# Relay message protocol > Understand how the Relay message protocol transmits data between clients and servers in the Relay service. If you’re using the Relay SDK [with NGO](../tutorials/relay-and-ngo) for your project, the Relay message protocol is already supported. However, if you want to use an alternative engine or networking solution, you’ll need to implement the Relay message protocol before using it. Use the Relay message protocol specifications to implement the Relay message protocol. The Relay message protocol expects you to express field values in big-endian order, also known as the “network order,” where the most significant byte occurs first. Messages aren't authenticated except for the [`BIND` message](#bind), which uses an HMAC signature. [Players](../players) can only connect and relay messages with other players from the same Unity project and environment. Relay rejects any attempts to communicate across Unity environments. All messages have a standard header and a message-specific body. Check out [Standard header](#standard) and [Message bodies](#message). ## Message types##message2 | Code | Name | Description | | ---- | -------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------- | | `0` | [`BIND message`](#bind) | A `BIND` message indicates a bind request sent from a client to a Relay server. | | `1` | [`BIND_RECEIVED message`](#bind-rec) | A `BIND_RECEIVED` message indicates a successful response from the Relay server to a client after receiving a `BIND request`. | | `2` | [`PING message`](#ping) | A `PING` message indicates a ping message sent between a client and a Relay server (both directions) to keep the connection alive. | | `3` | [`CONNECT_REQUEST message`](#connect-) | A `CONNECT_REQUEST` message indicates a request from a player (the requesting client) to connect to another player (the target client). | | `4` | Reserved. | Not available | | `5` | Reserved. | Not available | | `6` | [`ACCEPTED message`](#accepted) | An `ACCEPTED` message indicates a confirmation message sent from a Relay server to a requesting client after a successful connection to a target client. | | `7` | Reserved. | Not available | | `8` | Reserved. | Not available | | `9` | [`DISCONNECT message`](#disconne) | A `DISCONNECT` message indicates a request to disconnect two connected players. | | `10` | [`RELAY message`](#relay) | A `RELAY` message indicates a request to send a message between two players (clients). | | `11` | [`CLOSE message`](#close) | A `CLOSE` message indicates a request from a client to unbind from a Relay server. | | `12` | [`ERROR message`](#error) | An `ERROR` message indicates an error that has occurred. | ## Accept mode types##accept The accept mode defines how a [Relay server](../networking/relay-servers) handles requests from clients trying to connect. Relay only supports the `AUTO` accept mode. A connection mode of `AUTO` means the Relay server automatically accepts the connection if its capacity allows it (the number of connections must not exceed the maximum number of allowed connections). ## Standard header##standard All message types share a standard header that has a signature, the Relay message protocol version, and the message contained in the body of the packet. | Bytes | Purpose | | ------ | ----------- | | 1 .. 2 | `Signature` | | 3 | `Version` | | 4 | `Type` | The following table describes each field found in the standard header. ## Message bodies##message ### BIND message##bind `BIND` messages are sent from a client to a Relay server to create a mapping from a client’s IP address and port for a specific allocation. This message is authenticated to verify the identity of the client. The response received from the [Allocations service](./allocation-service.md) has the information necessary for the client to authenticate its `BIND` messages. The response from the Allocations service to the `BIND` message shows the binding status. Clients can send identical `BIND` messages as long as the client’s IP address and port haven't changed. For example, if a client sends a `BIND` message on a following connection after changing its IP address or port, the `Nonce` value must be greater than the previously supplied value. | Bytes | Purpose | | --------- | ---------------------- | | 1 .. 4 | Header (Type 0) | | 5 | `AcceptMode` | | 6 .. 7 | `Nonce` | | 8 | `ConnectionDataLength` | | 9 .. N | `ConnectionData` | | N+1 .. 