CONEX WG                                                     D. Kutscher
Internet-Draft                                                    F. Mir
Intended status: Informational                                 R. Winter
Expires: April 20, 2016                                              NEC
                                                             S. Krishnan
                                                                Y. Zhang
                                                           CJ. Bernardos
                                                        October 18, 2015

           Mobile Communication Congestion Exposure Scenario


   This memo describes a mobile communications use case for congestion
   exposure (ConEx) with a particular focus on those mobile
   communication networks that are architecturally similar to the 3GPP
   Evolved 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 these
   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 April 20, 2016.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Acronyms . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  ConEx Use Cases in Mobile Communication Networks . . . . . . .  4
     2.1.  ConEx as a Basis for Traffic Management  . . . . . . . . .  4
     2.2.  ConEx to Incentivize Scavenger Transports  . . . . . . . .  6
     2.3.  Accounting for Congestion Volume . . . . . . . . . . . . .  7
     2.4.  Partial vs. Full Deployment  . . . . . . . . . . . . . . .  7
     2.5.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   3.  CONEX in the EPS . . . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Possible Deployment Scenarios  . . . . . . . . . . . . . .  9
     3.2.  Implementing CONEX Functions in the EPS  . . . . . . . . . 14
       3.2.1.  CONEX Protocol Mechanisms  . . . . . . . . . . . . . . 14
       3.2.2.  CONEX Functions in the Mobile Network  . . . . . . . . 15
   4.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   7.  Informative References . . . . . . . . . . . . . . . . . . . . 19
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 21
   Appendix B.  Overview of 3GPP's Evolved Packet System (EPS)  . . . 21
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23

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1.  Introduction

   Mobile data traffic continues to grow rapidly.  The challenge
   wireless operators face is to support more subscribers with an
   increasing bandwidth demand.  To meet these bandwidth requirements,
   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 cellular network data traffic has shown that most flows
   are short-lived and low-volume, but a comparatively small number of
   high-volume flows constitute a large fraction of the overall traffic
   volume [lte-sigcomm2013].  That means that potentially a small
   fraction 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 (e.g. the wireless spectrum),
   resource allocation and usage accountability are two important issues
   for operators to solve in order to achieve both a better network
   resource utilization and fair resource sharing.  ConEx, as described
   in [RFC6789], is a technology that can be used to achieve these

   The ConEx congestion exposure mechanism is designed to be a general
   technology that could be applied as a key element of congestion
   management solutions for a variety of use cases.  The IETF CONEX WG
   decided to initially start to work on a specific use case, where the
   end hosts and the network that contains the destination end hosts 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) and current QoS mechanisms.  It
   further discusses how such networks can benefit from congestion
   exposure concepts and how they should be applied.  Using this use
   case as a basis, a set of requirements for ConEx mechanisms are

1.1.  Acronyms

   In the following we expand some acronyms that are used in throughout
   the text.  Most of them are explained and put in a system context in

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   Appendix B and the 3GPP specifications referenced there.

      Evolved NodeB: LTE base station

      Serving Gateway: mobility anchor and tunnel endpoint

      Packet Data Network (PDN) Gateway: tunnel endpoint for user and
      control plane protocol -- typically the GW to the Internet or an
      operator's service network

      User Equipment: mobile terminals

2.  ConEx Use Cases in Mobile Communication Networks

   In general, quality of service and good network resource utilization
   are important requirements for mobile communication network
   operators.  Radio access and backhaul capacity 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.
   These efforts often lead to complexity and cost.  ConEx could be
   simpler and more capable approach to efficient resource sharing in
   these networks.

   In the following, we discuss ways how congestion exposure could be
   beneficial for supporting resource management in such mobile
   communication networks.  [RFC6789] describes fundamental congestion
   exposure concepts and a set of use cases for applying congestion
   exposure mechanisms to realize different traffic management functions
   such as flow policy-based traffic management or traffic offloading.
   Readers that are not familiar with the 3GPP Evolved Packet System
   (EPS) should refer to Appendix B first.

2.1.  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,
   commercially operated mobile networks provide mechanisms for tight
   resource management (admission control for bearer establishment).
   However, sometimes these mechanisms are not easily applicable to IP-

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   and HTTP-dominated traffic mixes, for example, most Internet traffic
   in today's mobile network is transmitted over the (best-effort)
   default bearer.

   Given the above, and 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 -- despite
   its cost and complexity.  With the increase of encrypted data
   traffic, traffic management using DPI alone however will become even
   more challenging.

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

   1.  It can enable or enhance 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 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 operator decisions -- using partly
       static configuration and some estimation about the future per-
       flow bandwidth demand.  With congestion exposure it would be
       possible to assess the contribution to congestion of individual
       flows.  This information can then be used as input to a policer
       that can optimize network utilization more accurately and
       dynamically.  By using ConEx congestion contribution as a metric,
       such policers would not need to be aware of specific link loads
       (e.g., in wireless base stations) or flow application types.

