





Internet Engineering Task Force (IETF)                       D. Kutscher
Request for Comments: 7778                                        F. Mir
Category: Informational                                        R. Winter
ISSN: 2070-1721                                                      NEC
                                                             S. Krishnan
                                                                Ericsson
                                                                Y. Zhang
                                                    Hewlett Packard Labs
                                                           CJ. Bernardos
                                                                    UC3M
                                                              March 2016


           Mobile Communication Congestion Exposure Scenario

Abstract

   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).  This memo provides a brief overview of
   the architecture of these networks (both access and core networks)
   and current QoS mechanisms and then discusses how congestion exposure
   concepts could be applied.  Based on this discussion, this memo
   suggests a set of requirements for ConEx mechanisms that particularly
   apply to these mobile networks.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7778.









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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.  This document is subject to
   BCP 78 and the IETF Trust's Legal Provisions Relating to IETF
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   date of publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Acronyms  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  ConEx Use Cases in Mobile Communication Networks  . . . . . .   4
     2.1.  ConEx as a Basis for Traffic Management . . . . . . . . .   5
     2.2.  ConEx to Incentivize Scavenger Transports . . . . . . . .   7
     2.3.  Accounting for Congestion Volume  . . . . . . . . . . . .   7
     2.4.  Partial vs. Full Deployment . . . . . . . . . . . . . . .   8
     2.5.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .   9
   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 . . . . . . . . . . . . . .  15
       3.2.2.  ConEx Functions in the Mobile Network . . . . . . . .  15
   4.  Summary . . . . . . . . . . . . . . . . . . . . . . . . . . .  17
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   6.  Informative References  . . . . . . . . . . . . . . . . . . .  19
   Appendix A.  Overview of 3GPP's EPS . . . . . . . . . . . . . . .  22
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

















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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 data traffic in cellular networks 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 goals.

   The ConEx 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.  In particular, use cases that are of interest
   for initial deployment are those in which 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 EPS networks 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; that is,
   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
   described.








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1.1.  Acronyms

   In this section, we expand some acronyms that are used throughout the
   text.  Most are explained and put in a system context in Appendix A
   and the 3GPP, ECN, and ConEx specifications referenced there.

   eNB
      Evolved NodeB: LTE base station

   HSS
      Home Subscriber Server

   S-GW
      Serving Gateway: mobility anchor and tunnel endpoint

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

   UE
      User Equipment: mobile terminals

   GTP
      GPRS Tunneling Protocol [TS29060]

   GTP-U
      GTP User Data Tunneling [TS29060]

   GTP-C
      GTP Control [TS29060]

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
   lengths in order to provide network-controlled quality of service.
   These efforts often lead to complexity and cost.  ConEx could be a
   simpler and more capable approach to efficient resource sharing in
   these networks.







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   In the following sections, we discuss ways that 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 EPS should refer to
   Appendix A 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-
   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 Evolved Packet
   Core (EPC) -- despite its cost and complexity.  However, with the
   increase of encrypted data traffic, traffic management using DPI
   alone 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 more users can be served effectively
       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.



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   2.  It can reduce the need for complex DPI by allowing for a bulk
       packet traffic management system that does not have to consider
       either the application classes flows belong to or the 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); it would also perform better as decisions can be made
       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:

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

       B.  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 than, say,
           application-layer responsiveness in protocols such as Dynamic
           Adaptive Streaming over HTTP (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 Wi-Fi 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 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



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   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

   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 and
   application store downloads from web-based (i.e., browser-based)
   communication or other real-time communication.  For many of the bulk
   data transfers, completion times are not important within certain
   bounds; therefore, if scavenger transports (or transports that are
   less than best effort) such as Low Extra Delay Background Transport
   (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 giving a larger bandwidth allowance to users that
   contribute less to congestion or lowering 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, see the Policy Charging Control (PCC)
   architecture [TS23203].  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 from volume-based accounting to 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 that would allow for such congestion volume
   accounting.  This would not require any support from the global
   Internet, especially since the typical scarce resources such as the



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   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 Path Computation Client (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 a 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,
   senders need to be made ConEx-aware (assuming a congestion
   notification mechanism such as Explicit Congestion Notification (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 a
   handover.  There is existing work on distributed rate limiting (see
   [raghavan2007]) and on specific optimizations (see [nec.euronf-2011])
   for congestion exposure and policing in mobility scenarios.



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   Another aspect to consider is the addition of Selected IP Traffic
   Offload (SIPTO) and Local IP Access (LIPA) [TR23829]), i.e., the idea
   that some traffic such as 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; this is also illustrated by the current DPI-related work
   for EPS.

   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
   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 eNB
   interfaces, so that a more general approach, involving more than a
   single operator's network, can be feasible as well.

