ENRICHMENT OF SACK TCP PERFORMANCE BY DELAYING FAST RECOVERY Mr. R. D. Mehta 1, Dr. C. H. Vithalani 2, Dr. N. N. Jani 3

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1 Research Article ENRICHMENT OF SACK TCP PERFORMANCE BY DELAYING FAST RECOVERY Mr. R. D. Mehta 1, Dr. C. H. Vithalani 2, Dr. N. N. Jani 3 Address for Correspondence 1 Asst. Professor, Department of Electronics & Communication Engineering, MEFGI, Rajkot, Gujarat, India, rdmehta@hotmail.com 2 Head, Department of Electronics & Communication Engineering, GEC, Rajkot, Gujarat, India, chv@gecrajkot.org 3 Director, S. K. Patel Institute of Management & Computer studies, Gandhinagar, Gujarat, India, drnnjanicsd@gmail.com ABSTRACT Internet is growing rapidly over entire globe with heterogeneous networks. TCP/IP protocol suite is an inseparable part of Internet. Hence, the efficiency of the protocols plays a vital role in performance of Internet. The increasing use of wireless links in expanding Internet has brought out some serious performance issues of TCP. TCP was originally designed and deployed over a wired network. Wired links are less prone to channel errors. Due to lack of intelligence, it fails to differentiate losses by actual reason, so they are not treated fairly. Therefore, performance is often degraded by unnecessary congestion window reductions and Retransmission Time Outs. Basic TCP has gone through a number of revisions in order to improve performance. Among all of them, is considered to be the most efficient scheme because of its ability to avoid unnecessary retransmissions based on SACK information. also fails to discriminate the cause of the loss: congestion or corruption. If random losses can be categorized apart from losses due to severe congestion, then irrational decision of cwnd reduction can be avoided. This will surely lead to performance improvement due to effective bandwidth utilization and consistent flow in case of random packet losses. In this paper, is modified to delay fast recovery after one packet loss, until subsequent loss in the same windows is encountered. The technique of delaying fast recovery sometimes helps in avoiding fast recovery i.e. flow reductions when scattered/random losses are found on the links. This leads to significant performance enhancement when protocol is used over erroneous links. Testing TCP modifications in the global Internet may be difficult because of wide range of variables and implementations available. While in case of simulations, different situations can be easily created by changing the code or script. Simulation based comparison of modified and original shows significant improvement in case of random errors. There is no degradation in a network facing severe congestion. Results are demonstrated at the end with help of graphs of throughputs and percentage improvement in terms of packet delivered. KEYWORDS Fast retransmit, Fast recovery, ssthresh, TCP SACK, Congestion Window, Corruption 1. TCP in the ACK packet, that tells the sender how many The Transport Control Protocol, TCP, is a protocol that runs on the top of the IP network layer. In TCP, the receiver acknowledges all received data with an ACK message, and the sender buffers the sent data until an ACK packet is received. If no ACK packet is received, the sender retransmits the data. The TCP sender uses two parameters, also referred as windows, to control the send rate. Both parameters limit the number of the packets that may be sent by the sender without receiving the acknowledgment. The sender uses the minimal value provided by these two parameters. One parameter is the receiver-controlled offered window (also called advertised window), written bytes can be sent without overflowing the receiver s buffer. This parameter inhibits the sender from flooding the received buffer of a slow receiver. The other parameter is the sendercontrolled cwnd, which limits the number of packets that may be sent by the sender without receiving the acknowledgment. This parameter prevents network congestion. The packet losses are detected in TCP by using a timer that triggers after the time which is twice that network round-trip time. TCP protocol assumes that losses are mainly due to congestion rather than due to transmission errors. Therefore after detection of the packet losses, the sender

