Using border gateway protocol to expose maximum segment identifier depth to an external application

1. A method implemented by a network node, the method comprising: transmitting to a centralized controller an attribute Type Length Value (TLV), wherein a type of the attribute TLV includes a type of a maximum segment identifier depth (MSD) indicating that the MSD is a link MSD, a length of the attribute TLV includes a length of the MSD, and a value of the attribute TLV includes the MSD, which is a maximum number of segment routing (SR) labels supported at one link of a plurality of links of the network node, wherein the centralized controller is to use the attribute TLV to compute an SR tunnel with one or more SR labels, a label stack depth of the SR tunnel does not exceed the MSD, and the SR labels are used for steering a packet through an SR network.

2. The method of claim 1 , wherein the attribute TLV is a Border Gateway Protocol Link State (BGP-LS) extension message.

3. The method of claim 2 further comprising: negotiating capabilities of a BGP session; and verifying that the centralized controller supports reception of MSDs in BGP-LS extension messages.

4. The method of claim 3 , wherein transmitting to the centralized controller the attribute TLV is performed in response to verifying that the centralized controller supports reception of MSDs in BGP-LS extension messages.

5. The method of claim 1 , wherein the network node does not support Path Computation Element Communication Protocol (PCEP) and does not participate in Interior Gateway Protocol(s) (IGP).

6. The method of claim 1 , wherein the network node is an ingress node of the SR tunnel.

7. A network node, comprising: a processor and a memory, said memory containing instructions executable by the processor to cause the network node to: transmit to a centralized controller an attribute Type Length Value (TLV), wherein a type of the attribute TLV includes a type of a maximum segment identifier depth (MSD) indicating that the MSD is a link MSD, a length of the attribute TLV includes a length of the MSD, and a value of the attribute TLV includes the MSD, which is a maximum number of segment routing (SR) labels supported at one link of a plurality of links of the network node, wherein the centralized controller is to use the attribute TLV to compute an SR tunnel with one or more SR labels, a label stack depth of the SR tunnel does not exceed the MSD, and the SR labels are used for steering a packet through an SR network.

8. The network node of claim 7 , wherein the attribute TLV is a Border Gateway Protocol Link State (BGP-LS) extension message.

9. The network node of claim 8 , wherein the instructions executable by the processor are further to cause the network node to: negotiate capabilities of a BGP session; and verify that the centralized controller supports reception of MSDs in BGP-LS extension messages.

10. The network node of claim 9 , wherein to transmit to the centralized controller the attribute TLV is performed in response to verifying that the centralized controller supports reception of MSDs in BGP-LS extension messages.

11. The network node of claim 7 , wherein the network node does not support Path Computation Element Communication Protocol (PCEP) and does not participate in Interior Gateway Protocol(s) (IGP).

12. The network node of claim 7 , wherein the network node is an ingress node of the SR tunnel.

13. A non-transitory machine-readable storage medium that provides instructions that, if executed by a processor of a network node, will cause said processor to perform operations comprising: transmitting to a centralized controller an attribute Type Length Value (TLV), wherein a type of the attribute TLV includes a type of a maximum segment identifier depth (MSD) indicating that the MSD is a link MSD, a length of the attribute TLV includes a length of the MSD, and a value of the attribute TLV includes the MSD, which is a maximum number of segment routing (SR) labels supported at one link of a plurality of links of the network node, wherein the centralized controller is to use the attribute TLV to compute an SR tunnel with one or more SR labels, a label stack depth of the SR tunnel does not exceed the MSD, and the SR labels are used for steering a packet through an SR network.

14. The non-transitory machine-readable storage medium of claim 13 , wherein the attribute TLV is a Border Gateway Protocol Link State (BGP-LS) extension message.

15. The non-transitory machine-readable storage medium of claim 14 , wherein the operations further comprise: negotiating capabilities of a BGP session; and verifying that the centralized controller supports reception of MSDs in BGP-LS extension messages.

