1. SELECTIVE ACKNOWLEDGEMENT (SACK) DUPLICATE SELECTIVE ACKNOWLEDGMENT
CISC 856 – TCP OPTIONS
SELECTIVE ACKNOWLEDGEMENT (SACK)
RFC 2018
DUPLICATE SELECTIVE ACKNOWLEDGMENT
(DSACK)
RFC 2883
Thanks to Dr.Paul Amer and Pallavi Mahajan
Rajesh Ponnurangam
Computers & Information Sciences
University of Delaware

2. TCP without SACK TCP uses cumulative ACKs
Receiver identifies the last byte of data successfully received
Out of rrder segments are not ACKed
Receiver sends duplicate ACKs
TCP without SACK forces the TCP sender
Either to wait an RTT to find out a segment was lost
Or, unnecessarily retransmit data that has been correctly received
Can result in reduced overall throughput

3. TCP with Selective Ack (SACK)
SACK + Selective Repeat Retransmission Policy allows
receiver informs sender about all segments that are successfully received.
sender fast retransmits only the missing data segments
SACK is implemented using two TCP Options
SACK-Permitted Option
SACK Option

4. SACK-Permitted Option
is allowed only in a SYN Segment.
indicates sender handles SACKs, and receiver should send SACKs if possible.
SACK option can be used once connection is established
TCP Header
NOP
options
kind=1
Source port address
Destination port address
Sequence Number 6
TCP header length
Cumulative Ack No. 1
SYN bit
Window size
Checksum
Urgent pointer
SACK-permitted
kind=4
length=2

5. SACK-Permitted Option and SACK
SENDER
RECEIVER
SYN
“SACK-permitted”
TCP connection establishment phase
SYN/ACK
“SACK-permitted”
ACK
Cumulative Ack No.
Sequence Number
Source port address
Destination port address
SACK-permitted
kind=4
length=2
Window size
Urgent pointer
Checksum
NOP
options
kind=1 1
SYN bit
ACK bit
Cumulative Ack No.
Sequence Number
Source port address
Destination port address
SACK-permitted
kind=4
length=2
Window size
Urgent pointer
Checksum
NOP
options
kind=1 1
SYN bit
Cumulative Ack No.
Sequence Number
Source port address
Destination port address
Window size
Urgent pointer
Checksum
kind=1 1
ACK bit
data transfer phase
cum ack and optional SACKs

6. SACK Option Sequence Number Cumulative Ack No. HLEN Window size
Source port address
Destination port address
Length of SACK with n blocks?
= (2 + 8 * n) bytes
Sequence Number
Cumulative Ack No.
HLEN
Window size
Max number bytes available for TCP Options?
Checksum
Urgent pointer
= 40 bytes
Kind=1
Kind=1
Kind=5
Length=??
Left edge of 1st block
Max number of SACK blocks possible?
Right edge of 1st block
= 4 SACK blocks (barring no other TCP Options)
Left edge of nth block
Right edge of nth block

7. SACK Example 1 - 100 receiver’s buffer 101 - 200 ACK 201 201-300
1-100
ACK 201
sender
receiver
ACK 201 SACK
1-100

8. SACK Rules With SACKs, the ACK field is still a cum ACK
A SACK cannot be sent unless the SACK-Permitted option has been received (in the SYN)
The 1st SACK block MUST specify the contiguous block of data containing the segment which triggered this acknowledgment
If SACKs are sent, SACK option should be included in all ACK’s which do not ACK the highest sequence number in the data receiver’s queue

9. Generating SACKs – data receiver behavior
If the data receiver has not received a SACK-Permitted Option for a given connection, the receiver must not send SACK options on that connection
The receiver should send an ACK for every valid segment that arrives containing new data
The data receiver should include as many distinct SACK blocks as possible in the SACK option
SACK option should be filled out by repeating the most recently reported SACK blocks
The data receiver provides the sender with the most up-to-date info about the state of the network and the receiver’s queue

10. Interpreting SACKs - Data Sender behavior
The sender records the SACK for future reference
Maintains a retransmission queue containing unacknowledged segments
One possible implementation
Turns on SACK bit for the segment in retransmission queue when it receives a SACK
Skips SACKed data during any later fast retransmission
On fast retransmit, retransmits data not SACKed so far and less than the highest SACKed data
Turns off SACK bit after retransmission time out

11. Another SACK Example receiver sender Receiver Buffer 100 299 100-299
500
300
699
ACK 300, SACK
699
300
500
900
1099
ACK 300, SACK ,

12. Another SACK Example (cont’d)
699
300
500
900
1099
sender
receiver
699
300
500
900
1099
ACK 700, SACK
700
300
500
900
1099
ACK 1100
1100

13. Without SACK vs. With SACK
TCP with SACK
TCP without SACK
sender
receiver
ACK 200
ACK 200, SACK
ACK 200, SACK
fast retransmit
ACK 600
ACK 200, SACK
sender
receiver
ACK 200
fast retransmit
ACK 600

