Internet DRAFT - draft-wasserman-pcp-authentication

draft-wasserman-pcp-authentication






Network Working Group                                       M. Wasserman
Internet-Draft                                                S. Hartman
Intended status: Experimental                          Painless Security
Expires: September 12, 2012                                     D. Zhang
                                                                  Huawei
                                                          March 11, 2012


          Port Control Protocol (PCP) Authentication Mechanism
                 draft-wasserman-pcp-authentication-02

Abstract

   An IPv4 or IPv6 host can use the Port Control Protocol (PCP) to
   flexibly manage the IP address and port mapping information on
   Network Address Translators (NATs) or firewalls, to facilitate
   communications with remote hosts.  However, the un-controlled
   generation or deletion of IP address mappings on such network devices
   may cause security risks and should be avoided.  In some cases the
   client may need to prove that it is authorized to modify, create or
   delete PCP mappings.  This document proposes an in-band
   authentication mechanism for PCP that can be used in those cases.
   The Extensible Authentication Protocol (EAP) is used to perform
   authentication between PCP devices.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 12, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal



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   Provisions Relating to IETF Documents
   (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.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Protocol Details . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Session Initiation . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Session Termination  . . . . . . . . . . . . . . . . . . .  7
     3.3.  Result Codes . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  PA Security Association  . . . . . . . . . . . . . . . . . . .  7
   5.  Packet Format  . . . . . . . . . . . . . . . . . . . . . . . .  8
     5.1.  Authentication OpCode Format . . . . . . . . . . . . . . .  8
     5.2.  Nonce Option . . . . . . . . . . . . . . . . . . . . . . .  9
     5.3.  Authentication Tag Option  . . . . . . . . . . . . . . . . 10
     5.4.  EAP Payload Option . . . . . . . . . . . . . . . . . . . . 11
     5.5.  PRF Option . . . . . . . . . . . . . . . . . . . . . . . . 11
     5.6.  Hash Algorithm Option  . . . . . . . . . . . . . . . . . . 11
     5.7.  Session Lifetime Option  . . . . . . . . . . . . . . . . . 12
   6.  Processing Rules . . . . . . . . . . . . . . . . . . . . . . . 12
     6.1.  Authentication Data Generation . . . . . . . . . . . . . . 12
     6.2.  Authentication Data Validation . . . . . . . . . . . . . . 12
     6.3.  Sequence Number  . . . . . . . . . . . . . . . . . . . . . 13
     6.4.  Retransmission Policies  . . . . . . . . . . . . . . . . . 13
     6.5.  MTU Considerations . . . . . . . . . . . . . . . . . . . . 14
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   10. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     10.1. Changes from -00 to -01  . . . . . . . . . . . . . . . . . 16
     10.2. Changes from -01 to -02  . . . . . . . . . . . . . . . . . 16
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     11.2. Informative References . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17








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1.  Introduction

   Using the Port Control Protocol (PCP) [I-D.ietf-pcp-base], an IPv4 or
   IPv6 host can flexibly manage the IP address mapping information on
   its network address translators (NATs) and firewalls, and control
   their policies in processing incoming and outgoing IP packets.
   Because NATs and firewalls both play important roles in network
   security architectures, there are many situations in which
   authentication and access control are required to prevent un-
   authorized users from accessing such devices.  This document proposes
   a PCP security extension which enables PCP servers to authenticate
   the clients that they are communicating with using Extensible
   Authentication Protocol (EAP).  The following issues are considered
   in the design of this extension:

   o  Loss of EAP messages during transportation

   o  Disordered delivery of EAP messages

   o  Generation of transport keys

   o  Integrity protection and data origin authentication for PCP
      messages

   o  Algorithm agility

   The mechanism described in this document meets the security
   requirements to address the Advanced Threat Model described in the
   base PCP specification [I-D.ietf-pcp-base].  This mechanism can be
   used to secure PCP in the following situations::

   o  On security infrastructure equipment, such as corporate firewalls,
      that does not create implicit mappings.

   o  On equipment (such as CGNs or service provider firewalls) that
      serve multiple administrative domains and do not have a mechanism
      to securely partition traffic from those domains.

   o  For any implementation that wants to be more permissive in
      authorizing explicit mappings than it is in authorizing implicit
      mappings.

   o  For implementations that support the THIRD_PARTY Option (unless
      they can meet the constraints outlined in Section 14.1.2.2).

   o  For implementations that wish to support any deployment scenario
      that does not meet the constraints described in Section 14.1.




