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492 lines
21 KiB
Plaintext
492 lines
21 KiB
Plaintext
+--------------------+
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| Peers protocol 2.1 |
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+--------------------+
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Peers protocol has been implemented over TCP. Its aim is to transmit
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stick-table entries information between several haproxy processes.
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This protocol is symmetrical. This means that at any time, each peer
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may connect to other peers they have been configured for, to send
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their last stick-table updates. There is no role of client or server in this
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protocol. As peers may connect to each others at the same time, the protocol
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ensures that only one peer session may stay opened between a couple of peers
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before they start sending their stick-table information, possibly in both
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directions (or not).
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Handshake
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+++++++++
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Just after having connected to another one, a peer must identify itself
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and identify the remote peer, sending a "hello" message. The remote peer
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replies with a "status" message.
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A "hello" message is made of three lines terminated by a line feed character
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as follows:
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<protocol identifier> <version>\n
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<remote peer identifier>\n
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<local peer identifier> <process ID> <relative process ID>\n
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protocol identifier : HAProxyS
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version : 2.1
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remote peer identifier: the peer name this "hello" message is sent to.
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local peer identifier : the name of the peer which sends this "hello" message.
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process ID : the ID of the process handling this peer session.
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relative process ID : the haproxy's relative process ID (0 if nbproc == 1).
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The "status" message is made of a unique line terminated by a line feed
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character as follows:
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<status code>\n
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with these values as status code (a three-digit number):
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+-------------+---------------------------------+
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| status code | signification |
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+-------------+---------------------------------+
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| 200 | Handshake succeeded |
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+-------------+---------------------------------+
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| 300 | Try again later |
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+-------------+---------------------------------+
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| 501 | Protocol error |
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+-------------+---------------------------------+
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| 502 | Bad version |
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+-------------+---------------------------------+
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| 503 | Local peer identifier mismatch |
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+-------------+---------------------------------+
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| 504 | Remote peer identifier mismatch |
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+-------------+---------------------------------+
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As the protocol is symmetrical, some peers may connect to each other at the
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same time. For efficiency reasons, the protocol ensures there may be only
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one TCP session opened after the handshake succeeded and before transmitting
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any stick-table data information. In fact, for each couple of peers, this is
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the last connected peer which wins. Each time a peer A receives a "hello"
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message from a peer B, peer A checks if it already managed to open a peer
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session with peer B, so with a successful handshake. If it is the case,
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peer A closes its peer session. So, this is the peer session opened by B
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which stays opened.
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Peer A Peer B
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hello
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---------------------->
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status 200
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<----------------------
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hello
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<++++++++++++++++++++++
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TCP/FIN-ACK
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---------------------->
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TCP/FIN-ACK
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<----------------------
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status 200
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++++++++++++++++++++++>
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data
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<++++++++++++++++++++++
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data
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++++++++++++++++++++++>
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data
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++++++++++++++++++++++>
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data
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<++++++++++++++++++++++
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.
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.
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.
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As it is still possible that a couple of peers decide to close both their
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peer sessions at the same time, the protocol ensures peers will not reconnect
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at the same time, adding a random delay (50 up to 2050 ms) before any
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reconnection.
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Encoding
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++++++++
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As some TCP data may be corrupted, for integrity reason, some data fields
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are encoded at peer session level.
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The following algorithms explain how to encode/decode the data.
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encode:
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input : val (64bits integer)
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output: bitf (variable-length bitfield)
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if val has no bit set above bit 4 (or if val is less than 0xf0)
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set the next byte of bitf to the value of val
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return bitf
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set the next byte of bitf to the value of val OR'ed with 0xf0
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subtract 0xf0 from val
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right shift val by 4
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while val bit 7 is set (or if val is greater or equal to 0x80):
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set the next byte of bitf to the value of the byte made of the last
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7 bits of val OR'ed with 0x80
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subtract 0x80 from val
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right shift val by 7
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set the next byte of bitf to the value of val
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return bitf
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decode:
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input : bitf (variable-length bitfield)
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output: val (64bits integer)
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set val to the value of the first byte of bitf
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if bit 4 up to 7 of val are not set
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return val
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set loop to 0
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do
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add to val the value of the next byte of bitf left shifted by (4 + 7*loop)
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set loop to (loop + 1)
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while the bit 7 of the next byte of bitf is set
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return val
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Example:
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let's say that we must encode 0x1234.
