This document describes how to use PCP to retrieve the identify of a host behind a NAT. Two use cases are discussed and the PCP applicability is analyzed. This document extends PCP with a new OpCode: QUERY.

The proposed mechanism is valid for all NAT flavors including NAT44, NAT64 or NPTv6.

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/⁠.

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1. Introduction

As reported in [RFC6269], several issues are encountered when an IP address is shared among several subscribers. These issues are encountered in various deployment contexts: e.g., Carrier Grade NAT (CGN), application proxies or A+P [RFC6346].

This document extends Port Control Protocol [I-D.ietf-pcp-base] to identify a host among those sharing the same IP address in certain scenarios.

The proposed technique can be used independently or combined with the host identifier, denoted as HOST_ID defined in [I-D.ietf-intarea-nat-reveal-analysis].

2. Requirements Language and 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 [RFC2119].

3. Problem Space

3.1. Policy and Charging Control Architecture

                                +-----+    +-----+
                                | AAA |    | PCRF|
                                +-----+    +-----+
                                   \          /
                                    \        /                       /-
                                     \      /                       / I
  UE     BS                           \    /                       /  n
   |     /\    +-----+ /-----------\ +-----+ /-----------\ +----+ /   t
 +-+    /_ \---| ANG |/ Operator's  \| PCEF|/ Operator's  \| BR |/    e
 | |---/    \  +-----+\ IP Network  /+-----+\ IP Network  /+----+\    r
 +-+                   \-----------/         \-----------/        \   n
                                                                   \  e
                                                                    \ t
                                      +-----+
                                      |  AF |
                                      +-----+

Figure 1: Mobile Network Archtecture

The main involved functional elements are:

UE (User Equipment) is a mobile node.
Policy and Charging Rule Function (PCRF) which is responsible for determining which policy and charging control rules are to be applied [TS.23203].
Policy and Charging Enforcement Function (PCEF) which performs policy enforcement (e.g., Quality of Service (QoS)) and flow-based charging [TS.23203].
Application Function (AF) is an element offering applications that require dynamic policy and/or charging control [TS.23203].
Access Network Gateway (ANG), Base Station (BS) and Border Router (BR) are defined in [RFC6342].

Section 3.2 and Section 3.3 explain the encountered problems to identify individual UEs when a NAT is involved in the data path. The use of PCP to solve those problems is analyzed in Section 4.

3.2. NAT between the PCEF and AF

Figure 2 shows a scenario where a NAT function is located between the PCEF and AF.

                            PCP Client  
                             +------+
                             | PCRF |-----------------+
                             +------+                 |
                                |                     |
                 +----+      +------+   +-----+    +-----+
                 | UE |------| PCEF |---| NAT |----|  AF |
                 +----+      +------+   +-----+    +-----+
                                          PCP
                                         Aware

Figure 2: NAT between PCEF and AF

The main issue in this case is that PCEF, PCRF and AF all receive information bound to the same UE but cannot correlate between the piece of data visible for each entity. Concretely,

PCEF is aware of the IMSI (International Mobile Subscriber Identity) and an internal IP address assigned to the UE.
AF receives an external IP address and port as assigned by the NAT function.
PCRF is not able to correlate between the external IP address/port assigned by the NAT and the internal IP address and IMSI of the UE. For instance, the offered QoS on internal IPv4 address and the (shared) external IPv4 address may need to be correlated for accounting purposes.
The IP address seen by the AF is shared among multiple UEs using NAT, the PCRF needs to be able to inspect the NAT binding to disambiguate among the individual UEs. AF will not be able to treat UE traffic based on policy provided by PCRF.

This scenario can be generalized as follows (Figure 3):

Policy Enforcement Point (PEP, [RFC2753])
Policy Decision Point (PDP, [RFC2753])

                            PCP Client 
                             +------+
                             | PDP  |-----------------+
                             +------+                 |
                                |                     |
                 +----+      +------+   +-----+    +------+
                 |Host|------| PEP  |---| NAT |----|Server|
                 +----+      +------+   +-----+    +------+
                                         PCP 
                                        Aware

Figure 3: NAT between PEP and Server

3.3. NAT before PCEF

Figure 4 shows an alternative scenario in which a NAT function is located before PCEF. The main issue is that PCEF and AF are only aware of the external IP address and the external port number assigned by the NAT function. PCEF/AF will fail to identify each UE behind NAT since multiple UEs share the same external IP address and thus are unable to treat incoming connections differently.

