| Network Working Group | M. Wasserman |
| Internet-Draft | S. Hartman |
| Intended status: Standards Track | Painless Security |
| Expires: December 17, 2015 | D. Zhang |
| Huawei | |
| T. Reddy | |
| Cisco | |
| June 15, 2015 |
Port Control Protocol (PCP) Authentication Mechanism
draft-ietf-pcp-authentication-11
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 communication 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 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.
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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 December 17, 2015.
Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved.
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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 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 defines a PCP security extension that enables PCP servers to authenticate their clients with Extensible Authentication Protocol (EAP). The EAP messages are encapsulated within PCP messages during transportation.
The following issues are considered in the design of this extension:
[RFC6887]. This mechanism can be used to secure PCP in the following situations:
The mechanism described in this document meets the security requirements to address the Advanced Threat Model described in the base PCP specification
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 a 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 involved within a session includes the PCP Authentication (PA) messages used to perform EAP authentication, key distribution and session management, and 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 within a PA session. Each PA session has two session partners (a PCP server and a PCP client).
Session Lifetime: The lifetime associated with a PA session, which decides the lifetime of the current authorization given to the PCP client.
PCP Security Association (PCP SA): A PCP security association is 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 one defined in [RFC5448]).
PCP-Authentication (PA) message: A PCP message containing an Authentication Opcode. Particularly, a PA message sent from a PCP server to a PCP client is referred to as a PA-Server, while a PA message sent from a PCP client to a PCP server is referred to as a PA-Client. Therefore, a PA-Server is actually a PCP response message specified in [RFC6887], and a PA-Client is a PCP request message. This document specifies an option, the Authentication Tag Option defined in Section 5.4 for PCP authentication, to provide integrity protection and message origin authentication for PA messages.
Common PCP message: A PCP message which 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.
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 is attached with an Authentication Opcode and may optionally contain a set of Options for various purposes (e.g., transporting authentication messages and session management). The 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.
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 to have a result code. In the message, the Session ID and Sequence Number fields of the Authentication Opcode are set as 0. The PCP client SHOULD also append a nonce option defined in Section 5.3 which consists of a random nonce with the message.
After receiving the PA-Initiation, if the PCP server agrees to initiate a PA session with the PCP client, it will reply with a PA-Server message which contains an EAP Identity Request, and the result code field of this PA-Server message is set to AUTHENTICATION-REQUIRED. In addition, the server MUST assign a random session identifier to distinctly identify this session, and fill the identifier into the Session ID field of the Authentication Opcode in the PA-Server message. The Sequence Number field of the Authentication Opcode is set as 0. If there is a nonce option in the received PA-Initiation message, the PA-Server message MUST be attached with 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. From now on, every PCP message within this session will be attached with this session identifier. When receiving a PA message from an unknown session, a PCP device MUST discard the message silently. If the PCP client intends to simplify the authentication process, it MAY append an EAP Identity Response message within the PA-Initiation message so as to inform the PCP server that it would like to perform EAP authentication and skip the step of waiting for the EAP Identity Request.
PCP PCP
client server
|-- PA-Initiation-------------------------------->|
| (Seq=0, Session ID=0) |
| |
|<-- PA-Server -----------------------------------|
| (Seq=0, Session ID=X, EAP request) |
| |
|-- PA-Client ----------------------------------->|
| (Seq=1, Session ID=X, EAP response) |
| |
|<-- PA-Server -----------------------------------|
| (Seq=1, Session ID=X, EAP request) |
In the scenario where a PCP server receives a common PCP request message from a PCP client which needs to be authenticated, the PCP server can reply with a PA-Server message to initiate a PA session. The result code field of this PA-Server message is set to AUTHENTICATION-REQUIRED. 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 is set as 0. In the PA messages exchanged afterwards in this session, the Session ID will be used in order to help session partners distinguish the messages within this session from those not within. 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 which 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.
