Add comments to GRO and GSO control messages

Author: Gleb Pomykalov

src/sys/socket/mod.rs

@@ -464,6 +464,14 @@ pub enum ControlMessageOwned {
     ))]
     Ipv4RecvDstAddr(libc::in_addr),
 
+    /// UDP Generic Receive Offload (GRO) allows receiving multiple UDP
+    /// packets from a single sender.
+    /// Fixed-size payloads are following one by one in a receive buffer.
+    /// This Control Message indicates the size of all smaller packets,
+    /// except, maybe, the last one.
+    ///
+    /// `UdpGroSegment` socket option should be enabled on a socket
+    /// to allow receiving GRO packets.
     #[cfg(target_os = "linux")]
     UdpGroSegments(u16),
 
@@ -626,7 +634,14 @@ pub enum ControlMessage<'a> {
     ))]
     AlgSetAeadAssoclen(&'a u32),
 
-    ///Gso UDP
+    /// UDP GSO makes it possible for applications to generate network packets
+    /// for a virtual MTU much greater than the real one.
+    /// The length of the send data no longer matches the expected length on
+    /// the wire.
+    /// The size of the datagram payload as it should appear on the wire may be
+    /// passed through this control message.
+    /// Send buffer should consist of multiple fixed-size wire payloads
+    /// following one by one, and the last, possibly smaller one.
     #[cfg(target_os = "linux")]
     UdpGsoSegments(&'a u16),
 }