发送端缓冲时间是否被纳入RTT？含ping测得RTT的技术问询
Great question—this is a super common point of confusion since RTT definitions can feel pretty loose depending on the context! Let’s break this down clearly:
1. 发送端的缓冲时间是否被包含在RTT内？
First, let’s anchor ourselves to the standard definition of RTT (Round-Trip Time): it’s the time from the moment a packet leaves the sender’s network stack and hits the physical transmission link, to the moment the corresponding response packet comes back and is received by the sender’s network stack.
Here’s the key detail: if your packet is sitting in a user-space buffer or kernel send queue on the sender side, waiting to be picked up by the network card, that waiting time is not part of RTT. RTT only starts counting once the packet is actually sent out onto the network. Think of this buffer delay as "pre-transmission latency"—separate from the core round-trip measurement.
2. ping所测得的RTT是否包含发送节点的缓冲时间？
Nope, ping sticks strictly to the standard RTT definition when it calculates its values. Here’s how it works:
- When you run
ping, it builds an ICMP Echo Request packet and passes it to your OS kernel’s network stack. - The timer starts when the kernel hands the packet off to the network card driver (ready to be sent), not when you first run the command, and not while the packet is sitting in a local buffer waiting its turn.
- The timer stops when the kernel receives the ICMP Echo Reply and notifies the
pingtool that it’s arrived.
So if your sender’s network stack is backed up (say, a huge queue of packets waiting to be sent) and your ping packet waits 30ms just to get onto the wire, that 30ms won’t show up in the RTT value ping outputs. The RTT will only reflect the actual time the packet spends traveling to the target and back. The total time from you running the command to getting a reply would be 30ms + RTT, but the RTT itself doesn’t include that buffer wait.
内容的提问来源于stack exchange，提问作者S.Toonsi
