CONEX WG                                                     D. Kutscher
Internet-Draft                                                    F. Mir
Intended status: Informational                                 R. Winter
Expires: January 10, 2013                                            NEC
                                                             S. Krishnan
                                                                Y. Zhang
                                                           CJ. Bernardos
                                                            July 9, 2012

           Mobile Communication Congestion Exposure Scenario


   This memo describes a mobile communications use case for congestion
   exposure (CONEX) with a particular focus on mobile communication
   networks such as 3GPP Evoled Packet System (EPS).  The draft provides
   a brief overview of the architecture of these networks (both access
   and core networks), current QoS mechanisms and then discusses how
   congestion exposure concepts could be applied.  Based on this, this
   memo suggests a set of requirements for CONEX mechanisms that
   particularly apply to mobile networks.

Status of this Memo

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

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   This Internet-Draft will expire on January 10, 2013.

Copyright Notice

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

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Overview of 3GPP's Evolved Packet System (EPS) . . . . . . . .  3
   3.  CONEX Use Cases in the Mobile Communication Scenario . . . . .  5
     3.1.  CONEX as a Basis for Traffic Management  . . . . . . . . .  6
     3.2.  CONEX to Incentivize Scavenger Transports  . . . . . . . .  7
     3.3.  Accounting for Congestion Volume . . . . . . . . . . . . .  8
     3.4.  CONEX as a Form of Differential QoS  . . . . . . . . . . .  9
     3.5.  Partial vs. Full Deployment  . . . . . . . . . . . . . . .  9
     3.6.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.  CONEX in the EPS . . . . . . . . . . . . . . . . . . . . . . . 11
     4.1.  Possible Deployment Scenarios  . . . . . . . . . . . . . . 11
     4.2.  Implementing CONEX Functions in the EPS  . . . . . . . . . 14
       4.2.1.  CONEX Protocol Mechanisms  . . . . . . . . . . . . . . 15
       4.2.2.  CONEX Functions in the Mobile Network  . . . . . . . . 15
   5.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   8.  Informative References . . . . . . . . . . . . . . . . . . . . 19
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 21
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21

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1.  Introduction

   Mobile data traffic continues to grow rapidly.  The challenge
   wireless operators face is to support more subscribers with higher
   bandwidth requirements.  To meet the bandwidth demand, there is a
   need for new technologies that assist the operators in efficiently
   utilizing the available network resources.  Two specific areas where
   such new technologies could be deemed useful are resource allocation
   and flow management.  Analysis of widely available statistics for
   network traffic from cellular networks are available, reveals that
   most flows are short-lived and low-volume, but there a few large
   flows that constitute a large part of the overall traffic volume.
   Measurements have also shown that a small number of users is
   responsible for the majority of traffic in cellular networks.  In
   view of such highly skewed user behavior and limited and expensive
   resources (Wireless Spectrum), resource allocation and usage
   accountability are two important issues for operators to solve in
   order to achieve a better and fair network resource utilization.
   CONEX, as described in [I-D.ietf-conex-concepts-uses], is a
   technology that can be used to do so.

   The CONEX congestion exposure mechanism is intended as a general
   technology that could be applied as a key element of congestion
   management solutions in a variety of use cases.  The IETF CONEX WG
   will however work on a specific use case, where the end hosts and the
   network that contains the destination end host are CONEX-enabled but
   other networks need not be.

   A specific example of such a use case can be a mobile communication
   network such as a 3GPP Evolved Packet System (EPS) network, where UEs
   (User Equipment, i.e. mobile end hosts), servers and caches, the
   access network and possibly an operator's core network can be CONEX-
   enabled.  I.e., hosts support the CONEX mechanisms, and the network
   provides policing/auditing functions at its edges.

   This document provides a brief overview of the architecture of such
   networks (access and core networks), current QoS mechanisms and then
   discusses how congestion exposure concepts can benefit such networks
   and how they should be applied.  Using this use case as a basis, a
   set of requirements for CONEX mechanisms are described.

2.  Overview of 3GPP's Evolved Packet System (EPS)

   This section provides an overview of 3GPP's "Evolved Packet System"
   (EPS [3GPP.36.300]) as a specific example of a mobile communication
   architecture in order to illustrate congestion exposure applicability
   in this memo.  There are other mobile communication architectures.

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   The EPS architecture and its standardized interfaces are depicted in
   Figure 1.  The EPS provides IP connectivity to user equipment (UE)
   (i.e., mobile nodes) and access to operator services, such as global
   Internet access and voice communications.  The EPS comprises the
   radio access network called evolved UMTS Terrestrial Radio Access
   Network (E-UTRAN) and the core network called Evolved Packet Core
   (EPC).  QoS is supported through an EPS bearer concept, providing
   hierarchical bindings within the network.

   The evolved NodeB (eNB), the Long Term Evolution (LTE) base station,
   is part of the access network that provides radio resource
   management, header compression, security and connectivity to the core
   network through the S1 interface.  In an LTE network, the control
   plane signaling traffic and the data traffic are handled separately.
   The eNBs transmit the control traffic and data traffic separately via
   two logically separate interfaces.

   The Home Subscriber Server, HSS, is a database that contains user
   subscriptions and QoS profiles.  The Mobility Management Entity, MME,
   is responsible for user authentication, bearer establishment and
   modification and maintenance of the UE context.

   The Serving gateway, S-GW, is the mobility anchor and manages the
   user plane data tunnels during the inter-eNB handovers.  It tunnels
   all user data packets and buffers downlink IP packets destined for
   UEs that happen to be in idle mode.

   The Packet Data Network (PDN) Gateway, P-GW, is responsible for IP
   address allocation to the UE and is a tunnel endpoint for mobility
   protocols.  It is also responsible for charging, packet filtering,
   and policy-based control of flows.  It interconnects the mobile
   network to external IP networks, e.g. the Internet.

   In this architecture, data packets are not sent directly on an IP
   network between the eNB and the gateways.  Instead, every packet is
   tunneled over a tunneling protocol - the GPRS Tunneling Protocol (GTP
   [3GPP.29.060]) over UDP/IP.  A GTP path is identified in each node
   with the IP address and a UDP port number on the eNB/gateways.  The
   GTP protocol carries both the data traffic (GTP-U tunnels) and the
   control traffic (GTP-C tunnels [3GPP.29.274]).  Alternativly Proxy
   Mobile IP (PMIPv6) is used on the S5 interface.

   The above is very different from an end-to-end path on the Internet
   where the packet forwarding is performed at the IP level.
   Importantly, we observe that these tunneling protocols give the
   operator a large degree of flexibility to control the congestion
   mechanism incorporated with the GTP/PMIPv6 protocols.

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                             +-------+                | PCRF  |
                             |  HSS  |               /+-------+\
                             +-------+            Gx/           \Rx
                                 |                 /             \
                                 |                /               \
                                 |          +-------+    SGi  +-------+
                                 |          |  P-GW |=========|   AF  |
                                 |          +-------+         +-------+
   HPMN                          |              |
   VPLMN                         |              |
                             +-------+          |
                             |  MME  |          |
                            /+-------+\         |S8
                    S1-MME /           \        |
                          /             \S11    |
                         /               \      |
                 +-----------+            \     |
   +----+ LTE-Uu |           |             \    |
   | UE |========|           |    S1-U      +-------+
   +----+        |  E-UTRAN  |==============| S-GW  |
                 |   (eNBs)  |              +-------+
                 |           |

             Figure 1: EPS (non-roaming) architecture overview

3.  CONEX Use Cases in the Mobile Communication Scenario

   In general, quality of service and good network resource utilization
   are important requirements for mobile communication network
   operators.  Radio access and backhaul networks are considered scarce
   resources, and bandwidth (and radio resource) demand is difficult to
   predict precisely due to user mobility, radio propagation effects
   etc.  Hence today's architectures and protocols go to significant
   extent in order to provide network-controlled quality of service --
   for instance by 3GPP's EPS bearer model that enables the network to
   allocate service data flows (SDFs) to certain EPS bearers with
   specific quality of service classes (which can be used for fine-
   granular per-application service differentiation).

   In the following, we discuss ways how congestion exposure could be
   beneficial for supporting resource management in such mobile
   communication networks.  [I-D.ietf-conex-concepts-uses] describes
   fundamental congestion exposure concepts and a set of use cases for
   applying congestion exposure mechanisms to realize different traffic

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   management functions, accounting etc.  Here, we relate these CONEX
   use cases to the general mobile communication scenario in order to
   validate the use cases for this scenario.

3.1.  CONEX as a Basis for Traffic Management

   Traffic management is a very important function in mobile
   communication networks.  Since wireless resources are considered
   scarce and since user mobility and shared bandwidth in the wireless
   access create certain dynamics with respect to available bandwidth,
   these resources are traditionally managed very tightly (admission
   control for bearer establishment etc.).

