sctp: add the constants/variables and states and some APIs for transport

These are 4 constants described in rfc8899#section-5.1.2:

MAX_PROBES, MIN_PLPMTU, MAX_PLPMTU, BASE_PLPMTU;

And 2 variables described in rfc8899#section-5.1.3:

PROBED_SIZE, PROBE_COUNT;

And 5 states described in rfc8899#section-5.2:

DISABLED, BASE, SEARCH, SEARCH_COMPLETE, ERROR;

And these 4 APIs are used to reset/update PLPMTUD, check if PLPMTUD is
enabled, and calculate the additional headers length for a transport.

Note the member 'probe_high' in transport will be set to the probe
size when a probe fails with this probe size in the next patches.

Signed-off-by: Xin Long <[e-mail]>
Acked-by: Marcelo Ricardo Leitner <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/sctp/constants.h

···
 	SCTP_SS_CLOSING        = TCP_CLOSE_WAIT,
 };
 
+enum sctp_plpmtud_state {
+	SCTP_PL_DISABLED,
+	SCTP_PL_BASE,
+	SCTP_PL_SEARCH,
+	SCTP_PL_COMPLETE,
+	SCTP_PL_ERROR,
+};
+
+#define	SCTP_BASE_PLPMTU	1200
+#define	SCTP_MAX_PLPMTU		9000
+#define	SCTP_MIN_PLPMTU		512
+
+#define	SCTP_MAX_PROBES		3
+
+#define SCTP_PL_BIG_STEP	32
+#define SCTP_PL_MIN_STEP	4
+
 /* These functions map various type to printable names.  */
 const char *sctp_cname(const union sctp_subtype id);	/* chunk types */
 const char *sctp_oname(const union sctp_subtype id);	/* other events */

include/net/sctp/sctp.h

···
 /* Calculate max payload size given a MTU, or the total overhead if
  * given MTU is zero
  */
-static inline __u32 sctp_mtu_payload(const struct sctp_sock *sp,
-				     __u32 mtu, __u32 extra)
+static inline __u32 __sctp_mtu_payload(const struct sctp_sock *sp,
+				       const struct sctp_transport *t,
+				       __u32 mtu, __u32 extra)
 {
 	__u32 overhead = sizeof(struct sctphdr) + extra;
 
 	if (sp) {
 		overhead += sp->pf->af->net_header_len;
-		if (sp->udp_port)
+		if (sp->udp_port && (!t || t->encap_port))
 			overhead += sizeof(struct udphdr);
 	} else {
 		overhead += sizeof(struct ipv6hdr);
···
 		mtu = overhead;
 
 	return mtu ? mtu - overhead : overhead;
+}
+
+static inline __u32 sctp_mtu_payload(const struct sctp_sock *sp,
+				     __u32 mtu, __u32 extra)
+{
+	return __sctp_mtu_payload(sp, NULL, mtu, extra);
 }
 
 static inline __u32 sctp_dst_mtu(const struct dst_entry *dst)
···
 static inline __u32 sctp_min_frag_point(struct sctp_sock *sp, __u16 datasize)
 {
 	return sctp_mtu_payload(sp, SCTP_DEFAULT_MINSEGMENT, datasize);
+}
+
+static inline int sctp_transport_pl_hlen(struct sctp_transport *t)
+{
+	return __sctp_mtu_payload(sctp_sk(t->asoc->base.sk), t, 0, 0);
+}
+
+static inline void sctp_transport_pl_reset(struct sctp_transport *t)
+{
+	if (t->probe_interval && (t->param_flags & SPP_PMTUD_ENABLE) &&
+	    (t->state == SCTP_ACTIVE || t->state == SCTP_UNKNOWN)) {
+		if (t->pl.state == SCTP_PL_DISABLED) {
+			t->pl.state = SCTP_PL_BASE;
+			t->pl.pmtu = SCTP_BASE_PLPMTU;
+			t->pl.probe_size = SCTP_BASE_PLPMTU;
+		}
+	} else {
+		if (t->pl.state != SCTP_PL_DISABLED)
+			t->pl.state = SCTP_PL_DISABLED;
+	}
+}
+
+static inline void sctp_transport_pl_update(struct sctp_transport *t)
+{
+	if (t->pl.state == SCTP_PL_DISABLED)
+		return;
+
+	t->pl.state = SCTP_PL_BASE;
+	t->pl.pmtu = SCTP_BASE_PLPMTU;
+	t->pl.probe_size = SCTP_BASE_PLPMTU;
+}
+
+static inline bool sctp_transport_pl_enabled(struct sctp_transport *t)
+{
+	return t->pl.state != SCTP_PL_DISABLED;
 }
 
 static inline bool sctp_newsk_ready(const struct sock *sk)

include/net/sctp/structs.h

···
 		char cacc_saw_newack;
 	} cacc;
 
+	struct {
+		__u16 pmtu;
+		__u16 probe_size;
+		__u16 probe_high;
+		__u8 probe_count;
+		__u8 state;
+	} pl; /* plpmtud related */
+
 	/* 64-bit random number sent with heartbeat. */
 	__u64 hb_nonce;