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"set the next byte of bitf to the value of val OR'ed with 0xf0"
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=> bitf[0] = (0x1234 | 0xf0) & 0xff = 0xf4
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"subtract 0xf0 from val"
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=> val = 0x1144
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right shift val by 4
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=> val = 0x114
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"set the next byte of bitf to the value of the byte made of the last
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7 bits of val OR'ed with 0x80"
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=> bitf[1] = (0x114 | 0x80) & 0xff = 0x94
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"subtract 0x80 from val"
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=> val= 0x94
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"right shift val by 7"
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=> val = 0x1
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=> bitf[2] = 0x1
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So, the encoded value of 0x1234 is 0xf49401.
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To decode this value:
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"set val to the value of the first byte of bitf"
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=> val = 0xf4
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"add to val the value of the next byte of bitf left shifted by 4"
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=> val = 0xf4 + (0x94 << 4) = 0xf4 + 0x940 = 0xa34
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"add to val the value of the next byte of bitf left shifted by (4 + 7)"
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=> val = 0xa34 + (0x01 << 11) = 0xa34 + 0x800 = 0x1234
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Messages
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++++++++
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*** General ***
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After the handshake has successfully completed, peers are authorized to send
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some messages to each others, possibly in both direction.
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All the messages are made at least of a two bytes length header.
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The first byte of this header identifies the class of the message. The next
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byte identifies the type of message in the class.
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Some of these messages are variable-length. Others have a fixed size.
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Variable-length messages are identified by the value of the message type
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byte. For such messages, it is greater than or equal to 128.
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All variable-length message headers must be followed by the encoded length
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of the remaining bytes (so the encoded length of the message minus 2 bytes
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for the header and minus the length of the encoded length).
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There exist four classes of messages:
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+------------+---------------------+--------------+
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| class byte | signification | message size |
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+------------+---------------------+--------------+
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| 0 | control | fixed (2) |
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+------------+---------------------+--------------|
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| 1 | error | fixed (2) |
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+------------+---------------------+--------------|
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| 10 | stick-table updates | variable |
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+------------+---------------------+--------------|
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| 255 | reserved | |
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+------------+---------------------+--------------+
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At this time of this writing, only control and error messages have a fixed
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size of two bytes (header only). The stick-table updates messages are all
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variable-length (their message type bytes are greater than 128).
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*** Control message class ***
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At this time of writing, control messages are fixed-length messages used
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only to control the synchronizations between local and/or remote processes
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and to emit heartbeat messages.
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There exists five types of such control messages:
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+------------+--------------------------------------------------------+
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| type byte | signification |
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+------------+--------------------------------------------------------+
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| 0 | synchronisation request: ask a remote peer for a full |
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| | synchronization |
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+------------+--------------------------------------------------------+
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| 1 | synchronization finished: signal a remote peer that |
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| | local updates have been pushed and local is considered |
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| | up to date. |
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+------------+--------------------------------------------------------+
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| 2 | synchronization partial: signal a remote peer that |
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| | local updates have been pushed and local is not |
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| | considered up to date. |
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+------------+--------------------------------------------------------+
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| 3 | synchronization confirmed: acknowledge a finished or |
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| | partial synchronization message. |
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+------------+--------------------------------------------------------+
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| 4 | Heartbeat message. |
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+------------+--------------------------------------------------------+
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About hearbeat messages: a peer sends heartbeat messages to peers it is
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connected to after periods of 3s of inactivity (i.e. when there is no
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stick-table to synchronize for 3s). After a successful peer protocol
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handshake between two peers, if one of them does not send any other peer
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protocol messages (i.e. no heartbeat and no stick-table update messages)
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during a 5s period, it is considered as no more alive by its remote peer
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which closes the session and then tries to reconnect to the peer which
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has just disappeared.
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*** Error message class ***
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There exits two types of such error messages:
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+-----------+------------------+
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| type byte | signification |
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+-----------+------------------+
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| 0 | protocol error |
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+-----------+------------------+
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| 1 | size limit error |
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+-----------+------------------+
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*** Stick-table update message class ***
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This class is the more important one because it is in relation with the
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stick-table entries handling between peers which is at the core of peers
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protocol.
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All the messages of this class are variable-length. Their type bytes are
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all greater than or equal to 128.
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There exits five types of such stick-table update messages:
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+-----------+--------------------------------+
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| type byte | signification |
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+-----------+--------------------------------+
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| 128 | Entry update |
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+-----------+--------------------------------+
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| 129 | Incremental entry update |
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+-----------+--------------------------------+
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| 130 | Stick-table definition |
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+-----------+--------------------------------+
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| 131 | Stick-table switch (unused) |
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+-----------+--------------------------------+
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| 133 | Update message acknowledgement |
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+-----------+--------------------------------+
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Note that entry update messages may be multiplexed. This means that different
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entry update messages for different stick-tables may be sent over the same
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peer session.
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To do so, each time entry update messages have to sent, they must be preceded
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by a stick-table definition message. This remains true for incremental entry
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update messages.
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As its name indicate, "Update message acknowledgement" messages are used to
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acknowledge the entry update messages.
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In this following paragraph, we give some information about the format of
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each stick-table update messages. This very simple following legend will
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contribute in understanding it. The unit used is the octet.
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XX
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+-----------+
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| foo | Unique fixed sized "foo" field, made of XX octets.
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+-----------+
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+===========+
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| foo | Variable-length "foo" field.
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+===========+
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+xxxxxxxxxxx+
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| foo | Encoded variable-length "foo" field.
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+xxxxxxxxxxx+
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+###########+
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| foo | hereunder described "foo" field.
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+###########+
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With this legend, all the stick-table update messages have such a header:
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1 1
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+--------------------+------------------------+xxxxxxxxxxxxxxxx+
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| Message Class (10) | Message type (128-133) | Message length |
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+--------------------+------------------------+xxxxxxxxxxxxxxxx+
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Note that to help in making communicate different versions of peers protocol,
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such stick-table update messages may be extended adding non mandatory
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fields at the end of such messages, announcing a total message length
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which is greater than the message length of the previous versions of
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peers protocol. After having parsed such messages, the remaining ones
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will be skipped to parse the next message.
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- Definition message format:
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Before sending entry update messages, a peer must announce the configuration
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of the stick-table in relation with these messages thanks to a
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"Stick-table definition" message with such a following format:
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+xxxxxxxxxxxxxxxx+xxxxxxxxxxxxxxxxxxxxxxxxx+==================+
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| Stick-table ID | Stick-table name length | Stick-table name |
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+xxxxxxxxxxxxxxxx+xxxxxxxxxxxxxxxxxxxxxxxxx+==================+
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+xxxxxxxxxxxx+xxxxxxxxxxxxxx+xxxxxxxxxxxxxxxxxxxxxxx+xxxxxxxxx+
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| Key type | Key length | Data types bitfield | Expiry |
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+xxxxxxxxxxxx+xxxxxxxxxxxxxx+xxxxxxxxxxxxxxxxxxxxxxx+xxxxxxxxx+
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+xxxxxxxxxxxxxxxxxxxxxxxxxxx+xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx+
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| Frequency counter #1 | Frequency counter #1 period |
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+xxxxxxxxxxxxxxxxxxxxxxxxxxx+xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx+
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+xxxxxxxxxxxxxxxxxxxxxxxxxxx+xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx+
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| Frequency counter #2 | Frequency counter #2 period |
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+xxxxxxxxxxxxxxxxxxxxxxxxxxx+xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx+
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.
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.
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.
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Note that "Stick-table ID" field is an encoded integer which is used to
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identify the stick-table without using its name (or "Stick-table name"
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field). It is local to the process handling the stick-table. So we can have
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two peers attached to processes which generate stick-table updates for
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the same stick-table (same name) but with different stick-table IDs.
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Also note that the list of "Frequency counter #X" and their associated
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periods fields exists only if their underlying types are already defined
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in "Data types bitfield" field.
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"Expiry" field and the remaining ones are not used by all the existing
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version of haproxy peers. But they are MANDATORY, so that to make a
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stick-table aggregator peer be able to autoconfigure itself.
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- Entry update message format:
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4
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+-----------------+###########+############+
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| Local update ID | Key | Data |
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+-----------------+###########+############+
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with "Key" described as follows:
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+xxxxxxxxxxx+=======+
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| length | value | if key type is (non null terminated) "string",
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+xxxxxxxxxxx+=======+
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4
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+-------+
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| value | if key type is "integer",
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+-------+
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+=======+
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| value | for other key types: the size is announced in
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+=======+ the previous stick-table definition message.
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"Data" field is basically a list of encoded values for each type announced
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by the "Data types bitfield" field of the previous "Stick-table definition"
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message:
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+xxxxxxxxxxxxxxxxxxxx+xxxxxxxxxxxxxxxxxxxx+ +xxxxxxxxxxxxxxxxxxxx+
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| Data type #1 value | Data type #2 value | .... | Data type #n value |
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+xxxxxxxxxxxxxxxxxxxx+xxxxxxxxxxxxxxxxxxxx+ +xxxxxxxxxxxxxxxxxxxx+
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Most of these fields are internally stored as uint32_t (see STD_T_SINT,
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STD_T_UINT, STD_T_ULL C enumerations) or structures made of several uint32_t
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(see STD_T_FRQP C enumeration). The remaining one STD_T_DICT is internally
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used to store entries of LRU caches for others literal dictionary entries
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(couples of IDs associated to strings). It is used to transmit these cache
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entries as follows:
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+xxxxxxxxxxx+xxxx+xxxxxxxxxxxxxxx+========+
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| length | ID | string length | string |
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+xxxxxxxxxxx+xxxx+xxxxxxxxxxxxxxx+========+
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"length" is the length in bytes of the remaining data after this "length" field.
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"string length" is the length of "string" field which follows.
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Here the cache is used so that not to have to send again and again an already
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sent string. Indeed, the second time we have to send the same dictionary entry,
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if still cached, a peer sends only its ID:
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+xxxxxxxxxxx+xxxx+
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| length | ID |
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+xxxxxxxxxxx+xxxx+
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- Update message acknowledgement format:
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These messages are responses to "Entry update" messages only.
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Its format is very basic for efficiency reasons:
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4
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+xxxxxxxxxxxxxxxx+-----------+
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| Stick-table ID | Update ID |
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+xxxxxxxxxxxxxxxx+-----------+
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Note that the "Stick-table ID" field value is in relation with the one which
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has been previously announce by a "Stick-table definition" message.
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The following schema may help in understanding how to handle a stream of
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stick-table update messages. The handshake step is not represented.
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Stick-table IDs are preceded by a '#' character.
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Peer A Peer B
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stkt def. #1
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---------------------->
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updates (1-5)
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---------------------->
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stkt def. #3
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---------------------->
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updates (1000-1005)
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---------------------->
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stkt def. #2
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<----------------------
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updates (10-15)
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<----------------------
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ack 5 for #1
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<----------------------
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ack 1005 for #3
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<----------------------
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stkt def. #4
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<----------------------
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updates (100-105)
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<----------------------
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ack 10 for #2
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---------------------->
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ack 105 for #4
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---------------------->
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(from here, on both sides, all stick-table updates
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are considered as received)
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