Container Runtime Interface streaming explained

The Kubernetes Container Runtime Interface (CRI) acts as the main connection between the kubelet and the Container Runtime. Those runtimes have to provide a gRPC server which has to fulfill a Kubernetes defined Protocol Buffer interface. This API definition evolves over time, for example when contributors add new features or fields are going to become deprecated.

In this blog post, I'd like to dive into the functionality and history of three extraordinary Remote Procedure Calls (RPCs), which are truly outstanding in terms of how they work: Exec, Attach and PortForward.

Exec can be used to run dedicated commands within the container and stream the output to a client like kubectl or crictl. It also allows interaction with that process using standard input (stdin), for example if users want to run a new shell instance within an existing workload.

Attach streams the output of the currently running process via standard I/O from the container to the client and also allows interaction with them. This is particularly useful if users want to see what is going on in the container and be able to interact with the process.

PortForward can be utilized to forward a port from the host to the container to be able to interact with it using third party network tools. This allows it to bypass Kubernetes services for a certain workload and interact with its network interface.

What is so special about them?

All RPCs of the CRI either use the gRPC unary calls for communication or the server side streaming feature (only GetContainerEvents right now). This means that mainly all RPCs retrieve a single client request and have to return a single server response. The same applies to Exec, Attach, and PortForward, where their protocol definition looks like this:

// Exec prepares a streaming endpoint to execute a command in the container.
rpc Exec(ExecRequest) returns (ExecResponse) {}
// Attach prepares a streaming endpoint to attach to a running container.
rpc Attach(AttachRequest) returns (AttachResponse) {}
// PortForward prepares a streaming endpoint to forward ports from a PodSandbox.
rpc PortForward(PortForwardRequest) returns (PortForwardResponse) {}

The requests carry everything required to allow the server to do the work, for example, the ContainerId or command (Cmd) to be run in case of Exec. More interestingly, all of their responses only contain a url:

message ExecResponse {
    // Fully qualified URL of the exec streaming server.
    string url = 1;
message AttachResponse {
    // Fully qualified URL of the attach streaming server.
    string url = 1;
message PortForwardResponse {
    // Fully qualified URL of the port-forward streaming server.
    string url = 1;

Why is it implemented like that? Well, the original design document for those RPCs even predates Kubernetes Enhancements Proposals (KEPs) and was originally outlined back in 2016. The kubelet had a native implementation for Exec, Attach, and PortForward before the initiative to bring the functionality to the CRI started. Before that, everything was bound to Docker or the later abandoned container runtime rkt.

The CRI related design document also elaborates on the option to use native RPC streaming for exec, attach, and port forward. The downsides outweighed this approach: the kubelet would still create a network bottleneck and future runtimes would not be free in choosing the server implementation details. Also, another option that the Kubelet implements a portable, runtime-agnostic solution has been abandoned over the final one, because this would mean another project to maintain which nevertheless would be runtime dependent.

This means, that the basic flow for Exec, Attach and PortForward was proposed to look like this:

CRI Streaming flow

Clients like crictl or the kubelet (via kubectl) request a new exec, attach or port forward session from the runtime using the gRPC interface. The runtime implements a streaming server that also manages the active sessions. This streaming server provides an HTTP endpoint for the client to connect to. The client upgrades the connection to use the SPDY streaming protocol or (in the future) to a WebSocket connection and starts to stream the data back and forth.

This implementation allows runtimes to have the flexibility to implement Exec, Attach and PortForward the way they want, and also allows a simple test path. Runtimes can change the underlying implementation to support any kind of feature without having a need to modify the CRI at all.

Many smaller enhancements to this overall approach have been merged into Kubernetes in the past years, but the general pattern has always stayed the same. The kubelet source code transformed into a reusable library, which is nowadays usable from container runtimes to implement the basic streaming capability.

How does the streaming actually work?

At a first glance, it looks like all three RPCs work the same way, but that's not the case. It's possible to group the functionality of Exec and Attach, while PortForward follows a distinct internal protocol definition.

Exec and Attach

Kubernetes defines Exec and Attach as remote commands, where its protocol definition exists in five different versions:

1channel.k8s.ioInitial (unversioned) SPDY sub protocol (#13394, #13395) the issues present in the first version (#15961) support for resizing container terminals (#25273) support for exit codes using JSON errors (#26541) support for a CLOSE signal (#119157)

On top of that, there is an overall effort to replace the SPDY transport protocol using WebSockets as part KEP #4006. Runtimes have to satisfy those protocols over their life cycle to stay up to date with the Kubernetes implementation.

Let's assume that a client uses the latest (v5) version of the protocol as well as communicating over WebSockets. In that case, the general flow would be:

  1. The client requests an URL endpoint for Exec or Attach using the CRI.

    • The server (runtime) validates the request, inserts it into a connection tracking cache, and provides the HTTP endpoint URL for that request.
  2. The client connects to that URL, upgrades the connection to establish a WebSocket, and starts to stream data.

