Datagram Congestion Control Protocol

Transcription

1 Datagram Congestion Control Protocol Daniel Hein Advanced Computer Networks May 4th,

2 Outline Motivation Overview Connection Model Options and Features Headers and Associated Capabilities Connection Life Cycle Congestion Control Current Research - Performance 2

3 Motivation Problem: No congestion control => possible network congestion collapse Ratio of congestion controlled (TCP) and uncontrolled traffic (UDP) is changing New applications have arisen that prefer timeliness over reliability and thus use UDP Examples Video Streaming Internet Telephony Static preferable to echo Massive Multi-Player Online Games Only the latest information is important 3

4 Motivation Increase in not congestion controlled traffic threatens Internet stability Congestion control is difficult to implement Especially on the application layer Reinventing the wheel for every application Consequently, many applications that rely heavily on UDP do not implement congestion control 4

5 Overview - Features Unreliable datagrams with acknowledgments Connection oriented Reliable feature negotiation Selection of different congestion control strategies CCID2 TCP-like congestion control CCID3 TFRC congestion control Selective acknowledgment ECN support Designed for robustness against primitive attacks No state for unacknowledged connections Path MTU discovery Integrated support for mobility 5

6 Overview - Deliberate Omissions DCCP design is deliberately minimalistic it does not support: Flow Control No receive window Difficult to get right Selective Reliability No retransmitting of data packets Easily layered on top of DCCP Streams Packet, not byte oriented Trivial to implement on top of DCCP Multicasts Difficult to produce a one size fits all solution DCCP's design does not lend itself to multicasting 6

7 Connection Model A Request Response Ack Data/Ack/DataAck CloseReq Close Reset B DCCP connections are abstracted into two logical half connections AB and BA Half-connection: Data + corresponding acks Features: Set per halfconnection (including congestion control) Example: AB uses CCID2 BA uses CCID3 7

8 Options and Features Options All packets may contain options Appended after generic header Sample Options: Feature negotiation Init cookie Ack vector Timestamp Features Set of parameters specific to a half-connection Reliable, features requests are retransmitted Features other than congestion control include Sequence number parameters Checksum related parameters 8

9 DCCP Headers and Associated Capabilities Source Port Destination Port Data Offset CCVal CsCov Checksum Res. Type 1 Reserved Sequence Number (high bits) Sequence Number (low bits) DCCP uses 16-bit Port Numbers, similar to TCP and UDP 8-bit Data Offset => Up to 1020 bytes for options 4-bit CCval reserved for congestion control algorithm 4-bit Checksum coverage Defines packet corruption handling policy Sometimes corrupt data is preferable to no data (telephony, video) Packages with corrupt header are always discarded! 9

10 DCCP Headers and Associated Capabilities Source Port Destination Port Data Offset CCVal CsCov Checksum Res. Type 1 Reserved Sequence Number (high bits) Sequence Number (low bits) DCCP uses 16-bit Port Numbers, similar to TCP and UDP 8-bit Data Offset => Up to 1020 bytes for options 4-bit CCval reserved for congestion control algorithm 4-bit Checksum coverage Defines packet corruption handling policy Sometimes corrupt data is preferable to no data (telephony, video) Packages with corrupt header are always discarded! 10

11 DCCP Headers and Associated Capabilities Source Port Destination Port Data Offset CCVal CsCov Checksum Res. Type 1 Reserved Sequence Number (high bits) Sequence Number (low bits) DCCP uses 16-bit Port Numbers, similar to TCP and UDP 8-bit Data Offset => Up to 1020 bytes for options 4-bit CCval reserved for congestion control algorithm 4-bit Checksum coverage Defines packet corruption handling policy Sometimes corrupt data is preferable to no data (telephony, video) Packages with corrupt header are always discarded! 11

12 DCCP Headers and Associated Capabilities Source Port Destination Port Data Offset CCVal CsCov Checksum Res. Type 1 Reserved Sequence Number (high bits) Sequence Number (low bits) DCCP uses 16-bit Port Numbers, similar to TCP and UDP 8-bit Data Offset => Up to 1020 bytes for options 4-bit CCval reserved for congestion control algorithm 4-bit Checksum coverage Defines packet corruption handling policy Sometimes corrupt data is preferable to no data (telephony, video) Packages with corrupt header are always discarded! 12

13 DCCP Headers and Associated Capabilities Source Port Destination Port Data Offset CCVal CsCov Checksum Res. Type 1 Reserved Sequence Number (high bits) Sequence Number (low bits) DCCP uses 16-bit Checksum Depending on CsCov Protects the header and optionally also the data 4-bit Type Currently nine different types defined 13

14 DCCP Headers and Associated Capabilities Source Port Destination Port Data Offset CCVal CsCov Checksum Res. Type 1 Reserved Sequence Number (high bits) Sequence Number (low bits) DCCP uses 16-bit Checksum Depending on CsCov Protects the header and optionally also the data 4-bit Type Currently nine different types defined 14

15 DCCP Headers and Associated Capabilities Source Port Destination Port Data Offset CCVal CsCov Checksum Res. Type 1 Reserved Sequence Number (high bits) Sequence Number (low bits) Source Port Destination Port Data Offset CCVal CsCov Checksum Res. Type 0 Sequence Number DCCP supports two different Sequence Number and header lengths Connection establishment and tear-down with 48-bit Sequence Numbers Data may use shorter 24-bit Sequence Numbers to reduce overhead 15

16 DCCP Headers and Associated Capabilities Generic DCCP Header (12 bytes) with Type=2 (DCCP-Data) Options / [Padding]. Data. Sample Header: DCCP-Data Generic short sequence number header Options, e.g. a feature request Must be 32-bit aligned (padded) Data 16

