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Merge branch 'tcp-remove-obsolete-rfc3517-rfc6675-code'

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Neal Cardwell says:

====================
tcp: remove obsolete RFC3517/RFC6675 code

RACK-TLP loss detection has been enabled as the default loss detection
algorithm for Linux TCP since 2018, in:

 commit b38a51fec1 ("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:

(1) Removes the core RFC3517/RFC6675 logic.

(2) Removes the RFC3517/RFC6675 hint state and the first layer of logic that
    updates that state.

(3) Removes the emptied-out tcp_clear_retrans_hints_partial() helper function
    and all of its call sites.
====================

Link: https://patch.msgid.link/20250615001435.2390793-1-ncardwell.sw@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Jakub Kicinski 2025-06-17 16:19:04 -07:00
commit 4b7da7341b
7 changed files with 16 additions and 170 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
Documentation/networking/ip-sysctl.rst
View File

@ -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).

2
Documentation/networking/net_cachelines/tcp_sock.rst
View File

@ -115,7 +115,6 @@ u32 lost_out read_mostly read_m
u32 sacked_out read_mostly read_mostly tcp_left_out(tx);tcp_packets_in_flight(tx/rx);tcp_clean_rtx_queue(rx)
struct hrtimer pacing_timer
struct hrtimer compressed_ack_timer
struct sk_buff* lost_skb_hint read_mostly tcp_clean_rtx_queue
struct sk_buff* retransmit_skb_hint read_mostly tcp_clean_rtx_queue
struct rb_root out_of_order_queue read_mostly tcp_data_queue,tcp_fast_path_check
struct sk_buff* ooo_last_skb
@ -123,7 +122,6 @@ struct tcp_sack_block[1] duplicate_sack
struct tcp_sack_block[4] selective_acks
struct tcp_sack_block[4] recv_sack_cache
struct sk_buff* highest_sack read_write tcp_event_new_data_sent
int lost_cnt_hint
u32 prior_ssthresh
u32 high_seq
u32 retrans_stamp

3
include/linux/tcp.h
View File

@ -208,7 +208,6 @@ struct tcp_sock {
u32 notsent_lowat; /* TCP_NOTSENT_LOWAT */
u16 gso_segs; /* Max number of segs per GSO packet */
/* from STCP, retrans queue hinting */
struct sk_buff *lost_skb_hint;
struct sk_buff *retransmit_skb_hint;
__cacheline_group_end(tcp_sock_read_tx);
@ -419,8 +418,6 @@ struct tcp_sock {
struct tcp_sack_block recv_sack_cache[4];
int lost_cnt_hint;
u32 prior_ssthresh; /* ssthresh saved at recovery start */
u32 high_seq; /* snd_nxt at onset of congestion */

6
include/net/tcp.h
View File

@ -1811,14 +1811,8 @@ static inline void tcp_mib_init(struct net *net)
}
/* from STCP */
static inline void tcp_clear_retrans_hints_partial(struct tcp_sock *tp)
{
tp->lost_skb_hint = NULL;
}
static inline void tcp_clear_all_retrans_hints(struct tcp_sock *tp)
{
tcp_clear_retrans_hints_partial(tp);
tp->retransmit_skb_hint = NULL;
}

3
net/ipv4/tcp.c
View File

@ -5053,9 +5053,8 @@ static void __init tcp_struct_check(void)
CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_read_tx, reordering);
CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_read_tx, notsent_lowat);
CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_read_tx, gso_segs);
CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_read_tx, lost_skb_hint);
CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_read_tx, retransmit_skb_hint);
CACHELINE_ASSERT_GROUP_SIZE(struct tcp_sock, tcp_sock_read_tx, 40);
CACHELINE_ASSERT_GROUP_SIZE(struct tcp_sock, tcp_sock_read_tx, 32);
/* TXRX read-mostly hotpath cache lines */
CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_read_txrx, tsoffset);

