From 2de2bd43b1e4723ce91724a428eb591af22dfccf Mon Sep 17 00:00:00 2001
From: Demetrius Kanios <demetrius@kanios.net>
Date: Sat, 6 Jun 2026 00:15:42 -0700
Subject: [PATCH] fix: RakNet frame split threshold (#2198)

Signed-off-by: Demetrius Kanios <demetrius@kanios.net>
---
 pumpkin-protocol/src/bedrock/mod.rs        |  5 ++++-
 pumpkin/src/net/bedrock/mod.rs             | 19 ++++++++++++-------
 pumpkin/src/net/bedrock/open_connection.rs |  4 ++--
 3 files changed, 18 insertions(+), 10 deletions(-)

diff --git a/pumpkin-protocol/src/bedrock/mod.rs b/pumpkin-protocol/src/bedrock/mod.rs
index 7bb7b0724..b68cd6cb5 100644
--- a/pumpkin-protocol/src/bedrock/mod.rs
+++ b/pumpkin-protocol/src/bedrock/mod.rs
@@ -7,9 +7,12 @@ pub mod packet_encoder;
 pub mod server;
 
 pub const RAKNET_PROTOCOL_VERSION: u8 = 11;
-pub const UDP_HEADER_SIZE: u16 = 28;
+pub const UDP_HEADER_SIZE: usize = 28;
 pub const MTU: usize = 1400;
 
+// 26 bytes is RakNet header for FrameSet containing a single, ReliableOrdered, split/fragmented frame
+pub const SPLIT_FRAME_MAX_CONTENT: usize = MTU - UDP_HEADER_SIZE - 26;
+
 pub const RAKNET_MAGIC: [u8; 16] = [
     0x00, 0xff, 0xff, 0x0, 0xfe, 0xfe, 0xfe, 0xfe, 0xfd, 0xfd, 0xfd, 0xfd, 0x12, 0x34, 0x56, 0x78,
 ];
diff --git a/pumpkin/src/net/bedrock/mod.rs b/pumpkin/src/net/bedrock/mod.rs
index f0306e6d8..8c7c47fc1 100644
--- a/pumpkin/src/net/bedrock/mod.rs
+++ b/pumpkin/src/net/bedrock/mod.rs
@@ -18,7 +18,8 @@ use pumpkin_config::networking::compression::CompressionInfo;
 use pumpkin_protocol::{
     BClientPacket, PacketDecodeError, RawPacket,
     bedrock::{
-        MTU, RAKNET_ACK, RAKNET_GAME_PACKET, RAKNET_NACK, RakReliability, SubClient,
+        MTU, RAKNET_ACK, RAKNET_GAME_PACKET, RAKNET_NACK, RakReliability, SPLIT_FRAME_MAX_CONTENT,
+        SubClient, UDP_HEADER_SIZE,
         ack::Acknowledge,
         client::{
             disconnect_player::CDisconnectPlayer, level_chunk::CLevelChunk,
@@ -556,10 +557,14 @@ impl BedrockClient {
         let mut split_id = 0;
         let mut order_index = 0;
 
-        let count = if packet_buf.len() > MTU {
+        let mut max_content_len =
+            MTU - UDP_HEADER_SIZE - 12 - if reliability.is_ordered() { 4 } else { 0 };
+
+        let count = if packet_buf.len() > max_content_len {
             reliability = RakReliability::ReliableOrdered;
             split_id = self.output_split_number.fetch_add(1, Ordering::Relaxed);
-            split_size = packet_buf.len().div_ceil(MTU) as u32;
+            max_content_len = SPLIT_FRAME_MAX_CONTENT;
+            split_size = packet_buf.len().div_ceil(max_content_len) as u32;
             split_size as usize
         } else {
             1
@@ -570,12 +575,12 @@ impl BedrockClient {
         }
 
         for i in 0..count {
-            let end = if i + 1 == count && !packet_buf.len().is_multiple_of(MTU) {
-                packet_buf.len() % MTU
+            let end = if i + 1 == count && !packet_buf.len().is_multiple_of(max_content_len) {
+                packet_buf.len() % max_content_len
             } else {
-                MTU
+                max_content_len
             };
-            let chunk = &packet_buf[i * MTU..i * MTU + end];
+            let chunk = &packet_buf[i * max_content_len..i * max_content_len + end];
 
             let mut frame_set = FrameSet {
                 sequence: u24(0),
diff --git a/pumpkin/src/net/bedrock/open_connection.rs b/pumpkin/src/net/bedrock/open_connection.rs
index e7b815c78..c14084206 100644
--- a/pumpkin/src/net/bedrock/open_connection.rs
+++ b/pumpkin/src/net/bedrock/open_connection.rs
@@ -1,7 +1,7 @@
 use std::net::SocketAddr;
 
 use pumpkin_protocol::bedrock::{
-    RAKNET_PROTOCOL_VERSION,
+    MTU, RAKNET_PROTOCOL_VERSION,
     client::raknet::{
         incompatible_protocol::CIncompatibleProtocolVersion,
         open_connection::{COpenConnectionReply1, COpenConnectionReply2},
@@ -30,7 +30,7 @@ impl BedrockClient {
         }
 
         Self::send_offline_packet(
-            &COpenConnectionReply1::new(server.server_guid, false, 1400),
+            &COpenConnectionReply1::new(server.server_guid, false, MTU as u16),
             addr,
             socket,
         )