Clarify how to back a PacketAssembler

Author: benbrittain

src/iface/fragmentation.rs

@@ -10,6 +10,10 @@ use crate::Error;
 use crate::Result;
 
 /// Holds different fragments of one packet, used for assembling fragmented packets.
+///
+/// The buffer used for the `PacketAssembler` should either be dynamically sized (ex: Vec<u8>)
+/// or should be statically allocated based upon the MTU of the type of packet being
+/// assembled (ex: 1280 for a IPv6 frame).
 #[derive(Debug)]
 pub struct PacketAssembler<'a> {
     buffer: ManagedSlice<'a, u8>,

src/iface/interface.rs

@@ -1660,7 +1660,16 @@ impl<'a> InterfaceInner<'a> {
                     // This information is the total size of the packet when it is fully assmbled.
                     // We also pass the header size, since this is needed when other fragments
                     // (other than the first one) are added.
-                    check!(check!(fragments.reserve_with_key(&key)).start(
+                    let frag_slot = match fragments.reserve_with_key(&key) {
+                        Ok(frag) => frag,
+                        Err(Error::PacketAssemblerSetFull) => {
+                            net_debug!("No available packet assembler for fragmented packet");
+                            return Default::default();
+                        }
+                        e => check!(e),
+                    };
+
+                    check!(frag_slot.start(
                         Some(
                             frag.datagram_size() as usize - uncompressed_header_size
                                 + compressed_header_size