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Internet Engineering Task Force (IETF) M. Cullen
Request for Comments: 7652 S. Hartman
Updates: 6887 Painless Security
Category: Standards Track D. Zhang
ISSN: 2070-1721
T. Reddy
Cisco
September 2015
Port Control Protocol (PCP) Authentication Mechanism
Abstract
An IPv4 or IPv6 host can use the Port Control Protocol (PCP) to
flexibly manage the IP address-mapping and port-mapping information
on Network Address Translators (NATs) or firewalls to facilitate
communication with remote hosts. However, the uncontrolled
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 describes 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.
This document updates RFC 6887.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7652.
Cullen, et al. Standards Track [Page 1]
RFC 7652 PCP Authentication September 2015
Copyright Notice
Copyright (c) 2015 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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Cullen, et al. Standards Track [Page 2]
RFC 7652 PCP Authentication September 2015
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Session Initiation . . . . . . . . . . . . . . . . . . . 6
3.1.1. Authentication Triggered by the Client . . . . . . . 6
3.1.2. Authentication Triggered by the Server . . . . . . . 7
3.1.3. Authentication Using EAP . . . . . . . . . . . . . . 8
3.2. Recovery from Lost PA Session . . . . . . . . . . . . . . 10
3.3. Session Termination . . . . . . . . . . . . . . . . . . . 11
3.4. Session Re-authentication . . . . . . . . . . . . . . . . 11
4. PA Security Association . . . . . . . . . . . . . . . . . . . 12
5. Packet Format . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1. Packet Format of PCP Auth Messages . . . . . . . . . . . 14
5.2. Opcode-Specific Information of AUTHENTICATION Opcode . . 16
5.3. NONCE Option . . . . . . . . . . . . . . . . . . . . . . 16
5.4. AUTHENTICATION_TAG Option . . . . . . . . . . . . . . . . 17
5.5. PA_AUTHENTICATION_TAG Option . . . . . . . . . . . . . . 18
5.6. EAP_PAYLOAD Option . . . . . . . . . . . . . . . . . . . 19
5.7. PRF Option . . . . . . . . . . . . . . . . . . . . . . . 19
5.8. MAC_ALGORITHM Option . . . . . . . . . . . . . . . . . . 20
5.9. SESSION_LIFETIME Option . . . . . . . . . . . . . . . . . 20
5.10. RECEIVED_PAK Option . . . . . . . . . . . . . . . . . . . 21
5.11. ID_INDICATOR Option . . . . . . . . . . . . . . . . . . . 21
6. Processing Rules . . . . . . . . . . . . . . . . . . . . . . 22
6.1. Authentication Data Generation . . . . . . . . . . . . . 22
6.2. Authentication Data Validation . . . . . . . . . . . . . 23
6.3. Retransmission Policies for PA Messages . . . . . . . . . 24
6.4. Sequence Numbers for PCP Auth Messages . . . . . . . . . 25
6.5. Sequence Numbers for Common PCP Messages . . . . . . . . 26
6.6. MTU Considerations . . . . . . . . . . . . . . . . . . . 26
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
7.1. NONCE . . . . . . . . . . . . . . . . . . . . . . . . . . 28
7.2. AUTHENTICATION_TAG . . . . . . . . . . . . . . . . . . . 28
7.3. PA_AUTHENTICATION_TAG . . . . . . . . . . . . . . . . . . 29
7.4. EAP_PAYLOAD . . . . . . . . . . . . . . . . . . . . . . . 29
7.5. PRF . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
7.6. MAC_ALGORITHM . . . . . . . . . . . . . . . . . . . . . . 30
7.7. SESSION_LIFETIME . . . . . . . . . . . . . . . . . . . . 30
7.8. RECEIVED_PAK . . . . . . . . . . . . . . . . . . . . . . 30
7.9. ID_INDICATOR . . . . . . . . . . . . . . . . . . . . . . 31
8. Security Considerations . . . . . . . . . . . . . . . . . . . 31
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.1. Normative References . . . . . . . . . . . . . . . . . . 32
9.2. Informative References . . . . . . . . . . . . . . . . . 33
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 33
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
Cullen, et al. Standards Track [Page 3]
RFC 7652 PCP Authentication September 2015
1. Introduction
Using the Port Control Protocol (PCP) [RFC6887], an application can
flexibly manage the IP address-mapping information on its network
address translators (NATs) and firewalls and can 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 unauthorized users from
accessing such devices. This document defines a PCP security
extension that enables PCP servers to authenticate their clients with
the Extensible Authentication Protocol (EAP). The EAP messages are
encapsulated within PCP messages during transmission.
The following issues are considered in the design of this extension:
o Loss of EAP messages during transmission.
o Reordered 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 [RFC6887]. 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 for specific traffic.
o On equipment (such as Carrier-Grade NATs (CGNs) or service
provider firewalls) that serves 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 applications to create mappings for successful inbound
communications destined to machines located behind a NAT or a
firewall.
Cullen, et al. Standards Track [Page 4]
RFC 7652 PCP Authentication September 2015
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 [RFC6887].
PCP client: A PCP software instance that is responsible for issuing
PCP requests to a PCP server. In this document, a PCP client is also
an EAP peer [RFC3748], and it is the responsibility of a PCP client
to provide the credentials when authentication is required.
PCP server: A PCP software instance that resides on the
PCP-controlled device that receives PCP requests from the PCP client
and creates appropriate state in response to that request. In this
document, a PCP server is integrated with an EAP authenticator
[RFC3748]. Therefore, when necessary, a PCP server can verify the
credentials provided by a PCP client and make an access control
decision based on the authentication result.
PCP-Authentication (PA) session: A series of PCP message exchanges
transferred between a PCP client and a PCP server. The PCP messages
that are part of a given session include the PA messages used to
perform EAP authentication, key distribution, and session management,
as well as the common PCP messages secured with the keys distributed
during authentication. Each PA session is assigned a distinctive
Session ID.
Session partner: A PCP implementation involved in a PA session. Each
PA session has two session partners (a PCP server and a PCP client).
PCP device: A PCP client or a PCP server.
Session lifetime: The lifetime associated with a PA session. The
session lifetime of the PA session decides the lifetime of the
current authorization given to the PCP client.
PA Security Association (PCP SA): An association formed between a
PCP client and a PCP server by sharing cryptographic keying material
and associated context. The formed duplex security association is
used to protect the bidirectional PCP signaling traffic between the
PCP client and PCP server.
