TCP retransmission statistics from Google

Abstract

 

This article is an abstract from a paper which analyses TCP data from Google Web and vedio servers and

present key statistics on the nature of retransmissions.

 

Statistics

 

（1）TCP and HTTP statistics

TCP

    Total connections                                                     Billions

    Connections support SACK                                   96%

    Connections support Timestamp                         12%

    HTTP / 1.1 connections                                           94%

    Average requests per connection                         3.1

    Average retransmissions rate                               2.8%

HTTP

    Average response size                                            7.5KB

    Responses with TCP retransmissions               6.1%

 

Summary of Google TCP and HTTP statistics sampled for one week in May 2011.

The data include both port 80 and 443 but exclude YouTube videos and bulk downloads.

 

The majority of clients are Microsoft Windows which by default do not use TCP Timestamps.

The average user network bandwidth as observed from Google is 1.9Mbps.

 

（2）Breakdown of TCP retransmission types

                                                                                Web pages                  video

Fast retransmits                                                  24%                               54%

Timeout retransmits                                           43%                               17%

    Timeout in Open                                              30%                                8%

    Timeout in Disorder                                         2%                                 3%

    Timeout in Recovery                                        1%                                  2%

    Timeout Exp. Backoff                                      10%                                 4%

Slow start retransmits                                        17%                                29%

Failed retransmits                                               15%                                 0%

 

（3）Fast recovery related statistics

                                                                               Web pages                   video

Fast retransmits / FR                                         3.15                                2.93

DSACKs / FR                                                       12%                                4%

DSACKs / retransmit                                          3.8%                              1.4%

Lost (fast) retransmits / FR                               6%                                  9%

Lost retransmits / retransmit                            1.9%                               3.1%

 

（4）other

TCP recover losses in two ways

Fast recovery (1RTT) need dupacks

Timeout (often 5 ~ 10 RTTs)