Binary protocol

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

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.

Part types

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

ID	Name	Data type
0x0000	Host	String
0x0001	Time	Numeric
0x0002	Plugin	String
0x0003	Plugin instance	String
0x0004	Type	String
0x0005	Type instance	String
0x0006	Values	other (todo)
0x0007	Interval	Numeric
0x0100	Message (notifications)	String
0x0101	Severity	Numeric

Implementations

• Small Python script by Adrian Perez
• PacketWriter.java of jcollectd
• Of course the network plugin of collectd itself
• Ruby implementation by Astro.

See also

• Network plugin