Port Control Protocol (PCP) Proxy Function

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PCP Working Group                                           M. Boucadair
Internet-Draft                                            France Telecom
Intended status: Standards Track                               R. Dupont
Expires: September 30, 2012                  Internet Systems Consortium
                                                                R. Penno
                                                        Juniper Networks
                                                                 D. Wing
                                                          March 29, 2012

               Port Control Protocol (PCP) Proxy Function


   This document specifies the behavior of a PCP Proxy element, for
   instance embedded in Customer Premise routers.

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   This Internet-Draft will expire on September 30, 2012.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  PCP Server Discovery and Provisioning  . . . . . . . . . . . .  3
   3.  PCP Proxy as a PCP Server  . . . . . . . . . . . . . . . . . .  4
   4.  Control of the Firewall  . . . . . . . . . . . . . . . . . . .  4
   5.  Embedded NAT in the CP Router  . . . . . . . . . . . . . . . .  4
   6.  Simple PCP Proxy . . . . . . . . . . . . . . . . . . . . . . .  6
   7.  Advanced Functions . . . . . . . . . . . . . . . . . . . . . .  7
     7.1.  Multiple PCP Servers . . . . . . . . . . . . . . . . . . .  7
     7.2.  Epoch Handling . . . . . . . . . . . . . . . . . . . . . .  8
     7.3.  Request/Response Caching . . . . . . . . . . . . . . . . .  8
     7.4.  Retransmission Handling  . . . . . . . . . . . . . . . . .  9
     7.5.  Full State . . . . . . . . . . . . . . . . . . . . . . . .  9
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 10
     10.2. Informative References . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11

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1.  Introduction

   PCP [I-D.ietf-pcp-base] discusses the implementation of NAT control
   features that rely upon Carrier Grade NAT (CGN) devices such as DS-
   Lite AFTR [RFC6333].

   The Customer Premise router, the B4 element in DS-Lite, is in charge
   to enforce some security controls on PCP requests so implements a PCP
   Proxy function: it acts as a PCP server receiving PCP requests on
   internal interfaces, and as a PCP client forwarding accepted PCP
   requests on an external interface to a CGN PCP server.  The CGN PCP
   server in turn send replies (PCP responses) to the PCP Proxy external
   interface which are finally forwarded to PCP clients.

   The PCP Proxy can be simple, i.e., implement as transparent/minimal
   processing as possible, or it can be smart, i.e., handle multiple CGN
   PCP servers, cache requests/responses, etc.  A smart Proxy can be
   associated with UPnP IGD [I-D.ietf-pcp-upnp-igd-interworking] or/and
   NAT-PMP [I-D.bpw-pcp-nat-pmp-interworking] Interworking Function

      +------------+                       |
      | PCP Client |-----+                 |
      +--(Host 1)--+     |   +-----------+ |     +----------+
                         +---|           | |     |          |
                             | PCP Proxy |-------|PCP Server|
                         +---|           | |     |          |
      +------------+     |   +-----------+ |     +----------+
      | PCP Client |-----+                 |
      +--(Host 2)--+               possible boundary
                              <- Home side | ISP side ->

                     Figure 1: Reference Architecture

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  PCP Server Discovery and Provisioning

   The PCP Proxy MUST implement one of the discovery methods listed in
   [I-D.ietf-pcp-base] (e.g., DHCP [I-D.ietf-pcp-dhcp]).

   The address of the PCP Proxy is provisioned to local PCP Clients as
   their default PCP Server: If the PCP DHCP option is supported by an

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   internal PCP Client, it will retrieve the PCP Server IP address to
   use from its local DHCP server (usually embedded on the CP router);
   otherwise internal PCP Clients will assume their default router being
   the PCP Server.

3.  PCP Proxy as a PCP Server

   The PCP Proxy acts as a PCP server for internal hosts and accepts PCP
   requests on the interface(s) facing them, e.g., it creates servicing
   socket(s) and bound them to each address of this (these) interface(s)
   on UDP port 44323.

   When the topology makes a routing loop possible, the PCP Proxy MAY
   check it is not the source of a PCP message it's received.

