Internet Engineering Task Force A. Jain
Internet-Draft A. Terzis
Intended status: Informational Google
Expires: August 23, 2015 N. Sprecher
S. Arunachalam
Nokia Networks
K. Smith
G. Klas
Vodafone
February 19, 2015
Requirements and reference architecture for Mobile Throughput Guidance
Exposure
draft-sprecher-mobile-tg-exposure-req-arch-00.txt
Abstract
Rapidly-varying conditions in a cellular network can cause problems
for the Transmission Control Protocol (TCP), which in turn can
degrade application performance.
This document presents the problem statement and proposes solution
principles. It specifies the requirements and reference architecture
for a mobile throughput guidance exposure mechanism that can be used
to assist TCP in cellular networks, ensuring better network
efficiency and enhanced service delivery performance.
The proposed mechanism can be applied to any content or TCP/IP based
application content delivery. This document describes the
applicability of the mechanism for Intelligent Video Acceleration
over cellular networks.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on August 23, 2015.
Copyright Notice
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document authors. All rights reserved.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Contributing Authors . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Acronyms and Abbreviations . . . . . . . . . . . . . . . 3
1.4. Problem statement . . . . . . . . . . . . . . . . . . . . 3
1.5. Solution Principles . . . . . . . . . . . . . . . . . . . 4
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Requirements on the Mobile Throughput Guidance Exposure
Mechanism . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Security requirements . . . . . . . . . . . . . . . . . . 6
3. Reference Architecture . . . . . . . . . . . . . . . . . . . 7
4. Applicability to Mobile Video Delivery Optimization . . . . . 8
5. Manageability considerations . . . . . . . . . . . . . . . . 9
6. Security considerations . . . . . . . . . . . . . . . . . . . 9
7. IANA considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
The following sub-sections present the problem statement and the
solution principles.
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1.1. Contributing Authors
The editors gratefully acknowledge the following additional
contributors: Hannu Flinck, Helen Parsons, Peter Cosimini and Ram
Gopal.
1.2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
1.3. Acronyms and Abbreviations
HTTP Hypertext Transmission Protocol
IP Internet Protocol
LTE Long Term Evolution
RAN Radio Access Network
RTT Round Trip Time
TCP Transmission Control Protocol
UE User Equipment
1.4. Problem statement
Inefficient use of a cellular network's resources degrades
application performance, delivery of content and user experience.
Cellular networks are often required to deliver large, high bandwidth
files to end users, e.g from streaming media content providers. If
the available throughput from the Radio Access Network (RAN) to the
User Equipment (UE) falls below the bandwidth required then files are
delivered too slowly, resulting in a bad user experience. It may be
possible to take avoiding action and so limit the impact on the
network and the user experience. However to be able to do this in an
accurate and timely fashion, information on the available throughput
is required.
Internet media and file delivery are typically streamed or downloaded
today using Hypertext Transmission Protocol (HTTP) over the TCP. The
behavior of TCP assumes that network congestion is the primary cause
for packet loss and high delay. This may not be the case in cellular
networks where the bandwidth available for each UE can vary by an
order of magnitude within a few seconds due to changes in the
underlying radio channel conditions. Such changes can be caused by
the movement of devices or interference, as well as changes in system
load due to bursty traffic sources or when other devices enter and
leave the network. On the other hand, packet losses tend to be
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sporadic and temporary; retransmission mechanisms at the physical and
link layers repair most packet corruptions.
1.5. Solution Principles
This document proposes that the cellular network could provide near
real-time information on "Throughput Guidance" to the TCP server;
this throughput guidance information would indicate the throughput
estimated to be available at the radio downlink interface (between
the RAN and the UE) for the TCP connection.
While the implementation details will vary according to the cellular
access network technology, the resource allocation can be abstracted
as the capacity of the "radio link" between the network and the UE.
For example, in the case of an LTE network, the number of physical
resource blocks allocated to a UE, along with the modulation scheme
and coding rate used, can be translated into radio link capacity in
Megabits per second (Mbps). It can also include the quality of the
"radio link" which is reported by the UE.
