EAP peer [RFC3748], and it is the responsibility of a PCC to
provide the credentials when authentication is required.
PCP Server (PCS): A PCP device (e.g., a NAT or a firewall) that
implements the server-side of the PCP protocol, via which PCCs
request and manage explicit mappings. In this document, a PCS is
integrated with an EAP authenticator [RFC3748]. Therefore, when
necessary, a PCS can verify the credentials provided by a PCC and
make an access control decision based on the authentication result.
PCP Authentication (PA) Session: A series of PCP message exchanges
transferred between a PCC and a PCS in order to perform
authentication, authorization, key distribution and secured PCP
communication. Each PA session is assigned a distinctive Session ID.
The PCP devices involved within a PA session are called session
partners. A typical PA session has two session partners.
Session Lifetime: The life period associated with a PA session, which
decided the lifetime of the current authorization given to the PCC.
PCP Security Association (PCP SA): A PCP security association is
formed between a PCC and a PCS by sharing cryptographic keying
material and associated context. The formed duplex security
association is used to protect the bidirectional PCP signaling
traffic between the PCC and PCS.
Master Session Key (MSK): A key derived by the partners of a PA
session, using a EAP key generating method specified in [RFC3748].
PA (PCP for Authentication) message: A PCP message containing an
Authentication OpCode for EAP authentication.
3. Protocol Details
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3.1. Session Initiation
To carry out an EAP authentication process between two PCP devices, a
set of PA messages need to be exchanged. Each PA message contains an
Authentication OpCode (and additional Options if needed). The
Authentication OpCode consists of four fields: Session ID, Flag, EAP
Type, and Sequence Number. The Session ID field is used to identify
the session which the message belongs to. The Flag field indicates
the type of the PCP message, while EAP Type is used to identify the
type of the attached EAP message. The sequence number field is used
to detect the disorder or the duplication occurred during packet
delivery.
The message exchanges conveyed within an PA session is introduced in
the remainder section.
When a PCC intends to initiate a PA session with a PCS, it sends a
PCC-Initiation message to the PCS. The Session ID and Sequence
Number fields of the Authentication OpCode in the PCC-Initiation
message are set as 0, and the I bit is set. the PCC also needs to
select a random nonce and append it with the PCC-Initiation message
in order to deal with off-line attacks. Specifically, the nonce is
transported within a nonce option. After receiving the PCC-
Initiation, if the PCS would like to initiate a PA session, it will
reply with a PA-Request which contains an EAP Identity Request. The
Sequence Number field in the PA-Request is set as 0, and the Session
ID field MUST be filled with the session identifier assigned by the
PCS for this session. the PA-Request also needs to be attached with
the nonce. Form now on, every PA message within this session must be
attached with the session identifier. Otherwise, the session partner
receiving the message will discard the message silently. If the PCC
intends to simplify the authentication process, it can append an EAP
Identity Response message within the PCC-Initiation request so as to
skip over the step of waiting for the EAP Identity Request and inform
the PCS that it would like to perform EAP authentication.
In the scenario where a PCS receives a non-PA PCP message from a PCC
which needs to be authenticated, the PCS can reply with a PA-Request
to initiate a PA session; the result code field of the PA-Request is
set as AUTHENTICATION-REQUIRED. In addition, the PCS MUST assign a
session ID for the session and transfer it within the PA-Request. In
the PA messages exchanged afterwards in this session, the session ID
MUST be appended. Therefore, in the subsequent communication, the
PCC can distinguish the messages in this session from those in other
sessions through the PCS IP address and the session ID. When the PCC
receives the initial PA-Request message from the PCS, it can reply
with a PA-Answer message to continue the session or silently discards
the request message according to its local policies.
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In a PA session, PA-Request messages are sent from PCSs to PCCs while
PA-Answer messages are only sent from PCCs to PCSs. Correspondently,
an EAP request messages MUST be transported within a PA-Request
message, and an EAP answer messages MUST be transported within a PA-
Answer message. Particularly, when a PCP device receives a PA-
Request or a PA-Answer message from its partner, the PCP device needs
to reply with a PA-Acknowledge message to indicate that the message
has been received. This solution is used to deal with the conditions
where the device cannot generate a response within a pre-specified
period due to certain reasons (e.g., waiting for human input to
construct a EAP message). Therefore, the partner does not have to
un-necessarily retransfer the PCP message.