Master Session Key (MSK): A key derived by the partners of a
PA session, using an EAP key-generating method (e.g., the method
defined in [RFC5448]).
Cullen, et al. Standards Track [Page 5]
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PCP-Authentication (PA) message: A PCP message containing an
AUTHENTICATION Opcode. Specifically, a PA message sent from a
PCP server to a PCP client is referred to as a PA-Server message,
while a PA message sent from a PCP client to a PCP server is referred
to as a PA-Client message. Therefore, a PA-Server message is
actually a PCP response message as specified in [RFC6887], and a
PA-Client message is a PCP request message. This document specifies
an option -- the PA_AUTHENTICATION_TAG option defined in Section 5.5
for PCP authentication -- to provide integrity protection and message
origin authentication for PA messages.
Common PCP message: A PCP message that does not contain an
AUTHENTICATION Opcode. This document specifies an AUTHENTICATION_TAG
option to provide integrity protection and message origin
authentication for the common PCP messages.
3. Protocol Details
3.1. Session Initiation
At the beginning of a PA session, a PCP client and a PCP server need
to exchange a series of PA messages in order to perform an EAP
authentication process. Each PA message MUST contain an
AUTHENTICATION Opcode and may optionally contain a set of options for
various purposes (e.g., transporting authentication messages and
session management). The Opcode-specific information in an
AUTHENTICATION Opcode consists of two fields: Session ID and Sequence
Number. The Session ID field is used to identify the PA session to
which the message belongs. The Sequence Number field is used to
detect whether reordering or duplication occurred during message
delivery.
3.1.1. Authentication Triggered by the Client
When a PCP client intends to proactively initiate a PA session with a
PCP server, it sends a PA-Initiation message (a PA-Client message
with the result code INITIATION) to the PCP server. Section 5.1
updates the PCP request message format with result codes for the PCP
authentication mechanism. In the Opcode-specific information of the
message, the Session ID and Sequence Number fields are set to zero.
The PA-Client message MUST also contain a NONCE option (defined in
Section 5.3) that consists of a random nonce.
After receiving the PA-Initiation message, if the PCP server agrees
to initiate a PA session with the PCP client, it will reply with a
PA-Server message that contains an EAP request, and the Result Code
field of this PA-Server message is set to AUTHENTICATION_REQUEST. In
addition, the server MUST assign a unique session identifier to
Cullen, et al. Standards Track [Page 6]
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distinctly identify this session and insert the identifier into the
Session ID field in the Opcode-specific information of the PA-Server
message. The Sequence Number field of the message is set to zero.
The PA-Server message MUST contain a NONCE option so as to send the
nonce value back. The nonce will then be used by the PCP client to
check the freshness of this message. Subsequent PCP messages within
this PA session MUST contain this session identifier.
PCP PCP
client server
|-- PA-Initiation ------------------------------->|
| (Seq=0, rc=INITIATION, Session ID=0) |
| |
|<-- PA-Server -----------------------------------|
| (Seq=0, Session ID=X, EAP request, |
| rc=AUTHENTICATION_REQUEST) |
| |
|-- PA-Client ----------------------------------->|
| (Seq=1, Session ID=X, EAP response, |
| rc=AUTHENTICATION_REPLY) |
| |
|<-- PA-Server -----------------------------------|
| (Seq=1, Session ID=X, EAP request, |
| rc=AUTHENTICATION_REQUEST) |
3.1.2. Authentication Triggered by the Server
In the scenario where a PCP server receives a common PCP request
message from a PCP client that needs to be authenticated, the
PCP server rejects the request with an AUTHENTICATION_REQUIRED error
code and can reply with an unsolicited PA-Server message to initiate
a PA session. The Result Code field of this PA-Server message is set
to AUTHENTICATION_REQUEST. In addition, the PCP server MUST assign a
Session ID for the session and transfer it within the PA-Server
message. The Sequence Number field in the PA-Server message is set
to zero. If the PCP client retries the common request before EAP
authentication is successful, then it will receive an
AUTHENTICATION_REQUIRED error code from the PCP server. In
subsequent PA messages exchanged during this session, the Session ID
will be used in order to help session partners distinguish the
messages within this session from those not within it. When the
PCP client receives this initial PA-Server message from the
PCP server, it can reply with a PA-Client message or silently discard
the request message, according to its local policies. In the
PA-Client message, a NONCE option that consists of a random nonce MAY
be appended. If so, in the next PA-Server message, the PCP server
MUST forward the nonce back within a NONCE option.
Cullen, et al. Standards Track [Page 7]
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PCP PCP
client server
|-- Common PCP request -------------------------->|
| |
|<- Common PCP response --------------------------|
| (rc=AUTHENTICATION_REQUIRED) |
| |
|<-- PA-Server -----------------------------------|
| (Seq=0, Session ID=X, EAP request, |
| rc=AUTHENTICATION_REQUEST) |
| |
|-- PA-Client ----------------------------------->|
| (Seq=0, Session ID=X, EAP response, |
| rc=AUTHENTICATION_REPLY) |
| |
|<-- PA-Server -----------------------------------|
| (Seq=1, Session ID=X, EAP request, |
| rc=AUTHENTICATION_REQUEST) |
3.1.3. Authentication Using EAP
In a PA session, an EAP request message is transported within a
PA-Server message and an EAP response message is transported within a
PA-Client message. EAP relies on the underlying protocol to provide
reliable transmission; any reordered delivery or loss of packets
occurring during transmission must be detected and addressed.
Therefore, after sending out a PA-Server message, the PCP server will
not send a new PA-Server message in the same PA session until it
receives a PA-Client message with a proper sequence number from the
PCP client, and vice versa. If a PCP client receives a PA message
containing an EAP request and for some reason cannot generate an EAP
response immediately (e.g., waiting for human input in order to
construct an EAP message, or waiting for the additional PA messages
in order to assemble a complete EAP message from fragmented packets),
the PCP device MUST reply with a PA-Acknowledgement message (a
PA message with a RECEIVED_PAK option) to indicate that the message
has been received. This approach not only can avoid unnecessary
retransmission of the PA message but also can guarantee reliable
message delivery in conditions where a PCP device needs to receive
multiple PA messages carrying the fragmented EAP request before
generating an EAP response. The number of EAP messages exchanged
between the PCP client and PCP server depends on the EAP method used
for authentication.
