Protobuf Packets Used by the UE5 MMO Client

The UE5 client shares the same Packet.proto definition as the C++ GameServer. The generated C++ files live in MMOClient1/Source/MMOClient1/Proto/ — Packet.pb.h, Packet.pb.cpp, and the UE-specific Packet.pb.ue.h wrapper. All gameplay packets are serialized with Protocol Buffers v3 and framed with the 4-byte PacketHeader (2-byte size + 2-byte packet ID) described in the Packet Protocol reference.

The client proto file (MMOClient1/Source/MMOClient1/Proto/Packet.proto) and the server proto file (ServerSolution/ServerSolution/Proto/Packet.proto) must always be identical. If you change one, regenerate and update both.

Packets Sent by the Client

These are C_ (client-to-server) messages. Most travel over TCP; C_MoveReq and C_UdpHelloReq use the UDP channel.

C_EnterGameReq — `0x0201`

Sent over TCP immediately after the TCP connection is established. The enter_token is the short-lived token received from the AuthServer’s /auth/select-server response.

message C_EnterGameReq {
  string enter_token = 1;
  uint64 character_id = 2;
}

enter_token

string

required

Short-lived enter token issued by the AuthServer. Valid for EnterTokenSeconds (default 60 seconds).

character_id

uint64

required

Character ID returned by /auth/select-server in SelectServerResponse.CharacterId.

C_UdpHelloReq — `0x0204`

Sent over UDP to register the client’s UDP endpoint with the server after S_EnterGameRes is received.

message C_UdpHelloReq {
  string udp_token = 1;
  uint64 account_id = 2;
  uint64 character_id = 3;
}

udp_token

string

required

Token returned in S_EnterGameRes.udp_token for binding the UDP endpoint.

account_id

uint64

required

Account ID from S_EnterGameRes.account_id.

character_id

uint64

required

Character ID from S_EnterGameRes.character_id.

C_MoveReq — `0x0211`

Sent over UDP at the movement input rate. The server uses input_sequence for client-side prediction reconciliation.

message C_MoveReq {
  uint32 input_sequence = 1;
  float move_x = 2;
  float move_y = 3;
  float yaw = 4;
  uint64 client_time_ms = 5;
}

input_sequence

uint32

required

Monotonically increasing sequence number. The server echoes last_processed_input_sequence so the client can replay unacknowledged inputs.

move_x

float

required

Horizontal movement axis input, range [-1.0, 1.0].

move_y

float

required

Forward/backward movement axis input, range [-1.0, 1.0].

yaw

float

required

Character facing direction in degrees.

client_time_ms

uint64

required

Client-side timestamp in milliseconds for latency measurement.

C_ChatReq — `0x0221`

Sends a global chat message over TCP. Maximum length is 512 UTF-8 bytes (enforced client-side before sending).

message C_ChatReq {
  string message = 1;
}

C_AttackReq — `0x0231`

Sends a melee attack command over TCP. The server validates combo step ordering and applies server-authoritative hit detection.

message C_AttackReq {
  uint32 attack_sequence = 1;
  uint32 combo_step = 2;
  uint64 client_time_ms = 3;
}

attack_sequence

uint32

required

Monotonically increasing per-character sequence number for deduplication.

combo_step

uint32

required

Current step in the combo chain (1, 2, 3, …).

C_InventoryMoveReq — `0x0252`

Requests moving an item between two inventory slots. Uses a 16-byte idempotency key so retries are safe.

message C_InventoryMoveReq {
  bytes client_request_id = 1;
  uint64 expected_revision = 2;
  uint64 item_instance_id = 3;
  uint32 source_slot = 4;
  uint32 destination_slot = 5;
  uint32 quantity = 6;
}

client_request_id

bytes

required

16-byte UUID used as an idempotency key. The server returns this value in S_InventoryOperationResult so the client can match responses to requests.

expected_revision

uint64

required

The client’s current inventory revision. If the server’s revision differs, the operation is rejected and the client must re-read the snapshot.