   2.  It can reduce the need for complex DPI by allowing for a bulk
       packet traffic management system that does not have to consider
       the application classes flows belong to 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 contribution.  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.  For that, ConEx could be
       used in two different ways:

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       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.  Note that this level of
           responsiveness would be on a different level then, say,
           application-layer responsive in protocols such as DASH
           [dash], however it could interwork with such protocols, for
           example by triggering earlier responses.

   3.  It can further be used to more effectively 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 statically configured rules, whereas
       the use of ConEx mechanisms could also factor in real-time
       congestion information into the decision.

   In summary, it can be said that traffic management in the 3GPP EPS
   and other mobile communication architectures is 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 based on DPI.  Introducing ConEx could
   make these mechanisms more efficient or even remove the need for some
   of the DPI functions deployed today.

2.2.  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
   laptop PCs, home users with LTE access and others, capacity-sharing
   among different users and application flows becomes increasingly
   important in these mobile communication networks.

   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.  For many of the bulk data transfers,
   completion times aren't important within certain bounds and therefore

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   if scavenger (or less-than best effort) transports like e.g.  LEDBAT
   [RFC6817] were used, it would improve the overall utility of the
   network.  The use of these transports by the end user however needs
   to be incentivized.  ConEx could be used to build an incentive scheme
   e.g. by allowing users that contribute less to congestion to give
   them a larger bandwidth allowance or e.g. to lower the next monthly
   subscription fee.  In principle, this would be possible to implement
   with current specifications.

2.3.  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 [3GPP.23.203].  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 up their contractual data
   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, in
   particular since the typical scare resources such as the wireless
   access and the mobile backhaul are all within this domain.  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
   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.

2.4.  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,

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   senders need to be made ConEx-aware (assuming a congestion
   notification mechanisms such as ECN is in place).

   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 choose 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.

2.5.  Summary

   In summary, the 3GPP EPS is a system architecture that can benefit
   from congestion exposure in multiple ways.  Dynamic traffic and
   congestion management is an acknowledged and important requirement
   for the EPS, also illustrated by the current DPI-related work for

   Moreover, 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 operational domains, in

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   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.

3.  CONEX in the EPS

   At the time of writing, the CONEX mechanism is still work in progress
   in the IETF working group.  Still, discussing a few options for how
   such a mechanism (and possibly additional policing functions) could
   eventually be deployed in 3GPP's EPS is useful at this point.  Note
   that this description of options is not intended as a complete set of
   possible approaches -- it is merely intended for discussing the most
   promising options.

3.1.  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 four different deployment scenarios for congestion
   exposure in the figures below:

   1.  In Figure 1 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 (Packet Data Network -- PDN -- Gateway) can
       still benefit from congestion exposure without requiring any
       change on UEs.

   2.  CONEX is universally employed between operators (as depicted in
       Figure 2), 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
       agreements.  This deployment scenario would imply the willingness

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       of operators to expose congestion to each other.

   3.  Isolated CONEX domains as depicted in Figure 3, where 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.

   4.  [conex-lite] describes an approach called "ConEx Lite" for mobile
       networks that is intended for initial deployment of congestion
       exposure concepts in LTE, specifically in the backhaul and core
       network segments.  As depicted in Figure 4 ConEx Lite allows a
       tunnel receiver to monitor the volume of bytes that has been lost
       or dropped (or ECN-CE marked) between the tunnel sender and
       receiver.  For that purpose, a new field is introduced to the
       tunnel header called the Byte Sequence Marker (BSN) that
       identifies the byte in the flow of data from the tunnel sender to
       the tunnel receiver.  A policer at the tunnel sender is expected
       to re-act according to the tunnel congestion volume (see
       [conex-lite] for details.)

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

               Figure 1: 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 2: 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 3: CONEX deployment in a single operator domain

                   Backhaul Network     Core Network
                  +---------------+  +--------------+
                  |               |  |              |
                  | BSN or ECN-CE |  |              |
                  | marked        |  |              |
                  | packets       |  |              |
                  |    <---       |  |              |
   +----+     +-------+       +----------+       +-------+  +--------+
   |    |     |       | GTP-U |          | GTP-U |       |  |        |
   | UE |=====|  eNB  |=======|   S-GW   |=======|  P-GW |==|Internet|
   |    |     |       | Tunnel|          | Tunnel|       |  |        |
   +----+     +-------+       +----------+       +-------+  +--------+
                  |    --->       |  |              |
                  | User/control  |  | User/control |
                  | packets with  |  | packet with  |
                  | DL congestion |  | DL congestion|
                  | vol counters  |  | vol counters |
                  |               |  |              |
                  +---------------+  +--------------+

                      Figure 4: CONEX-lite deployment

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   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

3.2.  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.

   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.

3.2.1.  CONEX Protocol Mechanisms

   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

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   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).
   Specifically, a specification for tunneling ECN in GTP-U will be

3.2.2.  CONEX Functions in the Mobile Network

   In the following, we discuss some possible placement strategies for
   CONEX functional entities (addressing both policing and auditing
   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 (Serving Gateways) or
   the P-GWs (Packet Data Network -- PDN -- Gateways).  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

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   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 1, 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 5) 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
   (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 2, 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

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   persistent congestion contribution.  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.

