For localhost connections set the mtu to the localhost interface mtu

src/gui/windows/debug_network_advanced.zig

@@ -31,7 +31,7 @@ fn renderConnectionData(conn: *main.network.Connection, name: []const u8, y: *f3
 	conn.lossyChannel.getStatistics(&unconfirmed[0], &queued[0]);
 	conn.secureChannel.getStatistics(&unconfirmed[1], &queued[1]);
 	conn.slowChannel.getStatistics(&unconfirmed[2], &queued[2]);
-	draw.print("{s} | RTT = {d:.1} ms | {d:.0} kiB/RTT", .{name, conn.rttEstimate/1000.0, conn.bandwidthEstimateInBytesPerRtt/1024.0}, 0, y.*, 8);
+	draw.print("{s} | RTT = {d:.1} ms | {d:.0} kiB/RTT | MTU: {d}", .{name, conn.rttEstimate/1000.0, conn.bandwidthEstimateInBytesPerRtt/1024.0, conn.mtuEstimate}, 0, y.*, 8);
 	y.* += 8;
 	draw.print("Waiting in queue:      {: >6} kiB |{: >6} kiB |{: >6} kiB", .{queued[0] >> 10, queued[1] >> 10, queued[2] >> 10}, 0, y.*, 8);
 	y.* += 8;

src/network.zig

@@ -24,6 +24,7 @@ inline fn networkTimestamp() i64 {
 const Socket = struct {
 	const posix = std.posix;
 	socketID: if (builtin.os.tag == .windows) c.SOCKET else posix.socket_t,
+	interfaceMtu: ?u16 = null,
 
 	fn windowsError(err: c_int) !void {
 		if (err == 0) return;
@@ -57,7 +58,7 @@ const Socket = struct {
 	}
 
 	fn init(localPort: u16) !Socket {
-		const self = Socket{
+		var self = Socket{
 			.socketID = blk: {
 				if (builtin.os.tag == .windows) {
 					const socket = c.socket(c.AF_INET, c.SOCK_DGRAM, c.IPPROTO_UDP);
@@ -100,6 +101,7 @@ const Socket = struct {
 				},
 			}
 		}
+		self.interfaceMtu = self.getInterfaceMtu();
 		return self;
 	}
 
@@ -226,6 +228,25 @@ const Socket = struct {
 		}
 		return @byteSwap(addr.port);
 	}
+
+	fn getInterfaceMtu(self: Socket) ?u16 {
+		if (builtin.os.tag == .windows) {
+			return null;
+		} else {
+			var req: std.posix.ifreq = undefined;
+			req.ifrn.name = .{'l', 'o'} ++ @as([14]u8, @splat(0));
+			const result = std.c.ioctl(self.socketID, std.os.linux.SIOCGIFMTU, &req);
+			switch (std.posix.errno(result)) {
+				.SUCCESS => {
+					return @truncate(@as(u32, @bitCast(@min(req.ifru.mtu, 65535) - 20 - 8))); // its often reported to be 65536, but the IPv4 header has only a 16 bit length field
+				},
+				else => |err| {
+					std.log.warn("Failed to get the mtu of 'lo' interface from ioctl: {s}", .{@tagName(err)});
+					return null;
+				},
+			}
+		}
+	}
 };
 
 pub fn init() !void {
@@ -293,6 +314,12 @@ pub const SocketAddress = struct {
 			},
 		};
 	}
+
+	fn isLoopback(self: *const SocketAddress) bool {
+		return switch (self.address) {
+			inline else => |ip| ip.eql(.loopback(ip.port)),
+		};
+	}
 };
 
 const Request = struct {
@@ -1554,6 +1581,7 @@ pub const Connection = struct { // MARK: Connection
 		if (result.connectionIdentifier == 0) result.connectionIdentifier = 1;
 		result.remoteAddress = try SocketAddress.resolve(ipPort, settings.defaultPort);
 		result.bruteforcingPort = result.remoteAddress.isSymmetricNAT;
+		if (result.remoteAddress.isLoopback() and manager.socket.interfaceMtu != null) result.mtuEstimate = manager.socket.interfaceMtu.?;
 
 		try result.manager.addConnection(result);
 		return result;