2 voluntarily reduces the cwnd, which in its turn decreases the transmission rate avoiding in such way packet congestion. The senders decrease their sending rate in two ways. First, when a packet loss occurs, the cwnd size is reduced to some threshold, and afterwards it increases slowly. Second, for every consecutive loss, the retransmission interval is doubled, in order to prevent the network from being flooded by retransmissions. Different actual TCP implementations have various ways of increasing the window size again. The implementations are often named after their corresponding BSD UNIX releases, with names like Tahoe, Reno, or Vegas. Different versions work together, since the protocol does not require a particular sender behavior [1] [2]. Section 2 discusses different versions of TCP in brief. Section 3 gives an idea of performance issues related to. Section 4 proposes few modifications to improve the performance. Section 5 gives brief idea of different topologies used. Section 6 tabulates the observations obtained and finally Section 7 concludes the work [6]. 2. TCP IMPLEMENTATIONS 2.1 TCP Tahoe It includes Slow-Start, Congestion Avoidance and Fast Retransmit. The Tahoe implementation of TCP introduced significant improvements for working over a shared network. An algorithm Slow Start (for congestion control) and multiplicative decrease (for congestion avoidance) was introduced to control transmission following any detected congestion [7] [3]. 2.2 TCP Reno It adds Fast Recovery to TCP Tahoe. (a) The new algorithm prevents the communication path from going empty after Fast Retransmit, thereby avoiding the need to Slow-Start to re-fill it after a single packet loss. A TCP client enters fast Recovery after receiving an initial threshold of duplicated ACKs. This threshold is generally set to three. Once the threshold of duplicated ACKs is received, the server retransmits one packet and reduces its cwnd by one half. (b) Instead of slowstarting after detecting a packet loss, as is performed by a TCP Tahoe server, the TCP Reno server uses additional incoming duplicated ACKs to clock subsequent outgoing packets. TCP Reno's Fast Recovery algorithm is optimized for the case when a single packet is dropped during a window of data. The TCP Reno retransmits at most one dropped packet per round-trip time. TCP Reno significantly improves upon the behavior of TCP Tahoe when a single packet is dropped from a window of data, but can suffer from performance problems when multiple packets are dropped during a window of data [7] [3]. 2.3 TCP New-Reno TCP New-Reno is the enhanced TCP Reno using a modified version of Fast Recovery. Janey Hoe proposed a modification to TCP Reno usually called New-Reno, which addressed two problems in TCP Reno, these ideas are gradually finding acceptance within the IETF. First Hoe noted that a smaller value for ssthresh causes premature termination of the slow start phase and subsequent slow increase of the cwnd (i.e. the linear increase phase). A larger value causes the sender to overfeed packets to the network (i.e., transmit too long a burst of data packets) causing congestion. The second problem occurs since most TCP sessions last only for a short period of time, the initial slow start period is significant for the over all performance. Hoe proposed a method to estimate an optimum ssthresh value by calculating the byte equivalent of bandwidth delay product of the network, when a new connection is made [7] [5].