In this approach, a PCP client and a PCP server MUST perform a
key-generating EAP method in authentication. Specifically, a PCP
authentication implementation MUST support Extensible Authentication
Protocol Tunneled Transport Layer Security (EAP-TTLS) [RFC5281] and
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SHOULD support the Tunnel Extensible Authentication Protocol (TEAP)
[RFC7170]. Therefore, after a successful authentication procedure, a
Master Session Key (MSK) will be generated. If the PCP client and
the PCP server want to generate a transport key using the MSK, they
need to agree upon a Pseudorandom Function (PRF) for the transport
key derivation and a Message Authentication Code (MAC) algorithm to
provide data origin authentication for subsequent PCP messages. In
order to do this, the PCP server needs to append a set of PRF options
and MAC_ALGORITHM options to the initial PA-Server message. Each PRF
option contains a PRF that the PCP server supports, and each
MAC_ALGORITHM option contains a MAC algorithm that the PCP server
supports. Moreover, in the first PA-Server message, the server MAY
also attach an ID_INDICATOR option (defined in Section 5.11) to
direct the client to choose correct credentials. After receiving the
options, the PCP client MUST select the PRF and the MAC algorithm
that it would like to use; it then MUST add the associated PRF and
MAC Algorithm options to the next PA-Client message.
After the EAP authentication, the PCP server sends out a PA-Server
message to indicate the EAP authentication and PCP authorization
results. If the EAP authentication succeeds, the result code of the
PA-Server message is AUTHENTICATION_SUCCEEDED. In this case, before
sending out the PA-Server message, the PCP server MUST update the
PCP SA with the MSK and transport key and MUST use the derived
transport key to generate a digest for the message. The digest is
transported within a PA_AUTHENTICATION_TAG option for PCP Auth. A
more detailed description of generating the authentication data can
be found in Section 6.1. In addition, the PA-Server message MUST
also contain a SESSION_LIFETIME option (defined in Section 5.9) that
indicates the lifetime of the PA session (i.e., the lifetime of the
MSK). After receiving the PA-Server message, the PCP client then
needs to generate a PA-Client message in response. If the PCP client
also authenticates the PCP server, the result code of the PA-Client
message is AUTHENTICATION_SUCCEEDED. In addition, the PCP client
needs to update the PCP SA with the MSK and transport key, and it
uses the derived transport key to secure the message. From then on,
all the PCP messages within the session are secured with the
transport key and the MAC algorithm specified in the PCP SA. The
first secure PA-Client message from the client MUST include the set
of PRF and MAC_ALGORITHM options received from the PCP server. The
PCP server determines if the set of algorithms conveyed by the client
matches the set it had initially sent, to detect an algorithm
downgrade attack. If the server detects a downgrade attack, then it
MUST send a PA-Server message with result code
DOWNGRADE_ATTACK_DETECTED and terminate the session. If the
PCP client sends a common PCP request within the PA session without
an AUTHENTICATION_TAG option, then the PCP server rejects the request
by returning an AUTHENTICATION_REQUIRED error code.
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If a PCP client/server cannot authenticate its session partner, the
device sends out a PA message with the result code
AUTHENTICATION_FAILED. If the EAP authentication succeeds but
authorization fails, the device making the decision sends out a
PA message with the result code AUTHORIZATION_FAILED. In these two
cases, after the PA message is sent out, the PA session MUST be
terminated immediately. It is possible for independent PCP clients
on the host to create multiple PA sessions with the PCP server.
3.2. Recovery from Lost PA Session
If a PCP server resets or loses the PCP SA due to reboot, power
failure, or any other reason, then it sends an unsolicited ANNOUNCE
response, as explained in Section 14.1.3 of [RFC6887], to the
PCP client. Upon receiving the ANNOUNCE response with an anomalous
Epoch Time, the PCP client deduces that the server may have lost
state. The ANNOUNCE is either bogus (an attack), legitimate, or not
seen by the client. These three cases are described below:
o The PCP client sends an integrity-protected unicast ANNOUNCE
request to the PCP server to see whether the PCP server has indeed
lost state or an attacker has sent the ANNOUNCE response.
* If an integrity-protected success response is received from the
PCP server, then the PCP client determines that the PCP server
has not lost the PA session, and the unsolicited ANNOUNCE
response was sent by an attacker.
* If the PCP server responds to the ANNOUNCE request with an
UNKNOWN_SESSION_ID error code, then the PCP client MUST
initiate full EAP authentication with the PCP server, as
explained in Section 3.1.1. After EAP authentication is
successful, the PCP client updates the PCP SA and issues new
common PCP requests to recreate any lost mapping state.
o In a scenario where the PCP server has lost the PCP SA but did not
inform the PCP client, if the PCP client sends an integrity-
protected PCP request, then the PCP server rejects the request
with an UNKNOWN_SESSION_ID error code. The PCP client then
initiates full EAP authentication with the PCP server, as
explained in Section 3.1.1, and updates the PCP SA after
successful authentication.
If the PCP client resets or loses the PCP SA due to reboot, power
failure, or any other reason and sends a common PCP request, then the
PCP server rejects the request with an AUTHENTICATION_REQUIRED error
code. The PCP client MUST authenticate with the PCP server and,
after EAP authentication is successful, retry the common PCP request
Cullen, et al. Standards Track [Page 10]
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with an AUTHENTICATION_TAG option. The PCP server MUST update the
PCP SA after successful EAP authentication.
3.3. Session Termination
A PA session can be explicitly terminated by either session partner.
A PCP server may explicitly request termination of the session by
sending an unsolicited termination-indicating PA response (a
PA response with a result code of SESSION_TERMINATED). Upon
receiving a termination-indicating message, the PCP client MUST
respond with a termination-indicating PA message and MUST then remove
the associated PCP SA. To accommodate packet loss, the PCP server
MAY transmit the termination-indicating PA response up to ten times
(with an appropriate Epoch Time value in each to reflect the passage
of time between transmissions), provided that (1) the interval
between the first two notifications is at least 250 ms and (2) each
interval between subsequent notifications at least doubles.
A PCP client may explicitly request termination of the session by
sending a termination-indicating PA request (a PA request with a
result code of SESSION_TERMINATED). After receiving a termination-
indicating message from the PCP client, a PCP server MUST respond
with a termination-indicating PA message and remove the PCP SA
immediately. When the PCP client receives the termination-indicating
PA response, it MUST remove the associated PCP SA immediately.
