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Downloaded at Each Fig IEICE TRANS. INF. & SYST., VOL.E102–D, NO.5 MAY 2019 910 PAPER Special Section on the Architectures, Protocols, and Applications for the Future Internet Analysis of the State of ECN on the Internet Chun-Xiang CHEN†a) and Kenichi NAGAOKA††, Members SUMMARY ECN, as a decisive approach for TCP congestion control, called Explicit Congestion Notification (ECN) has been pro- has been proposed for many years. However, its deployment on the Inter- posed [4]. Unlike the traditional loss-based or delay-based net is much slower than expected. In this paper, we investigate the state of schemes, ECN extends the TCP/IP packet headers and lets the deployment of ECN (Explicit Congestion Notification) on the Internet from a different viewpoint. We use the data set of web domains published the intermediate points on the path of the network indicate by Alexa as the hosts to be tested. We negotiate an ECN-Capable and a Not their congestion state to the receiver. Then the receiver ECN-Capable connections with each host and collect all packets belonging echoes the state back to the transmitter so that the trans- to the connections. By analyzing the header fields of the TCP/IP packets, mitter can adjust the sending rate before the network gets we dig out the deployment rate, connectivity, variation of round-trip time and time to live between the Not ECN-Capable and ECN-Capable connec- crowded. Since ECN-Capable routers on the path are able tions as well as the rate of IPv6-Capable web servers. Especially, it is clear to report the impending congestion before the congestion re- that the connectivity is different from the domains (regions on the Internet). ally happens, ECN is highly expected as a decisive solution We hope that the findings acquired from this study would incentivize ISPs to avoid dropping packets. The effectiveness of using ECN and administrators to enable ECN in their network systems. has been shown in the previous works [6], [7]. Nevertheless, key words: Explicit Congestion Notification, active queueing, congestion control, connectivity other works [8]–[11] show that the deployment of ECN on the Internet is much slower than its expectation even though it was standardized as RFC 3168 [4] in 2001. Although ECN 1. Introduction has been implemented with TCP/IP in most modern operat- ing systems (see Sect. 6.4), it is almost inactivated at initial TCP congestion control has also been an open problem so values. It seems that the ISPs do not have enough incentive far. In the early Internet, the slow-start with AIMD (Ad- to enable ECN by default setting. / ditive Increase Multiplicative Decrease) worked pretty well In this paper, we practically investigate the state of art since the network bandwidth was not high at that time. With of deployment of ECN on the Internet. Through this investi- the rapid development of transmission technology, however, gation, we have managed to answer the following questions it has been approved that TCP slow-start mechanism (so- which are still unclear in the previous works: (1) What is called TCP Tahoe or TCP Reno) could not make good use the current deployment ratio of ECN on the Internet; (2) Is of the transmission bandwidth. To address this issue, a there a marginal (or extra) risk when a host negotiates a con- lot of modified TCP congestion control schemes have been nection with ECN-Capable; (3) Are there any benefits for developed, for example, HighSpeed TCP, Scalable TCP end users to use ECN-Capable TCP connection. (4) Can we and Fast TCP, etc. [1], [2]. These schemes go to conges- provide any convincing data for ISPs and/or end users to in- tion phase and activate their control mechanisms only af- centivize them to enable ECN in their networking devices? ter packet-loss happens in the intermediate network. An- The rest of this paper is organized as follows. Section 2 other approach is based on packet delay such as TCP Vegas, describes the relative works. Section 3 gives an examined etc. [3]. The advantage of these schemes is that they do not results of ECN in a test bed network. The deployment of require any changes at the middle points on the transmission ECN is measured in Sect. 4. The analytical results and dis- path, so they work compatibly with the existing schemes, cussion are illustrated in Sect. 5 and Sect. 6, and concluding which have been employed on the Internet. However, these remarks are given in Sect. 7. schemes are not able to drastically solve the problem exist- ing in the loss-based or delay-based schemes. 2. Related Works To completely solve the problem, another approach so- Manuscript received August 10, 2018. 