From: Neal Cardwell Date: Sun, 15 Jun 2025 00:14:33 +0000 (-0400) Subject: tcp: remove obsolete and unused RFC3517/RFC6675 loss recovery code X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=1c120191dcec510cc17d587ece48a7ae875a90c5;p=thirdparty%2Flinux.git tcp: remove obsolete and unused RFC3517/RFC6675 loss recovery code RACK-TLP loss detection has been enabled as the default loss detection algorithm for Linux TCP since 2018, in: commit b38a51fec1c1 ("tcp: disable RFC6675 loss detection") In case users ran into unexpected bugs or performance regressions, that commit allowed Linux system administrators to revert to using RFC3517/RFC6675 loss recovery by setting net.ipv4.tcp_recovery to 0. In the seven years since 2018, our team has not heard reports of anyone reverting Linux TCP to use RFC3517/RFC6675 loss recovery, and we can't find any record in web searches of such a revert. RACK-TLP was published as a standards-track RFC, RFC8985, in February 2021. Several other major TCP implementations have default-enabled RACK-TLP at this point as well. RACK-TLP offers several significant performance advantages over RFC3517/RFC6675 loss recovery, including much better performance in the common cases of tail drops, lost retransmissions, and reordering. It is now time to remove the obsolete and unused RFC3517/RFC6675 loss recovery code. This will allow a substantial simplification of the Linux TCP code base, and removes 12 bytes of state in every tcp_sock for 64-bit machines (8 bytes on 32-bit machines). To arrange the commits in reasonable sizes, this patch series is split into 3 commits. The following 2 commits remove bookkeeping state and code that is no longer needed after this removal of RFC3517/RFC6675 loss recovery. Suggested-by: Yuchung Cheng Signed-off-by: Neal Cardwell Reviewed-by: Yuchung Cheng Reviewed-by: Eric Dumazet Link: https://patch.msgid.link/20250615001435.2390793-2-ncardwell.sw@gmail.com Signed-off-by: Jakub Kicinski --- diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst index 0f1251cce3149..466bc3f5186ee 100644 --- a/Documentation/networking/ip-sysctl.rst +++ b/Documentation/networking/ip-sysctl.rst @@ -645,9 +645,11 @@ tcp_recovery - INTEGER features. ========= ============================================================= - RACK: 0x1 enables the RACK loss detection for fast detection of lost - retransmissions and tail drops. It also subsumes and disables - RFC6675 recovery for SACK connections. + RACK: 0x1 enables RACK loss detection, for fast detection of lost + retransmissions and tail drops, and resilience to + reordering. currently, setting this bit to 0 has no + effect, since RACK is the only supported loss detection + algorithm. RACK: 0x2 makes RACK's reordering window static (min_rtt/4). diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 8ec92dec321a9..dc234d3854aa4 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -2151,12 +2151,6 @@ static inline void tcp_init_undo(struct tcp_sock *tp) tp->undo_retrans = -1; } -static bool tcp_is_rack(const struct sock *sk) -{ - return READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_recovery) & - TCP_RACK_LOSS_DETECTION; -} - /* If we detect SACK reneging, forget all SACK information * and reset tags completely, otherwise preserve SACKs. If receiver * dropped its ofo queue, we will know this due to reneging detection. @@ -2182,8 +2176,7 @@ static void tcp_timeout_mark_lost(struct sock *sk) skb_rbtree_walk_from(skb) { if (is_reneg) TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_ACKED; - else if (tcp_is_rack(sk) && skb != head && - tcp_rack_skb_timeout(tp, skb, 0) > 0) + else if (skb != head && tcp_rack_skb_timeout(tp, skb, 0) > 0) continue; /* Don't mark recently sent ones lost yet */ tcp_mark_skb_lost(sk, skb); } @@ -2264,22 +2257,6 @@ static bool tcp_check_sack_reneging(struct sock *sk, int *ack_flag) return false; } -/* Heurestics to calculate number of duplicate ACKs. There's no dupACKs - * counter when SACK is enabled (without SACK, sacked_out is used for - * that purpose). - * - * With reordering, holes may still be in flight, so RFC3517 recovery - * uses pure sacked_out (total number of SACKed segments) even though - * it violates the RFC that uses duplicate ACKs, often these are equal - * but when e.g. out-of-window ACKs or packet duplication occurs, - * they differ. Since neither occurs due to loss, TCP should really - * ignore them. - */ -static inline int tcp_dupack_heuristics(const struct tcp_sock *tp) -{ - return tp->sacked_out + 1; -} - /* Linux NewReno/SACK/ECN state machine. * -------------------------------------- * @@ -2332,13 +2309,7 @@ static inline int tcp_dupack_heuristics(const struct tcp_sock *tp) * * If the receiver supports SACK: * - * RFC6675/3517: It is the conventional algorithm. A packet is - * considered lost if the number of higher sequence packets - * SACKed is greater than or equal the DUPACK thoreshold - * (reordering). This is implemented in tcp_mark_head_lost and - * tcp_update_scoreboard. - * - * RACK (draft-ietf-tcpm-rack-01): it is a newer algorithm + * RACK (RFC8985): RACK is a newer loss detection algorithm * (2017-) that checks timing instead of counting DUPACKs. * Essentially a packet is considered lost if it's not S/ACKed * after RTT + reordering_window, where both metrics are @@ -2353,8 +2324,8 @@ static inline int tcp_dupack_heuristics(const struct tcp_sock *tp) * is lost (NewReno). This heuristics are the same in NewReno * and SACK. * - * Really tricky (and requiring careful tuning) part of algorithm - * is hidden in functions tcp_time_to_recover() and tcp_xmit_retransmit_queue(). + * The really tricky (and requiring careful tuning) part of the algorithm + * is hidden in the RACK code in tcp_recovery.c and tcp_xmit_retransmit_queue(). * The first determines the moment _when_ we should reduce CWND and, * hence, slow down forward transmission. In fact, it determines the moment * when we decide that hole is caused by loss, rather than by a reorder. @@ -2381,79 +2352,8 @@ static bool tcp_time_to_recover(struct sock *sk, int flag) { struct tcp_sock *tp = tcp_sk(sk); - /* Trick#1: The loss is proven. */ - if (tp->lost_out) - return true; - - /* Not-A-Trick#2 : Classic rule... */ - if (!tcp_is_rack(sk) && tcp_dupack_heuristics(tp) > tp->reordering) - return true; - - return false; -} - -/* Detect loss in event "A" above by marking head of queue up as lost. - * For RFC3517 SACK, a segment is considered lost if it - * has at least tp->reordering SACKed seqments above it; "packets" refers to - * the maximum SACKed segments to pass before reaching this limit. - */ -static void tcp_mark_head_lost(struct sock *sk, int packets, int mark_head) -{ - struct tcp_sock *tp = tcp_sk(sk); - struct sk_buff *skb; - int cnt; - /* Use SACK to deduce losses of new sequences sent during recovery */ - const u32 loss_high = tp->snd_nxt; - - WARN_ON(packets > tp->packets_out); - skb = tp->lost_skb_hint; - if (skb) { - /* Head