diff options
author | Ingo Molnar <mingo@elte.hu> | 2008-10-12 12:49:27 +0200 |
---|---|---|
committer | Ingo Molnar <mingo@elte.hu> | 2008-10-12 12:49:27 +0200 |
commit | 1389ac4b976abdc0555280dfc1aa2c3abcc19641 (patch) | |
tree | 1ec51aa0b1e33201f6f4f3bebc657f9ec65f8539 /net/ipv4/tcp_input.c | |
parent | 69e13ad56f9e2cd81c4f8bfd6267211c10c14c08 (diff) | |
parent | fd048088306656824958e7783ffcee27e241b361 (diff) |
Merge branch 'linus' into x86/signal
Conflicts:
arch/x86/kernel/signal_64.c
Diffstat (limited to 'net/ipv4/tcp_input.c')
-rw-r--r-- | net/ipv4/tcp_input.c | 332 |
1 files changed, 170 insertions, 162 deletions
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 67ccce2a96b..d77c0d29e23 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -979,6 +979,39 @@ static void tcp_update_reordering(struct sock *sk, const int metric, } } +/* This must be called before lost_out is incremented */ +static void tcp_verify_retransmit_hint(struct tcp_sock *tp, struct sk_buff *skb) +{ + if ((tp->retransmit_skb_hint == NULL) || + before(TCP_SKB_CB(skb)->seq, + TCP_SKB_CB(tp->retransmit_skb_hint)->seq)) + tp->retransmit_skb_hint = skb; + + if (!tp->lost_out || + after(TCP_SKB_CB(skb)->end_seq, tp->retransmit_high)) + tp->retransmit_high = TCP_SKB_CB(skb)->end_seq; +} + +static void tcp_skb_mark_lost(struct tcp_sock *tp, struct sk_buff *skb) +{ + if (!(TCP_SKB_CB(skb)->sacked & (TCPCB_LOST|TCPCB_SACKED_ACKED))) { + tcp_verify_retransmit_hint(tp, skb); + + tp->lost_out += tcp_skb_pcount(skb); + TCP_SKB_CB(skb)->sacked |= TCPCB_LOST; + } +} + +void tcp_skb_mark_lost_uncond_verify(struct tcp_sock *tp, struct sk_buff *skb) +{ + tcp_verify_retransmit_hint(tp, skb); + + if (!(TCP_SKB_CB(skb)->sacked & (TCPCB_LOST|TCPCB_SACKED_ACKED))) { + tp->lost_out += tcp_skb_pcount(skb); + TCP_SKB_CB(skb)->sacked |= TCPCB_LOST; + } +} + /* This procedure tags the retransmission queue when SACKs arrive. * * We have three tag bits: SACKED(S), RETRANS(R) and LOST(L). @@ -1155,13 +1188,7 @@ static void tcp_mark_lost_retrans(struct sock *sk) TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS; tp->retrans_out -= tcp_skb_pcount(skb); - /* clear lost hint */ - tp->retransmit_skb_hint = NULL; - - if (!(TCP_SKB_CB(skb)->sacked & (TCPCB_LOST|TCPCB_SACKED_ACKED))) { - tp->lost_out += tcp_skb_pcount(skb); - TCP_SKB_CB(skb)->sacked |= TCPCB_LOST; - } + tcp_skb_mark_lost_uncond_verify(tp, skb); NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPLOSTRETRANSMIT); } else { if (before(ack_seq, new_low_seq)) @@ -1271,9 +1298,6 @@ static int tcp_sacktag_one(struct sk_buff *skb, struct sock *sk, ~(TCPCB_LOST|TCPCB_SACKED_RETRANS); tp->lost_out -= tcp_skb_pcount(skb); tp->retrans_out -= tcp_skb_pcount(skb); - - /* clear lost hint */ - tp->retransmit_skb_hint = NULL; } } else { if (!(sacked & TCPCB_RETRANS)) { @@ -1292,9 +1316,6 @@ static int tcp_sacktag_one(struct sk_buff *skb, struct sock *sk, if (sacked & TCPCB_LOST) { TCP_SKB_CB(skb)->sacked &= ~TCPCB_LOST; tp->lost_out -= tcp_skb_pcount(skb); - - /* clear lost hint */ - tp->retransmit_skb_hint = NULL; } } @@ -1324,7 +1345,6 @@ static int tcp_sacktag_one(struct sk_buff *skb, struct sock *sk, if (dup_sack && (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS)) { TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS; tp->retrans_out -= tcp_skb_pcount(skb); - tp->retransmit_skb_hint = NULL; } return flag; @@ -1726,6 +1746,8 @@ int tcp_use_frto(struct sock *sk) return 0; skb = tcp_write_queue_head(sk); + if (tcp_skb_is_last(sk, skb)) + return 1; skb = tcp_write_queue_next(sk, skb); /* Skips head */ tcp_for_write_queue_from(skb, sk) { if (skb == tcp_send_head(sk)) @@ -1867,6 +1889,7 @@ static void tcp_enter_frto_loss(struct sock *sk, int allowed_segments, int flag) if (!