[PATCH 6.12 331/337] rxrpc: Manage RTT per-call rather than per-peer
Greg Kroah-Hartman <[email protected]>
| Newsgroups | org.kernel.vger.stable,dev.linux.lists.patches |
|---|---|
| Message-ID | <[email protected]> |
6.12-stable review patch. If anyone has any objections, please let me know. ------------------ From: David Howells <[email protected]> [ Upstream commit b40ef2b85a7d117dd323b5910e504899e0a3e7dc ] Manage the determination of RTT on a per-call (ie. per-RPC op) basis rather than on a per-peer basis, averaging across all calls going to that peer. The problem is that the RTT measurements from the initial packets on a call may be off because the server may do some setting up (such as getting a lock on a file) before accepting the rest of the data in the RPC and, further, the RTT may be affected by server-side file operations, for instance if a large amount of data is being written or read. Note: When handling the FS.StoreData-type RPCs, for example, the server uses the userStatus field in the header of ACK packets as supplementary flow control to aid in managing this. AF_RXRPC does not yet support this, but it should be added. Signed-off-by: David Howells <[email protected]> cc: Marc Dionne <[email protected]> cc: [email protected] Signed-off-by: Jakub Kicinski <[email protected]> Stable-dep-of: e4d2878369d5 ("rxrpc: Fix irq-disabled in local_bh_enable()") Signed-off-by: Sasha Levin <[email protected]> Signed-off-by: Greg Kroah-Hartman <[email protected]> --- include/trace/events/rxrpc.h | 2 net/rxrpc/ar-internal.h | 39 +++++++++--------- net/rxrpc/call_event.c | 18 ++++---- net/rxrpc/call_object.c | 2 net/rxrpc/input.c | 8 +-- net/rxrpc/output.c | 14 +++--- net/rxrpc/peer_object.c | 8 --- net/rxrpc/proc.c | 6 +- net/rxrpc/rtt.c | 93 +++++++++++++++++++++---------------------- net/rxrpc/sendmsg.c | 2 10 files changed, 97 insertions(+), 95 deletions(-) --- a/include/trace/events/rxrpc.h +++ b/include/trace/events/rxrpc.h @@ -1333,7 +1333,7 @@ TRACE_EVENT(rxrpc_rtt_rx, __entry->rtt = rtt; __entry->srtt = srtt; __entry->rto = rto; - __entry->min_rtt = minmax_get(&call->peer->min_rtt) + __entry->min_rtt = minmax_get(&call->min_rtt) ), TP_printk("c=%08x [%d] %s sr=%08x rr=%08x rtt=%u srtt=%u rto=%u min=%u", --- a/net/rxrpc/ar-internal.h +++ b/net/rxrpc/ar-internal.h @@ -344,20 +344,9 @@ struct rxrpc_peer { int debug_id; /* debug ID for printks */ struct sockaddr_rxrpc srx; /* remote address */ - /* calculated RTT cache */ -#define RXRPC_RTT_CACHE_SIZE 32 - spinlock_t rtt_input_lock; /* RTT lock for input routine */ - ktime_t rtt_last_req; /* Time of last RTT request */ - unsigned int rtt_count; /* Number of samples we've got */ - unsigned int rtt_taken; /* Number of samples taken (wrapping) */ - struct minmax min_rtt; /* Estimated minimum RTT */ - - u32 srtt_us; /* smoothed round trip time << 3 in usecs */ - u32 mdev_us; /* medium deviation */ - u32 mdev_max_us; /* maximal mdev for the last rtt period */ - u32 rttvar_us; /* smoothed mdev_max */ - u32 rto_us; /* Retransmission timeout in usec */ - u8 backoff; /* Backoff timeout (as shift) */ + /* Calculated RTT cache */ + unsigned int