1. The Future of Transport
Hari Balakrishnan
LCS and EECS
Massachusetts Institute of Technology

2. Focus Congestion management Heterogeneous technologies
New applications
New application traffic patterns
Heterogeneous technologies
Wireless
Asymmetric networks
Large and small pipe size technologies

3. State-of-the-World, Yesterday (& Today!)
Independent TCP streams
r-n
1. Far too inefficient (multiple slow starts, etc.)
2. More alarmingly, far too aggressive
n connections, 1 sees loss; window
decreases only by (1 - 1/2n)

4. State-of-the-World, Today
Put everyone on same
ordered byte stream r2 r3
r-n
While this fixes some of the problems of independent
connections, it really is a step in the wrong direction!
1. Far too much coupling between objects
2. Far too application-specific

5. What is the World Heading Toward?u1 r1 u2 r2 u3 r3
u-m
r-n
The world won’t be just HTTP
The world won’t be just TCP
Logically different streams (objects) should be kept separate, yet congestion management must be performed.

6. What We Really Need…
Apps
Transport instances
Congestion management
Per-host & per-domain information IP
An integrated approach to end-to-end congestion management for the Internet

7. Some Salient Features Shared learning
Heterogeneous application support
Simple application interfaces to congestion manager
Robust and stable network behavior
Flexible bandwidth-apportioning using receiver hints
First step: Transport-Independent Congestion Control (TICC)

8. Heterogeneous Technologies
Non-congestion losses (“errors”)
Asymmetry
Bandwidth
Latency (delay variations)
Pipe sizes
Large pipes
Small pipes

9. Errors + Congestion
Some people think that we need to split connections to perform well: This is wrong!
Careful design of link-layer and transport-aware link protocols work very well
Explicit Loss Notification (ELN) helps sender decouple loss recovery from congestion control

10. Asymmetry
Network and traffic characteristics in one direction affect performance in the other
Bandwidth, latency (variability), media-access, loss rate…
TCP improvements
ACK filtering (purge “redundant” ACKs)
Sender adaptation (rate-controlled transmissions, byte-based window increases)
ACK reconstruction
ACK congestion control (Padmanabhan98)

11. Pipe sizes Large pipes are problematic
Timeouts when multiple losses occur
SACK fixes this (plus timestamp, PAWS, etc.)
The rtt-bias unfairness problem remains…
How big an rtt before TCP is unusable?
Small pipes are the more pressing problem!
Far too many timeouts
55% of all recovery in one traffic trace of a busy Web server (over 1.6 million connections)
A solution: Newreno + Enhanced Recovery (ER)
Follow packet conservation, sending new probe packets upon duplicate ACKs
No timeouts unless congestion is “persistent”

12. Conclusions: Revolution or Evolution?
A revolution in congestion management
To accommodate heterogeneous applications
But incremental deployability is critical
And then there’s multicast...
An evolutionary approach to changing TCP
But with revolutionary “local” techniques
Changes to end-to-end mechanisms (e.g., elements of rate control)