      
                                      PCP Client  
                                       +-------+
                                       | PCRF  |------+
                                       +-------+      |
                                           |          |
                 +----+      +------+   +-----+    +-----+
                 | UE |------| NAT  |---|PCEF |----|  AF |
                 +----+      +------+   +-----+    +-----+
                               PCP           
                              Aware

Figure 4: NAT before PCEF

This scenario can be generalized as follows (Figure 5):

        
                                      PCP Client  
                                        +------+
                                        |  PDP |------+
                                        +------+      |
                                           |          |
                 +----+      +------+   +-----+    +------+
                 |Host|------| NAT  |---| PEP |----|Server|
                 +----+      +------+   +-----+    +------+
                               PCP           
                              Aware

Figure 5: NAT before PEP

4. PCP Applicability

This section discusses how PCP can be used to solve the problems described in Section 3.2 and Section 3.3.

4.1. NAT between the PCEF and AF

A solution to solve the problem discussed in Section 3.2 is to enable a PCP Server to control the NAT and enable a PCP Client in the PCRF. The updated interaction between PCRF, PCEF and AF is detailed below.

The PCP server controlling the NAT is configured to accept PCP requests with THIRD_PARTY Option from authorized PCP clients (i.e., PCRF).
PCRF is configured with the IP address of the PCP Server.
The PCRF is aware of the following 5-tuple of each flow {Internal IP address of UE, Internal Port, Protocol, Remote Peer IP address, Remote Port number} learnt from PCEF. PCRF is also aware of the following 5-tuple of each flow {External IP address, External Port, Protocol, Remote Peer IP address, Remote Port number} learnt from AF.
The PCRF generates PCP PEER request with THIRD_PARTY option which has Internal IP Address set to the UE's Internal IP address provided by the PCEF.
- The Internal Port, Protocol, Remote Peer Port, Remote Peer IP Address fields of the PEER request are set by the PCRF according to the 5-tuple flow information provided by PCEF.
- Suggested External Port and Suggested External IP Address are set to zero.
- Requested Lifetime field is set to a very low value (see Section 12.3 of [I-D.ietf-pcp-base]).
Upon the receipt of the PEER response, the PCRF compares the External IP Address and Port learnt with the 5-tuple flow information provided by the AF to correlate external IP address/port associated with the mapping and the internal IP address/port of the flow.
PCRF notifies PCEF/AF to enforce relevant policies for the UE.

4.2. NAT before PCEF

A solution to solve the problem discussed in Section 3.3 is to extend PCP with a new OpCode called QUERY (see Section 5).

The updated interaction between PCRF, PCEF and AF is detailed below:Figure 6 shows an example of the use of QUERY OpCode. In this example, an HTTP connection is initiated by the UA (192.0.2.1:33041) to an HTTP server (198.51.100.2:80). The NAT assigns 198.51.100.1/23432 as external IP Address/Port. PCRF learns Internal IP Address and Port associated with the NAT mapping using PCP QUERY request/response.

The PCP server controlling the NAT is configured to accept QUERY requests Section 5 from authorized PCP clients such as PCRF. Query requests must not be received in the Internet-facing interface but from an internal interface (e.g., dedicated management interface).
PCRF generates a PCP QUERY request with External IP Address, External Port, Remote Peer IP address, Remote Peer Port and Protocol fields for the flow learnt from PCEF or AF.
PCRF learns the internal IP address and internal port number in the QUERY response. This correlation is used by the PCRF to retrieve the UE's policy to be passed to the PCEF.

 +------+             +-----+                +------+       +------+
 |  UE  |             | NAT |                | PCRF |       |  AF  |
 +------+             +-----+                +------+       +------+
 192.0.2.1                                       |       198.51.100.2   
    |                    |                                      | 
    |Src IP Address:Port |                                      |
    |  = 192.0.2.1:33041 |                                      |
    |Dst IP Address:Port |                                      |
    |  = 198.51.100.2:80 |                                      |                   
    |===================>|=====================================>|     
    |                    |Src IP Address:Port=198.51.100.1:23432|
    |                    |Dst IP Address:Port=198.51.100.2:80   |
    |                    |                                      |
    |                                   {PCRF learns 5-tuple    |
    |                                        from AF or PCEF}   |
    |                    |                        |             | 
    |                    | PCP QUERY Request      |             |
    |                    | External IP:Port =     |             |
    |                    |    198.51.100.1:23432  |             |
    |                    | Remote Peer IP:Port =  |             |
    |                    |    198.51.100.2:80     |             |   
    |                    | Protocol = TCP         |             | 
    |                    |<=======================|             |
    |                    | PCP QUERY Response     |             | 
    |                    | External IP:Port =     |             |
    |                    |    198.51.100.1:23432  |             |
    |                    | Internal IP:Port =     |             |
    |                    |    192.0.2.1:33041     |             |
    |                    | Protocol = TCP         |             | 
    |                    | Lifetime = 1000 sec    |             |
    |                    |=======================>|             |
    |                    |                        |             |  
    |                                     Generate Policy Rules

Figure 6: Usage Example

5. QUERY OpCode

This section defines a new PCP OpCode which can be used to query PCP-aware NAT to retrieve the Internal IP Address and Internal Port of a given mapping.