33 | `HMAC` | The following table describes each field found in a `BIND` message. | Field | Type | Description | | ---------------------- | --------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | `AcceptMode` | uint8 | `AcceptMode` defines how a Relay server should handle requests from clients trying to connect to the owner of this allocation.Currently, only the `AUTO` accept mode is supported. Refer to [Accept mode types](#accept). | | `Nonce` | uint16 | `Nonce` is a single-use cryptographic nonce. Each following `BIND` message must have an incremented nonce value. It's conventional to start the nonce value at 0. | | `ConnectionDataLength` | uint8 | `ConnectionDataLength` describes the byte length of `ConnectionData`.The maximum allowed value is `255`. | | `ConnectionData` | \[]byte | `ConnectionData` is the encrypted connection data of the \connecting client\.This field’s length is variable. The maximum length is `255` bytes. | | `HMAC` | \[32]byte | `HMAC` is a hash-based message authentication code generated using a known key and the preceding bytes in the message, including the header. | #### Security You must sign the HMAC with the secret key returned from the [Allocations service](../advanced-config/allocation-service). If the HMAC is invalid, the Relay server silently rejects the `BIND message`. The Relay server also validates the nonce value to mitigate message replay attacks by bad actors. If the Relay server determines that a nonce is invalid, it silently rejects it. If it’s the first time the client is binding to a Relay server with a `BIND message`, the nonce value can be `0`. ### BIND\_RECEIVED message##bind-rec Relay servers send `BIND_RECEIVED` messages to requesting clients to confirm they have successfully bound to the Relay server through a [`BIND message`](#bind-rec). After receiving the confirmation, the requesting client can initiate communication with a target client using a [`CONNECT_REQUEST message`](#connect-). > **Note:** > > The Relay server sends a `BIND_RECEIVED` message to the client for each successfully processed `BIND request`. | Bytes | Purpose | | ------ | --------------- | | 1 .. 4 | Header (Type 1) | ### PING message##ping `PING` messages are simple messages that keep the binding between a client and a [Relay server](../networking/relay-servers) alive by resetting the client timeout. Relay servers automatically disconnect clients after 10 seconds of inactivity. Any message received by the Relay server involving this client, either as a sender or receiver, resets this timeout. For games with a lower message frequency, the `PING` message allows clients to reset the disconnection timer. > **Tip:** > > Clients must send `PING` messages every second or two in addition to the other messages to ensure the connection doesn’t timeout. If you’re using [Relay with NGO](../tutorials/relay-and-ngo), the network manager automatically keeps the connection alive. Each `PING` message has the client’s allocation ID and an arbitrary number that identifies the message. When a Relay server receives a `PING` message from a client, it sends the packet back to the client without altering it. This allows you to use `PING` messages to check connectivity and measure round trip time. The Relay server doesn't send back an error message if the allocation ID has expired. > **Warning:** > > Clients must bind with the Relay server through a [`BIND` message](#bind) before sending a `PING` message. The following scenarios will result in the Relay server returning a `ErrClientPlayerMismatch`[`ERROR` message](#error): > > * The client sends a `PING` message before binding to the Relay server through a `BIND` message. > * The client sends a `PING` message after its IP address has changed and before rebinding to the Relay server with the new IP address. | Bytes | Purpose | | -------- | --------------- | | 1 .. 4 | Header (Type 2) | | 5 .. 20 | `AllocationID` | | 21 .. 22 | `Number` | The following table describes each field found in a `PING` message. | Field | Type | Description | | -------------- | --------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | `AllocationID` | \[16]byte | `AllocationID` is a 16-byte UUID identifying the allocated client sending the ping. | | `Number` | uint16 | `Number` is an arbitrary unsigned integer the Relay server echoes back to the client. This can be used by the client to correlate `PING` messages that are sent and received. | ### CONNECT\_REQUEST message##connect- `CONNECT_REQUEST` messages are sent from a requesting client to a Relay server to establish a connection to a target client. The target player the requesting client wants to connect to (or the target client) is represented by `ToConnectionData`, which the Relay server decrypts to determine which player to connect with. > **Warning:** > > The target client and the requesting client must be on the same Unity project and environment. > **Note:** > > Relay servers have no formal idea of player groups (often referred to as [sessions](../sessions). All players within a session are bound to the same Relay server. By extension of this, Relay doesn’t make any assumptions about the length of a player’s bindings and connections in relation to the session. It’s up to the game client to determine whether players unbind from the Relay server at the end of a session or use the same binding for multiple sessions. | Bytes | Purpose | | ------- | ------------------------- | | 1 .. 4 | Header (Type 3) | | 5 .. 20 | `AllocationID` | | 21 | `ToConnectionData Length` | | 22 .. | `ToConnectionData` | | Bytes | Purpose | | ------- | ------------------------- | | 1 .. 4 | Header (Type 3) | | 5 .. 