1. NR-SACKs (Non-Renegable Selective Acknowledgments )‏
I-D.natarajan-tsvwg-sctp-nrsack-00 (work in progress)
CISC 856 TCP/IP & UPPER LAYER PROTOCOLS
Ertuğrul Yılmaz
November 29, 2007
Thanks To:
Dr. Paul Amer

2. Data Acknowledgment
Cumulative Acknowledgment (ACK) Cum-Ack n: -all data up to but not including byte n (TCP) all TSNs up to and including TSN n (SCTP)
Selective Acknowledgment (SACK) Informs data sender of data received out-of-order
Duplicate SACK (D-SACK) Informs data sender of duplicate data
In SCTP, these three types of acknowledgment information are carried in a SACK chunk
I will refer to these Acks as: cum-acks, gap-acks, dup-TSNs

3. Reneging Example receiver sender TSN = 10 10 cum-ack 10 TSN = 11&12 1013
cum-ack 10; gap-ack 3-3
sender
receiver
Window decreases
TSN = 14
Window increases
TSN = 15 15
cum-ack 10; gap-ack 5-5
:Loss
:rwnd -receive buffer

4. Reneging reneging - receiver gap-acks data and later discards it
Reneging is discouraged, but permitted (Receiver might run out of buffer space)
Sender’s responsibility to recover from reneging:
keep gap-acked data chunks in retransmission queue until the data chunks are cum-acked
if the receiver reneges on a data chunk, then reconsider that chunk for retransmission
Retransmission queue contains the data chunks that have been transmitted but NOT yet cum-acked

5. SCTP multistreaming revisited5 6 7 8 9
sent from application
delivered to application
objects in send buffer
cum-ack 8
cum-ack 1 ;
Gap-ack 2-3
cum-ack 1 ;
Gap-ack 2-2
cum-ack 9
cum-ack 1 ;
Gap-ack 2-6
cum-ack 1 ;
Gap-ack 2-7
cum-ack 1 ;
Gap-ack 2-5
cum-ack 1
cum-ack 1 ;
Gap-ack 2-4
receive buffer
retransmission
loss
stream 1
stream 2
stream 3
SCTP association
Web client
Web server

6. NR-SACK
Cumulative Ack– cum-ack Cum-Ack n: ACKs all TSNs up to and including TSN n
Selective Ack (SACK) – gap-ack Informs data sender of data received out-of-order
Duplicate Selective Ack (D-SACK) – dup-TSN Informs the data sender of the duplicate data
Non-Renegable Selective Ack (NR-SACK) –nr-gap-ack Informs data sender of out-of-order TSNs that are NOT renegable
cum-ack, gap-ack, dup-TSN, and nr-gap-ack info are carried in NR-SACK chunk

7. 5 6 7 8 9 Now with NR-SACKs sent from application
delivered to application
objects in send buffer
cum-ack 8
cum-ack 1 ;
Gap-ack 2-3
cum-ack 1 ;
Gap-ack 2-2
cum-ack 9
cum-ack 1;gap-ack 2-6
nr-gap-ack 4-6
cum-ack 1;gap-ack 2-7
nr-gap-ack 4-7
cum-ack 1;gap-ack 2-5
nr-gap-ack 4-5
cum-ack 1
cum-ack 1;gap-ack 2-4
nr-gap-ack 4-4
receive buffer
retransmission
loss
stream 1
stream 2
stream 3
SCTP association
Web client
Web server

8. With NR-SACK
The receiver takes the responsibility for the nr-gap- acked data chunks
The sender is released from the responsibility of keeping those data chunks in its RQ
Benefits: send buffer is utilized better
Remove nr-gap-acked data chunks from RQ
New data chunks can be put into the send buffer in place of the removed nr-gap-acked data chunks
More data can be transmitted within the same amount of time, improved throughput
RQ : Retransmission Queue

