CompNet.Lab3 Report.md

CompNet.Lab3 Report

Can Su 1700012779

[TOC]

Writing Task

• How do you estimate the bandwidth and the delay of a path?

  When a HTTP/2 connection is established, we start to send HTTP/2 PING frames to the server continuously. And when we receive an ACK to the PING, the RTTTHIS_TIME is estimated by the interval between sending and receiving. The RTT is then estimated by the following formula

  ​ RTTNEW = ( 4 * RTTOLD + RTTTHIS_TIME ) / 5 Similarly, the bandwidth is estimate by

  ​ BANDWIDTHTHIS_TIME = BYTES_DOWNLOADED / ( TIMEPING_RECEIVED - TIMEDOWNLOAD_BEGIN ) and ​ BANDWIDTHNEW = ( 4 * BANDWIDTHOLD + BANDWIDTHTHIS_TIME ) / 5

• How do you assign jobs to the three paths?

  The first path initially request for the whole file. Once the response is received, we get the file size D. The other two paths are then set to begin downloading at [ D / 3 ] and 2 * [ D / 3 ] respectively, each requesting about [ D / 3 ] bytes. The range of the first path is also adjusted to 0 ~ [ D / 3 ] - 1. When the fastest path (let's say path i) is about to finish its job, it calculates the remaining time of the other two and chooses the slower one (suppose it's path j). Then we issue a new request on path i and adjust the range of path j based on the equation REMAINING_TIMEi + Di / BANDWIDTHi = Dj / BANDWIDTHj and Di + Dj = BYTES_TO_DOWNLOADj The adjusting process may iterate multiple times due to the imperfection estimation of RTTs and bandwidths.

• What features (pipelining, eliminating tail byes, etc.) do you implement? And how do you implement them?

  Pipelining is implemented based on HTTP/2 PING frames. We use HTTP/2 PING frames to estimate the bandwidth and therefore the remaining time to finish the download. So we can send the new request one RTT before the download finishes.

Drawing Task

• NetEm

  • Under static network conditions:

    Each path is set with 5ms delay and 10Mbps bandwidth.

  

  • Under varying bandwidth conditions:

    Bandwidth is set to vary from 5Mbps to 10Mbps. However, the download rate strangely seems to be unaffected.

  

  • With a high-bandwidth path and two low-bandwidth paths:

    The high-bandwidth path is set with 5ms delay and 10Mbps bandwidth, whereas the low-bandwidth paths are set with 5ms delay and 7Mbps bandwidth.

  

  • With a short-delay path and two long-delay paths:

    The short-delay path is set with 5ms delay and 10Mbps bandwidth, whereas the long-delay paths are set with 10ms delay and 10Mbps bandwidth.

  

• Mahimahi

  Random traces are chosen to demonstrate the download progress: