[GitHub] [incubator-nuttx] anchao commented on a change in pull request #1047: sim/tapdev: follow the tunnel MTU size

[GitBox <gi...@apache.org>]

anchao commented on a change in pull request #1047:
URL: https://github.com/apache/incubator-nuttx/pull/1047#discussion_r425528457



##########
File path: net/netdev/netdev_register.c
##########
@@ -333,6 +335,18 @@ int netdev_register(FAR struct net_driver_s *dev, enum net_lltype_e lltype)
             return -EINVAL;
         }
 
+      /* Update the package length */
+
+      if (dev->d_llhdrlen == 0)
+        {
+          dev->d_llhdrlen = llhdrlen;
+        }
+
+      if (dev->d_pktsize == 0)
+        {
+          dev->d_pktsize = pktsize;
+        }
+

Review comment:
       Configure the MTU packet size in the SIM scenario is a invalid choice,
   the MTU will only take effect at the link layer when configuring the tunnel bridge:
   
   tools/simbridge.sh:
   
   ```
    if [ "$2" == "on" ]; then
      ip link add nuttx0 type bridge
      ip addr flush dev $1
   +  ip link set dev nuttx0 mtu size?
      ip link set $1 master nuttx0
      ip link set dev nuttx0 up
      dhclient nuttx0
   ```
   
   
   ```
   nuttx0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
           inet 10.0.2.15  netmask 255.255.255.0  broadcast 10.0.2.255
           inet6 fe80::a00:27ff:fe63:d2e7  prefixlen 64  scopeid 0x20<link>
   ```
   
   If the nuttx MTU size is small than tunnel, the larger packets will do drop processing at:
   
   net/devif/ipv4_input.c:
   ...
   377   /* Check the size of the packet.  If the size reported to us in d_len is
   378    * smaller the size reported in the IP header, we assume that the packet
   379    * has been corrupted in transit.  If the size of d_len is larger than the
   380    * size reported in the IP packet header, the packet has been padded and
   381    * we set d_len to the correct value.
   382    */
   383 
   384   totlen = (ipv4->len[0] << 8) + ipv4->len[1];
   385   if (totlen <= dev->d_len)
   386     {
   387       dev->d_len = totlen;
   388     }
   389   else
   390     {
   391       nwarn("WARNING: IP packet shorter than length in IP header\n");
   392       goto drop;
   393     }
   ...
   
   in this situation, I think obtain the MTU size dynamically is a necessary choice.
   Regarding your concerns, We will check further whether there is side effects in the mainline.




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