mirror of
https://github.com/moparisthebest/xeps
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270 lines
12 KiB
XML
270 lines
12 KiB
XML
<?xml version='1.0' encoding='UTF-8'?>
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<!DOCTYPE xep SYSTEM 'xep.dtd' [
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<!ENTITY % ents SYSTEM 'xep.ent'>
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%ents;
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<!ENTITY nokia11 "<note>LTE Smartphone measurements <<link url='https://web.archive.org/web/20160624043050/http://networks.nokia.com/system/files/document/lte_measurements_final.pdf'>http://networks.nokia.com/system/files/document/lte_measurements_final.pdf</link>></note>">
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<!ENTITY huang12 "<note>A Close Examination of Performance and Power Characteristics of 4G LTE Networks <<link url='https://doi.org/10.1145/2307636.2307658'>doi:2307636.2307658</link>></note>">
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]>
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<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
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<xep>
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<header>
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<title>Mobile Considerations on LTE Networks</title>
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<abstract>
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This document provides background information for XMPP implementors
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concerned with mobile devices operating on an LTE cellular network.
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</abstract>
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&LEGALNOTICE;
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<number>0286</number>
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<status>Proposed</status>
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<lastcall>2017-11-15</lastcall>
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<type>Informational</type>
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<sig>Standards</sig>
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<approver>Council</approver>
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<dependencies>
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<spec>XMPP Core</spec>
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</dependencies>
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<supersedes/>
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<supersededby/>
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<shortname>NOT_YET_ASSIGNED</shortname>
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&dcridland;
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&sam;
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<revision>
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<version>0.4.1</version>
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<date>2017-09-17</date>
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<initials>ssw</initials>
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<remark>
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<ul>
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<li>Minor editorial fixes.</li>
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<li>Remove reference to EXI which has no implementations.</li>
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</ul>
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</remark>
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</revision>
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<revision>
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<version>0.4.0</version>
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<date>2017-01-17</date>
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<initials>ssw</initials>
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<remark>
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<ul>
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<li>Attempt to fix some confusing paragraphs.</li>
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<li>Add Client State Indication to Notable Extensions.</li>
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</ul>
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</remark>
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</revision>
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<revision>
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<version>0.3</version>
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<date>2015-07-24</date>
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<initials>ssw</initials>
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<remark>
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<p>
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Include real world compression numbers and additional recommended
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reading.
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</p>
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</remark>
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</revision>
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<revision>
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<version>0.2</version>
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<date>2015-07-22</date>
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<initials>ssw</initials>
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<remark><p>Overhaul to include LTE.</p></remark>
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</revision>
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<revision>
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<version>0.1</version>
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<date>2010-09-15</date>
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<initials>psa</initials>
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<remark><p>Initial published version.</p></remark>
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</revision>
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<revision>
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<version>0.0.1</version>
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<date>2010-07-13</date>
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<initials>dwd</initials>
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<remark><p>First draft. Also John's birthday.</p></remark>
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</revision>
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</header>
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<section1 topic='Introduction' anchor='intro'>
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<p>
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XMPP as a protocol was designed before the wide spread adoption of mobile
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devices, and is often cited as not being very mobile friendly as a result.
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However, this mostly stems from undocumented lore and outdated notions of
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how XMPP works. As the Internet and protocol design have changed to be
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more accommodating for mobile, so has XMPP.
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</p>
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<p>
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This XEP aims to provide useful background knowledge of mobile handset
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behavior, and those considerations that client and server designers can
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take to ensure that bandwidth and battery are used efficiently.
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</p>
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</section1>
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<section1 topic='Overview' anchor='overview'>
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<p>
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The two major constraints on mobile devices are power and bandwidth.
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With the wide spread proliferation of 3G and LTE technologies, mobile
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bandwidth and speeds have become broadly comparable to broadband.
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However, they are still relatively expensive compared to traditional wired
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networks, and therefore conserving them is still desirable.
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This XEP mostly focuses on LTE as it already has a very wide deployment
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and will only continue to further replace 3G technologies.
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</p>
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</section1>
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<section1 topic='Compression' anchor='compression'>
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<p>
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XML, and by extension XMPP, is known to be highly compressible.
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Compression of XMPP data can be achieved with the DEFLATE algorithm
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(&rfc1951;) via TLS compression (&rfc3749;) or &xep0138; (which also
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supports other compression algorithms).
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A description of the security implications of stream compression is beyond
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the scope of this document (See &rfc3749; or &xep0138; for more
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information), but the author does not recommend using TLS compression with
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XMPP (or in general).
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If compression must be used, stream level compression should be
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implemented instead, and the compressed stream should have a full flush
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performed on stanza boundaries to help prevent chosen plaintext attacks.
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While this may mitigate some of the benefits of compression by raising
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compression ratios, in a large, real world deployment, network traffic was
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still observed to decrease by a factor of 0.58 when enabling &xep0138;
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with ZLIB compression.
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</p>
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<p>
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While the CPU cost of compression may directly translate to higher power
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usage, it is vastly outweighed by the benefits of reduced network
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utilization, especially on modern LTE networks which use a great deal more
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power per bit than 3G networks as will be seen later in this document.
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However, CPU usage is also not guaranteed to rise due to compression.
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In the aforementioned deployment of stream compression, a
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<em>decrease</em> in CPU utilization by a factor of 0.60 was observed,
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presumably due to reductions in TLS and packet handling overhead.
