Comm Channel Programming Guide

Comm Channel Programming Guide (PDF)

NVIDIA DOCA Comm Channel Programming Guide

This document provides instructions on how to use the DOCA Comm Channel API.

The DOCA Comm Channel provides a secure, network-independent communication channel between the host and the DPU.

The communication channel allows the host to control services on the DPU or to activate certain offloads.

The DOCA Comm Channel is reliable, message-based, and supports multi-client connections to a single service. The API allows communication between a client using any PF/VF/SF on the host to a service on the DPU.

Both the host and the DPU must meet the following criteria:

  • Firmware version 24.33.0458 and higher
  • DOCA Base 5.5-1.0.3.2 and higher

The client is supported both for Linux and Windows, while the server only has Linux support.


3.1. doca_comm_channel_ep_t

This struct represents a Comm Channel endpoint either on the client or service side. The endpoint is needed for every other Comm Channel API function.

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struct doca_comm_channel_ep_t;

3.2. doca_comm_channel_init_attr

This struct contains the attributes for the newly created endpoint in doca_comm_channel_ep_create(). See further information under Limitations.

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struct doca_comm_channel_init_attr { uint32_t cookie; uint32_t flags; uint32_t maxmsgs; uint32_t msgsize; }

cookie
Cookie returned when polling the event_channel.
flags
0 or DOCA_CC_INIT_FLAG_NONBLOCK. Toggles non-blocking mode in created endpoint.
maxmsgs
queue size for created endpoint.
msgsize
maximal message size in created endpoint.

3.3. doca_comm_channel_ep_create

This function is used to create and initialize the endpoint used for all Comm Channel functions.

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doca_error_t doca_comm_channel_ep_create( struct doca_comm_channel_init_attr *attr, struct doca_comm_channel_ep_t **ep);

attr
Endpoint attributes structure.
ep
Output parameter, pointer to created endpoint object.

3.4. doca_comm_channel_addr_t

Referred to as peer_address, this structure represents a connection and whichrepresents a connection and can be used to identify the source of a received message. It is required to send a message using doca_comm_channel_ep_sendto().

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struct doca_comm_channel_addr_t;

3.5. doca_comm_channel_ep_listen

Used to listen on service endpoint, this function can only be called on the DPU. The service listens on all available devices. Calling listen allows clients to connect to the service.

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doca_error_t doca_comm_channel_ep_listen(struct doca_comm_channel_ep_t *local_ep, const char *name);

local_ep
Endpoint to listen on.
name
The name for the service to listen on. Clients must provide the same name to connect to the service.

3.6. doca_comm_channel_ep_connect

Used to create a connection between a client and a service, this function can only be called on the host.

The client connects to the first device it sees.

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doca_error_t doca_comm_channel_ep_connect( struct doca_comm_channel_ep_t *local_ep, const char *name, struct doca_comm_channel_addr_t **peer_addr);

local_ep
Endpoint to listen on.
name
The name for the service to listen on. Clients must provide the same name to connect to the service.
peer_addr
Output parameter. Contains the pointer to the new connection.

3.7. doca_comm_channel_ep_sendto

This function is used to send a packet of data from one side to the other (service to the client or vice versa). This function can be called either in blocking or non-blocking mode. Refer to chapter Usage for more details.

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doca_error_t doca_comm_channel_ep_sendto(struct doca_comm_channel_ep_t *local_ep, const void *msg, size_t len, int flags, struct doca_comm_channel_addr_t *peer_addr);

local_ep
Endpoint to send the message from.
msg
Pointer to the buffer that contains the data to send.
len
Length of data to be sent.
flags
0 or DOCA_CC_MSG_FLAG_DONTWAIT, runs on blocking/non-blocking mode respectively.
peer_addr
Peer address to send the message to (see also struct doca_comm_channel_addr_t).

3.8. doca_comm_channel_rp_recvfrom

This function is used to receive a packet of data on either the service or the host. This function can be called either in blocking or non-blocking mode. Refer to chapter Usage for more details.

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doca_error_t doca_comm_channel_ep_recvfrom( struct doca_comm_channel_ep_t *local_ep, void *msg, size_t *len, int flags, struct doca_comm_channel_addr_t **peer_addr);

local_ep
Endpoint to send the receive the message on.
msg
Buffer to write the message to.
len
Size of the given buffer
flags
0 or DOCA_CC_MSG_FLAG_DONTWAIT, runs on blocking or non-blocking mode respectively.
peer_addr
Output parameter. Handle to peer_addr that represents the connection the message arrived from.

3.9. doca_comm_channel_ep_disconnect

This function disconnects an endpoint from a specific peer_address. The disconnection is one-sided, and the other side is unaware of it. New connections can be created afterwards. Refer to chapter Usage for more details.

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doca_error_t doca_comm_channel_ep_disconnect( struct doca_comm_channel_ep_t *local_ep, struct doca_comm_channel_addr_t *peer_addr);

local_ep
Endpoint to disconnect.
peer_addr
Connection to disconnect from.

3.10. doca_comm_channel_ep_destroy

This function disconnects all connections of the endpoint, destroys the endpoint object, and frees all related resources.

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doca_error_t doca_comm_channel_ep_destroy(struct doca_comm_channel_ep_t *ep);

local_ep
Endpoint to destroy.