14. Data Receiver Reneging
Reneging – fail to fulfill a promise or obligation
Data receiver is permitted to discard data in its queue that has not been acknowledged to the data sender, even if the data has already been SACKed
Such discarding of SACKed segments is discouraged, but may occur if the receiver must give buffer space back to the OS
If reneging occurs
first SACK should reflect the newest segment even if its going to be discarded
Except for the newest segment, all SACK blocks MUST NOT report any old data which is no longer actually held by the receiver

15. reneg occurs; window decreases
Reneging Example
sender
receiver
100
199
ACK 200
200
399
300
ACK 200; SACK
reneg occurs; window decreases
200
200
window increases
599
500
200
ACK 200; SACK

16. Consequences of Reneging
Sender must maintain normal TCP timeouts
Data cannot be considered “communicated” until a cum ACK is sent
Sender must retransmit the data at the left window edge after a retransmit timeout, even if that data has been SACKed by the receiver
Sender MUST NOT discard data before being acked by the Cum Ack

17. SACK Observations
SACK TCP follows standard TCP congestion control; Adding SACK to TCP does not change the basic underlying congestion control algorithms
SACK TCP has major advantages when compared TCP Tahoe, Reno, Vegas and New Reno, as PDUs have been provided with additional information due to the SACK
Difference in behavior when multiple packets are dropped from one window of data
SACK information allows the sender to better decide what to retransmit and what not to

18. Duplicate SACK (D-SACK)
Extension to SACK – RFC 2883
How is SACK option used when duplicate segments are received?
D-SACK does not require separate negotiation between a TCP sender and receiver that have already negotiated SACK
When D-SACK is used, the first block of the SACK option should be a D-SACK block specifying a duplicate segment
A D-SACK block is only used to report a duplicate contiguous sequence of data received by the receiver in the most recent segment
Each duplicate contiguous sequence of data received is reported in at most one D-SACK block

19. Segment replicated by the network
D-SACK Example
Segment replicated by the network
Receiver Buffer
sender
receiver
200
399
ACK 400
600
400
799
ACK 400, SACK
800
400
600
999
ACK 400, SACK
800
400
600
999
ACK 400, SACK ,

20. DSACK – Another example
Receiver Buffer
sender
receiver
500
599
600
1100
1199
1100
600
699
1199
ACK 600, SACK
1100
600
699
1199
800
899
ACK 700, SACK
ACK 700, SACK ,
700
800
899
1100
1199
ACK 900, SACK ,

21. Interpreting D-SACK - Data Sender Behavior
The loss of a single ACK can prevent this information from reaching the sender.
How does sender knows the first SACK block is a D-SACK?
Compares the sequence space in the 1st SACK block to the cum ACK
if seq_space < cum_ACK, then duplicate data has been received
if seq_space > cum_ACK, then sender compares seq_space with the seq_space in 2nd SACK block (if there is one)
if the 1st SACK block is reporting duplicate data that lies above the cumulative ACK, then the 1st SACK block will be a subset of the 2nd SACK block.

22. DSACK Example TCP with SACK & without D-SACK TCP with SACK and D-SACK
sender
receiver
ACK 200
ACK 200, SACK
ACK 200, SACK
fast retransmit
ACK 600
ACK 200, SACK
cwnd =10
cwnd =5
sender
receiver
ACK 200
ACK 200, SACK
ACK 200, SACK
fast retransmit
ACK 600
ACK 200, SACK
ACK 600, SACK
cwnd =10
cwnd =5

23. D-SACK and Retransmissions
D-SACK allows TCP sender to determine when a retransmission was “spurious” (ie, unnecessary) and then undo congestion control measures
D-SACK allows TCP sender to determine if the network is duplicating TCP-PDUs
D-SACK does not allow a sender to determine if both the original and retransmitted data are received, or the original is lost and the retransmitted data is duplicated by the network.

24. SACK and D-SACK Interaction
There is no difference between SACK and D-SACK, except that the first SACK block is used to report a duplicate segment in D-SACK.
D-SACK does not require separate negotiation between a TCP sender and receiver that have already negotiated SACK capability.
D-SACK is compatible with current implementations of SACK option in TCP.

25. Current Implementations of SACK
Windows 2000/XP
Controlled by a registry parameter – SackOpts in “HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters” - SackOpts="1"
Windows Vista
Windows Server 2008 and Windows Vista support TCP SACK
Free BSD and NetBSD have optional modules
Solaris 7 and later

26. References RFC 2018 – TCP Selective Acknowledgement Options.
RFC 2883 – An Extension to SACK option for TCP.
Kevin Fall and Sally Floyd, “Simulation-based Comparisons of Tahoe, Reno, and SACK TCP”, Lawrence Berkley National Laboratory.