148  * Transmit the SYN,ACK fewer times than TCP_MAXRXTSHIFT specifies.
211     "Limit on SYN/ACK retransmissions");
498 		 * gone by the time we resend the SYN/ACK.  We do  in syncache_timer()
501 		 * host does the SYN/ACK->ACK.  in syncache_timer()
524 			    "retransmitting (%u) SYN|ACK\n",  in syncache_timer()
607  * If required send a challenge ACK.
627 	 * done and no SYN|ACK retransmissions will happen.  in syncache_chkrst()
683 				    "%s; %s: Our SYN|ACK was rejected, "  in syncache_chkrst()
690 			/* Send challenge ACK. */  in syncache_chkrst()
694 				    "sending challenge ACK\n",  in syncache_chkrst()
959 	 * If the SYN,ACK was retransmitted, indicate that CWND to be  in syncache_socket()
961 	 * NB: sc_rxmits counts all SYN,ACK transmits, not just retransmits.  in syncache_socket()
1042  * This function gets called when we receive an ACK for a
1063 	    ("%s: can handle only ACK", __func__));  in syncache_expand()
1086 		 * There is no syncache entry, so see if this ACK is  in syncache_expand()
1100 				log(LOG_DEBUG, "%s; %s: Spurious ACK, "  in syncache_expand()
1109 				log(LOG_DEBUG, "%s; %s: Spurious ACK, "  in syncache_expand()
1126 		/* If received ACK has MD5 signature, check it. */  in syncache_expand()
1130 			/* Drop the ACK. */  in syncache_expand()
1150 		 * received ACK has signature and it is correct.  in syncache_expand()
1151 		 * If not, drop the ACK and leave sc entry in th cache,  in syncache_expand()
1185 		 * XXXMT: RFC 7323 also requires to send an ACK.  in syncache_expand()
1204 		 * If timestamps were not negotiated during SYN/ACK and a  in syncache_expand()
1221 		 * If timestamps were negotiated during SYN/ACK and a  in syncache_expand()
1262 	 * ACK must match our initial sequence number + 1 (the SYN|ACK).  in syncache_expand()
1266 			log(LOG_DEBUG, "%s; %s: ACK %u != ISS+1 %u, segment "  in syncache_expand()
1342  *	<SEQ=ISS><ACK=RCV_NXT><CTL=SYN,ACK>
1345  * IMPORTANT NOTE: We do _NOT_ ACK data that might accompany the SYN.
1356  * ACK timer expires, whichever comes first.
1494 	 * If we do, resend the SYN,ACK, and reset the retransmit timer.  in syncache_add()
1550 		/* Retransmit SYN|ACK and reset retransmit count. */  in syncache_add()
1553 			    "resetting timer and retransmitting SYN|ACK\n",  in syncache_add()
1687 			 * or <SYN,ACK>) segment itself is never scaled.  in syncache_add()
1701 	 * with the SYN+ACK.  in syncache_add()
1780  * Send SYN|ACK or ACK to the peer.  Either in response to a peer's segment,
1781  * i.e. m0 != NULL, or upon 3WHS ACK timeout, i.e. m0 == NULL.
1976 	 * our SYN|ACK.  ip6_output() and ip_output() will not assign flowid  in syncache_respond()
1977 	 * to SYN|ACK due to lack of inp here.  in syncache_respond()
2051  * connection setup within the SYN|ACK that we send back.  That way we
2052  * can avoid keeping any local state until the ACK to our SYN|ACK returns
2060  * of our SYN|ACK.  The MAC can be recomputed when the ACK to our SYN|ACK
2061  * returns and signifies a legitimate connection if it matches the ACK.
2084  *   the ACK has them.
2106  * Vector 2: Collision attack on the MAC of a single ACK.  With a 24 bit MAC
2261 	tcp_seq ack, seq;  in syncookie_lookup()  local
2266 	 * Pull information out of SYN-ACK/ACK and revert sequence number  in syncookie_lookup()
2269 	ack = th->th_ack - 1;  in syncookie_lookup()
2276 	cookie.cookie = (ack & 0xff) ^ (ack >> 24);  in syncookie_lookup()
2283 	/* The recomputed hash matches the ACK if this was a genuine cookie. */  in syncookie_lookup()
2284 	if ((ack & ~0xff) != (hash & ~0xff))  in syncookie_lookup()
2293 	sc->sc_iss = ack;  in syncookie_lookup()