This preview shows pages 1–3. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.View Full Document
Unformatted text preview: 1 Csci 232: Computer Networks TCP Congestion Control 1 Transport Layer: TCP Congestion Control & Buffer Management Congestion Control What is congestion? Impact of Congestion Approaches to congestion control TCP Congestion Control End-to-end based: implicit congestion inference/notification Two Phases: slow start and congestion avoidance CongWin, theshold, AIMD, triple duplicates and fast recovery TCP Performance and Modeling; TCP Fairness Issues Router-Assisted Congestion Control and Buffer Management RED (random early detection) Fair queueing Readings: Sections 6.1-6.4 Csci 232: Computer Networks TCP Congestion Control 2 What is Congestion? Informally: too many sources sending too much data too fast for network to handle Different from flow control! Manifestations: Lost packets (buffer overflow at routers) Long delays (queuing in router buffers) Csci 232: Computer Networks TCP Congestion Control 3 Effects of Retransmission on Congestion Ideal case Every packet delivered successfully until capacity Beyond capacity: deliver packets at capacity rate Realistically As offered load increases, more packets lost More retransmissions more traffic more losses In face of loss, or long end-end delay Retransmissions can make things worse In other words, no new packets get sent! Decreasing rate of transmission in face of congestion Increases overall throughput (or rather goodput) ! Csci 232: Computer Networks TCP Congestion Control 4 Congestion: Moral of the Story When losses occur Back off, dont aggressively retransmit i.e., be a nice guy! Issue of fairness Social versus individual good What about greedy senders who dont back off? Csci 232: Computer Networks TCP Congestion Control 5 Approaches towards Congestion Control End-end congestion control: no explicit feedback from network congestion inferred from end-system observed as loss, delay approach taken by TCP Network-assisted congestion control: routers provide feedback to end systems single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM) explicit rate sender should send at Two broad approaches towards congestion control: Csci 232: Computer Networks TCP Congestion Control 6 TCP Approach End to End congestion control: How to limit, - How to predict, - What algorithm? Basic Ideas: Each source determines network capacity for itself Uses implicit feedback, adaptive congestion window Packet loss is regarded as indication of network congestion! ACKs pace transmission (self-clocking) Challenges Determining available capacity in the first place Adjusting to changes in the available capacity Available capacity depends on # of users and their traffic, which vary over time! 2 Csci 232: Computer Networks TCP Congestion Control 7 TCP Congestion Control Changes to TCP motivated by...
View Full Document
This note was uploaded on 10/25/2009 for the course CS 232 taught by Professor Susan during the Fall '09 term at GWU.
- Fall '09