spec: allow interfaces to be included in the delegated IPAM plugin result

[ruicao93]

Currently, the spec specifies the interfaces array should not be included in the delegated IPAM plugin result.

However, considering a common scenario where there are multiple interfaces on a Node, and each Pod use a dedicated network interface for ingress/egress traffic, the primary CNI plugin needs to know the uplink interface to install the corresponding routes. The muliple interface node is widely used in various clouds:

• Public cloud: using multiple ENIs to meet IP allocation and dedicated security group requirements.
  • AWS: EKS
  • Alibaba cloud: ACK
  • Bytedance cloud(Volcengine): VKE
• Private cloud: Use multiple interfaces for security and better performance.
• RDMA: multiple RDMA nics are common used on single GPU node and shared by Pods.

So we should allow the interfaces array be included to hint the uplink interface info(sandbox is emtpy) which can be consumed by the primary CNI plugin.

[ruicao93]

hi @squeed, could you please help review the PR?

[dakehero]

• RDMA: multiple RDMA nics are common used on single GPU node and shared by Pods.

It's a common case that different pods share a same RDMA NIC through macvlan (or ipvlan). For those nodes with multiple RDMA NICs, it's also helpful to use a single IPAM plugin to manage IPs on different devices.

[ruicao93]

cc @joestringer