allwpilib/ntcore/doc/networktables2.adoc

= NetworkTables Protocol Specification, Version 2.0
WPILib Developers <wpilib@wpi.edu>
Protocol Revision 2.0 (0x0200), 1/8/2013
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:toc-placement: preamble
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This document defines a network protocol for a key-value store that may be read
from and written to by multiple remote clients. A central server, most often
running on a FIRST FRC robot controller, is responsible for providing
information consistency and for facilitating communication between clients.
This document describes protocol revision 2.0 (0x0200).

Information consistency is guaranteed through the use of a sequence number
associated with each key-value pair. An update of a key-value pair increments
the associated sequence number, and this update information is shared with all
participating clients. The central server only applies and redistributes
updates which have a larger sequence number than its own, which guarantees that
a client must have received a server's most recent state before it can replace
it with a new value.

This is a backwards-incompatible rework of the NetworkTables network protocol
originally introduced for the 2012 FIRST Robotics Competition. Note that this
revision of the NetworkTables protocol no longer includes the concept of
sub-tables. We suggest that instead of representing sub-tables as first-class
data types in the network protocol, it would be easy for an implementation to
provide a similar API abstraction by adding prefixes to keys. For example, we
suggest using Unix-style path strings to define sub-table hierarchies. The
prefix ensures that sub-table namespaces do not collide in a global hashtable
without requiring an explicit sub-table representation.

In addition, the explicit concept of grouping multiple updates such that they
are all visible at the same time to user code on a remote device was discarded.
Instead, array types for all common elements are provided. By using an array
data type, users may achieve the same level of atomicity for common operations
(e.g. sending a cartesian coordinate pair) without requiring the complexity of
arbitrarily grouped updates.

This document conforms to <<rfc2119>> - Key words for use in RFCs to Indicate
Requirement Levels.

[[references]]
== References

[[rfc1982,RFC 1982]]
* RFC 1982, Serial Number Arithmetic, http://tools.ietf.org/html/rfc1982

[[rfc2119,RFC 2119]]
* RFC 2119, Key words for use in RFCs to Indicate Requirement Levels,
http://tools.ietf.org/html/rfc2119

[[definitions]]
== Definitions

[[def-client]]
Client:: An implementation of this protocol running in client configuration.
Any number of Clients may exist for a given Network.

[[def-entry]]
Entry:: A data value identified by a string name.

[[def-entry-id]]
Entry ID:: An unsigned 2-byte ID by which the Server and Clients refer to an
Entry across the network instead of using the full string key for the Entry.
Entry IDs range from 0x0000 to 0xFFFE (0xFFFF is reserved for an Entry
Assignment issued by a Client).

[[def-server]]
Server:: An implementation of this protocol running in server configuration.
One and only one Server must exist for a given Network.

[[def-network]]
Network:: One or more Client nodes connected to a Server.

[[def-user-code]]
User Code:: User-supplied code which may interact with a Client or Server. User
Code should be executed on the same computer as the Client or Server instance
it interacts with.

[[def-sequence-number]]
Sequence Number:: An unsigned number which allows the Server to resolve update
conflicts between Clients and/or the Server. Sequence numbers may overflow.
Sequential arithmetic comparisons, which must be used with Sequence Numbers,
are defined by RFC 1982.

[[def-protocol-revision]]
Protocol Revision:: A 16-bit unsigned integer which indicates the version of
the network tables protocol that a client wishes to use. The protocol revision
assigned to this version of the network tables specification is listed at the
top of this document. This number is listed in dot-decimal notation as well as
its equivalent hexadecimal value.

== Transport Layer

Conventional implementations of this protocol should use TCP for reliable
communication; the Server should listen on TCP port 1735 for incoming
connections.

== Example Exchanges

[[exchange-connect]]
=== Client Connects to the Server

Directly after client establishes a connection with the Server, the following
procedure must be followed:

. The Client sends a <<msg-client-hello>> message to the Server

. The Server sends one <<msg-assign>> for every field it currently recognizes.

. The Server sends a <<msg-server-hello-complete>> message.

. For all Entries the Client recognizes that the Server did not identify with a
Entry Assignment, the client follows the <<exchange-client-creates-entry>>
protocol.

In the event that the Server does not support the protocol revision that the
Client has requested in a Client Hello message, the Server must instead issue a
<<msg-protocol-unsupported>> message to the joining client and close the
connection.

[[exchange-client-creates-entry]]
=== Client Creates an Entry

When User Code on a Client assigns a value to an Entry that the Server has not
yet issued a Entry Assignment for, the following procedure must be followed:

. The Client sends an <<msg-assign>> with an Entry ID of 0xFFFF.

. The Server issues an <<msg-assign>> to all Clients (including the sender) for
the new field containing a real Entry ID and Sequence Number for the new field.

