Great question! Let’s break down this common point of confusion with Wireshark’s time fields.

Is your judgment correct? #

Your observation that the two fields appear identical is only true if your trace file contains a single TCP stream. If you were to analyze a capture with multiple distinct TCP flows, you’d immediately see clear differences between them.

What do these fields actually measure? #

Let’s clarify their core definitions:

• frame.time_relative: This is the relative time of the entire packet compared to the very first packet captured in the trace. It’s a global timestamp, anchored to the start of the entire capture session—regardless of which network flow the packet belongs to.
• tcp.time_relative: This is the relative time of the TCP packet compared to the first packet of its own TCP stream. It’s a per-flow timestamp, anchored to the start of the specific TCP conversation (like the SYN packet that initiated the connection), not the start of the overall capture.

Example to highlight the difference #

Suppose you have a capture with two separate TCP streams:

1. Stream 1 starts at capture time 0.0s (SYN packet)
2. Stream 2 starts at capture time 5.0s (another SYN packet)

• For the first packet in Stream 2:
  • frame.time_relative will be 5.0s (since it’s 5 seconds after the first captured packet)
  • tcp.time_relative will be 0.0s (since it’s the first packet of its own stream)

In this scenario, the two fields clearly diverge.

Why do both fields exist? #

They serve distinct, complementary use cases:

• Use frame.time_relative when you need to analyze the global timeline of your capture—for example, understanding how different network flows overlap in time, or correlating events across multiple connections.
• Use tcp.time_relative when you’re focused on a single TCP stream’s internal behavior—like measuring round-trip times (RTT), analyzing retransmission delays, or debugging sequence/acknowledgment timing within one conversation. It isolates the stream’s timing from other traffic in the capture.

Since your trace.raw likely contains only one TCP stream, the two fields align perfectly. But once you work with multi-stream captures, their unique purposes will become immediately obvious.

内容的提问来源于stack exchange，提问作者tehemaroo