1. Flow Rate Fairness Many slides are borrowed from Bob Briscoe
Presented by: Yang Guan
March 25, 2010
CISC 856: TCP/IP & Upper Layer Protocols

2. Resource Sharing in the Internet
The Internet is based on a simple premise:
Sharing communication links are more efficient than dedicated channels
The primary sharing algorithm is built into Transmission Control Protocol (TCP)
TCP provides mechanisms to guard people how to share Internet capacity politely
For economical reasons: dedicated is expensive
For channel utilization: a physical channel may remain idle if only one user
We share local area networks at work and neighborhood links from home. Indeed, a multi-gigabit
backbone cable is shared among thousands of folks surfing the Web, downloading videos, and talking on Internet phones.
That is congestion control: TCP constantly does congestion detection that lows its
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3. How TCP shares the Internet
The protocol allows you to seem to be polite
TCP constantly increases transmission rate if it can
Until it sees some sign of congestion, TCP politely reduces bit rate
TCP-friendly if new protocol does not consume more bit rate than TCP does.
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4. TCP-Friendliness TCP is even used as a standard
For applications that do not utilize TCP in transport layer, they are called TCP-friendly if they consume about the same data rate as TCP does.
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5. Does TCP make the world perfect?
The answer is of course NO!
Methods to circumvent TCP-friendliness rules:
Running multiple TCP sessions
Running each TCP session for long time
It is really the application software that determines how to share the Internet fairly
So,does TCP make the world perfect? When I ask this question, you know the answer is NO. Otherwise this is just the end of my talk.
It is really the end application software that determine how to share the Internet fairly
It is not right to solely blame P2P software, the first Web browser opens 4 TCP connections. The TCP itself is problematic.
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6. Fig 1. TCP overlooks users’ activity over time [4]
What does TCP overlook?
The plot visualizes the shares of the ISP's 10Mbps link taken by 10
sample users. Those active at any one moment take equal rates,
regardless of how active each user is over time.
In general, TCP gives 20 shares of bottleneck capacity to the heavy users and only 4 to the light users.
Fig 1. TCP overlooks users’ activity over time [4]
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7. What does TCP overlook? (cont’d)
When there are only 20 active downloading, bandwidth available to each light user can be as low as 20 kilobits, which is worse than dial-up
We see
Simply upgrade physical link does not solve the fairness problem.
Fig 2. TCP overlooks multiple TCP instances [4]
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8. Rethink: What is fair? Equal flow rate?
It is not about how much a TCP consumes
It is about how much a TCP can affect others
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9. Rethink: What is fair? (cont’d)
How to measure the effect on others?
Congestion volume: the amount of data that is sent during network congestion
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10. Rethink: What is fair? (cont’d)
Network neutrality debate, whether ISPs should limit the usage of certain applications.
The same as discussing with fox on how to set up barrier on a chicken coop……
Fig 3. Different TCP sharing schemes [4]
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11. Rethink: What is fair? (cont’d)
Fair is faster:
Light browsing goes blisteringly faster
Heavy downloading is not obviously prolonged
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12. Problems with TCP
Congestion is only detected and managed solely by computers at the edge
ISPs cannot set congestion limits
The few ruin the life of the many
Massive capacity is required
But poor incentive to invest in capacity
To make router as simple as possible.
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13. A New TCP Routine Parameterize TCP with weight The key is
Behave like 12 TCP flows, or
Behave like 0.25 of a TCP flow
The key is
High weights for light interactive usage (web surfing)
Low weights for heavy usage (movie downloads)
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14. A New TCP Routine (cont’d)
Whenever congestion happens
Higher weighted TCP goes much faster
Lower weighted TCP expands back to fast rate afterwards
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15. A New TCP Routine (cont’d)
On today’s Internet, the balance of weights is the wrong way around
How to persuade people to reasonably choose weights?
We should limit people by the effects they have on others—the incremental cost of their usage
Congestion volume: the volume of data sent during congestion
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16. A New TCP Routine (cont’d)
Solution:
ISPs provide a monthly congestion-volume allowance (CVA)
High weights TCPs consumes CVA while low weights ones doesn’t
Heavy usage does not consume CVA since weights are set to be low
Light intensive usage does not consume too much CVA due to short lifetime
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17. congestion policer – one example: per-user policerNB NA R1 S1
overdraft
non-interactive long flows (e.g. P2P, ftp)
congestion volume allowance
interactive short flows (e.g. Web, IM)

18. Making Congestion Visible to Network Layer
Why?
Healthy supply of bandwidth
Reroute data around congested links
Costumers draw down the limited allowances if congestion can not be avoided.
Currently, only the router that drops a packet knows the drop 9 8 7 6 5 3
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19. First Step: ECN Explicit Congestion Notification
Standardized into TCP/IP in 2001
ECN allows end-to-end notification of network congestion without dropping packets[3]
Routers set CE (Congestion Experience) bit when the average queue length exceeds configured threshold levels.
Receivers feedback congestion information back to senders
Traditionally, TCP/IP networks signal congestion by dropping packets. When ECN is successfully negotiated, an ECN-aware router may set a bit in the IP header instead of dropping a packet in order to signal the beginning of congestion. The receiver of the packet echoes the congestion indication to the sender, which must react as though a packet drop were detected.
implemented by most router vendors – very lightweight mechanism
but rarely turned on by operators (yet) – mexican stand-off with OS vendors
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20. First Step: ECN (cont’d)
VER
HLEN
Service Type
Total length
Identification
Flag
Fragmentation offset
TTL
Protocol
Header checksum
Source IP address
Destination IP address
Fig 4. IPv4 header format
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21. First Step: ECN (cont’d)
feedback 2 3 4 5 6 7 8 9 NB NA R S 8 7 6 5 4 3 2
Fig 5. ECN mechanism [5]
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22. Second Step: re-feedback
Fig 6. Re-feedback mechanism [4]
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23. Conclusion
Ready to be implemented as ECN has been included into TCP/IP
Sticks to the Internet e2e principle
Makes congestion visible to the networks in the middle
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24. References
[1] Bob Briscoe (BT), Illustrations by QuickHoney, A Fairer, Faster Internet Protocol, IEEE Spectrum, Dec 2008 pp38-43 [2] B. Briscoe. Flow rate fairness: Dismantling a religion. Computer Communications Review, 37(2):63–74, Apr [3] Explicit Congestion Notification, [4] Bob Briscoe, Internet: Fairer is Faster, BT White Paper [5] Bob Briscoe, et al, Policing congestion response in an internetwork using re-feedback, Sigcomm 2005
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25. Questions
Thanks
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