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Network Working Group                                            G. Chen
Internet-Draft                                                    Z. Cao
Intended status: Informational                              China Mobile
Expires: January 15, 2014                                   M. Boucadair
                                                          France Telecom
                                                               A. Vizdal
                                                     Deutsche Telekom AG
                                                             L. Thiebaut
                                                           July 14, 2013

          Analysis of Port Control Protocol in Mobile Network


   This memo provides a motivation description for the Port Control
   Protocol (PCP) deployment in a 3GPP mobile network environment.  The
   document focuses on a mobile network specific issues (e.g. cell phone
   battery power consumption, keep-alive traffic reduction), PCP
   applicability to these issues is further studied and analyzed.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on January 15, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of

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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Benefits of Introducing PCP in Mobile Networks  . . . . . . .   3
     2.1.  Restoring Internet Reachability . . . . . . . . . . . . .   3
     2.2.  Radio Resource Optimization . . . . . . . . . . . . . . .   3
     2.3.  Energy Saving . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Overviews of PCP Deployment in Mobile Network . . . . . . . .   4
   4.  PCP Server Discovery  . . . . . . . . . . . . . . . . . . . .   5
   5.  MN and multi-homing . . . . . . . . . . . . . . . . . . . . .   6
   6.  Retransmission Consideration  . . . . . . . . . . . . . . . .   6
   7.  Unsolicited Messages Delivery . . . . . . . . . . . . . . . .   7
   8.  SIPTO Architecture  . . . . . . . . . . . . . . . . . . . . .   8
   9.  Authentication Consideration  . . . . . . . . . . . . . . . .   9
   10. Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   11. Security Considerations . . . . . . . . . . . . . . . . . . .   9
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     14.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     14.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   The Port Control Protocol[RFC6887] allows an IPv6 or IPv4 host to
   control how incoming IPv6 or IPv4 packets are translated and
   forwarded by a network address translator (NAT) or simple
   firewall(FW), and also allows a host to optimize its outgoing NAT
   keepalive messages.  A 3rd Generation Partnership Project (3GPP)
   network can benefit from the use of the PCP service.  Traffic in a
   mobile network is becoming a complex mix of various protocols,
   different applications and user behaviors.  Mobile networks are
   currently facing several issues such as a frequent keepalive message,
   terminal battery consumption and etc.  In order to mitigate these
   issues, PCP could be used to improve terminal behavior by managing
   how incoming packets are forwarded by upstream devices such as NAT64,
   NAT44 translators and firewall devices.

   It should be noticed that mobile networks have particular
   characteristics and therefore, there are several factors that should

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   be investigated before implementing PCP in a mobile context.  Without
   the particular considerations , PCP may not provide desirable
   outcomes.  Some default behaviors may even cause negative impacts or
   system failures in a mobile environment.  Considering very particular
   environment of mobile networks , it's needed to have a document
   describing specific concerns from mobile network side.  That would
   also encourage PCP support in mobile network as well.

   This memo covers PCP-related considerations in mobile networks.  The
   intension of publishing this memo is to elaborate major issues during
   the deployment and share the thoughts for potential usages in mobile
   networks.  Such considerations would provide a pointer to parties
   interested (e.g. mobile operators) to be included in their UE profile
   requirements.  Some adaptation of PCP protocol might be derived from
   this document.  Such a work would be documented in separated memo(s).

2.  Benefits of Introducing PCP in Mobile Networks

2.1.  Restoring Internet Reachability

   Many Mobile networks are making use of a Firewall to protect their
   customers from an unwanted Internet originated traffic.  The firewall
   is usually configured to reject all unknown inbound connections and
   only permit inbound traffic that belongs to a connection initiated
   from the Firewall or NAT/PAT device.  The behavior is described as
   Category I in [I-D.ietf-opsawg-firewalls].There are applications that
   can be running on the mobile device that require to be reachable from
   the Internet or there could be services running behind the terminal
   that require reachability from the Internet.  For example, mobile
   phones should be able to reachable for instant message or online
   game.  PCP enabled applications / devices could request a port from
   the Firewall to ensure Internet reachability, and thus would lighten
   the traffic flow of keep-alive by reducing the number of sending
   packets.  This would result in resource savings on the Firewall node
   whilst still keeping the customer protected from the unwanted

