how is the delay defined in BDP

[Steve Zhou]

HI,

As we know, BDP = Bandwidth x Delay (RTT). However, RTT is different from the perspective of protocol. For example, the following request/response pair can be used to measure the RTT.

• ICMP Request/Response
• TCP SYN, SYN ACK
• SMB Request/Response
• HTTP Request/Reponse
• etc.

Apparently, SMB have the relatively higher RTT as compared with others on the same network. Maybe 10x more. We usually take TCP as an example to set the receive window to be greater or at least equal to the BDP so that the transmission can fill the pipe before having to wait for an ACK.

What if we are talking about SMB throughput? I think it should be the same that we will still use TCP or ICMP as the way to measure the BDP rather than using the SMB delay because the data filling the pipe will be over TCP anyway. All these kinda of application protocol are usually for management or control purpose, the real data will finally fly on TCP.

If my understanding was wrong, please feel free to correct me.

thank you!

- Steve Zhou

[Georg Pauwen · Accepted solution]

Steve,

interesting question. SMB is layer 5, and HTTP layer 7, while TCP is obviously layer 4. I guess you are right, since TCP is used for transporting the very vast majority of data, taking it as a reference for BDP would make sense.