   In EPS, the QoS requirements for different applications running on a
   UE are supported by a bearer concept which is managed by the network.
   Each bearer has an associated QoS Class identifier (QCI) and an
   Allocation and Retention Policy (ARP) that has been standardized for
   uniform traffic handling (across implementations).  For the necessary
   QoS across the mobile network, an EPS bearer is maintained that
   crosses different interfaces in the network and maps to lower layer
   bearers for packet forwarding.  A radio bearer transports traffic
   between a UE and eNB whereas S1 bearer transports traffic between the
   eNB and S-GW.  Primarily LTE offers two types of bearer: Guaranteed
   Bit rate bearer for real time communication, e.g., Voice calls etc.
   and Non-Guaranteed bit rate, e.g., best effort traffic for web access
   etc.  Packets mapped to the same EPS bearer receive the same bearer
   level packet forwarding treatment.

   In the light of the significant increase of overall data volume in 3G
   networks, Deep-Packet-Inspection (DPI) is often considered a
   desirable function to have in the EPC -- on, for example, a PDN
   (Packet Data Network) gateway, and some operators do in fact deploy
   DPI today. 3GPP has a current work item on "Service Awareness and
   Privacy Policies" that is chartered to add DPI-related extensions to
   the PCC architecture [3GPP.23.203].  The (optional) DPI entity in the
   EPC is called "Traffic Detection Function" (TDF), and it performs
   application detection and reporting of detected application and its
   service data flow description to the Policy Control and Charging
   Rules Function (PCRF) for performing functions such as traffic
   blocking, redirection, policing for selected flows.

   Congestion exposure can be employed to address these requirements for
   tight resource management in different ways:

   1.  It can enhance DPI by providing flow policy-based traffic
       management.  At present, DPI-based resource management is often
       used to prioritize certain application classes with respect to
       others in overload situations, so that effectively more users can

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       be served on the network.  In overload situations, operators use
       DPI to identify dispensable flows and make them yield to other
       flows (of different application classes) through policing.  Such
       traffic management is thus based on static configuration and some
       estimation about the future per-flow bandwidth demand.  With
       congestion exposure it would be possible to assess, in a more
       accurate and dynamic fashion, the congestion that certain flows
       are causing.  This information can then be input to a policer
       that can optimize network utilization (better than a pure DPI-
       based approach can do).

   2.  It can reduce the need for DPI by allowing for a bulk packet
       traffic management system that does not have to consider flows'
       application classes and individual sessions.  Instead traffic
       management would be based on the current cost (contribution to
       congestion) incurred by different flows and enable operators to
       apply policing/accounting depending on their preference.  Such
       traffic management would be simpler and more robust (no real-time
       flow application type identification required, no static
       configuration of application classes) and perform better as
       decisions can be taken based on real-time actual cost

   3.  It can be used to more efectively trigger the offload of selected
       traffic to a non-3GPP network.  Nowadays, it is common that users
       are equipped with dual mode mobile phones (e.g., integrating
       third/fourth generation cellular and WiFi radio devices) capable
       of attaching to available networks either sequentially or
       simultaneously.  With this scenario in mind, 3GPP is currently
       looking at mechanisms to seamlessly and selectively switch over a
       single IP flow (e.g., user application) to a different radio
       access, while keeping all other ongoing connections untouched.
       The decision on when and which IP flows move is typically based
       on static configured rules, whereas the use of CONEX mechanisms
       could also factor in real-time congestion events in the decision.

   In summary, it can be said that traffic management in 3GPP EPS and
   other mobile communication architectures in very important.
   Currently, more static approaches based on admission control and
   static QoS are in use, but recently, there has been a perceived need
   for more dynamic mechanisms such as DPI.  Adding CONEX support might
   thus require slight changes the PCC architecture, depending on the
   scope and impact of a CONEX-based traffic management approach.

3.2.  CONEX to Incentivize Scavenger Transports

   As 3G and LTE networks are turning into universal access networks
   that are shared between mobile (smart) phone users, mobile users with

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   laptop PCs, home users with LTE access etc., it is likely that
   capacity-sharing among different users and application flows becomes
   more important in the mobile communication network as a fine-granular
   differentiation would be too costly.

   Most of this traffic is likely to be classified as best-effort
   traffic, without differentiating (for example) periodic OS updates,
   application store downloads from web (browser)-based or other more
   real-time communication.  Having said that, the general argument for
   scavenger transports apply.  Especially when wireless and backhaul
   resources are scarce, incentivizing users to use less-than best
   effort transport for non-interactive background communication would
   improve the overall utility of the network.  It can be argued that,
   if this would be done with a CONEX approach, it could be done in a
   more effective and cost-efficient way compared to the aforementioned
   DPI mechanisms.

   This would work best if the network did not do any traffic class
   segregation below the IP layer, i.e., if all traffic would be in the
   same traffic class.  In principle, this would be possible to
   implement with current specifications.

