Rather than TCP, which is a stream-oriented protocol, ATP is message-oriented.
ref. /docs/features.md
An ATP packet can be wrapped within a UDP packet
IP Header
UDP Header
ATP Header
Payload(including ATP options and user data)
An ATP packet as a byte array can be re-interpreted as a POD struct ATPPacket. In ATP sockets, ATP packets are managed by class OutgoingPacket which takes ownship of an ATPPacket to provide a whole lifetime management.
The ATP Header(10 bytes):
0 7 8 f
+........+........+
+ seq +
+........+........+
+ ack +
+........+........+
+ sock_id +
+........+........+
+ opts + flags +
+........+........+
+ window +
+........+........+
The flags are
0 7 8 f
+........+........+
+ + UAPRSF+
+ opts + RCSSYI+
+ + GKHTNN+
+........+........+
Both seq and ack are packet-indexed rather than byte-indexed(as TCP does).
ATP allows multiple connections established through one port. The sock_id field is used to distiguish ATP packets sent to the same port.
The SYN/FIN flags work similarly with TCP.
The ACK flag acknowledges sequence number of the last processed in-coming package(which may be buffered for a while) with user data. For example, if ATP processed a packet whose real_payload() > 0 and seq_nr == n, then it will update ack_nr to n and create an packet with ACK number equal to n(not n + 1). However if the packet has real_payload() == 0, then ack_nr will not be updated and no ACK packet will be created, because actually reused seq_nr of the previous packet.
The PSH flag works differently from TCP's. Notice that all in-coming data in UDP are buffered and signaled one packet by one packet, so all packets are handled without deferring and packet splicing problems.
The URG flag works differently from TCP's. An URG flag marks an OOB packet which will be sent immediately and handled as soon as possible, regardless any of the window restrictions. URG packets share sequence number with normal packets. URG packets are promised to be acknowledged from peer within a given interval. Otherwise user will be notified by a callback.
The RST flags work similarly with TCP's.
When opts is set not equal to 0, there are opts number of options at the beginning of payload. Notice that the option fields are part of payload, so use real_payload() instead of payload to check if there's user data.
An option has layout as following
0 7 8 f
+........+........+
+ kind + length +
+........+........+
+ data +
+........+........+
ref. /docs/socket.md
Enable detailed logging by opening the following macros: ATP_LOG_AT_NOTE, ATP_LOG_UDP