2018-02-23 18:58:03 +00:00
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---
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created_at: '2013-11-11T14:56:03.000Z'
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title: What is the theoretical maximum number of open TCP connections? (2010)
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url: http://stackoverflow.com/q/2332741/1257977
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author: georgecmu
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points: 42
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story_text: ''
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comment_text:
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num_comments: 21
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story_id:
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story_title:
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story_url:
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parent_id:
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created_at_i: 1384181763
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_tags:
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- story
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- author_georgecmu
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- story_6711866
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objectID: '6711866'
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2018-06-08 12:05:27 +00:00
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year: 2010
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2018-02-23 18:58:03 +00:00
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---
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2018-03-03 09:35:28 +00:00
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A single listening port can accept more than one connection
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simultaneously.
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2018-02-23 18:19:40 +00:00
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2018-03-03 09:35:28 +00:00
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There is a '64K' limit that is often cited, but that is per client per
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server port, and needs clarifying.
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2018-02-23 18:19:40 +00:00
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2018-03-03 09:35:28 +00:00
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Each TCP/IP packet has basically four fields for addressing; these are:
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2018-02-23 18:19:40 +00:00
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2018-03-03 09:35:28 +00:00
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source_ip source_port destination_ip destination_port
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< client > < server >
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Inside the TCP stack, these four fields are used as a compound key to
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match up packets to connections (e.g. file descriptors).
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If a client has many connections to the same port on the same
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destination, then three of those fields will be the same - only
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`source_port` varies to differentiate the different connections. Ports
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are 16-bit numbers, therefore the maximum number of connections any
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given client can have to any given host port is 64K.
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However, multiple clients can each have up to 64K connections to some
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server's port, and if the server has multiple ports or either is
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multi-homed then you can multiply that further.
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So the real limit is file descriptors. Each individual socket connection
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is given a file descriptor, so the limit is really the number of file
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descriptors that the system has been configured to allow and resources
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to handle. The maximum limit is typically up over 300K, but is
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configurable e.g. with [sysctl](http://linux.die.net/man/8/sysctl).
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The realistic limits being boasted about for normal boxes are around 80K
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for example single threaded Jabber messaging servers.
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