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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   Most of the terms used in this document are introduced in
   [I-D.ietf-pcp-base].

   PCP Client (PCC): A PCP device (e.g., a host) which is responsible
   for issuing PCP requests to a PCP server.  In this document, a PCC is
   also a EAP peer [RFC3748], and it is the responsibility of a PCC to
   provide the credentials when authentication is required.

   PCP Server (PCS): A PCP device (e.g., a NAT or a firewall) that
   implements the server-side of the PCP protocol, via which PCCs
   request and manage explicit mappings.  In this document, a PCS is
   integrated with an EAP authenticator [RFC3748].  Therefore, when
   necessary, a PCS can verify the credentials provided by a PCC and
   make an access control decision based on the authentication result.

   PCP Authentication (PA) Session: A series of PCP message exchanges
   transferred between a PCC and a PCS in order to perform
   authentication, authorization, key distribution and secured PCP
   communication.  Each PA session is assigned a distinctive Session ID.
   The PCP devices involved within a PA session are called session
   partners.  A typical PA session has two session partners.

   Session Lifetime: The life period associated with a PA session, which
   decided the lifetime of the current authorization given to the PCC.

   PCP Security Association (PCP SA): A PCP security association is
   formed between a PCC and a PCS by sharing cryptographic keying
   material and associated context.  The formed duplex security
   association is used to protect the bidirectional PCP signaling
   traffic between the PCC and PCS.

   Master Session Key (MSK): A key derived by the partners of a PA
   session, using a EAP key generating method specified in [RFC3748].

   PA (PCP for Authentication) message: A PCP message containing an
   Authentication OpCode for EAP authentication.


3.  Protocol Details






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3.1.  Session Initiation

   To carry out an EAP authentication process between two PCP devices, a
   set of PA messages need to be exchanged.  Each PA message contains an
   Authentication OpCode (and additional Options if needed).  The
   Authentication OpCode consists of four fields: Session ID, Flag, EAP
   Type, and Sequence Number.  The Session ID field is used to identify
   the session which the message belongs to.  The Flag field indicates
   the type of the PCP message, while EAP Type is used to identify the
   type of the attached EAP message.  The sequence number field is used
   to detect the disorder or the duplication occurred during packet
   delivery.

   The message exchanges conveyed within an PA session is introduced in
   the remainder section.

   When a PCC intends to initiate a PA session with a PCS, it sends a
   PCC-Initiation message to the PCS.  The Session ID and Sequence
   Number fields of the Authentication OpCode in the PCC-Initiation
   message are set as 0, and the I bit is set. the PCC also needs to
   select a random nonce and append it with the PCC-Initiation message
   in order to deal with off-line attacks.  Specifically, the nonce is
   transported within a nonce option.  After receiving the PCC-
   Initiation, if the PCS would like to initiate a PA session, it will
   reply with a PA-Request which contains an EAP Identity Request.  The
   Sequence Number field in the PA-Request is set as 0, and the Session
   ID field MUST be filled with the session identifier assigned by the
   PCS for this session. the PA-Request also needs to be attached with
   the nonce.  Form now on, every PA message within this session must be
   attached with the session identifier.  Otherwise, the session partner
   receiving the message will discard the message silently.  If the PCC
   intends to simplify the authentication process, it can append an EAP
   Identity Response message within the PCC-Initiation request so as to
   skip over the step of waiting for the EAP Identity Request and inform
   the PCS that it would like to perform EAP authentication.

   In the scenario where a PCS receives a non-PA PCP message from a PCC
   which needs to be authenticated, the PCS can reply with a PA-Request
   to initiate a PA session; the result code field of the PA-Request is
   set as AUTHENTICATION-REQUIRED.  In addition, the PCS MUST assign a
   session ID for the session and transfer it within the PA-Request.  In
   the PA messages exchanged afterwards in this session, the session ID
   MUST be appended.  Therefore, in the subsequent communication, the
   PCC can distinguish the messages in this session from those in other
   sessions through the PCS IP address and the session ID.  When the PCC
   receives the initial PA-Request message from the PCS, it can reply
   with a PA-Answer message to continue the session or silently discards
   the request message according to its local policies.