3.4. Session Re-authentication
A session partner may choose to perform EAP re-authentication if it
would like to update the PCP SA without initiating a new PA session.
For example, a re-authentication procedure could be triggered for the
following reasons:
o The session lifetime needs to be extended.
o The sequence number is going to reach the maximum value.
Specifically, when the sequence number reaches 2**32 - 2**16, the
session partner MUST trigger re-authentication.
When the PCP server would like to initiate a re-authentication, it
sends the PCP client a PA-Server message. The result code of the
message is set to RE-AUTHENTICATION, which indicates that the message
is for a re-authentication process. If the PCP client would like to
start the re-authentication, it will send a PA-Client message to the
PCP server, with the result code of the PA-Client message set to
RE-AUTHENTICATION. Then, the session partners exchange PA messages
to transfer EAP messages for the re-authentication. During the
re-authentication procedure, the session partners protect the
Cullen, et al. Standards Track [Page 11]
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integrity of PA messages with the key and MAC algorithm specified in
the current PCP SA; the sequence numbers associated with the message
will continue to keep increasing as specified in Section 6.4. The
result code for a PA-Server message carrying an EAP request will be
set to AUTHENTICATION_REQUIRED, and a PA-Client message carrying an
EAP response will be set to AUTHENTICATION_REPLY.
If the EAP re-authentication succeeds, the result code of the last
PA-Server message is AUTHENTICATION_SUCCEEDED. In this case, before
sending out the PA-Server message, the PCP server MUST update the SA
and use the new key to generate a digest for the PA-Server message
and subsequent PCP messages. In addition, the PA-Server message MUST
be appended with a SESSION_LIFETIME option that indicates the new
lifetime of the PA session. PA and PCP message sequence numbers must
also be reset to zero.
If the EAP authentication fails, the result code of the last
PA-Server message is AUTHENTICATION_FAILED. If the EAP
authentication succeeds but authorization fails, the result code of
the last PA-Server message is AUTHORIZATION_FAILED. In the latter
two cases, the PA session MUST be terminated immediately after the
last PA message exchange. If for some unknown reason
re-authentication is not performed and the session lifetime has
expired, then the PA session MUST be terminated immediately.
During re-authentication, the session partners can also exchange
common PCP messages in parallel. The common PCP messages MUST be
protected with the current SA until the new SA has been generated.
The sequence of EAP messages exchanged for re-authentication will not
change, regardless of the PCP device triggering re-authentication.
If the PCP server receives a re-authentication request from the
PCP client after the PCP server itself had sent a re-authentication
request, then it should discard its request and respond to the
re-authentication request from the PCP client.
4. PA Security Association
At the beginning of a new PA session, each PCP device must create and
initialize state information for a new PA Security Association
(PCP SA) to maintain its state information for the duration of the
PA session. The parameters of a PCP SA are as follows:
o IP address and UDP port number of the PCP client.
o IP address and UDP port number of the PCP server.
o Session identifier.
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o Sequence number for the next outgoing PA message.
o Sequence number for the next incoming PA message.
o Sequence number for the next outgoing common PCP message.
o Sequence number for the next incoming common PCP message.
o Last outgoing message payload.
o Retransmission interval.
o The Master Session Key (MSK) generated by the EAP method.
o The MAC algorithm that the transport key should use to generate
digests for PCP messages.
o The pseudorandom function negotiated in the initial PA-Server and
PA-Client message exchange for the transport key derivation.
o The transport key derived from the MSK to provide integrity
protection and data origin authentication for the messages in the
PA session. The lifetime of the transport key SHOULD be identical
to the lifetime of the session.
o The nonce selected by the PCP client at the initiation of the
session.
o The key ID associated with the transport key.
Specifically, the transport key is computed in the following way:
transport key = prf(MSK, "IETF PCP" || Session ID || Nonce ||
key ID), where:
o prf is the pseudorandom function assigned in the PRF option
(Section 5.7).
o MSK is the master session key generated by the EAP method.
o "IETF PCP" is the ASCII code representation of the
non-null-terminated string (excluding the double quotes
around it).
o '||' is the concatenation operator.
o Session ID is the ID of the session from which the MSK is derived.
Cullen, et al. Standards Track [Page 13]
RFC 7652 PCP Authentication September 2015
o Nonce is the nonce selected by the client and transported in the
initial PA-Client message.
o Key ID is the ID assigned for the transport key.
5. Packet Format
5.1. Packet Format of PCP Auth Messages
The format of the PA-Server message is identical to the response
message format specified in Section 7.2 of [RFC6887]. The result
code for a PA-Server message carrying an EAP request MUST be set to
AUTHENTICATION_REQUEST.
This document updates the Reserved field (see Figure 1) in the
Request header specified in Section 7.1 of [RFC6887] to carry
Opcode-specific data.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version = 2 |R| Opcode | Reserved |Opcode-specific|
| | | | | data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Requested Lifetime (32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| PCP Client's IP Address (128 bits) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Opcode-specific information :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: (optional) PCP Options :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Request Packet Format
Cullen, et al. Standards Track [Page 14]
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The PA-Client messages (as shown in Figure 2) use the Request header
specified in Figure 1. The Opcode-specific data is used to transfer
the result codes (e.g., INITIATION, AUTHENTICATION_FAILED). Other
fields in Figure 2 are described in Section 7.1 of [RFC6887]. The
result code for a PA-Client message carrying an EAP response MUST be
set to AUTHENTICATION_REPLY.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version = 2 |R| Opcode | Reserved | Result Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Requested Lifetime (32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| PCP Client's IP Address (128 bits) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Opcode-specific information :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: (optional) PCP Options :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: PA-Client Message Format
The Requested Lifetime field of a PA-Client message and the Lifetime
field of a PA-Server message are both set to zero on transmission and
ignored on reception.
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5.2. Opcode-Specific Information of AUTHENTICATION Opcode
The following diagram shows the format of the Opcode-specific
information for the 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Session ID: This field contains a 32-bit PA session identifier.
Sequence Number: This field contains a 32-bit sequence number. A
sequence number needs to be incremented on every new
(non-retransmission) outgoing PA message in order to provide an
ordering guarantee for PA messages.
