XEP-0286: Retab and rewrap

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<!DOCTYPE xep SYSTEM 'xep.dtd' [
<!ENTITY % ents SYSTEM 'xep.ent'>
<!ENTITY % ents SYSTEM 'xep.ent'>
%ents;
]>
<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
<xep>
<header>
<title>Mobile Considerations</title>
<abstract>
This document provides background information for XMPP implementors
concerned with mobile devices operating on an LTE cellular network.
</abstract>
&LEGALNOTICE;
<number>0286</number>
<status>Experimental</status>
<type>Informational</type>
<sig>Standards</sig>
<approver>Council</approver>
<dependencies>
<spec>XMPP Core</spec>
</dependencies>
<supersedes/>
<supersededby/>
<shortname>NOT_YET_ASSIGNED</shortname>
<author>
<firstname>Dave</firstname>
<surname>Cridland</surname>
<email>dave.cridland@isode.com</email>
<jid>dave.cridland@isode.com</jid>
</author>
<author>
<firstname>Sam</firstname>
<surname>Whited</surname>
<email>sam@samwhited.com</email>
<jid>sam@samwhited.com</jid>
</author>
<revision>
<version>0.4.0</version>
<date>2017-01-17</date>
<initials>ssw</initials>
<remark>
<ul>
<li>Attempt to fix some confusing paragraphs.</li>
<li>Add Client State Indication to Notable Extensions.</li>
</ul>
</remark>
</revision>
<revision>
<version>0.3</version>
<date>2015-07-24</date>
<initials>ssw</initials>
<remark><p>Include real world compression numbers and additional recommended reading.</p></remark>
</revision>
<revision>
<version>0.2</version>
<date>2015-07-22</date>
<initials>ssw</initials>
<remark><p>Overhaul to include LTE.</p></remark>
</revision>
<revision>
<version>0.1</version>
<date>2010-09-15</date>
<initials>psa</initials>
<remark><p>Initial published version.</p></remark>
</revision>
<revision>
<version>0.0.1</version>
<date>2010-07-13</date>
<initials>dwd</initials>
<remark><p>First draft. Also John's birthday.</p></remark>
</revision>
</header>
<section1 topic='Introduction' anchor='intro'>
<p>
XMPP as a protocol was designed before the wide spread adoption of mobile
devices, and is often cited as not being very mobile friendly as a result.
However, this mostly stems from undocumented lore and outdated notions of
how XMPP works. As the Internet and protocol design have changed to be more
accommodating for mobile, so has XMPP.
</p>
<p>
This XEP aims to provide useful background knowledge of mobile handset
behavior, and those considerations that client and server designers can
take to ensure that bandwidth and battery are used efficiently.
</p>
</section1>
<section1 topic='Overview' anchor='overview'>
<p>
The two major constraints on mobile devices are power and bandwidth.
With the wide spread proliferation of 3G and LTE technologies, mobile
bandwidth and speeds have become broadly comparable to broadband.
However, they are still relatively expensive compared to traditional wired
networks, and therefore conserving them is still desirable.
This XEP mostly focuses on LTE as it already has a very wide deployment and
will only continue to further replace 3G technologies.
</p>
</section1>
<header>
<title>Mobile Considerations</title>
<abstract>
This document provides background information for XMPP implementors
concerned with mobile devices operating on an LTE cellular network.
</abstract>
&LEGALNOTICE;
<number>0286</number>
<status>Experimental</status>
<type>Informational</type>
<sig>Standards</sig>
<approver>Council</approver>
<dependencies>
<spec>XMPP Core</spec>
</dependencies>
<supersedes/>
<supersededby/>
<shortname>NOT_YET_ASSIGNED</shortname>
<author>
<firstname>Dave</firstname>
<surname>Cridland</surname>
<email>dave.cridland@isode.com</email>
<jid>dave.cridland@isode.com</jid>
</author>
<author>
<firstname>Sam</firstname>
<surname>Whited</surname>
<email>sam@samwhited.com</email>
<jid>sam@samwhited.com</jid>
</author>
<revision>
<version>0.4.0</version>
<date>2017-01-17</date>
<initials>ssw</initials>
<remark>
<ul>
<li>Attempt to fix some confusing paragraphs.</li>
<li>Add Client State Indication to Notable Extensions.</li>
</ul>
</remark>
</revision>
<revision>
<version>0.3</version>
<date>2015-07-24</date>
<initials>ssw</initials>
<remark>
<p>
Include real world compression numbers and additional recommended
reading.