In the event that User Code on the Client updates the value of the
to-be-announced field again before the expected Entry Assignment is received,
then the Client must save the new value and take no other action (the most
recent value of the field should be issued when the Entry Assignment arrives,
if it differs from the value contained in the received Entry Assignment).

In the event that the Client receives a Entry Assignment from the Server for
the Entry that it intended to issue an Entry Assignment for, before it issued
its own Entry Assignment, the procedure may end early.

In the event that the Server receives a duplicate Entry Assignment from a
Client (likely due to the client having not yet received the Server's Entry
Assignment), the Server should ignore the duplicate Entry Assignment.

[[exchange-client-updates-entry]]
=== Client Updates an Entry

When User Code on a Client updates the value of an Entry, the Client must send
an <<msg-update>> message to the Server. The Sequence Number included in the
Entry Update message must be the most recently received Sequence Number for the
Entry to be updated incremented by one.

.Example:

. Client receives Entry Assignment message for Entry "a" with integer value 1,
Entry ID of 0, and Sequence Number 1.

. User Code on Client updates value of Entry "a" to 16 (arbitrary).

. Client sends Entry Update message to Server for Entry 0 with a Sequence
Number of 2 and a value of 16.

When the Server receives an Entry Update message, it first checks the Sequence
Number in the message against the Server's value for the Sequence Number
associated with the Entry to be updated. If the received Sequence Number is
strictly greater than (aside: see definition of "greater than" under the
definition of Sequence Number) the Server's Sequence Number for the Entry to be
updated, the Server must apply the new value for the indicated Entry and repeat
the Entry Update message to all other connected Clients.

If the received Sequence Number is less than or equal (see definition of "less
than or equal" in RFC 1982) to the Server's Sequence Number for the Entry to be
updated, this implies that the Client which issued the Entry Update message has
not yet received one or more Entry Update message(s) that the Server recently
sent to it; therefore, the Server must ignore the received Entry Update
message. In the event that comparison between two Sequence Numbers is undefined
(see RFC 1982), then the Server must always win (it ignores the Entry Update
message under consideration).

[[update-rate]]
NOTE: If User Code modifies the value of an Entry too quickly, 1) users may not
see every value appear on remote machines, and 2) the consistency protection
offered by the Entry's Sequence Number may be lost (by overflowing before
remote devices hear recent values). It is recommended that implementations
detect when user code updates an Entry more frequently than once every 5
milliseconds and print a warning message to the user (and/or offer some other
means of informing User Code of this condition).

[[exchange-server-creates-entry]]
=== Server Creates an Entry

When User Code on the Server assigns a value to a Entry which does not exist,
the Server must issue an <<msg-assign>> message to all connected clients.

[[exchange-server-updates-entry]]
=== Server Updates an Entry

When User Code on the Server updates the value of an Entry, the Server must
apply the new value to the Entry immediately, increment the associated Entry's
Sequence Number, and issue a <<msg-update>> message containing the new value
and Sequence Number of the associated Entry to all connected Clients.

NOTE: See <<update-rate,Note>> under <<exchange-client-updates-entry>>.

[[exchange-keep-alive]]
=== Keep Alive

To maintain a connection and prove a socket is still open, a Client or Server
may issue <<msg-keep-alive>> messages. Clients and the Server should ignore
incoming Keep Alive messages.

The intent is that by writing a Keep Alive to a socket, a Client forces its
network layer (TCP) to reevaluate the state of the network connection as it
attempts to deliver the Keep Alive message. In the event that a connection is
no longer usable, a Client's network layer should inform the Client that it is
no longer usable within a few attempts to send a Keep Alive message.

To provide timely connection status information, Clients should send a Keep
Alive message to the Server after every 1 second period of connection
inactivity (i.e. no information is being sent to the Server). Clients should
not send Keep Alive messages more frequently than once every 100 milliseconds.

Since the Server does not require as timely information about the status of a
connection, it is not required to send Keep Alive messages during a period of
inactivity.

[[bandwidth]]
== Bandwidth and Latency Considerations

To reduce unnecessary bandwidth usage, implementations of this protocol should:

* Send an Entry Update if and only if the value of an Entry is changed to a
value that is different from its prior value.

* Buffer messages and transmit them in groups, when possible, to reduce
transport protocol overhead.

* Only send the most recent value of an Entry. When User Code updates the value
of an Entry more than once before the new value is transmitted, only the latest
value of the Entry should be sent.

It is important to note that these behaviors will increase the latency between
when a Client or Server updates the value of an Entry and when all Clients
reflect the new value. The exact behavior of this buffering is left to
implementations to determine, although the chosen scheme should reflect the
needs of User Code. Implementations may include a method by which User Code can
specify the maximum tolerable send latency.

