About eBPF

7 MINUTE READ

This guide provides optional background education, including education that is not specific to Calico.

eBPF is a Linux kernel feature that allows fast yet safe mini-programs to be loaded into the kernel in order to customise its operation.

In this guide you will learn:

  • General background on eBPF.
  • Various uses of eBPF.
  • How Calico uses eBPF in the eBPF dataplane.

What is eBPF?

eBPF is a virtual machine embedded within the Linux kernel. It allows small programs to be loaded into the kernel, and attached to hooks, which are triggered when some event occurs. This allows the behaviour of the kernel to be (sometimes heavily) customised. While the eBPF virtual machine is the same for each type of hook, the capabilities of the hooks vary considerably. Since loading programs into the kernel could be dangerous; the kernel runs all programs through a very strict static verifier; the verifier sandboxes the program, ensuring it can only access allowed parts of memory and ensuring that it must terminate quickly.

Why is it called eBPF?

eBPF stands for “extended Berkeley Packet Filter”. The Berkeley Packet Filter was an earlier, more specialised virtual machine that was tailored for filtering packets. Tools such as tcpdump use this “classic” BPF VM to select packets that should be sent to userspace for analysis. eBPF is a considerably extended version of BPF that is suitable for general purpose use inside the kernel. While the name has stuck, eBPF can be used for a lot more than just packet filtering.

What can eBPF do?

Types of eBPF program

There are several classes of hooks to which eBPF programs can be attached within the kernel. The capabilities of an eBPF program depend hugely on the hook to which it is attached:

  • Tracing programs can be attached to a significant proportion of the functions in the kernel. Tracing programs are useful for collecting statistics and deep-dive debugging of the kernel. Most tracing hooks only allow read-only access to the data that the function is processing but there are some that allow data to be modified. The Calico team use tracing programs to help debug Calico during development; for example, to figure out why the kernel unexpectedly dropped a packet.

  • Traffic Control (tc) programs can be attached at ingress and egress to a given network device. The kernel executes the programs once for each packet. Since the hooks are for packet processing, the kernel allows the programs to modify or extend the packet, drop the packet, mark it for queueing, or redirect the packet to another interface. Calico’s eBPF dataplane is based on this type of hook; we use tc programs to load balance Kubernetes services, to implement network policy, and, to create a fast-path for traffic of established connections.

  • XDP, or “eXpress Data Path”, is actually the name of an eBPF hook. Each network device has an XDP ingress hook that is triggered once for each incoming packet before the kernel allocates a socket buffer for the packet. XDP can give outstanding performance for use cases such as DoS protection (as supported in Calico’s standard Linux dataplane) and ingress load balancing (as used in facebook’s Katran). The downside of XDP is that it requires network device driver support to get good performance, and it doesn’t inter-work with pod networking very well.

  • Several types of socket programs hook into various operations on sockets, allowing the eBPF program to, for example, change the destination IP of a newly-created socket, or force a socket to bind to the “correct” source IP address. Calico uses such programs to do connect-time load balancing of Kubernetes Services; this reduces overhead because there is no DNAT on the packet processing path.

  • There are various security-related hooks that allow for program behaviour to be policed in various ways. For example, the seccomp hooks allow for syscalls to be policed in fine-grained ways.

  • And… probably a few more hooks by the time you ready this; eBPF is under heavy development in the kernel.

The kernel exposes the capabilities of each hook via “helper functions”. For example, the tc hook has a helper function to resize the packet, but that helper would not be available in a tracing hook. One of the challenges of working with eBPF is that different kernel versions support different helpers and lack of a helper can make it impossible to implement a particular feature.

BPF maps

Programs attached to eBPF hooks are able to access BPF “maps”. BPF maps have two main uses:

  • They allow BPF programs to store and retrieve long-lived data.

  • They allow communication between BPF programs and user-space programs. BPF programs can read data that was written by userspace and vice versa.

There are many types of BPF maps, including some special types that allow jumping between programs, and, some that act as queues and stacks rather than strictly as key/value maps. Calico uses maps to keep track of active connections, and, to configure the BPF programs with policy and service NAT information. Since map accesses can be relatively expensive, Calico aims to do a single map lookup only for each packet on an established flow.

The contents of bpf maps can be inspected using the command-line tool, bpftool, which is provided with the kernel.

Calico’s eBPF dataplane

Calico’s eBPF dataplane is an alternative to our standard Linux dataplane (which is iptables based). While the standard dataplane focuses on compatibility by inter-working with kube-proxy, and your own iptables rules, the eBPF dataplane focuses on performance, latency and improving user experience with features that aren’t possible in the standard dataplane. As part of that, the eBPF dataplane replaces kube-proxy with an eBPF implementation. The main “user experience” feature is to preserve the source IP of traffic from outside the cluster when traffic hits a NodePort; this makes your server-side logs and network policy much more useful on that path.

Feature comparison

While the eBPF dataplane has some features that the standard Linux dataplane lacks, the reverse is also true:

Factor Standard Linux Dataplane eBPF dataplane
Throughput Designed for 10GBit+ Designed for 40GBit+
First packet latency Low (kube-proxy service latency is bigger factor) Lower
Subsequent packet latency Low Lower
Preserves source IP within cluster Yes Yes
Preserves external source IP Only with externalTrafficPolicy: Local Yes
Direct Server Return Not supported Supported (requires compatible underlying network)
Connection tracking Linux kernel’s conntrack table (size can be adjusted) BPF map (fixed size)
Policy rules Mapped to iptables rules Mapped to BPF instructions
Policy selectors Mapped to IP sets Mapped to BPF maps
Kubernetes services kube-proxy iptables or IPVS mode BPF program and maps
IPIP Supported Supported (no performance advantage due to kernel limitations)
VXLAN Supported Supported
Wireguard Supported Supported
Other routing Supported Supported
Supports third party CNI plugins Yes (compatible plugins only) Yes (compatible plugins only)
Compatible with other iptables rules Yes (can write rules above or below other rules) Partial; iptables bypassed for workload traffic
XDP DoS Protection Supported Not supported (yet)
IPv6 Supported Not supported (yet)
Host endpoint policy Supported Not supported (yet)
Enterprise version Available Not supported (yet)

Architecture overview

Calico’s eBPF dataplane attaches eBPF programs to the tc hooks on each Calico interface as well as your data and tunnel interfaces. This allows Calico to spot workload packets early and handle them through a fast-path that bypasses iptables and other packet processing that the kernel would normally do.

Diagram showing the packet path for pod-to-pod networking; a BPF program is attached to the client pod's veth interface; it does a conntrack lookup in a BPF map, and forwards the packet to the second pod directly, bypassing iptables

The logic to implement load balancing and packet parsing is pre-compiled ahead of time and relies on a set of BPF maps to store the NAT frontend and backend information. One map stores the metadata of the service, allowing for externalTrafficPolicy and “sticky” services to be honoured. A second map stores the IPs of the backing pods.

In eBPF mode, Calico converts your policy into optimised eBPF bytecode, using BPF maps to store the IP sets matched by policy selectors.

Detail of BPF program showing that packets are sent to a separate (generated) policy program,

To improve performance for services, Calico also does connect-time load balancing by hooking into the socket BPF hooks. When a program tries to connect to a Kubernetes service, Calico intercepts the connection attempt and configures the socket to connect directly to the backend pod’s IP instead. This removes all NAT overhead from service connections.

Diagram showing BPF program attached to socket connect call; it does NAT at connect time,

Above and beyond