recent_srtt_us; + unsigned int recent_rto_us; u8 cong_ssthresh; /* Congestion slow-start threshold */ }; @@ -742,6 +731,18 @@ struct rxrpc_call { rxrpc_seq_t acks_hard_ack; /* Latest hard-ack point */ rxrpc_seq_t acks_lowest_nak; /* Lowest NACK in the buffer (or ==tx_hard_ack) */ rxrpc_serial_t acks_highest_serial; /* Highest serial number ACK'd */ + + /* Calculated RTT cache */ + ktime_t rtt_last_req; /* Time of last RTT request */ + unsigned int rtt_count; /* Number of samples we've got */ + unsigned int rtt_taken; /* Number of samples taken (wrapping) */ + struct minmax min_rtt; /* Estimated minimum RTT */ + u32 srtt_us; /* smoothed round trip time << 3 in usecs */ + u32 mdev_us; /* medium deviation */ + u32 mdev_max_us; /* maximal mdev for the last rtt period */ + u32 rttvar_us; /* smoothed mdev_max */ + u32 rto_us; /* Retransmission timeout in usec */ + u8 backoff; /* Backoff timeout (as shift) */ }; /* @@ -1222,10 +1223,12 @@ static inline int rxrpc_abort_eproto(str /* * rtt.c */ -void rxrpc_peer_add_rtt(struct rxrpc_call *, enum rxrpc_rtt_rx_trace, int, - rxrpc_serial_t, rxrpc_serial_t, ktime_t, ktime_t); -ktime_t rxrpc_get_rto_backoff(struct rxrpc_peer *peer, bool retrans); -void rxrpc_peer_init_rtt(struct rxrpc_peer *); +void rxrpc_call_add_rtt(struct rxrpc_call *call, enum rxrpc_rtt_rx_trace why, + int rtt_slot, + rxrpc_serial_t send_serial, rxrpc_serial_t resp_serial, + ktime_t send_time, ktime_t resp_time); +ktime_t rxrpc_get_rto_backoff(struct rxrpc_call *call, bool retrans); +void rxrpc_call_init_rtt(struct rxrpc_call *call); /* * rxkad.c --- a/net/rxrpc/call_event.c +++ b/net/rxrpc/call_event.c @@ -44,8 +44,8 @@ void rxrpc_propose_delay_ACK(struct rxrp trace_rxrpc_propose_ack(call, why, RXRPC_ACK_DELAY, serial); - if (call->peer->srtt_us) - delay = (call->peer->srtt_us >> 3) * NSEC_PER_USEC; + if (call->srtt_us) + delay = (call->srtt_us >> 3) * NSEC_PER_USEC; else delay = ms_to_ktime(READ_ONCE(rxrpc_soft_ack_delay)); ktime_add_ms(delay, call->tx_backoff); @@ -72,7 +72,7 @@ void rxrpc_resend(struct rxrpc_call *cal struct rxrpc_txbuf *txb; rxrpc_seq_t transmitted = call->tx_transmitted; ktime_t next_resend = KTIME_MAX; - ktime_t rto = rxrpc_get_rto_backoff(call->peer, false); + ktime_t rto = rxrpc_get_rto_backoff(call, false); ktime_t resend_at = KTIME_MAX, now, delay; bool unacked = false, did_send = false; unsigned int i; @@ -174,7 +174,7 @@ void rxrpc_resend(struct rxrpc_call *cal no_further_resend: no_resend: if (resend_at < KTIME_MAX) { - delay = rxrpc_get_rto_backoff(call->peer, did_send); + delay = rxrpc_get_rto_backoff(call, did_send); resend_at = ktime_add(resend_at, delay); trace_rxrpc_timer_set(call, resend_at - now, rxrpc_timer_trace_resend_reset); } @@ -189,7 +189,7 @@ no_resend: */ if (!did_send) { ktime_t next_ping = ktime_add_us(call->acks_latest_ts, - call->peer->srtt_us >> 3); + call->srtt_us >> 3); if (ktime_sub(next_ping, now) <= 0) rxrpc_send_ACK(call, RXRPC_ACK_PING, 0, @@ -306,8 +306,8 @@ static void rxrpc_transmit_some_data(str */ static void rxrpc_send_initial_ping(struct rxrpc_call *call) { - if (call->peer->rtt_count < 3 || - ktime_before(ktime_add_ms(call->peer->rtt_last_req, 1000), + if (call->rtt_count < 3 || + ktime_before(ktime_add_ms(call->rtt_last_req, 1000), ktime_get_real())) rxrpc_send_ACK(call, RXRPC_ACK_PING, 0, rxrpc_propose_ack_ping_for_params); @@ -438,10 +438,10 @@ bool rxrpc_input_call_event(struct rxrpc rxrpc_propose_ack_rx_idle); if (call->ackr_nr_unacked > 2) { - if (call->peer->rtt_count < 3) + if (call->rtt_count < 3) rxrpc_send_ACK(call, RXRPC_ACK_PING, 0, rxrpc_propose_ack_ping_for_rtt); - else if (ktime_before(ktime_add_ms(call->peer->rtt_last_req, 1000), + else if (ktime_before(ktime_add_ms(call->rtt_last_req, 1000), ktime_get_real())) rxrpc_send_ACK(call, RXRPC_ACK_PING, 0, rxrpc_propose_ack_ping_for_old_rtt); --- a/net/rxrpc/call_object.c +++ b/net/rxrpc/call_object.c @@ -178,6 +178,8 @@ struct rxrpc_call *rxrpc_alloc_call(stru call->cong_cwnd = RXRPC_MIN_CWND; call->cong_ssthresh = RXRPC_TX_MAX_WINDOW; + rxrpc_call_init_rtt(call); + call->rxnet = rxnet; call->rtt_avail = RXRPC_CALL_RTT_AVAIL_MASK; atomic_inc(&rxnet->nr_calls); --- a/net/rxrpc/input.c +++ b/net/rxrpc/input.c @@ -83,11 +83,11 @@ static void rxrpc_congestion_management( /* We analyse the number of packets that get ACK'd per RTT * period and increase the window if we managed to fill it. */ - if (call->peer->rtt_count == 0) + if (call->rtt_count == 0) goto out; if (ktime_before(skb->tstamp, ktime_add_us(call->cong_tstamp, - call->peer->srtt_us >> 3))) + call->srtt_us >> 3))) goto out_no_clear_ca; change = rxrpc_cong_rtt_window_end; call->cong_tstamp = skb->tstamp; @@ -197,7 +197,7 @@ void rxrpc_congestion_degrade(struct rxr if (__rxrpc_call_state(call) == RXRPC_CALL_CLIENT_AWAIT_REPLY) return; - rtt = ns_to_ktime(call->peer->srtt_us * (1000 / 8)); + rtt = ns_to_ktime(call->srtt_us * (NSEC_PER_USEC / 8)); now = ktime_get_real(); if (!ktime_before(ktime_add(call->tx_last_sent, rtt), now)) return; @@ -664,7 +664,7 @@ static void rxrpc_complete_rtt_probe(str clear_bit(i + RXRPC_CALL_RTT_PEND_SHIFT, &call->rtt_avail); smp_mb(); /* Read data before setting avail bit */ set_bit(i, &call->rtt_avail); - rxrpc_peer_add_rtt(call, type, i, acked_serial, ack_serial, + rxrpc_call_add_rtt(call, type, i, acked_serial, ack_serial, sent_at, resp_time); matched = true; } --- a/net/rxrpc/output.c +++ b/net/rxrpc/output.c @@ -220,7 +220,7 @@ static void rxrpc_send_ack_packet(struct if (ack->reason == RXRPC_ACK_PING) rxrpc_begin_rtt_probe(call, txb->serial, now, rxrpc_rtt_tx_ping); if (txb->flags & RXRPC_REQUEST_ACK) - call->peer->rtt_last_req = now; + call->rtt_last_req = now; rxrpc_set_keepalive(call, now); } rxrpc_tx_backoff(call, ret); @@ -358,9 +358,9 @@ static void rxrpc_prepare_data_subpacket why = rxrpc_reqack_slow_start; else if (call->tx_winsize <= 2) why = rxrpc_reqack_small_txwin; - else if (call->peer->rtt_count < 3 && txb->seq & 1) + else if (call->rtt_count < 3) why = rxrpc_reqack_more_rtt; - else if (ktime_before(ktime_add_ms(call->peer->rtt_last_req, 1000), ktime_get_real())) + else if (ktime_before(ktime_add_ms(call->rtt_last_req, 1000), ktime_get_real())) why = rxrpc_reqack_old_rtt; else goto dont_set_request_ack; @@ -407,9 +407,9 @@ static void rxrpc_tstamp_data_packets(st if (ack_requested) { rxrpc_begin_rtt_probe(call, txb->serial, now, rxrpc_rtt_tx_data); - call->peer->rtt_last_req = now; - if (call->peer->rtt_count > 1) { - ktime_t delay = rxrpc_get_rto_backoff(call->peer, false); + call->rtt_last_req = now; + if (call->rtt_count > 1) { + ktime_t delay = rxrpc_get_rto_backoff(call, false); call->ack_lost_at = ktime_add(now, delay); trace_rxrpc_timer_set(call, delay, rxrpc_timer_trace_lost_ack); @@ -727,7 +727,7 @@ void rxrpc_transmit_one(struct rxrpc_cal rxrpc_instant_resend(call, txb); } } else { - ktime_t delay = ns_to_ktime(call->peer->rto_us * NSEC_PER_USEC); + ktime_t delay = ns_to_ktime(call->rto_us * NSEC_PER_USEC); call->resend_at = ktime_add(ktime_get_real(), delay); trace_rxrpc_timer_set(call, delay, rxrpc_timer_trace_resend_tx); --- a/net/rxrpc/peer_object.c +++ b/net/rxrpc/peer_object.c @@ -222,11 +222,8 @@ struct rxrpc_peer *rxrpc_alloc_peer(stru peer->service_conns = RB_ROOT; seqlock_init(&peer->service_conn_lock); spin_lock_init(&peer->lock); - spin_lock_init(&peer->rtt_input_lock); peer->debug_id = atomic_inc_return(&rxrpc_debug_id); - - rxrpc_peer_init_rtt(peer); - + peer->recent_srtt_us = UINT_MAX; peer->cong_ssthresh = RXRPC_TX_MAX_WINDOW; trace_rxrpc_peer(peer->debug_id, 1, why); } @@ -244,7 +241,6 @@ static void rxrpc_init_peer(struct rxrpc peer->hash_key = hash_key; rxrpc_assess_MTU_size(local, peer); peer->mtu = peer->if_mtu; - peer->rtt_last_req = ktime_get_real(); switch (peer->srx.transport.family) { case AF_INET: @@ -480,7 +476,7 @@ EXPORT_SYMBOL(rxrpc_kernel_get_call_peer */ unsigned int rxrpc_kernel_get_srtt(const struct rxrpc_peer *peer) { - return peer->rtt_count > 0 ? peer->srtt_us >> 3 : UINT_MAX; + return READ_ONCE(peer->recent_srtt_us); } EXPORT_SYMBOL(rxrpc_kernel_get_srtt); --- a/net/rxrpc/proc.c +++ b/net/rxrpc/proc.c @@ -299,15 +299,15 @@ static int rxrpc_peer_seq_show(struct se now = ktime_get_seconds(); seq_printf(seq, "UDP %-47.47s %-47.47s %3u" - " %3u %5u %6ds %8u %8u\n", + " %3u %5u %6ds %8d %8d\n", lbuff, rbuff, refcount_read(&peer->ref), peer->cong_ssthresh, peer->mtu, (s32)now - (s32)READ_ONCE(peer->last_tx_at), - peer->srtt_us >> 3, - peer->rto_us); + READ_ONCE(peer->recent_srtt_us), + READ_ONCE(peer->recent_rto_us)); return 0; } --- a/net/rxrpc/rtt.c +++ b/net/rxrpc/rtt.c @@ -12,17 +12,17 @@ #include "ar-internal.h" #define RXRPC_RTO_MAX (120 * USEC_PER_SEC) -#define RXRPC_TIMEOUT_INIT ((unsigned int)(1 * MSEC_PER_SEC)) /* RFC6298 2.1 initial RTO value */ +#define RXRPC_TIMEOUT_INIT ((unsigned int)(1 * USEC_PER_SEC)) /* RFC6298 2.1 initial RTO value */ #define rxrpc_jiffies32 ((u32)jiffies) /* As rxrpc_jiffies32 */ -static u32 rxrpc_rto_min_us(struct rxrpc_peer *peer) +static u32 rxrpc_rto_min_us(struct rxrpc_call *call) { return 200; } -static u32 __rxrpc_set_rto(const struct rxrpc_peer *peer) +static u32 __rxrpc_set_rto(const struct rxrpc_call *call) { - return (peer->srtt_us >> 3) + peer->rttvar_us; + return (call->srtt_us >> 3) + call->rttvar_us; } static u32 rxrpc_bound_rto(u32 rto) @@ -40,10 +40,10 @@ static u32 rxrpc_bound_rto(u32 rto) * To save cycles in the RFC 1323 implementation it was better to break * it