Datagram Congestion Control Protocol (DCCP) CISC TCP/IP and Upper Layer Protocols Presentation by Xiaofeng Han Thanks for Kireeti Valicherla’s slides

2 Overview Motivation Connection process Unreliable packets flow with acknowledgement Features negotiation Choice of congestion control Miscellaneous features

3 DCCP Figure 2-11 TCP/IP Protocol Suite, Behrouz A. Forouzan DCCP: Which Layer?

4 Streaming Media Source: What streaming media needs? Timeliness of data What streaming media doesn’t need? Retransmissions of lost and expired packets

5 D12 D13 A12 D14 - D16 D13 Data is not useful ServerClient Streaming Media Over TCP

6 Streaming Media Over UDP No congestion control in UDP flows Harmful to Internet health ServerClient

7 Streaming Media with SCTP Multi-streams over a single association Uses TCP-like congestion control Retransmission IP network IP A2 IP B2IP B1 IP B3 IP A1

8 Solution: DCCP Reliable connection establishment and termination Unreliable packet flow On-demand congestion control Optional features Security concerns

9 DCCP Response DCCP Request DCCP Ack ClientServer DCCP Connection Setup DCCP ADCCP B Similar to TCP connection setup

10 DCCP Data Transfer Phase ClientServer DCCP DATA DCCP ACK DCCP DATA ACK DCCP DATA

11 DCCP Connection Termination DCCP Close DCCP Reset ClientServer DCCP CloseReq Wait 2 MSL DCCP Close DCCP Reset ClientServer Wait 2 MSL DCCP Reset

12 DCCP Data Transfer Example - 1 Pure Seq #, not bytes Each packet carries a Seq # Seq # increase per packet Pure Acks also consume Seq # DCCP-DATA (Seq #1) DCCP-DATA(seq # 2) DCCP-ACK(seq # 11, ACK # 2) Client Server DCCP-ACK (Seq # 10, Ack #1)

13 No Retransmissions Acks the largest Seq # received DCCP-DATA (Seq #1) DCCP-DATA(seq # 2) DCCP-ACK(seq # 11, ACK # 3) Client Server DCCP-ACK (Seq # 10, Ack #1) DCCP Data Transfer Example - 2 DCCP-DATA(seq # 3) DCCP-DATA(seq # 4) DCCP-ACK(seq # 12, ACK # 4)

14 GSR – Greatest Sequence Number Received GSS – Greatest Ack Number Received Window Size = 8 DCCP Data Transfer Example GSR + 3 * (W/4) GSR + 1 – (W/4) DATA (Seq #20) DATA(seq # 21) ACK(seq # 61, ACK # 24) Client Server DATA(seq # 22) ACK (Seq #60, Ack #20) DATA(seq # 23) DATA (Seq #24) 24

15 GSR – Greatest Sequence Number Received GSS – Greatest Ack Number Received Window Size = 8 DCCP Data Transfer Example - 4 GSR + 3 * (W/4) GSR + 1 – (W/4) DATA (Seq #20) DATA(Seq # 21) : DATA(Seq # 30) Sync(Seq # 61, ACK # 31) Client Server ACK (Seq #60, Ack #20) DATA (Seq #31) Seq # out of range SyncAck (Seq #32, Ack 61) 2032

16 DCCP Packet Types DCCP-Request DCCP-Response DCCP-Ack DCCP-Data DCCP-DataAck DCCP-CloseReq DCCP-Close DCCP-Reset DCCP-Sync, DCCP-SyncAck

17 DCCP Packet Formats Generic Header Additional Fields (depending on type) Options (optional ) Application Data Area DCCP header can be from 12 to 1020 bytes Generic header: 12 bytes Additional fields: fixed length field Options: variable length field

18 DCCP Generic Header Source PortDestination Port Data OffsetCCValCsCovChecksum ResType X=0X=0 Sequence Number

19 DCCP Generic Header Source PortDestination Port Data OffsetCCValCsCovChecksum ResType X=1X=1 ReservedSequence Number (high bits) Sequence Number (low bits)

20 Acknowledgement Sub-Header ReservedAcknowledgement Number (high bits) Acknowledgement Number (low bits) X =1 X =0

21 DCCP Checksum Checksum Coverage (CsCov): 4 bits CsCov = 0: covers the DCCP header, DCCP options, network- layer pseudoheader, and all application data in the packet (possibly some padding) CsCov = 1-15: covers the DCCP header, DCCP options, network- layer pseudoheader, and the initial (CsCov-1)*4 bytes of the packet's application data. Option provides CRC checksum for all application data CsCovChecksum

22 Congestion Control in DCCP Each congestion control mechanism supported by DCCP is assigned a congestion control identifier, or CCID: a number from 0 to 255. CCID 0 and CCID 1 are reserved TCP-like congestion control – CCID 2 TCP friendly rate control (TFRC) – CCID 3 CCID are reserved

23 CCID 2: TCP-like Congestion Control Congestion control as in TCP/IP: Slow start Timeouts Congestion event -> Halve congestion window Abrupt rate changes time cwnd Loss, e.g. timeout (initial) ssthresh

24 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

25 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 and less burst traffic as provided by TFRC CCID 3: TCP Friendly Rate Control

26 CCID 3: TCP Friendly Rate Control s 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.

27 CCID 3: TCP Friendly Rate Control The receiver measures the loss event rate and feeds this information back to the sender The sender 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

28 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.

29 Explicit Congestion Notification ECT set ECN enabled sender ECN enabled receiver CWR set ECE set ACK

30 Features Negotiation Connection attribute on what value two endpoints agree Examples Congestion control identifier (CCID) ECN capable / incapable Data checksum Sequence Window DCCP features are identified by a feature number and an endpoint Notation “F/X” is used

31 F/X Notation A B Feature location for all F/A Feature location for all F/B Feature Remote for all F/B Feature Remote for all F/A Change L (Local) Change R (Remote) Confirm L (Local) Confirm R (Remote)

32 Feature Negotiation Anytime during the connection process Carried in a reliable way Multiple values, priority order Endpoints keep sending packets containing change options, until agreement is reached ( and signalled by Confirm Option) TypeLengthFeature #Value(s)

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

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

35 DCCP: Miscellaneous Features Security Concerns Prevents DDoS attacks – init cookie Prevents Sequence Number Attack Large Sequence Number Sequence and Acknowledgement Number Windows Data Corruption CRC data Checksum Option

36 DCCP: Miscellaneous Features Path MTU discovery DCCP should NOT fragment data DCCP must maintain maximum packet size (MPS) Applications can usually get better error tolerance by producing packets smaller than the PMTU Methods On IPv4 connections whose applications have requested fragmentation, the sender SHOULD send packets with the DF bit not set On IPv6 connections whose applications have requested fragmentation, the sender SHOULD use fragmentation extension headers to fragment packets larger than PMTU into suitably-sized chunks (Those chunks are, of course, unfragmentable.)

37 DCCP and RTP DCCP Figure 28-8 TCP/IP Protocol Suite, Behrouz A. Forouzan

38 DCCP: Summary Transport layer protocol Unreliable datagrams Negotiable features Optional congestion control

39 References Datagram Congestion Control Protocol (DCCP) Eddie Kohler, Mark Handley, and Sally Floyd[May 2005] DCCP Overview Eddie Kohler and Sally Floyd DCCP Eddie Kohler, Mark Handley, Sally Floyd, Jitendra Padhye TCP Friendly Rate Control (TFRC) RFC 3448

40 Questions and Comments ?