I seen everywhere in slow start mechanism if initial sender size denoted by cwnd= 1mss , then after 1 RTT cwnd becomes = 2 mss, and after 2 RTT cwnd becomes = 4 mss and so on. My question is why transmission time (TT) isn't added with RTT? That means if cwnd= 1mss , then it should after 1 RTT + 1 TT cwnd becomes = 2 mss, and after 3 TT+ 2 RTT cwnd becomes = 4 mss and so on. So my assumption is that cwnd becomes 2^n after nRTT + (2n+1) TT where we take 1 TT per 1 mss and n=0,1,2,3,4..........................
My question is why transmission time (TT) isn't added with RTT?
In the context of TCP, RTT can mean two things.
- meaning one: RTT that is supposed to be measured by TCP to calculate retransmission timeout.
- meaning two: RTT is used in the description of congestion control algorithms
meaning one of RTT
TT is inside RTT.
RTT is measured at the implementation of layer 4. It measures the time between layer 4 send the packet to layer 3 and layer 4 receives ACK from layer 3 [at least, some l4 processing might end up in RTT as well].
Thus RTT includes host processing at layer 3 and below, sending of the packet, all network processing, receiving the corresponding ACK and processing up to layer 3 on the host for the ACK.
Note, that it is impossible for TCP implementation to know when the packet is actually sent out of the system. This is done by a network card, which does not notify OS about this. It is especially impossible if the packet is sent over Wi-Fi. Who knows how much time the packet needs to wait before the channel is free to send it. Also, OS implements buffers for outgoing interfaces. This queueing delay is in RTT as well.
meaning two of RTT
this RTT is quite fuzzy.
The idea is the same - for a single segment it means the time between layer 4 sending segment to layer 3 and layer 4 receiving corresponding acknowledgement
However, in a lot of cases it actually means the time between first packet of current window is sent and first ACK of the current window is received. It works under the assumption that sending time of a window is smaller than RTT, i.e., by the time the first ACK of a window is received, the whole window is already sent.
This allows to reason of congestion control in rounds, where each round corresponds to one RTT. In each round, congestion control specifies window size, aka number of packets sent in this round.
Obviously actual TCP congestion control cannot work like this. This is an abstraction that is used to simplify understanding.
Note. When diving deeper in congestion control, this abstraction can become somewhat confusing. It is reasonable to assume that round time (in this case RTT) is constant and independent of congestion window, but it is not. When congestion window becomes large enough, it causes excessive packets to be buffered on bottleneck link, which increases RTT. Thus the RTT is actually not constant and slightly increases with congestion window once the window has reached certain threshold.