up into three procedures. -- erics */ -static void rxrpc_rtt_estimator(struct rxrpc_peer *peer, long sample_rtt_us) +static void rxrpc_rtt_estimator(struct rxrpc_call *call, long sample_rtt_us) { long m = sample_rtt_us; /* RTT */ - u32 srtt = peer->srtt_us; + u32 srtt = call->srtt_us; /* The following amusing code comes from Jacobson's * article in SIGCOMM '88. Note that rtt and mdev @@ -66,7 +66,7 @@ static void rxrpc_rtt_estimator(struct r srtt += m; /* rtt = 7/8 rtt + 1/8 new */ if (m < 0) { m = -m; /* m is now abs(error) */ - m -= (peer->mdev_us >> 2); /* similar update on mdev */ + m -= (call->mdev_us >> 2); /* similar update on mdev */ /* This is similar to one of Eifel findings. * Eifel blocks mdev updates when rtt decreases. * This solution is a bit different: we use finer gain @@ -78,31 +78,31 @@ static void rxrpc_rtt_estimator(struct r if (m > 0) m >>= 3; } else { - m -= (peer->mdev_us >> 2); /* similar update on mdev */ + m -= (call->mdev_us >> 2); /* similar update on mdev */ } - peer->mdev_us += m; /* mdev = 3/4 mdev + 1/4 new */ - if (peer->mdev_us > peer->mdev_max_us) { - peer->mdev_max_us = peer->mdev_us; - if (peer->mdev_max_us > peer->rttvar_us) - peer->rttvar_us = peer->mdev_max_us; + call->mdev_us += m; /* mdev = 3/4 mdev + 1/4 new */ + if (call->mdev_us > call->mdev_max_us) { + call->mdev_max_us = call->mdev_us; + if (call->mdev_max_us > call->rttvar_us) + call->rttvar_us = call->mdev_max_us; } } else { /* no previous measure. */ srtt = m << 3; /* take the measured time to be rtt */ - peer->mdev_us = m << 1; /* make sure rto = 3*rtt */ - peer->rttvar_us = max(peer->mdev_us, rxrpc_rto_min_us(peer)); - peer->mdev_max_us = peer->rttvar_us; + call->mdev_us = m << 1; /* make sure rto = 3*rtt */ + call->rttvar_us = umax(call->mdev_us, rxrpc_rto_min_us(call)); + call->mdev_max_us = call->rttvar_us; } - peer->srtt_us = max(1U, srtt); + call->srtt_us = umax(srtt, 1); } /* * Calculate rto without backoff. This is the second half of Van Jacobson's * routine referred to above. */ -static void rxrpc_set_rto(struct rxrpc_peer *peer) +static void rxrpc_set_rto(struct rxrpc_call *call) { u32 rto; @@ -113,7 +113,7 @@ static void rxrpc_set_rto(struct rxrpc_p * is invisible. Actually, Linux-2.4 also generates erratic * ACKs in some circumstances. */ - rto = __rxrpc_set_rto(peer); + rto = __rxrpc_set_rto(call); /* 2. Fixups made earlier cannot be right. * If we do not estimate RTO correctly without them, @@ -124,73 +124,73 @@ static void rxrpc_set_rto(struct rxrpc_p /* NOTE: clamping at RXRPC_RTO_MIN is not required, current algo * guarantees that rto is higher. */ - peer->rto_us = rxrpc_bound_rto(rto); + call->rto_us = rxrpc_bound_rto(rto); } -static void rxrpc_update_rtt_min(struct rxrpc_peer *peer, ktime_t resp_time, long rtt_us) +static void rxrpc_update_rtt_min(struct rxrpc_call *call, ktime_t resp_time, long rtt_us) { /* Window size 5mins in approx usec (ipv4.sysctl_tcp_min_rtt_wlen) */ u32 wlen_us = 5ULL * NSEC_PER_SEC / 1024; - minmax_running_min(&peer->min_rtt, wlen_us, resp_time / 1024, + minmax_running_min(&call->min_rtt, wlen_us, resp_time / 1024, (u32)rtt_us ? : jiffies_to_usecs(1)); } -static