C_InventoryUseReq — `0x0254`

Requests using (consuming) an item from a specific slot.

message C_InventoryUseReq {
  bytes client_request_id = 1;
  uint64 expected_revision = 2;
  uint64 item_instance_id = 3;
  uint32 slot_index = 4;
}

Packets Received by the Client

These are S_ (server-to-client) messages received over TCP, except movement batch notifications which arrive over UDP.

S_EnterGameRes — `0x0202`

Response to C_EnterGameReq. On success, contains the UDP token and port to use for C_UdpHelloReq.

message S_EnterGameRes {
  bool success = 1;
  string message = 2;
  uint64 account_id = 3;
  uint64 character_id = 4;
  string udp_token = 5;
  uint32 udp_port = 6;
}

S_UdpHelloRes — `0x0205`

Response to C_UdpHelloReq. On success, includes the current server time for clock synchronization.

message S_UdpHelloRes {
  bool success = 1;
  string message = 2;
  uint64 server_time_ms = 3;
}

S_SpawnMyCharacter — `0x0203`

Sent after S_EnterGameRes succeeds. Provides the character’s initial world position.

message S_SpawnMyCharacter {
  uint64 character_id = 1;
  float x = 2;
  float y = 3;
  float z = 4;
}

S_SpawnOtherCharacter — `0x0206`

Sent when another player enters this client’s Area of Interest.

message S_SpawnOtherCharacter {
  uint64 character_id = 1;
  float x = 2;
  float y = 3;
  float z = 4;
  float yaw = 5;
}

S_DespawnCharacter — `0x0207`

Sent when another player exits this client’s Area of Interest or disconnects.

message S_DespawnCharacter {
  uint64 character_id = 1;
}

S_MoveNotify — `0x0212`

Sent per-player over UDP when MoveBatchNotify is not used. Carries the server-authoritative position.

message S_MoveNotify {
  uint64 character_id = 1;
  uint32 last_processed_input_sequence = 2;
  float x = 3;
  float y = 4;
  float z = 5;
  float velocity_x = 6;
  float velocity_y = 7;
  float velocity_z = 8;
  float yaw = 9;
  uint64 server_time_ms = 10;
}

S_ChatNtf — `0x0222`

Global chat broadcast received by all authenticated sessions.

message S_ChatNtf {
  uint64 sender_id = 1;
  string sender_name = 2;
  string message = 3;
}

S_AttackAck — `0x0232`

Server acknowledgement of a C_AttackReq.

message S_AttackAck {
  bool success = 1;
  string message = 2;
  uint32 attack_sequence = 3;
  uint32 combo_step = 4;
  uint64 server_time_ms = 5;
}

S_InventorySnapshot — `0x0251`

Full inventory state sent on enter-game and after any inventory operation.

message InventoryItem {
  uint64 item_instance_id = 1;
  uint32 template_id = 2;
  uint32 quantity = 3;
  uint32 slot_index = 4;
  uint32 durability = 5;
}

message S_InventorySnapshot {
  bool success = 1;
  string message = 2;
  uint64 revision = 3;
  uint32 capacity = 4;
  repeated InventoryItem items = 5;
}

S_InventoryOperationResult — `0x0253`

Result of a C_InventoryMoveReq or C_InventoryUseReq. Contains an updated inventory snapshot on success.

message S_InventoryOperationResult {
  bool success = 1;
  string message = 2;
  bytes client_request_id = 3;
  S_InventorySnapshot snapshot = 4;
}

S_ErrorRes — `0x0003`

General error response. Sent when any request cannot be processed.

