JTCP: Congestion Distinction by the Jitter-based Scheme over Wireless Networks

Transcription

1 JTCP: Congeston stncton by the Jtter-based Scheme over Wreless Networks Erc Hsao-Kuang Wu, Mng-I Hseh, Me-Zhen Chen and Shao-Y Hung ept. of Computer Scence and Informaton Engneerng, Natonal Central Unversty, Chung-L, Tawan Abstract TCP, a wdely used transport protocol performs well over the tradtonal network whch s constructed by purely wred lnks. As wreless network s growng rapdly, the wred/wreless mxed nter-network, a heterogeneous envronment wll get wde deployment n the next generaton ALL-IP wreless networks. TCP whch detects the losses as congeston events could not sut the heterogeneous network n whch the losses wll be ntroduced by hgher bt error rates or handoffs. There exst some unsolved challenges for applyng TCP over wreless lnks. End-to-end congeston control and farness ssues are two sgnfcant factors. To satsfy these two crtera, we propose a tter-based scheme to adapt sendng rates by the packet losses and tter ratos. The experment results show that our tter-based TCP (JTCP conducts good performance over the heterogeneous network. Keywords TCP; tter; wreless; congeston control I. INTOUCTION In the recent years, the wreless and moble communcatons [] have shown sgnfcant populartes n areas such as commerce, educaton, entertanment, and defense. As the number of moble hosts grows exponentally, the smooth and effectve communcatons over wred and wreless lnks, a heterogeneous network draw a great amount of research efforts. Besdes, the ubquty of the Internet s drven by the network-technology-ndependent desgn of IP [3]. Wthout surprse, a heterogeneous network envronment wll get wde deployment n the next generaton ALL-IP wreless networks. In ths smple IP archtecture whch allows a multtude of servces, the transport layer s prmary role s to provde end-to-end communcatons servce. Most applcatons are employng one of two protocols: the user datagram protocol (UP or the transmsson control protocol (TCP [4]. UP s a smple, datagram-orented, transport layer protocol. It s sutable for best effort uncast streamng multmeda or real-tme applcatons. However t provdes no relablty and no congeston control. It does not assure that the sent datagram reaches ther destnaton. Some enhancements were proposed to reform ths protocol such as equaton-based congeston control for uncast traffc [6]. Unlke UP, TCP offers a connecton-orented, relable delvery servce for a stream of data sent from one machne to another wthout duplcaton or data loss over an IP network. One of the most crtcal ssues for TCP s congeston control. It won t overshoot the bottleneck bandwdth va properly slowng down ts congeston wndow. It s also a gentle protocol to share the network capacty wth others. Accordng to these characterstcs, TCP s not only wdely adopted but also carefully nvestgated on recent advanced networks. The applcatons such as FTP, Web, Telnet, etc. are developed on top of TCP. It s well desgned for the best effort envronment for ts congeston avodance scheme [2]. The TCP sender deploys the addtve ncrease and multplcatve decrease (AIM scheme to enforce congeston avodance. It dctates that the sender ncreases the congeston wndow (cwnd by at most one segment each round trp tme (TT. As three or more duplcate acknowledgements (ACKs are receved by the sender n a row, t ndcates that the segment has been lost. TCP assumes the packet losses as the consequence of network congeston and wll reduce ts wndow accordngly. The congeston avodance mechansm makes TCP robust n the purely wred envronment but not n wreless lnks whch wll experence hgher bt error rates n transmsson [7]. In the wred lnks, the losses are usually caused by congeston; however, the non-congeston losses may occur due to random or burst errors n the wreless lnks. TCP wll unnecessarly half down hs transt wndow sze before retransmttng lost packets or ntate ts congeston wndow and back off ts retransmsson tmer (TO. It s not worth to reduce the sendng rate especally when the event s not caused by congeston. Therefore, TCP wll experence performance degradaton snce t can not dstngush the lost event caused by real congeston or random drop of the wreless characterstcs. A number of researches have been devoted to the ssue of TCP performance over the heterogeneous network and a great effort s underway to fnd solutons of the problem. There are several categores of solutons to mprove TCP performance over the heterogeneous network [8]. The Splt connecton approach hdes the lossy parts of the nternet so that only congeston losses are detected at the source such as the Indrect-TCP []. The Snoop Agent approach [9][0], a lnkaware protocol nvolves the nstallaton of a specalzed agent n the base staton. The drawback of both schemes s that endto-end semantcs of TCP acknowledges are volated. One set of research efforts enhance TCP to dstngush dfferent types of losses. The congeston coherence approach [2][3] utlzes Explct Congeston Notfcaton (ECN [4] to enhance ts udgment on wred or wreless losses; however, the routers are requred to support ECN and there mght not be enough tme to obtan ECN as the buffer at a congested router s overflowng. Another set of research efforts propose schemes to estmate the lnk capablty. TCP Westwood [5] estmates the capacty of the network connecton but t wll have poor performance when the sender estmates the ncorrect