(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)) { TCP_SKB_CB(skb)->sacked |= TCPCB_LOST; tp->lost_out += tcp_skb_pcount(skb); + tp->retransmit_high = TCP_SKB_CB(skb)->end_seq; } } tcp_verify_left_out(tp); @@ -1883,7 +1906,7 @@ static void tcp_enter_frto_loss(struct sock *sk, int allowed_segments, int flag) tp->high_seq = tp->snd_nxt; TCP_ECN_queue_cwr(tp); - tcp_clear_retrans_hints_partial(tp); + tcp_clear_all_retrans_hints(tp); } static void tcp_clear_retrans_partial(struct tcp_sock *tp) @@ -1934,12 +1957,11 @@ void tcp_enter_loss(struct sock *sk, int how) /* Push undo marker, if it was plain RTO and nothing * was retransmitted. */ tp->undo_marker = tp->snd_una; - tcp_clear_retrans_hints_partial(tp); } else { tp->sacked_out = 0; tp->fackets_out = 0; - tcp_clear_all_retrans_hints(tp); } + tcp_clear_all_retrans_hints(tp); tcp_for_write_queue(skb, sk) { if (skb == tcp_send_head(sk)) @@ -1952,6 +1974,7 @@ void tcp_enter_loss(struct sock *sk, int how) TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_ACKED; TCP_SKB_CB(skb)->sacked |= TCPCB_LOST; tp->lost_out += tcp_skb_pcount(skb); + tp->retransmit_high = TCP_SKB_CB(skb)->end_seq; } } tcp_verify_left_out(tp); @@ -2157,19 +2180,6 @@ static int tcp_time_to_recover(struct sock *sk) return 0; } -/* RFC: This is from the original, I doubt that this is necessary at all: - * clear xmit_retrans hint if seq of this skb is beyond hint. How could we - * retransmitted past LOST markings in the first place? I'm not fully sure - * about undo and end of connection cases, which can cause R without L? - */ -static void tcp_verify_retransmit_hint(struct tcp_sock *tp, struct sk_buff *skb) -{ - if ((tp->retransmit_skb_hint != NULL) && - before(TCP_SKB_CB(skb)->seq, - TCP_SKB_CB(tp->retransmit_skb_hint)->seq)) - tp->retransmit_skb_hint = NULL; -} - /* Mark head of queue up as lost. With RFC3517 SACK, the packets is * is against sacked "cnt", otherwise it's against facked "cnt" */ @@ -2217,11 +2227,7 @@ static void tcp_mark_head_lost(struct sock *sk, int packets) cnt = packets; } - if (!(TCP_SKB_CB(skb)->sacked & (TCPCB_SACKED_ACKED|TCPCB_LOST))) { - TCP_SKB_CB(skb)->sacked |= TCPCB_LOST; - tp->lost_out += tcp_skb_pcount(skb); - tcp_verify_retransmit_hint(tp, skb); - } + tcp_skb_mark_lost(tp, skb); } tcp_verify_left_out(tp); } @@ -2263,11 +2269,7 @@ static void tcp_update_scoreboard(struct sock *sk, int fast_rexmit) if (!tcp_skb_timedout(sk, skb)) break; - if (!(TCP_SKB_CB(skb)->sacked & (TCPCB_SACKED_ACKED|TCPCB_LOST))) { - TCP_SKB_CB(skb)->sacked |= TCPCB_LOST; - tp->lost_out += tcp_skb_pcount(skb); - tcp_verify_retransmit_hint(tp, skb); - } + tcp_skb_mark_lost(tp, skb); } tp->scoreboard_skb_hint = skb; @@ -2378,10 +2380,6 @@ static void tcp_undo_cwr(struct sock *sk, const int undo) } tcp_moderate_cwnd(tp); tp->snd_cwnd_stamp = tcp_time_stamp; - - /* There is something screwy going on with the retrans hints after - an undo */ - tcp_clear_all_retrans_hints(tp); } static inline int tcp_may_undo(struct tcp_sock *tp) @@ -2838,7 +2836,8 @@ static u32 tcp_tso_acked(struct sock *sk, struct sk_buff *skb) * is before the ack sequence we can discard it as it's confirmed to have * arrived at the other end. */ -static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets) +static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets, + u32 prior_snd_una) { struct tcp_sock *tp = tcp_sk(sk); const struct inet_connection_sock *icsk = inet_csk(sk); @@ -2848,6 +2847,7 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets) int flag = 0; u32 pkts_acked = 0; u32 reord = tp->packets_out; + u32 prior_sacked = tp->sacked_out; s32 seq_rtt = -1; s32 ca_seq_rtt = -1; ktime_t last_ackt = net_invalid_timestamp(); @@ -2904,9 +2904,6 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets) if (sacked & TCPCB_LOST) tp->lost_out -= acked_pcount; - if (unlikely(tp->urg_mode && !before(end_seq, tp->snd_up))) - tp->urg_mode = 0; - tp->packets_out -= acked_pcount; pkts_acked += acked_pcount; @@ -2929,9 +2926,16 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets) tcp_unlink_write_queue(skb, sk); sk_wmem_free_skb(sk, skb); - tcp_clear_all_retrans_hints(tp); + tp->scoreboard_skb_hint = NULL; + if (skb == tp->retransmit_skb_hint) + tp->retransmit_skb_hint = NULL; + if (skb == tp->lost_skb_hint) + tp->lost_skb_hint = NULL; } + if (likely(between(tp->snd_up, prior_snd_una, tp->snd_una))) + tp->snd_up = tp->snd_una; + if (skb && (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)) flag |= FLAG_SACK_RENEGING; @@ -2948,6 +2952,15 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets) /* Non-retransmitted hole got filled? That's reordering */ if (reord < prior_fackets) tcp_update_reordering(sk, tp->fackets_out - reord, 0); + + /* No need to care for underflows here because + * the lost_skb_hint gets NULLed if we're past it + * (or something non-trivial happened) + */ + if (tcp_is_fack(tp)) + tp->lost_cnt_hint -= pkts_acked; + else + tp->lost_cnt_hint -= prior_sacked - tp->sacked_out; } tp->fackets_out -= min(pkts_acked, tp->fackets_out); @@ -3299,7 +3312,7 @@ static int tcp_ack(struct sock *sk, struct sk_buff *skb, int flag) goto no_queue; /* See if we can take anything off of the retransmit queue. */ - flag |= tcp_clean_rtx_queue(sk, prior_fackets); + flag |= tcp_clean_rtx_queue(sk, prior_fackets, prior_snd_una); if (tp->frto_counter) frto_cwnd = tcp_process_frto(sk, flag); @@ -3442,6 +3455,22 @@ void tcp_parse_options(struct sk_buff *skb, struct tcp_options_received *opt_rx, } } +static int tcp_parse_aligned_timestamp(struct tcp_sock *tp, struct tcphdr *th) +{ + __be32 *ptr = (__be32 *)(th + 1); + + if (*ptr == htonl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16) + | (TCPOPT_TIMESTAMP << 8) | TCPOLEN_TIMESTAMP)) { + tp->rx_opt.saw_tstamp = 1; + ++ptr; + tp->rx_opt.rcv_tsval = ntohl(*ptr); + ++ptr; + tp->rx_opt.rcv_tsecr = ntohl(*ptr); + return 1; + } + return 0; +} + /* Fast parse options. This hopes to only see timestamps. * If it is wrong it falls back on tcp_parse_options(). */ @@ -3453,16 +3482,8 @@ static int tcp_fast_parse_options(struct sk_buff *skb, struct tcphdr *th, return 0; } else if (tp->rx_opt.tstamp_ok && th->doff == (sizeof(struct tcphdr)>>2)+(TCPOLEN_TSTAMP_ALIGNED>>2)) { - __be32 *ptr = (__be32 *)(th + 1); - if (*ptr == htonl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16) - | (TCPOPT_TIMESTAMP << 8) | TCPOLEN_TIMESTAMP)) { - tp->rx_opt.saw_tstamp = 1; - ++ptr; - tp->rx_opt.rcv_tsval = ntohl(*ptr); - ++ptr; - tp->rx_opt.rcv_tsecr = ntohl(*ptr); + if (tcp_parse_aligned_timestamp(tp, th)) return 1; - } } tcp_parse_options(skb, &tp->rx_opt, 1); return 1; @@ -4138,7 +4159,7 @@ drop: skb1 = skb1->prev; } } - __skb_insert(skb, skb1, skb1->next, &tp->out_of_order_queue); + __skb_queue_after(&tp->out_of_order_queue, skb1, skb); /* And clean segments covered by new one as whole. */ while ((skb1 = skb->next) != @@ -4161,6 +4182,18 @@ add_sack: } } +static struct sk_buff *tcp_collapse_one(struct sock *sk, struct sk_buff *skb, + struct sk_buff_head *list) +{ + struct sk_buff *next = skb->next; + + __skb_unlink(skb, list); + __kfree_skb(skb); + NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPRCVCOLLAPSED); + + return next; +} + /* Collapse contiguous sequence of skbs head..tail with * sequence numbers start..end. * Segments with FIN/SYN are not collapsed (only because this @@ -4178,11 +4211,7 @@ tcp_collapse(struct sock *sk, struct sk_buff_head *list, for (skb = head; skb != tail;) { /* No new bits? It is possible on ofo queue. */ if (!before(start, TCP_SKB_CB(skb)->end_seq)) { - struct sk_buff *next = skb->next; - __skb_unlink(skb, list); - __kfree_skb(skb); - NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPRCVCOLLAPSED); - skb = next; + skb = tcp_collapse_one(sk, skb, list); continue; } @@ -4228,7 +4257,7 @@ tcp_collapse(struct sock *sk, struct sk_buff_head *list, memcpy(nskb->head, skb->head, header); memcpy(nskb->cb, skb->cb, sizeof(skb->cb)); TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(nskb)->end_seq = start; - __skb_insert(nskb, skb->prev, skb, list); + __skb_queue_before(list, skb, nskb); skb_set_owner_r(nskb, sk); /* Copy data, releasing collapsed skbs. */ @@ -4246,11 +4275,7 @@ tcp_collapse(struct sock *sk, struct sk_buff_head *list, start += size; } if (!before(start, TCP_SKB_CB(skb)->end_seq)) { - struct sk_buff *next = skb->next; - __skb_unlink(skb, list); - __kfree_skb(skb); - NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPRCVCOLLAPSED); - skb = next; + skb = tcp_collapse_one(sk, skb, list); if (skb == tail || tcp_hdr(skb)->syn || tcp_hdr(skb)->fin) @@ -4436,8 +4461,8 @@ static void tcp_new_space(struct sock *sk) if (tcp_should_expand_sndbuf(sk)) { int sndmem = max_t(u32, tp->rx_opt.mss_clamp, tp->mss_cache) + - MAX_TCP_HEADER + 16 + sizeof(struct sk_buff), - demanded = max_t(unsigned int, tp->snd_cwnd, + MAX_TCP_HEADER + 16 + sizeof(struct sk_buff); + int demanded = max_t(unsigned int, tp->snd_cwnd, tp->reordering + 1); sndmem *= 2 * demanded; if (sndmem > sk->sk_sndbuf) @@ -4691,6 +4716,67 @@ out: } #endif /* CONFIG_NET_DMA */ +/* Does PAWS and seqno based validation of an incoming segment, flags will + * play significant role here. + */ +static int tcp_validate_incoming(struct sock *sk, struct sk_buff *skb, + struct tcphdr *th, int syn_inerr) +{ + struct tcp_sock *tp = tcp_sk(sk); + + /* RFC1323: H1. Apply PAWS check first. */ + if (tcp_fast_parse_options(skb, th, tp) && tp->rx_opt.saw_tstamp && + tcp_paws_discard(sk, skb)) { + if (!th->rst) { + NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED); + tcp_send_dupack(sk, skb); + goto discard; + } + /* Reset is accepted even if it did not pass PAWS. */ + } + + /* Step 1: check sequence number */ + if (!tcp_sequence(tp, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq)) { + /* RFC793, page 37: "In all states except SYN-SENT, all reset + * (RST) segments are validated by checking their SEQ-fields." + * And page 69: "If an incoming segment is not acceptable, + * an acknowledgment should be sent in reply (unless the RST + * bit is set, if so drop the segment and return)". + */ + if (!th->rst) + tcp_send_dupack(sk, skb); + goto discard; + } + + /* Step 2: check RST bit */ + if (th->rst) { + tcp_reset(sk); + goto discard; + } + + /* ts_recent update must be made after we are sure that the packet + * is in window. + */ + tcp_replace_ts_recent(tp, TCP_SKB_CB(skb)->seq); + + /* step 3: check security and precedence [ignored] */ + + /* step 4: Check for a SYN in window. */ + if (th->syn && !before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) { + if (syn_inerr) + TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_INERRS); + NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONSYN); + tcp_reset(sk); + return -1; + } + + return 1; + +discard: + __kfree_skb(skb); + return 0; +} + /* * TCP receive function for the ESTABLISHED state. * @@ -4718,6 +4804,7 @@ int tcp_rcv_established(struct sock *sk, struct sk_buff *skb, struct tcphdr *th, unsigned len) { struct tcp_sock *tp = tcp_sk(sk); + int res; /* * Header