void rxrpc_ack_update_rtt(struct rxrpc_peer *peer, ktime_t resp_time, long rtt_us) +static void rxrpc_ack_update_rtt(struct rxrpc_call *call, ktime_t resp_time, long rtt_us) { if (rtt_us < 0) return; /* Update RACK min RTT [RFC8985 6.1 Step 1]. */ - rxrpc_update_rtt_min(peer, resp_time, rtt_us); + rxrpc_update_rtt_min(call, resp_time, rtt_us); - rxrpc_rtt_estimator(peer, rtt_us); - rxrpc_set_rto(peer); + rxrpc_rtt_estimator(call, rtt_us); + rxrpc_set_rto(call); /* Only reset backoff on valid RTT measurement [RFC6298]. */ - peer->backoff = 0; + call->backoff = 0; } /* * Add RTT information to cache. This is called in softirq mode and has - * exclusive access to the peer RTT data. + * exclusive access to the call RTT data. */ -void rxrpc_peer_add_rtt(struct rxrpc_call *call, enum rxrpc_rtt_rx_trace why, +void rxrpc_call_add_rtt(struct rxrpc_call *call, enum rxrpc_rtt_rx_trace why, int rtt_slot, rxrpc_serial_t send_serial, rxrpc_serial_t resp_serial, ktime_t send_time, ktime_t resp_time) { - struct rxrpc_peer *peer = call->peer; s64 rtt_us; rtt_us = ktime_to_us(ktime_sub(resp_time, send_time)); if (rtt_us < 0) return; - spin_lock(&peer->rtt_input_lock); - rxrpc_ack_update_rtt(peer, resp_time, rtt_us); - if (peer->rtt_count < 3) - peer->rtt_count++; - peer->rtt_taken++; - spin_unlock(&peer->rtt_input_lock); + rxrpc_ack_update_rtt(call, resp_time, rtt_us); + if (call->rtt_count < 3) + call->rtt_count++; + call->rtt_taken++; + + WRITE_ONCE(call->peer->recent_srtt_us, call->srtt_us / 8); + WRITE_ONCE(call->peer->recent_rto_us, call->rto_us); trace_rxrpc_rtt_rx(call, why, rtt_slot, send_serial, resp_serial, - rtt_us, peer->srtt_us, peer->rto_us); + rtt_us, call->srtt_us, call->rto_us); } /* * Get the retransmission timeout to set in nanoseconds, backing it off each * time we retransmit. */ -ktime_t rxrpc_get_rto_backoff(struct rxrpc_peer *peer, bool retrans) +ktime_t rxrpc_get_rto_backoff(struct rxrpc_call *call, bool retrans) { u64 timo_us; - u32 backoff = READ_ONCE(peer->backoff); + u32 backoff = READ_ONCE(call->backoff); - timo_us = peer->rto_us; + timo_us = call->rto_us; timo_us <<= backoff; if (retrans && timo_us * 2 <= RXRPC_RTO_MAX) - WRITE_ONCE(peer->backoff, backoff + 1); + WRITE_ONCE(call->backoff, backoff + 1); if (timo_us < 1) timo_us = 1; @@ -198,10 +198,11 @@ ktime_t rxrpc_get_rto_backoff(struct rxr return ns_to_ktime(timo_us * NSEC_PER_USEC); } -void rxrpc_peer_init_rtt(struct rxrpc_peer *peer) +void rxrpc_call_init_rtt(struct rxrpc_call *call) { - peer->rto_us = RXRPC_TIMEOUT_INIT; - peer->mdev_us = RXRPC_TIMEOUT_INIT; - peer->backoff = 0; - //minmax_reset(&peer->rtt_min, rxrpc_jiffies32, ~0U); + call->rtt_last_req = KTIME_MIN; + call->rto_us = RXRPC_TIMEOUT_INIT; + call->mdev_us = RXRPC_TIMEOUT_INIT; + call->backoff = 0; + //minmax_reset(&call->rtt_min, rxrpc_jiffies32, ~0U); } --- a/net/rxrpc/sendmsg.c +++ b/net/rxrpc/sendmsg.c @@ -133,7 +133,7 @@ static int rxrpc_wait_for_tx_window_wait rxrpc_seq_t tx_start, tx_win; signed long rtt, timeout; - rtt = READ_ONCE(call->peer->srtt_us) >> 3; + rtt = READ_ONCE(call->srtt_us) >> 3; rtt = usecs_to_jiffies(rtt) * 2; if (rtt < 2) rtt = 2;