5.3. NONCE Option
Because the session identifier of a PA session is determined by the
PCP server, a PCP client does not know the session identifier that
will be used when it sends out a PA-Initiation message. In order to
prevent an attacker from interrupting the authentication process by
sending spoofed PA-Server messages, the PCP client needs to generate
a random number as a nonce in the PA-Initiation message. The
PCP server will append the nonce within the initial PA-Server
message. If the PA-Server message does not carry the correct nonce,
the message MUST be silently discarded.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Code: 4.
Reserved: 8 bits. MUST be set to zero on transmission and MUST be
ignored on reception.
Option-Length: 4 octets.
Cullen, et al. Standards Track [Page 16]
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Nonce: A random 32-bit number that is transported within a
PA-Initiation message and the corresponding reply message from the
PCP server.
5.4. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Authentication Data (Variable) |
~ ~
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Because there is no authentication Opcode in common PCP messages, the
authentication tag for common PCP messages needs to carry the
Session ID and Sequence Number.
Option Code: 5.
Reserved: 8 bits. MUST be set to zero on transmission and MUST be
ignored on reception.
Option-Length: The length of the AUTHENTICATION_TAG option for the
common PCP message (in octets), including the 12-octet
fixed-length header and the variable-length authentication data.
Session ID: A 32-bit field used to identify the session to which
the message belongs and identify the secret key used to create the
message digest appended to the PCP message.
Sequence Number: A 32-bit sequence number. In this option, a
sequence number needs to be incremented on every new
(non-retransmission) outgoing common PCP message in order to
provide an ordering guarantee for common PCP messages.
Cullen, et al. Standards Track [Page 17]
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Key ID: The ID associated with the transport key used to generate
authentication data. This field is filled with zeros if the MSK
is directly used to secure the message.
Authentication Data: A variable-length field that carries the
Message Authentication Code for the common PCP message. The
generation of the digest varies according to the algorithms
specified in different PCP SAs. This field MUST end on a 32-bit
boundary, padded with zeros when necessary.
5.5. PA_AUTHENTICATION_TAG Option
This option is used to provide message authentication for
PA messages. In contrast to the AUTHENTICATION_TAG option for common
PCP messages, the Session ID field and the Sequence Number field are
removed because such information is provided in the Opcode-specific
information of the 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Code | Reserved | Option-Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Authentication Data (Variable) |
~ ~
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Code: 6.
Reserved: 8 bits. MUST be set to zero on transmission and MUST be
ignored on reception.
Option-Length: The length of the PA_AUTHENTICATION option for the
PCP Auth message (in octets), including the 4-octet fixed-length
header and the variable-length authentication data.
Key ID: The ID associated with the transport key used to generate
authentication data. This field is filled with zeros if the MSK
is directly used to secure the message.
Authentication Data: A variable-length field that carries the
Message Authentication Code for the PCP Auth message. The
generation of the digest varies according to the algorithms
Cullen, et al. Standards Track [Page 18]
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specified in different PCP SAs. This field MUST end on a 32-bit
boundary, padded with null characters when necessary.
5.6. 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 |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Code: 7.
Reserved: 8 bits. MUST be set to zero on transmission and MUST be
ignored on reception.
Option-Length: Variable.
EAP Message: The EAP message transferred. Note that this field
MUST end on a 32-bit boundary, padded with zeros when necessary.
5.7. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Code: 8.
Reserved: 8 bits. MUST be set to zero on transmission and MUST be
ignored on reception.
Option-Length: 4 octets.
PRF: The pseudorandom function that the sender supports to
generate an MSK. This field contains a value indicating Internet
Key Exchange Protocol version 2 (IKEv2) Transform Type 2 [RFC7296]
[RFC4868]. A PCP implementation MUST support PRF_HMAC_SHA2_256
(transform ID = 5).
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5.8. MAC_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 ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Code: 9.
Reserved: 8 bits. MUST be set to zero on transmission and MUST be
ignored on reception.
Option-Length: 4 octets.
MAC Algorithm ID: Indicates the MAC algorithm that the sender
supports to generate authentication data. The MAC Algorithm ID
field contains a value indicating IKEv2 Transform Type 3 [RFC7296]
[RFC4868]. A PCP implementation MUST support
AUTH_HMAC_SHA2_256_128 (transform ID = 12).
5.9. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Code: 10.
Reserved: 8 bits. MUST be set to zero on transmission and MUST be
ignored on reception.
Option-Length: 4 octets.
Session Lifetime: An unsigned 32-bit integer, in seconds, ranging
from 0 to 2^32-1 seconds. The lifetime of the PA session, which
is decided by the authorization result.
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5.10. RECEIVED_PAK Option
This option is used in a PA-Acknowledgement message to indicate that
a PA message with the contained sequence number has been received.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Received Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Code: 11.
Reserved: 8 bits. MUST be set to zero on transmission and MUST be
ignored on reception.
Option-Length: 4 octets.
Received Sequence Number: The sequence number of the last received
PA message.
5.11. ID_INDICATOR Option
The ID_INDICATOR option is used by the PCP client to determine which
credentials to provide to the PCP server.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ID Indicator |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Code: 12.
Reserved: 8 bits. MUST be set to zero on transmission and MUST be
ignored on reception.
Option-Length: Variable.
ID Indicator: The identity of the authority that issued the EAP
credentials to be used to authenticate the client. The field
Cullen, et al. Standards Track [Page 21]
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MUST NOT be null terminated, and its length is indicated by the
Option-Length field. In particular, when a client receives an
ID_INDICATOR option, it MUST NOT rely on the presence of a null
character in the wire format data to identify the end of the
ID Indicator field.
The field MUST end on a 32-bit boundary, padded with zeros when
necessary. The ID Indicator field is a UTF-8 encoded [RFC3629]
Unicode string conforming to the UsernameCaseMapped profile of the
PRECIS IdentifierClass [RFC7613]. The PCP client validates that
the ID Indicator field conforms to the UsernameCaseMapped profile
of the PRECIS IdentifierClass. The PCP client enforces the rules
specified in Section 3.2.2 of [RFC7613] to map the ID Indicator
field. The PCP client compares the resulting string with the ID
indicators stored locally on the PCP client to pick the
credentials for authentication. The two indicator strings are to
be considered equivalent by the client if and only if they are an
exact octet-for-octet match.
