tcp: SACK Lost Retransmission Detection (LRD)

Recover from excessive losses without reverting to a
retransmission timeout (RTO). Disabled by default, enable
with sysctl net.inet.tcp.do_lrd=1

Reviewed By: #transport, rrs, tuexen, #manpages
Sponsored by: Netapp, Inc.
Differential Revision: https://reviews.freebsd.org/D28931

newlib/libc/sys/rtems/include/netinet/tcp.h

@@ -199,6 +199,7 @@ struct tcphdr {
 #define TCP_PROC_ACCOUNTING 76	/* Do accounting on tcp cpu usage and counts */
 #define TCP_USE_CMP_ACKS 77 	/* The transport can handle the Compressed mbuf acks */
 #define	TCP_PERF_INFO	78	/* retrieve accounting counters */
+#define	TCP_LRD		79	/* toggle Lost Retransmission Detection for A/B testing */
 #define	TCP_KEEPINIT	128	/* N, time to establish connection */
 #define	TCP_KEEPIDLE	256	/* L,N,X start keeplives after this period */
 #define	TCP_KEEPINTVL	512	/* L,N interval between keepalives */