Abstract
This article is an abstract from a 易做图 which 易做图yses 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)
