Difference between revisions of "Binary protocol"

Revision as of 12:11, 24 September 2010

The binary protocol is the protocol implemented by the Network plugin and some external implementations to exchange data collected by collectd or to send data to an instance of collectd.

Until the Network plugin has been factorized out into a library, it is useful to have some documentation to reimplement it.

Well-known numbers

Default UDP port

25826

Default IPv4 Multicast group

239.192.74.66

Default IPv6 multicast group

ff18::efc0:4a42

Maximum packet size

1024 bytes (payload only, not including UDP / IP headers)
In versions 4.0 through 4.7, the receive buffer has a fixed size of 1024 bytes. When longer packets are received, the trailing data is simply ignored. Since version 4.8, the buffer size can be configured. Version 5.0 will increase the default buffer size to 1452 bytes (the maximum payload size when using UDP/IPv6 over Ethernet).

Protocol structure

Beginning of each “part”: Type and length.

Each packet consists of one or more so called “parts”. Each part starts with the same four bytes: Two bytes that specify the “part type” (what kind of information is enclosed in the part) and two bytes which specify the length of the part, including the four header bytes itself. The maximum length of payload in any part is therefore 65531 bytes.

Using this layout, clients can determine the length of a part they don't know and lets them skip unknown data. This makes the protocol forward compatible so that new features can be added easily.

There are two part layouts that are used for a couple of “types”: numeric (an 8 byte integer) and string.

Numeric parts

Structure of the “number” parts

Numeric integer values, e. g. the interval and time values, are transferred using 8 byte integers. The length field of those parts must therefore always be set to 12.

String parts

Structure of “string” parts. The example shows the encoding of the string “foobar”.

Strings are transferred including a null byte at the end. In the example you can see the encoding of the string “foobar”. The string is six characters long, followed by a null-byte and appended to a four byte header, leading to a length of 11 bytes for this part.

Value parts

Value parts encode an actual data sample. Preceding time, plugin, plugin instance, type, and type instance parts must have set the context for the data sample. The value part consists of:

• Type 0x0006
• Length (16 bit field)
• Number of values (16 bit field)
• Values, for each one:
  • Data type code (8 bit field)

    COUNTER → 0

    GAUGE → 1

    DERIVE → 2

    ABSOLUTE → 3

  • Value (64 bit field)

    COUNTER → network (big endian) unsigned integer

    GAUGE → x86 (little endian) double

    DERIVE → network (big endian) signed integer

    ABSOLUTE → network (big endian) unsigned integer

Many data samples have a single value, such as "cpu" (see types.db(5)), but others have multiple values, such as "disk_merged", which has read and write values.

Part types

The following numeric types are currently used to identify the type of a “part”. Defines are available from src/network.h.

Implementations

• Small Python script by Adrian Perez
• PacketWriter.java of jcollectd
• Of course the Network plugin (src/network.c) of collectd itself
• Ruby implementation by Astro.
• erlang-collectd, an Erlang implementation by Astro

See also

• Network plugin
• The network protocol analyzer Wireshark supports dissecting this binary protocol since version 1.4. The code can be found in epan/dissectors/packet-collectd.c.