In this work, it is mandated for a PCC and a PCS to perform a key-
generating EAP method in authentication, and so a successful EAP
authentication process will result in a Master Session Key (MSK). If
the PCC and the PCS want to generate a traffic key using the MSK,
they need to agree upon a Pseudo-Random Function (PRF) for the
transport key derivation and a MAC algorithm to provide data origin
authentication for subsequent PCP packets. On this occasion, the PCS
needs to append the initial PA-Request message with a set of PRF
Options and MAC Algorithm Options. Each PRF Option (MAC Algorithm
Option) contains a PRF (MAC (Message Authentication Code) algorithm)
that the PCS supports. After receiving the request, the PCC selects
a PRF and a MAC algorithm which it intends to support, and sends back
a PA-Answer with a PRF Option and a MAC Algorithm Option for the
selected algorithms.
The last PA-Request message transported within a PA session carries
the EAP authentication and PCP authorization results. The last PA-
Request and PA-Answer messages MUST have their the 'C' (Complete) bit
set.
If the EAP authentication successes, the result code of the last PA-
Request is AUTHENTICATION-SUCCESS. In this case, before sending out
the PA-Request, the PCS must derive a transport key and use it to
generate digests to protect the integrity and authenticity of the PA-
Request and any subsequent PCP message. Such digests are transported
within Authentication Tag Options. In addition, the PA-Request needs
to be appended with a Session Lifetime Option which indicates the
life time of the PA session (i.e., the life time of the MSK).
If the EAP authentication fails, the result code of the last PA-
Request is AUTHENTICATION-FAILED. If the EAP authentication
successes but Authorization fails, the result code of the last PA-
Request is AUTHORIZATION-FAILED. In the latter two cases, the PA
session MUST be terminated immediately after the last PCP
authentication message exchange.
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3.2. Session Termination
A PA session can be explicitly terminated by sending a termination-
indicating PA acknowledge message from either session partner. After
receiving a termination-indicating message from the session partner,
a PCP device MUST response with a termination-indicating PA
Acknowledge message and remove the PA SA immediately. When the
session partner initiating the termination process receives the
acknowledge message, it will remove the associated PA SA immediately.
3.3. Result Codes
Following result codes are defined in the solution:
XXX AUTHENTICATION-REQUIRED
XXX AUTHENTICATION-FAILED
XXX AUTHENTICATION-SUCCESS
XXX AUTHORIZATION-FAILED
4. PA Security Association
At the beginning a PA session, a session SHOULD generate a PA SA to
maintain its state information during the session. A The parameters
of a PA SA is listed as follows:
o IP address and UDP port number of the PCC
o IP address and UDP port number of the PCS
o Session Identifier
o Sequence number for the next outgoing PCP message
o Sequence number for the next incoming PCP message
o Last outgoing message payload
o Retransmission interval
o MSK
o MAC algorithm: The algorithm that the transport key should use to
generate digests for PCP messages.
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o Pseudo-random function: The pseudo random function negotiated in
the initial PA-Request and PA-Answer exchange for the transport
key derivation
o Transport key: the key derived from the MSK to provide integrity
protection and data origin authentication for the messages in the
PA session. The life time of the transport key SHOULD be
identical to the life time of the session.
Particularly, the transport key is computed in the following way:
Transport key = prf(MSK, "IETF PCP"| Session_ID, key ID), where:
o The prf: The pseudo-random function assigned in the Pseudo-random
function parameter.
o MSK: The master session key generated by the EAP method.
o "IETF PCP": The ASCII code representation of the non-NULL
terminated string (excluding the double quotes around it).
o Session_ID: The ID of the session which the MSK is derived from
o Key ID: The ID assigned for the traffic key
5. Packet Format
5.1. Authentication OpCode Format
The following figure illustrates the format of an authentication
Opcode:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | EAP Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Flags: The Flags field is two octets. The following bits are
assigned:
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0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|I C R A K T S E r r r r r r r r|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* I (Initiation): This bit is set in a PCC-Initiation message.
* C (Complete): If the message is the last PA-Request or PA-
Answer message in the session, this bit MUST be set. For other
messages, this bit MUST be cleared.
* R (Request): This bit is set in a PA-Request message.
* A (Answer): This bit is set in a PA-Answer message.
* K (acKnowledgement): This bit is set and only set in a PA-
Acknowledgement message.
* T (Termination): If this bit is set in a PA-Acknowledgement
message, the message is used for session-termination
indication.
* S (Fragmentation start): This bit is set in a PA message which
contain the first fragment of a EAP message.
* E (Fragmentation start): This bit is set in a PA message which
contain the last fragment of a EAP message.
Message Type: The Message Type field is two octets. This field is
used to indicate the type of the EAP message attached within the
message. Message Type allocation is managed by IANA [IANAWEB].
Session ID: This field contains a 32-bit PA session identifier.
Sequence Number: This field contains a 32-bit sequence number. In
this solution, a sequence number needs to be incremented on every
new (non-retransmission) outgoing packet in order to provide
ordering guarantee for PCP.
