Understanding Packet class and Request class in gem5

Packet class

A packet is used to encapsulate a transaction between 2 objects in the memory system.

headerDelay is the extra delay from seeing the packet until the header is transmitted.

payloadDelay is the extra pipelining delay from seeing the packet until the end of payload is transmitted by the component that provided it (if any). This includes the header delay. Similar to the header delay, this is used to make up for the fact that the crossbar does not make the packet wait. As the delay is relative, a 32-bit unsigned should be sufficient.

Request class

A request travels all the way from the requestor to the ultimate destination and back, possibly conveyed by several different packets along the way.