4.  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
   allowing operators to apply these mechanisms in different ways, e.g.,
   for implementing different use cases as described in Section 2.  It
   is important to note that CONEX is fundamentally a mechanism that can
   be applied in different ways -- to realize different operators

   CONEX may also be used to complement 3GPP-based mechanisms for
   congestion management which are currently under development, such as
   in the User Plane Congestion Management (UPCON) work item described
   in [3GPP.23.705].

   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.

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   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 networks, 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.

   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.

5.  IANA Considerations

   No IANA considerations.

6.  Security Considerations

   For any ConEx deployment, it is important to apply appropriate

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   mechanisms to preclude applications and senders from mis-stating
   their congestion contribution.  [I-D.ietf-conex-abstract-mech]
   discusses this problem in detail and introduces the ConEx auditing
   concept.  ConEx auditing can be performed in different ways -- for
   example flows can be constantly audited or only on-demand, when
   network operators decide to do so.  Also, coarse-grained auditing may
   operate on flow aggregates for efficiency reasons, whereas fine-
   grained auditing would inspect individual flow.  In mobile networks,
   there may be deployment strategies that favor efficiency over very
   exact auditing.  It is important to understand the trade-offs and to
   apply ConEx auditing appropriately.

   The ConEx protocol specifications in [I-D.ietf-conex-destopt] and
   [I-D.ietf-conex-tcp-modifications] are discussing additional security
   considerations that would also apply to mobile network deployments.

7.  Informative References

              3GPP, "Policy and charging control architecture", 3GPP
              TS 23.203 10.9.0, September 2013.

              3GPP, "General Packet Radio Service (GPRS) enhancements
              for Evolved Universal Terrestrial Radio Access Network
              (E-UTRAN) access", 3GPP TS 23.401 10.10.0, March 2013.

              3GPP, "Architecture enhancements for non-3GPP accesses",
              3GPP TS 23.402 10.8.0, September 2012.

              3GPP, "System Enhancements for User Plane Congestion
              Management", 3GPP TR 23.705 0.8.0, October 2013.

              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
              telephony; Media handling and interaction", 3GPP TS 26.114
              10.7.0, June 2013.

              3GPP, "General Packet Radio Service (GPRS); GPRS
              Tunnelling Protocol (GTP) across the Gn and Gp interface",

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              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 10.11.0,
              June 2013.

              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
              10.11.0, September 2013.

              Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
              Concepts, Abstract Mechanism and Requirements",
              draft-ietf-conex-abstract-mech-13 (work in progress),
              October 2014.

              Krishnan, S., Kuehlewind, M., and C. Ucendo, "IPv6
              Destination Option for Congestion Exposure (ConEx)",
              draft-ietf-conex-destopt-09 (work in progress),
              August 2015.

              Kuehlewind, M. and R. Scheffenegger, "TCP modifications
              for Congestion Exposure",
              draft-ietf-conex-tcp-modifications-10 (work in progress),
              October 2015.

   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, DOI 10.17487/RFC6040,
              November 2010, <>.

   [RFC6789]  Briscoe, B., Ed., Woundy, R., Ed., and A. Cooper, Ed.,
              "Congestion Exposure (ConEx) Concepts and Use Cases",
              RFC 6789, DOI 10.17487/RFC6789, December 2012,

   [RFC6817]  Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
              "Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
              DOI 10.17487/RFC6817, December 2012,

              Baillargeon and Johansson, "ConEx Lite for Mobile

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              Networks", in proceedings of ACM SIGCOMM-2014 Capacity
              Sharing Workshop (CSWS-2014), August 2014.

   [dash]     ISO/IEC, "ISO/IEC 23009-1: Information Technology --
              Dynamic Adaptive Streaming over HTTP (DASH) --",
              April 2012.

              Huang, Qian, Guo, Zhou, Xu, Mao, Sen, and Spatscheck, "An
              In-depth Study of LTE: Effect of Network Protocol and
              Application Behavior on Performance", in proceedings
              of ACM SIGCOMM-2013, August 2013.

              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.  We would also like to thank Andreas
   Maeder and Dirk Staehle for reviewing the draft and for providing
   helpful comments.

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

   This section provides an overview of 3GPP's "Evolved Packet System"
   (EPS [3GPP.36.300], [3GPP.23.401]) as a specific example of a mobile
   communication architecture.  Of course other architectures exist but
   the EPS is used as one example to demonstrate the applicability of
   congestion exposure concepts and mechanisms.

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   The EPS architecture and some of its standardized interfaces are
   depicted in Figure 5.  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 bindings to resource reservation within the network.

                             +-------+                | 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 5: EPS architecture overview (Roaming Case)

   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 mobility management, user authentication, bearer

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   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 user and
   control plane 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]).  Alternatively Proxy
   Mobile IP (PMIPv6) is used on the S5 interface between S-GW and P-GW.

   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.

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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