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[tcp] Avoid rewinding sequence numbers on receiving old duplicate ACKs 

Commit 558c1a4 ("[tcp] Improve robustness in the presence of duplicated
received packets") introduced a regression in that an old duplicate
ACK received while in the ESTABLISHED state would pass through normal
ACK processing, including updating tcp->snd_seq.

Fix by ensuring that ACK processing ignores all duplicate ACKs.
This commit is contained in:
Michael Brown 2009-06-23 16:10:34 +01:00
parent 99e64f5806
commit 58f60df66c
1 changed files with 31 additions and 27 deletions
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58
src/net/tcp.c
View File

@ -726,6 +726,37 @@ static int tcp_rx_ack ( struct tcp_connection *tcp, uint32_t ack,
size_t len;
unsigned int acked_flags;
/* Check for out-of-range or old duplicate ACKs */
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 ) );
if ( TCP_HAS_BEEN_ESTABLISHED ( tcp->tcp_state ) ) {
/* Just ignore what might be old duplicate ACKs */
return 0;
} else {
/* Send RST if an out-of-range ACK is received
* on a not-yet-established connection, as per
* RFC 793.
*/
return -EINVAL;
}
}
/* Ignore ACKs that don't actually acknowledge any new data.
* (In particular, do not stop the retransmission timer; this
* avoids creating a sorceror's apprentice syndrome when a
* duplicate ACK is received and we still have data in our
* transmit queue.)
*/
if ( ack_len == 0 )
return 0;
/* Stop the retransmission timer */
stop_timer ( &tcp->timer );
/* Determine acknowledged flags and data length */
len = ack_len;
acked_flags = ( TCP_FLAGS_SENDING ( tcp->tcp_state ) &
@ -733,33 +764,6 @@ static int tcp_rx_ack ( struct tcp_connection *tcp, uint32_t ack,
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;