SELECTIVE ACKNOWLEDGEMENT SACK

1
SELECTIVE ACKNOWLEDGEMENT (SACK)

• Presented by
• David R. Cook
• Bruce N. Phan

2
Overview

• Introduction to SACK
• How SACK Works
• Duplicate SACK An extension of SACK
• A Conservative SACK-based Loss Recovery Algorithm
  for TCP
• References

3
Motivation for SACK

• Cumulative acknowledgments give the data sender
  limited information
• Unnecessary retransmissions can reduce network
  throughput
• A SACK mechanism can help overcome this problem
• Using SACK, a data receiver can inform the sender
  about all segments that have arrived successfully

4
How SACK Works

• 2-byte Sack-Permitted Option
• must be sent in a SYN segment to enable the SACK
  option.
• SACK Option
• conveys extended acknowledgement information.

Kind4
Length2
Length2
Kind5
Left Edge of 1st Block
Right Edge of 1st Block
. . .
Left Edge of nth Block
Right Edge of nth Block
5
How SACK Works (continued)

• Normally, a maximum of 3 blocks can be used for
  the TCP SACK Option
• If SACK-Permitted option has not been received,
  the data receiver MUST NOT send SACK options
• If the data receiver SHOULD only use the SACK
  option if it is to be used under all
  circumstances
• The first SACK block indicates the most recent
  contiguous segment that was received successfully
  which has not been acknowledged.
• Data receiver SHOULD include as many distinct
  SACK blocks as possible.

6
Examples

• Assumptions
• Left window edge 5000
• Number of transmitted segments 8
• Segment size 500 bytes
• Case 1 The first 4 segments are received, but
  the last 4 are dropped.
• The data receiver will return a normal TCP ACK
  segment acknowledging sequence number 7000, with
  no SACK option.

7
Examples (continued)

• Case 2 The first segment is dropped but the
  remaining 7 are received.
• Upon receiving each of the last seven
  packets, the data
• receiver will return a TCP ACK segment
  that acknowledges
• sequence number 5000 and contains a SACK
  option specifying
• one block of queued data
• Triggering ACK Left Edge
  Right Edge
• Segment
• 5000 (lost)
• 5500 5000 5500
  6000
• 6000 5000 5500
  6500
• 6500 5000 5500
  7000
• 7000 5000 5500
  7500
• 7500 5000 5500
  8000
• 8000 5000 5500
  8500
• 8500 5000 5500
  9000

8
Examples (continued)

• Case 3 The 2nd, 4th, 6th, and 8th (last)
  segments are dropped.
• The data receiver ACKs the first packet
  normally. The third,
• fifth, and seventh packets trigger SACK options
  as follows
• Triggering ACK First Block 2nd
  Block 3rd Block
• Segment Left Right
  Left Right Left Right
• Edge Edge
  Edge Edge Edge Edge
• 5000 5500
• 5500 (lost)
• 6000 5500 6000 6500
• 6500 (lost)
• 7000 5500 7000 7500
  6000 6500
• 7500 (lost)
• 8000 5500 8000 8500
  7000 7500 6000 6500
• 8500 (lost)

9
Examples (continued)

• At this point, if 4th packet is received, data
  receiver replies with this SACK
• Triggering ACK First Block 2nd
  Block 3rd Block
• Segment Left Right
  Left Right Left Right
• Edge Edge
  Edge Edge Edge Edge
• 6500 5500 6000 7500
  8000 8500
• At this point, if 2nd packet is received, data
  receiver replies with this SACK
• Triggering ACK First Block 2nd
  Block 3rd Block
• Segment Left Right
  Left Right Left Right
• Edge Edge
  Edge Edge Edge Edge
• 5500 7500 8000 8500

10
Data Receiver Reneging

• The data receiver has the choice to discard
  unacknowledged (cumulatively) data even if the
  data has been previously SACKed.
• This is discouraged, but may be used by the data
  receiver in case the receiver runs out of buffer
  space.
• In this case, the first SACK block must report,
  at a minimum, the left and right edges of the
  newest segment.
• Except for the newest segment, the receiver
  should not report any SACKed data that are no
  longer being held.
• Because the receiver can discard previously
  SACKed data, the data sender MUST NOT discard any
  data that have not been cumulatively acknowledged.

