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[tcp] Improve robustness in the presence of duplicated received packets 

gPXE responds to duplicated ACKs with an immediate retransmission,
which can lead to a sorceror's apprentice syndrome.  It also responds
to out-of-range (or old duplicate) ACKs with a RST, which can cause
valid connections to be dropped.

Fix the sorceror's apprentice syndrome by leaving the retransmission
timer running (and so inhibiting the immediate retransmission) when we
receive a potential duplicate ACK.  This seems to match the behaviour
of Linux observed via wireshark traces.

Fix the RST issue by sending RST only on out-of-range ACKs that occur
before the connection is fully established, as per RFC 793.

These problems were exposed during development of the 802.11 wireless
link layer; the 802.11 protocol has a failure mode that can easily
cause duplicated packets.  The fixes were tested in a controlled way
by faking large numbers of duplicated packets in the rtl8139 driver.

Originally-fixed-by: Joshua Oreman <oremanj@rwcr.net>
This commit is contained in:
Michael Brown 2009-06-23 01:29:43 +01:00
parent 68973f1c49
commit 558c1a45fe
2 changed files with 38 additions and 13 deletions
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10
src/include/gpxe/tcp.h
View File

@ -229,6 +229,16 @@ struct tcp_options {
TCP_STATE_SENT ( TCP_FIN ) ) ) \
== TCP_STATE_ACKED ( TCP_SYN ) )
/** Have ever been fully established
*
* We have been fully established if we have both received a SYN and
* had our own SYN acked.
*/
#define TCP_HAS_BEEN_ESTABLISHED(state) \
( ( (state) & ( TCP_STATE_ACKED ( TCP_SYN ) | \
TCP_STATE_RCVD ( TCP_SYN ) ) ) \
== ( TCP_STATE_ACKED ( TCP_SYN ) | TCP_STATE_RCVD ( TCP_SYN ) ) )
/** Have closed gracefully
*
* We have closed gracefully if we have both received a FIN and had

41
src/net/tcp.c
View File

@ -704,30 +704,45 @@ static int tcp_rx_ack ( struct tcp_connection *tcp, uint32_t ack,
size_t len;
unsigned int acked_flags;
/* Ignore duplicate or out-of-range ACK */
if ( ack_len > tcp->snd_sent ) {
DBGC ( tcp, "TCP %p received ACK for [%08x,%08zx), "
"sent only [%08x,%08x)\n", tcp, tcp->snd_seq,
( tcp->snd_seq + ack_len ), tcp->snd_seq,
( tcp->snd_seq + tcp->snd_sent ) );
return -EINVAL;
}
/* Acknowledge any flags being sent */
/* Determine acknowledged flags and data length */
len = ack_len;
acked_flags = ( TCP_FLAGS_SENDING ( tcp->tcp_state ) &
( TCP_SYN | TCP_FIN ) );
if ( acked_flags )
len--;
/* Stop retransmission timer if necessary */
if ( ack_len == 0 ) {
/* Duplicate ACK (or just a packet that isn't
* intending to ACK any new data). If the
* retransmission timer is running, leave it running
* so that we don't immediately retransmit and cause a
* sorceror's apprentice syndrome.
*/
} else if ( ack_len <= tcp->snd_sent ) {
/* ACK of new data. Stop the retransmission timer. */
stop_timer ( &tcp->timer );
} else {
/* Out-of-range (or old duplicate) ACK. Leave the
* timer running, as for the ack_len==0 case, to
* handle old duplicate ACKs.
*/
DBGC ( tcp, "TCP %p received ACK for [%08x,%08zx), "
"sent only [%08x,%08x)\n", tcp, tcp->snd_seq,
( tcp->snd_seq + ack_len ), tcp->snd_seq,
( tcp->snd_seq + tcp->snd_sent ) );
/* Send RST if an out-of-range ACK is received on a
* not-yet-established connection.
*/
if ( ! TCP_HAS_BEEN_ESTABLISHED ( tcp->tcp_state ) )
return -EINVAL;
}
/* Update SEQ and sent counters, and window size */
tcp->snd_seq = ack;
tcp->snd_sent = 0;
tcp->snd_win = win;
/* Stop the retransmission timer */
stop_timer ( &tcp->timer );
/* Remove any acknowledged data from transmit queue */
tcp_process_queue ( tcp, len, NULL, 1 );