2.1 Previous Works Manuscript revised December 10, 2018. Manuscript publicized February 27, 2019. The benefits of ECN could be achieved under the condition †The author is with Prefectural University of Hiroshima, that routers on the path between the transmitter and receiver Hiroshima-shi, 734–8558 Japan. †† operate on active queuing management (AQM) [26], where The author is with Dept. of Electronics and Information En- the routers are able to accurately measure the occupancy of gineering, National Institute of Technology, Ishikawa College, Ishikawa, 929–0392 Japan. the queuing and indicate the status of congestion to the re- a) E-mail: [email protected] ceiver [22], [23]. The ECN-Capable receiver feeds the con- DOI: 10.1587/transinf.2018NTP0006 gestion status back to the transmitter so that the transmitter Copyright c 2019 The Institute of Electronics, Information and Communication Engineers CHEN and NAGAOKA: ANALYSIS OF THE STATE OF ECN ON THE INTERNET 911 can adjust the sending rate before the network becomes con- to indicate ECT. ECN-Capable routers treat ECT(0) and gested. This is an ideal scenario. However, it is difficult to ECT(1) codepoints as equivalent [4]. If a router supporting make the changeover from the autonomous distributed Inter- the ECN along the path forwards the data packet with code- net to a whole ECN-Capable network. As a matter of fact, point ECT(0) (or ECT(1)), it is allowed to change the code- the prevalence of ECN is much slower than expected. point from ECT(0) (or ECT(1)) to CE instead of dropping There are many factors which might obstruct the de- the packet according to the occupation of its active queuing. ployment of ECN. ECN works well as long as every point In transport layer, ECN is implemented as an option in along the path is available for supporting ECN. Unfortu- TCP packet header. Two bits in the reserved field of TCP nately, it is likely that a lot of old-model routers, still work- header are assigned to ECN. The first one is ECN-Echo flag ing on the Internet, are incapable of dealing with the ECN (ECE), and the other one is congestion Window Reduced fields in TCP/IP packet header. In extreme cases, they might (CWR) flag. When a TCP source negotiates an ECN-Capable scrape away ECN option or discard the packet with ECN connection, it sets the ECE and CWR bits as well as the option. Also, security gateways or firewalls along the path SYN bit (ECN-setup SYN packet [4]) to indicate that the might block the SYN packet with the ECN option in TCP sender expects an ECN-Capable connection. As reply to packet header and make the ECN negotiation unsuccess- the ECN-setup SYN packet, the TCP receiver replies by ful [5]. Nevertheless, ECN has been considered as an ap- setting the ACK and ECE bits if ECN is available. When proach that is able to avoid the congestion as well as to an ECN-Capable connection has been established, the TCP achieve high throughput efficiency in high-bandwidth and/or sender sets an ECT codepoint in the IP packet header of long-distance networks. data packet to express that ECN is capable. Thus the ECN- To know the current state of ECN, several academic Capable routers along the path could mark the packet with researches have been done [8]–[12]. [8] examined the ECN- CE codepoint to indicate an imminent congestion to the TCP capability about 84,394 web servers on the Internet. The receiver. The receiver echoes back this congestion notifica- percentage of ECN-capability was 1.1% in 2000 and 2.1% tion by marking all acknowledgment packets with the ECE in 2004. [9] also shows that the percentage was about 2.2% flag. When the TCP sender receives the acknowledgment in 2004 and 1.1% in 2008. [10] reported the state of ECN packet with ECE flag, it can appropriately adjust its sending and TCP options on the Internet measured by another ap- rate to avoid the occurrence of congestion. proach. It shows that 25.16% of web servers were ECN- Capable in April 2012, and 29.48% in August 2012. [11] 3. Verifying the Effectiveness of ECN shows the results measured in 3 different vantage points (in London, New York and Singapore). In [12], the au- Performance of ECN and its effectiveness was evaluated by thors measured and analyzed the deployment of ECN in Jan. computer simulation [6] and by testbed [7] as it was pro- 2017. It shows that the ratio of ECN-capability was around posed. However, as hardware and software have greatly 68.11%. However, the number of the web servers used to changed in network environments in the last two decades, be tested is relative small (50000 web servers), and the mea- the effectiveness of ECN should be verified again at a surement was performed at one point.
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