20 | `AllocationID` | | 21 | `ToConnectionData Length` | | 22 .. | `ToConnectionData` | The following table describes each field found in a `CONNECT_REQUEST` message. | Field | Type | Description | | ------------------------ | --------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | `AllocationID` | \[16]byte | `AllocationID` is a 16-byte UUID that identifies the allocated client that's requesting the connection. | | `ToConnectionDataLength` | uint8 | `ToConnectionDataLength` is the byte length of `ToConnectionData`.The maximum allowed value is `255`. | | `ToConnectionData` | \[]byte | `ToConnectionData` is an encrypted data blob that describes the client to connect to. Refer to connection data.This field has a variable length. The maximum byte length is 255. | ### ACCEPTED message##accepted A Relay server sends an `ACCEPTED` message to a requesting client after a successful connection to a target client. > **Note:** > > The Relay server returns the `ACCEPTED` message to the requesting client if the target client has fewer than their specified maximum connections. | Bytes | Purpose | | -------- | ------------------ | | 1 .. 4 | Header (Type 6) | | 5 .. 20 | `FromAllocationID` | | 21 .. 36 | `ToAllocationID` | | Bytes | Purpose | | -------- | ------------------ | | 1 .. 4 | Header (Type 6) | | 5 .. 20 | `FromAllocationID` | | 21 .. 36 | `ToAllocationID` | The following table describes each field found in an `ACCEPTED` message. | Field | Type | Description | | ------------------ | --------- | ------------------------------------------------------------------------------------------------------------------------------ | | `FromAllocationID` | \[16]byte | `FromAllocationID` is a 16-byte UUID that identifies the target client. | | `ToAllocationID` | \[16]byte | `ToAllocationID` is a 16-byte UUID that identifies the allocated client that requested the connection (the requesting client). | ### DISCONNECT message##disconne The `DISCONNECT` message is the inverse of the [`CONNECT_REQUEST`](#connect-) handshake and allows a client to disconnect from another client. A client can disconnect from any other client it has connected with by sending a `DISCONNECT` message to the Relay server. When the Relay server receives the message, it removes the specified allocation ID from the requesting client’s list of connected players. The Relay server also forwards the `DISCONNECT` request to the host client so the client can update its map of connected players to remove the requesting client's Allocation ID. The Relay server then sends the `DISCONNECT` message back to the client as a confirmation. If either of the allocation IDs in the message body is invalid, the Relay server sends an [`ERROR message`](#error) instead. Once a client has disconnected, the Relay server rejects all [`RELAY` messages](#relay) sent to that client’s allocation ID. A disconnected client can re-establish a connection with a [`CONNECT_REQUEST message`](#connect-). > **Note:** > > The host and connecting clients can both send a `DISCONNECT` messages. However, if a host client sends a `DISCONNECT` message, the client must migrate the joined players to a new host or disconnect them all. Relay doesn't handle host migration. | Bytes | Purpose | | -------- | ------------------ | | 1 .. 4 | Header (Type 9) | | 5 .. 20 | `FromAllocationID` | | 21 .. 36 | `ToAllocationID` | The following table describes each field found in a `DISCONNECT` message. | Field | Type | Description | | ------------------ | --------- | -------------------------------------------------------------------------------------------------------------------------------------------- | | `FromAllocationID` | \[16]byte | `FromAllocationID` is a 16-byte UUID that identifies the allocated client that's requesting to close the connection (the requesting client). | | `ToAllocationID` | \[16]byte | `ToAllocationID` is a 16-byte UUID that identifies the allocated client whose connection is being closed (the target client). | ### RELAY message##relay `RELAY` messages allow clients to send messages containing any arbitrary payload of bytes between each other without awareness of each other’s IP addresses and ports. Before sending a `RELAY` message between clients, the Relay server ensures the client sending the packet has previously been authenticated as the `FromAllocationID` through a `BIND message`. If the client hasn't previously bound, the Relay server returns an `ErrClientPlayerMismatch` [`ERROR` message](#error). The Relay server also ensures the two clients have an established connection through a earlier [`CONNECT_REQUEST`](#connect-) exchange. The Relay server returns an `ErrNotConnected`[`ERROR message`](#error) if the clients aren't connected. If all validations pass, the Relay server sends the entire message to the `ToAllocationID` as-is. The Relay server doesn't send any confirmation message to the `FromAllocationID`. | Bytes | Purpose | | -------- | ------------------ | | 1 .. 4 | Header (Type 10) | | 5 .. 20 | `FromAllocationID` | | 21 .. 36 | `ToAllocationID` | | 37 .. 