The%20Future%20of%20Transport

1
The Future of Transport

• Hari Balakrishnan
• LCS and EECS
• Massachusetts Institute of Technology
• http//www.sds.lcs.mit.edu/hari
• hari_at_lcs.mit.edu

2
Focus

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

3
State-of-the-World, Yesterday ( Today!)
r1
r2
r3
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
r1
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 wont be just HTTP
• The world wont be just TCP

Logically different streams (objects) should be
kept separate, yet congestion management must be
performed.
6
What We Really Need
Apps
Transportinstances
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 theres multicast...
• An evolutionary approach to changing TCP
• But with revolutionary local techniques
• Changes to end-to-end mechanisms (e.g., elements
  of rate control)