TCP Flow Control How large files moves across the Internet
John Hawkinson
Massachusetts Institute of Technology
March 20, 2008 / 6.UAT Concept Talk v1.7
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
1 / 11
Flow Control
“Flow Control” or “Congestion Control” How data moves across the Internet
The Internet is all about moving data When you view a web page, you’re moving a file across the Internet Other ways?
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
2 / 11
Flow Control
“Flow Control” or “Congestion Control” How data moves across the Internet
The Internet is all about moving data When you view a web page, you’re moving a file across the Internet Other ways? Downloading software (e.g., FireFox) Transferring files Watching movies Sending e-mail
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
2 / 11
Flow Control
“Flow Control” or “Congestion Control” How data moves across the Internet
The Internet is all about moving data When you view a web page, you’re moving a file across the Internet Other ways? Downloading software (e.g., FireFox) Transferring files Listening to music Watching movies Sending e-mail Games
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
2 / 11
Flow Control
Packets Data is divided into small packets
Definition To send anything across the Internet, it is divided into small pieces, called packets. Most packets are 1500 bytes long. How big is 1500 bytes?
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
3 / 11
Flow Control
Packets Data is divided into small packets
Definition To send anything across the Internet, it is divided into small pieces, called packets. Most packets are 1500 bytes long. How big is 1500 bytes? 1.5 kilobytes
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
3 / 11
Flow Control
Packets Data is divided into small packets
Definition To send anything across the Internet, it is divided into small pieces, called packets. Most packets are 1500 bytes long. How big is 1500 bytes? 1.5 kilobytes About 250 words
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
3 / 11
Flow Control
Packets Data is divided into small packets
Definition To send anything across the Internet, it is divided into small pieces, called packets. Most packets are 1500 bytes long. How big is 1500 bytes? 1.5 kilobytes About 250 words 110 pixel by 110 pixel black-and-white picture
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
3 / 11
Flow Control
Packets Data is divided into small packets
Definition To send anything across the Internet, it is divided into small pieces, called packets. Most packets are 1500 bytes long. How big is 1500 bytes? 1.5 kilobytes About 250 words 110 pixel by 110 pixel black-and-white picture 39 pixel by 39 pixel color picture (525 packets make a screen!)
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
3 / 11
Flow Control
Packets Data is divided into small packets
Definition To send anything across the Internet, it is divided into small pieces, called packets. Most packets are 1500 bytes long. How big is 1500 bytes? 1.5 kilobytes About 250 words 110 pixel by 110 pixel black-and-white picture 39 pixel by 39 pixel color picture (525 packets make a screen!) An MP3 of this year’s Oscar-winning Best Original Song, Falling Slowly, is 3.7 megabytes, or 1265 packets.
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
3 / 11
Flow Control
Packets Data is divided into small packets
Definition To send anything across the Internet, it is divided into small pieces, called packets. Most packets are 1500 bytes long. How big is 1500 bytes? 1.5 kilobytes About 250 words 110 pixel by 110 pixel black-and-white picture 39 pixel by 39 pixel color picture (525 packets make a screen!) An MP3 of this year’s Oscar-winning Best Original Song, Falling Slowly, is 3.7 megabytes, or 1265 packets. This presentation: 684kb, 468 packets.
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
3 / 11
Flow Control
Links Don’t drop a packet!
Every link on the Internet has finite “bandwidth,” or capacity. Usually measured in bits-per-second, but we can think of it in packets-per-second.
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
4 / 11
TCP
TCP Transmission Control Protocol
Chunks data into packets
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
5 / 11
TCP
TCP Transmission Control Protocol
Chunks data into packets Handles reassembly on the far end
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
5 / 11
TCP
TCP Transmission Control Protocol
Chunks data into packets Handles reassembly on the far end Guarantees in-order delivery
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
5 / 11
TCP
TCP Transmission Control Protocol
Chunks data into packets Handles reassembly on the far end Guarantees in-order delivery Handles packet acknowledgement
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
5 / 11
TCP
TCP Transmission Control Protocol
Chunks data into packets Handles reassembly on the far end Guarantees in-order delivery Handles packet acknowledgement Handles packet retransmission
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
5 / 11
TCP
The Problem Without flow control, shared links fill up; how much can the network support?
Alice, Courteny, and Eileen are in Los Angeles. Bob, Donald, and Frank are in Boston. Three conversations are sharing the same link.
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
6 / 11
Slow Start
Slow Start TCP Solves this with “Slow Start”; gradual window increase
TCP defines a window, maximum number of packets in-flight
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
7 / 11
Slow Start
Slow Start TCP Solves this with “Slow Start”; gradual window increase
TCP defines a window, maximum number of packets in-flight Window starts out as one packet long
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
7 / 11
Slow Start
Slow Start TCP Solves this with “Slow Start”; gradual window increase
TCP defines a window, maximum number of packets in-flight Window starts out as one packet long Every time a packet is acknowledged, window is increased by one
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
7 / 11
Slow Start
More Slow Start
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
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Slow Start
Plot without Slow Start
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
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Slow Start
Plot with Slow Start
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
10 / 11
Appendix
Acknowledgements & References
Excellent further reading, and the source of these graphs: Jacobson, Van. Congestion Avoidance and Control. Proceedings of SIGCOMM ’88 (Palo Alto, CA, Aug. 1988) http://www-nrg.ee.lbl.gov.
John Hawkinson (MIT)
TCP Flow Control
20 Mar 2008 / 6.UAT CT v1.7
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