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   In a PA session, PA-Request messages are sent from PCSs to PCCs while
   PA-Answer messages are only sent from PCCs to PCSs.  Correspondently,
   an EAP request messages MUST be transported within a PA-Request
   message, and an EAP answer messages MUST be transported within a PA-
   Answer message.  Particularly, when a PCP device receives a PA-
   Request or a PA-Answer message from its partner, the PCP device needs
   to reply with a PA-Acknowledge message to indicate that the message
   has been received.  This solution is used to deal with the conditions
   where the device cannot generate a response within a pre-specified
   period due to certain reasons (e.g., waiting for human input to
   construct a EAP message).  Therefore, the partner does not have to
   un-necessarily retransfer the PCP message.

   In this work, it is mandated for a PCC and a PCS to perform a key-
   generating EAP method in authentication, and so a successful EAP
   authentication process will result in a Master Session Key (MSK).  If
   the PCC and the PCS want to generate a traffic key using the MSK,
   they need to agree upon a Pseudo-Random Function (PRF) for the
   transport key derivation and a MAC algorithm to provide data origin
   authentication for subsequent PCP packets.  On this occasion, the PCS
   needs to append the initial PA-Request message with a set of PRF
   Options and MAC Algorithm Options.  Each PRF Option (MAC Algorithm
   Option) contains a PRF (MAC (Message Authentication Code) algorithm)
   that the PCS supports.  After receiving the request, the PCC selects
   a PRF and a MAC algorithm which it intends to support, and sends back
   a PA-Answer with a PRF Option and a MAC Algorithm Option for the
   selected algorithms.

   The last PA-Request message transported within a PA session carries
   the EAP authentication and PCP authorization results.  The last PA-
   Request and PA-Answer messages MUST have their the 'C' (Complete) bit
   set.

   If the EAP authentication successes, the result code of the last PA-
   Request is AUTHENTICATION-SUCCESS.  In this case, before sending out
   the PA-Request, the PCS must derive a transport key and use it to
   generate digests to protect the integrity and authenticity of the PA-
   Request and any subsequent PCP message.  Such digests are transported
   within Authentication Tag Options.  In addition, the PA-Request needs
   to be appended with a Session Lifetime Option which indicates the
   life time of the PA session (i.e., the life time of the MSK).

   If the EAP authentication fails, the result code of the last PA-
   Request is AUTHENTICATION-FAILED.  If the EAP authentication
   successes but Authorization fails, the result code of the last PA-
   Request is AUTHORIZATION-FAILED.  In the latter two cases, the PA
   session MUST be terminated immediately after the last PCP
   authentication message exchange.



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3.2.  Session Termination

   A PA session can be explicitly terminated by sending a termination-
   indicating PA acknowledge message from either session partner.  After
   receiving a termination-indicating message from the session partner,
   a PCP device MUST response with a termination-indicating PA
   Acknowledge message and remove the PA SA immediately.  When the
   session partner initiating the termination process receives the
   acknowledge message, it will remove the associated PA SA immediately.

3.3.  Result Codes

   Following result codes are defined in the solution:

      XXX AUTHENTICATION-REQUIRED

      XXX AUTHENTICATION-FAILED

      XXX AUTHENTICATION-SUCCESS

      XXX AUTHORIZATION-FAILED


4.  PA Security Association

   At the beginning a PA session, a session SHOULD generate a PA SA to
   maintain its state information during the session.  A The parameters
   of a PA SA is listed as follows:

   o  IP address and UDP port number of the PCC

   o  IP address and UDP port number of the PCS

   o  Session Identifier

   o  Sequence number for the next outgoing PCP message

   o  Sequence number for the next incoming PCP message

   o  Last outgoing message payload

   o  Retransmission interval

   o  MSK

   o  MAC algorithm: The algorithm that the transport key should use to
      generate digests for PCP messages.




