[SCTP] Optimize sctp packet marshalling

sctp/CHANGELOG.md

@@ -3,7 +3,8 @@
 ## Unreleased
 
 * Performance improvements
-  * The lock for the internal association was contended badly because marshaling was done while still in a critical section and also tokio was scheduling tasks badly[#363](https://github.com/webrtc-rs/webrtc/pull/363)
+  * reuse as many allocations as possible when marshaling [#364](https://github.com/webrtc-rs/webrtc/pull/364)
+  * The lock for the internal association was contended badly because marshaling was done while still in a critical section and also tokio was scheduling tasks badly [#363](https://github.com/webrtc-rs/webrtc/pull/363)
 
 ## v0.7.0
 

sctp/src/chunk/chunk_payload_data.rs

@@ -233,7 +233,7 @@ impl Chunk for ChunkPayloadData {
         writer.put_u16(self.stream_identifier);
         writer.put_u16(self.stream_sequence_number);
         writer.put_u32(self.payload_type as u32);
-        writer.extend(self.user_data.clone());
+        writer.extend_from_slice(&self.user_data);
 
         Ok(writer.len())
     }

sctp/src/packet.rs

@@ -20,7 +20,6 @@ use crate::util::*;
 
 use crate::chunk::chunk_unknown::ChunkUnknown;
 use bytes::{Buf, BufMut, Bytes, BytesMut};
-use crc::{Crc, CRC_32_ISCSI};
 use std::fmt;
 
 ///Packet represents an SCTP packet, defined in https://tools.ietf.org/html/rfc4960#section-3
@@ -155,30 +154,29 @@ impl Packet {
         writer.put_u16(self.destination_port);
         writer.put_u32(self.verification_tag);
 
+        // This is where the checksum will be written
+        let checksum_pos = writer.len();
+        writer.extend_from_slice(&[0, 0, 0, 0]);
+
         // Populate chunks
-        let mut raw = BytesMut::new();
         for c in &self.chunks {
-            let chunk_raw = c.marshal()?;
-            raw.extend(chunk_raw);
+            c.marshal_to(writer)?;
 
-            let padding_needed = get_padding_size(raw.len());
+            let padding_needed = get_padding_size(writer.len());
             if padding_needed != 0 {
-                raw.extend(vec![0u8; padding_needed]);
+                // padding needed if < 4 because we pad to 4
+                writer.extend_from_slice(&[0u8; 16][..padding_needed]);
             }
         }
-        let raw = raw.freeze();
 
-        let hasher = Crc::<u32>::new(&CRC_32_ISCSI);
-        let mut digest = hasher.digest();
+        let mut digest = ISCSI_CRC.digest();
         digest.update(writer);
-        digest.update(&FOUR_ZEROES);
-        digest.update(&raw[..]);
         let checksum = digest.finalize();
 
         // Checksum is already in BigEndian
         // Using LittleEndian stops it from being flipped
-        writer.put_u32_le(checksum);
-        writer.extend(raw);
+        let checksum_place = &mut writer[checksum_pos..checksum_pos + 4];
+        checksum_place.copy_from_slice(&checksum.to_le_bytes());
 
         Ok(writer.len())
     }

sctp/src/util.rs

@@ -11,10 +11,11 @@ pub(crate) fn get_padding_size(len: usize) -> usize {
 /// We need to use it for the checksum and don't want to allocate/clear each time.
 pub(crate) static FOUR_ZEROES: Bytes = Bytes::from_static(&[0, 0, 0, 0]);
 
+pub(crate) const ISCSI_CRC: Crc<u32> = Crc::<u32>::new(&CRC_32_ISCSI);
+
 /// Fastest way to do a crc32 without allocating.
 pub(crate) fn generate_packet_checksum(raw: &Bytes) -> u32 {
-    let hasher = Crc::<u32>::new(&CRC_32_ISCSI);
-    let mut digest = hasher.digest();
+    let mut digest = ISCSI_CRC.digest();
     digest.update(&raw[0..8]);
     digest.update(&FOUR_ZEROES[..]);
     digest.update(&raw[12..]);