diff --git a/sys/netinet/tcp_syncache.h b/sys/netinet/tcp_syncache.h
index 520b484b9b0b..fb9a6c64c452 100644
--- a/sys/netinet/tcp_syncache.h
+++ b/sys/netinet/tcp_syncache.h
@@ -75,6 +75,7 @@ struct syncache {
 	struct label	*sc_label;		/* MAC label reference */
 	struct ucred	*sc_cred;		/* cred cache for jail checks */
 
+	void		*sc_pspare;		/* TCP_SIGNATURE */
 	u_int32_t	sc_spare[2];		/* UTO */
 };
 
diff --git a/sys/netinet/tcp_var.h b/sys/netinet/tcp_var.h
index aaaa4a44e1fd..e3197e5406e2 100644
--- a/sys/netinet/tcp_var.h
+++ b/sys/netinet/tcp_var.h
@@ -211,7 +211,7 @@ struct tcpcb {
 	u_int	t_tsomax;		/* tso burst length limit */
 
 	uint32_t t_ispare[8];		/* 5 UTO, 3 TBD */
-	void	*t_pspare2[4];		/* 4 TBD */
+	void	*t_pspare2[4];		/* 1 TCP_SIGNATURE, 3 TBD */
 	uint64_t _pad[6];		/* 6 TBD (1-2 CC/RTT?) */
 };
 
@@ -353,6 +353,8 @@ struct tcptw {
 	u_int		t_starttime;
 	int		tw_time;
 	TAILQ_ENTRY(tcptw) tw_2msl;
+	void		*tw_pspare;	/* TCP_SIGNATURE */
+	u_int		*tw_spare;	/* TCP_SIGNATURE */
 };
 
 #define	intotcpcb(ip)	((struct tcpcb *)(ip)->inp_ppcb)