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xep-0286: remove EXI reference
Other minor editorial and citation fixes.
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xep-0286.xml
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xep-0286.xml
@ -2,11 +2,13 @@
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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</title>
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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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@ -30,6 +32,17 @@
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<jid>dave.cridland@isode.com</jid>
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</author>
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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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@ -103,17 +116,17 @@
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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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While the security implications of stream compression are beyond the scope
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of this document (See the aforementioned RFC or XEP for more info), the
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author does not recommend using TLS compression with XMPP (or in general).
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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 a class of chosen plaintext
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attacks which can cause data leakage in compressed streams.
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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 at HipChat, network
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traffic was still observed to decrease by a factor of 0.58 when enabling
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&xep0138; with ZLIB compression!
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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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@ -122,13 +135,10 @@
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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 due
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to the fact that there were fewer packets that needed to be handled by the
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OS (which also takes CPU time), and, potentially more importantly, less
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data that needed to be TLS-encrypted (which is a much more CPU-expensive
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operation than compression).
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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) should be considered negligable.
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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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@ -140,12 +150,10 @@
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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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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) instead of the more modern VoLTE (Voice Over LTE) standard, or even
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CSFB (Circuit-switched fallback) this number would (presumably) be even
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higher.
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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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@ -153,12 +161,11 @@
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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
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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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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 (non linearly-precoded) OFDM, slightly
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offsetting the fact that broadcasting requires more power than receiving.
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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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@ -182,14 +189,14 @@
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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='Transmit as much data as you can at once'>
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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-times (the time it takes
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for the radio to change state).
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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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@ -199,10 +206,8 @@
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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 send any pending
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data.
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Batching data to be sent and sending it all at once will help reduce
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power consumption.
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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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@ -213,7 +218,6 @@
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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>&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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@ -247,8 +251,8 @@
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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 for allowing me to release hard numbers from their
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XMPP compression deployment.
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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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