6. Processing Rules
6.1. Authentication Data Generation
After a successful EAP authentication process, every subsequent
PCP message within the PA session MUST carry an authentication tag
that contains the digest of the PCP message for data origin
authentication and integrity protection.
o Before generating a digest for a PA message, a device needs to
first locate the PCP SA according to the session identifier and
then get the transport key. Then, the device appends a
PA_AUTHENTICATION_TAG option for PCP Auth at the end of the
PCP Auth message. The length of the Authentication Data field is
decided by the MAC algorithm adopted in the session. The device
then fills the Key ID field with the key ID of the transport key
and sets the Authentication Data field to zero. After this, the
device generates a digest for the entire PCP message (including
the PCP header and PA_AUTHENTICATION_TAG option) using the
transport key and the associated MAC algorithm, and inserts the
generated digest into the Authentication Data field.
o Similar to generating a digest for a PA message, before generating
a digest for a common PCP message, a device needs to first locate
the PCP SA according to the session identifier and then get the
transport key. Then, the device appends the AUTHENTICATION_TAG
option at the end of the common PCP message. The length of the
Authentication Data field is decided by the MAC algorithm adopted
in the session. The device then uses the corresponding values
Cullen, et al. Standards Track [Page 22]
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derived from the SA to fill the Session ID field, the Sequence
Number field, and the Key ID field, and sets the Authentication
Data field to zero. After this, the device generates a digest for
the entire PCP message (including the PCP header and
AUTHENTICATION_TAG option) using the transport key and the
associated MAC algorithm, and inserts the generated digest into
the Authentication Data field.
6.2. Authentication Data Validation
When a device receives a common PCP message with an
AUTHENTICATION_TAG option for common PCP messages, the device needs
to use the Session ID transported in the option to locate the proper
SA and then find the associated transport key (using the key ID in
the option) and the MAC algorithm. If no proper SA or transport key
is found or the sequence number is invalid (see Section 6.5), the PCP
device stops processing the PCP message and silently discards the
message. After storing the value of the Authentication field of the
AUTHENTICATION_TAG option, the device fills the Authentication field
with zeros. Then, the device generates a digest for the message
(including the PCP header and AUTHENTICATION_TAG option) with the
transport key and the MAC algorithm. If the value of the newly
generated digest is identical to the stored one, the device can
ensure that the message has not been tampered with, and the
validation succeeds. Otherwise, the PCP device stops processing the
PCP message and silently discards the message.
Similarly, when a device receives a PA message with a
PA_AUTHENTICATION_TAG option for PCP authentication, the device needs
to use the Session ID transported in the Opcode to locate the proper
SA and then find the associated transport key (using the key ID in
the option) and the MAC algorithm. If no proper SA or transport key
is found or the sequence number is invalid (see Section 6.4), the PCP
device stops processing the PCP message and silently discards the
message. After storing the value of the Authentication field of the
PA_AUTHENTICATION_TAG option, the device fills the Authentication
field with zeros. Then, the device generates a digest for the
message (including the PCP header and PA_AUTHENTICATION_TAG option)
with the transport key and the MAC algorithm. If the value of the
newly generated digest is identical to the stored one, the device can
ensure that the message has not been tampered with, and the
validation succeeds. Otherwise, the PCP device stops processing the
PCP message and silently discards the message.
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6.3. Retransmission Policies for PA Messages
Because EAP relies on the underlying protocols to provide reliable
transmission, after sending a PA message, a PCP client/server
MUST NOT send out any subsequent messages until it has received a
PA message with a proper sequence number from the peer. If no such
message is received, the PCP device will resend the last message
according to retransmission policies. This specification uses the
retransmission policies specified in Section 8.1.1 of the base PCP
specification [RFC6887]. In base PCP, such retransmission policies
are only applied by PCP clients. However, in this specification,
such retransmission policies are also applied by the PCP servers. If
the "maximum retransmission" duration (in seconds) has elapsed and no
expected response is received, the device will terminate the session
and discard the current SA.
As discussed in Section 3.1.3, in order to avoid unnecessary
retransmission, the device receiving a PA message MUST send a
PA-Acknowledgement message to the sender of the PA message when it
cannot send a PA response immediately. The PA-Acknowledgement
message is used to indicate the receipt of the PA message. When the
sender receives the PA-Acknowledgement message, it will stop the
retransmission.
Note that the last PA messages transported within the phases of
session initiation, session re-authentication, and session
termination do not have to follow the above policies, since the
devices sending out those messages do not expect any further
PA messages.
When a device receives a retransmitted last incoming PA message from
its session partner, it MUST try to answer it by sending the last
outgoing PA message again. However, if the duplicate message has the
same sequence number but is not bitwise identical to the original
message, then the device MUST discard it. In order to perform this
function, the device may need to maintain the last incoming message
and the associated outgoing messages. In this case, if no outgoing
PA message has been generated for the received duplicate PA message
yet, the device needs to send a PA-Acknowledgement message. The rate
of replying to duplicate PA messages MUST be limited to provide
robustness against denial-of-service (DoS) attacks. The details of
rate limiting are outside the scope of this specification.
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6.4. Sequence Numbers for PCP Auth Messages
PCP uses UDP to transport signaling messages. As an unreliable
transport protocol, UDP does not guarantee ordered packet delivery
and does not provide any protection from packet loss. In order to
ensure that the EAP messages are exchanged in a reliable way, every
PCP message exchanged during EAP authentication must carry a
monotonically increasing sequence number. During a PA session, a PCP
device needs to maintain two sequence numbers for PA messages: one
for incoming PA messages and one for outgoing PA messages. When
generating an outgoing PA message, the device adds the associated
outgoing sequence number to the message and increments the sequence
number maintained in the SA by 1. When receiving a PA message from
its session partner, the device will not accept it if the sequence
number carried in the message does not match the incoming sequence
number maintained in the device. After confirming that the received
message is valid, the device increments the incoming sequence number
maintained in the SA by 1.
The above rules are not applicable to PA-Acknowledgement messages
(i.e., PA messages containing a RECEIVED_PAK option). A
PA-Acknowledgement message does not transport any EAP message and
only indicates that a PA message is received. Therefore, reliable
transmission of PA-Acknowledgement messages is not required. For
instance, after sending out a PA-Acknowledgement message, a device
generates an EAP response. In this case, the device does not have to
confirm whether the PA-Acknowledgement message has been received by
its session partner or not. Therefore, when receiving or sending out
a PA-Acknowledgement message, the device MUST NOT increase the
corresponding sequence number stored in the SA. Otherwise, loss of a
PA-Acknowledgement message will cause a mismatch in sequence numbers.
