Calculating the Timestamp Delta

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

When the host stack offloads a TCP connection, it supplies the following timestamp-related variables in the TCP delegated state for the connection:

TsRecent
The timestamp value to send in the next acknowledgement (Ts.Recent in RFC 1323)

TsRecentAge
The amount of time, in clock ticks, since the most recent timestamp was received (RFC 1323)

TsTime
The current value of the adjusted time stamp

The offload target uses TsRecent and TsRecentAge in accordance with RFC 1323. The offload target uses TsTime to calculate its own timestamp delta.

TsTime specifies the current value of the host stack's timestamp value, which is equivalent to the host stack's current clock time plus the host's timestamp delta, if any:

TsTime= StackTime+ StackDelta

When a connection is initially offloaded, the timestamp delta is always zero.

The offload target subtracts its current time from the value of TsTime that is supplied by the host stack to arrive at the offload target's timestamp delta:

NicDelta= TsTime- NicTime

For example, if the host stack supplies 100 in the TsTime variable and if the offload target's current time is 200, the offload target subtracts 200 from 100 to get an offload target timestamp delta of -100.

Similarly, when the offload target terminates of the offload of a connection, it supplies a value in TsTime in the TCP delegated state for the connection. In this situation, the value of TsTime is the offload target's current time plus the offload target's timestamp delta. The host stack subtracts its current time from the value TsTime to arrive at the host stack timestamp delta.