2.2.2.6 Probegap Probe Message

An initiator device sends a train of Probegap Probe messages to a sink device if it wishes to perform the probegap probing experiment. For every Probegap Probe message that a sink device receives from the initiator, it sends a copy of that message back to the initiator.

The Probegap Probe message MUST contain the following message-specific header following the Handshake header.

Sequence_Number (4 bytes): An unsigned 32-bit integer that uniquely (in the context of a probegap experiment) identifies the specific instance of a message, in network byte order. A sink device MUST NOT change this field.

Initiator_Send_Timestamp (8 bytes): An unsigned 64-bit integer that specifies the timestamp of the initiator just before the message is sent, in network byte order. The unit of measurement MUST be in 100 nanosecond units. A sink device MUST NOT change this field.

Sink_Recv_Timestamp (8 bytes): An unsigned 64-bit integer that specifies the timestamp of the sink device immediately when it receives the message from the initiator device, in network byte order. The unit of measurement MUST be in 100 nanosecond units. The initiator device SHOULD set this field to 0.

Sink_Send_Timestamp (8 bytes): An unsigned 64-bit integer that specifies the timestamp of the sink device just before it sends a copy of the message back to the initiator device, in network byte order. The unit of measurement MUST be in 100 nanosecond units. The initiator device SHOULD set this field to 0.

Optional_Data_Payload (variable): This optional field specifies an array of random byte values, generated using any pseudo-random number generator, to pad the probe message to an arbitrary size. The size of this payload plus the size of all relevant qWave, TCP, IP, and Ethernet headers must not exceed 1510 bytes so a 4-byte IEEE 802.1Q header can be accommodated without breaking some networks that cannot handle Ethernet packets larger than 1514 bytes. The actual size of the payload is carefully chosen by the initiator (see [ProbeGap]), taking into account factors such as the size of a packet directly impacts the latency of the transmission, which in turn affects the calculations of the probegap experiment. The payload consists of random byte values only for the purpose of minimizing the effect of networks that employs packet compression; the sink does not validate the content of the payload itself. On the other hand, the sink MUST send this exact payload back to the initiator.