Performance Interactions Between P-HTTP and TCP Implementations J. Heidemann ACM Computer Communication Review April 1997 김호중 CA Lab., KAIST

2 / 20 Contents Introduction Initial experiment The performance problems Related works Conclusion Critique

3 / 20 Introduction (1/2) HTTP HTTP request per each TCP connection Several connections per a page P-HTTP (Persistent-HTTP) Multiple HTTP requests over a TCP connection Advantages of HTTP 1.1 (RFC2068) Saving CPU time, memory Pipelining request without waiting each response Reducing network congestion Evolving HTTP

4 / 20 Introduction (2/2) Problem : P-HTTP is not HTTP HTTP : similar to FTP P-HTTP : similar to SMTP or NNTP SMTP : batch protocol NNTP : interactive uses are usually in LAN

5 / 20 Initial Experiment (1/2) Experimental framework 10Mb/s Ethernet Sun SPARC 20/71 SunOS with some TCP modification 16KB default TCP window size Slow-start enabled Apache 1.1b4 HTTP 1.0 with “Keep-Alive” on No pipelining 3 types of workload 6651, 3883, 1866 bytes : yahoo’s title page (May 1996)

6 / 20 Initial Experiment (2/2) Initial performance HTTP P-HTTP ServerProtocol 1.1b4 1.1b4 (1 st fix) 1.1b4 (both fixes) Retrieval time 43 ms 605 ms 195 ms 26 ms Std. variation 4.0 ms 10 ms 1.9 ms 8.8 ms

7 / 20 Short-Initial-Segment Problem (1/4) Conflict between TCP’s slow-start algorithm and piggybacking Slow-start algorithm The connection opens its congestion window exponentially Initial window size = 2 segments Piggybacking ACK on return traffic RFC1122 : must ACK at >2 segments

8 / 20 Short-Initial-Segment Problem (2/4) Connection setup request ACK HTTP header & 1 st data segment Waiting ACK … ACK timer expired SERVERCLIENT TCP MSS(max. segment size) : 1460bytes for Ethernet 512 or 536bytes for wide area TCP connection HTTP header : 200~300bytes

9 / 20 Short-Initial-Segment Problem (3/4)

10 / 20 Short-Initial-Segment Problem (4/4) Solution Send HTTP header with initial data Problem of piggybacking Unidirectional data traffic dominates HTTP P-HTTP ServerProtocol 1.1b4 1.1b4 (1 st fix) Retrieval time 43 ms 605 ms 195 ms Std. variation 4.0 ms 10 ms 1.9 ms P-HTTP1.1b4 (both fixes)26 ms8.8 ms

11 / 20 Odd/Short-Final-Segment Problem (1/4) Conflict between SWS avoidance algorithm and BSD TCP small message transfer BSD TCP prevent sending until … A full-size segment can be sent We can send half of the client’s advertised window We can send everything and… Not expecting ACK Nagle algorithm is disabled

12 / 20 Odd/Short-Final-Segment Problem (2/4) Silly Window Syndrome (SWS) Example MSS : 200bytes, initial window : 1000bytes Send 200bytes * 5 segments Receive 1 segment and ACK 800bytes pending, 200bytes available Send 200bytes * 1 segment, … Send 50bytes segment, followed by 150bytes segment Receive 50bytes segment and ACK 950byes pending, 50bytes available Send 50bytes again? Nagle’s algorithm

13 / 20 Odd/Short-Final-Segment Problem (3/4)

14 / 20 Odd/Short-Final-Segment Problem (4/4) Solution Disable Nagle’s algorithm Nagle’s algorithm is for small, interactive packet HTTP P-HTTP ServerProtocol 1.1b4 1.1b4 (1 st fix) 1.1b4 (both fixes) Retrieval time 43 ms 605 ms 195 ms 26 ms Std. variation 4.0 ms 10 ms 1.9 ms 8.8 ms

15 / 20 Slow-Start Re-Start Problem (1/2) Conservative congestion control All data sent has been acknowledged + Nothing has been sent for a retransmission time- out period  Reinitialize congestion window to 1  Slow-start again time for browsing a page > time-out period

16 / 20 Slow-Start Re-Start Problem (2/2) Solutions Omitting to reset the congestion window SunOS Bursty traffic can cause packet loss Reinitiate the window after an idle period Decay the window size over time Still bursty traffic Rate-based flow control Hard to use for new connection : unknown network status

17 / 20 Other Problems Inefficient I/O package Disk  file system cache  (input stream buffer  user buffer  output buffer)  network buffer  network device Memory-mapping the input file Too small socket buffer 2~16Kbytes Can’t utilize the high-speed network

18 / 20 Related Works Moldeklev and Gunningberg Effect of MTU size to TCP transfer efficiency Large-MTU network (ATM) Crowcroft, Wakerman, Wang and Strovica Relation of user- and TCP-level buffering SunRPC traffic over TCP Similar to HTTP request / P-HTTP response

19 / 20 Conclusion P-HTTP over TCP Short-initial-segment problem Send HTTP header with initial data Odd/short-final-segment problem Disable SWS-avoidance algorithm Slow-start re-start problem Rate-based flow control

20 / 20 Critique Good analysis Evaluation Is it realistic P-HTTP traffic? Large images, many objects, … Approaches HTTP header with initial data browser support needed SWS-avoidance Disabled No problem in real environments?