1. PR-SCTP (Partially Reliable) Ethan M Giordano CISC865 TCP/IP & Upper Layer Protocols University of Delaware November 22, 2005 Some slides/graphics courtesy of: Dr. Paul Amer, Jana Iyengar

2. Some quick background… RFC 3758 May 2004 Implemented technology Designed in part by Dr. Philip Conrad of our department

3. 3 Overview Motivation for PR Service SCTP Extensions – How? PR-SCTP Extension Some Examples Summary

4. 4 Shades of Reliability Reliable (TCP/SCTP) –No-loss –In-order –No duplicates –Data integrity “Unreliable” (UDP) –Maybe loss –Maybe unordered –Maybe duplicates –Data integrity (optional) Partial Reliability (PR-SCTP) –Controlled-loss –In-order –No duplicates –Data integrity

5. 5 The anatomy of a loss event Received & delivered Received & buffered Free buffer Lost Chunk!! What happens to blue chunks while red one is missing? –Waiting! What if blue chunks are time sensitive? –Application is waiting on information that is already available!

6. 6 Partial Reliability Application defined controlled loss –Sender can give up on a message –We will call this “abandonment”

7. “Undo” or “Expire” Application GPS transport Not PR-SCTP!!

8. Controlled loss / partial reliability acceptable  3 frames/sec unacceptable < 3 frames/sec 10 second video 5 frames/sec … 1231087645950 35 … 10 … 13 50 1089 5 179 retransmissions Not PR-SCTP!!

9. 9 Association Setup First part of using an extension is to negotiate it during association setup (similar to TCP SACK) INIT w/ Fwd TSN Supported INIT-ACK w/ Fwd TSN Supported Forward TSN Supported is Param Type 49152 Assigned by ICANN

10. 10 An Example – Actual PR-SCTP FWD TSN 3 Delivery 1 2 3 6-10 Lifetimes 1 2 3 4 10 9 8 4 5 7 3 6 5 2 2 2 2 2 3 3 FWD TSN 4 FWD TSN 5 4 10

11. PR-SCTP Sender can artificially advance the expected TSN value of the receiver –Accomplished by sending a Forward-TSN Receiver then makes available all received data up to this new point and then continues on

12. 12 Message Abandonment An “abandoned” chunk is one which the sender has marked as such for various possible reasons –Expired lifetime without being ACK'd –Explicit decision from upper layer Once abandoned: –Treated as ACK'd and not outstanding –Does not count toward expanding cwnd! –Need to notify the receiver...

13. 13 Notifying the Receiver The heart of the PR extension Send a Forward Cumulative TSN –Otherwise receiver would forever be expecting this chunk which is not coming! But a Fwd-TSN could be lost!! –SACK's from the receiver generate Fwd-TSN's

14. 14 An Example – Actual PR-SCTP FWD TSN 3 Delivery 1 2 3 6-10 Lifetimes 1 2 3 4 10 9 8 4 5 7 3 6 5 2 2 2 2 2 3 3 FWD TSN 4 FWD TSN 5 4 10

15. 15 An Example Adv.Ack.Pt = new variable for PR that tracks where we want the receiver to be expecting. On arrival of a SACK, if Sack.CumAck < Adv.Ack.Pt then Adv.Ack.Pt = Sack.CumAck 2 acked Adv.Ack.Pt -> 3 abandoned 4 abandoned 5 6 2 acked 3 abandoned Adv.Ack.Pt -> 4 abandoned 5 6

16. 16 An Example – Actual PR-SCTP FWD TSN 3 Delivery 1 2 3 6-10 Lifetimes 1 2 3 4 10 9 8 4 5 7 3 6 5 2 2 2 2 2 3 3 FWD TSN 4 FWD TSN 5 4 10

17. 17 An Example 2 acked 3 abandoned Adv.Ack.Pt -> 4 abandoned 5 abandoned 6 2 acked 3 abandoned 4 abandoned Adv.Ack.Pt -> 5 abandoned 6

18. 18 RULE If after moving Adv.Ack.Pt it is greater than the SACK.CumAck the sender must send a Fwd-TSN How is the Fwd-TSN constructed??

19. 19 How does one extend a protocol? Designers allowed 8 bits for the chunk type —256 chunk types!! —Base protocol uses 13 types —That's 243 types for extensions!! FlagsLength Chunk Data Type

20. 20 Introducing: Forward Cumulative TSN Type = 192LengthFlags = 0x00 New Cumulative TSN Stream-1 Stream-n Stream Sequence-1 Stream Sequence-n... Chunk Type of 192 assigned by ICANN Sender-only chunk type!

21. 21 FWD-TSN Construction The basic piece of information is the new CUMULATIVE TSN for the receiver to use Plus an entry for each affected stream Stream data waits in a reorder buffer at the receiver This information in the Fwd-TSN makes it simple for the receiver to know which chunks to now deliver

22. 22 Streams in Detail TCP Receive Buffer 0123 Sequential Sequence (Byte) Numbers SCTP Receive/Reorder Buffers 012 3689 45 10 7 1112 Stream 1 Stream 2 Stream 3 Sequential Stream Sequence (Chunk) Numbers

23. TSN vs Stream Seq. # TSN –Used to ensure reliability Stream Seq. # –Used to provide ordering

24. 24 FDW-TSN Illustrated Stream 2 Stream 1 Stream 3 FWD TSN 3 Delivery 1 2 3,8 6 Lifetimes 1 2 3 4 10 9 8 4 5 7 3 6 5 2 2 2 2 2 3 3 FWD TSN 4 FWD TSN 5 4 10 1 3 2 4 7 5 8 9 6 7 9

25. 25 Why stream info? Type = 192LengthFlags = 0x00 New Cumulative TSN = z Stream-1 Stream-2 Stream Sequence = x Stream Sequence = y Since each chunk is reviewed at sender to create Fwd-TSN, we pass that work to the receiver!

26. RFC – What it defines… “Timed reliability” –All of the examples with lifetime used this Can we think of others?? –Of course…. –RFC gives guidelines for their creation!

27. Questions/Comments/Flames?