The PCP Server MUST provide a configuration option to allow administrators to enable/disable QUERY OpCode.

5.1. QUERY Request Format

The following diagram shows the format of the OpCode-specific information in a request for the QUERY 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                   QUERY Nonce (96 bits)                       |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Protocol    |          Reserved (24 bits)                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        External Port        |    Remote Peer Port             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |              External IP Address (128 bits)                   |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |              Remote Peer IP Address (128 bits)                |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 7: Query Opcode Request

These fields are described below:

Requested Lifetime (in common header):: This field is positioned to 0.
Mapping Nonce:: Random value chosen by the PCP client. See Section 12.2 of [I-D.ietf-pcp-base]
Protocol:: Upper-layer protocol associated with this OpCode. Values are taken from the IANA protocol registry [proto_numbers]. For example, this field contains 6 (TCP) if the OpCode is describing a TCP mapping. Protocol MUST NOT be zero.
Reserved:: 24 reserved bits, MUST be set to 0 on transmission and MUST be ignored on reception.
External Port:: External port allocated by NAT for the flow. External Port MUST NOT be zero
Remote Peer Port:: Remote peer's port for the flow. Remote Peer Port MUST NOT be zero
External IP address:: External IP address allocated by NAT for the flow. External IP address MUST NOT be zero
Remote Peer IP address:: Remote peer IP address for the flow. Remote Peer IP address MUST NOT be zero.

5.2. QUERY Response Format

The following diagram shows the format of OpCode-specific information in a response packet for the QUERY 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                   QUERY Nonce (96 bits)                       |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Protocol    |          Reserved (24 bits)                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        External Port        |      Internal Port              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |              External IP Address (128 bits)                   |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |              Internal IP Address (128 bits)                   |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 8: Query Opcode Response

These fields are described below:

Lifetime (in common header):: On a success response, this indicates the lifetime for this mapping, in seconds. On an error response, this indicates that mapping does not exist.
Mapping Nonce:: Copied from the request.
Protocol:: Copied from the request.
Reserved:: 24 reserved bits, MUST be set to 0.
External Port:: Copied from the request.
External IP address:: Copied from the request.
Internal Port:: Internal Port as assigned by the PCP-controlled device.
Internal IP address:: Internal IP address as assigned by the PCP-controlled controlled device.

5.3. Generating a QUERY Request

This section describes the operation of a PCP client when sending requests with the QUERY OpCode.

PCP QUERY request is used by an authorized third party PCP client that is only aware of the 5-tuple {External IP address and Port, Protocol, Remote Peer IP address and Port} and needs to learn the Internal IP address and Port associated with the NAT mapping. The request MUST contain non-zero values of Protocol, External Port, Remote Peer Port, External IP address and Remote Peer IP address. The Requested Lifetime MUST be set to zero.

5.4. Processing a QUERY Request

This section describes the operation of a PCP server when processing a QUERY request.

For EIM/EIF port-mapping NAT, the processing of the QUERY request is as follows:

If any of the values Protocol, External Port and External IP address are equal to zero, the request is invalid and the PCP server MUST return a MALFORMED_REQUEST to the client.
If Protocol, External Port and External IP address do not match any existing implicit dynamic mapping, then the PCP server MUST return NONEXIST_MAP error response (also needed in [I-D.boucadair-pcp-failure]).
If Protocol, External Port and External IP address match an existing implicit dynamic mapping, then the PCP server MUST build a QUERY response with the Internal IP address, Internal Port and the lifetime associated with the mapping.

For EDM port-mapping NAT, the processing of the QUERY request is as follows:

If any of the values Protocol, External Port, Remote Peer Port, External IP address and Remote Peer IP Address are zero, the request is invalid and PCP server MUST return a MALFORMED_REQUEST to the client.
If Protocol, External Port, Remote Peer Port, External IP address and Remote Peer IP address do not match any existing implicit dynamic mapping then the PCP server MUST return NONEXIST_MAP error response (also needed in [I-D.boucadair-pcp-failure]).
If Protocol, External Port, Remote Peer Port, External IP address and Remote Peer IP address matches an existing implicit dynamic mapping then the PCP server builds a QUERY response with the Internal IP address, Internal Port and the lifetime associated with the mapping.

PCP QUERY requests received on the Internet-facing interface MUST be silently drooped.

