tcp: Add the TCP Prague congestion control module

TCP Prague is a variant of DCTCP aimed to be used over the public
Internet. As such, it implements a couple of safety requirements and
uses the ECT(1) ECN codepoint, as defined in the L4S experiment.

This base version enhances DCTCP by:
- Requesting the use of ECT(1) for all packets.
- Supporting various RTT independence modes
- Updating the EWMA to support low marking probabilities
- Improved the accuracy of the cwnd manipulations, notably preserving
  remainders
- Switching to unsaturated marking during additive increase
- Controlling pacing/GSO sizing

See:
https://tools.ietf.org/html/draft-ietf-tsvwg-ecn-l4s-id-09#page-23
https://tools.ietf.org/html/draft-ietf-tsvwg-l4s-arch-05
https://arxiv.org/abs/1904.07605

Signed-off-by: Chia-Yu Chang <[email protected]>
Signed-off-by: Olivier Tilmans <[email protected]>
Signed-off-by: Koen De Schepper <[email protected]>
Signed-off-by: Bob Briscoe <[email protected]>
Signed-off-by: Ilpo Järvinen <[email protected]>

include/uapi/linux/inet_diag.h

@@ -161,6 +161,7 @@ enum {
 	INET_DIAG_SK_BPF_STORAGES,
 	INET_DIAG_CGROUP_ID,
 	INET_DIAG_SOCKOPT,
+	INET_DIAG_PRAGUEINFO,
 	__INET_DIAG_MAX,
 };
 
@@ -231,9 +232,22 @@ struct tcp_bbr_info {
 	__u32	bbr_cwnd_gain;		/* cwnd gain shifted left 8 bits */
 };
 
+/* INET_DIAG_PRAGUEINFO */
+
+struct tcp_prague_info {
+	__u64	prague_alpha;
+	__u64	prague_frac_cwnd;
+	__u64	prague_rate_bytes;
+	__u32	prague_max_burst;
+	__u32	prague_round;
+	__u32	prague_rtt_target;
+	bool	prague_enabled;
+};
+
 union tcp_cc_info {
 	struct tcpvegas_info	vegas;
 	struct tcp_dctcp_info	dctcp;
+	struct tcp_prague_info	prague;
 	struct tcp_bbr_info	bbr;
 };
 #endif /* _UAPI_INET_DIAG_H_ */

net/ipv4/Kconfig

@@ -679,6 +679,15 @@ config TCP_CONG_BBR
 	  AQM schemes that do not provide a delay signal. It requires the fq
 	  ("Fair Queue") pacing packet scheduler.
 
+config TCP_CONG_PRAGUE
+	tristate "TCP Prague"
+	default n
+	help
+	  TCP Prague is an enhancement for DCTCP to make DCTCP congestion
+	  control deployable into general Internet.
+
+	  To be completed ...
+
 choice
 	prompt "Default TCP congestion control"
 	default DEFAULT_CUBIC
@@ -716,6 +725,9 @@ choice
 	config DEFAULT_BBR
 		bool "BBR" if TCP_CONG_BBR=y
 
+	config DEFAULT_PRAGUE
+		bool "Prague" if TCP_CONG_PRAGUE=y
+
 	config DEFAULT_RENO
 		bool "Reno"
 endchoice
@@ -740,6 +752,7 @@ config DEFAULT_TCP_CONG
 	default "dctcp" if DEFAULT_DCTCP
 	default "cdg" if DEFAULT_CDG
 	default "bbr" if DEFAULT_BBR
+	default "prague" if DEFAULT_PRAGUE
 	default "cubic"
 
 config TCP_SIGPOOL

net/ipv4/Makefile

@@ -60,6 +60,7 @@ obj-$(CONFIG_TCP_CONG_SCALABLE) += tcp_scalable.o
 obj-$(CONFIG_TCP_CONG_LP) += tcp_lp.o
 obj-$(CONFIG_TCP_CONG_YEAH) += tcp_yeah.o
 obj-$(CONFIG_TCP_CONG_ILLINOIS) += tcp_illinois.o
+obj-$(CONFIG_TCP_CONG_PRAGUE) += tcp_prague.o
 obj-$(CONFIG_TCP_SIGPOOL) += tcp_sigpool.o
 obj-$(CONFIG_NET_SOCK_MSG) += tcp_bpf.o
 obj-$(CONFIG_BPF_SYSCALL) += udp_bpf.o