3.  ConEx in the EPS

   In this section, we discuss a few options for how such a mechanism
   (and possibly additional policing functions) could eventually be
   deployed in the 3GPP EPS.  Note that this description of options is
   not intended to be a complete set of possible approaches; it merely
   discusses the most promising options.

3.1.  Possible Deployment Scenarios

   There are different possible ways for 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.




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   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 (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 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
       of operators to expose congestion to each other.

   3.  For Isolated ConEx domains as depicted in Figure 3, 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, dropped, or ECN-CE (Congestion Experienced) marked between
       the tunnel sender and receiver.  For that purpose, a new field
       called the Byte Sequence Marker (BSN) is introduced to the tunnel
       header to identify the byte in the flow of data from the tunnel
       sender to the tunnel receiver.  A policer at the tunnel sender is
       expected to react 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


























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                   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

   Note: DL stands for "downlink".

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 the
   3GPP EPS.  [RFC7713] describes 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.



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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
   implemented.

   Congestion exposure information for IPv6 [CONEX-DESTOPT] is contained
   in a destination option header field, which requires minimal changes
   at senders and nodes that want to assess path congestion.  The
   destination option header field 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 Proxy Mobile IPv6 (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 header.

   For 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 the congestion contribution of their flows.  ECN is already
   partially introduced into 3GPP networks: Section 11.6 in [TS36300]
   specifies the usage of ECN for congestion notification on the radio
   link (between eNB and UE), and [TS26114] 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 needed.

3.2.2.  ConEx Functions in the Mobile Network

   In this section, 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.



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   In order to provide a comprehensive ConEx-based capacity management
   framework for the 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
   not, however, imply that uplink congestion is not an issue, since the
   asymmetric maximum bandwidth configuration can create a smaller
   bottleneck for uplink traffic.  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 Policy and Charging Rule
   Function (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 possibly 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).  For example, the traffic is policed at the ingress before it
   reaches concentration points in the core network.



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   Such a setup would enable all the ConEx use cases described in
   Section 2 without requiring significant changes to the EPS
   architecture.  It would also enable operators to re-use existing
   infrastructure, specifically wireless base stations, PCRF, and Home
   Subscriber Server (HSS) systems.

   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: PMIPv6 [TS23402] and the
   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.  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.
   This calls for efficient, scalable, and 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 the policies of different
   operators.

   ConEx may also be used to complement 3GPP-based mechanisms for
   congestion management that are currently under development, such as
   in the User Plane Congestion Management (UPCON) work item described
   in [TR23705].

   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.



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   For 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
   networks:

   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 of cellular networks
      when compared to wired networks is the presence of user mobility.
      As the user location changes, the same device can be connected to
      the network via different base stations (eNBs) or even go through
      switching gateways.  Thus, the ConEx scheme must to be able to
      carry the 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



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   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.  Security Considerations

   For any ConEx deployment, it is important to apply appropriate
   mechanisms to preclude applications and senders from misstating their
   congestion contribution.  [RFC7713] 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 audited 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 flows.  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 [CONEX-DESTOPT] and [TCP-MOD]
   discuss additional security considerations that would also apply to
   mobile network deployments.

6.  Informative References

   [CONEX-DESTOPT]
              Krishnan, S., Kuehlewind, M., Briscoe, B., and C. Ralli,
              "IPv6 Destination Option for Congestion Exposure (ConEx)",
              Work in Progress, draft-ietf-conex-destopt-12, January
              2016.

   [conex-lite]
              Baillargeon, S. and I. Johansson, "ConEx Lite for Mobile
              Networks", In Proceedings of the 2014 ACM SIGCOMM Capacity
              Sharing Workshop, DOI 10.1145/2630088.2630091, August
              2014.

   [dash]     ISO/IEC, "Information Technology -- Dynamic Adaptive
              Streaming over HTTP (DASH) -- Part 1: Media presentation
              description and segment formats", ISO/IEC 23009-1:2014,
              May 2014.








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   [lte-sigcomm2013]
              Huang, J., Qian, F., Guo, Y., Zhou, Y., Xu, Q., Mao, Z.,
              Sen, S., and O. Spatscheck, "An In-depth Study of LTE:
              Effect of Network Protocol and Application Behavior on
              Performance", In Proceedings of the 2013 ACM SIGCOMM
              Conference, DOI 10.1145/2486001.2486006, August 2013.

   [nec.euronf-2011]
              Mir, F., Kutscher, D., and M. Brunner, "Congestion
              Exposure in Mobility Scenarios", In Proceedings of the 7th
              Euro-NF Conference on Next Generation Internet (NGI),
              DOI 10.1109/NGI.2011.5985948, June 2011.

   [nec.globecom2010]
              Kutscher, D., Lundqvist, H., and F. Mir, "Congestion
              Exposure in Mobile Wireless Communications", In
              Proceedings of 2010 IEEE Global Telecommunications
              Conference (GLOBECOM), DOI 10.1109/GLOCOM.2010.5684362,
              December 2010.

