Truncate GSO after assembling a short packet

quinn-proto/src/connection/mod.rs

@@ -683,6 +683,20 @@ impl Connection {
                     }
 
                     if num_datagrams > 1 {
+                        // If too many padding bytes would be required to continue the GSO batch
+                        // after this packet, end the GSO batch here. Ensures that fixed-size frames
+                        // with heterogeneous sizes (e.g. application datagrams) won't inadvertently
+                        // waste large amounts of bandwidth. The exact threshold is a bit arbitrary
+                        // and might benefit from further tuning, though there's no universally
+                        // optimal value.
+                        const MAX_PADDING: usize = 16;
+                        let packet_len_unpadded =
+                            cmp::max(builder.min_size, buf.len()) + builder.tag_len;
+                        if packet_len_unpadded + MAX_PADDING < segment_size as usize {
+                            trace!("GSO truncated by {} byte packet", packet_len_unpadded);
+                            break;
+                        }
+
                         // Pad the current packet to GSO segment size so it can be included in the
                         // GSO batch.
                         builder.pad_to(segment_size);