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   o  Pseudo-random function: The pseudo random function negotiated in
      the initial PA-Request and PA-Answer exchange for the transport
      key derivation

   o  Transport key: the key derived from the MSK to provide integrity
      protection and data origin authentication for the messages in the
      PA session.  The life time of the transport key SHOULD be
      identical to the life time of the session.

   Particularly, the transport key is computed in the following way:
   Transport key = prf(MSK, "IETF PCP"| Session_ID, key ID), where:

   o  The prf: The pseudo-random function assigned in the Pseudo-random
      function parameter.

   o  MSK: The master session key generated by the EAP method.

   o  "IETF PCP": The ASCII code representation of the non-NULL
      terminated string (excluding the double quotes around it).

   o  Session_ID: The ID of the session which the MSK is derived from

   o  Key ID: The ID assigned for the traffic key


5.  Packet Format

5.1.  Authentication OpCode Format

   The following figure illustrates the format of an authentication
   Opcode:
         0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |             Flags             |     EAP Message Type          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Session ID                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Sequence Number                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



      Flags: The Flags field is two octets.  The following bits are
      assigned:






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          0                   1
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |I C R A K T S E r r r r r r r r|
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




      *  I (Initiation): This bit is set in a PCC-Initiation message.

      *  C (Complete): If the message is the last PA-Request or PA-
         Answer message in the session, this bit MUST be set.  For other
         messages, this bit MUST be cleared.

      *  R (Request): This bit is set in a PA-Request message.

      *  A (Answer): This bit is set in a PA-Answer message.

      *  K (acKnowledgement): This bit is set and only set in a PA-
         Acknowledgement message.

      *  T (Termination): If this bit is set in a PA-Acknowledgement
         message, the message is used for session-termination
         indication.

      *  S (Fragmentation start): This bit is set in a PA message which
         contain the first fragment of a EAP message.

      *  E (Fragmentation start): This bit is set in a PA message which
         contain the last fragment of a EAP message.

      Message Type: The Message Type field is two octets.  This field is
      used to indicate the type of the EAP message attached within the
      message.  Message Type allocation is managed by IANA [IANAWEB].

      Session ID: This field contains a 32-bit PA session identifier.

      Sequence Number: This field contains a 32-bit sequence number.  In
      this solution, a sequence number needs to be incremented on every
      new (non-retransmission) outgoing packet in order to provide
      ordering guarantee for PCP.

5.2.  Nonce Option







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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Option Code  |  Reserved     |       Option-Length           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Nonce                                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      Nonce: A random 32 bits number which is transported within a PCC-
      Initiate message and the correspondent reply message from the PCS.

5.3.  Authentication Tag Option



        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Option Code  |  Reserved     |       Option-Length           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Session ID                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Key ID                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                Authentication Data (Variable)                 |
       ~                                                               ~
       |                                                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      Option-Length: The length of the Authentication Tag Option (in
      octet), including the 8 octet fixed header and the variable length
      of the authentication data.

      Session ID: A 32-bit field used to indicates the identifier of the
      session that the message belongs to and identifies the secret key
      used to create the message digest appended to the PCP message.

      Key ID: The ID associated with the traffic key used to generate
      authentication data.  This field is filled with zero if MSK is
      directly used to secure the message.

      Authentication Data: A Variable length field that carries the
      Message Authentication Code for the PCP packet.  The generation of
      the digest can be various according to the algorithms specified in



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      different PCP SAs.

5.4.  EAP Payload Option



        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Option Code  |  Reserved     |       Option-Length           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                           EAP Message                         |
       ~                                                               ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      EAP Message: The EAP message transferred.  Note this field MUST
      end on a 32-bit boundary, padded with 0's when necessary.

5.5.  PRF Option


        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Option Code  |  Reserved     |       Option-Length           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          PRF                                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      PRF: The pseudo-random Function which the sender supports to
      generate a MSK.

5.6.  Hash Algorithm Option

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Option Code  |  Reserved     |       Option-Length           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                    MAC Algorithm                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   MAC Algorithm: The MAC algorithm which the sender supports to
   generate authentication data.



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5.7.  Session Lifetime Option

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Option Code  |  Reserved     |       Option-Length           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Session Lifetime                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Session Lifetime: The life period of the PA Session, which is decided
   by the authorization result.


