1. Stream Control Transmission Protocol Partial Reliability Extension (PR-SCTP)
Course: CISC 856: TCP/IP and Upper Layer Protocols
Presented by Nasif Ekiz
Computer & Information Sciences
University of Delaware
Thanks to Prof. Paul Amer, Ethan M Giordano

2. Outline Motivation Introduction Partially Reliable Service
Negotiation between end-points
Sender side implementation
Receiver side implementation
Examples
Questions

3. Motivation TCP (Reliable) UDP (Unreliable)
PR-SCTP (Partially Reliable)

4. Motivation

5. Introduction
PR-SCTP is an extension to the Stream Control Transmission Protocol (SCTP) that allows an SCTP endpoint to signal its peer to move the cumulative ACK point forward.
RFC 3758
Implemented Technology:
FreeBSD
Linux
Solaris

6. Reliability Comparison of Transport Layer Protocols
Reliable
Partially Reliable
Unreliable
TCP, SCTP*
PR-SCTP
UDP
No loss
Controlled loss
Loss is possible
In order/unordered* data delivery
In order/unordered data delivery
Unordered data delivery
No duplicates
Duplicates are possible
Data integrity
Data integrity (optional)
Flow control
No flow control
Congestion control
No congestion control

7. Partial Reliability Extension
is achieved through two new elements:
A parameter (Forward-TSN-Supported) in the INIT/INIT-ACK indicates whether the endpoint supports the extension.
A chunk type (FORWARD TSN) indicates the receiver should move its cumulative ACK point forward.

8. Negotiation of Partial Reliability
INIT with Forward-TSN-Supported parameter
INIT–ACK with Forward-TSN-Supported parameter
COOKIE–ECHO
COOKIE–ACK
data

9. Forward-TSN-Supported Parameter
in INIT chunk
Common fields of INIT chunk
Chunk Type 0x01 (1)
Flags = 0
Length = 0x18 (24)
Initiation Tag
Receiver Window
Outbound Streams
Maximum Inbound Streams
Initial Transmission Sequence Number (TSN)
Parameter type 0xC000 (49152)
Parameter Length = 0x0004 (4)
Optional Forward-TSN-Supported parameter

10. Definition of “abandoned” chunk
A PR-SCTP data sender MAY determine that a particular data chunk SHOULD NOT be transmitted or retransmitted further. Such a data chunk is referred as “abandoned”.
When a data chunk is “abandoned”:
The sender MUST treat the data chunk as ACK’ed and no longer outstanding.
The sender MUST NOT advance the cwnd based on the “abandoned” data chunk.

11. Generating a Forward TSN
The data sender maintains a variable called “Advanced Peer Ack Point”.
Data sender MUST process the SACK
When a SACK is received:
If (adv.peer.ack.point < SACKCumAckPoint)
adv.peer.ack.point = SACKCumAckPoint;
Try to further advance adv.peer.ack.point
When a data chunk is marked “abandoned”
Try to advance adv.peer.ack.point
If adv.peer.ack.point > SACKCumAckPoint
Data sender MUST send data receiver a Forward TSN chunk containing the latest value of adv.peer.ack.point.

12. Advancing adv.peer.ack.point with SACK information
A SACK with cum. ack. = 102 arrives
Sender’s Out Queue
TSN
Status …
102
103
abandoned
104
105
106
acked
adv.peer.ack.pnt 
acked
adv.peer.ack.pnt 
adv.peer.ack.pnt 
A Forward TSN with New Cum. TSN = 104 is sent!

13. a.p.a.p. = 0
tsn = 1
Receiver Buffer
tsn = 2
tsn = 3
c.a. = 1 1 1
tsn = 4
tsn = 5
c.a. = 2 1 2 1 2
tsn = 6
tsn = 7
c.a. = 2 6 1 2
a.p.a.p. = 1
c.a. = 2 6 1 2 7
a.p.a.p. = 2
a.p.a.p. = 2
a.p.a.p. = 2
tsn = 3
tsn = 4
tsn = 5
c.a. = 3 6 1 2 3 7 6 1 2 3 7
a.p.a.p. = 3
fwd tsn = 3
a.p.a.p. = 4
fwd tsn = 4
c.a. = 3 6 1 2 3 7
a.p.a.p. = 5
fwd tsn = 5
c.a. = 4 4 6 1 2 3 7
c.a. = 7 4 5 6 1 2 3 7 4 5 6 1 2 3 7
a.p.a.p. = Advanced Peer Ack Point
c.a. = Cumulative ACK