17 DCCP Headers and Associated Capabilities Generic DCCP Header (12 bytes) with Type=2 (DCCP-Data) Options / [Padding]. Data. Sample Header: DCCP-Data Generic short sequence number header Options, e.g. a feature request Must be 32-bit aligned (padded) Data 17

18 DCCP Headers and Associated Capabilities Generic DCCP Header (12 bytes) with Type=2 (DCCP-Data) Options / [Padding]. Data. Sample Header: DCCP-Data Generic short sequence number header Options, e.g. a feature request Must be 32-bit aligned (padded) Data 18

19 Connection Life Cycle - Establishment Client Request Response Ack Data/Ack/DataAck Server Client sends DCCP-Request Request MUST contain Service code 48-Bit sequence number Request MAY contain Feature requests Data CloseReq Close Reset 19

20 Connection Life Cycle - Establishment Client Request Response Ack Data/Ack/DataAck CloseReq Close Reset Server Server answers with DCCP- Response Response must contain Service code 48-bit sequence number Acknowledgment number subheader Request may contain Feature confirmation and request Data 20

21 Connection Life Cycle - Establishment Client Request Response Ack Server Clients responds with a DCCP- Ack and establishes connection Data/Ack/DataAck CloseReq Close Reset 21

22 Connection Life Cycle Data Transfer Client Request Response Ack Data/Ack/DataAck Server Both half-connections can send data DCCP-Data, DCCP-Ack, DCCP- DataAck packets may Use short packet headers Contain options CloseReq Close Reset 22

23 Connection Life Cycle Tear Down Client Request Response Ack Data/Ack/DataAck Server Server transmits a DCCP- CloseReq Starts connection tear down Only server can send CloseReq, Client can use DCCP-Close Server does not want to hold timewait state Must use long sequence numbers CloseReq Close Reset 23

24 Connection Life Cycle Tear Down Client Request Response Ack Server Client acknowledges request with DCCP-Close Again a long sequence number header is mandatory Data/Ack/DataAck Client can always initiate connection tear down on its own with a DCCP-Close CloseReq Close Reset 24

25 Connection Life Cycle Tear Down Client Request Response Ack Data/Ack/DataAck Server DCCP-Reset unconditionally shuts down connection Client holds timewait state to allow remaining packets to clear network CloseReq Close Reset 25

26 Congestion Control Congestion control algorithm depends on application is selected via Congestion Control ID is negotiated upon at connection startup Two congestion control mechanisms have currently been developed CCID2 - TCP-like congestion control CCID3 - TFRC congestion control 26

27 Congestion Control CCID2 TCP-like congestion control, similar behaviour to TCP Abrupt rate changes Fast adaption to rapid fluctuations in available bandwidth Uses Ack Vector Option Equivalent to TCP SACK Uses congestion window Capable to detect reverse path congestion Uses Ack Ratio Feature to control rate of acknowledgment 27

28 Congestion Control CCID3 Suitable for applications that have a more or less constant send rate (voice) Uses sending rate instead of a congestion window Feedback (loss event rate) approx. once per RTT Sender uses loss event rate to set sending rate Must protect against misbehaving receivers Loss interval Uses CCVal header field as timestamp 28

29 Current Research - Performance Most research concentrated on performance DCCP coupled with efficient API performs well for video data on bandwidth limited channels in comparison to UDP Video applications using DCCP-CCID3 fare well against TCP and other TFRC flows, but it is not ideally suited for video because of slow response to rate changes Evaluation of burst traffic using DCCP-CCID2 shows that it works well in most cases, but fairness between DCCP/TCP and DCCP/DCCP is not yet perfect 29

30 E. Kohler, M. Handley, S. Floyd References RFC 4340: Datagram Congestion Control Protocol, March S. Floyd, M. Handley, E. Kohler RFC 4336: Problem Statement for Datagram Congestion Control Protocol, March S. Floyd, E. Kohler RFC 4341: Profile for Datagram Congestion Control Protocol (DCCP) Congestion Control ID 2: TCP-like Congestion Control, March S. Floyd, E. Kohler, J. Padhye RFC 4342: Profile for Datagram Congestion Control Protocol (DCCP) Congestion Control ID 3: TCP-Friendly Rate Control (TFRC) 30

31 E. Kohler, M. Handley, S. Floyd References Designing DCCP: Congestion Control Without Reliability SIGCOMM'06, September 11-15, 2006, Pisa, Italy, E. Kohler, S. Floyd Datagram Congestion Control Protocol (DCCP) Overview, July Junwen Lai, E. Kohler Efficiency and Late Data Choice in a User-Kernel Interface for Congestion- Controlled Datagrams Proc. 12 th Annual SPIE Conference on Multimedia Computing and Networking (MMCN'05) 31

32 References J. Van Velthoven, K.Spaey and C.Blondia Performance of Constant Quality Video Applications using the DCCP Transport Protocol Local Computer Networks, Proceedings st IEEE Conference on Nov Page(s): Shigeki Takeuchi, Hiroyuki Koga, Katsuyoshi Iida, Youki Kadobayashi, Suguru Yamaguchi Performance Evaluations of DCCP for Bursty Traffic in Real-time Applications Applications and the Internet, Proceedings. The 2005 Symposium on 31 Jan.-4 Feb Page(s): Burak Görkemli, M. Reha Civanlar SVC Coded Video Streaming over DCCP Multimedia, ISM'06. Eighth IEEE International Symposium on Dec Page(s):

33 Thank you for your attention! Questions? 33