[[entry-types]]
== Entry Types

Entry Type must assume one the following values:

[cols="1,3"]
|===
|Numeric Value |Type

|0x00
|Boolean

|0x01
|Double

|0x02
|String

|0x10
|Boolean Array

|0x11
|Double Array

|0x12
|String Array
|===

[[entry-values]]
== Entry Values

Entry Value must assume the following structure as indicated by Entry Type:

[cols="1,3"]
|===
|Entry Type |Entry Value Format

|[[entry-value-boolean]]Boolean
|1 byte, unsigned; True = 0x01, False = 0x00

|[[entry-value-double]]Double
|8 bytes, IEEE 754 floating-point "double format" bit layout; (big endian)

|[[entry-value-string]]String
|2 byte, unsigned length prefix (big endian) of the number of raw bytes to
follow, followed by the string encoded in UTF-8

|[[entry-value-boolean-array]]Boolean Array
|1 byte, unsigned, number of elements within the array to follow

N bytes - The raw bytes representing each Boolean element contained within the
array, beginning with the item at index 0 within the array.

|[[entry-value-double-array]]Double Array
|1 byte, unsigned, number of elements within the array to follow

N bytes - The raw bytes representing each Double element contained within the
array, beginning with the item at index 0 within the array.

|[[entry-value-string-array]]String Array
|1 byte, unsigned, number of elements within the array to follow

N bytes - The raw bytes representing each String element contained within the
array, beginning with the item at index 0 within the array.
|===

== Message Structures

All messages are of the following format:

[cols="1,3"]
|===
|Field Name |Field Type

|Message Type
|1 byte, unsigned

|Message Data
|N bytes (length determined by Message Type)
|===

[[msg-keep-alive]]
=== Keep Alive

Indicates that the remote party is checking the status of a network connection.

[cols="1,3"]
|===
|Field Name |Field Type

|0x00 - Keep Alive
|1 byte, unsigned; Message Type
|===

[[msg-client-hello]]
=== Client Hello

A Client issues a Client Hello message when first establishing a connection.
The Client Protocol Revision field specifies the Network Table protocol
revision that the Client would like to use.

[cols="1,3"]
|===
|Field Name |Field Type

|0x01 - Client Hello
|1 byte, unsigned; Message Type

|Client Protocol Revision
|2 bytes, Unsigned 16-bit integer (big-endian).

See <<def-protocol-revision,Protocol Revision>>
|===

[[msg-protocol-unsupported]]
=== Protocol Version Unsupported

A Server issues a Protocol Version Unsupported message to a Client to inform it
that the requested protocol revision is not supported. It also includes the
most recent protocol revision which it supports, such that a Client may
reconnect under a prior protocol revision if able.

[cols="1,3"]
|===
|Field Name |Field Type

|0x02 - Protocol Version Unsupported
|1 byte, unsigned; Message Type

|Server Supported Protocol Revision
|2 bytes, Unsigned 16-bit integer (big-endian).

See <<def-protocol-revision,Protocol Revision>>
|===

[[msg-server-hello-complete]]
=== Server Hello Complete

A Server issues a Server Hello Complete message when it has finished informing
a newly-connected client of all of the fields it currently recognizes.
Following the receipt of this message, a Client should inform the Server of
any/all additional fields that it recognizes that the Server did not announce.

[cols="1,3"]
|===
|Field Name |Field Type

|0x03 - Server Hello Complete
|1 byte, unsigned; Message Type
|===

[[msg-assign]]
=== Entry Assignment

A Entry Assignment message informs the remote party of a new Entry. An Entry
Assignment's value field must be the most recent value of the field being
assigned at the time that the Entry Assignment is sent.

[cols="1,3"]
|===
|Field Name |Field Type

|0x10 - Entry Assignment
|1 byte, unsigned; Message Type

|Entry Name
|<<entry-value-string,String>>

|Entry Type
|1 byte, unsigned; see <<entry-types,Entry Types>>

|Entry ID
|2 bytes, unsigned

|Entry Sequence Number
|2 bytes, unsigned

|Entry Value
|N bytes, length depends on Entry Type
|===

If the Entry ID is 0xFFFF, then this assignment represents a request from a
Client to the Server. In this event, the Entry ID field and the Entry Sequence
Number field must not be stored or relied upon as they otherwise would be.

[[msg-update]]
=== Entry Update

An Entry Update message informs a remote party of a new value for an Entry.

[cols="1,3"]
|===
|Field Name |Field Type

|0x11 - Entry Update
|1 byte, unsigned; Message Type

|Entry ID
|2 bytes, unsigned

|Entry Sequence Number
|2 bytes, unsigned

|Entry Value
|N bytes, length dependent on value type
|===