Start Me Up: Determining and Sharing TCPs Initial Congestion Window - - PowerPoint PPT Presentation

Start Me Up: Determining and Sharing TCP’s Initial Congestion Window

Safiqul Islam and Michael Welzl Department of Informatics University of Oslo

Motivation

• Each TCP connection maintains states in a data

structure called Transport Control Block (TCB)

• Sharing TCB across parallel connections and

combining their congestion controllers between two endpoints can be beneficial

– Reduce the Flow Completion Time (FCT) of short flows: skipping slow start, immediately using large cwnd, applying priorities

• Do parallel connections follow the same route?

– When they are encapsulated, e.g. VPNs; more in [1]

2 [1] M. Welzl, S. Islam, K. Hiorth, and J. You. TCP in UDP. Internet-Draft draft-welzl-irtf-iccrg-tcp-in-udp-00, Internet Engineering Task Force, Mar. 2016. Work in Progress.

The Problem

100 200 300 400 500 600 700 5 5.5 6 6.5 7

Packet sequence number Time (s) Connection 2 Connection 3

3

• Short flows joining an

aggregate can immediately increase their cwnds

– Lead to sudden bursts – if not paced

The Solution

4

100 200 300 400 500 600 700 5 5.5 6 6.5 7

Packet sequence number Time (s) Connection 2 Connection 3

100 200 300 400 500 600 700 5 5.5 6 6.5 7

Packet sequence number Time (s) Connection 2 Connection 3

• Timer based pacing used by

prior works

• Our approach:

– Maintain the ack-clock of TCP – Using the ACKs of conn 1 to clock packet transmissions of connection 2 over the course

• f the first RTT when

connection 2 joins – Similarly, we make use of the ACKs of connections 1 and 2 to clock packet transmissions of connection 3

FCTs of Short Flows

5

FCT of short flows coupled with our ack-clocked mechanism reduces the FCT

2 4 6 8 10

Capacity (Mbps)

10 20 30 40 50 60 70 80 90

FCT of short flows (RTTs)

• 1. Short flow
• 2. Short flow - coupled (no-ack-clock)
• 3. Short flow - coupled (ack-clocked)

Reduction 2 vs. 1 (%) Reduction 3 vs. 1 (%)

20 40 60 80 100

Reduction (%)