The TCP server can use this explicit information to inform several
congestion control decisions. For example: (1) selecting the initial
window size, (2) deciding the value of the congestion window during
the congestion avoidance phase, and (3) reducing the size of the
congestion window when the conditions on the "radio link"
deteriorate. In other words, with this additional information, TCP
does neither have to congest the network when probing for available
resource, nor rely on heuristics to reduce its sending rate after a
congestion episode.
The same explicit information can also be used to optimize
application behavior given the available resources. For example,
when video is encoded in multiple bitrates, the application server
can select the appropriate encoding for the network conditions.
Note that the throughput estimation for the upstream traffic between
the UE and the RAN, and the throughput of the network path between
the RAN and the server communicating with the UE are beyond the scope
of the document.
It is also important to note that the validity of the throughput
guidance and the distance between the originating server and the
cellular network (in terms of the number of Internet hops) are
inversely proportional. This is due to the fact that the latency
incurred at each hop increases the time that elapses between issuing
and consuming the guidance.
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2. Requirements
The requirements set out in section 2.1 are for the behavior of the
mobile throughput guidance exposure mechanism and the related
functional elements. The related security requirements are specified
in section 2.2.
2.1. Requirements on the Mobile Throughput Guidance Exposure Mechanism
1. The throughput guidance information SHALL indicate the expected
available bandwidth in the downlink interface. Depending on the
solution mechanism, the information MAY be provided per TCP flow
or per user. If the solution mechanism supports both options,
then granularity SHOULD be configurable.
2. The throughput guidance information SHALL be provided for TCP
based traffic.
3. A functional element, residing in the RAN and acting as
Throughput Guidance Provider, SHOULD supply the TCP server a
near real-time indication (in sub-seconds) on the throughput
estimated to be available at the radio downlink interface (i.e.,
mobile throughput guidance information). It SHOULD keep up with
the rapid changes in the radio network conditions, the network
traffic and the user movement, in order to provide the most
accurate guidance information.
4. The introduction of the Throughput Guidance exposure mechanism
SHALL NOT require any update to the TCP client software.
5. The mobile throughput guidance exposure mechanism SHALL work
when the user traffic is end-to-end encrypted (e.g., HTTPS,
etc.). This requirement is compliant with the IAB Statement on
Internet Confidentiality (see [IAB_Statement]), saying that the
IAB "strongly encourage developers to include encryption in
their implementations, and to make them encrypted by default.
We similarly encourage network and service operators to deploy
encryption where it is not yet deployed, and we urge firewall
policy administrators to permit encrypted traffic.".
6. The mobile throughput guidance exposure mechanism SHALL NOT
adversely impact the behavior of the TCP flows (e.g., it SHOULD
NOT cause an increase in retransmissions or degradation in
performance, etc.).
7. The throughput guidance information SHALL be opaque to the
intermediate elements between the Throughput Guidance Provider
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and the TCP server. The intermediate elements SHOULD NOT modify
or remove the throughput guidance information.
8. The TCP server MAY reside within the mobile operator's network
(behind the mobile core network) or in the Internet.
9. The TCP server MAY use the mobile throughput guidance
information to assist TCP.
10. It SHOULD be possible for the TCP server to provide the exposed
mobile throughput guidance information to an authorized higher
layer application. The application may use the mobile
throughput guidance information to optimize its behavior.
11. The Throughput Guidance Provider SHOULD provide the mobile
throughput guidance information periodically, starting from the
initiation of the flow.
12. The frequency (in milliseconds) at which mobile throughput
guidance needs to be exposed SHALL be configurable.
13. The mobile Throughput Guidance Provider SHALL be able to supply
mobile throughput guidance information to more than one TCP
server simultaneously, with independent configurable parameters
for each server.
14. There SHOULD be a mechanism to configure the Mobile Throughput
Guidance Provider with a list of TCP flows for which mobile
throughput guidance information shall be exposed.