</p>
</remark>
</revision>
<revision>
<version>0.2</version>
<date>2015-07-22</date>
<initials>ssw</initials>
<remark><p>Overhaul to include LTE.</p></remark>
</revision>
<revision>
<version>0.1</version>
<date>2010-09-15</date>
<initials>psa</initials>
<remark><p>Initial published version.</p></remark>
</revision>
<revision>
<version>0.0.1</version>
<date>2010-07-13</date>
<initials>dwd</initials>
<remark><p>First draft. Also John's birthday.</p></remark>
</revision>
</header>
<section1 topic='Introduction' anchor='intro'>
<p>
XMPP as a protocol was designed before the wide spread adoption of mobile
devices, and is often cited as not being very mobile friendly as a result.
However, this mostly stems from undocumented lore and outdated notions of
how XMPP works. As the Internet and protocol design have changed to be
more accommodating for mobile, so has XMPP.
</p>
<p>
This XEP aims to provide useful background knowledge of mobile handset
behavior, and those considerations that client and server designers can
take to ensure that bandwidth and battery are used efficiently.
</p>
</section1>
<section1 topic='Overview' anchor='overview'>
<p>
The two major constraints on mobile devices are power and bandwidth.
With the wide spread proliferation of 3G and LTE technologies, mobile
bandwidth and speeds have become broadly comparable to broadband.
However, they are still relatively expensive compared to traditional wired
networks, and therefore conserving them is still desirable.
This XEP mostly focuses on LTE as it already has a very wide deployment
and will only continue to further replace 3G technologies.
</p>
</section1>
<section1 topic='Compression' anchor='compression'>
<p>
XML, and by extension XMPP, is known to be highly compressible.
Compression of XMPP data can be achieved with the DEFLATE algorithm
(&rfc1951;) via TLS compression (&rfc3749;) or &xep0138; (which also
supports other compression algorithms). While the security implications of
stream compression are beyond the scope of this document (See the
aforementioned RFC or XEP for more info), the author does not recommend
using TLS compression with XMPP (or in general). If compression must be
used, stream level compression should be implemented instead, and the
compressed stream should have a full flush performed on stanza boundaries
to help prevent a class of chosen plaintext attacks which can cause data
leakage in compressed streams. While this may mitigate some of the benefits
of compression by raising compression ratios, in a large, real world
deployment at HipChat, network traffic was still observed to decrease by a
factor of 0.58 when enabling &xep0138; with ZLIB compression!
</p>
<p>
While the CPU cost of compression may directly translate to higher power
usage, it is vastly outweighed by the benefits of reduced network
utilization, especially on modern LTE networks which use a great deal more
power per bit than 3G networks as will be seen later in this document.
However, CPU usage is also not guaranteed to rise due to compression. In
the aforementioned deployment of stream compression, a <em>decrease</em> in
CPU utilization by a factor of 0.60 was observed due to the fact that there
were fewer packets that needed to be handled by the OS (which also takes
CPU time), and, potentially more importantly, less data that needed to be
TLS-encrypted (which is a much more CPU-expensive operation than
compression). Therefore CPU time spent on compression (for ZLIB, at least;
other algorithms were not tested) should be considered negligable.
</p>
<p>
Supporting compression and performming a full flush on stanza boundaries is
recommended for mobile devices.
</p>
</section1>
<section1 topic='Power Consumption' anchor='power'>
<p>
While the wide spread adoption of LTE has dramatically increased available
bandwidth on mobile devices, it has also increased power consumption.
According to one study, early LTE devices consumed 5&#x2013;20% more power
than their 3G counterparts
<note>LTE Smartphone measurements &lt;<link url='http://networks.nokia.com/system/files/document/lte_measurements_final.pdf'>http://networks.nokia.com/system/files/document/lte_measurements_final.pdf</link>&gt;</note>.
