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author | Michael Brown | 2009-06-23 02:29:43 +0200 |
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committer | Michael Brown | 2009-06-23 10:40:26 +0200 |
commit | 558c1a45fe30ffe3d6c67b2321e0fc973f13e9b7 (patch) | |
tree | 16153ed9464185891dabb5eaee235b318ff672bc /src/net/tcp.c | |
parent | [settings] Fix setting_cmp() to handle nameless settings (diff) | |
download | ipxe-558c1a45fe30ffe3d6c67b2321e0fc973f13e9b7.tar.gz ipxe-558c1a45fe30ffe3d6c67b2321e0fc973f13e9b7.tar.xz ipxe-558c1a45fe30ffe3d6c67b2321e0fc973f13e9b7.zip |
[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>
Diffstat (limited to 'src/net/tcp.c')
-rw-r--r-- | src/net/tcp.c | 41 |
1 files changed, 28 insertions, 13 deletions
diff --git a/src/net/tcp.c b/src/net/tcp.c index 1de2abf0..29e233fa 100644 --- a/src/net/tcp.c +++ b/src/net/tcp.c @@ -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 ); |