docs(guide): explain how Rattan synchronize trace (#158)

Co-authored-by: BobAnkh <[email protected]>

Author: Lethe10137

guide/src/guide/flexible-configuration.md

@@ -202,8 +202,10 @@ Possible cell types:
 - `Bw`: Fixed bandwidth
 - `BwReplay`: Bandwidth trace replayer
 - `Delay`: Fixed delay
+- `DelayReplay`: Delay trace replayer
 - `DelayPerPacket`: Delay specified per-packet
 - `Loss`: Loss pattern
+- `LossReplay`: Loss trace replayer
 - `Spy`: Log all packets
 - `Custom`: Custom cell, only specify the cell ID used in link section, and the cell must be built by the user
 
@@ -220,6 +222,25 @@ queue_config = { packet_limit = 60 }
 
 For detailed parameters, you can check the [documentation of `rattan-core`](https://docs.rs/rattan-core/latest/rattan-core) or the example config file [config.example.toml](https://github.com/stack-rs/rattan/blob/main/config.example.toml).
 
+To make the result more duplicable, Rattan synchronize the start time of all cell types that are time-dependent, including `DelayReplay`, `BwReplay`, and `LossReplay`, for them to replay traces at the same time base.
+The start time is conditionally defined. By default, we use the `FIRST_PACKET` time as the start time. But users can also choose to use the Rattan start-up time as the start time.
+
+The `FIRST_PACKET` is defined as the first L2 unicast packet to enter Rattan. An L2 unicast packet has a **destination** MAC address with the 8th bit (the Individual/Group bit) set to `0`.
+
+For example, `38:7e:58:e7:02:01` is a unicast MAC address because the 8th bit (the least-significant bit of the first byte, `0x38`) is `0`. In contrast, MAC addresses like `01:00:5e:00:00:fb` or `33:33:00:00:02` are **not** unicast.
+
+**Examples of a `FIRST_PACKET`:**
+
+- A TCP SYN packet from the client.
+- An ICMP Echo Request (Ping).
+
+**Packets that cannot be a `FIRST_PACKET` (often generated by the Linux network stack):**
+
+- An ICMPv6 Router Solicitation.
+- An mDNS (Multicast Domain Name System) query.
+
+This design ensures that the trace does not begin replaying until actual application traffic has started.
+
 ### Links Section
 
 This section describes how to link the cells.