On some networks that support the legacy SVLTE (Simultaneous Voice and LTE)
instead of the more modern VoLTE (Voice Over LTE) standard, or even CSFB
(Circuit-switched fallback) this number would (presumably) be even higher.
</p>
<p>
XMPP server and client implementers, bearing this increased power usage in
mind, and knowing a bit about how LTE radios work, can optimize their
traffic to minimize network usage. For the downlink, LTE user equipment
(UE) utilizes Orthogonal Frequency Division Multiplexing (OFDM), which is
somewhat inefficient
<note>A Close Examination of Performance and Power Characteristics of 4G LTE Networks &lt;<link url='http://www.cs.columbia.edu/~lierranli/coms6998-7Spring2014/papers/rrclte_mobisys2012.pdf'>http://www.cs.columbia.edu/~lierranli/coms6998-7Spring2014/papers/rrclte_mobisys2012.pdf</link>&gt;</note>.
On the uplink side a different technology, Single-carrier frequency
division multiple access (SC-FDMA) is used, which is slightly more
efficient than traditional (non linearly-precoded) OFDM, slightly
offsetting the fact that broadcasting requires more power than receiving.
LTE UE also implements a Discontinuous reception (DRX) mode in which the
hardware can sleep until it is woken by a paging message or is needed to
perform some task. LTE radios have two power modes: RRC_CONNECTED and
RRC_IDLE. DRX is supported in both of these power modes. By attempting to
minimize the time which the LTE UE state machine spends in the
RCC_CONNECTED state, and maximize the time it stays in the DRX state (for
RCC_CONNECTED and RRC_IDLE), we can increase battery life without degrading
the XMPP experience. To do so, the following rules should be observed:
</p>
<section2 topic='Transmit no data'>
<p>
Whenever possible, data that is not strictly needed should not be
transmitted (by the server or client).
Supporting &xep0352; is recommended.
Most importantly, XMPP pings should be kept as far apart as possible and
only used when necessary.
Server operators are encouraged to set high ping timeouts, and client
implementors are advised to only send pings when absolutely necessary to
prevent the server from closing the socket.
</p>
</section2>
<section2 topic='Transmit as much data as you can at once'>
<p>
If one is on 3G, transmitting a small amount of data will cause the radio
to enter FACH mode which is significantly cheaper than its high power
mode.
On LTE radios, however, transmitting small amounts of data is vastly more
expensive per bit due to the higher tail-times (the time it takes for the
radio to change state).
On LTE radios, one should transmit as much data from the client as
possible when the radio is already on (eg. by placing messages in a send
queue and executing the queue as a batch when the radio is on).
Similarly, when data is being received from the server, the mobile devices
radio is already in a high power state and therefore any data that needs
to be sent to the server should be transmitted.
</p>
<p>
These rules also apply to server operators: If the server receives data,
the phones radio is already on therefore you should send any pending data.
Batching data to be sent and sending it all at once will help reduce power
consumption.
</p>
</section2>
</section1>
<section1 topic='Notable Extensions' anchor='xeps'>
<p>
This section provides pointers to other documents which may be of interest
to those developing mobile clients, or considering implementing
optimizations for them in servers.
</p>
<p>&xep0138; provides stream level compression.</p>
<p>&xep0322; allows XMPP streams to use the EXI XML format.</p>
<p>
&xep0115; provides a mechanism for caching, and hence eliding, the
disco#info requests needed to negotiate optional features.
</p>
<p>
&xep0237; provides a relatively widely deployed extension for reducing
roster fetch sizes.
</p>
<p>
&xep0198; allows the client to send and receive smaller keep-alive messages,
and resume existing sessions without the full handshake.
This is useful on unstable connections.
</p>
<p>
&xep0352; allows clients to indicate to the server that they are inactive,
allowing the server to optimize and reduce unnecessary traffic.
</p>
<p>
&xep0357; implements push notifications (third party message delivery),
which are often used on mobile devices and highly optimized to conserve
battery.