14. Forward Cumulative TSN chunk format
Generated only by data sender
Chunk Type 0xC0 (192)
Flags 0x00 (0)
Length = Variable
New Cumulative TSN
Stream-1
Streams Sequence-1 …
Stream-N
Streams Sequence-N

15. Forward Cumulative TSN chunk construction
For each “abandoned” TSN, if chunk has valid stream and sequence number (ordered delivery), the data sender MUST include those numbers.
Each stream SHOULD be reported once.
If a Forward TSN is sent, the data sender MUST assure that at least one T3-rtx timer is running.

16. Generating a FORWARD TSN Example
A SACK with cum. ack. = 102 arrives
Sender’s Out Queue
TSN
STR
SSN
Status …
102 1 54
103 55
abandoned
104 2 32
105 56
106 33
Chunk Type 0xC0 (192)
Flags 0x00 (0)
Length = Variable
New Cumulative TSN = 105 1 56 2 32
adv.peer.ack.pnt 
acked
adv.peer.ack.pnt 
adv.peer.ack.pnt 
adv.peer.ack.pnt 

17. Receiver Side Implementation of PR-SCTP
When a FORWARD TSN chunk arrives, receiver MUST update its cumulative TSN point
Try to further advance cumulative TSN point
Process FORWARD TSN chunk using stream and stream sequence numbers

18. Advancing Cumulative TSN Point
A FORWARD TSN with new cum. tsn = 103 arrives
Receiver’s In Queue
TSN
Status …
102
received
103
missing
104
105
106
107
cum.TSN.pnt 
cum.TSN.pnt 
cum.TSN.pnt 
cum.TSN.pnt 
The new Cum. TSN point = 105!

19. Stream Reorder Queues Stream 1 1 3 1 1 1 3 Stream 2 2 7 2 7 2 2
a.p.a.p. = 0
tsn = 1 str = 1 seq = 1
tsn = 2 str = 2 seq = 1
tsn = 3 str = 1 seq = 2
c.a. = 1
tsn = 4 str = 3 seq = 1
c.a. = 2
tsn = 5 str = 3 seq = 2
tsn = 6 str = 3 seq = 3
Stream Reorder
Queues
tsn = 7 str = 2 seq = 2
c.a. = 2
a.p.a.p. = 1
c.a. = 2
Stream 1
a.p.a.p. = 2 1 3 1 1 1 3
Stream 2
a.p.a.p. = 2
a.p.a.p. = 2 2 7 2 7 2 2
tsn = 3 str = 1 seq = 2
Stream 3
tsn = 4 str = 3 seq = 1
tsn = 5 str = 3 seq = 2
c.a. = 3 4 6 4 5 6 6 4 5 6
a.p.a.p. = 3
fwd tsn = 3 str = 1 seq = 2
a.p.a.p. = 4
fwd tsn = 4 str = 3 seq = 1, str = 1 seq = 2
c.a. = 3
a.p.a.p. = 5
fwd tsn = 5 str = 3 seq = 2, str = 1 seq = 2
c.a. = 4
c.a. = 7

20. Special Cases A FORWARD TSN is lost! A SACK for a FORWARD TSN is lost!
SACK’s from the receiver will generate FORWARD-TSN’s
If T3-rtx timer expires, a new FORWARD TSN will be generated
A SACK for a FORWARD TSN is lost!
New SACKs from the receiver can inform data sender that the FORWARD TSN is received

21. Special Cases
A FORWARD TSN with new cum ack <= cum TSN pnt arrives at data receiver
Out of date
Send a SACK ( may indicate prev. SACK is lost!)
A FORWARD TSN updates the cum TSN point and a TSN that was skipped (abandoned) arrives
Drop the data chunk
Report the data chunk in SACK as duplicate

22. QUESTIONS?
THANK YOU!