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

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

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

The number of bytes that the kernel believes the client TCP stack is using 
for its maximum segment size (MSS) sending parameter. It is based on the 
largest packet size previously received on the same connection.

`client.socket.tcpi_rcv_mss` is primarily used to govern
the rate of ACK generation by the TCP stack when implementing the
delayed ACK algorithm. The value of `client.socket.tcpi_rcv_mss` will change 
over the lifetime of the connection as new data is received. Its value can also
provide limited insight into how efficiently the client was able to
use its TCP send channel.