Mobile comn.pptx
- 1. TCP FLAVOUR MAJ DAMANDEEP SINGH MAJ ARPIT GULERIA
- 2. Topics • Retransmission mechanisms • Timeout • Fast retransmission • TCP congestion control • Slow start • Additive increase multiplicative decrease (AIMD) • Fast recovery • TCP flavors • TCP Tahoe • TCP Reno • TCP New Reno • TCP Vegas • TCP freeze • Conclusion
- 3. Main Function of TCP(recap) • Connection –orientation mech • Ensure presence of receiver before Tx • Seq and ACK • Ensure proper order of segments • Flow control • To handle the slow receiver pbm • Retransmission control • Take care of lost or error packets • Congestion control • Take care of network bottlenecks
- 4. Retransmission Mechanism (recap) Lack of ack is an indication that the packet is lost and retransmission is required. Retransmission Mechanism • Time out based retransmission • Fast retransmission
- 5. Timeout • Packet is retransmitted if ack is not received within Timeout • In TCP timeout is a function of RTT. • How RTT is calculated ? • Measure sample_RTT for each seg- ack pair • Compute weighted average of RTT(Est_RTT) Est_RTT = a Est_RTT + b sample_RTT a + b=1 • Set timeout= 2 X Est_RTT
- 6. Fast Retransmission • If packet n is lost but packet n+1,n+2,n+3 ……… reached the destination, the receiver will keep on sending ack indicating that nth packet is awaited • nth packet is lost • All packets after nth packets were received • In Fast Retransmission, only the nth packet is retransmitted before timeout IN TCP SENDER RETRANSMIT DATA AFTER 3 DUP ACK
- 7. TCP Congestion Control TCP requires to have some mech of congestion control TCP assumes that if there is an ACK loss then it is due to congestion TCP starts taking steps for congestion control TCP starts taking steps for congestion control
- 8. Congestion control Mechanism • Slow Start • Additive increase and multiplicated decrease
- 9. Slow Start Idea • Start with congestion window = 1 packet • Double congestion window each RTT • Till reaching ss_th or packet loss
- 10. AIMD • TCP maintain a congestion window for each connection • Congestion window is used to limit how much data can be transmitted to prevent congestion Algo – • Inc cwnd by one packet per RTT (Linear Inc) • Divide cwnd by 2 whenever a timeout occurs (Multiplicative dec)
- 11. Fast Recovery This tech possible to quickly recover lost data packets Idea • After receiving 3 duplicate ACKs in a row: . Set ss_thresh to one-half of the current congestion window. . Retransmit the missing segment. . set cwnd = ss_thresh + 3. . Each time another duplicate ACK arrives, set cwnd = cwnd + 1. . Once receive a new ACK, exit fast recovery. Set cwnd to ssthresh. Then, continue with linear increasing.
- 12. TCP Flavours
- 13. Evolution Of TCP (congestion control) • TCP Tahoe • TCP Reno • TCP new Reno • TCP Vegas • Slow Start • Additive increase • Fast Retransmission • Fast Recovery
- 14. TCP Tahoe
- 15. TCP Tahoe Idea : Slow start + Additive Inc + Fast transmission Algo: Set cwnd to 1 for both timeout and 3 dup ack
- 16. TCP Tahoe – State Diagram Congestion avoidance Fast Retransmission Start cwnd >= ss_th cwnd = cwnd +1/cwnd cwnd = 1 cwnd = cwnd +1 Slow Start ACK DATA Retransmission timeout All ack Dup ack >=3 Send missing packet ss_th= cwnd/2 cwnd =1 Congestion avoidance Slow Start
- 17. TCP Tahoe Cwnd = 8 Sender Receiver 1. Cwnd = 8 2. Cwnd = 1 3. Cwnd = 1 4. Comments 31 32 33 34 35 36 37 38 Sender sent segment 31 - 38 30 30 30 30 31 Receiver replied seven duplicate ACKs of segment 30 Sender received three dupacks and cwnd is changed to 1 packet. The lost segment 31 is retransmitted 39 32 Receiver replied the ACK of packet 38 when it received the retransmitted packet 31. 32 07 Dupacks On 3 DuPacks Retx 31 ACK 38 cwnd =1 Send 39
- 18. TCP Reno
- 19. TCP RENO Slow start + Additive Inc + Fast transmission + Fast Recovery Idea: In case of Dup Ack , 3 Dup Ack indicates the network capability of delivering seg so . Cwnd is cut to half ( instead of 1 as in Tahoe) and then it increase linearly
- 20. Congestion avoidance Fast Retransmission Start cwnd = cwnd +1/cwnd cwnd = cwnd +1 Slow Start ACK DATA Dup ack >=3 Dup ack >=3 Fast Recovery cwnd >= ss_th Send missing packet ss_th= cwnd/2 cwnd =ss_th cwnd = cwnd +1 ACK DATA cwnd = ss_th Non dup ack> x cwnd = cwnd +1 cwnd = 1 All ack Retransmission timeout TCP Reno
- 21. Cwnd = 8 Sender Receiver 1. Cwnd = 8 2. Cwnd = 11 3. Cwnd = 4 4. Comments 31 32 33 34 35 36 37 38 Sender sent segment 31 - 38 30 30 30 30 30 31 30 30 Receiver replied seven duplicate ACKs of segment 30 Sender received three dupacks and cwnd is changed to (8/2)+3 packets. The lost segment 31 is retransmitted 39 32 Receiver replied the ACK of 38 when it received the retransmitted packet 31. 32 Pkts 31 get lost 07 Dupacks On 3 DuPacks Retx 31 Send 3 new pkts Ack 38 for re tx Back to ai cwnd =4 Sender exist fast retx cwnd=4
- 22. Comparison between TCP Reno and TCP Tahoe Optional statement
- 23. Why TCP is Fair?
