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xeps/xep-0286.xml
2015-07-23 10:02:55 +02:00

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<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE xep SYSTEM 'xep.dtd' [
<!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>0.2</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However, this mostly stems from undocumented lore and outdated notions of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
should therefore still be considered. 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. In a simple
test of a small (266089 byte) XMPP stream (connection, stream
initialization, feature discovery, roster loading, several presence stanzas
sent and received, disconnect), the entropy of the stream was found to be
5.616313 bits per byte. Using the `gzip` tool to apply Lempel-Ziv coding
(LZ77) without concern for server-side CPU usage resulted in a compression
ratio of 21% (a 79% reduction in bandwidth). In one test with a much larger
dataset typical of a corporate environment (many hundreds of users in the
roster), the ratio was as low as 13%, an 87% reduction in bandwidth!
</p>
<p>
Compression of XMPP data can be achieved with the DEFLATE algorithm
(&rfc1951;) via TLS compression (&rfc3749;) or &xep0138;. While the
security implications of stream compression are beyond the scope of this
document (See the aforementioned RFC or XEP for more info), mitigating them
may affect compression ratios. 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. Compressing at the stream
level gives us the benefit of being able to flush the compression stream on
stanza boundaries to help prevent information from leaking. This, however,
may drastically increase compression ratios.
</p>
<p>
While the CPU cost of compression directly translates 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.
</p>
<p>
Supporting compression and flushing on stanza boundaries is highly
recommended.
</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 highly
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 significantly higher tail-times
(the time it takes for the radio to change state). On LTE radios, one
should transmit as much data as possible when the radio is already on
(eg. by placing messages in a send queue and executing the queue as a
batch). Similarly, when data is being received the radio is already in a
high power state and therefore any data that needs to be sent should be.
</p>
<p>
These rules also apply to server operators: If you receive data, the
phones radio is already on therefore you should send anything you have.
Otherwise, batching data to be sent and sending it all at once (and as
much as possible) 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 support for them in
servers.
</p>
<p>&xep0138; provides stream level compression.</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. Useful
on unstable connections.
</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 flakey 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.
</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>