3.3.  Accounting for Congestion Volume

   3G and LTE networks provide extensive support for accounting and
   charging already, for example cf. the Policy Charging Control (PCC)
   architecture.  In fact, most operators today account transmitted data
   volume on a very fine granular basis and either correlate monthly
   charging to the exact number of packets/bytes transmitted, or employ
   some form of flat rate (or flexible flat rate), often with a so-
   called fair-use policy.  With such policies, users are typically
   limited to an administratively configured maximum bandwidth limit,
   after they have used their data contractual volume budget for the
   charging period.

   Changing this data volume-based accounting to a congestion-based
   accounting would be possible in principle, especially since there
   already is an elaborate per-user accounting system available.  Also,
   an operator-provided mobile communication network can be seen as a
   network domain within such congestion volume accounting would be
   possible, without requiring any support from the global Internet.
   Traffic normally leaves/enters the operator's network via well-
   defined egress/ingress points that would be ideal candidates for
   policing functions.  Moreover, in most commercially operated
   networks, accounting is performed for both received and sent data,
   which would facilitate congestion volume accounting as well.

   With respect to the current PCC framework, accounting for congestion

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   volume could be added as another feature to the "Usage Monitoring
   Control" capability that is currently based on data volume.  This
   would not require any new interface (reference points) at all.

3.4.  CONEX as a Form of Differential QoS

   As mentioned above, 3GPP mobile communication networks provide an
   elaborate QoS architecture.  In LTE, the idea is to map different
   traffic classes onto different logical channels (bearers) with
   individual QoS configuration.

   It can be argued whether this approach is sufficient in a world where
   most traffic is on TCP port 80 and whether some more application
   control would be useful.

   With CONEX, accurate downstream path information would be visible to
   ingress network operators, which can respond to incipient congestion
   in time.  This can be equivalent to offering different levels of QoS,
   e.g. premium service with zero congestion response.

   Again, CONEX could be used in two different ways:

   1.  as additional information to assist network functions to impose
       different QoS for different application sessions; and

   2.  as a tool to let applications decide on their response to
       congestion notification, while incentivizing them to react (in
       general) appropriately, e.g., by enforcing overall limits for
       congestion contribution or by accounting and charging for such
       congestion contribution.

3.5.  Partial vs. Full Deployment

   In general CONEX lends itself to partial deployment as the mechanism
   does not require all routers and hosts to support congestion
   exposure.  Moreover, assuming a policing infrastructure has been put
   in place, it is not required to modify all hosts.  Since CONEX is
   about senders exposing congestion contribution to the network,
   senders need to be made CONEX-aware (assuming a congestion
   notification mechanisms such as ECN is in place).

   [I-D.briscoe-conex-initial-deploy] provides specific examples of how
   CONEX deployment can be initiated, focusing unilaterial deployment by
   single networks, i.e., by partial deployment.

   In mobile communication networks that would for example allow early
   partial CONEX deployment in the downlink direction only, i.e.,
   servers, gateways and caches would support CONEX but UEs (mobile

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   hosts) would not.

   When moving towards full deployment in a specific operator's network,
   different ways for introducing CONEX support on UEs are feasible.
   Since mobile communication networks are multi-vendor networks,
   standardizing CONEX support on UEs (e.g., in 3GPP specifications)
   appears useful.  Still, not all UEs would have to support CONEX, and
   operators would be free to choose their policing approach in such
   deployment scenarios.  Leveraging existing PCC architectures, 3GPP
   network operators could for example decide policing/accounting
   approaches per UE -- i.e., apply fixed volume caps for non-CONEX UEs
   and more flexible schemes for CONEX-enabled UEs.

   Moreover, it should be noted that network support for CONEX is a
   feature that some operators may implement to deploy if they wish, but
   it is not required that all operators (or all other networks) do so.

   Depending on the extent of CONEX support, specific aspects such as
   roaming have to be taken into account.  I.e., what happens when a
   user is roaming in a CONEX-enabled network, but their UE is not
   CONEX-enabled and vice versa.  Although these may not be fundamental
   problems, they need to be considered.  For supporting mobility in
   general, it can be required to shift users' policing state during
   hand-over.  There is existing work in [raghavan2007] on distributed
   rate limiting and in [nec.euronf-2011] on specific optimizations for
   congestion exposure and policing in mobility scenarios.

   Another aspect to consider is the addition of Selected IP Traffic
   Offload (SIPTO) and Local Breakout (LIPA), also see [3GPP.23.829],
   i.e., the idea that some traffic (e.g., high-volume Internet traffic)
   is actually not passed through the EPC but is offloaded at a "break-
   out point" closer to (or in) the access network.  On the other hand,
   CONEX can also enable more dynamic decisions on what traffic to
   actually offload by considering congestion exposure in bulk traffic
   aggregates -- thus making traffic offload more effective.