5.2. Nonce Option
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Code | Reserved | Option-Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Nonce: A random 32 bits number which is transported within a PCC-
Initiate message and the correspondent reply message from the PCS.
5.3. Authentication Tag Option
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Code | Reserved | Option-Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Authentication Data (Variable) |
~ ~
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option-Length: The length of the Authentication Tag Option (in
octet), including the 8 octet fixed header and the variable length
of the authentication data.
Session ID: A 32-bit field used to indicates the identifier of the
session that the message belongs to and identifies the secret key
used to create the message digest appended to the PCP message.
Key ID: The ID associated with the traffic key used to generate
authentication data. This field is filled with zero if MSK is
directly used to secure the message.
Authentication Data: A Variable length field that carries the
Message Authentication Code for the PCP packet. The generation of
the digest can be various according to the algorithms specified in
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different PCP SAs.
5.4. EAP Payload Option
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Code | Reserved | Option-Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| EAP Message |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
EAP Message: The EAP message transferred. Note this field MUST
end on a 32-bit boundary, padded with 0's when necessary.
5.5. PRF Option
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Code | Reserved | Option-Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PRF |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PRF: The pseudo-random Function which the sender supports to
generate a MSK.
5.6. Hash Algorithm Option
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Code | Reserved | Option-Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MAC Algorithm: The MAC algorithm which the sender supports to
generate authentication data.
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5.7. Session Lifetime Option
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Code | Reserved | Option-Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Session Lifetime: The life period of the PA Session, which is decided
by the authorization result.
6. Processing Rules
6.1. Authentication Data Generation
If a PCP SA is generated as the result of an successful EAP
authentication process, every subsequent PCP message within the
session needs carry an Authentication Tag Option which contains the
digest of the PCP message for data orgin authentication and integrity
protection.
Before generating a digest for a PCP message, a device needs to first
select a traffic key in the session and append the Authentication Tag
Option at the end of the protected PCP message. The length of the
Authentication Data field is decided by the MAC algorithm adopted in
the session. The device then fills the Session ID field and the PCP
SA ID field, and sets the Authentication Data field as 0. After
this, the device generates a digest for the PCP message with the MAC
algorithm and the selected traffic key, and input the generated
digest into the Authentication Data field.
6.2. Authentication Data Validation
When a device receives a PCP pacekt with an Authentication Tag
Option, it needs to use the session ID transported in the option to
locate the proper SA, and then find out the associated transport key
and the MAC algorithm. After storing the value of the Authentication
field of the Authentication Tag Option, the device fills the the
Authentication field with zeros. Then, the device generates a digest
for the packet with the transport key and the MAC algorithm found in
the first step. If the value of the newly generated digest is
identical to the stored one, the device can ensure that the packet
has not been tampered during the transportation. The validation
successes. Otherwise, the packet MUST be discarded.
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6.3. Sequence Number
PCP adopts UDP to transport signaling messages. As an un-reliable
transporting protocol, UDP does not guarantee the ordered packet
delivery and does not provide any protection from packet loss. In
order to ensure the EAP messages are exchanged in a reliable way,
every PCP packet exchanged during EAP authentication must carries an
monotonically increased sequence number. During a PA session, a PCP
device needs to maintain two sequence numbers, one for incoming
packets and one for outgoing packets. When generating an outgoing
PCP packet, the device attaches the outgoing sequence number to the
packet and increments the sequence number by 1. When receiving a PCP
packet from its session partner, the device will not accept it if the
sequence number carried in the packet does not matche the incoming
sequence number the device maintains.
After confirming that the received packet is valid, the device
increments the incoming sequence number by 1. However, the above
rules are not applied to PA-Acknowledgement messages. When receiving
or sending out a PA-Acknowledgement message, the device does not
inicrease the correspondent sequence number. Another exception is
message retransmission. When a device does not receive any response
message from its session partner in a certain period, it needs to
retransmit the last sent message with a limited rate. The duplicate
messages and the original message MUST use the identical sequence
number. When the device receives such duplicate messages from its
session partner, it MUST tries to answer them by sending the last
outgoing message with a limited rate unless it has received another
valid message with a larger sequence number from its session. Note
that in these cases the incoming and outgoing sequence number will
not be affected by the message retransmission.
6.4. Retransmission Policies
This work provides a retransmission mechanism for reliable PA message
delivery. The timer, the variables, and the rules used in this
mechanism is mostly brought from PANA[RFC5191].
The retransmission behavior is controlled and described by the
following variables:
RT: Retransmission timeout from the previous (re)transmission
IRT: Base value for RT for the initial retransmission
MRC: Maximum retransmission count
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MRT: Maximum retransmiting time interval
RAND: Randomization factor
With each message transmission or retransmission, the sender sets RT
according to the rules given below.
