Max parallel http connections in a browser

Question

I am creating some suspended connections to an HTTP server  comet  reverse ajax  etc   It works ok  but I see the browser only allows two suspended connections to a given domain simultaneously  So if a user is looking at my website in Tab1 of their browser  then also tries loading it in Tab2  they ve used up the two allowed connections to my site   I think I can do some wildcard domain thing  where I have my http server resolve any address to my site like     example com webapp  - gt  192 0 2 1  the actual ip of my server    so   a example com webapp b example com webapp c example com webapp   all still point to  www example com webapp  but the browser considers them different domains  so I don t run into the 2 connection limit  Is this true   Even if that is true - is there any limit to the number of active connections per browser  across all domains  Say I use the scheme above - does Firefox for example only allow 24 parallel connections at any given time  Something like   1  a example com webapp 2  www download example hugefile zip 3  b example com webapp 4  c example com webapp     24  x example com webapp 25     Error - all 24 possible connections currently in use    I just picked 24 connections Firefox as an example

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BrowserVersion   ConnectionsPerHostname   MaxConnections ----------------------------------------------------------  Chrome34 32      6                        10  IE9              6                        35  IE10             8                        17  IE11             13                       17  Firefox27 26     6                        17  Safari7 0 1      6                        17  Android4         6                        17  ChromeMobile18   6                        16  IE Mobile9       6                        60   The first value is ConnectionsPerHostname and the second value is MaxConnections   Source  http   www browserscope org  category network amp v top  Note  ConnectionsPerHostname is the maximum number of concurrent http requests that browsers will make to the same domain   To increase the number of concurrent connections  one can host resources  e g  images  in different domains  However  you cannot exceed  MaxConnections   the maximum number of connections a browser will open in total - across all domains    2020 Update  Number of parallel connections per browser    Browser                Connections per Domain           Max Connections                    --------------------   ------------------------------   ------------------------------     Chrome 81              6   note1                        256  note2                         Edge 18                 same as Internet Explorer 11     same as Internet Explorer 11      Firefox 68             9   note1  or 6   note3          1000   note2                       Internet Explorer 11   12   note4                       1000   note2                       Safari 13              6   note1                        1000   note2                          note1   tested with 72 requests   1 domain 127 0 0 1    note2   tested with 1002 requests  6 requests per domain   167 domains   127 0 0      note3   when called in async context  e g  in callback of setTimeout    requestAnimationFrame  then      note4   of which the last 6 are follow-ups  2 4 6 available at 0 5s 1s 1 5s respectively

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HTTP 1 1  IE 6 and 7       2 IE 8             6 IE 9             6 IE 10            8 IE 11            8 Firefox 2        2 Firefox 3        6 Firefox 4 to 46  6 Opera 9 63       4 Opera 10         8 Opera 11 and 12  6 Chrome 1 and 2   6 Chrome 3         4 Chrome 4 to 23   6 Safari 3 and 4   4   source  http   p2p wrox com book-professional-website-performance-optimizing-front-end-back-end-705   HTTP 2 SPDY   Multiplexed support one single TCP connection for all requests

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The 2 concurrent requests is an intentional part of the design of many browsers  There is a standard out there that  good http clients  adhere to on purpose  Check out this RFC to see why

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Note that increasing a browser s max connections per server to an excessive number  as some sites suggest  can and does lock other users out of small sites with hosting plans that limit the total simultaneous connections on the server

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Firefox stores that number in this setting  you find it in about config   network http max-connections-per-server  For the max connections  Firefox stores that in this setting  network http max-connections

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Max Number of default simultaneous persistent connections per server proxy  Firefox 2   2 Firefox 3   6 Opera 9 26  4 Opera 12    6 Safari 3    4 Safari 5    6 IE 7        2 IE 8        6 IE 10       8 Edge        6 Chrome      6  The limit is per-server proxy  so your wildcard scheme will work  FYI  this is specifically related to HTTP 1 1  other protocols have separate concerns and limitations  i e   SPDY  TLS  HTTP 2

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Various browsers have various limits for maximum connections per host name  you can find the exact numbers at http   www browserscope org  category network and here is an interesting article about connection limitations from web performance expert Steve Souders http   www stevesouders com blog 2008 03 20 roundup-on-parallel-connections

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There is no definitive answer to this  as each browser has its own configuration for this  and this configuration may be changed  If you search on the internet you can find ways to change this limit  usually they re branded as  performance enhancement methods    It might be worth advising your users to do so if it is required by your website

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Looking at about config on Firefox 33 on GNU Linux  Ubuntu   and searching connections I found   network http max-connections  256  That is likely to answer the part is there any limit to the number of active connections per browser  across all domain  network http max-persistent-connections-per-proxy  32 network http max-persistent-connections-per-server  6  skipped two properties     network websocket max-connections  200   interesting  seems like they are not limited per server but have a default value lower than global http connections

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My understanding is that the connection limit is not changeable on the client side   The connection limit must be changed on the server to have any effect   By default  many servers will only allow 2 connections per unique client   The client is not the browser  it is the client machine issuing the TCP IP requests   To see the effect very clearly  use something like JMeter to fire off a bunch of web service calls to your server host - it will accept the first two and will not accept another until one of the two is completed   The amazing thing about this is that for a SOA shop  this is critical  yet hardly anyone is really aware of it

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Yes  wildcard domain will work for you  Not aware of any limits on connections  Limits if any will be browser specific

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