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Keepalive Timer

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[The TCP chimney offload feature is deprecated and should not be used.]

During an offload or an update operation, the host stack can set the following information in the TCP_OFFLOAD_STATE_CACHED structure that is supplied for a TCP connection:

  • The TCP_FLAG_KEEP_ALIVE_ENABLED flag in the Flags member enables the keepalive option for a TCP connection. If the host stack does not set this flag, the keepalive option for a TCP connection is disabled.

  • The TCP_FLAG_KEEP_ALIVE_RESTART flag in the Flags member causes the offload target to restart its keepalive timer with a value of 0.

  • The KaProbeCount member specifies the maximum number of keepalive probes that the offload target can send to determine whether a TCP connection is intact.

  • The KaTimeout member specifies the timeout interval for inactivity before sending the first keepalive probe.

  • The KaInterval member specifies the timeout after which to retransmit a keepalive frame if no response is received to a keepalive probe.

The TCP_OFFLOAD_STATE_DELEGATED structure contains ProbeCount and TimeoutDelta members for the keepalive option. The value in the keepalive ProbeCount specifies the number of keepalive probes that have been sent that have not received a response. The value in the keepalive TimeoutDelta specifies the time remaining until the next keepalive timeout.

When offloading a TCP connection, the host stack passes its current keepalive values for the connection to the offload target. The offload uses these values to resume keepalive processing on the offloaded connection. If the host stack target was not running the keepalive timer, it supplies a value of -1 in the TimeoutDelta member.

Similarly, when terminating the offload of a TCP connection, an offload target passes its current keepalive values for the connection to the host stack. The host stack uses these values to resume keepalive processing on the uploaded connection. If the offload target was not running the keepalive timer, it should supply a value of -1 in the TimeoutDelta member. If the offload target supplies a value of 0 in the TimeoutDelta member, the host stack assumes that the keepalive timer was running, and the host stack's keepalive timer expires immediately.