Push notifications also allow delivery of notifications to mobile clients
that are currently offline (eg. in an XEP-0198 "zombie" state).
</p>
<p>
&xep0313; lets clients fetch messages which they missed (eg. due to poor
mobile coverage and a flaky network connection).
</p>
</section1>
<section1 topic='Acknowledgements' anchor='acks'>
<p>
This XEP was originally written by Dave Cridland, and parts of his original
work were used in this rewrite.
Thanks to Atlassian for allowing me to release hard numbers from their XMPP
compression deployment.
</p>
</section1>
<section1 topic='Security Considerations' anchor='security'>
<p>This document introduces no new security considerations.</p>
</section1>
<section1 topic='IANA Considerations' anchor='iana'>
<p>
This document requires no interaction with the Internet Assigned Numbers
Authority (IANA).
</p>
</section1>
<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
<p>
No namespaces or parameters need to be registered with the XMPP Registrar
as a result of this document.
</p>
</section1>
<section1 topic='Compression' anchor='compression'>
<p>
XML, and by extension XMPP, is known to be highly compressible.
Compression of XMPP data can be achieved with the DEFLATE algorithm
(&rfc1951;) via TLS compression (&rfc3749;) or &xep0138; (which also
supports other compression algorithms).
While the security implications of stream compression are beyond the scope
of this document (See the aforementioned RFC or XEP for more info), the
author does not recommend using TLS compression with XMPP (or in general).
If compression must be used, stream level compression should be
implemented instead, and the compressed stream should have a full flush
performed on stanza boundaries to help prevent a class of chosen plaintext
attacks which can cause data leakage in compressed streams.
While this may mitigate some of the benefits of compression by raising
compression ratios, in a large, real world deployment at HipChat, network
traffic was still observed to decrease by a factor of 0.58 when enabling
&xep0138; with ZLIB compression!
</p>
<p>
While the CPU cost of compression may directly translate to higher power
usage, it is vastly outweighed by the benefits of reduced network
utilization, especially on modern LTE networks which use a great deal more
power per bit than 3G networks as will be seen later in this document.
However, CPU usage is also not guaranteed to rise due to compression.
In the aforementioned deployment of stream compression, a
<em>decrease</em> in CPU utilization by a factor of 0.60 was observed due
to the fact that there were fewer packets that needed to be handled by the
OS (which also takes CPU time), and, potentially more importantly, less
data that needed to be TLS-encrypted (which is a much more CPU-expensive
operation than compression).
Therefore CPU time spent on compression (for ZLIB, at least; other
algorithms were not tested) should be considered negligable.
</p>
<p>
Supporting compression and performming a full flush on stanza boundaries
is recommended for mobile devices.
</p>
</section1>
<section1 topic='Power Consumption' anchor='power'>
<p>
While the wide spread adoption of LTE has dramatically increased available
bandwidth on mobile devices, it has also increased power consumption.
According to one study, early LTE devices consumed 5&#x2013;20% more power
than their 3G counterparts
<note>LTE Smartphone measurements &lt;<link url='http://networks.nokia.com/system/files/document/lte_measurements_final.pdf'>http://networks.nokia.com/system/files/document/lte_measurements_final.pdf</link>&gt;</note>.
On some networks that support the legacy SVLTE (Simultaneous Voice and
LTE) instead of the more modern VoLTE (Voice Over LTE) standard, or even
CSFB (Circuit-switched fallback) this number would (presumably) be even
higher.
</p>
<p>
XMPP server and client implementers, bearing this increased power usage in
mind, and knowing a bit about how LTE radios work, can optimize their
traffic to minimize network usage.
For the downlink, LTE user equipment
(UE) utilizes Orthogonal Frequency Division Multiplexing (OFDM), which is
somewhat inefficient
<note>A Close Examination of Performance and Power Characteristics of 4G LTE Networks &lt;<link url='http://www.cs.columbia.edu/~lierranli/coms6998-7Spring2014/papers/rrclte_mobisys2012.pdf'>http://www.cs.columbia.edu/~lierranli/coms6998-7Spring2014/papers/rrclte_mobisys2012.pdf</link>&gt;</note>.