2.2.  Radio Resource Optimization

   3GPP network use different radio channels to transmit control
   messages(e.g. signaling) and data packages(e.g. voice packages or
   data flows).  Always-on applications, e.g. IM(Instant Message), VoIP
   or P2P based applications always generate a fair amount of keepalive
   messages periodically.  It's observed that a number of trivial
   keepalive messages may occupy the data channel.  For example, 16% of
   traffic caused by instant signaling message would consume 50%~70%
   radio resource in some area.  It likely causes the air congestion
   with voice calls and service data transmission.  PCP could help to

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   reduce the frequency of periodic messages aimed at refreshing a NAT/
   FW binding by indicating to the mobile device the Life time of a

2.3.  Energy Saving

   Devices with low battery resources exist widely in mobile
   environments, such as mobile terminals, advanced sensors, etc. mobile
   terminals often go to "sleep" (IDLE) mode to extend battery life and
   save air resources.  Host initiated message needs to "wake-up" mobile
   terminals by changing the state to CONNECTED.  That would cause more
   energy on such terminals.  Testing reports show that energy
   consumption is dramatically reduced with prolonged sending interval
   of signaling messages [VTC2007_Energy_Consumption].

3.  Overviews of PCP Deployment in Mobile Network

   The Figure 1 shows the architecture of a mobile network.  Radio
   access network would provide wireless connectivity to the MN.
   Packets are transmitted through Packet Switch(PS) domain heading to
   MGW.  MGW bear the responsibilities of address allocation, routing
   and transfer.  The connection between MN and MGW normally is a point-
   to- point link, on which MGW is the default router for MN.  NAT/
   Firewall could either be integrated with MGW or deployed behind MGW
   as standalone.  The traffic is finally destined to application
   servers, which manage subscriber service.

      MN                                                        Internet
       |               +---------------+                     +----------------+
     +-+     +-----+   |            +--|--+     +-------+    |  +----------+  |
     | | /|/ | RAN |---| PS Network | MGW |---- |NAT/FW |----|  |APP Server|  |
     +-+     +-----+   |            +--|--+     +-------+    |  +----------+  |
                       +---------------+                     +----------------+
     MN:  Mobile Nodes
     RAN: Radio Access Network
     PS:  Packet Switch
     MGW:Mobile GateWay
     NAT/FW: Network Address Translator or Firewall

                    Figure 1: Mobile Networks Scenario

   A PCP client could be located on MN to control the outbound and
   inbound traffic on PCP servers.  The PCP server is hosted by the NAT/
   FW respectively.  Corresponding to the various behaviors of PCP
   client, MN would perform PCP operation using MAP, PEER or ANNOUNCE
   opcodes.  A specific application programming interface may be
   provided to applications.  More discussions and recommendations are
   presented in following sub-sections.

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4.  PCP Server Discovery

   A straightforward solution seems that MN assume their default router
   as the PCP Server.  However, NAT/FW normally is deployed in a
   different node than the MGW.  Thus there is the need to ensure that
   MN get information allowing them to discover a PCP server.

   [I-D.ietf-pcp-dhcp] specified name options in DHCPv4 and DHCPv6 to
   discover PCP server.  It's expected the same mechanism could be used
   in mobile network. 3GPP network allocates IP address and respective
   parameter during the PDP (Packet Data Protocol)/PDN(Packet Data
   Network) context activation phase (PDP and PDN represent terminology
   in 3G and LTE network respectively ).  On the UE, a PDP/PDN context
   has same meaning which is equivalent to a network interface.

   It should be noted that the Stateful DHCPv6-based address
   configuration[RFC3315]is not supported by 3GPP specifications. 3GPP
   adopts IPv6 Stateless Address Auto-configuration (SLAAC) [RFC4861]to
   allocate IPv6 address.  The UE uses stateless DHCPv6[RFC3736] for
   additional parameter configuration.  The MGW acts as the DHCPv6
   server.  PCP servers discovery could leverage current process to
   perform the functionalities.  The M-bit is set to zero and the O-bit
   may be set to one in the Router Advertisement (RA) sent to the UE.
   To carry out PCP sever discovery, a MN should thus send an
   Information-request message that includes an Option Request Option
   (ORO) requesting the DHCPv6 PCP Server Name option.

