Internet DRAFT - draft-ietf-conex-mobile
draft-ietf-conex-mobile
CONEX WG D. Kutscher
Internet-Draft F. Mir
Intended status: Informational R. Winter
Expires: April 20, 2016 NEC
S. Krishnan
Y. Zhang
Ericsson
CJ. Bernardos
UC3M
October 18, 2015
Mobile Communication Congestion Exposure Scenario
draft-ietf-conex-mobile-06
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). 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
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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
goals.
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
described.
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.
eNB:
Evolved NodeB: LTE base station
S-GW:
Serving Gateway: mobility anchor and tunnel endpoint
P-GW:
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
UE:
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
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
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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
document.
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
implemented.
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
header.
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
needed.
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
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: 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
policies.
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
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.
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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.23.203]
3GPP, "Policy and charging control architecture", 3GPP
TS 23.203 10.9.0, September 2013.
[3GPP.23.401]
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.23.402]
3GPP, "Architecture enhancements for non-3GPP accesses",
3GPP TS 23.402 10.8.0, September 2012.
[3GPP.23.705]
3GPP, "System Enhancements for User Plane Congestion
Management", 3GPP TR 23.705 0.8.0, October 2013.
[3GPP.23.829]
3GPP, "Local IP Access and Selected IP Traffic Offload
(LIPA-SIPTO)", 3GPP TR 23.829 10.0.1, October 2011.
[3GPP.26.114]
3GPP, "IP Multimedia Subsystem (IMS); Multimedia
telephony; Media handling and interaction", 3GPP TS 26.114
10.7.0, June 2013.
[3GPP.29.060]
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.29.274]
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.36.300]
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.
[I-D.ietf-conex-abstract-mech]
Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
Concepts, Abstract Mechanism and Requirements",
draft-ietf-conex-abstract-mech-13 (work in progress),
October 2014.
[I-D.ietf-conex-destopt]
Krishnan, S., Kuehlewind, M., and C. Ucendo, "IPv6
Destination Option for Congestion Exposure (ConEx)",
draft-ietf-conex-destopt-09 (work in progress),
August 2015.
[I-D.ietf-conex-tcp-modifications]
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, <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>.
[conex-lite]
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.
[lte-sigcomm2013]
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.
[nec.euronf-2011]
Mir, Kutscher, and Brunner, "Congestion Exposure in
Mobility Scenarios", in proceedings of 7th EURO-NF
CONFERENCE ON NEXT GENERATION INTERNET, June 2011.
[nec.globecom2010]
Kutscher, Lundqvist, and Mir, "Congestion Exposure in
Mobile Wireless Communications", in proceedings of IEEE
GLOBECOM 2010, December 2010.
[raghavan2007]
Raghavan, Vishwanath, Ramabhadran, Yocum, and Snoeren,
"Cloud Control with Distributed Rate Limiting", in
proceedings of ACM SIGCOMM 2007, 2007.
DOI: http://doi.acm.org/10.1145/1282427.1282419
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
NEC
Kurfuersten-Anlage 36
Heidelberg,
Germany
Phone:
Email: kutscher@neclab.eu
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Faisal Ghias Mir
NEC
Kurfuersten-Anlage 36
Heidelberg,
Germany
Phone:
Email: faisal.mir@neclab.eu
Rolf Winter
NEC
Kurfuersten-Anlage 36
Heidelberg,
Germany
Phone:
Email: rolf.winter@neclab.eu
Suresh Krishnan
Ericsson
8400 Blvd Decarie
Town of Mount Royal, Quebec
Canada
Phone:
Email: suresh.krishnan@ericsson.com
Ying Zhang
Ericsson
200 Holger Way
San Jose, CA 95134
USA
Phone:
Email: ying.zhang@ericsson.com
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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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