already handled? */ - if (mark_head && after(TCP_SKB_CB(skb)->seq, tp->snd_una)) - return; - cnt = tp->lost_cnt_hint; - } else { - skb = tcp_rtx_queue_head(sk); - cnt = 0; - } - - skb_rbtree_walk_from(skb) { - /* TODO: do this better */ - /* this is not the most efficient way to do this... */ - tp->lost_skb_hint = skb; - tp->lost_cnt_hint = cnt; - - if (after(TCP_SKB_CB(skb)->end_seq, loss_high)) - break; - - if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED) - cnt += tcp_skb_pcount(skb); - - if (cnt > packets) - break; - - if (!(TCP_SKB_CB(skb)->sacked & TCPCB_LOST)) - tcp_mark_skb_lost(sk, skb); - - if (mark_head) - break; - } - tcp_verify_left_out(tp); -} - -/* Account newly detected lost packet(s) */ - -static void tcp_update_scoreboard(struct sock *sk, int fast_rexmit) -{ - struct tcp_sock *tp = tcp_sk(sk); - - if (tcp_is_sack(tp)) { - int sacked_upto = tp->sacked_out - tp->reordering; - if (sacked_upto >= 0) - tcp_mark_head_lost(sk, sacked_upto, 0); - else if (fast_rexmit) - tcp_mark_head_lost(sk, 1, 1); - } + /* Has loss detection marked at least one packet lost? */ + return tp->lost_out != 0; } static bool tcp_tsopt_ecr_before(const struct tcp_sock *tp, u32 when) @@ -2990,17 +2890,8 @@ static void tcp_process_loss(struct sock *sk, int flag, int num_dupack, *rexmit = REXMIT_LOST; } -static bool tcp_force_fast_retransmit(struct sock *sk) -{ - struct tcp_sock *tp = tcp_sk(sk); - - return after(tcp_highest_sack_seq(tp), - tp->snd_una + tp->reordering * tp->mss_cache); -} - /* Undo during fast recovery after partial ACK. */ -static bool tcp_try_undo_partial(struct sock *sk, u32 prior_snd_una, - bool *do_lost) +static bool tcp_try_undo_partial(struct sock *sk, u32 prior_snd_una) { struct tcp_sock *tp = tcp_sk(sk); @@ -3025,9 +2916,6 @@ static bool tcp_try_undo_partial(struct sock *sk, u32 prior_snd_una, tcp_undo_cwnd_reduction(sk, true); NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPPARTIALUNDO); tcp_try_keep_open(sk); - } else { - /* Partial ACK arrived. Force fast retransmit. */ - *do_lost = tcp_force_fast_retransmit(sk); } return false; } @@ -3041,7 +2929,7 @@ static void tcp_identify_packet_loss(struct sock *sk, int *ack_flag) if (unlikely(tcp_is_reno(tp))) { tcp_newreno_mark_lost(sk, *ack_flag & FLAG_SND_UNA_ADVANCED); - } else if (tcp_is_rack(sk)) { + } else { u32 prior_retrans = tp->retrans_out; if (tcp_rack_mark_lost(sk)) @@ -3068,10 +2956,8 @@ static void tcp_fastretrans_alert(struct sock *sk, const u32 prior_snd_una, { struct inet_connection_sock *icsk = inet_csk(sk); struct tcp_sock *tp = tcp_sk(sk); - int fast_rexmit = 0, flag = *ack_flag; + int flag = *ack_flag; bool ece_ack = flag & FLAG_ECE; - bool do_lost = num_dupack || ((flag & FLAG_DATA_SACKED) && - tcp_force_fast_retransmit(sk)); if (!tp->packets_out && tp->sacked_out) tp->sacked_out = 0; @@ -3120,7 +3006,7 @@ static void tcp_fastretrans_alert(struct sock *sk, const u32 prior_snd_una, if (!(flag & FLAG_SND_UNA_ADVANCED)) { if (tcp_is_reno(tp)) tcp_add_reno_sack(sk, num_dupack, ece_ack); - } else if (tcp_try_undo_partial(sk, prior_snd_una, &do_lost)) + } else if (tcp_try_undo_partial(sk, prior_snd_una)) return; if (tcp_try_undo_dsack(sk)) @@ -3175,11 +3061,8 @@ static void tcp_fastretrans_alert(struct sock *sk, const u32 prior_snd_una, /* Otherwise enter Recovery state */ tcp_enter_recovery(sk, ece_ack); - fast_rexmit = 1; } - if (!tcp_is_rack(sk) && do_lost) - tcp_update_scoreboard(sk, fast_rexmit); *rexmit = REXMIT_LOST; }