In DS-Lite context [RFC6333], the Internal IP address returned in the QUERY response MUST be the IPv6 address of the remote tunnel endpoint and not the internal private IPv4 address.

5.5. Processing a QUERY Response

After performing common PCP response processing by the PCP Client, the response is further matched with a previously-sent QUERY request by comparing the QUERY Nonce, External IP Address, External Port and Protocol. On a SUCCESS response, the PCP Client can use the Internal IP Address and Port in the QUERY response as needed.

6. Applicability Scope of QUERY OpCode

The PCP-Reveal solution is designed for needs within one single administrative domain (i.e., the PCP Client and PCP Server are managed by the same entity). Considerations related to the activation of the PCP-Reveal solution in an inter-domain context is out of scope of this document.

7. IANA Considerations

Authors of this document request the following OpCode:

QUERY

The following error code is requested:

NONEXIST_MAP

8. Security Considerations

Security considerations discussed in [I-D.ietf-pcp-base] are to be taken into account. In particular, QUERY OpCode MUST NOT be implemented or used unless the network on which the PCP QUERY messages are to be sent is fully trusted. For example if Access Control Lists (ACLs) are installed on the PCP server, and the network between the PCP client and the PCP server, so those ACLs allow only communications from a trusted PCP client to the PCP server.

QUERY OpCode may be generated by non legitimate PCP Clients; the PCP Server MUST enforce some policies such as rate limit QUERY messages. QUERY requests received from non legitimate PCP Clients are silently dropped.

PCP authentication [I-D.ietf-pcp-authentication] MAY be used.

9. References

9.1. Normative References

[RFC2119]	Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[I-D.ietf-pcp-base]	Wing, D, Cheshire, S, Boucadair, M, Penno, R and P Selkirk, "Port Control Protocol (PCP)", Internet-Draft draft-ietf-pcp-base-29, November 2012.
[proto_numbers]	IANA, , "Protocol Numbers", 2010.

9.2. Informative References

[TS.23203]	3GPP, , "Policy and charging control architecture", September 2012.
[RFC6342]	Koodli, R., "Mobile Networks Considerations for IPv6 Deployment", RFC 6342, August 2011.
[RFC2753]	Yavatkar, R., Pendarakis, D. and R. Guerin, "A Framework for Policy-based Admission Control", RFC 2753, January 2000.
[RFC6269]	Ford, M., Boucadair, M., Durand, A., Levis, P. and P. Roberts, "Issues with IP Address Sharing", RFC 6269, June 2011.
[RFC6333]	Durand, A., Droms, R., Woodyatt, J. and Y. Lee, "Dual-Stack Lite Broadband Deployments Following IPv4 Exhaustion", RFC 6333, August 2011.
[RFC6346]	Bush, R., "The Address plus Port (A+P) Approach to the IPv4 Address Shortage", RFC 6346, August 2011.
[I-D.boucadair-intarea-host-identifier-scenarios]	Boucadair, M, Binet, D, Durel, S and T Reddy, "HOST_ID: Use Cases", Internet-Draft draft-boucadair-intarea-host-identifier-scenarios-01, October 2012.
[I-D.ietf-intarea-nat-reveal-analysis]	Boucadair, M, Touch, J, Levis, P and R Penno, "Analysis of Solution Candidates to Reveal a Host Identifier (HOST_ID) in Shared Address Deployments", Internet-Draft draft-ietf-intarea-nat-reveal-analysis-04, August 2012.
[I-D.boucadair-pcp-failure]	Boucadair, M, Dupont, F and R Penno, "Port Control Protocol (PCP) Failure Scenarios", Internet-Draft draft-boucadair-pcp-failure-04, August 2012.
[I-D.ietf-pcp-authentication]	Wasserman, M, Hartman, S and D Zhang, "Port Control Protocol (PCP) Authentication Mechanism", Internet-Draft draft-ietf-pcp-authentication-01, October 2012.

Authors' Addresses

Mohamed Boucadair France Telecom Rennes, 35000 France EMail: mohamed.boucadair@orange.com

Tirumaleswar Reddy Cisco Systems, Inc. Cessna Business Park, Varthur Hobli Sarjapur Marathalli Outer Ring Road Bangalore, Karnataka 560103 India EMail: tireddy@cisco.com

Prashanth Patil Cisco Systems, Inc. Cessna Business Park, Varthur Hobli Sarjapur Marthalli Outer Ring Road Bangalore, Karnataka 560103 India EMail: praspati@cisco.com

Dan Wing Cisco Systems, Inc. 170 West Tasman Drive San Jose, California 95134 USA EMail: dwing@cisco.com

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