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  <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>
    &sam;
    <revision>
      <version>0.4.0</version>
      <date>2017-01-17</date>
      <initials>ssw</initials>
      <remark>
        <ul>
          <li>Attempt to fix some confusing paragraphs.</li>
          <li>Add Client State Indication to Notable Extensions.</li>
        </ul>
      </remark>
    </revision>
    <revision>
      <version>0.3</version>
      <date>2015-07-24</date>
      <initials>ssw</initials>
      <remark>
        <p>
          Include real world compression numbers and additional recommended
          reading.
        </p>
      </remark>
    </revision>
    <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      However, this mostly stems from undocumented lore and outdated notions of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 therefore conserving them is still desirable.
      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.
      Compression of XMPP data can be achieved with the DEFLATE algorithm
      (&rfc1951;) via TLS compression (&rfc3749;) or &xep0138; (which also
      supports other compression algorithms).
      While the security implications of stream compression are beyond the scope
      of this document (See the aforementioned RFC or XEP for more info), 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, and the compressed stream should have a full flush
      performed on stanza boundaries to help prevent a class of chosen plaintext
      attacks which can cause data leakage in compressed streams.
      While this may mitigate some of the benefits of compression by raising
      compression ratios, in a large, real world deployment at HipChat, network
      traffic was still observed to decrease by a factor of 0.58 when enabling
      &xep0138; with ZLIB compression!
    </p>
    <p>
      While the CPU cost of compression may directly translate 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.
      However, CPU usage is also not guaranteed to rise due to compression.
      In the aforementioned deployment of stream compression, a
      <em>decrease</em> in CPU utilization by a factor of 0.60 was observed due
      to the fact that there were fewer packets that needed to be handled by the
      OS (which also takes CPU time), and, potentially more importantly, less
      data that needed to be TLS-encrypted (which is a much more CPU-expensive
      operation than compression).
      Therefore CPU time spent on compression (for ZLIB, at least; other
      algorithms were not tested) should be considered negligable.
    </p>
    <p>
      Supporting compression and performming a full flush on stanza boundaries
      is recommended for mobile devices.
    </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 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 higher tail-times (the time it takes
        for the radio to change state).
        On LTE radios, one should transmit as much data from the client as
        possible when the radio is already on (eg. by placing messages in a send
        queue and executing the queue as a batch when the radio is on).
        Similarly, when data is being received from the server, the mobile
        devices radio is already in a high power state and therefore any data
        that needs to be sent to the server should be transmitted.
      </p>
      <p>
        These rules also apply to server operators: If the server receives data,
        the phones radio is already on therefore you should send any pending
        data.
        Batching data to be sent and sending it all at once 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 implementing
      optimizations for them in servers.
    </p>
    <p>&xep0138; provides stream level compression.</p>
    <p>&xep0322; allows XMPP streams to use the EXI XML format.</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.
      This is useful on unstable connections.
    </p>
    <p>
      &xep0352; allows clients to indicate to the server that they are inactive,
      allowing the server to optimize and reduce unnecessary traffic.
    </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 flaky 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.
      Thanks to Atlassian for allowing me to release hard numbers from their
      XMPP compression deployment.
    </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>
