Transport- TCP.ppt
《Transport- TCP.ppt》由会员分享,可在线阅读,更多相关《Transport- TCP.ppt(44页珍藏版)》请在麦多课文档分享上搜索。
1、Transport: TCP,Manpreet Singh(Slides borrowed from various sources on the web),Announcements (1/2),Everybody needs to join the class mailing list.else I cant communicate class info. Check the class archives to see if someone else has picked the same lecture or TCP application We have a group of mach
2、ines you can use for simulation (snoopy, linus, etc.). You need CSUG accounts to access these machines. Well dig up more machines for those who want to do kernel hacking.,Announcements (2/2),Need a volunteer to give the “post-modern“ E2E lecture 9/9 (in class.). The non-research track students will
3、have to do an initial demo by 11/9. Most of the functionality should be there Allows us to give feed back You time to do performance measurements.,Roadmap,Why is TCP fair ? Loss-based congestion schemes Tahoe Reno NewReno Sack Delay-based congestion control (Vegas) Modeling TCP throughput Equation-b
4、ased congestion control,The Desired Properties of a Congestion Control Scheme,Efficiency (high utilization)Optimality (high throughput, utility)Fairness (resource sharing)Distributedness (no central knowledge for scalability)Convergence and stability (fast convergence after disturbance, low oscillat
5、ion),TCP Fairness,Fairness goal: if N TCP sessions share same bottleneck link, each should get 1/N of link capacity,TCP congestion avoidance: AIMD: additive increase, multiplicative decrease increase window by 1 per RTT decrease window by factor of 2 on loss event,AIMD,TCP connection 1,bottleneck ro
6、uter capacity R,TCP connection 2,Why is TCP fair?,Two competing sessions: Additive increase gives slope of 1, as throughout increases multiplicative decrease decreases throughput proportionally,R,R,equal bandwidth share,Connection 1 throughput,Connection 2 throughput,congestion avoidance: additive i
7、ncrease,loss: decrease window by factor of 2,congestion avoidance: additive increase,loss: decrease window by factor of 2,Loss vs Delay as signal ?,TCP,oscillation,Loss is a binary signal Delay is a multi-bit signal,Simulation-based Comparisons of Tahoe, Reno, and SACK TCP,Kevin Fall Sally Floyd,Int
8、roduction,SACK compared with Tahoe, Reno and New-RenoSimulations designed to highlight performance differences with and without SACK,Comparison,Tahoe: Slow start, congestion avoidance and fast retransmit Reno: Tahoe + fast recovery New-Reno: Reno with modified fast recovery SACK: Reno + selective AC
9、Ks,TCP Slowstart,exponential increase (per RTT) in window size (not so slow!) loss event: timeout (Tahoe TCP) and/or or three duplicate ACKs (Reno TCP),initialize: Congwin = 1 for (each segment ACKed)Congwin+ until (loss event ORCongWin threshold),Host A,one segment,RTT,Host B,two segments,four segm
10、ents,TCP Congestion Avoidance,/* slowstart is over */ /* Congwin threshold */ Until (loss event) every w segments ACKed:Congwin+ threshold = Congwin/2 Congwin = 1 perform slowstart,Congestion avoidance (linear phase),1,1: TCP Reno skips slowstart (fast recovery) after three duplicate ACKs,Fast Retra
11、nsmit,Receiving small number of duplicate ACKs (3) signals packet lossLost packet can be retransmitted before timeout This improves channel utilization,TCP/Reno Congestion Control,Initially:cwnd = 1;ssthresh = infinite (64K); For each newly ACKed segment:if (cwnd ssthresh) /* slow start*/cwnd = cwnd
12、 + 1;else/* congestion avoidance; cwnd increases (approx.) by 1 per RTT */ cwnd += 1/cwnd; Triple-duplicate ACKs:/* multiplicative decrease */cwnd = ssthresh = cwnd/2; Timeout:ssthresh = cwnd/2;cwnd = 1; (if already timed out, double timeout value; this is called exponential backoff),TCP/Reno: Big P
13、icture,Time,cwnd,slow start,congestion avoidance,TD,TD: Triple duplicate acknowledgements TO: Timeout,TO,ssthresh,ssthresh,ssthresh,ssthresh,congestion avoidance,TD,congestion avoidance,slow start,congestion avoidance,TD,Tahoe + Fast Recovery,Fast Recovery,Observation: Each duplicate ACK indicates s
14、ome packet has left pipe,New-Reno extension,New-Reno continues with fast recovery if a partial ACK is received,Old cwnd,New cwnd = (old cwnd)/2,Usable window increased by 1 for each duplicate ACK until ACK for LP is received,Packet 1 lost,Packet 2 lost,LP: Last Packet sent before loss detection,Why
15、use SACK?,Without SACK sender has to use one of following retransmission strategies- Retransmit 1 dropped packet / RTTReno, New-Reno- Retransmit packets that might have been successfully deliveredTahoe,SACK option RFC2018,Ex: 2nd segment dropped (each segment has 500 bytes),SACK Congestion Control (
- 1.请仔细阅读文档,确保文档完整性,对于不预览、不比对内容而直接下载带来的问题本站不予受理。
- 2.下载的文档,不会出现我们的网址水印。
- 3、该文档所得收入(下载+内容+预览)归上传者、原创作者;如果您是本文档原作者,请点此认领!既往收益都归您。
下载文档到电脑,查找使用更方便
2000 积分 0人已下载
下载 | 加入VIP,交流精品资源 |
- 配套讲稿:
如PPT文件的首页显示word图标,表示该PPT已包含配套word讲稿。双击word图标可打开word文档。
- 特殊限制:
部分文档作品中含有的国旗、国徽等图片,仅作为作品整体效果示例展示,禁止商用。设计者仅对作品中独创性部分享有著作权。
- 关 键 词:
- TRANSPORTTCPPPT
