Packet Processing Delay (PPD)

The PPD parameter is a 16-bit word exchanged during the Call Control phase that represents tenths of a second (64 means 6.4 seconds). The protocol only specifies that the parameter is exchanged; it does not specify how it is calculated. The way in which values for PPD are calculated is implementation-dependent and need not be variable (static time-outs). The PPD must be exchanged in the call connect sequence even if it remains constant in an implementation. One possible way to calculate the PPD is the following:

The header size consists of the IP and GRE v2 header, and is 36. The MTU is the overall MTU for the internetwork link between the FEP and NTS. WindowSize represents the number of packets in the sliding window, and its calculation or size is implementation-dependent. The amount of latency in the internetwork could be used to pick a window size sufficient to keep the current connection's pipe full. The constant 8 converts bytes to bits (assuming ConnectRate is in bits per second). If ConnectRate is in bytes per second, omit the 8. FEPFudge is not required but can be used to take overall processing overhead of the FEP into account.

The PPD is used to seed the adaptive algorithm with the initial RTT(n-1) value.