    • In the case of Attach, the server has to stream the main container process data to the client.
    • In the case of Exec, the server has to create the subprocess command within the container and then streams the output to the client.

    If stdin is required, then the server needs to listen for that as well and redirect it to the corresponding process.

Interpreting data for the defined protocol is fairly simple: The first byte of every input and output packet defines the actual stream:

First ByteTypeDescription
0standard inputData streamed from stdin
1standard outputData streamed to stdout
2standard errorData streamed to stderr
3stream errorA streaming error occurred
4stream resizeA terminal resize event
255stream closeStream should be closed (for WebSockets)

How should runtimes now implement the streaming server methods for Exec and Attach by using the provided kubelet library? The key is that the streaming server implementation in the kubelet outlines an interface called Runtime which has to be fulfilled by the actual container runtime if it wants to use that library:

// Runtime is the interface to execute the commands and provide the streams.
type Runtime interface {
        Exec(ctx context.Context, containerID string, cmd []string, in io.Reader, out, err io.WriteCloser, tty bool, resize <-chan remotecommand.TerminalSize) error
        Attach(ctx context.Context, containerID string, in io.Reader, out, err io.WriteCloser, tty bool, resize <-chan remotecommand.TerminalSize) error
        PortForward(ctx context.Context, podSandboxID string, port int32, stream io.ReadWriteCloser) error

Everything related to the protocol interpretation is already in place and runtimes only have to implement the actual Exec and Attach logic. For example, the container runtime CRI-O does it like this pseudo code:

func (s StreamService) Exec(
    ctx context.Context,
    containerID string,
    cmd []string,
    stdin io.Reader, stdout, stderr io.WriteCloser,
    tty bool,
    resizeChan <-chan remotecommand.TerminalSize,
) error {
    // Retrieve the container by the provided containerID
    // …

    // Update the container status and verify that the workload is running
    // …

    // Execute the command and stream the data
    return s.runtimeServer.Runtime().ExecContainer(
        s.ctx, c, cmd, stdin, stdout, stderr, tty, resizeChan,


Forwarding ports to a container works a bit differently when comparing it to streaming IO data from a workload. The server still has to provide a URL endpoint for the client to connect to, but then the container runtime has to enter the network namespace of the container, allocate the port as well as stream the data back and forth. There is no simple protocol definition available like for Exec or Attach. This means that the client will stream the plain SPDY frames (with or without an additional WebSocket connection) which can be interpreted using libraries like moby/spdystream.

Luckily, the kubelet library already provides the PortForward interface method which has to be implemented by the runtime. CRI-O does that by (simplified):

func (s StreamService) PortForward(
    ctx context.Context,
    podSandboxID string,
    port int32,
    stream io.ReadWriteCloser,
) error {
    // Retrieve the pod sandbox by the provided podSandboxID
    sandboxID, err := s.runtimeServer.PodIDIndex().Get(podSandboxID)
    sb := s.runtimeServer.GetSandbox(sandboxID)
    // …

    // Get the network namespace path on disk for that sandbox
    netNsPath := sb.NetNsPath()
    // …

    // Enter the network namespace and stream the data
    return s.runtimeServer.Runtime().PortForwardContainer(
        ctx, sb.InfraContainer(), netNsPath, port, stream,

Future work

The flexibility Kubernetes provides for the RPCs Exec, Attach and PortForward is truly outstanding compared to other methods. Nevertheless, container runtimes have to keep up with the latest and greatest implementations to support those features in a meaningful way. The general effort to support WebSockets is not only a plain Kubernetes thing, it also has to be supported by container runtimes as well as clients like crictl.

For example, crictl v1.30 features a new --transport flag for the subcommands exec, attach and port-forward (#1383, #1385) to allow choosing between websocket and spdy.

CRI-O is going an experimental path by moving the streaming server implementation into conmon-rs (a substitute for the container monitor conmon). conmon-rs is a Rust implementation of the original container monitor and allows streaming WebSockets directly using supported libraries (#2070). The major benefit of this approach is that CRI-O does not even have to be running while conmon-rs can keep active Exec, Attach and PortForward sessions open. The simplified flow when using crictl directly will then look like this:

sequenceDiagram autonumber participant crictl participant runtime as Container Runtime participant conmon-rs Note over crictl,runtime: Container Runtime Interface (CRI) crictl->>runtime: Exec, Attach, PortForward Note over runtime,conmon-rs: Cap’n Proto runtime->>conmon-rs: Serve Exec, Attach, PortForward conmon-rs->>runtime: HTTP endpoint (URL) runtime->>crictl: Response URL crictl-->>conmon-rs: Connection upgrade to WebSocket conmon-rs-)crictl: Stream data

All of those enhancements require iterative design decisions, while the original well-conceived implementation acts as the foundation for those. I really hope you've enjoyed this compact journey through the history of CRI RPCs. Feel free to reach out to me anytime for suggestions or feedback using the official Kubernetes Slack.