3.6.  Summary

   In summary, the 3GPP EPS is a system architecture that can benefit
   from congestion exposure in multiple ways, as we have shown by this
   brief description of CONEX use cases in this environment.  Dynamic
   traffic and congestion management is an acknowledged important
   requirement for the EPS, also illustrated by the current DPI-related
   work for EPS.

   Moreover, we believe that networks such as an EPS mobile
   communication network would be quite amenable for deploying CONEX as
   a mechanism, since they represent clearly defined and well separated

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   operational domains, in which local CONEX deployment would be
   possible.  Aside from roaming (which needs to be considered for a
   specific solution), such a deployment is fully under the control of a
   single operator, which can enable operator-local enhancement without
   the need for major changes to the architecture.

   In 3GPP EPS, interfaces between all elements of the architecture are
   subject to standardization, including UE interfaces and eNodeB
   interfaces, so that a more general approach, involving more than one
   single operator's network, can be feasible as well.

4.  CONEX in the EPS

   The CONEX mechanism is still work in progress in the IETF working
   group.  Still, we would like to discuss a few options for how such a
   mechanism (and possibly additional policing functions) could
   eventually be deployed in 3GPP's EPS.  Note that this description of
   options is not intended as a complete set of possible approaches --
   it is merely intended for discussing a few options.  More details
   will be provided in a future revision of this document.

4.1.  Possible Deployment Scenarios

   There are different possible ways how CONEX functions on hosts and
   network elements can be used.  For example, CONEX could be used for a
   limited part of the network only -- e.g., for the access network --
   congestion exposure and sender adaptation could involve the mobile
   nodes or not, or, finally, the CONEX feedback loop could extend
   beyond a single operator's domain or not.

   We present three different deployment scenarios for congestion
   exposure in the figures below:

   1.  In Figure 2 CONEX is supported by servers for sending data (here:
       web servers in the Internet and caches in an operator's network)
       but not by UEs (neither for receiving nor sending).  An operator
       who chooses to run a policing function on the network ingress
       (e.g., on the P-GW) can still benefit from congestion exposure
       without requiring any change on UEs.

   2.  CONEX is universally employed between operators (as depicted in
       Figure 3), with an end-to-end CONEX feedback loop.  Here,
       operators could still employ local policies, congestion
       accounting schemes etc., and they could use information about
       congestion contribution for determining interconnection

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   3.  Isolated CONEX domains as depicted in Figure 4, CONEX is solely
       applied locally, in the operator network, and there is no end-to-
       end congestion exposure.  This could be the case when CONEX is
       only implemented in a few networks, or when operators decide to
       not expose ECN and account for congestion for inter-domain
       traffic.  Independent of the actual scenario, it is likely that
       there will be border gateways (as in today's deployments) that
       are associated with policing and accounting functions.

                                     | Web server |
                                     | w/ CONEX   |
                            |                  |  |
                            |     Internet     |  |
                            |                  |  |
   |                                           |       |
   |                                     +-----------+ |
   |                                     | Web cache | |
   |                                     | w/ CONEX  | |
   |                                     +-----------+ |
   |                                           |       |
   |  +----+     +-------+     +-------+     +-------+ |
   |  | UE |=====|  eNB  |=====|  S-GW |=====|  P-GW | |
   |  +----+     +-------+     +-------+     +-------+ |
   |                                                   |
   |              Operator A                           |

               Figure 2: CONEX support on servers and caches

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   |  +----+     +-------+     +-------+     +-------+ |
   |  | UE |=====|  eNB  |=====|  S-GW |=====|  P-GW | |
   |  +----+     +-------+     +-------+     +-------+ |
   |                                           |       |
   |              Operator A                   |       |
                            |                     |
                            |     Internet        |
                            |                     |
   |  +----+     +-------+     +-------+     +-------+ |
   |  | UE |=====|  eNB  |=====|  S-GW |=====|  P-GW | |
   |  +----+     +-------+     +-------+     +-------+ |
   |                                                   |
   |              Operator B                           |

            Figure 3: CONEX deployment across operator domains

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   |   |---            CONEX path            ---|      |
   |   v                                        v
   |  +----+     +-------+     +-------+     +-------+ |
   |  | UE |=====|  eNB  |=====|  S-GW |=====|  P-GW | |
   |  +----+     +-------+     +-------+     +-------+ |
   |                                           |       |
   |              Operator A                   |       |
                            |                     |
                            |     Internet        |
                            |                     |
   |  +----+     +-------+     +-------+     +-------+ |
   |  | UE |=====|  eNB  |=====|  S-GW |=====|  P-GW | |
   |  +----+     +-------+     +-------+     +-------+ |
   |                                                   |
   |              Operator B                           |