PCP PCP
client server
|-- Common PCP request--------------------------->|
| |
|<-- PA-Server -----------------------------------|
| (Seq=0, Session ID=X, EAP request) |
| |
|-- PA-Client ----------------------------------->|
| (Seq=0, Session ID=X, EAP response) |
| |
|<-- PA-Server -----------------------------------|
| (Seq=1, Session ID=X, EAP request) |
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 transportation must be detected and addressed. Therefore, after sending out a PA-Server message, the PCP server will not send a new PA-Server message until it receives a PA-Client message with a proper sequence number from the PCP client, and vice versa. If a PCP device receives a PA message from its partner and cannot generate an EAP response immediately due to certain reasons (e.g., waiting for human input to construct a EAP message or waiting for the additional PA messages in order to construct a complete EAP message), the PCP device MUST reply with a PA-Acknowledgement message (PA message with a Received Packet 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 the reliable message delivery in the conditions where a PCP device needs to receive multiple PA messages before generating an EAP response.
In this approach, it is mandated for a PCP client and a PCP server to perform a key-generating EAP method in authentication. Particularly, a PCP authentication implementation MUST support EAP-TTLS [RFC5281] and SHOULD support 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 Pseudo-Random Function (PRF) for the transport key derivation and a 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 (Message Authentication Code) 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 selects the PRF and the MAC algorithm which it would like to use, and then adds 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 generate a PCP SA and use the derived transport key to generate a digest for the message. The digest is transported within an 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 MAY also contain a Session Lifetime Option defined in Section 5.9 which 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 as response. If the PCP client also authenticates the PCP server, the result code of the PA-Client is AUTHENTICATION-SUCCEEDED. In addition, the PCP client needs to generate a PCP SA and 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, unless a re-authentication is performed. The first secure PA-client response 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 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.
A PA session can be explicitly terminated by sending a termination-indicating PA message (a PA message with a result code "SESSION-TERMINATED" ) from either session partner. After receiving a Termination-Indicating message from the session partner, a PCP device MUST respond with a Termination-Indicating PA message and remove the PA SA immediately. When the session partner initiating the termination process receives the PA message, it will remove the associated PA SA immediately.
A session partner may select to perform EAP re-authentication if it would like to update the PCP SA without initiating a new PA session. A re-authentication procedure could be triggered for the following reasons:Section 6.3.
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 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 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 according to
If the EAP re-authentication succeeds, the result code of the last PA-Server 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 and subsequent PCP messages. In addition, the PA-Server message MAY be appended with a Session Lifetime Option which 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 is "AUTHENTICATION-FAILED". If the EAP authentication succeeds but authorization fails, the result code of the last PA-Server is "AUTHORIZATION-FAILED". In the latter two cases, the PA session MUST be terminated immediately after the last PA message exchange.
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.
At the beginning of a PA session, a session SHOULD generate a PA SA to maintain its state information during the session. The parameters of a PA SA are listed as follows:
Particularly, the transport key is computed in the following way: Transport key = prf(MSK, "IETF PCP" || Session ID || Nonce || key ID), where:
The format of the PA-Server message is identical to the response message format specified in Section 7.2 of [RFC6887].
As illustrated in Figure 1, the PA-Client messages use the request header specified in Section 7.1 of[RFC6887]. The only difference is that eight reserved bits are used to transfer the result codes (e.g., "INITIATION", "AUTHENTICATION-FAILED"). Other fields in Figure 1 are described in Section 7.1 of [RFC6887].
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 1. PA-Client message Format
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Because the session identifier of a PA session is determined by the PCP server, a PCP client does not know the session identifier which will be used when it sends out a PA-Initiation message. In order to prevent an attacker from interrupting the authentication process by sending off-line generated 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 will be discarded silently.
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.
This option is used to provide message authentication for PA messages. Compared with 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 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) |
~ ~
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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: TBA-134.
Reserved: 8 bits. MUST be set to 0 on transmission and MUST be ignored on reception.
Option-Length: 4 octets.
PRF: The Pseudo-Random Function which the sender supports to generate an MSK. This field contains an IKEv2 Transform ID of Transform Type 2 [RFC7296][RFC4868]. A PCP implementation MUST support PRF_HMAC_SHA2_256 (5).
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: TBA-135.
Reserved: 8 bits. MUST be set to 0 on transmission and MUST be ignored on reception.
Option-Length: 4 octets.