158
net/ipv4/tcp_input.c
View File

@ -1451,11 +1451,6 @@ static u8 tcp_sacktag_one(struct sock *sk,
tp->sacked_out += pcount;
/* Out-of-order packets delivered */
state->sack_delivered += pcount;
/* Lost marker hint past SACKed? Tweak RFC3517 cnt */
if (tp->lost_skb_hint &&
before(start_seq, TCP_SKB_CB(tp->lost_skb_hint)->seq))
tp->lost_cnt_hint += pcount;
}
/* D-SACK. We can detect redundant retransmission in S|R and plain R
@ -1496,9 +1491,6 @@ static bool tcp_shifted_skb(struct sock *sk, struct sk_buff *prev,
tcp_skb_timestamp_us(skb));
tcp_rate_skb_delivered(sk, skb, state->rate);
if (skb == tp->lost_skb_hint)
tp->lost_cnt_hint += pcount;
TCP_SKB_CB(prev)->end_seq += shifted;
TCP_SKB_CB(skb)->seq += shifted;
@ -1531,10 +1523,6 @@ static bool tcp_shifted_skb(struct sock *sk, struct sk_buff *prev,
if (skb == tp->retransmit_skb_hint)
tp->retransmit_skb_hint = prev;
if (skb == tp->lost_skb_hint) {
tp->lost_skb_hint = prev;
tp->lost_cnt_hint -= tcp_skb_pcount(prev);
}
TCP_SKB_CB(prev)->tcp_flags |= TCP_SKB_CB(skb)->tcp_flags;
TCP_SKB_CB(prev)->eor = TCP_SKB_CB(skb)->eor;
@ -2151,12 +2139,6 @@ 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 +2164,7 @@ 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 +2245,6 @@ 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 +2297,7 @@ 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 +2312,8 @@ 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 +2340,8 @@ 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)
@ -2881,8 +2769,6 @@ void tcp_simple_retransmit(struct sock *sk)
tcp_mark_skb_lost(sk, skb);
}
tcp_clear_retrans_hints_partial(tp);
if (!tp->lost_out)
return;
@ -2990,17 +2876,8 @@ 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 +2902,6 @@ 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 +2915,7 @@ 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 +2942,8 @@ 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 +2992,7 @@ 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 +3047,8 @@ 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;
}
@ -3435,8 +3304,6 @@ static int tcp_clean_rtx_queue(struct sock *sk, const struct sk_buff *ack_skb,
next = skb_rb_next(skb);
if (unlikely(skb == tp->retransmit_skb_hint))
tp->retransmit_skb_hint = NULL;
if (unlikely(skb == tp->lost_skb_hint))
tp->lost_skb_hint = NULL;
tcp_highest_sack_replace(sk, skb, next);
tcp_rtx_queue_unlink_and_free(skb, sk);
}
@ -3494,14 +3361,9 @@ static int tcp_clean_rtx_queue(struct sock *sk, const struct sk_buff *ack_skb,
if (flag & FLAG_RETRANS_DATA_ACKED)
flag &= ~FLAG_ORIG_SACK_ACKED;
} else {
int delta;
/* Non-retransmitted hole got filled? That's reordering */
if (before(reord, prior_fack))
tcp_check_sack_reordering(sk, reord, 0);
delta = prior_sacked - tp->sacked_out;
tp->lost_cnt_hint -= min(tp->lost_cnt_hint, delta);
}
} else if (skb && rtt_update && sack_rtt_us >= 0 &&
sack_rtt_us > tcp_stamp_us_delta(tp->tcp_mstamp,

6
net/ipv4/tcp_output.c
View File

@ -1554,11 +1554,6 @@ static void tcp_adjust_pcount(struct sock *sk, const struct sk_buff *skb, int de
if (tcp_is_reno(tp) && decr > 0)
tp->sacked_out -= min_t(u32, tp->sacked_out, decr);
if (tp->lost_skb_hint &&
before(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(tp->lost_skb_hint)->seq) &&
(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
tp->lost_cnt_hint -= decr;
tcp_verify_left_out(tp);
}
@ -3252,7 +3247,6 @@ static bool tcp_collapse_retrans(struct sock *sk, struct sk_buff *skb)
TCP_SKB_CB(skb)->eor = TCP_SKB_CB(next_skb)->eor;
/* changed transmit queue under us so clear hints */
tcp_clear_retrans_hints_partial(tp);
if (next_skb == tp->retransmit_skb_hint)
tp->retransmit_skb_hint = skb;