Description

The motivation behind adding a new routing mode is discussed in this CFP .

This is Part 1 of feature:

• adds the subnet map in datapath
• add lookup functions for getting subnet id
• skip tunneling if IPs belong to same subnet ID
• won't interfere with how datapath works today (allows checking this PR in as a standalone change)

For reference, here's a draft PR with the entire feature implemented - #41405 .

Overview

This feature would allow user to configure Cilium cluster(s) with multiple subnets, and benefit from both tunnel and native routing mode for maximum efficiency (no overhead of tunneling when not needed).

Motivation

1. Given that tunneling can always route a packet as long as nodes have connectivity, the entire change is designed to use tunnel as default, with exceptions built in to skip tunneling when IPs belong to same subnet.
2. I have tried to adhere to existing patterns that I could identify - the eBPF map and lookup function closely resembles the IPCache library.

Changes Overview

Datapath

This covers all the C code changes. Files edited/added:

1. bpf/lib/subnet.h - This has the LPM trie based subnet map, similar to the IPCache map. Has functions for lookup.
2. bpf_lxc.c - Added an extra check to skip_tunnel. Now, skip tunneling if IPs belong to same subnet.
3. bpf_host.c - Added an extra check to skip_tunnel. Now, skip tunneling if IPs belong to same subnet.
4. Added two debug keys - DBG_SUBNET_CHECK and DBG_TUNNEL_TRACE for debugging purposes. Will remove these before merging.

Testing Done

The full feature is tested using the change draft PR.
I tested this change against a cluster and verified:

• the subnet map doesn't exist
• all IPs return ID=0
• No packet skips tunneling

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Fixes: #41867

Add support for hybrid routing in datapath