15. The mobile throughput guidance exposure mechanism SHOULD ensure
backward compatibility. Normal TCP processing at the TCP server
SHOULD be performed if the TCP server does not recognize the
throughput guidance information.
16. The mobile throughput guidance exposure mechanism MUST be
extensible, ensuring that additional information can be provided
in the future in a non-disruptive, backward-compatible way.
2.2. Security requirements
1. A trustful relationship between the Mobile Throughput Provider
and the TCP server SHOULD be formed before any information is
exposed.
2. There SHOULD be a mechanism to configure the Mobile Throughput
Guidance Provider with a list of destinations to which throughput
guidance should be provided.
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3. The identifier of the network that exposes the throughput
guidance information SHALL be explicitly known to the TCP server
which receives the information. The TCP server SHALL be able to
authenticate the identity of the information provider. The
Throughput Guidance Provider MUST NOT reveal any other identity
or address of network elements that can compromise the security
of the network.
4. The mobile throughput guidance information SHOULD be secured to
ensure confidentiality and integrity.
5. There SHOULD be a mechanism to configure the required security
level and parameters for the encryption and the authentication if
supported.
6. The exposure of the Mobile throughput guidance information SHALL
NOT introduce any additional security threats and privacy
concerns to the mobile operator's network, the Internet and the
users.
7. The throughput guidance SHOULD be treated only as an estimate to
the optimization algorithm running at the TCP server. The TCP
server that receives this information SHOULD NOT assume that it
is always accurate and up to date. Specifically, the TCP server
SHOULD check the validity of the information received and if it
finds it erroneous it SHOULD discard it and possibly take other
corrective actions (e.g., discard all future throughput guidance
information from a particular IP prefix).
3. Reference Architecture
Figure 1 below, depicts the functional elements and their interfaces
that comprise the mobile network guidance solution (based on the
requirements for mobile throughput guidance).
A Throughput Guidance Provider functional element signals to the TCP
server the information on the (near-real time) throughput estimated
to be available at the radio downlink interface. The TCP server
resides within the mobile operator's network or in the Internet.
Note that the Throughput Guidance Provider functional element and the
TCP server can belong to the same Administrative System (AS) or to
different Administrative Systems.
The TCP server MAY use the information to optimize the TCP behavior.
The information MAY also be used by the application to adapt its
behavior accordingly and to optimize service delivery performance.
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TCP flow behaviour based on
+-+-+-+-+-+-+-+ Throughput Guidance Information +-+-+-+-+-+-+-+
| | <---------------------------------------- | |
| TCP client | +-+-+-+-+-+-+-+-+-+-+-+ | TCP Server |
| | | Througput Guidance | (x) | |
| | | Provider | ----------> | |
+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+
UE <--------------- RAN -------------------> x = Mobile Thourghput
Guidance Signaling
Mobile Throughput Guidance Reference Architecture
Figure 1
The information source and the algorithm used by the Throughput
Guidance Provider to calculate the throughput guidance are beyond the
scope of this document.
The TCP server MAY use the throughput guidance information to assist
TCP in any of the following ways:
o Determine the size of the initial congestion window
o Determine when to exit the slow start phase
o Determine the size of the congestion window during the congestion
avoidance phase
o Determine the size of the window after a congestion event
4. Applicability to Mobile Video Delivery Optimization
The mobile throughput guidance exposure mechanism applies to mobile
video delivery optimization.
In this use case the Throughput Guidance Provider sends to the video
server throughput guidance information for a TCP flow. The video
server may use this information to assist TCP congestion control
decisions, for example in selecting the initial congestion window
size, and adjusting the size of the congestion window when the
conditions on the radio link change. In other words, with this
additional information, TCP does not need to overload the network
when probing for available resources, nor does it need to rely on
heuristics to reduce its sending rate after a congestion episode.
Slow start and buffering of content delivery can be eliminated.
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The same information may also be used to ensure that the application
level coding matches the estimated capacity at the radio downlink.
The aim of all of these improvements is to enhance the end user's
quality of experience. For example, the content's time-to-start as
well as video buffering occurrences can be reduced, the utilization
of the radio network's resources and its throughput can be optimized,
etc.
