First draft.
- 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 - However, this mostly stems from undocumented lore and outdated notions of - how XMPP works.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. In a simple - test of a small (266089 byte) XMPP stream (connection, stream - initialization, feature discovery, roster loading, several presence stanzas - sent and received, disconnect), the entropy of the stream was found to be - 5.616313 bits per byte. Using the `gzip` tool to apply Lempel-Ziv coding - (LZ77) without concern for server-side CPU usage resulted in a compression - ratio of 21% (a 79% reduction in bandwidth). In one test with a much larger - dataset typical of a corporate environment (many hundreds of users in the - roster), the ratio was as low as 13%, an 87% reduction in bandwidth! -
-- Compression of XMPP data can be achieved with the DEFLATE algorithm - (&rfc1951;) via TLS compression (&rfc3749;) or &xep0138;. While the - security implications of stream compression are beyond the scope of this - document (See the aforementioned RFC or XEP for more info), mitigating them - may affect compression ratios. 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. Compressing at the stream - level gives us the benefit of being able to flush the compression stream on - stanza boundaries to help prevent information from leaking. This, however, - may drastically increase compression ratios. -
-- While the CPU cost of compression directly translates 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. - Setting security considerations aside and thinking only of power - consumption and bandwidth, supporting compression 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
-
- 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
-
- Whenever possible, data that is not strictly needed should not be - transmitted (by the server or client). Supporting &xep0352; is highly - 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. -
-- 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 significantly higher tail-times - (the time it takes for the radio to change state). On LTE radios, one - should transmit as much data as possible when the radio is already on - (eg. by placing messages in a send queue and executing the queue as a - batch). Similarly, when data is being received the radio is already in a - high power state and therefore any data that needs to be sent should be. -
-- These rules also apply to server operators: If you receive data, the - phones radio is already on therefore you should send anything you have. - Otherwise, batching data to be sent and sending it all at once (and as - much as possible) will help reduce power consumption. -
-- 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.
-- &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. -
-- The original mobile XEP, &xep0286;, was written by Dave Cridland, and parts - of it were used when writing this XEP. -
-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. -
-Overhaul to include LTE.
The use of XMPP on mobile devices is little understood, since few XMPP implementors have good mobile knowledge, and few mobile engineers have good XMPP knowledge. In addition, as the mobile landscape has changed, optimal protocol designs and usage patterns have also changed. This has led to the sub-optimal combination of a large amount of mostly undocumented lore, as well as several outdated concepts being discussed as fact.
-This XEP aims to provide useful background knowledge of mobile handset behaviours, and essentially distills a number of conversations with experienced mobile engineers and XMPP implementors, providing useful background as general suggestions.
++ 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. +
Mobile handsets typically have two constraints - power and bandwidth. The advent of 3G technology and beyond has tended to mean that bandwidth is radically higher, and comparable to broadband speeds - however many operators still charge based on transferred data, hence bandwidth remains an important issue for cost purposes.
-The major cost of power in the handset for our purposes is the radio - here, too, bandwidth plays a part, but as this document will show, the time the radio is forced to be available to receive also costs substantially.
-Whilst this document refers to &rfc3920;, implementors are advised to take note of &rfc6120;.
++ 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. +
XMPP is known to compress well. Both TLS, part of &xmppcore;, and &xep0138; can provide access to the DEFLATE codec (RFC 1951
Compression ratios vary with usage, however, typical usage by a general client appears to show a 20% ratio (an 80% reduction in bandwidth) in longer sessions
At an exemplary point in one experiment, the author found the following figures
| Window Bits | -Compression Ratio (approx) | -
|---|---|
| 15 | -20% | -
| 14 | -22% | -
| 13 | -25% | -
| 12 | -30% | -
| 11 | -38% | -
| 10 | -43% | -
| 9 | -60% | -
Although there is an equal cost for the mobile device to compress, it is considered that the compression codec memory and CPU costs - while certainly translating into power cost - are outweighed by two factors. Firstly, they're likely to reduce the transmission cost by a greater amount, and secondly they will also reduce the encryption cost in TLS.
-Care, however, should be taken not to use XEP-0138 compression when TLS compression is in effect.
