Packet Sequencing

EO embeds a sequence number in every client packet to detect out-of-order or replayed packets. The sequence start is established during the handshake and then increments by one per packet, wrapping every 10 steps.

Initialization from a server init packet:

// Server sends two bytes (seq1, seq2) that encode the start value.
int32_t seq1 = ...; // from ServerInitInitPacket
int32_t seq2 = ...; // from ServerInitInitPacket
 
int32_t start = eo_sequence_start_from_init(seq1, seq2);
EoSequencer sequencer = eo_sequencer_init(start);

Re-synchronisation from a server ping packet:

int32_t seq1 = ...; // from server ping
int32_t seq2 = ...; // from server ping
 
int32_t start = eo_sequence_start_from_ping(seq1, seq2);
sequencer = eo_sequencer_init(start);

Stamping outgoing packets:

int32_t seq;
eo_sequencer_next(&sequencer, &seq);
eo_writer_add_char(&writer, seq);  // written into the packet header

Server-side: generating and encoding a new start:

int32_t start = eo_generate_sequence_start();
 
uint8_t bytes[2];
if (eo_sequence_init_bytes(start, bytes) == EO_SUCCESS) {
    // Send bytes[0] and bytes[1] in the init packet.
}
 
EoSequencer sequencer = eo_sequencer_init(start);