1 / 14

Wireless TCP

Wireless TCP. Prasun Dewan. Department of Computer Science University of North Carolina dewan@unc.edu. Problem. TCP layered on top of of IP IP interface provided to TCP is independent of physical layer Implementation dependent on physical layer Wireless just another physical Layer

oshin
Download Presentation

Wireless TCP

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Wireless TCP Prasun Dewan Department of Computer Science University of North Carolina dewan@unc.edu

  2. Problem • TCP layered on top of of IP • IP interface provided to TCP is independent of physical layer • Implementation dependent on physical layer • Wireless just another physical Layer • Problem?

  3. Tuning problem • Working correctly not an issue • Working efficiently is • On wire links losses (normally) due to congestion • On wireless losses can be due to • Unreliable physical medium • Intermittent connectivity • Handoff losses • Can be reduced • With old base station buffering messages • With adjacent base stations joining a multicast group and buffering messages • Reduces delay • TCP/IP policies for wired links will mistake wireless losses and delays for router congestion

  4. TCP/IP Response to Losses • Assume losses due to congestion • Drops (halves) transmission window size • Window size determines how many packets can be sent before waiting for ack • Determines throughput • If losses due to unreliable physical layer needless reduction of throughput

  5. Solution • Distinguish between congestion and other losses • Do not reduce window in response to non congestion losses

  6. Issues • Where in the path from sender to receiver to solve problem? • How to distinguish between the two reasons for losses?

  7. Possible Adaptations • Where in the path from sender to receiver to solve problem? • End-to-end • Sender and receiver together addresses problem • They address congestion loss • They should also address medium loss • Link layer addresses problem • Problem occurred in the link • Should be solved there too • Split connection • One TCP connection from wired end to base, another from base to wireless end • Problem solved locally • But solved at TCP layer (more semantics)

  8. End-to-end • How to distinguish between the two reasons for losses?

  9. Traditional TCP Loss Detection • Timeout • average round trip time + 4*mean deviation • Duplicate acks from receiver • Ack indicates sequence number of next expected message • If message mi gets lost then acks of subsequent messages will have sequence # i

  10. Explicit Loss Notification • When packet dropped over wireless • Subsequent acks indicate loss occurred • How loss detected in Wireless LAN • If corrupted packet • Receiver detects CRC errors • Passes to transport layer • If entire packet is lost • Base station observes duplicate acks • Attaches ELN to them • What if wireless link is sending? • Congestion vs. loss

  11. Problem with End to End • Un-necessary duplicate acks sent all the way to source • Un-necessary retransmissions from source to destination • Does not address wireless sender

  12. Link Level • Handle the problem at the link level,that is where the loss occurred. • Local retransmission instead of end to end retransmission • Link-level timer much smaller (~20ms) • TCP timers larger (multiples of 500 ms) • Depends on end to end delay • Several possibilities • Window of size 1 • Larger window • Cumulative acks • SMART scheme • Cumulative ack mentions sequence # of received packet • Previous packets presumed to be lost rather than reordered

  13. Link Level Problems • Both end to end and link level retransmissions are working • “Incompatible” timers cause retransmission by both parties • Unless > 10% loss retransmissions occur • End to end timeout should not occur • Link-level does not try to deliver in order • Duplicate retransmissions can reach receiver because of out of order delivery • If messages in order, dup acks and end to end retransmissions do not occur, unlike end to end • Can add TCP awareness to link level so dup acks do not occur • Layering broken

  14. Split Connection Algorithm • Break a single TCP connection from wired end to wireless end into • TCP connection from wired end to base station • TCP connection from base station to wireless end • SMART scheme better than RENO since sender does not wait for timeout or duplicate acks • Out of order messages do not occur on single link • End to end out of order delivery does not occur • Sender never gets duplicate acks • Two TCP stacks encountered • Sharing of pointers between stacks at base station helps • End to end semantics violated • Sender can get ack before receiver gets message • Buffer space at base station bounded • Does not ack wired end when this happens? • Original TCP sender can stall • Link level with TCP awareness best

More Related