3 2.4 SACK (Selective Acknowledgment) The probability of multiple packet losses in a window is much greater for a fast network; where many more packets are transmitted. Although TCP is able to recover multiple packet losses without waiting for expiry of the retransmission timer, frequent packet loss may still not be efficiently recovered. The SACK extension improves TCP performance over such a network, and has been included in some recent TCP implementations. The SACK option is triggered when the receiver buffer holds in-sequence data segments following a packet loss. The receiver then sends duplicate ACKs bearing the SACK option to inform the transmitter which segments have been correctly received. When the third duplicate ACK is received, a transmitter retransmits only the missing packets starting with the sequence number acknowledged by the duplicate ACKs, and followed by any subsequent unacknowledged segments. The Fast Retransmission and Recovery algorithms are also modified to avoid retransmitting already SACKed segments. The explicit information carried by SACKs enables the transmitter to also accurately estimate the number of transmitted data packets that have left the network (this procedure is known as Forward Acknowledgment (FACK)), allowing transmission of new data to continue during retransmission. The SACK option is able to sustain high throughput over a network subject to high packet loss and is therefore desirable for bulk transfers over a network. Availability of the SACK information allows the TCP sender to make congestion control more accurately. When three duplicate ACKs are arrived by, the sender cuts the cwnd uniformly by half and an estimating outstanding data variable pipe, to control the number of segment sent during fast recovery phase. Considering three duplicate acknowledgements to implicitly indicate that receiver receives three outstanding packets, the pipe size is estimated to (cwnd 3) and then cwnd is halved. The variable pipe is increased by one segment when sending a packet and decreased by one segment when receiving a duplicate acknowledgement or decreased by two when received a partial acknowledgement. Packets can be sent if pipe is smaller then cwnd, in order to keep the outstanding data within permissible limits [7] [4]. 3. TCP SACK PERFORMANCE ISSUES To detect a loss and retransmit the corresponding segment, Basic TCP sender has to wait till timeout occurrence [2]. Moreover, timeout also resets congestion window to 1. Tahoe TCP uses Fast Retransmission algorithm after getting 3 dupacks, but reduces cwnd to 1. Reno TCP enters fast recovery after three dupacks but not able to deal with multiple losses in same window as it leaves fast recovery after 1 retransmission only. New- Reno TCP deals with multiple losses in same window but still bandwidth is not fully utilized. carries information about the particular segment which was lost or corrupted. More information is available to the transmitter. Still none of the TCP is able to differentiate between congestion or corruption loss and the actions taken for different reasons for loss remain same. 4. PROPOSED MODIFICATIONS In TCP, random losses (due to corruption) are not treated differently, though it does not add to congestion. Still cwnd reduction is done in this case. One way to discriminate between congestion and corruption (random) loss and to behave accordingly is to use the information available to the transmitter. Generally, losses due to channel noise are random. Whenever network is facing congestion, many packets within short time

4 duration are dropped at routers because of unavailability of resources. Random loss can be considered as a single loss without any subsequent loss in the window. If random loss can be confirmed somehow, then recovery from that loss is the only issue. At sender side, SACK blocks report number of packets successfully reaching the receiver out of order after a loss. Sender can use this information effectively to be sure about a random loss. After reception of a dupack, sender can check right edge of the SACK block. As long as the loss is the only one, continuous increment in right edge is reported to sender. Whenever another loss is detected, right edge discontinuous to increment as expected at sender side and a gap will be noticed. Thus, sender can identify another loss with help of SACK information. SACK_OK flag is introduced to avoid or delay a fast recovery. First loss in the window is always considered as a random loss. SACK_OK is true as long as the SACK information supports the assumption of the random loss. After having a gap on the sender, SACK_OK is made false because a gap proves assumption of a random loss is invalid as there is another loss. 4.1 Delayed Fast Recovery The losses are not random now, i.e. congestion might have dropped the packets. Even if both of them are due to channel noise, they should not be categorized as random losses to be on safer side. They should be now qualified as congestive losses. Fast recovery is evoked now. As it was delayed till a subsequent loss, it is named "delayed fast recovery" in this paper. cwnd and ssthresh reduction is now followed as in case of original. Modified 's behavior is resumed like original one once it enters into fast recovery. dup_wnd is initialized by cwnd after a loss. It is reduced by 1 on arrival of each dupack. When dup_wnd reached zero and if SACK_OK is also true till now, then each parameter is reset because network could carry all the packets of the window except the lost one. Therefore, it was certainly a random loss and definition of random loss would be applied to next loss which will actually first loss in the next window. That will be again considered as a random loss and all parameters will be initialized according to it. If two losses are found within three dupacks, then fast recovery is followed at three dupacks like base line. 4.2 Distance Another parameter distance is introduced to handle the fast retransmission in delayed fast recovery. Distance indicates displacement of second loss from first loss. Once last_pack+1 packet was fast retransmitted in condition of SACK_OK true. Fast recovery was delayed till next loss. To recover from second loss, fast retransmission of second loss should be attempted in fast recovery. As per the definition of distance, it is clear that it is used in delayed fast recovery for fast retransmission of second loss. pipe_ indicates number of outstanding packets on the network. In conventional SACK TCP, pipe_ is initialized by maxseq_last_ack_3. pipe_ can not be initialized the same way in presence of modifications because packets on flight would be different as delayed fast recovery was evoked not directly after three dupacks. When fast recovery is delayed, more dupacks reach to sender. Receiver sends dupacks only on arrival of those segments, i.e. they are no more on the network. Hence, pipe_ should be set as maxseq_last_distance+2. This is one of the important factors because pipe governs transmission of new packs or retransmission if it is less than cwnd.fast recovery is ended like original, either on arrival of acknowledgement of recover or timeout. Flow chart shown in Figure 1 reflects an algorithm discussed above