38 | `Length` | | 39 .. | `Content` | > **Note:** > > The maximum length of a `RELAY` message isn't guaranteed at the protocol level. It's defined by the configuration of the Relay server itself, which has a default maximum content length of \*\*1400 \*\*bytes and results in a `RELAY`message maximum size of \*\*1438 \*\*bytes. The following table describes each field found in a `RELAY` message. | Field | Type | Description | | ------------------ | --------- | ---------------------------------------------------------------------------------------------------------------------- | | `FromAllocationID` | \[16]byte | `FromAllocationID` is a 16-byte UUID that identifies the allocated client sending the message (the requesting client). | | `ToAllocationID` | \[16]byte | `ToAllocateID` is a 16-byte UUID that identifies the allocated client receiving the message (the target client). | | `Length` | uint16 | `Length` specifies the length of the message content in bytes. The maximum value is `1400`. | | `Content` | \[]byte | `Content` has the content of the message. | ### CLOSE message##close `CLOSE` messages are idempotent messages that allow a client to unbind and deallocate from a Relay server. Clients should send a `CLOSE` message to the [Relay server](../networking/relay-servers) when leaving their session or when closing the game client. Clients can only close their own bindings to the Relay server; they can't close another client’s binding. Clients should send `CLOSE` messages multiple times to increase the chance of successful delivery. Because `CLOSE` messages are idempotent, there’s no risk of the Relay server sending an [`ERROR` message](#error) if the client has already been disconnected. The `CLOSE` message is a best effort to gracefully terminate an allocation, and there is no guarantee it will succeed. Because Relay servers can deallocate clients with a timeout, Relay servers don't rely on a `CLOSE` message to remove a client allocation. The client sending the `CLOSE` message must be bound to the Relay server through a earlier [`BIND` request](#bind-rec). If the client isn't bound to the Relay server (or the client’s IP address has changed), the Relay server will silently reject the message. Use `PING` messages to prevent unintended allocation timeouts. > **Note:** > > Unbound clients still expire through timeout. | Bytes | Purpose | | ------- | ---------------- | | 1 .. 4 | Header (Type 11) | | 5 .. 20 | `AllocationID` | The following table describes each field found in a `CLOSE` message. | Field | Type | Description | | -------------- | --------- | ------------------------------------------------------------------------- | | `AllocationID` | \[16]byte | `AllocationID` is a 16-byte UUID that identifies the client's allocation. | ### ERROR message##error Relay servers use `ERROR` messages strictly to inform the client that an error has occurred. | Bytes | Purpose | | ------- | ---------------- | | 1 .. 4 | Header (Type 12) | | 5 .. 20 | `AllocationID` | | 21 | `ErrorCode` | The following table describes each field found in an `ERROR` message. | Field | Type | Description | | -------------- | --------- | --------------------------------------------------------------------------- | | `AllocationID` | \[16]byte | `AllocationID` is a 16-byte UUID that identifies the client's allocation. | | `ErrorCode` | uint8 | `ErrorCode` represents an error code. Refer to [Error codes](#error) below. | #### Error codes The following table describes each of the possible error messages a Relay server can send to a client. | Code | Name | Description | | ---- | --------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | `0` | Invalid protocol version (`ErrInvalidProtocolVersion`) | `ErrInvalidProtocolVersion` is returned when a parsed message has an unexpected protocol version. | | `1` | Player timed out due to inactivity (`ErrTimeout`) | `ErrTimeout` is returned when a client’s binding to a Relay server has timed out. Clients should use [`PING messages`](#ping) to prevent this error. | | `2` | Unauthorized (`ErrUnauthorized`) | `ErrUnauthorized` is returned when a client attempts to perform an operation that it's not authorized to perform. | | `3` | Allocation ID client mismatch (`ErrClientPlayerMismatch`) | `ErrClientPlayerMismatch` is returned when a client has sent a `RELAY` message to the server using an unknown network connection.It's intended to inform the client that the IP address they're using has changed from the one the Relay server is aware of.This error message indicates the client should re-bind with a new `BIND message`. | | `4` | Allocation ID not found (`ErrAllocationNotFound`) | `ErrAllocationNotFound` is returned when the Relay server can't find an allocation for the given allocation ID. | | `5` | Not connected (`ErrNotConnected`) | `ErrNotConnected` is returned when a client attempts to communicate with a client to whom it's not connected. | | `6` | Self-connect not allowed (`ErrSelfConnectNotAllowed`) | `ErrSelfConnectNotAllowed` is returned when a client sends a `CONNECT_REQUEST message` to a client with the same allocation ID. |