MAC Algorithm ID: Indicate the MAC algorithm which the sender supports to generate authentication data. The MAC Algorithm ID field contains an IKEv2 Transform ID of Transform Type 3 [RFC7296][RFC4868]. A PCP implementation MUST support AUTH_HMAC_SHA2_256_128 (12).
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: TBA-136.
Reserved: 8 bits. MUST be set to 0 on transmission and MUST be ignored on reception.
Option-Length: 4 octets.
Session Lifetime: The lifetime of the PA Session, which is decided by the authorization result.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This option is used in a PA-Acknowledgement message to indicate that a message with the contained sequence number has been received.
Option Code: TBA-137.
Reserved: 8 bits. MUST be set to 0 on transmission and MUST be ignored on reception.
Option-Length: 4 octets.
Received Sequence Number: The sequence number of the last received PCP message.
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 |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If a PCP SA is generated as the result of a successful EAP authentication process, every subsequent PCP message within the session MUST carry an Authentication Tag Option which contains the digest of the PCP message for data origin authentication and integrity protection.
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 message MUST be discarded silently. 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 message MUST be discarded.
Similarly, when a device receives a PA message with an 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 message MUST be discarded silently. 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 message MUST be discarded.
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 receiving a PA message with a proper sequence number from the peer. If no such a message is received the PCP device will re-send the last message according to retransmission policies. This work reuses the retransmission policies specified in the base PCP protocol (Section 8.1.1 of [RFC6887]). In the base PCP protocol, such retransmission policies are only applied by PCP clients. However, in this work, such retransmission policies are also applied by the PCP servers. If Maximum retransmission duration seconds have elapsed and no expected response is received, the device will terminate the session and discard the current SA.
As illustrated in Section 3.1.3, in order to avoid unnecessary re-transmission, 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 re-transmitted 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 bit-wise identical to the original message then the device MUST discard it. In order to achieve this function, the device may need to maintain the last incoming 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.
PCP uses UDP to transport signaling messages. As an un-reliable transport protocol, UDP does not guarantee 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 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 the device maintains. 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 Packet 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 need 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 bit-wise 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.
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 the device maintains. 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 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.
EAP methods are responsible for MTU handling, so no special facilities are required in PCP to deal with MTU issues. Particularly, EAP lower layers indicate to EAP methods and 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-GPSK [RFC5433] assume they will never send packets larger than the MTU and use small EAP packets.
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.
The following PCP Opcode is to be allocated in the mandatory-to-process range from the standards action range (the registry for PCP Opcodes is maintained in http://www.iana.org/assignments/pcp-parameters):
TBA Authentication Opcode.
The following PCP Option Codes are to be allocated in the mandatory-to-process range from the standards action range (the registry for PCP Options is maintained in http://www.iana.org/assignments/pcp-parameters):
The following PCP result codes are to be allocated in the mandatory-to-process range from the standards action range (the registry for PCP result codes is maintained in http://www.iana.org/assignments/pcp-parameters):
In this work, 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 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 choose the weakest algorithms. Therefore, the server needs to guarantee that all the PRF and MAC algorithms it provides support for 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 exchanges. The choice of EAP method is also very important. The selected EAP method must be resilient to the attacks possible in an insecure network environment, provide user-identity confidentiality, protection against dictionary attacks, and support session-key establishment.
When a PCP proxy 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, 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.
Thanks to Dan Wing, Prashanth Patil, Dave Thaler, Peter Saint-Andre, Carlos Pignataro and Brian Haberman for the valuable comments.
[Note: This section should be removed by the RFC Editor upon publication]
| [RFC5216] | Simon, D., Aboba, B. and R. Hurst, "The EAP-TLS Authentication Protocol", RFC 5216, March 2008. |
| [RFC5433] | Clancy, T. and H. Tschofenig, "Extensible Authentication Protocol - Generalized Pre-Shared Key (EAP-GPSK) Method", RFC 5433, February 2009. |
| [RFC5448] | Arkko, J., Lehtovirta, V. and P. Eronen, "Improved Extensible Authentication Protocol Method for 3rd Generation Authentication and Key Agreement (EAP-AKA')", RFC 5448, May 2009. |