6.  Processing Rules

6.1.  Authentication Data Generation

   If a PCP SA is generated as the result of an successful EAP
   authentication process, every subsequent PCP message within the
   session needs carry an Authentication Tag Option which contains the
   digest of the PCP message for data orgin authentication and integrity
   protection.

   Before generating a digest for a PCP message, a device needs to first
   select a traffic key in the session and append the Authentication Tag
   Option at the end of the protected PCP message.  The length of the
   Authentication Data field is decided by the MAC algorithm adopted in
   the session.  The device then fills the Session ID field and the PCP
   SA ID field, and sets the Authentication Data field as 0.  After
   this, the device generates a digest for the PCP message with the MAC
   algorithm and the selected traffic key, and input the generated
   digest into the Authentication Data field.

6.2.  Authentication Data Validation

   When a device receives a PCP pacekt with an Authentication Tag
   Option, it needs to use the session ID transported in the option to
   locate the proper SA, and then find out the associated transport key
   and the MAC algorithm.  After storing the value of the Authentication
   field of the Authentication Tag Option, the device fills the the
   Authentication field with zeros.  Then, the device generates a digest
   for the packet with the transport key and the MAC algorithm found in
   the first step.  If the value of the newly generated digest is
   identical to the stored one, the device can ensure that the packet
   has not been tampered during the transportation.  The validation
   successes.  Otherwise, the packet MUST be discarded.





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6.3.  Sequence Number

   PCP adopts UDP to transport signaling messages.  As an un-reliable
   transporting protocol, UDP does not guarantee the ordered packet
   delivery and does not provide any protection from packet loss.  In
   order to ensure the EAP messages are exchanged in a reliable way,
   every PCP packet exchanged during EAP authentication must carries an
   monotonically increased sequence number.  During a PA session, a PCP
   device needs to maintain two sequence numbers, one for incoming
   packets and one for outgoing packets.  When generating an outgoing
   PCP packet, the device attaches the outgoing sequence number to the
   packet and increments the sequence number by 1.  When receiving a PCP
   packet from its session partner, the device will not accept it if the
   sequence number carried in the packet does not matche the incoming
   sequence number the device maintains.

   After confirming that the received packet is valid, the device
   increments the incoming sequence number by 1.  However, the above
   rules are not applied to PA-Acknowledgement messages.  When receiving
   or sending out a PA-Acknowledgement message, the device does not
   inicrease the correspondent sequence number.  Another exception is
   message retransmission.  When a device does not receive any response
   message from its session partner in a certain period, it needs to
   retransmit the last sent message with a limited rate.  The duplicate
   messages and the original message MUST use the identical sequence
   number.  When the device receives such duplicate messages from its
   session partner, it MUST tries to answer them by sending the last
   outgoing message with a limited rate unless it has received another
   valid message with a larger sequence number from its session.  Note
   that in these cases the incoming and outgoing sequence number will
   not be affected by the message retransmission.

6.4.  Retransmission Policies

   This work provides a retransmission mechanism for reliable PA message
   delivery.  The timer, the variables, and the rules used in this
   mechanism is mostly brought from PANA[RFC5191].

   The retransmission behavior is controlled and described by the
   following variables:

      RT: Retransmission timeout from the previous (re)transmission

      IRT: Base value for RT for the initial retransmission

      MRC: Maximum retransmission count





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      MRT: Maximum retransmiting time interval

      RAND: Randomization factor

   With each message transmission or retransmission, the sender sets RT
   according to the rules given below.

   If RT expires before receiving any reply, the sender re-calculates RT
   and retransmits the message.  Each of the computations of a new RT
   include a randomization factor (RAND), which is a random number
   chosen with a uniform distribution between -0.1 and +0.1.  The
   randomization factor is included to minimize the synchronization of
   messages.  The algorithm for choosing a random number does not need
   to be cryptographically sound.  The algorithm SHOULD produce a
   different sequence of random numbers from each invocation.  RT for
   the first message retransmission is based on IRT:

   RT = IRT

   RT for each subsequent message retransmission is based on the
   previous value of RT (RTprev):

   RT = (2+RAND) * RTprev

   MRT specifies an upper bound on the value of RT (disregarding the
   randomization added by the use of RAND).  If MRT has a value of 0,
   there is no upper limit on the value of RT.  Otherwise:

   if (RT > MRT)

   RT = (1+RAND) * MRT

   MRC specifies an upper bound on the number of times a sender may
   retransmit a message.  Unless MRC is zero, the message exchange fails
   once the sender has transmitted the message MRC times.  In this case,
   the sender needs to start a session termination process illustrated
   in Section 3.2.