          Figure 4: CONEX deployment in a single operator domain

   We consider all three scenarios to be relevant and believe that all
   of them are within the scope of the CONEX WG charter.  A more
   detailed description will be provided in a future version of this

4.2.  Implementing CONEX Functions in the EPS

   We expect a CONEX solution to consist of different functions that
   should be considered when implementing congestion exposure in 3GPP's
   EPS.  [I-D.ietf-conex-abstract-mech] is describing the following
   congestion exposure components:

   o  Modified senders that send congestion exposure information in
      response to congestion feedback).

   o  Receivers that generate congestion feedback (leveraging existing
      behavior or requiring new functions).

   o  Audit functions that audit CONEX signals against actual
      congestion, e.g., by monitoring flows or aggregate of flows.

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   o  Policy devices that monitor congestion exposure information and
      act on the flows according to the operator's policy.

   Two aspects are important to consider: 1) how the CONEX protocol
   mechanisms would be implemented and what modifications to existing
   networks would be required and 2) where CONEX functional entities
   would be placed best (to allow for a non-invasive addition).  We
   discuss these two aspects in the following sections.

4.2.1.  CONEX Protocol Mechanisms

   As described in [I-D.briscoe-conex-initial-deploy], the most
   important step in introducing CONEX (initially) is adding the
   congestion exposure functionality to senders.  For an initial
   deployment, no further modification to senders and receivers would be
   required.  Specifically, there is no fundamental dependency on ECN,
   i.e., CONEX can be introduced without requiring ECN to be

   Congestion exposure information for IPv6 [I-D.ietf-conex-destopt] is
   contained in a destination option header field, which requires
   minimal changes at senders and nodes that want to assess path
   congestion -- and that does not affect non-CONEX nodes in a network.

   In 3GPP networks, IP tunneling is used intensively, i.e., using
   either IP-in-GTP-U or PMIPv6 (i.e., IP-in-IP) tunnels.  In general,
   the CONEX destination option of encapsulated packets should be made
   available for network nodes on the tunnel path, i.e., a tunnel
   ingress should copy the CONEX destination option field to the outer

   For an effective and efficient capacity sharing, we envisage the
   deployment of ECN in conjunction with CONEX so that ECN-enabled
   receivers and senders get more accurate and more timely information
   about their flows congestion contribution.  ECN is already partially
   introduced into 3GPP networks: Section 11.6 in [3GPP.36.300]
   specifies the usage of ECN for congestion notification on the radio
   link (between eNB and UE), and [3GPP.26.114] specifies how this can
   be leveraged for voice codec adaptation.  A complete, end-to-end
   support of ECN would require specification of tunneling behaviour,
   which should be based on [RFC6040] (for IP-in-IP tunnels) and on
   [I-D.briscoe-tsvwg-ecn-encap-guidelines].  Specifically, a
   specification for tunneling ECN in GTP-U will be needed.

4.2.2.  CONEX Functions in the Mobile Network

   In the following, we discuss some possible placement strategies for
   CONEX functional entities (addressing both policing and auditing

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   functions) in the EPS and for possible optimizations for both the
   uplink and the downlink.

   In general, CONEX information (exposed congestion) is declared by a
   sender and remains unchanged on the path, hence reading CONEX
   information (e.g., by policing functions) is placement-agnostic.
   Auditing CONEX normally requires assessing declared congestion
   contribution and current actual congestion.  If the latter is, for
   example, done using ECN, such a function would best be placed at the
   end of the path.

   In order to provide a comprehensive CONEX-based capacity management
   framework for EPS, it would be advantageous to consider user
   contribution to congestion for both the radio access and the core
   network.  For a non-invasive introduction of CONEX, it can be
   beneficial to combine CONEX functions with existing logical EPS
   entities.  For example, potential places for CONEX policing and
   auditing functions would then be eNBs, S-GWs or the P-GWs.  Operator
   deployments may of course still provide additional intermediary
   CONEX-enabled IP network elements.

   For a more specific discussion it will be beneficial to distinguish
   downlink and uplink traffic directions (also see [nec.globecom2010]
   for a more detailed discussion).  In today's networks and usage
   models, downlink traffic is dominating (also reflected by the
   asymmetric capacity provided by the LTE radio interface).  That does
   however not imply that uplink congestion is not an issue, since the
   asymmetric maximum bandwidth configuration can create a smaller
   bottleneck for uplink traffic -- and there are of course backhaul
   links, gateways etc. that could be overloaded as well.