5 5. SIMULATION TOPOLOGY Two types of topologies are used to test and measure performance of new algorithm. Topology shown in Figure 2 gives a scenario of erroneous environment without congestion. Topology shown in Figure 3 gives a scenario of congestion and erroneous environment acting together. Later one is a more practical scenario one can find in wireless networks. FTP is used to generate traffic which uses TCP at transport layer. Throughput is measured in terms of packets delivered to the network by a sender. (1) Erroneous Environment with no Congestion Figure 2: Simulation Topology 1 Figure 1: Flow chart of algorithm (2) Congested Network with an Erroneous Link Figure 3: Simulation Topology 2 6 SIMULATION RESULTS Results are obtained for initial and sustained response for 10 seconds and for 100 seconds respectively. Behavior of protocol and algorithm is measured for small size and bulky size types of file transfer. Plot also reflects few observations of performance comparison through Figure 4 to 9. Initial Response: Table 1: Number of Packets Delivered (Only Errors 10 Seconds) Error Rate Modified SACK TCP

6 International Journal of Advanced Engineering Technology E-ISSN Sustained Traffic: Table 2: Number of Packets Delivered (Only Errors 100 Seconds) Error Rate Modified Initial Response: Table 3: Number of Packets Delivered (Errors & Congestion 10 Seconds) Error Modified SACK Rate TCP Sustained Traffic (Congested Network with Errors): Table 4: Number of Packets Delivered (Errors & Congestion 100 Seconds) Error Rate Figure 5: Modified SACK (Only Errors 10 Sec) Modified Plots: Top curve, middle curve and bottom curve in the graphs are plotted for error rates 0.00, and 0.01 respectively. Figure 6: SACK (Only Errors 100 Sec) Figure 4: SACK (Only Errors 10 Sec) Figure 7: Modified SACK (Only Errors 100 Sec)

7 International Journal of Advanced Engineering Technology The performance of improves significantly in presence of proposed modifications by avoiding cwnd reduction. For erroneous or noisy links, performance rise is exceptional, which is a case of wireless environment. Greater performance improvement is observed in higher error rates. As cwnd reduction is not avoided on congested network, improvement. Figure 8: and Modified SACK (Congested network with Errors 10 Sec) E-ISSN there is no The modifications suggested here do not require any changes on receiver side as well as in TCP header. 8 REFERENCES 1. Figure 9: and Modified SACK (Congested network with Errors 100 Sec) 7 CONCLUSIONS Simulations were carried out on ns-2 with two types of topologies: erroneous but congestion free environment & congested network along with random errors. Following observations were made from the results of simulations attempted on basic and modified schemes. Basic s is unable to differentiate losses which makes it conservative. It uses congestion friendly approach even when the losses are due to channel errors. This results into performance degradation. Behrouz Forouzan, TCP/IP Protocol Suite, TMH 2. Transmission Control Protocol, RFC 793, September TCP Slow Start, Congestion Avoidance, Fast Retransmit & Fast recovery algorithms, RFC M. Mathis, J. Mahadavi & S. Floyd, TCP Selective ACK options, RFC 2018 October, S. Floyd, T. Henderson, A. Gurtov, The NewReno Modification to TCP s Fast Recovery Algorithm, RFC 3782, April V. Jacobson & R. Braden, TCP evaluation for long delay path, RFC 1072 October, Kevin Fall & Sally Floyd, Simulation based comparisons of Tahoe, Reno and

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