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f6b6883aff
Update compression section with better numbers
242 lines
11 KiB
XML
242 lines
11 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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]>
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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</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>Experimental</status>
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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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<author>
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<firstname>Dave</firstname>
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<surname>Cridland</surname>
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<email>dave.cridland@isode.com</email>
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<jid>dave.cridland@isode.com</jid>
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</author>
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<author>
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<firstname>Sam</firstname>
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<surname>Whited</surname>
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<email>sam@samwhited.com</email>
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<jid>sam@samwhited.com</jid>
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</author>
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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><p>Include real world compression numbers and additional recommended reading.</p></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 more
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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. With
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the wide spread proliferation of 3G and LTE technologies, mobile bandwidth
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and speeds have become broadly comparable to broadband. However, they are
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still relatively expensive compared to traditional wired networks, and
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should therefore still be considered. This XEP mostly focuses on LTE as it
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already has a very wide deployment and will only continue to further
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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). While the security implications of
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stream compression are beyond the scope of this document (See the
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aforementioned RFC or XEP for more info), the author does not recommend
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using TLS compression with XMPP (or in general). If compression must be
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used, stream level compression should be implemented instead, and the
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compressed stream should have a full flush performed on stanza boundaries
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to help prevent a class of chosen plaintext attacks which can cause data
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leakage in compressed streams. While this may mitigate some of the benefits
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of compression by raising compression ratios, in a large, real world
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deployment at HipChat, network traffic was still observed to decrease by a
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factor of 0.58 when enabling &xep0138; 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. In
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the aforementioned deployment of stream compression, a <em>decrease</em> in
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CPU utilization by a factor of 0.60 was observed due to the fact that there
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were fewer packets that needed to be handled by the OS (which also takes
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CPU time), and, potentially more importantly, less data that needed to be
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TLS-encrypted (which is a much more CPU-expensive operation than
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compression). Therefore CPU time spent on compression (for ZLIB, at least;
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other algorithms were not tested) should be considered negligable.
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</p>
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<p>
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Supporting compression and flushing on stanza boundaries is highly
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recommended.
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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
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<note>LTE Smartphone measurements <<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>></note>.
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On some networks that support the legacy SVLTE (Simultaneous Voice and LTE)
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instead of the more modern VoLTE (Voice Over LTE) standard, or even CSFB
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(Circuit-switched fallback) 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. 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
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<note>A Close Examination of Performance and Power Characteristics of 4G LTE Networks <<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>></note>.
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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 (non linearly-precoded) OFDM, slightly
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offsetting the fact that 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. LTE radios have two power modes: RRC_CONNECTED and
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RRC_IDLE. DRX is supported in both of these power modes. By attempting to
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minimize the time which the LTE UE state machine spends in the
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RCC_CONNECTED state, and maximize the time it stays in the DRX state (for
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RCC_CONNECTED and RRC_IDLE), we can increase battery life without degrading
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the XMPP experience. 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). Supporting &xep0352; is highly
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recommended. Most importantly, XMPP pings should be kept as far apart as
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possible and only used when necessary. Server operators are encouraged to
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set high ping timeouts, and client implementors are advised to only send
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pings when absolutely necessary to prevent the server from closing the
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socket.
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</p>
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</section2>
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<section2 topic='Transmit as much data as you can at once'>
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<p>
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If one is on 3G, transmitting a small amount of data will cause the radio
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to enter FACH mode which is significantly cheaper than its high power
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mode. On LTE radios, however, transmitting small amounts of data is
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vastly more expensive per bit due to the significantly higher tail-times
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(the time it takes for the radio to change state). On LTE radios, one
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should transmit as much data as possible when the radio is already on
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(eg. by placing messages in a send queue and executing the queue as a
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batch). Similarly, when data is being received the radio is already in a
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high power state and therefore any data that needs to be sent should be.
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</p>
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<p>
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These rules also apply to server operators: If you receive data, the
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phones radio is already on therefore you should send anything you have.
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Otherwise, batching data to be sent and sending it all at once (and as
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much as possible) will help reduce power consumption.
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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 support for them in
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servers.
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</p>
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<p>&xep0138; provides stream level compression.</p>
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<p>&xep0322; allows XMPP streams to use the EXI XML format.</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. Useful
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on unstable connections.
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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. Push notifications also allow delivery of notifications to mobile
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clients 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 original
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work were used in this rewrite. Thanks to Atlassian for allowing me to
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release hard numbers from 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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