enum ErrorCode {
  ERROR_CODE_UNSPECIFIED = 0;
  ERROR_CODE_MALFORMED_PACKET = 1;
  ERROR_CODE_UNKNOWN_PACKET = 2;
  ERROR_CODE_INVALID_DATA = 3;
  ERROR_CODE_INVALID_STATE = 4;
  ERROR_CODE_AUTH_REQUIRED = 5;
  ERROR_CODE_RATE_LIMITED = 6;
}

message S_ErrorRes {
  uint32 request_packet_id = 1;
  ErrorCode error_code = 2;
  string message = 3;
}

Regenerating Proto Files

If Packet.proto changes, regenerate the C++ bindings for both the server and client:

# Generate server-side files
protoc --cpp_out=ServerSolution/ServerSolution/Generated/ \
  ServerSolution/ServerSolution/Proto/Packet.proto

# Generate client-side files
protoc --cpp_out=MMOClient1/Source/MMOClient1/Proto/ \
  MMOClient1/Source/MMOClient1/Proto/Packet.proto

The server (ServerSolution/ServerSolution/Proto/Packet.proto) and client (MMOClient1/Source/MMOClient1/Proto/Packet.proto) proto files must remain identical. Mismatched proto definitions cause silent deserialization errors that are difficult to debug.

After regenerating, rebuild both the C++ GameServer solution in Visual Studio and the UE5 client in the Unreal Editor to pick up the new generated headers.

UE5 Client

Protobuf Packets Used by the UE5 MMO Client

Packets Sent by the Client

C_EnterGameReq — `0x0201`

C_UdpHelloReq — `0x0204`

C_MoveReq — `0x0211`

C_ChatReq — `0x0221`

C_AttackReq — `0x0231`

C_InventoryMoveReq — `0x0252`

C_InventoryUseReq — `0x0254`

Packets Received by the Client

S_EnterGameRes — `0x0202`

S_UdpHelloRes — `0x0205`

S_SpawnMyCharacter — `0x0203`

S_SpawnOtherCharacter — `0x0206`

S_DespawnCharacter — `0x0207`

S_MoveNotify — `0x0212`

S_ChatNtf — `0x0222`

S_AttackAck — `0x0232`

S_InventorySnapshot — `0x0251`

S_InventoryOperationResult — `0x0253`

S_ErrorRes — `0x0003`

Regenerating Proto Files

Build docs developers (and LLMs) love

UE5 Client

Documentation Index

​Packets Sent by the Client

​C_EnterGameReq — 0x0201

​C_UdpHelloReq — 0x0204

​C_MoveReq — 0x0211

​C_ChatReq — 0x0221

​C_AttackReq — 0x0231

​C_InventoryMoveReq — 0x0252

​C_InventoryUseReq — 0x0254

​Packets Received by the Client

​S_EnterGameRes — 0x0202

​S_UdpHelloRes — 0x0205

​S_SpawnMyCharacter — 0x0203

​S_SpawnOtherCharacter — 0x0206

​S_DespawnCharacter — 0x0207

​S_MoveNotify — 0x0212

​S_ChatNtf — 0x0222

​S_AttackAck — 0x0232

​S_InventorySnapshot — 0x0251

​S_InventoryOperationResult — 0x0253

​S_ErrorRes — 0x0003

​Regenerating Proto Files

Build docs developers (and LLMs) love

Packets Sent by the Client

C_EnterGameReq — `0x0201`

C_UdpHelloReq — `0x0204`

C_MoveReq — `0x0211`

C_ChatReq — `0x0221`

C_AttackReq — `0x0231`

C_InventoryMoveReq — `0x0252`

C_InventoryUseReq — `0x0254`

Packets Received by the Client

S_EnterGameRes — `0x0202`

S_UdpHelloRes — `0x0205`

S_SpawnMyCharacter — `0x0203`

S_SpawnOtherCharacter — `0x0206`

S_DespawnCharacter — `0x0207`

S_MoveNotify — `0x0212`

S_ChatNtf — `0x0222`

S_AttackAck — `0x0232`

S_InventorySnapshot — `0x0251`

S_InventoryOperationResult — `0x0253`

S_ErrorRes — `0x0003`

Regenerating Proto Files