1

SELECTIVE ACKNOWLEDGEMENT (SACK)

RFC 2018

DUPLICATE SELECTIVE ACKNOWLEDGMENT

(DSACK)

RFC 2883

Rajesh Ponnurangam

Computers & Information Sciences

University of Delaware

CISC 856 – TCP OPTIONS

Thanks to Dr.Paul Amer and Pallavi Mahajan

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

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SACK-Permitted Option

• 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

Cumulative Ack No.

6

TCP header length

Sequence Number

Source port address

Destination port address

NOP NOP

options

kind=1 kind=1

SACK-permitted

kind=4

length=2

Window size

Urgent pointer

Checksum

1

SYN bit

TCP Header

5

SENDER

SACK-Permitted Option and SACK

RECEIVER

SYN“SACK-permitted”

SYN/ACK“SACK-permitted”ACK

TCP connection

establishment phase

data transfer phase

cum ack and optional

SACKs

Cumulative Ack No.

Sequence Number

Source port address Destination port address

SACK-permitted

kind=4

length=2

Window size

Urgent pointer

Checksum

NOP NOP

options

kind=1 kind=1

1

SYN bit

Cumulative Ack No.

Sequence Number

Source port address Destination port address

SACK-permitted

kind=4

length=2

Window size

Urgent pointer

Checksum

NOP NOP

options

kind=1 kind=1

1

SYN bit

1

ACK bit

Cumulative Ack No.

Sequence Number

Source port address Destination port address

Window size

Urgent pointer

Checksum

kind=1 kind=1

1

ACK bit

6

SACK Option

Cumulative Ack No.

Sequence Number

Source port address

Window size

Urgent pointer

Checksum

HLEN

Destination port address

Right edge of nth block

Left edge of nth block

Right edge of 1st block

Left edge of 1st block

Kind=1

Length=??Kind=1 Kind=5

= (2 + 8 * n) bytes

• Max number of SACK blocks possible?

= 4 SACK blocks (barring no other TCP Options)

• Max number bytes available for TCP Options?

= 40 bytes

• Length of SACK with n blocks?

7

SACK Example

1 - 100 receiver’s buffer

sen

der

receiv

er

101 - 200

ACK 201201-300301-400

401 - 500501 - 600

ACK 201 SACK 401-601

1-100 101-200

1-100 101-200

401-500

501-600

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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

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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

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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

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Another SACK Example

sen

der

receiv

er

100-299

ACK 300

100 299

Receiver Buffer

300-499

500-699

ACK 300, SACK 500-700

500300 699

700-899

900-1099

ACK 300, SACK 900-1100, 500-

700

699300 500 900 1099

1100-1299

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sen

der

receiv

er

1100-1299

300-499

699300 500 900 1099

ACK 700, SACK 900-1100

699300 500 900 1099

700-899

ACK 1100700300 500 900 1099

1100

Another SACK Example (cont’d)

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Without SACK vs. With SACK

sen

der

receiv

er

100-199

ACK 200

200-299

300-399

400-499

500-599

ACK 200

ACK 200

ACK 200

fast retransmit

200-599

ACK 600

sen

der

receiv

er

100-199

ACK 200

200-299

300-399

400-499

500-599

ACK 200, SACK 300-400

ACK 200, SACK 300-500

fast retransmit

200-299

ACK 600

ACK 200, SACK 300-600

TCP without SACK TCP with SACK

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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

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Reneging Example

sen

der

receiv

er

200-299

100-199 100 199

ACK 200; SACK 300-400

500-599

ACK 200

ACK 200; SACK 500-600

400-499

300-399200 399300

200

200

599500200

window increases

reneg occurs; window decreases

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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

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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

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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

19D-SACK Example

Segment replicated by the network

sen

der

receiv

er

200-399

ACK 400

200 399

Receiver Buffer

400-599

600-799

ACK 400, SACK 600-800

600400 799

800-999

ACK 400, SACK 800-1000, 600-

1000

800400 600 999

ACK 400, SACK 600-1000

800400 600 999

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DSACK – Another example

sen

der

receiv

er

500-599 500 599

Receiver Buffer

700-799

ACK 700, SACK 800-900,1100-

1200

600 1100 1199

700-899

ACK 900, SACK 800-900,1100-

1200

ACK 600, SACK 1100-1200

600-699

800-899900-999

1000-1099

1100-1199

ACK 700, SACK 1100-1200

1100600 699 1199

700 800 899 1100 1199

1100600 699 1199800 899

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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.

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sen

der

receiv

er

100-199

ACK 200

300-399

400-499

500-599

ACK 200, SACK 300-400

ACK 200, SACK 300-500

fast retransmit

200-299ACK 600

ACK 200, SACK 300-600

200-299

ACK 600

cwnd =10

cwnd =5

cwnd =5

cwnd =5

sen

der

receiv

er

100-199

ACK 200

300-399

400-499

500-599

ACK 200, SACK 300-400

ACK 200, SACK 300-500

fast retransmit

200-299ACK 600

ACK 200, SACK 300-600

200-299

ACK 600, SACK 200-300

cwnd =10

cwnd =5

cwnd =5

cwnd =10

TCP with SACK & without D-SACK

TCP with SACK and D-SACK

DSACK Example

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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.

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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.

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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

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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.