+ ++ XML, and by extension XMPP, is known to be highly compressible. In a simple + test of a small (266089 byte) XMPP stream (connection, stream + initialization, feature discovery, roster loading, several presence stanzas + sent and received, disconnect), the entropy of the stream was found to be + 5.616313 bits per byte. Using the `gzip` tool to apply Lempel-Ziv coding + (LZ77) without concern for server-side CPU usage resulted in a compression + ratio of 21% (a 79% reduction in bandwidth). In one test with a much larger + dataset typical of a corporate environment (many hundreds of users in the + roster), the ratio was as low as 13%, an 87% reduction in bandwidth! +
++ Compression of XMPP data can be achieved with the DEFLATE algorithm + (&rfc1951;) via TLS compression (&rfc3749;) or &xep0138;. While the + security implications of stream compression are beyond the scope of this + document (See the aforementioned RFC or XEP for more info), mitigating them + may affect compression ratios. 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. Compressing at the stream + level gives us the benefit of being able to flush the compression stream on + stanza boundaries to help prevent information from leaking. This, however, + may drastically increase compression ratios. +
++ While the CPU cost of compression directly translates 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. +
++ Supporting compression and flushing on stanza boundaries is highly + recommended. +
Mobile handsets have a number of levels for radio activity. 3G radios can be either Idle, or else in an increasingly capable - and increasingly power-hungry - series of levels, through FACH to DCH.
-For the purposes of investigating this, power consumption (or rather battery depletion rate, as current) and timeouts where measured on the 3UK network, with a Nokia E71, using the Energy Profiler. A "typical handset" mentioned here has a 1000mAh battery - some smartphones have up to 1500mAh. Note that all timeouts are under the control of the network operator, not the handset or application.
-Idle state is when the radio is neither receiving nor transmitting. It may have live (although silent) TCP connections. The cost is low. There is also a PCH level, which is similarly low-power, and again is only used when the radio is silent.
-The current here was measured as 8mA - likely affected more by the energy profiler than much else.
-FACH uses a shared channel for low-bandwidth communications. Packet sizes must be small - around 128 octets maximum, although this is operator controlled. Raising to this state takes around 2.5 seconds before the data can flow - although the power cost rises instantly - and after the session has returned to silence, it will remain at FACH level for some time.
-Note that this threshold includes the overheads from TCP and TLS, which are 52 and 5 octets respectively, leaving around 70 octets for the payload data.
-Here, the current was measured as 140mA, and a timeout of 8 seconds - this timeout is at the lower end of the expected range, which could be up to around 2 minutes. On a typical handset, this will exhaust the battery in around 7 hours.
-DCH uses a dedicated channel for high bandwidth communications. Again, raising to this state takes 2.5 seconds at the DCH power level, and there is a timeout before dropping back. Some operators will drop back to FACH for the duration fo the FACH timeout - others will drop back to Idle/PCH.
-Sending more than the FACH threshold will raise the radio all the way to DCH - taking, again, 2.5 seconds.
-The measurements here were 380mA and 8 seconds - this is sufficient to flatten a typical handset battery in less than 3 hours, and the figures are considered normal.
-Transmission of data can use up to 2W
Experimentation suggests that uncompressed XMPP will never trigger the FACH state, leaping directly into the more costly DCH state. However, compression does make FACH possible, if rare.
+
+ 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
+
+ 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
+
+ Whenever possible, data that is not strictly needed should not be + transmitted (by the server or client). Supporting &xep0352; is highly + 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. +
++ 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 significantly higher tail-times + (the time it takes for the radio to change state). On LTE radios, one + should transmit as much data as possible when the radio is already on + (eg. by placing messages in a send queue and executing the queue as a + batch). Similarly, when data is being received the radio is already in a + high power state and therefore any data that needs to be sent should be. +
++ These rules also apply to server operators: If you receive data, the + phones radio is already on therefore you should send anything you have. + Otherwise, batching data to be sent and sending it all at once (and as + much as possible) will help reduce power consumption. +
+As with anything, there are no hard and fast rules. If there were, they might look like these. First, for devices:
-And for servers, similar rules apply:
-Finally, protocol designers should aim to minimize any responses required from the handset, and ensure keepalive traffic, if any, fits inside FACH wherever possible.
-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 application stream level compression, useful if the device TLS stack does not support TLS-based compression.
-&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 the roster fetch bandwidth, in most cases reducing it to a simple affirmation that the client has the current roster. This saves not only bandwidth, but also reduces local storage writes.
-&xep0198; provides session resumption over TCP, enabling a client to handle the case where the coverage is patchy. The <r/> and <a/> elements also provide a keepalive facility in a small number of octets.
-&xep0273; provides a mechanism which, amongst other things, would allow a presence "hush", buffering presence during certain states.
++ 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.
++ &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 flakey network connection). +
The author is not a mobile expert, and relied on the knowledge and patient help of several others. In particular, thanks are due to Jussi Laako, Markku Vampari, and Markus Isomaki of Nokia, and Simon Tennant of Buddycloud.
-The attribution of any mistakes herein is zealously guarded by the author, however.
++ This XEP was originally written by Dave Cridland, and parts of his original + work were used in this rewrite. +
This document does not discuss a protocol, thus introduces no new security considerations.
+This document introduces no new security considerations.
None.
++ This document requires no interaction with the Internet Assigned Numbers + Authority (IANA). +
None.
++ No namespaces or parameters need to be registered with the XMPP Registrar + as a result of this document. +