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 However, this mostly stems from undocumented lore and outdated notions of how XMPP works.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.

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.

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 should therefore still be considered. This XEP mostly focuses on LTE as it already has a very wide deployment and will only continue to further replace 3G technologies.

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!

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 decrease 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.

Supporting compression and flushing on stanza boundaries is highly recommended.

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 5–20% more power than their 3G counterparts LTE Smartphone measurements <http://networks.nokia.com/system/files/document/lte_measurements_final.pdf>. 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.

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 A Close Examination of Performance and Power Characteristics of 4G LTE Networks <http://www.cs.columbia.edu/~lierranli/coms6998-7Spring2014/papers/rrclte_mobisys2012.pdf>. 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:

This section provides pointers to other documents which may be of interest to those developing mobile clients, or considering support for them in servers.

&xep0138; provides stream level compression.

&xep0322; allows XMPP streams to use the EXI XML format.

&xep0115; provides a mechanism for caching, and hence eliding, the disco#info requests needed to negotiate optional features.

&xep0237; provides a relatively widely deployed extension for reducing roster fetch sizes.

&xep0198; allows the client to send and receive smaller keep-alive messages, and resume existing sessions without the full handshake. Useful on unstable connections.

&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).

&xep0313; lets clients fetch messages which they missed (eg. due to poor mobile coverage and a flaky network connection).

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.

This document introduces no new security considerations.

This document requires no interaction with the Internet Assigned Numbers Authority (IANA).

No namespaces or parameters need to be registered with the XMPP Registrar as a result of this document.