4.  Control of the Firewall

   A security policy to accept PCP messages from the provisioned PCP
   Server is to be enabled on the CP router.  This policy can be for
   instance triggered by DHCP configuration or by outbound PCP requests
   issued from the PCP Proxy to the provisioned PCP Server.

   In order to accept inbound and outbound traffic associated with PCP
   mappings instantiated in the upstream PCP Server, appropriate
   security policies are to be configured on the firewall.

   For instance if the firewall rules have a lifetime, PCP response can
   be snooped in order to instantiate the corresponding firewall rules
   with the same lifetime.  If they have no lifetime, an explicit
   dynamic mapping table can be kept in the PCP Proxy state in order to
   instantiate and remove corresponding firewall rules.  This is in fact
   an easy subcase of Section 5.

   REMOTE_PEER_FILTER Options can be installed into the local firewall,
   forwarded to the PCP Server so installed into the remote NAT/firewall
   or both.

      [Ed.  Note: should we say the firewall function is already handled
      by the PCP controlled device so it is useless at the local level?]

5.  Embedded NAT in the CP Router

   When no NAT is embedded in the CP router, the port number included in
   received PCP messages (from the PCP Server or PCP Client(s)) are not
   altered by the PCP Proxy.

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      [Ed.  Note: NAT444 seems to be the only exception?]

   When the PCP Proxy is co-located with a NAT function in the CP
   router, it MUST update the content of received requested messages
   with the mapped port number and the address belonging to the external
   interface of the CP router (i.e., after the NAT operation) and not as
   initially positioned by the PCP Client.  For the reverse path, PCP
   response messages MUST be updated by the PCP Proxy to replace the
   target port number to what has been initially positioned by the PCP
   Client.  For this purpose the PCP Proxy has an access to the local
   NAT state.  Note PCP messages with an unknown OpCode or Option can
   carry a hidden target address or internal port which will not be

   o  a PCP Proxy co-located with a NAT SHOULD reject by an
      UNSUPP_OPCODE error response a received request with an unknown

   o  a PCP Proxy co-located with a NAT SHOULD reject by an
      UNSUPP_OPTION error response a received request with a mandatory-
      to-process unknown Option;

   o  a PCP Proxy co-located with a NAT SHOULD remove any optional-to-
      process unknown Options from received requests before forwarding

   When a PCP request is received and accepted by the PCP Proxy the
   corresponding mapping (explicit dynamic mapping for a MAP request,
   implicit dynamic mapping for a PEER request) is looked for in the
   local NAT state and temporary created if it does not exist.
   Temporary means it is deleted if no SUCCESS response is received,
   either explicitly or because of its short lifetime at creation.

   If the local NAT associates explicit dynamic mappings to a lifetime,
   the requested lifetime in MAP requests SHOULD be adjusted to be in
   the accepted range of the local NAT, and the assigned lifetime copied
   from MAP responses to the corresponding mapping in the local NAT.
   The same processing applies to implicit dynamic mappings and PEER
   requests/responses (but the valid requested lifetime range begins by
   zero in this case).

   Otherwise explicit dynamic mappings have an undefined lifetime in the
   local NAT and the PCP Proxy SHOULD maintain an explicit dynamic
   mapping table and SHOULD delete corresponding explicit dynamic
   mappings in the local NAT when they expire or are deleted by the MAP
   request with a zero requested lifetime.

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6.  Simple PCP Proxy

   A simple PCP Proxy performs minimal modifications to PCP requests and
   responses, in particular it does not change the Epoch value in
   responses.  So it does not handle more than one PCP server.

   The detailed behavior at the reception of a PCP request on an
   internal interface is as follows:

   o  check if the source IP address and the PCP target address are the

   o  apply security controls, including with the result of the previous

   o  if the request is rejected, build a synthetic error response and
      send it back to the PCP client.

   o  if the request is accepted, adjust it (e.g., adding a THIRD_PARTY
      Option, updating the internal address and port to their translated
      values as specified in Section 5 and forward it on a fresh UDP
      socket connected to the PCP server.

   o  Wait for the response during a reasonable delay.

   o  when the response is received from the PCP server, adjust it back
      (e.g., removing the THIRD_PARTY Option added previously, updating
      the internal address and port to their initial values as specified
      in Section 5), forward it to the source PCP client and close the
      socket to the PCP server.