prediction. @@ -4756,19 +4843,10 @@ int tcp_rcv_established(struct sock *sk, struct sk_buff *skb, /* Check timestamp */ if (tcp_header_len == sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) { - __be32 *ptr = (__be32 *)(th + 1); - /* No? Slow path! */ - if (*ptr != htonl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16) - | (TCPOPT_TIMESTAMP << 8) | TCPOLEN_TIMESTAMP)) + if (!tcp_parse_aligned_timestamp(tp, th)) goto slow_path; - tp->rx_opt.saw_tstamp = 1; - ++ptr; - tp->rx_opt.rcv_tsval = ntohl(*ptr); - ++ptr; - tp->rx_opt.rcv_tsecr = ntohl(*ptr); - /* If PAWS failed, check it more carefully in slow path */ if ((s32)(tp->rx_opt.rcv_tsval - tp->rx_opt.ts_recent) < 0) goto slow_path; @@ -4879,7 +4957,8 @@ int tcp_rcv_established(struct sock *sk, struct sk_buff *skb, goto no_ack; } - __tcp_ack_snd_check(sk, 0); + if (!copied_early || tp->rcv_nxt != tp->rcv_wup) + __tcp_ack_snd_check(sk, 0); no_ack: #ifdef CONFIG_NET_DMA if (copied_early) @@ -4899,51 +4978,12 @@ slow_path: goto csum_error; /* - * RFC1323: H1. Apply PAWS check first. - */ - if (tcp_fast_parse_options(skb, th, tp) && tp->rx_opt.saw_tstamp && - tcp_paws_discard(sk, skb)) { - if (!th->rst) { - NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED); - tcp_send_dupack(sk, skb); - goto discard; - } - /* Resets are accepted even if PAWS failed. - - ts_recent update must be made after we are sure - that the packet is in window. - */ - } - - /* * Standard slow path. */ - if (!tcp_sequence(tp, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq)) { - /* RFC793, page 37: "In all states except SYN-SENT, all reset - * (RST) segments are validated by checking their SEQ-fields." - * And page 69: "If an incoming segment is not acceptable, - * an acknowledgment should be sent in reply (unless the RST bit - * is set, if so drop the segment and return)". - */ - if (!th->rst) - tcp_send_dupack(sk, skb); - goto discard; - } - - if (th->rst) { - tcp_reset(sk); - goto discard; - } - - tcp_replace_ts_recent(tp, TCP_SKB_CB(skb)->seq); - - if (th->syn && !before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) { - TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_INERRS); - NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONSYN); - tcp_reset(sk); - return 1; - } + res = tcp_validate_incoming(sk, skb, th, 1); + if (res <= 0) + return -res; step5: if (th->ack) @@ -5225,6 +5265,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb, struct tcp_sock *tp = tcp_sk(sk); struct inet_connection_sock *icsk = inet_csk(sk); int queued = 0; + int res; tp->rx_opt.saw_tstamp = 0; @@ -5277,42 +5318,9 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb, return 0; } - if (tcp_fast_parse_options(skb, th, tp) && tp->rx_opt.saw_tstamp && - tcp_paws_discard(sk, skb)) { - if (!th->rst) { - NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED); - tcp_send_dupack(sk, skb); - goto discard; - } - /* Reset is accepted even if it did not pass PAWS. */ - } - - /* step 1: check sequence number */ - if (!tcp_sequence(tp, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq)) { - if (!th->rst) - tcp_send_dupack(sk, skb); - goto discard; - } - - /* step 2: check RST bit */ - if (th->rst) { - tcp_reset(sk); - goto discard; - } - - tcp_replace_ts_recent(tp, TCP_SKB_CB(skb)->seq); - - /* step 3: check security and precedence [ignored] */ - - /* step 4: - * - * Check for a SYN in window. - */ - if (th->syn && !before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) { - NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONSYN); - tcp_reset(sk); - return 1; - } + res = tcp_validate_incoming(sk, skb, th, 0); + if (res <= 0) + return -res; /* step 5: check the ACK field */ if (th->ack) { |