Communications method and apparatus

1. A communications method, comprising:

generating, by a terminal device, a first message, wherein the first message is used to request to establish a protocol data unit (PDU) session home-routed to a first network; and

sending, by the terminal device, the first message to a first access and mobility management function (AMF) entity, wherein the first AMF entity is a network element in a second networks and wherein the first network is a private network and the second network is a public network, or the first network is a public network and the second network is a private network.

2. The method according to claim 1 , wherein the first message comprises first indication information, and wherein the first indication information is used to indicate that the PDU session is a PDU session home-routed to the first network.

3. The method according to claim 2 , wherein the first indication information comprises at least one of a data network name, slice selection information, network type indication information, or identifier information of the first network.

4. The method according to claim 1 , wherein the first message comprises at least one of a data network name or slice selection information.

5. A communications method, comprising:

receiving, by a first access and mobility management function (AMF) entity, a first message from a terminal device, wherein the first message is used to request to establish a protocol data unit (PDU) session home-routed to a first network;

sending, by the first AMF entity, a second message to a first session management function (SMF) entity based on the first message, wherein the second message is used to request core network tunnel information of the PDU session;

receiving, by the first SMF entity, the second message;

sending, by the first SMF entity, first core network tunnel information to the first AMF entity based on the second message, wherein the first core network tunnel information is used to establish a data channel, of the PDU session, between a first user plane function (UPF) entity and a UPF entity in the first network; and

receiving, by the first AMF entity, the first core network tunnel information, and sending the first core network tunnel information to a second AMF entity;

wherein the first AMF entity, the first SMF entity, and the first UPF entity are network elements in a second network, and wherein the second AMF entity is a network element in the first network.

6. The method according to claim 5 , wherein the first message comprises first indication information, and wherein the first indication information is used to indicate that the PDU session is a PDU session home-routed to the first network.

7. The method according to claim 6 , wherein the first indication information comprises at least one of a data network name, slice selection information, network type indication information, or identifier information of the first network.

8. The method according to claim 6 , wherein the second message comprises the first indication information.

9. The method according to claim 5 , wherein the first message comprises at least one of a data network name or slice selection information, and wherein the method further comprises:

determining, by the first AMF entity based on a correspondence between indication information and at least one of a data network name or slice selection information, indication information corresponding to at least one of the data network name or the slice selection information in the first message; and

when the indication information corresponding to at least one of the data network name or the slice selection information in the first message is used to indicate that the PDU session is not allowed to be used for local breakout, determining, by the first AMF entity based on an identifier of the terminal device, that a home network of the PDU session is the first network; or determining, by the first AMF entity based on a correspondence that is between a home-routed network identifier and at least one of a data network name or slice selection information and that is comprised in subscription information of the terminal device, that a home network of the PDU session is the first network.

10. The method according to claim 5 , wherein the second message comprises second indication information; and

when the first network is a private network and the second network is a public network, the second indication information is used to indicate that a type of the PDU session requested to be established is a private network type; or

when the first network is a public network and the second network is a private network, the second indication information is used to indicate that a type of the PDU session requested to be established is a public network type.

11. The method according to claim 5 , wherein the method further comprises:

selecting, by the first AMF entity, a second SMF entity, wherein the second SMF entity is located in the first network.

12. The method according to claim 11 , wherein the second message comprises an identifier of the second SMF entity; and

when the first network is a private network, the identifier of the second SMF entity is used to indicate that the second SMF entity is an SMF entity in the private network; or

when the first network is a public network, the identifier of the second SMF entity is used to indicate that the second SMF entity is an SMF entity in the public network.

13. The method according to claim 5 , wherein the method further comprises:

sending, by the first AMF entity, at least one of slice selection information, data network name, a PDU session establishment request, or identifier information of the second network to the second AMF entity.

14. The method according to claim 5 , wherein the method further comprises:

receiving, by the second AMF entity, the first core network tunnel information;

sending, by the second AMF entity, the first core network tunnel information to a second SMF entity in the first network; and

receiving, by the second SMF entity, the first core network tunnel information.

15. The method according to claim 14 , wherein the method further comprises:

sending, by the second SMF entity, second core network tunnel information to the second AMF entity, wherein the second core network tunnel information is used to establish the data channel, of the PDU session, between the first UPF entity and a second UPF entity in the first network;

receiving, by the second AMF entity, the second core network tunnel information; and

sending, by the second AMF entity, the second core network tunnel information to the first AMF entity.

16. A communications system, comprising: a first access and mobility management function (AMF) entity, a first session management function (SMF) entity, and a first user plane function (UPF) entity, wherein:

the first AMF entity is configured to:

receive a first message from a terminal device, wherein the first message is used to request to establish a protocol data unit (PDU) session home-routed to a first network; and

send a second message to the first SMF entity based on the first message, wherein the second message is used to request core network tunnel information of the PDU session;

the first SMF entity is configured to:

receive the second message; and

send first core network tunnel information to the first AMF entity based on the second message, wherein the first core network tunnel information is used to establish a data channel, of the PDU session, between the first UPF entity and a UPF entity in the first network; and

the first AMF entity is further configured to:

receive the first core network tunnel information; and

send the first core network tunnel information to a second AMF entity;

wherein the first AMF entity, the first SMF entity, and the first UPF entity are network elements in a second network, and wherein the second AMF entity is a network element in the first network.

17. The system according to claim 16 , wherein the first message comprises first indication information, and wherein the first indication information is used to indicate that the PDU session is a PDU session home-routed to the first network.

18. The system according to claim 16 , wherein the first message comprises at least one of a data network name or slice selection information, and wherein the first AMF entity is further configured to:

determine, based on a correspondence between indication information and at least one of a data network name or slice selection information, indication information corresponding to at least one of the data network name or the slice selection information in the first message; and

when the indication information corresponding to at least one of the data network name or the slice selection information in the first message is used to indicate that the PDU session is not allowed to be used for local breakout, determine, based on an identifier of the terminal device, that a home network of the PDU session is the first network; or determine, based on a correspondence that is between a home-routed network identifier and at least one of a data network name or slice selection information and that is comprised in subscription information of the terminal device, that a home network of the PDU session is the first network.

19. The system according to claim 16 , wherein the system further comprises the second AMF entity and a second SMF entity in the first network;

wherein the second AMF entity is configured to receive the first core network tunnel information, and send the first core network tunnel information to the second SMF entity; and

wherein the second SMF entity is configured to receive the first core network tunnel information.

20. The system according to claim 19 , wherein the second SMF entity is further configured to send second core network tunnel information to the second AMF entity, wherein the second core network tunnel information is used to establish the data channel, of the PDU session, between the first UPF entity and a second UPF entity in the first network; and

wherein the second AMF entity is further configured to receive the second core network tunnel information, and send the second core network tunnel information to the first AMF entity.