Abstract: Embodiments of this application provide a packet transmission method, apparatus, and system. A first network device sends a probe packet to a second network device, the probe packet includes an identifier of the first network device, and the identifier of the first network device is used to indicate the second network device to add the identifier of the first network device to a destination address in a response packet of the probe packet; and receives the response packet from the second network device, where the destination address in the response packet is the identifier of the first network device; or receives an updated response packet from a third network device, where a destination address in the updated response packet is the identifier of the first network device. Both the third network device and the first network device are connected to a same user-side network device.

Abstract: A path detection method and apparatus for forwarding at least two layers of labels includes generating, by a first network device, a first request packet used for path detection, where the first request packet includes a label stack and a time to live (TTL), and an initial quantity of layers of the label stack matches an initial value of the TTL, and sending the first request packet, where the first request packet instructs a second network device to send a first response packet to the first network device when the TTL is 0 after being reduced by 1, and the first response packet reaches the first network device through an internet protocol route.

Abstract: A path detection method and apparatus for forwarding at least two layers of labels includes generating, by a first network device, a first request packet used for path detection, where the first request packet includes a label stack and a time to live (TTL), and an initial quantity of layers of the label stack matches an initial value of the TTL, and sending the first request packet, where the first request packet instructs a second network device to send a first response packet to the first network device when the TTL is 0 after being reduced by 1, and the first response packet reaches the first network device through an internet protocol route.