2 bandwdth or the random packet loss rate exceeds a few percent. Among the above solutons, few could conform to the TCP s manpulaton prncple especally end-to-end semantcs. Even f ther schemes can get end-to-end congeston control, they may not have well performance or farness over lossy lnks. In ths paper, we propose congeston dstncton by the tter-based scheme over wreless networks (JTCP. JTCP carres the followng key functons: Jtter rato: Frst we propose tter rato scheme to reflect queue. If any packet s queued, the queued length wll grow and congeston event occurs. Va the tter rato estmaton, we wll detect congeston events. Cooperatng the tter rato and TCP: Snce TCP tself s not suffcent to detect the losses caused by the wreless lnk we apply the tter rato estmaton to ts congeston detecton mechansm. That wll reduce the un-necessary retransmsson by TCP. Next TT: The TCP s reacton duraton s about one TT. If mult-losses occur n the same TT, we consder them as the same congeston event and ust reduce the sendng rate once. Complete performance nvestgaton to JTCP: we smulate several scenaros to fnd out the well parameters for the proposed tter-base scheme. JTCP supports the above functons as a real end-to-end, far, and wreless compatble approach for the current nternet envronment. II. JITTE AN JITTE ATIO The man deal of our proposed scheme s to apply the tter rato on TCP congeston control mechansm. The tter rato s extended form nter-arrval tter. The nter-arrval tter s defned n TP (real-tme protocol s FC document. From the value of tter, we can trace the delay varance of packet-by-packet. Ths characterstc can help us to nvestgate the lnk status. A. Inter-arrval Jtter The nter-arrval tter s the value of packet spacng at the recever compared wth packet spacng at the sender for a par of packets. For example, f S s the sendng tme for packet, and s the recevng tme for packet, then for two packets and, nter-arrval tter (, may be expressed as n: (, ( ( S S ( S ( S ( In other words, the value of nter-arrval tter can demonstrate the packet-by-packet delay. Accordng to the above equaton, there s no delay between packet and packet f s equal to zero. As s greater than zero, t means that the transmt tme of packet s longer than packet. We can assume packet s queued n a whle. There s a bref vew n Fg.. epends on ths phenomenon, and then we defne the tter rato to drect the rato of queued packets. Sender Fgure. Inter-arrval tter B. Jtter ato The tter rato (r s defned as the rato of queued packet. Consder the queue at the bottleneck, and one traffc flow of fxed rate s consumng the bandwdth of the bottleneck. If the packet arrval rate at the router s greater than the router s servce rate, the probablty of un-servced/queued packets wll be proportonal to the arrval rate subtractng the servce rate of the router over the arrval rate. To express the stuaton, let t A (sec be the packet-by-packet delay of the packets arrval at the router, t (sec be the delay of the packets departure from the router, and B (packet/sec be the servce rate of the router. (2 B t The rato of queued packets at the router could be calculated as follows: t t A B t t t t t t t t ta ta ta A A A A ( t( t( ( ts( ts( ( t ( t ( ( t ( t ( When the ncreasng queue reaches the maxmum lmt of the buffer, the followng arrvng packets at the router wll be dropped. Therefore, the rato of packets dropped (lost wll be approxmated as the rato of queued packets. Ths rato can be treated as the loss rato predctor that s formulated by the nter-arrval tter and s defned as the followng: Jr t t Queue ( ( Jr (Jtter rato s an mportant gude to determne whether the packets are queued or not. In our proposed scheme, the congeston event s defned as the tme when the trple duplcate ACKs are trggered and the tter rato s much sgnfcant. Now we wll enter the practcal method to enhance the TCP s performance shown n next secton. < 0 > 0 ecever (3 (4