Another exception is the message retransmission scenario. As
discussed in Section 6.3, when a PCP device does not receive any
response from its session partner, it needs to retransmit the last
outgoing PA message, following the retransmission procedure specified
in Section 8.1.1 of [RFC6887]. The original message and duplicate
messages MUST be bitwise identical. When the device receives such a
duplicate PA message from its session partner, it MUST send the last
outgoing PA message again. In such cases, the maintained incoming
and outgoing sequence numbers will not be affected by the message
retransmission.
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6.5. Sequence Numbers for Common PCP Messages
When transporting common PCP messages within a PA session, a PCP
device needs to maintain a sequence number for outgoing common
PCP messages and a sequence number for incoming common PCP messages.
When generating a new outgoing PCP message, the PCP device updates
the Sequence Number field in the AUTHENTICATION_TAG option with the
outgoing sequence number maintained in the SA and increments the
outgoing sequence number by 1.
When receiving a PCP message from its session partner, the PCP device
will not accept it if the sequence number carried in the message is
smaller than the incoming sequence number maintained in the device.
This approach can protect the PCP device from replay attacks. After
confirming that the received message is valid, the PCP device will
update the incoming sequence number maintained in the PCP SA with the
sequence number of the incoming message.
Note that the sequence number in the incoming message may not exactly
match the incoming sequence number maintained locally. As discussed
in the base PCP specification [RFC6887], if a PCP client is no longer
interested in the PCP transaction and has not yet received a
PCP response from the server, then it will stop retransmitting the
PCP request. After that, the PCP client might generate new
PCP requests for other purposes, using the current SA. In this case,
the sequence number in the new request will be larger than the
sequence number in the old request and so will be larger than the
incoming sequence number maintained in the PCP server.
Note that, as discussed in the base PCP specification [RFC6887], a
PCP client needs to select a nonce in each MAP or PEER request, and
the nonce is sent back in the response. However, it is possible for
a client to use the same nonce in multiple MAP or PEER requests, and
this may cause a potential risk of replay attacks. This attack is
addressed by using the sequence number in the PCP response.
6.6. MTU Considerations
EAP methods are responsible for MTU handling, so no special
facilities are required in PCP to deal with MTU issues.
Specifically, EAP lower layers indicate to EAP methods and
Authentication, Authorization, and Accounting (AAA) servers the MTU
of the lower layer. EAP methods such as EAP-TLS [RFC5216], TEAP
[RFC7170], and others that are likely to exceed reasonable MTUs
provide support for fragmentation and reassembly. Others, such as
EAP - Generalized Pre-Shared Key (EAP-GPSK) [RFC5433], assume that
they will never send packets larger than the MTU and use small EAP
packets.
Cullen, et al. Standards Track [Page 26]
RFC 7652 PCP Authentication September 2015
If an EAP message is too long to be transported within a single
PA message, it will be divided into multiple sections and sent within
different PA messages. Note that the receiver may not be able to
know what to do in the next step until it has received all the
sections and reconstructed the complete EAP message. In this case,
in order to guarantee reliable message transmission, after receiving
a PA message, the receiver replies with a PA-Acknowledgement message
to notify the sender to send the next PA message.
7. IANA Considerations
The following PCP Opcode has been allocated from the Standards Action
range of the "PCP Opcodes" registry (which is maintained in
<http://www.iana.org/assignments/pcp-parameters>):
3 AUTHENTICATION.
The following PCP result codes have been allocated from the Standards
Action range of the "PCP Result Codes" registry (which is maintained
in <http://www.iana.org/assignments/pcp-parameters>):
14 INITIATION: The client includes this PCP result code in its
request to the server for authentication.
15 AUTHENTICATION_REQUIRED: This error response is sent to the
client if EAP authentication is required.
16 AUTHENTICATION_FAILED: This error response is sent to the
client if EAP authentication failed.
17 AUTHENTICATION_SUCCEEDED: This success response is sent to the
client if EAP authentication succeeded.
18 AUTHORIZATION_FAILED: This error response is sent to the client
if EAP authentication succeeded but authorization failed.
19 SESSION_TERMINATED: This PCP result code indicates to the
partner that the PA session must be terminated.
20 UNKNOWN_SESSION_ID: This error response is sent from the
PCP server if there is no known PA session associated with the
Session ID sent in the PA request or common PCP request from the
PCP client.
21 DOWNGRADE_ATTACK_DETECTED: This PCP result code indicates to
the client that the server detected a downgrade attack.
Cullen, et al. Standards Track [Page 27]
RFC 7652 PCP Authentication September 2015
22 AUTHENTICATION_REQUEST: The server indicates to the client that
the PA message contains an EAP request.
23 AUTHENTICATION_REPLY: The client indicates to the server that
the PA message contains an EAP response.
The following PCP options have been allocated from the Standards
Action range (the registry for PCP options is maintained in
<http://www.iana.org/assignments/pcp-parameters>):
7.1. NONCE
Name: NONCE.
Value: 4.
Purpose: See Section 5.3.
Valid for Opcodes: AUTHENTICATION.
Length: 4 octets.
May appear in: Request and response.
Maximum occurrences: 1.
7.2. AUTHENTICATION_TAG
Name: AUTHENTICATION_TAG.
Value: 5.
Purpose: See Section 5.4.
Valid for Opcodes: MAP, PEER, ANNOUNCE.
Length: variable.
May appear in: Request and response.
Maximum occurrences: 1.
Cullen, et al. Standards Track [Page 28]
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7.3. PA_AUTHENTICATION_TAG
Name: PA_AUTHENTICATION_TAG.
Value: 6.
Purpose: See Section 5.5.
Valid for Opcodes: AUTHENTICATION.
Length: variable.
May appear in: Request and response.
Maximum occurrences: 1.
7.4. EAP_PAYLOAD
Name: EAP_PAYLOAD.
Value: 7.
Purpose: See Section 5.6.
Valid for Opcodes: AUTHENTICATION.
Length: variable.
May appear in: Request and response.
Maximum occurrences: 1.
7.5. PRF
Name: PRF.
Value: 8.