   Regarding the IPv4 bearer, MN generally indicates that it prefers to
   obtain an IPv4 address as part of the PDP context activation
   procedure.  In such a case, the MN relies on the network to provide
   IPv4 parameters as part of the PDP context activation/ PDN connection
   set-up procedure.  The MN may nevertheless indicate that it prefers
   to obtain the IPv4 address and configuration parameter after the PDP
   Context activation by DHCPv4, but it is not available on a wide
   scale[RFC6459].  MN usually receive those configurations in
   PCO(Protocol Configuration Options) .  PCP server name options in
   DHCPv4 would not help the PCP servers discovery in that case.

   A specific method in 3GPP is to extend PCO [TS24.008]information
   element to transfer a request of PCP server name.  However,
   additional specification efforts are required in 3GPP to make that

   [I-D.kiesel-pcp-ip-based-srv-disc]propose anycast-based solutions to
   discover the closest PCP server on the data path.  It may be worth to
   consider the case when a subscriber roams to different areas, where
   anycast configurations may be unavailable or operators use other

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   provisioning method, for example [I-D.ietf-pcp-dhcp].  Asymmetric
   routing should also be considered in the anycast-based solution.
   Otherwise, the traffic would likely loses the mapping information for
   the inbound traffic.

5.  MN and multi-homing

   As a MN may activate multiple PDP context / PDN connection, it may be
   multi-homed (the UE receives at least an IP address / an IPv6 prefix
   per PDN connection).  Different MGWs are likely to be associated with
   each of these PDP context / PDN connection and may thus advertise
   different PCP servers (using the mechanism described in the previous
   section).  In that case, a MN has to be able to manage multiple PCP
   servers and to associate an IP flow with the PCP server corresponding
   to the PDP context / PDN connection used to carry that IP flow.

6.  Retransmission Consideration

   Mobile devices are usually powered with limited battery . Users would
   like to use such MN for several days without charging, even several
   weeks in sensor case.  Many applications do not send or receive
   traffic constantly; instead, the network interface is idle most of
   the time.  That could help to save energy unless there is data
   leading the link to be activated.  Such state changes is based on
   network-specific timer values corresponding to a number of Radio
   Resource Control (RRC) states(see more at Section 8.2.2
   3GPP[TS23.060].  In order to maximize battery life, it's desirable
   that all activities on battery-powered devices needs to be
   coordinated and synchronized.  It's not specific to PCP.  Whereas ,
   those concerns also can be applied to PCP retransmission behavior.

   PCP designed retransmission mechanisms on the client for reliable
   delivery of PCP request.  If a PCP client fails to receive an
   expected response from a server, the client must retransmit its
   message.  The retransmission method may cause unnecessary power
   consumption when a subscriber roams to a network, in which PCP is not
   deployed.  Several timers are specified to control the retransmission
   behavior.  Therefore, an appropriate implementation and configuration
   are desirable to help to alleviate the concern.  For example, the
   time transiting to idle is normally less than default Maximum
   Retransmission Time (MRT), i.e. 1024 seconds.  With "no maximum"
   setting of Maximum Retransmission Duration (MRD), it would cause
   devices activating their uplink radio in order to retransmit the
   request messages.  Furthermore, the state transition and the
   transmission take some times, which causes significant power
   consumption.  The MRD should be configured with an optimal time which
   in line with activated state duration on the device.

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   The power consumption problem is made complicated if several PCP
   clients residing on a MN.  Several clients are potentially sending
   requests at random times and by so doing causing MN uplink radio into
   a significantly power consuming state for unnecessarily often.  It's
   necessary to perform a synchronization process for tidy up several
   PCP clients retransmission.  A time-line observer maybe required to
   control different PCP clients resending requests in an optimal
   transmission window.  If the uplink radio of MN is active at the time
   of sending retransmission from several clients, a proper MRD
   described as above should be set in a client.  If the uplink radio of
   MN is in idle mode, the time-line observer should hold Initial
   Retransmission Time(IRT) for while to synchronize different
   retransmitted PCP requests into same optimal transmission window.