   [raghavan2007]
              Raghavan, B., Vishwanath, K., Ramabhadran, S., Yocum, K.,
              and A. Snoeren, "Cloud Control with Distributed Rate
              Limiting", ACM SIGCOMM Computer Communication Review,
              DOI 10.1145/1282427.1282419, October 2007.

   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, DOI 10.17487/RFC6040, November
              2010, <http://www.rfc-editor.org/info/rfc6040>.

   [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,
              <http://www.rfc-editor.org/info/rfc6789>.

   [RFC6817]  Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
              "Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
              DOI 10.17487/RFC6817, December 2012,
              <http://www.rfc-editor.org/info/rfc6817>.

   [RFC7713]  Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
              Concepts, Abstract Mechanism, and Requirements", RFC 7713,
              DOI 10.17487/RFC7713, December 2015,
              <http://www.rfc-editor.org/info/rfc7713>.

   [TCP-MOD]  Kuehlewind, M. and R. Scheffenegger, "TCP modifications
              for Congestion Exposure", Work in Progress, draft-ietf-
              conex-tcp-modifications-10, October 2015.



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   [TR23705]  3GPP, "System Enhancements for User Plane Congestion
              Management", 3GPP TR 23.705 13.0.0, December 2015.

   [TR23829]  3GPP, "Local IP Access and Selected IP Traffic Offload
              (LIPA-SIPTO)", 3GPP TR 23.829 10.0.1, October 2011.

   [TS23203]  3GPP, "Policy and charging control architecture", 3GPP
              TS 23.203 13.6.0, December 2015.

   [TS23401]  3GPP, "General Packet Radio Service (GPRS) enhancements
              for Evolved Universal Terrestrial Radio Access Network
              (E-UTRAN) access", 3GPP TS 23.401 13.5.0, December 2015.

   [TS23402]  3GPP, "Architecture enhancements for non-3GPP accesses",
              3GPP TS 23.402 13.4.0, December 2015.

   [TS26114]  3GPP, "IP Multimedia Subsystem (IMS); Multimedia
              telephony; Media handling and interaction", 3GPP TS 26.114
              13.2.0, December 2015.

   [TS29060]  3GPP, "General Packet Radio Service (GPRS); GPRS
              Tunnelling Protocol (GTP) across the Gn and Gp interface",
              3GPP TS 29.060 13.3.0, December 2015.

   [TS29274]  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 13.4.0,
              December 2015.

   [TS36300]  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
              13.2.0, January 2016.


















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Appendix A.  Overview of 3GPP's EPS

   This section provides an overview of the 3GPP "Evolved Packet System"
   (EPS [TS36300] [TS23401]) 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.

   The EPS architecture and some of its standardized interfaces are
   depicted in Figure 5.  The EPS provides IP connectivity to 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 Universal Terrestrial Radio
   Access Network (E-UTRAN) and the core network called the Evolved
   Packet Core (EPC).  QoS is supported through an EPS bearer concept,
   providing bindings to resource reservation within the network.



































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

            Figure 5: EPS Architecture Overview (Roaming Case)

   Note:
   HPLMN - Home Public Land Mobile Network
   VPLMN - Visited Public Land Mobile Network
   AF - Application Function
   SGi - Service Gateway Interface
   LTE-Uu - LTE Radio Interface

   The Evolved NodeB (eNB), the 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
   establishment and modification, and maintenance of the UE context.




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   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 PDN Gateway (P-GW) is responsible for IP address allocation to
   the UE and is a tunnel endpoint for user-plane 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) [TS29060] 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 [TS29274]).  Alternatively, 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.

Acknowledgements

   We would like to thank Bob Briscoe and Ingemar Johansson for their
   support in shaping the overall idea and in improving the document by
   providing constructive comments.  We would also like to thank Andreas
   Maeder and Dirk Staehle for reviewing the document and for providing
   helpful comments.

Authors' Addresses

   Dirk Kutscher
   NEC
   Kurfuersten-Anlage 36
   Heidelberg
   Germany

   Email: kutscher@neclab.eu








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   Faisal Ghias Mir
   NEC
   Kurfuersten-Anlage 36
   Heidelberg
   Germany

   Email: faisal.mir@gmail.com


   Rolf Winter
   NEC
   Kurfuersten-Anlage 36
   Heidelberg
   Germany

   Email: rolf.winter@neclab.eu


   Suresh Krishnan
   Ericsson
   8400 Blvd Decarie
   Town of Mount Royal, Quebec
   Canada

   Email: suresh.krishnan@ericsson.com


   Ying Zhang
   Hewlett Packard Labs
   3000 Hannover Street
   Palo Alto, CA  94304
   United States

   Email: ying.zhang13@hp.com


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

   Phone: +34 91624 6236
   Email: cjbc@it.uc3m.es
   URI:   http://www.it.uc3m.es/cjbc/






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