   For managing downlink traffic -- e.g., in scenarios such as the one
   depicted in Figure 2, operators can have different requirements for
   policing traffic.  Although policing is in principle location-
   agnostic, it is important to consider requirements related to the EPS
   architecture (Figure 1) such as tunneling between P-GWs and eNBs.
   Policing can require access to subscriber information (e.g.,
   congestion contribution quota) or user-specific accounting, which
   suggests that the CONEX function could be co-located with the P-GW
   that already has an interface towards the PCRF.

   Still, policing can serve different purposes.  For example, if the
   objective is to police bulk traffic induced by peer networks,
   additional monitoring functions can be placed directly at
   corresponding ingress points to monitor traffic and possible drive
   out-of-band functions such as triggering border contract penalties.

   The auditing function which should be placed at the end of the path

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   (at least after/at the last bottleneck) would likely be placed best
   on the eNB (wireless base station).

   For the uplink direction, there are naturally different options for
   designing monitoring and policy enforcement functions.  A likely
   approach can be to monitor congestion exposure on central gateway
   nodes (such as P-GWs) that provide the required interfaces to the
   PCRF, but to perform policing actions in the access network, i.e., in
   eNBs, e.g., to police traffic at the ingress, before it reaches
   concentration points in the core network.

   Such a setup would enable all the CONEX use cases described in
   Section 3, without requiring significant changes to the EPS
   architecture, while enabling operators to re-use existing
   infrastructure, specifically wireless base stations, PCRF and HSS

   For CONEX functions on elements such as the S-GWs and P-GWs, it is
   important to consider mobility and tunneling protocol requirements.
   LTE provides two alternative approaches: Proxy-Mobile-IPv6 (PMIPv6,
   [3GPP.23.402]) and GPRS Tunneling Protocol (GTP).  For the
   propagation of congestion information (responses) tunneling
   considerations are therefore very important.

   In general, policing will be done based on per-user (per subscriber)
   information such as congestion quota, current quota usage etc. and
   network operator policies, e.g., specifying how to react to
   persistent congestion contribution etc.  In the EPS, per-user
   information is normally part of the user profile (stored in the HSS)
   that would be accessed by PCC entities such as the PCRF for dynamic
   updates, enforcement etc.

   A more detailed description of the different approaches and their
   respective advantages will be provided in a future revision of this

5.  Summary

   We have shown how congestion exposure can be useful for efficient
   resource management in mobile communication networks.  The premise
   for this discussion was the observation that data communication,
   specifically best-effort bulk data transmission, is becoming a
   commodity service whereas resources are obviously still limited --
   which calls for efficient, scalable, yet effective capacity sharing
   in such networks.

   CONEX can be a mechanism that enables such capacity sharing, while

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   allowing operators to apply these mechanisms in different ways, e.g.,
   for implementing different use cases as described in Section 3.  It
   is important to note that CONEX is fundamentally a mechanism that can
   be applied in different ways -- to realize different operators

   We have described a few possibilities for adding CONEX as a mechanism
   to 3GPP LTE-based networks and have shown how this could be done
   incrementally (starting with partial deployment).  It is quite
   feasible that such partial deployments be done on a per-operator-
   domain basis, without requiring changes to standard 3GPP interfaces.
   For a network-wide deployment, e.g., with congestion exposure between
   operators, more considerations might be needed.

   We have also identified a few implications/requirements that should
   be taken into consideration when enabling congestion exposure in such

   Performance:  In mobile communication networks -- with more expensive
      resources and more stringent QoS requirements -- the feasibility
      of applying CONEX as well as its performance and deployment
      scenarios need to be examined closer.  For instance, a mobile
      communication network may encounter longer delay and higher loss
      rates, which can impose specific requirements on the timeliness
      and accuracy of congestion exposure information.

   Mobility:  One of the unique characteristics in cellular network is
      the presence of user mobility compared to wired networks.  As the
      user location changes, the same device can be connected to the
      network via different base stations (eNodeBs) or even go through
      switching gateways.  Thus, the CONEX scheme must to be able to
      carry latest congestion information per user/flow across multiple
      network nodes in real time.

   Multi-access:  In cellular network, multiple access technologies can
      co-exist.  In such cases, a user can use multiple access
      technologies for multiple applications or even a single
      application simultaneously.  If the congestion policies are set
      based on each user, then CONEX should have the capability to
      enable information exchange across multiple access domains.

   Tunneling:  Both 3G and LTE networks make extensive usage of
      tunneling.  The CONEX mechanism should be designed in a way to
      support usage with different tunneling protocols such as PMIPv6
      and GTP.  For ECN-based congestion notification, [RFC6040]
      specifies how the ECN field of the IP header should be constructed
      on entry and exit from IP-in-IP tunnels, and
      [I-D.briscoe-tsvwg-ecn-encap-guidelines] provides guidelines for

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      adding congestion notification to protocols that encapsulate IP.

