refactor: name packet size headroom

Author: mjc

russh/src/cipher/mod.rs

@@ -311,9 +311,14 @@ pub(crate) async fn read<R: AsyncRead + Unpin>(
 pub(crate) const PACKET_LENGTH_LEN: usize = 4;
 
 const MINIMUM_PACKET_LEN: usize = 16;
-const MAXIMUM_PACKET_LEN: usize = 256 * 1024 + 36;
-
+const MAXIMUM_PACKET_LEN_BASELINE: usize = 256 * 1024;
+const CHANNEL_DATA_PACKET_OVERHEAD: usize = 1 + 4 + 4;
+const CHANNEL_EXTENDED_DATA_PACKET_OVERHEAD: usize = CHANNEL_DATA_PACKET_OVERHEAD + 4;
 const PADDING_LENGTH_LEN: usize = 1;
+const MAXIMUM_PADDING_LEN: usize = 19;
+const MAXIMUM_PACKET_LEN_HEADROOM: usize =
+    PADDING_LENGTH_LEN + CHANNEL_EXTENDED_DATA_PACKET_OVERHEAD + MAXIMUM_PADDING_LEN;
+const MAXIMUM_PACKET_LEN: usize = MAXIMUM_PACKET_LEN_BASELINE + MAXIMUM_PACKET_LEN_HEADROOM;
 
 #[cfg(feature = "_bench")]
 pub mod benchmark;