3 III. POPOSE SCHEME: JTCP Snce the tter rato could pont out the connecton condton between the source and the destnaton, we apply the tter rato on TCP. The JTCP wll be able to dstngush the congeston loss and non-congeston loss. A. Combnaton of tter rato and TCP From the descrpton of secton 2, TCP wll send packets when recevng a new ACK. We know the reacton tme of TCP s about one round trp tme so the JTCP s sample tme for an average tter rato wll be set one TT. Accordng to the TCP s behavor of the sldng wndow, f the current wndow sze s w, t means TCP sends w segments n one round trp tme prevously. Now, we know the requred date to estmate JTCP s average tter rato s the current congeston wndow sze and sequence number, see Fg. 2. Acked cwnd n-w n- n w current cwnd w JTCP wll determne f these ACKs arrve wthn a TT. If the perod for duplcate ACKs has been extended to the next TT, we consder them as one congeston event (no matter how many duplcate ACKs occur durng one TT such as TCP Newreno. Otherwse, an mmedate recovery wll proceed. Snce the reacton tme of TCP s about one TT and the senders whch half ther congeston wndows redundantly wll degrade the performance. The value of w dentfes the current wndow sze and k s the control parameter. The k/w rato denotes that k packets wll be queued at the router when TCP nects w packets nto the network. Snce TCP only adds one segment per TT n AIM scheme, we know the k shouldn t greater than. JTCP qualfes a congeston event wth three factors: ( the sender receves n (e.g. trple duplcate ACKs, (2 the event s longer than one TT (Next TT and (3 the tter rato s more than the value of k/w. As long as the three stuatons are satsfed, JTCP wll half down the congeston wndows as TCP eno. On the other hand, f the tter rato s less than the value of k/w, the duplcate ACKs are caused by lossy lnk loss and JTCP only enters the mmedate recovery phase. One TT Sendng packets Fgure 2. congeston wndow of TCP durng one TT TACKs We can get TCP s average tter durng one TT as the followng: cwnd cwnd/2 ssthreshold cwnd Next TT NO Immedate recovery n n S S n w n w ( ( (5 ( ( S S n n w n n w We defne the average tter rato of JTCP as r ( m ( n w, n n m ( n w Note m s a parameter that determnates the amount of pervous TT taken nto account. The greater m s set and the more hstory we consder. If the value of m s huge, t wll make low response to adust the sendng rate. In our experments, we set the m to make fast response and acheve good performance. The prevous one TT s enough to udge the state of the communcaton lnk. The value of n denotes the sequence number of earlest non-acked packets and w denotes the current congeston wndow sze. In our proposed scheme, we apply the value of tter rato (r to TCP s congeston avodance algorthm. Accordng to the sgnfcance of r, we want to fnd the rule between congeston event and r value. The rule can help us to know that the congeston event s composed of packet losses and tter rato. B. JTCP acton after recevng trple duplcate ACKs Fg. 3 shows the state dagram of JTCP after recevng trple duplcaton ACKs. As three duplcate ACKs occur, (6 YES - Congeston Event YES Jr > k/w Fgure 3. agram of JTCP after recevng TAC NO - Non-congeston Even Next TT: The tme between recent and prevous TACKs s longer than one TT If ( TACKs are receved If (r > k / cwnd & In Next TT // Congeston E ssthreshold / 2 cwnd else // packets lost but not caused by congeston endf endf // Fast ecovery // Immedate ecovery ssthreshold * cwnd Fgure 4. code of JTCP after recevng TACKs

4 Fg. 4 s the pseudo code of JTCP after recevng TACKs. In the mmedate recovery, we set a decrease factor ( to whch wll be greater than /2. If s, t means the sender only sends out the lost packet and doesn t reduce ts sendng rate. We have expermental evdence for the approprate value of k and (see next secton. The purpose of JTCP s to reduce the unnecessary degradaton to mprove TCP s performance. C. JTCP acton after the tmer expres After the tmer expres, we stll check whether the tter rato s greater than k/w or not. If r s less than k/w, then we ust half down the congeston wndow nstead of applyng slow start. We consder the lossy lnk may have burst losses to lead to the tmer expraton. The burst losses may be caused by weak sgnals n long dstance or handoff of moble devce. We decrease one-half wndow sze and try to do fast recovery because of the non-congeston event. The pseudo code of the algorthm s n Fg. 5. If ( Tmer expre If (r > k / cwnd // enter slow start phase sstrheshold / 2 cwnd cwnd else // the loss s occurred by lossy lnks Send the loss packet endf endf Fgure 5. Pseudo code of JTCP after tmer expres IV. SIMULATION AN ANALYSIS A set of smulaton experments are performed to evaluate the effectveness of JTCP. All smulatons are performed wth network smulator 2 (NS-2 [23] by mplementng a par of JTCP agents. Fg. 6 shows the typcal network topology of a wred/wreless mxed network. S denotes the source node, and 2 are routers, and s the destnaton node. The wreless lnk whch has been modeled wth an error module represents the last hop transmsson between 2 and. Errors are