16. The non-transitory machine-readable storage medium of claim 15 , wherein transmitting to the centralized controller the attribute TLV is performed in response to verifying that the centralized controller supports reception of MSDs in BGP-LS extension messages.

17. The non-transitory machine-readable storage medium of claim 13 , wherein the network node does not support Path Computation Element Communication Protocol (PCEP) and does not participate in Interior Gateway Protocol(s) (IGP).

18. The non-transitory machine-readable storage medium of claim 13 , wherein the network node is an ingress node of the SR tunnel.

19. A method in a network controller, the method comprising: receiving from a network node an attribute Type Length Value (TLV), wherein a type of the attribute TLV includes a type of a maximum segment identifier depth (MSD) indicating that the MSD is a link MSD, a length of the attribute TLV includes a length of the MSD, and a value of the attribute TLV includes a value of the MSD, which is a maximum number of segment routing (SR) labels supported at one link of a plurality of links of the network node; and using the attribute TLV to compute an SR tunnel with one or more SR labels, wherein a label stack depth of the SR tunnel does not exceed the MSD, and the SR labels are used for steering a packet through an SR network that includes the network node.

20. The method of claim 19 , wherein the attribute TLV is a Border Gateway Protocol Link State (BGP-LS) extension message.

21. The method of claim 20 further comprising: negotiating capabilities of a BGP session; and verifying that the network node supports transmission of MSDs in BGP-LS extension messages.

22. The method of claim 19 , wherein the network node does not support Path Computation Element Communication Protocol (PCEP) and does not participate in Interior Gateway Protocol(s) (IGP).

23. A centralized controller comprising: a processor and a memory, said memory containing instructions executable by the processor to cause the centralized controller to: receive from a network node an attribute Type Length Value (TLV), wherein a type of the attribute TLV includes a type of a maximum segment identifier depth (MSD) indicating that the MSD is a link MSD, a length of the attribute TLV includes a length of the MSD, and a value of the attribute TLV includes a value of the MSD, which is a maximum number of segment routing (SR) labels supported at one link of a plurality of links of the network node, and use the attribute TLV to compute an SR tunnel with one or more SR labels, wherein a label stack depth of the SR tunnel does not exceed the MSD, and the SR labels are used for steering a packet through an SR network that includes the network node.

24. The centralized controller of claim 23 , wherein the attribute TLV is a Border Gateway Protocol Link State (BGP-LS) extension message.

25. The centralized controller of claim 24 , wherein the instructions executable by the processor are further to cause the centralized controller to: negotiate capabilities of a BGP session; and verify that the network node supports transmission of MSDs in BGP-LS extension messages.

26. The centralized controller of claim 23 , wherein the network node does not support Path Computation Element Communication Protocol (PCEP) and does not participate in Interior Gateway Protocol(s) (IGP).

27. A non-transitory machine-readable storage medium that provides instructions that, if executed by a processor of a centralized controller, will cause said processor to perform operations comprising: receiving from a network node an attribute Type Length Value (TLV), wherein a type of the attribute TLV includes a type of a maximum segment identifier depth (MSD) indicating that the MSD is a link MSD, a length of the attribute TLV includes a length of the MSD, and a value of the attribute TLV includes a value of the MSD, which is a maximum number of segment routing (SR) labels supported at one link of a plurality of links of the network node; and using the attribute TLV to compute an SR tunnel with one or more SR labels, wherein a label stack depth of the SR tunnel does not exceed the MSD, and the SR labels are used for steering a packet through an SR network that includes the network node.

28. The non-transitory machine-readable storage medium of claim 27 , wherein the attribute TLV is a Border Gateway Protocol Link State (BGP-LS) extension message.

29. The non-transitory machine-readable storage medium of claim 28 further comprising: negotiating capabilities of a BGP session: and verifying that the network node supports transmission of MSDs in BGP-LS extension messages.

30. The non-transitory machine-readable storage medium of claim 27 , wherein the network node does not support Path Computation Element Communication Protocol (PCEP) and does not participate in Interior Gateway Protocol(s) (IGP).