-// Receive incoming data. Check if it contains a complete record, if so, handle it.
-bool sptps_receive_data(sptps_t *s, const char *data, size_t len) {
- while(len) {
- // First read the 2 length bytes.
- if(s->buflen < 6) {
- size_t toread = 6 - s->buflen;
- if(toread > len)
- toread = len;
-
- memcpy(s->inbuf + s->buflen, data, toread);
-
- s->buflen += toread;
- len -= toread;
- data += toread;
-
- // Exit early if we don't have the full length.
- if(s->buflen < 6)
- return true;
-
- // Decrypt the length bytes
-
- if(s->instate) {
- if(!cipher_counter_xor(&s->incipher, s->inbuf + 4, 2, &s->reclen))
- return false;
- } else {
- memcpy(&s->reclen, s->inbuf + 4, 2);
+static bool sptps_check_seqno(sptps_t *s, uint32_t seqno, bool update_state) {
+ // Replay protection using a sliding window of configurable size.
+ // s->inseqno is expected sequence number
+ // seqno is received sequence number
+ // s->late[] is a circular buffer, a 1 bit means a packet has not been received yet
+ // The circular buffer contains bits for sequence numbers from s->inseqno - s->replaywin * 8 to (but excluding) s->inseqno.
+ if(s->replaywin) {
+ if(seqno != s->inseqno) {
+ if(seqno >= s->inseqno + s->replaywin * 8) {
+ // Prevent packets that jump far ahead of the queue from causing many others to be dropped.
+ bool farfuture = s->farfuture < s->replaywin >> 2;
+
+ if(update_state) {
+ s->farfuture++;
+ }
+
+ if(farfuture) {
+ return update_state ? error(s, EIO, "Packet is %d seqs in the future, dropped (%u)\n", seqno - s->inseqno, s->farfuture) : false;
+ }
+
+ // Unless we have seen lots of them, in which case we consider the others lost.
+ if(update_state) {
+ warning(s, "Lost %d packets\n", seqno - s->inseqno);
+ }
+
+ if(update_state) {
+ // Mark all packets in the replay window as being late.
+ memset(s->late, 255, s->replaywin);
+ }
+ } else if(seqno < s->inseqno) {
+ // If the sequence number is farther in the past than the bitmap goes, or if the packet was already received, drop it.
+ if((s->inseqno >= s->replaywin * 8 && seqno < s->inseqno - s->replaywin * 8) || !(s->late[(seqno / 8) % s->replaywin] & (1 << seqno % 8))) {
+ return update_state ? error(s, EIO, "Received late or replayed packet, seqno %d, last received %d\n", seqno, s->inseqno) : false;
+ }
+ } else if(update_state) {
+ // We missed some packets. Mark them in the bitmap as being late.
+ for(int i = s->inseqno; i < seqno; i++) {
+ s->late[(i / 8) % s->replaywin] |= 1 << i % 8;
+ }