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[tcp] Calculate correct MSS from peer address 

iPXE currently advertises a fixed MSS of 1460, which is correct only
for IPv4 over Ethernet.  For IPv6 over Ethernet, the value should be
1440 (allowing for the larger IPv6 header).  For non-Ethernet link
layers, the value should reflect the MTU of the underlying network
device.

Use tcpip_mtu() to calculate the transport-layer MTU associated with
the peer address, and calculate the MSS to allow for an optionless TCP
header as per RFC 6691.

As a side benefit, we can now fail a connection immediately with a
meaningful error message if we have no route to the destination
address.

Reported-by: Anton D. Kachalov <mouse@yandex-team.ru>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
pull/17/head
Michael Brown 2014-03-04 13:14:13 +00:00
parent 6414b5ca03
commit e191298a1d
2 changed files with 14 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

10
src/include/ipxe/tcp.h
View File

@ -330,16 +330,6 @@ struct tcp_options {
#define TCP_PATH_MTU \
( 1280 - 40 /* IPv6 */ - 20 /* TCP */ - 12 /* TCP timestamp */ )
/**
* Advertised TCP MSS
*
* We currently hardcode this to a reasonable value and hope that the
* sender uses path MTU discovery. The alternative is breaking the
* abstraction layer so that we can find out the MTU from the IP layer
* (which would have to find out from the net device layer).
*/
#define TCP_MSS 1460
/** TCP maximum segment lifetime
*
* Currently set to 2 minutes, as per RFC 793.

15
src/net/tcp.c
View File

@ -43,6 +43,8 @@ struct tcp_connection {
struct sockaddr_tcpip peer;
/** Local port */
unsigned int local_port;
/** Maximum segment size */
size_t mss;
/** Current TCP state */
unsigned int tcp_state;
@ -250,6 +252,7 @@ static int tcp_open ( struct interface *xfer, struct sockaddr *peer,
struct sockaddr_tcpip *st_peer = ( struct sockaddr_tcpip * ) peer;
struct sockaddr_tcpip *st_local = ( struct sockaddr_tcpip * ) local;
struct tcp_connection *tcp;
size_t mtu;
int port;
int rc;
@ -271,6 +274,16 @@ static int tcp_open ( struct interface *xfer, struct sockaddr *peer,
INIT_LIST_HEAD ( &tcp->rx_queue );
memcpy ( &tcp->peer, st_peer, sizeof ( tcp->peer ) );
/* Calculate MSS */
mtu = tcpip_mtu ( &tcp->peer );
if ( ! mtu ) {
DBGC ( tcp, "TCP %p has no route to %s\n",
tcp, sock_ntoa ( peer ) );
rc = -ENETUNREACH;
goto err;
}
tcp->mss = ( mtu - sizeof ( struct tcp_header ) );
/* Bind to local port */
port = tcpip_bind ( st_local, tcp_port_available );
if ( port < 0 ) {
@ -552,7 +565,7 @@ static int tcp_xmit ( struct tcp_connection *tcp ) {
mssopt = iob_push ( iobuf, sizeof ( *mssopt ) );
mssopt->kind = TCP_OPTION_MSS;
mssopt->length = sizeof ( *mssopt );
mssopt->mss = htons ( TCP_MSS );
mssopt->mss = htons ( tcp->mss );
wsopt = iob_push ( iobuf, sizeof ( *wsopt ) );
wsopt->nop = TCP_OPTION_NOP;
wsopt->wsopt.kind = TCP_OPTION_WS;