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ProtoXEP.modile-concerns v0.0.1: first draft.
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Matthew A. Miller
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inbox/mobile-concerns.xml
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inbox/mobile-concerns.xml
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<?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>NOT_YET_ASSIGNED</number>
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<status>ProtoXEP</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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<spec>XEP-0286</spec>
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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>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>0.0.1</version>
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<date>2015-07-18</date>
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<initials>ssw</initials>
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<remark><p>First draft.</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. In a simple
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test of a small (266089 byte) XMPP stream (connection, stream
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initialization, feature discovery, roster loading, several presence stanzas
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sent and received, disconnect), the entropy of the stream was found to be
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5.616313 bits per byte. Using the `gzip` tool to apply Lempel-Ziv coding
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(LZ77) without concern for server-side CPU usage resulted in a compression
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ratio of 21% (a 79% reduction in bandwidth). In one test with a much larger
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dataset typical of a corporate environment (many hundreds of users in the
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roster), the ratio was as low as 13%, an 87% reduction in bandwidth!
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</p>
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<p>
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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;. While the
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security implications of stream compression are beyond the scope of this
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document (See the aforementioned RFC or XEP for more info), mitigating them
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may affect compression ratios. The author does not recommend using TLS
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compression with XMPP (or in general). If compression must be used, stream
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level compression should be implemented instead. Compressing at the stream
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level gives us the benefit of being able to flush the compression stream on
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stanza boundaries to help prevent information from leaking. This, however,
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may drastically increase compression ratios.
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</p>
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<p>
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While the CPU cost of compression directly translates 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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Setting security considerations aside and thinking only of power
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consumption and bandwidth, supporting compression is highly 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 than
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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>
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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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</section1>
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<section1 topic='Acknowledgements' anchor='acks'>
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<p>
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The original mobile XEP, &xep0286;, was written by Dave Cridland, and parts
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of it were used when writing this XEP.
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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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1
xep.ent
1
xep.ent
@ -405,6 +405,7 @@ THE SOFTWARE.
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<!ENTITY rfc1939 "<span class='ref'><link url='http://tools.ietf.org/html/rfc1939'>RFC 1939</link></span> <note>RFC 1939: Post Office Protocol - Version 3 <<link url='http://tools.ietf.org/html/rfc1939'>http://tools.ietf.org/html/rfc1939</link>>.</note>" >
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<!ENTITY rfc1945 "<span class='ref'><link url='http://tools.ietf.org/html/rfc1945'>RFC 1945</link></span> <note>RFC 1945: Hypertext Transfer Protocol -- HTTP/1.0 <<link url='http://tools.ietf.org/html/rfc1945'>http://tools.ietf.org/html/rfc1945</link>>.</note>" >
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<!ENTITY rfc1950 "<span class='ref'><link url='http://tools.ietf.org/html/rfc1950'>RFC 1950</link></span> <note>RFC 1950: ZLIB Compressed Data Format Specification version 3.3 <<link url='http://tools.ietf.org/html/rfc1950'>http://tools.ietf.org/html/rfc1950</link>>.</note>" >
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<!ENTITY rfc1951 "<span class='ref'><link url='http://tools.ietf.org/html/rfc1951'>RFC 1951</link></span> <note>RFC 1951: DEFLATE Compressed Data Format Specification version 1.3 <<link url='http://tools.ietf.org/html/rfc1951'>http://tools.ietf.org/html/rfc1951</link>>.</note>" >
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<!ENTITY rfc2026 "<span class='ref'><link url='http://tools.ietf.org/html/rfc2026'>RFC 2026</link></span> <note>RFC 2026: The Internet Standards Process <<link url='http://tools.ietf.org/html/rfc2026'>http://tools.ietf.org/html/rfc2026</link>>.</note>" >
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<!ENTITY rfc2045 "<span class='ref'><link url='http://tools.ietf.org/html/rfc2045'>RFC 2045</link></span> <note>RFC 2045: Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies <<link url='http://tools.ietf.org/html/rfc2045'>http://tools.ietf.org/html/rfc2045</link>>.</note>" >
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<!ENTITY rfc2068 "<span class='ref'><link url='http://tools.ietf.org/html/rfc2068'>RFC 2068</link></span> <note>RFC 2068: Hypertext Transport Protocol -- HTTP/1.1 <<link url='http://tools.ietf.org/html/rfc2068'>http://tools.ietf.org/html/rfc2068</link>>.</note>" >
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