Purpose: See Section 5.7.
Valid for Opcodes: AUTHENTICATION.
Length: 4 octets.
May appear in: Request and response.
Maximum occurrences: as many as fit within maximum PCP message size.
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7.6. MAC_ALGORITHM
Name: MAC_ALGORITHM.
Value: 9.
Purpose: See Section 5.8.
Valid for Opcodes: AUTHENTICATION.
Length: 4 octets.
May appear in: Request and response.
Maximum occurrences: as many as fit within maximum PCP message size.
7.7. SESSION_LIFETIME
Name: SESSION_LIFETIME.
Value: 10.
Purpose: See Section 5.9.
Valid for Opcodes: AUTHENTICATION
Length: 4 octets.
May appear in: Response.
Maximum occurrences: 1.
7.8. RECEIVED_PAK
Name: RECEIVED_PAK.
Value: 11.
Purpose: See Section 5.10.
Valid for Opcodes: AUTHENTICATION.
Length: 4 octets.
May appear in: Request and response.
Maximum occurrences: 1.
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7.9. ID_INDICATOR
Name: ID_INDICATOR.
Value: 12.
Purpose: See Section 5.11.
Valid for Opcodes: AUTHENTICATION.
Length: variable.
May appear in: Response.
Maximum occurrences: 1.
8. Security Considerations
As described in this specification, after a successful EAP
authentication process is performed between two PCP devices, an MSK
will be exported. The MSK will be used to derive the transport keys
to generate MAC digests for subsequent PCP message exchanges.
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-Server and
PA-Client message exchange is vulnerable to spoofing attacks, as
these messages are not authenticated and integrity protected. In
addition, because the PRF and MAC algorithms are transported at this
stage, an attacker may try to remove the PRF and MAC options
containing strong algorithms from the initial PA-Server message and
force the client to choose the weakest algorithms. Therefore, the
server needs to guarantee that all the PRF and MAC algorithms for
which it provides support are strong enough.
In order to prevent very basic DoS attacks, a PCP device SHOULD
generate state information as little as possible in the initial
PA-Server and PA-Client message exchanges. The choice of EAP method
is also very important. The selected EAP method must (1) be
resilient to attacks that are possible in an insecure network
environment, (2) provide user-identity confidentiality and protection
against dictionary attacks, and (3) support session-key
establishment.
When a PCP proxy [RFC7648] is located between a PCP server and
PCP clients, the proxy may perform authentication with the PCP server
before it processes requests from the clients. In addition,
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re-authentication between the PCP proxy and PCP server will not
interrupt the service that the proxy provides to the clients, since
the proxy is still allowed to send common PCP messages to the
PCP server during that period.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <http://www.rfc-editor.org/info/rfc3629>.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, Ed., "Extensible Authentication Protocol
(EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,
<http://www.rfc-editor.org/info/rfc3748>.
[RFC4868] Kelly, S. and S. Frankel, "Using HMAC-SHA-256, HMAC-SHA-
384, and HMAC-SHA-512 with IPsec", RFC 4868,
DOI 10.17487/RFC4868, May 2007,
<http://www.rfc-editor.org/info/rfc4868>.
[RFC5281] Funk, P. and S. Blake-Wilson, "Extensible Authentication
Protocol Tunneled Transport Layer Security Authenticated
Protocol Version 0 (EAP-TTLSv0)", RFC 5281,
DOI 10.17487/RFC5281, August 2008,
<http://www.rfc-editor.org/info/rfc5281>.
[RFC6887] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,
DOI 10.17487/RFC6887, April 2013,
<http://www.rfc-editor.org/info/rfc6887>.
[RFC7170] Zhou, H., Cam-Winget, N., Salowey, J., and S. Hanna,
"Tunnel Extensible Authentication Protocol (TEAP) Version
1", RFC 7170, DOI 10.17487/RFC7170, May 2014,
<http://www.rfc-editor.org/info/rfc7170>.
[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
2014, <http://www.rfc-editor.org/info/rfc7296>.
Cullen, et al. Standards Track [Page 32]
RFC 7652 PCP Authentication September 2015
[RFC7613] Saint-Andre, P. and A. Melnikov, "Preparation,
Enforcement, and Comparison of Internationalized Strings
Representing Usernames and Passwords", RFC 7613,
DOI 10.17487/RFC7613, August 2015,
<http://www.rfc-editor.org/info/rfc7613>.
[RFC7648] Perreault, S., Boucadair, M., Penno, R., Wing, D., and S.
Cheshire, "Port Control Protocol (PCP) Proxy Function",
RFC 7648, DOI 10.17487/RFC7648, September 2015,
<http://www.rfc-editor.org/info/rfc7648>.
9.2. Informative References
[RFC5216] Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
Authentication Protocol", RFC 5216, DOI 10.17487/RFC5216,
March 2008, <http://www.rfc-editor.org/info/rfc5216>.
[RFC5433] Clancy, T. and H. Tschofenig, "Extensible Authentication
Protocol - Generalized Pre-Shared Key (EAP-GPSK) Method",
RFC 5433, DOI 10.17487/RFC5433, February 2009,
<http://www.rfc-editor.org/info/rfc5433>.
[RFC5448] Arkko, J., Lehtovirta, V., and P. Eronen, "Improved
Extensible Authentication Protocol Method for 3rd
Generation Authentication and Key Agreement (EAP-AKA')",
RFC 5448, DOI 10.17487/RFC5448, May 2009,
<http://www.rfc-editor.org/info/rfc5448>.
Acknowledgements
Thanks to Dan Wing, Prashanth Patil, Dave Thaler, Peter Saint-Andre,
Carlos Pignataro, Brian Haberman, Paul Kyzivat, Jouni Korhonen,
Stephen Farrell, and Terry Manderson for their valuable comments.
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RFC 7652 PCP Authentication September 2015
Authors' Addresses
Margaret Cullen
Painless Security
356 Abbott Street
North Andover, MA 01845
United States
Phone: +1 781 405 7464
Email: margaret@painless-security.com
URI: http://www.painless-security.com
Sam Hartman
Painless Security
356 Abbott Street
North Andover, MA 01845
United States
Email: hartmans@painless-security.com
URI: http://www.painless-security.com
Dacheng Zhang
Beijing, China
China
Email: zhang_dacheng@hotmail.com
Tirumaleswar Reddy
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: tireddy@cisco.com
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