If RT expires before receiving any reply, the sender re-calculates RT
and retransmits the message. Each of the computations of a new RT
include a randomization factor (RAND), which is a random number
chosen with a uniform distribution between -0.1 and +0.1. The
randomization factor is included to minimize the synchronization of
messages. The algorithm for choosing a random number does not need
to be cryptographically sound. The algorithm SHOULD produce a
different sequence of random numbers from each invocation. RT for
the first message retransmission is based on IRT:
RT = IRT
RT for each subsequent message retransmission is based on the
previous value of RT (RTprev):
RT = (2+RAND) * RTprev
MRT specifies an upper bound on the value of RT (disregarding the
randomization added by the use of RAND). If MRT has a value of 0,
there is no upper limit on the value of RT. Otherwise:
if (RT > MRT)
RT = (1+RAND) * MRT
MRC specifies an upper bound on the number of times a sender may
retransmit a message. Unless MRC is zero, the message exchange fails
once the sender has transmitted the message MRC times. In this case,
the sender needs to start a session termination process illustrated
in Section 3.2.
6.5. MTU Considerations
The fragmentation and reassembly of EAP messages must be provided in
order to ensure the length of a PA message is not larger than the MTU
of the link that it will be transported through. Therefore, a PA
message may only transport a fragment of an EAP message. Becasue any
loss or tamper of a EAP frgament will be detected and sequencing
information is provided, fragmentation support can be added in a
simple manner. Particularly, the S bit is set in a PA message which
contain the first fragment of a EAP message, and the The E bit is set
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in a PA message which contain the last fragment of a EAP message.
7. IANA Considerations
TBD
8. Security Considerations
In this work, a successful EAP authentication process performed
between two PCP devices will result in the generation of a MSK which
can be used to derive the transport keys to generate MAC digests for
subsequent PCP message exchanges. This work does not exclude the
possibility of using the MSK to generate keys for different security
protocols to enable per-packet cryptographic protection. The methods
of deriving the transport key for the security protocols is out of
scope of this document.
However, before a transport key has been generated, the PA messages
exchanged within a PA session have little cryptographic protection,
and if there is no already established security channel between two
session partners, these messages are subject to man-in-the-middle
attacks and DOS attacks. For instance, the initial PA-Request and
PA-Answer exchange is vulnerable to spoofing attacks as these
messages are not authenticated and integrity protected. In order to
prevent very basic DOS attacks, a PCP device SHOULD generate state
information as little as possible in the initial PA-Request and PA-
Answer exchanges. The choice of EAP method is also very important.
The selected EAP method must be resilient to the attacks possibly
occurred in a insecure network environment, and the user-identity
confidentiality, protection against dictionary attacks, and session-
key establishment must be supported.
9. Acknowledgements
This document was written using the xml2rfc tool described in RFC
2629 [RFC2629].
Some of the ideas in this document were adopted from PANA[RFC5191].
10. Change Log
Wasserman, et al. Expires September 12, 2012 [Page 15]
Internet-Draft PCP Authentication March 2012
10.1. Changes from -00 to -01
o Editorial changes, added use cases to introduction.
10.2. Changes from -01 to -02
o Add a nonce into the first two exchanged PA message between the
PCC and PCS. When a PCC initiate the session, it can use the
nonce to detect offline attacks.
o Add the key ID field into the authentication tag option so that a
MSK can generate multiple traffic keys.
o Specify that when a PCP device receives a PA-Request or a PA-
Answer message from its partner the PCP device needs to reply with
a PA-Acknowledge message to indicate that the message has been
received.
o Add the support of fragmenting EAP messages.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
11.2. Informative References
[I-D.ietf-pcp-base]
Cheshire, S., Boucadair, M., Selkirk, P., Wing, D., and R.
Penno, "Port Control Protocol (PCP)",
draft-ietf-pcp-base-23 (work in progress), February 2012.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.
[RFC5191] Forsberg, D., Ohba, Y., Patil, B., Tschofenig, H., and A.
Yegin, "Protocol for Carrying Authentication for Network
Access (PANA)", RFC 5191, May 2008.
Wasserman, et al. Expires September 12, 2012 [Page 16]
Internet-Draft PCP Authentication March 2012
Authors' Addresses
Margaret Wasserman
Painless Security
356 Abbott Street
North Andover, MA 01845
USA
Phone: +1 781 405 7464
Email: mrw@painless-security.com
URI: http://www.painless-security.com
Sam Hartman
Painless Security
356 Abbott Street
North Andover, MA 01845
USA
Email: hartmans@painless-security.com
URI: http://www.painless-security.com
Dacheng Zhang
Huawei
Beijing,
China
Phone:
Fax:
Email: zhangdacheng@huawei.com
URI:
Wasserman, et al. Expires September 12, 2012 [Page 17]