- 24. TCP : NEW RENO
- 25. Multiple Packet loss problem Cwnd = 8 Sender Receiver 1. Cwnd = 8 2. Cwnd = 7 3. Cwnd = 9 4. Comments 31 32 33 34 35 36 37 38 Sender sent segment 31 - 38 30 30 30 30 30 31 30 30 Receiver replied five duplicate ACKs of segment 30 Sender received three dupacks and cwnd is changed to (8/2)+3 packets. The lost segment 31 is retransmitted 39 32 Receiver replied the ACK of packet 32 when it received the retransmitted packet 31. This is Partial ACK 32 Pkts 31, 33 & 34 lost 05 Dupacks On 3 DuPacks Retx 31 Partial ACK
- 26. Multiple Packet loss problem Sender Receiver 5. Cwnd = 4 Cwnd = 4 6. Comments Sender exits from Fast Recovery and entered the congestion avoidance state after receiving the Partial ACK. Cwnd is changed to 4 MSS Sender waited until timeout 32 Exit FastRecovery Cwnd = 4 Sender Timeout
- 27. Limitation of Reno • Multiple Packet Loss • Partial ack will be received when first packet is retransmitted • Ack causes moving out of Fast Recovery state • Cwnd halved – wait for timeout • Affects Reno performance • Solution : TCP New Reno • Extend fast recovery Phase – Do not exit Fast recovery Phase after getting Partial acknowledgement • Alternate Solution • TCP SACK : Explicitly ACK lost segments • TCP Vegas : Using RTT to Signal Congestion
- 28. TCP NewReno Cwnd = 8 Sender Receiver 1. Cwnd = 8 2. Cwnd = 7 3. Cwnd = 9 4. Comments 31 32 33 34 35 36 37 38 Sender sent segment 31 - 38 30 30 30 30 30 31 30 30 Receiver replied five duplicate ACKs of segment 30 Sender received three dupacks and cwnd is changed to (8/2)+3 packets. The lost segment 31 is retransmitted 39 32 Receiver replied the ACK of packet 32 when it received the retransmitted packet 31. This is Partial ACK 32 Pkts 31, 33 & 34 lost 05 Dupacks On 3 DuPacks Retx 31 Partial ACK
- 29. TCP NewReno Cwnd = 8 Sender Receiver 5. Cwnd = 9 6. Cwnd = 9 7. Cwnd = 10 8. Comments 33 40 Sender receives the partial Ack of pkt 32 and immediately retransmitted the lost segment 33. Cwnd is changed to 9-2+1 33 33 41 33 Sender receivesed a DupAck and added Cwnd by 1, thus segment 41 is kicked out. Receiver replied a partial ACK and a duplicate ACK of pkt 33 The partial ACK triggered the sender Re-Tx pkt 34. Receiver replied an ACK of pkt 33 upon receiving segment 34 34 ReTX 33 Tx 40 32 32 34 33 Cwnd ++ Tx 41 Partial + Duplicate ACK 33 ReTx 34 42 43 33 Cwnd ++ Tx 42,43 Upon receiving the duplicate ACK of pkt 33, Cwnd was advanced by 1. Two new pkts were also sent. (since Awnd < Cwnd)
- 30. TCP NewReno Cwnd = 11 Sender Receiver 9. Cwnd = 11 10. Cwnd = 4 11. Comments 34 42 43 On receiving the duplicate ACK of pkt 33, Cwnd was advanced by 1 and thus pkt 44 was Triggered out 43 42 41 44 Receiver replied ACKs of Pkt 30, 42, 43 & 44 Sender Exited Fast recovery upon receiving the ACK pkt 40. Cwnd = 4 44 43 44 42
- 31. TCP : VEGAS
- 32. TCP : VEGAS • Totally different approach • Emphasizes pkt delay, rather than packet loss, as a signal to help determine the rate at which to send packets • Change in triggering Fast Retransmit • On receiving first dupAck check the following:- If((Current_time – Sending_time ) + MinRTT > Timeoutvalue) Trigger Fast-Retransmit without 3 dupAcks • Use of fine-grained RTT to improve congestion control • Anticipate Congestion based on minimum RTT
- 33. TCP VEGAS – Anticipating Congestion Control • Base RTT = Minimum RTT • Expected sending rate & Actual sending rate • Expected = Cwnd/BaseRTT • Diff = Expected – Actual • If Diff < a, Cwnd ++ per RTT else if Diff > b, Cwnd -- (a and b are two thresholds that control utilization of bandwidth without overloading) • Implementation with Runo ??? Performance of Vegas degrades because Vegas reduces its sending rate before Reno, as it detects congestion early and hence gives greater bandwidth to co-existing TCP Reno flows
- 34. TCP : FREEZE
- 35. SNOOP TCP • SNOOP Agent : TCP aware with limited powers • SNOOP can buffer TCP Segments • Upto certain numbers of lost ACKs are allowed SH/FH SNOOP MH Effective Not much effective Congestion Window will be controlled by mobile host in this case
- 36. M-TCP • Delayed duplicate ACK • Drawback : Base station is central entity SH/FH BS MH MH ZWA FWA
- 37. TCP : FREEZE • Involve MH as TCP receiver • During handoff, ZWA is sent by MH to FH • On reconnection FWA is sent by MH to FH How MH will get to know that Handoff is initiated ? When Antenna Signal strength is getting down and pkt loss increases consequtively. It implies that handoff has been initiated • On reconnection, FWA along with three Dupacks will be forwarded by MH to BS as well. From three Dupacks BS will send buffered data to MH SH/FH BS MH MH ZWA FWA
Editor's Notes
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