[netdevice] Limit MTU by hardware maximum frame length

Separate out the concept of "hardware maximum supported frame length"
and "configured link MTU", and limit the latter according to the
former.

In networks where the DHCP-supplied link MTU is inconsistent with the
hardware or driver capabilities (e.g. a network using jumbo frames),
this will result in iPXE advertising a TCP MSS consistent with a size
that can actually be received.

Note that the term "MTU" is typically used to refer to the maximum
length excluding the link-layer headers; we adopt this usage.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

diff --git a/src/include/ipxe/netdevice.h b/src/include/ipxe/netdevice.h
index a1d207ffc47dcae88aacd0e34487c455e79eb4a6..d498ab697dccbc248d8ff137da916abbf70d9d93 100644 (file)
--- a/src/include/ipxe/netdevice.h
+++ b/src/include/ipxe/netdevice.h
@@ -397,9 +397,16 @@ struct net_device {
        struct retry_timer link_block;
        /** Maximum packet length
         *
-        * This length includes any link-layer headers.
+        * This is the maximum packet length (including any link-layer
+        * headers) supported by the hardware.
         */
        size_t max_pkt_len;
+       /** Maximum transmission unit length
+        *
+        * This is the maximum transmission unit length (excluding any
+        * link-layer headers) configured for the link.
+        */
+       size_t mtu;
        /** TX packet queue */
        struct list_head tx_queue;
        /** Deferred TX packet queue */

diff --git a/src/net/infiniband/xsigo.c b/src/net/infiniband/xsigo.c
index 91b7b71f1eb9966fb86f9ef7fd49001ea6da60a5..0ee753c38a1c113bd6969edc6ca3ad346b04a7a2 100644 (file)
--- a/src/net/infiniband/xsigo.c
+++ b/src/net/infiniband/xsigo.c
@@ -323,6 +323,7 @@ static int xve_update_mtu ( struct xsigo_nic *xve, struct eoib_device *eoib,
         * not the EoIB header.
         */
        netdev->max_pkt_len = ( mtu + sizeof ( struct ethhdr ) );
+       netdev->mtu = mtu;
        DBGC ( xve, "XVE %s has MTU %zd\n", xve->name, mtu );
 
        return 0;

diff --git a/src/net/netdev_settings.c b/src/net/netdev_settings.c
index 67a45bedee1ce10e25e1a6aff0e15dfba4ba326d..c54288d4f86ef2c0469fe47db80fc942aa371cfa 100644 (file)
--- a/src/net/netdev_settings.c
+++ b/src/net/netdev_settings.c
@@ -393,7 +393,8 @@ static int apply_netdev_settings ( void ) {
        struct net_device *netdev;
        struct settings *settings;
        struct ll_protocol *ll_protocol;
-       size_t old_max_pkt_len;
+       size_t max_mtu;
+       size_t old_mtu;
        size_t mtu;
        int rc;
 
@@ -410,18 +411,25 @@ static int apply_netdev_settings ( void ) {
                if ( ! mtu )
                        continue;
 
-               /* Update maximum packet length */
+               /* Limit MTU to maximum supported by hardware */
                ll_protocol = netdev->ll_protocol;
-               old_max_pkt_len = netdev->max_pkt_len;
-               netdev->max_pkt_len = ( mtu + ll_protocol->ll_header_len );
-               if ( netdev->max_pkt_len != old_max_pkt_len ) {
+               max_mtu = ( netdev->max_pkt_len - ll_protocol->ll_header_len );
+               if ( mtu > max_mtu ) {
+                       DBGC ( netdev, "NETDEV %s cannot support MTU %zd (max "
+                              "%zd)\n", netdev->name, mtu, max_mtu );
+                       mtu = max_mtu;
+               }
+
+               /* Update maximum packet length */
+               old_mtu = netdev->mtu;
+               netdev->mtu = mtu;
+               if ( mtu != old_mtu ) {
                        DBGC ( netdev, "NETDEV %s MTU is %zd\n",
                               netdev->name, mtu );
                }
 
                /* Close and reopen network device if MTU has increased */
-               if ( netdev_is_open ( netdev ) &&
-                    ( netdev->max_pkt_len > old_max_pkt_len ) ) {
+               if ( netdev_is_open ( netdev ) && ( mtu > old_mtu ) ) {
                        netdev_close ( netdev );
                        if ( ( rc = netdev_open ( netdev ) ) != 0 ) {
                                DBGC ( netdev, "NETDEV %s could not reopen: "

diff --git a/src/net/netdevice.c b/src/net/netdevice.c
index 9df21196c1db5097697e800f2165141e708e7dc7..41ece77f0c5e9c48bc72a1d8a04476678bce8b2e 100644 (file)
--- a/src/net/netdevice.c
+++ b/src/net/netdevice.c
@@ -663,6 +663,12 @@ int register_netdev ( struct net_device *netdev ) {
                ll_protocol->init_addr ( netdev->hw_addr, netdev->ll_addr );
        }
 
+       /* Set MTU, if not already set */
+       if ( ! netdev->mtu ) {
+               netdev->mtu = ( netdev->max_pkt_len -
+                               ll_protocol->ll_header_len );
+       }
+
        /* Reject network devices that are already available via a
         * different hardware device.
         */

diff --git a/src/net/tcpip.c b/src/net/tcpip.c
index c9e4ee789c979fde93ace4b80d5c8a0406a4252b..cc7d02005024bc4b8e4a7fa62ef982e649e51768 100644 (file)
--- a/src/net/tcpip.c
+++ b/src/net/tcpip.c
@@ -144,8 +144,7 @@ size_t tcpip_mtu ( struct sockaddr_tcpip *st_dest ) {
                return 0;
 
        /* Calculate MTU */
-       mtu = ( netdev->max_pkt_len - netdev->ll_protocol->ll_header_len -
-               tcpip_net->header_len );
+       mtu = ( netdev->mtu - tcpip_net->header_len );
 
        return mtu;
 }