11
Duplicate SACK (D-SACK) An Extension

• This extension to SACK addresses the issue of
  duplicate segments.
• The first SACK block will identify the duplicate
  packet.
• This allows the sender to infer that it has sent
  a duplicate packet.
• This extension does not make changes to TCP
  cumulative acknowledgement. It is compatible
  with other TCP nodes which have not implemented
  this extension.
• Using this implementation does not require any
  additional negotiation between a TCP sender and
  receiver.

12
Guidelines for Reporting Duplicate Segments

• The D-SACK block is only used to report a
  duplicate found in the most recently received
  packet.
• Each duplicate is only reported once.
• The D-SACK block follows the same format as a
  SACK block with left and right edges.
• If a duplicate contiguous segment is found in
  data above cumulative ACK, then the D-SACK block
  will specify the duplicate segment and the 2nd
  SACK block will specify the contiguous segment
  within which the duplicate was detected.
• The remaining SACK blocks can be used to report
  additional received data segments.

13
Examples

• Example 1 Reporting a duplicate segment.
• Because some ACKs were lost, the sender sent a
  duplicate segment, 3000-3499.
• Transmitted Received ACK Sent
• Segment Segment (Including SACK
  Blocks)
• 3000-3499 3000-3499 3500 (ACK dropped)
• 3500-3999 3500-3999 4000 (ACK dropped)
• 3000-3499 3000-3499 4000, SACK3000-3500

14
Examples (continued)

• Example 2 Reporting an out-of-order segment and
  a duplicate segment
• Transmitted Received ACK Sent
• Segment Segment (Including SACK
  Blocks)
• 3000-3499 3000-3499 3500 (ACK dropped)
• 3500-3999 3500-3999 4000 (ACK dropped)
• 4000-4499 (data packet dropped)
• 4500-4499 4500-4499 4000, SACK4500-5000
  (ACK dropped)
• 3000-3499 3000-3499 4000,
  SACK3000-3500, 4500-5000

15
Examples (continued)

• Example 3 Reporting a duplicate of an
  out-of-order segment
• The receiver receives 2 out-of-order segments
  and then receives a duplicate of one of these.
• Transmitted Received ACK Sent
• Segment Segment (Including SACK
  Blocks)
• 3000-3999 3000-3999 4000
• 4000-4499 (data packet dropped)
• 4500-4499 4500-4499 4000, SACK4500-5000
• 5000-5499 5000-5499 4000, SACK4500-5500
• (duplicated packet)
• 5000-5499 4000,
  SACK5000-5500, 4500-5500

16
A Conservative SACK-based Loss Recovery Algorithm
for TCP

• As of April 2003, there was evidence that TCP
  hosts were beginning to implement the SACK
  option however, they were not utilizing the SACK
  information when making retransmission and
  congestion control decisions.
• To encourage TCP implementations to use SACK
  information to increase performance, the authors
  of RFC 3517 presents a conservative loss-recovery
  algorithm.
• The algorithm helps TCP implementations make
  various retransmission decisions, such as
  determining which segments to send next.
• This algorithm is considered to be conservative
  because it allows implementers to use a more
  careful RTO management variant that re-arms the
  RTO timer on each retransmission that is sent
  during recovery.

17
References

• RFC2018 Mathis, M., Mahdavi, J., Floyd, S. and
  A. Romanow, TCP Selective Acknowledgement
  Options, RFC 2018, October 1996.
• RFC2883 Floyd. S, Mahdavi J., Mathis, M.,
  Podolsky, M., An Extension to the Selective
  Acknowledgment (SACK) Option for TCP, RFC 2883,
  July 2000.
• RFC3517 Blanton, E., Allman, M., Fall, K.,
  Wang, L., A Conservative Selective
  Acknowledgment (SACK)-based Loss Recovery
  Algorithm for TCP, RFC 3517, April 2003.
• Fall, K., Floyd S, Comparisons of Tahoe, Reno,
  and Sack TCP, December 1995.
• Floyd S., Issues of TCP with SACK, March 1996.