7.  Unsolicited Messages Delivery

   When the states on NAT/FW have been changed like reboot or changed
   configuration, PCP servers can send unsolicited messages (e.g.
   ANNOUNCE messages or unicast PCP MAP or PEER responses ) to clients
   informing them of the new state of their mappings.  This aims at
   achieving rapid detection of PCP failure, rapid PCP recovery or PCP
   mapping update.  When those messages are delivered in a mobile
   environment, it should be noted multicast delivery may not be
   available in 3GPP network.  PCP servers would use unicast delivery.
   More considerations are listed as the below.

   o  This requires PCP servers to retain knowledge of the IP
      address(es) and port(s) of their clients, for example using
      redundancy design based on hot-standby, even though they have

   o  Care should be taken not to generate floods of unicast messages,
      e.g. to multiple thousands of MN that were served by a PCP server
      that has rebooted.  Such flood may have impacts on Mobile Networks
      as it may imply the simultaneous generation of Paging process(see
      more at Section 8.2.4 3GPP[TS23.060]) for very big numbers of MN.

   o  Paging function is optionally supported at some particular nodes,
      e.g. Traffic Offload Function (TOF) in Selected IP Traffic Offload
      architecture (more discussions on this issues is described in
      Section 7).  The delivery of unsolicited messages would fail in
      this case.

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8.  SIPTO Architecture

   Since Release 10, 3GPP starts supporting of Selected IP Traffic
   Offload (SIPTO) function defined in [TS23.060], [TS23.401].The SIPTO
   function allows an operator to offload certain types of traffic at a
   network node close to the UE's point of attachment to the access
   network.  It can be achieved by selecting a set of MGWs that is
   geographically/topologically close to a UE's point of attachment.
   Two variants of solutions has specified in 3GPP.

   The mainstream standard deployment relies on selecting a MGW that is
   geographically/ topologically close to a UE's point of attachment.
   This deployment may apply to both 3G and LTE.  The MN may sometimes
   be requested to re-activate its PDP context / PDN connection, in
   which case it is allocated a new MGW and thus a new IP address and a
   new PCP server.  In this case, host renumbering is inevitable.  Some
   considerations have been described as Address Change Events at
   Section11.5 of [RFC6887].  The deletions of the mapping information
   on the old MGW is necessary in order to avoid traffic sending to the
   old IP address.  In a mobile device context, PCP client may take the
   NAS(Non-Access Stratum) layer message (e.g. "reactivation request" or
   "detach request" message) as a notification to delete the old mapping
   information before the subscriber moved to new MGW.  Afterwards, PCP
   clients install new mappings for its new IP address.

   As an implementation option dedicated to 3G networks, it is also
   possible to carry out Selected IP Traffic Offload in a TOF(Traffic
   Offload Function) entity [TS23.060]located at the interface of the
   Radio Access Network, i.e. in the path between the radio stations and
   the Mobile Gateway.  The TOF decides on which traffic to offload and
   enforces NAT for that traffic.  The deployment of a TOF is totally
   transparent for user's equipments that even cannot know which traffic
   is subject to TOF (NATed at the TOF) and which traffic is processed
   by the MGW.  The PCP server advertised by the MGW does not take into
   account the NAT carried out by the TOF function.  Therefore, PCP
   client doesn't know which PCP servers should be selected to send the
   request.  [I-D.rpcw-pcp-pmipv6-serv-discovery]provides a solution in
   the similar architecture, in which a PCP proxy with advanced
   functions[I-D.ietf-pcp-proxy] is required on the offloading point to
   dispatch requests to a right PCP server.  Additional consideration
   will be given for determining the each traffic flow, since TOF
   inspects the NAS and RANAP(Radio Access Network Application Part)
   messages to build the local UE context and local session context.
   The traffic flow can't be identified with 5 tuples.  The offloaded IP
   flow is indicated with Radio Access Bearer Identifier (RAB-ID).  PCP
   proxy must understand RAB-ID and map the identifier with each IP

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9.  Authentication Consideration

   The general authentication requirements have been analyzed in
   [I-D.reddy-pcp-auth-req].  In mobile networks, it is desirable to
   reuse the existing credentials on the UE for the pcp authentication
   between involved entities.  This way makes the deployment of
   authentication easiler.

   The [I-D.ietf-pcp-authentication] has provided solutions for PCP
   authentication, in which an EAP option is included in the PCP
   requests from the devices.  In the EAP framework, the EAP
   authentication server could be the co-located with the PCP server or
   separated and located on a third-party entity.  If the EAP
   authentication server is placed on the AAA/Radius server, there is a
   need of an interface between the PCP server and AAA.  But per our
   investigation of 3GPP networks, most exisiting NAT devices do not
   have such an interface with AAA.  So in practical deployment, this
   could be taken into consideration.

