[FS#210] ltq-RFC4638 PPPoE MTU support

[LEDE Bugs]

The following task has a new comment added:

FS#210 - ltq-RFC4638 PPPoE MTU support
User who did this - dwmw2 (dwmw2)

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The really naïve test seems to work...


--- a/package/kernel/lantiq/ltq-ptm/src/ifxmips_ptm_vdsl.c
+++ b/package/kernel/lantiq/ltq-ptm/src/ifxmips_ptm_vdsl.c
@@ -107,6 +107,14 @@ static int g_queue_gamma_map[4];
 
 static struct ptm_priv_data g_ptm_priv_data;
 
+static int ptm_change_mtu(struct net_device *dev, int new_mtu)
+{
+        if (new_mtu < 68 || new_mtu > ETH_DATA_LEN + 8)
+                return -EINVAL;
+        dev->mtu = new_mtu;
+        return 0;
+}
+
 static struct net_device_ops g_ptm_netdev_ops = {
     .ndo_get_stats       = ptm_get_stats,
     .ndo_open            = ptm_open,
@@ -114,7 +122,7 @@ static struct net_device_ops g_ptm_netdev_ops = {
     .ndo_start_xmit      = ptm_hard_start_xmit,
     .ndo_validate_addr   = eth_validate_addr,
     .ndo_set_mac_address = eth_mac_addr,
-    .ndo_change_mtu      = eth_change_mtu,
+    .ndo_change_mtu      = ptm_change_mtu,
     .ndo_do_ioctl        = ptm_ioctl,
     .ndo_tx_timeout      = ptm_tx_timeout,
 };


I can then ping it from the outside:

 $ ping 217.169.22.43 -s 1472 -M do
PING 217.169.22.43 (217.169.22.43) 1472(1500) bytes of data.
1480 bytes from 217.169.22.43: icmp_seq=1 ttl=59 time=20.4 ms


I'm slightly confused though, because I have a VLAN too; ptm0.101. 


config switch_vlan 'ptm0_101'
        option device 'ptm0'
        option vlan '101'
        option vid '101'

config interface 'aa101'
        option ifname 'ptm0.101'
        option proto 'static'
        option mtu '1508'

config interface 'ptm0'
        option mtu '1508'


If I run tcpdump on the ptm0.101 interface, that looks fine:

11:21:32.598772 PPPoE  [ses 0x24de] IP (tos 0x0, ttl 64, id 40501, offset 0, flags [none], proto ICMP (1), length 1500)
    adsl-2.infradead.org > casper.infradead.org: ICMP echo reply, id 30233, seq 5, length 1480
        0x0000:  0030 881e 9065 0020 da86 2375 8864 1100  .0...e....#u.d..
        0x0010:  24de 05de 0021 4500 05dc 9e35 0000 4001  $....!E....5.. at .
        ...
        0x05e0:  aeaf b0b1 b2b3 b4b5 b6b7 b8b9 babb bcbd  ................
        0x05f0:  bebf                                     ..



But... on the ptm0 interface, obviously I have extra VLAN overhead:


11:22:48.287214 PPPoE  [ses 0x24de] IP (tos 0x0, ttl 64, id 43141, offset 0, flags [none], proto ICMP (1), length 1500)
    adsl-2.infradead.org > casper.infradead.org: ICMP echo reply, id 30370, seq 2, length 1480
        0x0000:  0030 881e 9065 0020 da86 2375 8100 0065  .0...e....#u...e
        0x0010:  8864 1100 24de 05de 0021 4500 05dc a885  .d..$....!E.....
        ...
        0x05e0:  aaab acad aeaf b0b1 b2b3 b4b5 b6b7 b8b9  ................
        0x05f0:  babb bcbd bebf                           ......



It's working, but why? That packet is larger than the 1508 bytes I configured as the MTU for the ptm0 interface. Why don't I need to configure 1512 for ptm0 and 1508 for ptm0.101? Is that a bug somewhere?

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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=210#comment589