On the uplink side a different technology, Single-carrier frequency
division multiple access (SC-FDMA) is used, which is slightly more
efficient than traditional (non linearly-precoded) OFDM, slightly
offsetting the fact that broadcasting requires more power than receiving.
LTE UE also implements a Discontinuous reception (DRX) mode in which the
hardware can sleep until it is woken by a paging message or is needed to
perform some task.
LTE radios have two power modes: RRC_CONNECTED and RRC_IDLE.
DRX is supported in both of these power modes.
By attempting to minimize the time which the LTE UE state machine spends
in the RCC_CONNECTED state, and maximize the time it stays in the DRX
state (for RCC_CONNECTED and RRC_IDLE), we can increase battery life
without degrading the XMPP experience.
To do so, the following rules should be observed:
</p>
<section2 topic='Transmit no data'>
<p>
Whenever possible, data that is not strictly needed should not be
transmitted (by the server or client).
Supporting &xep0352; is recommended.
Most importantly, XMPP pings should be kept as far apart as possible and
only used when necessary.
Server operators are encouraged to set high ping timeouts, and client
implementors are advised to only send pings when absolutely necessary to
prevent the server from closing the socket.
</p>
</section2>
<section2 topic='Transmit as much data as you can at once'>
<p>
If one is on 3G, transmitting a small amount of data will cause the
radio to enter FACH mode which is significantly cheaper than its high
power mode.
On LTE radios, however, transmitting small amounts of data is vastly
more expensive per bit due to the higher tail-times (the time it takes
for the radio to change state).
On LTE radios, one should transmit as much data from the client as
possible when the radio is already on (eg. by placing messages in a send
queue and executing the queue as a batch when the radio is on).
Similarly, when data is being received from the server, the mobile
devices radio is already in a high power state and therefore any data
that needs to be sent to the server should be transmitted.
</p>
<p>
These rules also apply to server operators: If the server receives data,
the phones radio is already on therefore you should send any pending
data.
Batching data to be sent and sending it all at once will help reduce
power consumption.
</p>
</section2>
</section1>
<section1 topic='Notable Extensions' anchor='xeps'>
<p>
This section provides pointers to other documents which may be of interest
to those developing mobile clients, or considering implementing
optimizations for them in servers.
</p>
<p>&xep0138; provides stream level compression.</p>
<p>&xep0322; allows XMPP streams to use the EXI XML format.</p>
<p>
&xep0115; provides a mechanism for caching, and hence eliding, the
disco#info requests needed to negotiate optional features.
</p>
<p>
&xep0237; provides a relatively widely deployed extension for reducing
roster fetch sizes.
</p>
<p>
&xep0198; allows the client to send and receive smaller keep-alive
messages, and resume existing sessions without the full handshake.
This is useful on unstable connections.
</p>
<p>
&xep0352; allows clients to indicate to the server that they are inactive,
allowing the server to optimize and reduce unnecessary traffic.
</p>
<p>
&xep0357; implements push notifications (third party message delivery),
which are often used on mobile devices and highly optimized to conserve
battery.
Push notifications also allow delivery of notifications to mobile clients
that are currently offline (eg. in an XEP-0198 "zombie" state).
</p>
<p>
&xep0313; lets clients fetch messages which they missed (eg. due to poor
mobile coverage and a flaky network connection).
</p>
</section1>
<section1 topic='Acknowledgements' anchor='acks'>
<p>
This XEP was originally written by Dave Cridland, and parts of his
original work were used in this rewrite.
Thanks to Atlassian for allowing me to release hard numbers from their
XMPP compression deployment.
</p>
</section1>
<section1 topic='Security Considerations' anchor='security'>
<p>This document introduces no new security considerations.</p>
</section1>
<section1 topic='IANA Considerations' anchor='iana'>
<p>
This document requires no interaction with the Internet Assigned Numbers
Authority (IANA).
</p>
</section1>
<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
<p>
No namespaces or parameters need to be registered with the XMPP Registrar
as a result of this document.
</p>
</section1>
</xep>