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Reduced TCP Window Size for Legacy LAN QoS

Reduced TCP Window Size for Legacy LAN QoS. Niko F ä rber July 26, 2000. Scenario: Single Switch LAN. 10/100 Legacy Switch. Voice is received from WAN File is loaded from file server Both have to go through buffer at output port. File Server. 100. A. B. Router. C. bottle neck. D.

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Reduced TCP Window Size for Legacy LAN QoS

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  1. Reduced TCP Window Sizefor Legacy LAN QoS Niko Färber July 26, 2000

  2. Scenario: Single Switch LAN 10/100 Legacy Switch • Voice is received from WAN • File is loaded from file server • Both have to go through buffer at output port File Server 100 A B Router C bottle neck D 10 WAN E T2

  3. Outline • TCP flow control basics • Reduced TCP window size for legacy LAN QoS • Simulation results based on ns • Measurements on Stanford campus LAN • Related work • Future work

  4. B = 40 packet/sec D = 0.1 sec N = 2 connections TCP Flow Control Basics • TCP flow control based on window size W (number of packets source is allowed to send without ACK) • Receiver can signal max. window size in ACK • Steady state: W = BxD/N W=2 • TCP does not know B, D, N! • Use loss as implicit sign for congestion: Increase until loss Back off

  5. Reduced TCP Window Size • No loss in throughput as long as output buffer is full – but a few packets are as good as many! • Max. delay in 802.3 LAN segment is 512 bit times TCP could operate in stop-and-wait mode (W=1)! • If we allow a maximum delay of DMAX we can even be more generous, e.g., • DMAX = 10 ms • B = 10 Mbps • 1460 Byte packets • N=1 connection • Advertised window in ACK can be reduced without knowledge of sender/receiver W = 8.56 packets

  6. NS Simulation Scenario • Voice traffic: UDP, 30 ms, 240 byte, 5 s • Data traffic: 3 x TCP, 1024 x 1024 byte (1 MB), start after 1 s • Links: 10 Mbps, full duplex, 0.1 ms delay • Switch: 64 KB buffer TCP bottle neck switch UDP host

  7. NS Simulation Results 64 KB tcp start voice delay [ms] 3 KB data throughput [Mbps] time [s]

  8. Stanford Campus LAN “File Servers” 100-BASE-T tree1 “Gateway” tree1> pftp 5555 oolong bigfile1 4 elaine3 tree2 Campus LAN tree2> pftp 5555 oolong bigfile2 5 bottle neck 10-BASE-T “VoIP Terminal” 10-BASE-T Legacy Switch oolong oolong> pftp 5555 –a –B4096 oolong> tcpdump host oolong > trace.1.4096 oolong> hping elaine3 –1 –i u30000 –d 240 –c 500 1 2 3

  9. 10-BASE-T Measurements 16 KB tcp start voice delay [ms] 4 KB data throughput [Mbps] time [s]

  10. 10-BASE-T Measurements (Cont) • Window size = 2, 4, 8, 16, 32 KByte max voice delay [ms] mean min data throughput [Mbps]

  11. 100-BASE-T Measurements • TCP window size = 4, 8, 16, 32, 64 KByte max voice delay [ms] mean min data throughput [Mbps]

  12. Related Work • Packeteer: PacketShaper • Application classification (Web, SAP, … , VoIP) • Bandwidth management policy • Policy enforcement (TCP window size + ACK pacing) • IETF: Subnet Bandwidth Management (SBM) • RFC-2814, May 2000, now on standards track • QoS in mixed legacy 802.3/802.3p LANs • RSVP-based protocol for coordinating resource reservation between subnets and/or end systems • Just signaling, not enforcement • See WWW-links on project page!

  13. Future Work • More extended NS simulations • Dynamic ACK modification • Set window size a priori or based on delay measurements? • Add traffic in reverse direction • Add multiple hosts – simulate Netergy LAN • Comparison with class based queuing • … • Look at patent by Packeteer • Consider more complex LAN topologies • When is cooperation between T2 modules necessary? • How is cooperation done (e.g. RFC-2816)?

  14. Conclusions • Pros • No changes at server or client required • No changes to other hosts required • No coordination between VoIP terminals required • No/little loss in throughput for data • Simple • Cons • Requires modification of ACKs depending on IP address and number of current connections • Already used by Packeteer • Does not extend directly to more general LAN topologies

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