---
title: client.socket.tcpi_rtt
summary: null
url: >-
  https://www.fastly.com/documentation/reference/vcl/variables/client-connection/client-socket-tcpi-rtt
---

**Type:** INTEGER  
**Access:** read-only

**Available in:** recv, deliver, log, error

The TCP stack's smoothed round trip time (RTT) estimate, in
microseconds, for the current connection to the client. As the client
acknowledges the data being sent to it on the current connection, the
value is updated with new timer measurements. The RTT is used by the
loss detection algorithms and sometimes for congestion control as
well.