         [Ed.  Note: is there extra validation useful?  The response
         comes from the PCP server and the PCP client will validated it

   o  on a hard error on the UDP socket, build a synthetic ICMP error
      and send it to the source PCP client.

   The reasonable delay minimum value is 20 seconds, request
   retransmission is handled by PCP clients.

   For each pending request, the proxy MUST maintain in a data record:

   o  the request payload

   o  the interface where the request was received

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   o  the source IP address of the request

   o  the source UDP port of the request

   o  the UDP socket connected to the PCP server

   o  an expire timeout

   Receiving interfaces can be implemented by a set of servicing
   sockets, each socket bound to an address of an internal interface.
   Interface, source address and port are used to send back packets to
   the source PCP client.  The request payload is used to generate
   synthetic ICMP.  Responses are received on the UDP socket.

   There is no (not yet) standardized way to build a synthetic error
   response, in particular no way to determine which Epoch value to put
   into it.  This is why it is better to build a synthetic ICMP error
   than a synthetic error response with NETWORK_FAILURE on a socket hard

   Too large requests SHOULD be forwarded to the PCP server in order to
   relay back the error response, i.e., the PCP Proxy is not in charge
   to enforce the message size limit and in general the PCP Proxy SHOULD
   NOT generate error response for a reason other than security
   controls.  No behavior is specified in the case the PCP Proxy
   processing (e.g., adding a THIRD_PARTY Option) makes a valid request
   too large when it is sent to the PCP Server.

7.  Advanced Functions

   When a simple PCP Proxy uses as global variables only the CGN PCP
   server IP address, a set of servicing sockets and a list of pending
   request handlers, a smart PCP Proxy implements more services.

   Even if most services rely on the Epoch handling one Section 7.2,
   services are described below in a natural order.

7.1.  Multiple PCP Servers

   A smart PCP Proxy MAY offer to handle multiple PCP servers at the
   same time, each PCP server is associated to each own handled Epoch
   value according to Section 7.2.

   The only constraint is to maintain a reasonable coherency as PCP
   clients cannot be assumed to be prepared to this, i.e., this has to
   be transparent for / hidden to them.

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      [Ed.  Note: we propose to require a partition of clients, clients
      on the same host or sharing a target address SHOULD be in the same
      subset, i.e., the same PCP server and the same Epoch.]

      [Ed.  Note: the Proxy can get per PCP server capabilities, for
      instance from the error responses.]

7.2.  Epoch Handling

   With Epoch handling the Epoch value is related to internal timers and
   not blindly copied from PCP responses.  There should be no advantages
   to have more than one managed Epoch per PCP server.

   The Epoch MUST be reset when explicit dynamic mappings are lost,

   o  at startup if the PCP proxy can't recover the state.

         [Ed.  Note: as it is very optional to manage state in the Proxy
         it should be the default.]

   o  when the WAN address is changed or any similar events which show
      any previous state is no longer valid.

   o  when the Epoch value in a PCP response is too small (cf. Epoch
      value validation rules in [I-D.ietf-pcp-base]).

   o  when the External Address has changed

   The last two rules are per PCP server, a PCP Proxy MAY check these
   conditions in all received responses for a PCP server, including when
   the PCP Proxy is a part of an IWF

7.3.  Request/Response Caching

   A PCP Proxy providing request/response caching checks each time it
   receives a PCP request if it has already seen the same request
   recently and got the corresponding PCP response.  In this case, it
   sends back directly the cached response with the proper Epoch value
   and not forward the request to the PCP server.

      [Ed.  Note: this is an easy optimization, the only difficult point
      can be solved by the Epoch handling.]

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7.4.  Retransmission Handling

   An extension of the previous service is to manage the retransmission
   of pending requests to the server internally, i.e., no longer driven
   by the PCP client.  A cache entry SHOULD be expired after a delay
   short enough to keep it easy to distinguish it from a replay.

      [Ed.  Note: this allows smart retransmission scheduling as the
      Proxy "sees" all PCP exchanges with the PCP server.]

7.5.  Full State

   A smart PCP Proxy can keep the full state: an image of all active
   explicit dynamic mappings is kept in memory.  This service is not
   interesting by itself but it can be necessary to support embedded
   firewall or NAT Section 5 and if the PCP Proxy is integrated in an
   IWF (e.g., to support UPnP IGD [I-D.ietf-pcp-upnp-igd-interworking]).

   In conclusion this service MAY be supported.  Note when it is
   supported the state SHOULD be recovered in case of failures according
   to [I-D.boucadair-pcp-failure].

8.  IANA Considerations

   This document makes no request of IANA.

9.  Security Considerations

   The security controls are applied on PCP requests and are about:

   o  authorized target addresses, in particular in case of a third

   o  authorized internal and external ports (note the external port is
      in general assigned by the CGN PCP server).

   The default policy for requests for a third party when such a policy
   exists is be to not allow them.  The exact rule is: PCP requests
   including a THIRD_PARTY option enclosing an IP address distinct than
   the source IP address of the request MUST be rejected (by a
   NOT_AUTHORIZED error response).

   When a PCP Proxy is at the boundary of two trust domains (named
   "internal" and "external" sides), it MUST provide at least these two
   security controls:

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   o  split horizon anti-spoofing: requests from the external side and
      responses from the internal side MUST be dropped.

   o  a policy about requests on the behalf of a third party MUST be

   A PCP Proxy MAY implement only the simple rule about third party: all
   received requests including a THIRD_PARTY option are rejected.

      [Ed.  Note: this is stricter than the default but keeps the
      minimal implementation as simple as possible.]

   A received request carrying an unknown OpCode or Option SHOULD be
   dropped (or in the case of an unknown Option which is not mandatory-
   to-process the Option be removed) if it is not a priori compatible
   with security controls or correct processing.  This includes at least
   all cases where received requests are scanned for elements like the
   protocol, an address or a port.

      [Ed.  Note: magically a minimal implementation in favorable
      environments (no embedded NAT!)  MAY accept unknown Opcodes and
      Options.  There is no need for a similar rule for responses as the
      proxy can do nothing with a "bad" response anyway...]

10.  References

10.1.  Normative References

              Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
              Selkirk, "Port Control Protocol (PCP)",
              draft-ietf-pcp-base-24 (work in progress), March 2012.

              Boucadair, M., Penno, R., and D. Wing, "DHCP Options for
              the Port Control Protocol (PCP)", draft-ietf-pcp-dhcp-02
              (work in progress), January 2012.

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

10.2.  Informative References

              Boucadair, M., Dupont, F., and R. Penno, "Port Control
              Protocol (PCP) Failure Scenarios",
              draft-boucadair-pcp-failure-02 (work in progress),

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              September 2011.

              Boucadair, M., Penno, R., Wing, D., and F. Dupont, "Port
              Control Protocol (PCP) NAT-PMP Interworking Function",
              draft-bpw-pcp-nat-pmp-interworking-00 (work in progress),
              March 2011.

              Boucadair, M., Dupont, F., Penno, R., and D. Wing,
              "Universal Plug and Play (UPnP) Internet Gateway Device
              (IGD)-Port Control Protocol (PCP) Interworking Function",
              draft-ietf-pcp-upnp-igd-interworking-01 (work in
              progress), March 2012.

   [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
              Stack Lite Broadband Deployments Following IPv4
              Exhaustion", RFC 6333, August 2011.

Authors' Addresses

   Mohamed Boucadair
   France Telecom
   Rennes  35000

   Email: mohamed.boucadair@orange.com

   Francis Dupont
   Internet Systems Consortium

   Email: fdupont@isc.org

   Reinaldo Penno
   Juniper Networks
   1194 N Mathilda Avenue
   Sunnyvale, California  94089

   Email: rpenno@juniper.net

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   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, California  95134

   Email: dwing@cisco.com

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