assumed to occur n both drectons of the wreless lnk. The propagaton tme over the wred lnk s ntally assumed to be 45ms. The bottleneck bandwdth s 2Mbps between and 2 and the packet sze s 000 bytes. S sstrheshold / 2 cwnd 00M 5ms 2M 40ms 2 0M 0.0ms Wreless Lnk Fgure 6. Smulaton Network topology A. Wreless TCP comparson The losses may be random or burst drops over wreless lnk. In followng scenaros, we apply the error models to smulate the lost packets. Compared wth other verson modfcatons of TCP (eno, Newreno, and Westwood, we wll show JTCP has a great mprovement upon the lossy lnks. Frst the case of the random drop errors wll be shown and the case of the varyng propagaton tme wll be ntroduced later. The total tme of ths smulaton s about 300 seconds. The normal error model s attached on the wreless lnk. We compare the throughputs of JTCP wth several verson TCP assumng ndependent errors rangng from 0 to 0% packet loss probablty. In Fg.7, JTCP has more than 20% mprovement for the to 5% packet error rates. Even thought the packet dscard rate reaches 0%, the average throughput for JTCP s stll hgher than Westwood whch s desgned as end-to-end semantcs to mprove TCP over the heterogeneous network. Average Throughput (Mbps Fgure 7. Average throughput vs. packet loss rate JTCP eno Newreno Westwood B. Inter-protocol Farness and Utlzaton One of the key concerns for TCP mplementatons s farness that every connecton can get the far share bandwdth of the bottleneck. In ths subsecton, a set of experments are performed to address the ssue. Ther defntons are: Throughput total _ packet packet _ sze smulaton _ tme (7 Utlzaton Throughput 00(% bottlenect _ bandwdth Farness n 2 b n 2 n b b denotes the fracton of the bandwdth taken up by connecton on the lnks. From the farness equaton, we can see that the value of Farness ranges form /n to, wth correspondng to best allocaton among all connectons.

5 In ths scenaro, we nect multple flows (2 to 30 nto the lnks and calculate ther utlzaton and farness values. TCP eno s well known can get far network capacty. We can contrast the farness of JTCP wth eno over the lossy lnks wth % and 5% loss rate. In the Fg. 8 and Fg. 20, three wreless TCP (JTCP, eno, Westwood almost have the same utlzaton whch s hgher than nnety percent when the flow number s hgh. It s sgnfcant target that the JTCP gans better utlzaton than others when there are fewer flows. The lossy lnk wll drop any packet for each connecton. If there are fewer flows, the packet loss rate wll have a great effect upon each flow. The results of farness are shown n Fg. 8. Even though the JTCP s farness sn t the best, the farness stll keeps upon Farness JTCP eno Westwood The number of flows Fgure 8. Farness wth % loss r V. CONCLUSION Ths artcle presented a novel protocol, JTCP to mprove the performance of TCP n the networks wth wreless lnks and moble hosts. The proposed transport soluton has acheved end-to-end congeston control semantcs, farness and wreless adaptablty. JTCP has been expermented through several smulatons to show consderable throughput gans over other versons of TCP. The artcle ntroduces the concept of tter rato and the mplementaton of JTCP. It uses extra nformaton n the TCP header to store the sendng and recevng tmestamp. Accordng to the tmestamps, we can estmate the tter rato. When the losses occur, the sender receves trple duplcate acknowledgements or the retransmt tmer expres, we wll compare the value of tter rato wth the nverse value of current congeston wndow sze to dstngush the network congeston losses or non-congeston losses. Ths s a smple scheme but enhances TCP robustness for the heterogeneous wreless network. In smulaton experments, JTCP not only performs as well as regular TCP, eno verson, n wred envronment but also gans better utlzaton and farness than other versons of TCP lke New eno, Westwood. The artcle demonstrates that as the tter rato s greater than /w the queued packets are ncreasng n the bottleneck buffer. As long as the packets are queued, the congeston event occurs. On the contrary, the packet losses may occur by the characterstcs on the wreless lnk. It s crtcal for next generaton wreless networks to avod the unnecessary retransmt and degradaton. As mentoned n the above sectons, we use the course tter rato scheme to enhance TCP eno. Under extremely hgher bt error rate, JTCP mght not be able to dstngush the wreless losses by comparng the tter rato value and /w. In the experment observatons, the tter rato value may be utlzed to estmate the number of the packets whch are queued at the router accordng to the sendng rate and the growng rate of the tter rato. The contnuous efforts wll go to the analyss of the tter-base model and desgn a precse scheme to adapt the varous wreless networks. 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