   Roaming:  Independent of the specific architecture, mobile
      communication networks typically differentiate between non-roaming
      and roaming scenarios.  Roaming scenarios are typically more
      demanding regarding implementing operator policies, charging etc.
      It can be expected that this would also hold for deploying CONEX.
      A more detailed analysis of this problem will be provided in a
      future revision of this document.

   It is important to note that CONEX is intended to be used as a
   supplement and not a replacement to the existing QoS mechanisms in
   mobile networks.  For example, CONEX deployed in 3GPP mobile networks
   can provide useful input to the existing 3GPP PCC mechanisms by
   supplying more dynamic network information to supplement the fairly
   static information used by the PCC.  This would enable the mobile
   network to make better policy control decisions than is possible with
   only static information.

6.  IANA Considerations

   No IANA considerations.

7.  Security Considerations

   Security considerations for applying CONEX to EPS include, but are
   not limited to, the security considerations that apply to the CONEX

8.  Informative References

              3GPP, "Policy and charging control architecture", 3GPP
              TS 23.203 10.7.0, June 2012.

              3GPP, "Architecture enhancements for non-3GPP accesses",
              3GPP TS 23.402 10.7.0, March 2012.

              3GPP, "Local IP Access and Selected IP Traffic Offload
              (LIPA-SIPTO)", 3GPP TR 23.829 10.0.1, October 2011.

              3GPP, "IP Multimedia Subsystem (IMS); Multimedia

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              telephony; Media handling and interaction", 3GPP TS 26.114
              10.4.0, June 2012.

              3GPP, "General Packet Radio Service (GPRS); GPRS
              Tunnelling Protocol (GTP) across the Gn and Gp interface",
              3GPP TS 29.060 3.19.0, March 2004.

              3GPP, "3GPP Evolved Packet System (EPS); Evolved General
              Packet Radio Service (GPRS) Tunnelling Protocol for
              Control plane (GTPv2-C); Stage 3", 3GPP TS 29.274 10.7.0,
              June 2012.

              3GPP, "Evolved Universal Terrestrial Radio Access (E-UTRA)
              and Evolved Universal Terrestrial Radio Access Network
              (E-UTRAN); Overall description; Stage 2", 3GPP TS 36.300
              10.8.0, July 2012.

              Briscoe, B., "Initial Congestion Exposure (ConEx)
              Deployment Examples",
              draft-briscoe-conex-initial-deploy-02 (work in progress),
              March 2012.

              Briscoe, B., "Guidelines for Adding Congestion
              Notification to Protocols that Encapsulate IP",
              draft-briscoe-tsvwg-ecn-encap-guidelines-00 (work in
              progress), March 2011.

              Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
              Concepts and Abstract Mechanism",
              draft-ietf-conex-abstract-mech-04 (work in progress),
              March 2012.

              Briscoe, B., Woundy, R., and A. Cooper, "ConEx Concepts
              and Use Cases", draft-ietf-conex-concepts-uses-04 (work in
              progress), March 2012.

              Krishnan, S., Kuehlewind, M., and C. Ucendo, "IPv6
              Destination Option for Conex", draft-ietf-conex-destopt-02
              (work in progress), March 2012.

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   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, November 2010.

              Mir, Kutscher, and Brunner, "Congestion Exposure in
              Mobility Scenarios", in proceedings of 7th EURO-NF

              Kutscher, Lundqvist, and Mir, "Congestion Exposure in
              Mobile Wireless Communications", in proceedings of IEEE
              GLOBECOM 2010, December 2010.

              Raghavan, Vishwanath, Ramabhadran, Yocum, and Snoeren,
              "Cloud Control with Distributed Rate Limiting", in
              proceedings of ACM SIGCOMM 2007, 2007.


Appendix A.  Acknowledgments

   We would like to thank Bob Briscoe and Ingemar Johansson for their
   support in shaping the overall idea and in improving the draft by
   providing constructive comments.

Authors' Addresses

   Dirk Kutscher
   Kurfuersten-Anlage 36


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   Faisal Ghias Mir
   Kurfuersten-Anlage 36


   Rolf Winter
   Kurfuersten-Anlage 36


   Suresh Krishnan
   8400 Blvd Decarie
   Town of Mount Royal, Quebec


   Ying Zhang
   200 Holger Way
   San Jose, CA  95134


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   Carlos J. Bernardos
   Universidad Carlos III de Madrid
   Av. Universidad, 30
   Leganes, Madrid  28911

   Phone: +34 91624 6236

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