internet networking recitation 11 n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Internet Networking recitation #11 PowerPoint Presentation
Download Presentation
Internet Networking recitation #11

Loading in 2 Seconds...

play fullscreen
1 / 12

Internet Networking recitation #11 - PowerPoint PPT Presentation


  • 97 Views
  • Uploaded on

Internet Networking recitation #11. TCP Eiffel (RFC 3522). Motivation. Allows a TCP sender to detect if it had entered loss recovery unnecessarily. Detects whether a fast retransmit or a timeout was spurious upon the first acceptable ACK that arrives during loss recovery.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Internet Networking recitation #11' - cirila


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.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
internet networking recitation 11

Internet Networkingrecitation #11

TCP Eiffel (RFC 3522)

Spring Semester 2007, Dept. of Computer Science, Technion

motivation
Motivation
  • Allows a TCP sender to detect if it had entered loss recovery unnecessarily.
    • Detects whether a fast retransmit or a timeout was spurious upon the first acceptable ACK that arrives during loss recovery.
    • Robust against the loss of ACKs.
  • Requires TCP Timestamps option be enabled for a connection (RFC 1323).
    • Eliminates the retransmission ambiguity in TCP
    • Based on the timestamp of the first acceptable ACK that arrives during loss recovery
    • Decides whether loss recovery was entered unnecessarily.
  • Performs a response algorithms to back out of loss recovery by restoring a TCP sender's congestion control state.
    • Restoring the TCP sender's congestion control state.
    • Avoiding unnecessary go-back-N retransmits.

Internet Networking

retransmission ambiguity
Retransmission Ambiguity
  • A TCP sender unable to distinguish whether the first acceptable ACK that arrives after a retransmit was sent in response to the original transmit or the retransmit.
    • This problem occurs after a timeout-based retransmit and after a fast retransmit.
  • The Eiffel detection algorithm uses the TCP Timestamps option to eliminate the retransmission ambiguity.
    • Useful in environments where TCP's loss recovery and congestion control algorithms may often get falsely triggered.
      • This can be caused by packet reordering, packet duplication, or a sudden delay increase in the data or the ACK path that results in a spurious timeout.

Internet Networking

falsely trigger tcp loss recovery
Falsely Trigger TCP Loss Recovery
  • Spurious timeout
    • May be caused by increased delay that suddenly occurs in the data and/or the ACK path.
    • Then an acceptable ACK to arrive too late, i.e., only after a TCP sender's retransmission timer has expired.
    • Unnecessarily forces a TCP sender into slow start and causes a go-back-N retransmissions.

1

Ack(2)

2

Ack(3)

2

Time out

Internet Networking

falsely trigger tcp loss recovery cont
Falsely Trigger TCP Loss Recovery (cont.)
  • Packet reordering
    • IP does not guarantee in-order delivery of packets.
    • May cause a single spurious retransmit (the fast retransmit), and the unnecessary halving of a TCP sender's congestion window as a result of the subsequent fast recovery phase.

Ack(1)

1

2

Ack(1)

3

Ack(1)

4

Ack(1)

Ack(4)

Internet Networking

falsely trigger tcp loss recovery cont1
Falsely Trigger TCP Loss Recovery (cont.)
  • Packet duplication
    • May cause a single spurious retransmit (the fast retransmit), and the unnecessary halving of a TCP sender's congestion window as a result of the subsequent fast recovery phase.

Internet Networking

the idea
The Idea
  • Allows a TCP sender to detect a posteriori whether it has entered loss recovery unnecessarily.
  • TCP sender should be able to make this decision upon the first acceptable ACK that arrives after the timeout-based retransmit or the fast retransmit has been sent.
  • Requires extra information in ACKs by which the TCP sender can unambiguously distinguish whether that first acceptable ACK was sent in response to the original transmit or the retransmit.
  • Such extra information is provided by the TCP Timestamps option.

Internet Networking

tcp timestamps option rfc 1323
TCP Timestamps Option (RFC 1323)
  • The sender places a timestamp in each data segment.
  • The receiver reflects these timestamps back in ACK segments.
  • Can be used to measure the RTT.
  • Can be used to correlate between data segments and their ACK’s.

Internet Networking

the detection algorithm
The Detection Algorithm
  • A TCP sender always stores the timestamp of the retransmit sent in the beginning of loss recovery.
    • Retransmit as a result of timeout or dup acks.

Ack(1)

1

2

Ack(1)

3

Ack(1)

4

Ack(1)

Ack(4)

Resend packet 1,

record time stamp

Internet Networking

the detection algorithm cont
The Detection Algorithm (Cont.)
  • If the echo timestamp of the first acceptable ACK, that arrives after the retransmit was sent, is smaller then the stored timestamp of that retransmit
    • that ACK must have been sent in response to an original transmit.
    • ‘Acceptable ACK’ - an ACK that acknowledges previously unacknowledged data.
  • Hence, the TCP sender must have entered loss recovery unnecessarily.

Ack(1)

1

2

Ack(1)

3

Ack(1)

4

Ack(1)

Ack(4)

Resend packet 1,

record time stamp

Smaller time-stamp here

Internet Networking

the detection algorithm example
The Detection Algorithm Example

Without Eiffel Detection

With Eiffel Detection

Detect a spurious timeout

1

1

Ack(2)

Ack(2)

2

2

Ack(3)

Ack(3)

3

3

Ack(4)

Ack(4)

4

4

Time out event

Time out event

2

2

3

5

4

6

Internet Networking

conclusions
Conclusions
  • The Eiffel algorithm eliminates the retransmission ambiguity.
  • Allows to recognize whether a retransmit was triggered by congestion in the network.
  • Detects unnecessary congestion control algorithm activation caused by:
    • packet reordering,
    • packet duplication,
    • spurious timeout.
  • Alleviates the consequences of a falsely triggered loss recovery.

Internet Networking