4.1. Message Size

When creating an endpoint using doca_comm_channel_ep_create(), the user must provide the maximal message size value.

This value must be between 256 and 4080 bytes. If a smaller value is provided, it is raised internally to 256 bytes.

4.2. Queue Size

The maximal queue size (as provided in doca_comm_channel_ep_create(), maxmsgs) cannot exceed 8191 messages.

The minimal queue size is at least 16 messages. If a smaller value is provided, it is raised internally to 16.

4.3. Service Name

The service name must be between one character and 120 bytes long, including the terminating null byte.

4.4. Multi-client

A single service on the DPU can serve multiple clients, but a client can only connect to a single service.

In addition, two clients cannot connect to the same service from the same VF. If two clients are present on the same VF, each must connect to a different service.

4.5. Threads

The DOCA Comm Channel is not thread-safe. To use a single endpoint over multiple threads, one must use locks. Different endpoints can be used over different threads.

4.6. Devices

DOCA Comm Channel does not allow the user to choose the devices used. The Comm Channel listens on all devices available on the service side. On the client’s side, the endpoint uses the first available device.


5.1. Objects

While working with DOCA Comm Channel, one must maintain two objects:

  • struct doca_comm_channel_ep_t, referred to as "endpoint"
  • struct doca_comm_channel_addr_t, referred to as "peer_address"

5.1.1. Endpoint

The endpoint object represents the endpoint of the Comm Channel, either on the client or service side. The endpoint is created by calling the doca_comm_channel_ep_create() function. It is required for every other Comm Channel function.

5.1.2. Peer_address

The peer_address structure represents a connection. It is created when a new connection is made (i.e., client calls doca_comm_channel_ep_connect() or a service receives a connection through doca_comm_channel_ep_recvfrom()). Refer to section Connection Flow for more details.

The peer_address structure can be used to identify the source of a received message and is needed to send a message using doca_comm_channel_ep_sendto().

The peer_address has an identifier named "user data" that can be set by the user using doca_comm_channel_peer_addr_user_data_set(), and retrieved using doca_comm_channel_peer_addr_user_data_get(). The default value for user data is 0.

5.2. Endpoint Initialization

To start using the DOCA Comm Channel, the user should initialize a doca_comm_channel_init_attr struct that contains the endpoint's configuration attributes and use it to create an endpoint object, using the doca_comm_channel_ep_create() function.

5.3. Connection Flow

The following diagram illustrates the process of establishing a connection between the host and a service.

establishing-connection.png

  1. After initializing the endpoint on the service side, one should call doca_comm_channel_ep_listen() with a legal service name (see Limitations) to start listening.
  2. After the service starts listening and the client endpoint is created, the client calls doca_comm_channel_ep_connect() with the same service name used for listening. As part of the connect function, the client starts a handshake protocol with the server. If connect is called before the service is listening, or the handshake process fails, then the connect function fails.
  3. The service receiving new connections is done using doca_comm_channel_ep_recvfrom(). No indication is given that a new connection is made. The server keeps waiting to receive packets. If the handshake fails or is done for an existing client, then the receive function fails.

5.4. Data Transfer Flow

After a connection is established between client and service, both sides can send and receive data using the doca_comm_channel_ep_sendto() and doca_comm_channel_ep_recvfrom() functions, respectively.

If multiple clients are connected to the same service, then the doca_comm_channel_ep_recvfrom() function reads the messages in the order of their arrival, regardless of their source.

To send a message, the endpoint needs to obtain the target's peer_address object. This restriction leads to the client being the one that must start the communication (not including the handshake) for the server to obtain the client's peer_address object and send data back.

The user can choose between working in blocking or non-blocking mode. When running in non-blocking mode, the send/receive function returns immediately, either in success or failure. When running in blocking mode, if the conditions to send/receive are not met, the function waits internally for the requirements to be met.

5.4.1. Blocking Mode

If doca_comm_channel_ep_sendto() or doca_comm_channel_ep_recvfrom() are called without the DOCA_CC_MSG_FLAG_DONTWAIT flag and also the sending/receiving endpoint is not created with the DOCA_CC_INIT_FLAG_NONBLOCK flag, then those functions run in blocking mode. And if the other side is not ready to receive (for send), or a new message is not available to receive, then the function is blocked until the receiver is ready or a new message arrives.

5.4.2. Non-blocking Mode

If the doca_comm_channel_ep_sendto() or doca_comm_channel_ep_recvfrom() functions are called with the DOCA_CC_MSG_FLAG_DONTWAIT flag, or the endpoint is created with the DOCA_CC_INIT_FLAG_NONBLOCK flag, then:

  • The send function runs in non-blocking mode and returns with DOCA_ERROR_AGAIN if the other side is not ready to receive
  • The receive function runs in non-blocking mode and returns with DOCA_ERROR_AGAIN if no new message is available

5.5. Disconnection Flow

Disconnection can occur specifically by using doca_comm_channel_ep_disconnect() or when destroying the whole endpoint.

Disconnection is one-sided, which means that the other side is unaware of the channel being closed and experiences errors when sending data. It is up to the application to synchronize the connection teardown.

It is possible to perform another handshake and establish a new channel connection after disconnection.

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