Life of a Packet

Endpoint to Endpoint

First we show the local endpoint to endpoint flow with optional L7 Policy on egress and ingress. Followed by the same endpoint to endpoint flow with socket layer enforcement enabled. With socket layer enforcement enabled for TCP traffic the handshake initiating the connection will traverse the endpoint policy object until TCP state is ESTABLISHED. Then after the connection is ESTABLISHED only the L7 Policy object is still required.


Egress from Endpoint

Next we show local endpoint to egress with optional overlay network. In the optional overlay network traffic is forwarded out the Linux network interface corresponding to the overlay. In the default case the overlay interface is named cilium_vxlan. Similar to above, when socket layer enforcement is enabled and a L7 proxy is in use we can avoid running the endpoint policy block between the endpoint and the L7 Policy for TCP traffic. An optional L3 encryption block will encrypt the packet if enabled.


Ingress to Endpoint

Finally we show ingress to local endpoint also with optional overlay network. Similar to above socket layer enforcement can be used to avoid a set of policy traversals between the proxy and the endpoint socket. If the packet is encrypted upon receive it is first decrypted and then handled through the normal flow.


veth-based versus ipvlan-based datapath


This feature is currently only in technology preview and to be used for experimentation purposes. This restriction will be lifted in future Cilium releases.

By default Cilium CNI operates in veth-based datapath mode which allows for more flexibility in that all BPF programs are managed by Cilium out of the host network namespace such that containers can be granted privileges for their namespaces like CAP_NET_ADMIN without affecting security since BPF enforcement points in the host are unreachable for the container. Given BPF programs are attached from the host’s network namespace, BPF also has the ability to take over and efficiently manage most of the forwarding logic between local containers and host since there always is a networking device reachable. However, this also comes at a latency cost as in veth-based mode the network stack internally needs to be re-traversed when handing the packet from one veth device to its peer device in the other network namespace. This egress-to-ingress switch needs to be done twice when communicating between local Cilium endpoints, and once for packets that are arriving or sent out of the host.

For a more latency optimized datapath, Cilium CNI also supports ipvlan L3/L3S mode with a number of restrictions. In order to support older kernel’s without ipvlan’s hairpin mode, Cilium attaches BPF programs at the ipvlan slave device inside the container’s network namespace on the tc egress layer, which means that this datapath mode can only be used for containers which are not running with CAP_NET_ADMIN and CAP_NET_RAW privileges! ipvlan uses an internal forwarding logic for direct slave-to-slave or slave-to-master redirection and therefore forwarding to devices is not performed from the BPF program itself. The network namespace switching is more efficient in ipvlan mode since the stack does not need to be re-traversed as in veth-based datapath case for external packets. The host-to-container network namespace switch happens directly at L3 layer without having to queue and reschedule the packet for later ingress processing. In case of communication among local endpoints, the egress-to-ingress switch is performed once instead of having to perform it twice.

For Cilium in ipvlan mode there are a number of additional restrictions in the current implementation which are to be addressed in upcoming work: NAT64 cannot be enabled at this point as well as L7 policy enforcement via proxy. Service load-balancing to local endpoints is currently not enabled as well as container to host-local communication. If one of these features are needed, then the default veth-based datapath mode is recommended instead.

The ipvlan mode in Cilium’s CNI can be enabled by running the Cilium daemon with e.g. --datapath-mode=ipvlan --ipvlan-master-device=bond0 where the latter typically specifies the physical networking device which then also acts as the ipvlan master device. Note that in case ipvlan datapath mode is deployed in L3S mode with Kubernetes, make sure to have a stable kernel running with the following ipvlan fix included: d5256083f62e.

This completes the datapath overview. More BPF specifics can be found in the BPF and XDP Reference Guide. Additional details on how to extend the L7 Policy exist in the Envoy section.