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[netdevice] Limit MTU by hardware maximum frame length

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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>
This commit is contained in:
Michael Brown
2017-01-25 14:48:24 +00:00
parent f450c75dad
commit 70fc25ad6e
5 changed files with 31 additions and 10 deletions
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22
src/net/netdev_settings.c
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@@ -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: "