5. Manageability considerations
Manageability of mobile throughput guidance exposure will be
discussed in the solution documents. Section 2 specifies a set of
requirements on the management of the mobile throughput guidance
exposure functional elements and protocol operation.
6. Security considerations
The exposure of mobile throughput guidance information from the
cellular network to the TCP server introduces a set of security
considerations.
First, an identifier of the network that provides the throughput
guidance must be explicitly known to the endpoint receiving the
information. Omitting such information would enable malicious third
parties to provide erroneous information. To avoid compromising
network and user security, other identities or addresses MUST NOT be
exposed.
Furthermore, the throughput guidance information should be treated
only as an estimate to the congestion control algorithm running at
the transport endpoint. The endpoint that receives this information
should not assume that it is always correct and accurate.
Specifically, endpoints should check the authenticity and integrity
of the information received and if they find it erroneous they should
discard it and possibly take other corrective actions (e.g., discard
all future throughput guidance information from a particular IP
prefix).
One way to check if the throughput guidance information overestimates
the capacity available on the radio link is to check whether any
packet losses or other signs of congestion (e.g., increasing RTT)
occur after the guidance is used. Notably, the same mechanism can be
used to deal with bottlenecks in other parts of the end-to-end
network path. To check if the throughput guidance underestimates the
available network capacity, the source can periodically attempt to
send faster and then check for signs of congestion.
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Section 2 above, specifies a set of requirements on the mobile
throughput guidance exposure protocol to ensure secured communication
and operation.
7. IANA considerations
This requirements and architecture document does not introduce any
requests for IANA actions.
8. Acknowledgements
We would like to thank Peter Szilagyi, Meir Cohen and Csaba Vulkan
for conversations on these issues.
9. References
9.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6994] Touch, J., "Shared Use of Experimental TCP Options", RFC
6994, August 2013.
9.2. Informative References
[I-D.narten-iana-considerations-rfc2434bis]
Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", draft-narten-iana-
considerations-rfc2434bis-09 (work in progress), March
2008.
[IAB_Statement]
"IAB Statement on Internet Confidentiality", November
2014, <http://www.ietf.org/mail-archive/web/ietf-
announce/current/msg13460.html>.
[MEC_White_Paper]
"Mobile-Edge Computing - Introductory Technical White
Paper", September 2014,
<http://portal.etsi.org/Portals/0/TBpages/MEC/Docs/Mobile-
edge_Computing_-
_Introductory_Technical_White_Paper_V1%2018-09-14.pdf>.
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[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552, July
2003.
[RFC4413] West, M. and S. McCann, "TCP/IP Field Behavior", RFC 4413,
March 2006.
Authors' Addresses
Ankur Jain
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Phone: +1-925-526-5879
Email: jankur@google.com
Andreas Terzis
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Phone: +1-650-214-5270
Email: aterzis@google.com
Nurit Sprecher
Nokia Networks
Hod HaSharon
IL
Phone: +97297751229
Email: nurit.sprecher@nsn.com
Swaminathan Arunachalam
Nokia Networks
Irving
FUS
Phone: +19723303204
Email: swaminathan.arunachalam@nsn.com
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Kevin Smith
Vodafone
One Kingdom Street, Paddington Central
London W2 6BY
UK
Email: kevin.smith@vodafone.com
Guenter Klas
Vodafone
One Kingdom Street, Paddington Central
Newbury RG14 2FN
UK
Email: kguenter.klas@vodafone.com
Hannu Flinck
Nokia Networks
Helsinki
FI
Phone: +358504839522
Email: hannu.flinck@nsn.com
Helen Parsons
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Email: helenparsons@google.com
Peter Cosimini
Vodafone
Vodafone House, The Connection
Newbury, RG14 2FN
UK
Email: peter.cosimini@vodafone.com
Ram Gopal
Nokia Networks
Mountain View
US
Phone: +17815264572
Email: ram.gopal@nsn.com
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