2 votes 2 votes After receiving 3 duplicate ACKs sender will go to slow start phase or congestion avoidance? Somewhere, I read that, 3 duplicacy means the weak-possibility of congestion. When does global timer expire? Computer Networks computer-networks tcp congestion-control + – Shubhanshu asked Jan 16, 2018 Shubhanshu 873 views answer comment Share Follow See all 3 Comments See all 3 3 Comments reply SHUBHAM SHASTRI commented Jan 16, 2018 reply Follow Share here we will not start with slow phase...rather ..we will find new threshold by (CURRENT WINDOW)/2 and start from new threshold instead slow phase... 0 votes 0 votes Rishabh Gupta 2 commented Jan 16, 2018 reply Follow Share @Shubhanshu There are different versions of TCP. in TCP Tahoe: After receiving 3 duplicate ACKs it goes to slow start state. in TCP Reno: After receiving 3 duplicate ACKs, it goes to fast recovery phase, i.e it retransmits the packet and wait for its ACK, and does not go to slow start phase immediately. If the retransmitted packet is deliverd correctly, it goes to congestion avoidance phase. https://en.wikipedia.org/wiki/TCP_congestion_control#TCP_Tahoe_and_Reno 0 votes 0 votes Shubhanshu commented Jan 16, 2018 reply Follow Share Thanks, @Rishabh Gupta 2 based on what you quote, I collect some more information about Tahoe and Reno, please correct if those are wrong. in TCP Tahoe: After receiving 3 duplicate ACKs it goes to slow start state. In TCP Tahoe:- congestion policy moves are:- I am considering from the beginning:- 1. Slow start Initially go up to Advertised window exponentially and when packet loss occurred, set cwnd = cwnd /2. Note that packet loss will be considered only after receiving three duplicate ACKs. Sent the lost packets. Now, here I am confused that it will retransmit the lost packets before Timeout or after Timeout. And then it will again start Slow start. Till SSthreshold and then enters into Congestion Avoidance Phase 2. Congestion Avoidance phase(increases linearly by 1) here also it will continue to send packets linearly i.e. +1 every time when it receives all sent packet's acks. and if packet loss occur i.e. three duplicate ack. retransmit those packets again the same confusion. retransmit before TO or after TO. 3. Fast Retransmission:- retransmit all lost packets and again start slow start phase and continue. Graph of TCP Tahoe:- You can see the operation of TCP Tahoe clearly from the above figure: At approximately time 0, TCP Tahoe starts and it is in the slow start mode: the congestion window size increases exponentially At approximately time 5, packet loss is detected. TCP marks SSThresh = 25 (approximately) and begins another slow start When it reaches CWND = 25 (approximately), the CWND increases linearly - here TCP Tahoe enters the congestion avoidance mode At approximately time 19, TCP Tahoe detects packet loss and begins a slow start. SSThresHold is approximately 22. TCP begins another slow start and so on.. Ref:- http://www.mathcs.emory.edu/~cheung/Courses/455/Syllabus/A1-congestion/tcp2.html 0 votes 0 votes Please log in or register to add a comment.
2 votes 2 votes When Global timer expires, it's Strong possibility of congestion. Rest you can check in diagram. targate2018 answered Jan 16, 2018 targate2018 comment Share Follow See all 2 Comments See all 2 2 Comments reply Shubhanshu commented Jan 16, 2018 reply Follow Share @targate2018 check my comment above, and let me know what is wrong in that. 0 votes 0 votes Swastik commented Jan 25, 2019 reply Follow Share @targate2018 During receiving three duplicate acknowledge whether we will increase the congestion window or not? 0 votes 0 votes Please log in or register to add a comment.