10.  Conclusion

   PCP mechanism could be potentially adopted in different usage
   contexts.  The deployment in mobile network described applicability
   analysis, which could give mobile operators a explicit recommendation
   for PCP implementation.  Operators would benefit from such particular
   considerations.  The memo would take the role to document such
   considerations for PCP deployment in mobile network.

11.  Security Considerations


12.  IANA Considerations

   This document makes no request of IANA.

13.  Acknowledgements

   The authors would like to thank Dan Wing, Stuart Cheshire, Ping Lin
   and Tao Sun for their discussion and comments.

   Many thanks to Reinaldo Penno and Tirumaleswar Reddy for their
   detailed reviews.

14.  References

14.1.  Normative References

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              Wasserman, M., Hartman, S., and D. Zhang, "Port Control
              Protocol (PCP) Authentication Mechanism", draft-ietf-pcp-
              authentication-01 (work in progress), October 2012.

              Boucadair, M., Penno, R., and D. Wing, "DHCP Options for
              the Port Control Protocol (PCP)", draft-ietf-pcp-dhcp-07
              (work in progress), March 2013.

              Boucadair, M., Penno, R., and D. Wing, "Port Control
              Protocol (PCP) Proxy Function", draft-ietf-pcp-proxy-03
              (work in progress), June 2013.

              Reddy, T., Patil, P., Wing, D., and R. Penno, "PCP
              Authentication Requirements", draft-reddy-pcp-auth-req-04
              (work in progress), July 2013.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", BCP 84, RFC 3704, March 2004.

   [RFC3736]  Droms, R., "Stateless Dynamic Host Configuration Protocol
              (DHCP) Service for IPv6", RFC 3736, April 2004.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC6887]  Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
              Selkirk, "Port Control Protocol (PCP)", RFC 6887, April

              , "General Packet Radio Service (GPRS); Service
              description; Stage 2", June 2012.

              , "General Packet Radio Service (GPRS) enhancements for
              Evolved Universal Terrestrial Radio Access Network
              (E-UTRAN) access", June 2012.

14.2.  Informative References

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              Cheshire, S., "PCP Anycast Address", draft-cheshire-pcp-
              anycast-01 (work in progress), March 2013.

              Baker, F. and P. Hoffman, "On Firewalls in Internet
              Security", draft-ietf-opsawg-firewalls-01 (work in
              progress), October 2012.

              Kiesel, S. and R. Penno, "PCP Server Discovery based on
              well-known IP Address", draft-kiesel-pcp-ip-based-srv-
              disc-00 (work in progress), February 2013.

              Reddy, T., Patil, P., Chandrasekaran, R., and D. Wing,
              "PCP Server Discovery with IPv4 traffic offload for Proxy
              Mobile IPv6", draft-rpcw-pcp-pmipv6-serv-discovery-02
              (work in progress), February 2013.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC6459]  Korhonen, J., Soininen, J., Patil, B., Savolainen, T.,
              Bajko, G., and K. Iisakkila, "IPv6 in 3rd Generation
              Partnership Project (3GPP) Evolved Packet System (EPS)",
              RFC 6459, January 2012.

              , "Mobile radio interface Layer 3 specification; Core
              network protocols; Stage 3", 9.11.0 3GPP TS 24.008, June

              , "Generic Authentication Architecture (GAA); Generic
              Bootstrapping Architecture (GBA)", 10.1.0 3GPP TS 33.220,
              March 2012.

              , "Energy Consumption of Always-On Applications in WCDMA
              Networks", 2007.

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Authors' Addresses

   Gang Chen
   China Mobile
   No.32 Xuanwumen West Street
   Xicheng District
   Beijing  100053

   Email: phdgang@gmail.com

   Zhen Cao
   China Mobile
   No.32 Xuanwumen West Street
   Xicheng District
   Beijing  100053

   Email: caozhen@chinamobile.com

   Mohamed Boucadair
   France Telecom
   No.32 Xuanwumen West Street

   Email: mohamed.boucadair@orange.com

   Vizdal Ales
   Deutsche Telekom AG
   Tomickova 2144/1
   Prague 4,  149 00
   Czech Republic

   Email: ales.vizdal@t-mobile.cz

   Laurent Thiebaut

   Email: laurent.thiebaut@alcatel-lucent.com

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