6.5.  MTU Considerations

   The fragmentation and reassembly of EAP messages must be provided in
   order to ensure the length of a PA message is not larger than the MTU
   of the link that it will be transported through.  Therefore, a PA
   message may only transport a fragment of an EAP message.  Becasue any
   loss or tamper of a EAP frgament will be detected and sequencing
   information is provided, fragmentation support can be added in a
   simple manner.  Particularly, the S bit is set in a PA message which
   contain the first fragment of a EAP message, and the The E bit is set



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   in a PA message which contain the last fragment of a EAP message.


7.  IANA Considerations

   TBD


8.  Security Considerations

   In this work, a successful EAP authentication process performed
   between two PCP devices will result in the generation of a MSK which
   can be used to derive the transport keys to generate MAC digests for
   subsequent PCP message exchanges.  This work does not exclude the
   possibility of using the MSK to generate keys for different security
   protocols to enable per-packet cryptographic protection.  The methods
   of deriving the transport key for the security protocols is out of
   scope of this document.

   However, before a transport key has been generated, the PA messages
   exchanged within a PA session have little cryptographic protection,
   and if there is no already established security channel between two
   session partners, these messages are subject to man-in-the-middle
   attacks and DOS attacks.  For instance, the initial PA-Request and
   PA-Answer exchange is vulnerable to spoofing attacks as these
   messages are not authenticated and integrity protected.  In order to
   prevent very basic DOS attacks, a PCP device SHOULD generate state
   information as little as possible in the initial PA-Request and PA-
   Answer exchanges.  The choice of EAP method is also very important.
   The selected EAP method must be resilient to the attacks possibly
   occurred in a insecure network environment, and the user-identity
   confidentiality, protection against dictionary attacks, and session-
   key establishment must be supported.


9.  Acknowledgements

   This document was written using the xml2rfc tool described in RFC
   2629 [RFC2629].

   Some of the ideas in this document were adopted from PANA[RFC5191].


10.  Change Log







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10.1.  Changes from -00 to -01

   o  Editorial changes, added use cases to introduction.

10.2.  Changes from -01 to -02

   o  Add a nonce into the first two exchanged PA message between the
      PCC and PCS.  When a PCC initiate the session, it can use the
      nonce to detect offline attacks.

   o  Add the key ID field into the authentication tag option so that a
      MSK can generate multiple traffic keys.

   o  Specify that when a PCP device receives a PA-Request or a PA-
      Answer message from its partner the PCP device needs to reply with
      a PA-Acknowledge message to indicate that the message has been
      received.

   o  Add the support of fragmenting EAP messages.


11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

11.2.  Informative References

   [I-D.ietf-pcp-base]
              Cheshire, S., Boucadair, M., Selkirk, P., Wing, D., and R.
              Penno, "Port Control Protocol (PCP)",
              draft-ietf-pcp-base-23 (work in progress), February 2012.

   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              June 1999.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, "Extensible Authentication Protocol (EAP)",
              RFC 3748, June 2004.

   [RFC5191]  Forsberg, D., Ohba, Y., Patil, B., Tschofenig, H., and A.
              Yegin, "Protocol for Carrying Authentication for Network
              Access (PANA)", RFC 5191, May 2008.






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Authors' Addresses

   Margaret Wasserman
   Painless Security
   356 Abbott Street
   North Andover, MA  01845
   USA

   Phone: +1 781 405 7464
   Email: mrw@painless-security.com
   URI:   http://www.painless-security.com


   Sam Hartman
   Painless Security
   356 Abbott Street
   North Andover, MA  01845
   USA

   Email: hartmans@painless-security.com
   URI:   http://www.painless-security.com


   Dacheng Zhang
   Huawei
   Beijing,
   China

   Phone:
   Fax:
   Email: zhangdacheng@huawei.com
   URI:



















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