1. Datagram Congestion Control Protocol
CISC 856
TCP/IP and Upper Layer Protocols
Presentation by:
Kireeti Valicherla
11/30/2004

2. DCCP: Which Layer?
DCCP
SCTP
Figure TCP/IP Protocol Suite, Behrouz A. Forouzan

3. Streaming Media What streaming media needs?
Source:
What streaming media needs?
Timeliness of data.
What streaming media doesn’t need?
Retransmissions of lost packets.

4. Streaming media with TCP
Arbitrary
Delay
D9-D12
A12
D12-D16
A16
D17-D19
D20-D22
Data is not useful
A16
D17

5. Streaming Media with UDP
No Congestion Control in UDP flows. Harmful to internet health

6. Problems with UDP From Network’s point of view:
Congestion
Unfairness
From the application’s point of view:
Firewalls and NAT’s do not always pass UDP traffic

7. Solution: UDP with congestion control
Handshaking during connection setup and teardown

8. Questions?

9. DCCP : Overview Connection setup & teardown: Reliable handshaking
Unreliable packet flow
Bi-directional
Unicast connections
Reliable feedback from receiver to sender
Choice of congestion control
Minimal overhead

10. Similar to TCP connection setup
DCCP connection setup
Client
Server
DCCP A
DCCP B
Similar to TCP connection setup
DCCP Request
DCCP Response
DCCP ACK

11. DCCP Data transfer phase
Client
Server
DCCP DATA
DCCP ACK
DCCP DATA
DCCP DATA ACK

12. DCCP connection termination
Client
Server
DCCP A
DCCP B
DCCP CloseReq
DCCP Close
DCCP Reset

13. Connection dynamics echo those of TCP
Server
Client
Connection dynamics echo those of TCP
Initiation
DCCP Request
DCCP Response
DCCP ACK
Data Transfer
DCCP Data
DCCP ACK
DCCP DATA ACK
Termination
DCCP CloseReq
DCCP Close
DCCP Reset

14. Additional Fields (depending on type)
DCCP Packet Formats
Generic Header
Additional Fields (depending on type)
Options (optional )
Application Data Area
DCCP header can be from 12 to 1020 bytes long
Generic Header -> 12 bytes
Additional Fields -> Fixed length field
Options -> Variable length field

15. DCCP Generic Header Source Port Destination Port Data Offset CCVal
CsCov
Checksum
Res
Type X =
Sequence Number

16. Sequence Number (low bits)
DCCP Generic Header
Source Port
Destination Port
Data Offset
CCVal
CsCov
Checksum
Res
Type X = 1
Reserved
Sequence Number (high bits)
Sequence Number (low bits)

17. Acknowledgement Sub-Header
Reserved
Acknowledgement Number (high bits)
Acknowledgement Number (low bits)
X =1
X =0
Acknowledgement Number(low bits)

18. DCCP data transfer ExampleB A
Each packet carries a seq #
Seq # incremented per packet
Pure Acks also increment seq #
DCCP-DATA(seq # 1)
DCCP-DATA(seq # 2)
DCCP-ACK(seq # 10, ACK # 2)
DCCP-DATA ACK(seq # 3, ACK # 10)
DCCP-ACK(seq # 11, ACK # 3)

19. DCCP data tranfer exampleB
No Retransmissions
Ack # = Greatest Seq # received
DCCP-DATA(seq # 1)
DCCP-DATA(seq # 2)
DCCP-ACK(seq # 10, ACK # 2)

20. Questions ?

21. Features Connection attribute on whose value two endpoints agree
Examples
CCID
ECN incapable
Data checksum
DCCP features are identified by a feature number and an endpoint
Notation “F/X” is used

22. F/X Notation Feature location for all F/B Feature location for all F/A
Feature Remote for all F/B
Feature Remote for all F/A

23. Feature Negotiation
At the connection setup, and whenever either endpoint wants
Carried in a reliable way
Endpoints send packets containing Change options, until agreement is reached( and signalled by Confirm Option)

24. Feature Negotiation Example
CCID/Server agreed as 2
Change R(CCID, 2)
Confirm L(CCID, 2)
CCID/Server agreed as 4
Change R(CCID, 3 4)
Confirm L(CCID, 4, 4 2)

25. Feature Negotiation Example
Change R(CCID, 2)
CCID/Server agreed as 3
Prefer L(CCID, 3)
Confirm R(CCID, 3)

26. Feature Negotiation Example
Change R(CCID, 2)
CCID/Server agreed as 2
Change R(CCID, 2)
Confirm L(CCID, 2)

27. Questions?

28. Congestion Control Mechanisms in DCCP
TCP Like Congestion Control – CCID 2
TFRC Congestion Control – CCID 3

29. CCID2: TCP Like Congestion Control
AIMD behavior exhibited as in TCP/IP:
Slow start
Timeouts
Congestion event -> Halve congestion window
Abrupt rate changes

30. CCID 2: TCP-like Congestion Control
Applications using this:
Respond quickly to changes in available bandwidth.
Must tolerate abrupt changes
Online Interactive Games prefer this kind of congestion control

31. CCID 3 TFRC, [RFC 3448] Equation-based congestion control
Minimizes abrupt changes in sending rate
Maintains longer-term fairness with TCP
Streaming Media doesn’t need responsiveness but prefer
steadier less bursty traffic as provided by TFRC

32. TCP-Friendly Rate Control
The receiver measures the loss event rate and feeds this information back to the sender.
The sender also uses these feedback messages to measure the round-trip time (RTT).
The loss event rate and RTT are then fed into TFRC's throughput equation, giving the acceptable transmit rate.
The sender then adjusts its transmit rate to match the calculated rate.

33. The throughput equations
X =
{R*sqrt(2*b*p/3) + (t_RTO * (3*sqrt(3*b*p/8) * p * (1+32*p^2)))}
Where:
X is the transmit rate in bytes/second.
s is the packet size in bytes.
R is the round trip time in seconds.
p is the loss event rate, between 0 and 1.0, of the number of loss events as a fraction of the number of packets transmitted.
t_RTO is the TCP retransmission timeout value in seconds.
b is the number of packets acknowledged by a single TCP acknowledgement.

34. Congestion related options
Slow receiver option
Receiver sends this option to its sender to indicate it is having trouble keeping up with the sender’s data
Data dropped option
Option indicates that some packets reported as received actually had their data dropped before it reached the application.

35. Explicit Congestion Notification
A network assisted congestion control method
Uses two bits of IP header plus two bits in transport level header:
Sender sets an ECT bit in outgoing packets
If a router is congested, it may set a CE bit in ECT marked packets
Receiver reports CE flagged packets as congestion events
The sender then reduces its sending rate

36. DCCP: Miscellaneous Features
Prevents DDoS attacks
Multihoming and Mobility
Path MTU discovery

37. DCCP and RTP
DCCP
Figure TCP/IP Protocol Suite, Behrouz A. Forouzan

38. DCCP: Summary Transport layer protocol Unreliable datagrams
Bi-directional
Congestion-controlled
unicast

39. References
Datagram Congestion Control Protocol (DCCP) Eddie Kohler, Mark Handley, and Sally Floyd[Nov 2004]
DCCP Overview Eddie Kohler and Sally Floyd
DCCP
Eddie Kohler, Mark Handley, Sally Floyd, Jitendra Padhye