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Therefore CPU time spent on compression (for ZLIB, at least; other
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algorithms were not tested) can be considered negligable.
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</p>
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<p>
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Supporting compression and performming a full flush on stanza boundaries
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is recommended for mobile devices.
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</p>
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</section1>
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<section1 topic='Power Consumption' anchor='power'>
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<p>
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While the wide spread adoption of LTE has dramatically increased available
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bandwidth on mobile devices, it has also increased power consumption.
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According to one study, early LTE devices consumed 5–20% more power
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than their 3G counterparts &nokia11;.
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On some networks that support the legacy SVLTE (Simultaneous Voice and
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LTE) or CSFB (Circuit-switched fallback) instead of the more modern VoLTE
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(Voice Over LTE) standard, this number would (presumably) be even higher.
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</p>
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<p>
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XMPP server and client implementers, bearing this increased power usage in
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mind, and knowing a bit about how LTE radios work, can optimize their
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traffic to minimize network usage.
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For the downlink, LTE user equipment
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(UE) utilizes Orthogonal Frequency Division Multiplexing (OFDM), which is
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somewhat inefficient &huang12;.
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On the uplink side a different technology, Single-carrier frequency
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division multiple access (SC-FDMA) is used, which is slightly more
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efficient than traditional OFDM, slightly offsetting the fact that
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broadcasting requires more power than receiving.
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LTE UE also implements a Discontinuous reception (DRX) mode in which the
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hardware can sleep until it is woken by a paging message or is needed to
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perform some task.
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LTE radios have two power modes: RRC_CONNECTED and RRC_IDLE.
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DRX is supported in both of these power modes.
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By attempting to minimize the time which the LTE UE state machine spends
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in the RCC_CONNECTED state, and maximize the time it stays in the DRX
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state (for RCC_CONNECTED and RRC_IDLE), we can increase battery life
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without degrading the XMPP experience.
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To do so, the following rules should be observed:
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</p>
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<section2 topic='Transmit no data'>
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<p>
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Whenever possible, data that is not strictly needed should not be
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transmitted (by the server or client).
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Supporting &xep0352; is recommended.
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Most importantly, XMPP pings should be kept as far apart as possible and
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only used when necessary.
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Server operators are encouraged to set high ping timeouts, and client
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implementors are advised to only send pings when absolutely necessary to
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prevent the server from closing the socket.
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</p>
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</section2>
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<section2 topic='When transmitting, transmit as much as you can'>
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<p>
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If one is on 3G, transmitting a small amount of data will cause the
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radio to enter FACH mode which is significantly cheaper than its high
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power mode.
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On LTE radios, however, transmitting small amounts of data is vastly
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more expensive per bit due to the higher tail-time (the time it takes
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for the radio to change state) of approximately 11 seconds&huang12;.
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On LTE radios, one should transmit as much data from the client as
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possible when the radio is already on (eg. by placing messages in a send
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queue and executing the queue as a batch when the radio is on).
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Similarly, when data is being received from the server, the mobile
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devices radio is already in a high power state and therefore any data
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that needs to be sent to the server should be transmitted.
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</p>
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<p>
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These rules also apply to server operators: If the server receives data,
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the phones radio is already on, therefore you should flush any pending
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data as soon as possible after receiving data from a client.
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</p>
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</section2>
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</section1>
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<section1 topic='Notable Extensions' anchor='xeps'>
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<p>
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This section provides pointers to other documents which may be of interest
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to those developing mobile clients, or considering implementing
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optimizations for them in servers.
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</p>
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<p>&xep0138; provides stream level compression.</p>
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<p>
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&xep0115; provides a mechanism for caching, and hence eliding, the
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disco#info requests needed to negotiate optional features.
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</p>
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<p>
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&xep0237; provides a relatively widely deployed extension for reducing
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roster fetch sizes.
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</p>
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<p>
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&xep0198; allows the client to send and receive smaller keep-alive
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messages, and resume existing sessions without the full handshake.
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This is useful on unstable connections.
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</p>
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<p>
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&xep0352; allows clients to indicate to the server that they are inactive,
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allowing the server to optimize and reduce unnecessary traffic.
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</p>
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<p>
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&xep0357; implements push notifications (third party message delivery),
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which are often used on mobile devices and highly optimized to conserve
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battery.
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Push notifications also allow delivery of notifications to mobile clients
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that are currently offline (eg. in an XEP-0198 "zombie" state).
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</p>
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<p>
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&xep0313; lets clients fetch messages which they missed (eg. due to poor
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mobile coverage and a flaky network connection).
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</p>
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</section1>
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<section1 topic='Acknowledgements' anchor='acks'>
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<p>
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This XEP was originally written by Dave Cridland, and parts of his
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original work were used in this rewrite.
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Thanks to Atlassian (HipChat) for allowing me to release numbers from
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their XMPP compression deployment.
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</p>
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</section1>
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<section1 topic='Security Considerations' anchor='security'>
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<p>This document introduces no new security considerations.</p>
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</section1>
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<section1 topic='IANA Considerations' anchor='iana'>
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<p>
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This document requires no interaction with the Internet Assigned Numbers
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Authority (IANA).
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</p>
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</section1>
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<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
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<p>
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No namespaces or parameters need to be registered with the XMPP Registrar
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as a result of this document.
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</p>
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</section1>
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</xep>
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