9. What data can be nr-gap-acked
With multistreamed SCTP
Any out-of-order data that is in-sequence within its stream can be delivered to the application
SCTP allows for unordered data transfer
Any data tagged as unordered –U flag in data chunk, can be delivered to the application
data delivered to the application should be nr-gap-acked –impossible to be reneged
Any other data that the receiver guarantees never to renege can also be nr-gap-acked

10. Web client never reneges5 6 7 8 9
sent from application
delivered to application
objects in send buffer
cum-ack 8
cum-ack 1 ;
nr-gap-ack 2-3
cum-ack 1 ;
nr-gap-ack 2-2
cum-ack 9
cum-ack 1
nr-gap-ack 2-6
cum-ack 1
nr-gap-ack 2-7
cum-ack 1
nr-gap-ack 2-5
cum-ack 1
cum-ack 1
nr-gap-ack 2-4
receive buffer
retransmission
loss
stream 1
stream 2
stream 3
SCTP association
Web client
Web server

11. SACK CHUNK cum-ack ……………………………………… ……………………………………… gap-acks dup-TSNs
Type = 0x03
Chunk Flags
Chunk Length
gap-acks
Cumulative TSN Ack
Advertised Receiver Window Credit
Number of Gap-Ack Blocks = N
Number of Duplicate TSNs = X
dup-TSNs
Gap Ack Block #1 Start
Gap Ack Block #1 End
………………………………………
Gap Ack Block #N Start
Gap Ack Block #N End
Duplicate TSN 1
………………………………………
Duplicate TSN X

12. NR-SACK CHUNK nr-gap-acks cum-ack ……………………………………… ………………………………………
Type = 0x10
Chunk Flags
Chunk Length
Cumulative TSN Ack
gap-acks
Advertised Receiver Window Credit
Number of Gap-Ack Blocks = N
Number of NR-Gap-Ack Blocks = M
Number of Duplicate TSNs = X
U N U S E D
Gap Ack Block #1 Start
Gap Ack Block #1 End
………………………………………
Gap Ack Block #N Start
Gap Ack Block #N End
dup-TSNs
NR-Gap Ack Block #1 Start
nr-gap-acks
NR-Gap Ack Block #1 End
………………………………………
NR-Gap Ack Block #M Start
NR-Gap Ack Block #M End
Duplicate TSN 1
………………………………………
Duplicate TSN X

13. SCTP With SACKs DATA SACKs(cum-ack,gap-ack,dup-TSNs) INIT INIT-ACK
SCTP Association Startup
INIT
INIT-ACK
COOKIE-ECHO
COOKIE-ACK
SCTP Data Transfer
DATA
SACKs(cum-ack,gap-ack,dup-TSNs)

14. NR-SACK Negotiation DATA NR-SACKs(cum-ack,gap-ack,dup-TSN,nr-gap-ack)
SCTP Association Startup
INIT – NR-SACKs Supported
INIT-ACK – NR-SACKs Supported
COOKIE-ECHO
COOKIE-ACK
SCTP Data Transfer
DATA
NR-SACKs(cum-ack,gap-ack,dup-TSN,nr-gap-ack)

15. Send Buffer Blocking Send window = min (cwnd, rwnd, send buffer)
Assume cwnd is large enough to transmit new data chunks
Assume rwnd is large enough to transmit new data chunks
The send buffer is relatively small
Send Buffer
TSNs sent but not yet cum-acked
Retransmission Queue
TSNs not yet transmitted
min (cwnd, rwnd)
► Application cannot pass new T-SDUs to the transport sctp
► SCTP transport sender cannot transmit new T-PDUs

16. RQ Utilization With SACKs cum-ack and gap-acks With NR-SACKs
cum-ack and gap-acks and nr-gap-acks
TSNs sent but not yet cum-acked
Retransmission Queue
With NR-SACKs
not yet nr-gap-acked or cum-acked
Retransmission Queue
: Freed up buffer space
RQ : Retransmission Queue,

17. 64K Send Buffer (RTT=90ms)
Throughput(bits/s)
error rate

18. 16K Send Buffer (RTT=90ms)
Throughput(bits/s)
error rate

19. THANK YOU!!!