As is discussed in ACK flag, ATP uses the ACK flag to acknowledge packets with user data from peer.
Use function OutgoingPacket::is_promised_packet to check whether a packet can be re-sent. Basicly, ATP only resend the following packets:
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Packets with SYN/FIN flags
SYN and FIN are important control message thus they should always been re-sent until acknowledged.
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Packets with user data
In order to reduce network cost, a packet with no user data(even if it contains payload of option) will not be re-sent.
When my_max_sack_count is set to non-zero, a ATP_OPT_SACKOPT option will be attached to the SYN packets at the connection establishing stage. When handling the ATP_OPT_SACKOPT option, my_max_sack_count will be updated.
Nagle's algorithm inhibit the sending of new TCP segments when new outgoing data arrives from the user if any previously transmitted data on the connection remains unacknowledged.
ATP use ATPSocket::cur_window_packets to control numbers of the in-flight packets which haven't been acknowledged. Nagle's algorithm is enabled when cur_window_packets = 1.
Every ATP socket has a distinct sock_id, multiple ATP sockets can read/write bi-directionally through the same UDP socket. Thus when a ATP Socket is destructed, it will not wait very short TIME_WAIT time.
A Sliding Window Protocol implementation is provided in ATP.
ATP is recommended to be used in between relatively healthy network connections. Slow start is disabled by default.