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Network Abstraction Layer

The network abstraction (NA) layer is internally used by both the RPC layer and the bulk layer. The NA layer uses a plugin mechanism so that support for various network protocols can be easily added and selected at runtime.


The NA interface should not be directly used if you intend to use Mercury's RPC layer (HG calls). In that case, please directly jump to the available plugins section for a list of plugins that can be used when initializing Mercury—Mercury's initialization is then further described in the RPC layer section.

NA Interface

NA provides a minimal set of function calls that abstract the underlying network fabric and that can be used to provide: target address lookup, point-to-point messaging with both unexpected and expected messaging, remote memory access (RMA), progress and cancelation. The API is non-blocking and uses a callback mechanism so that upper layers can provide asynchronous execution more easily: when progress is made (either internally or after a call to NA_Progress()) and an operation completes, the user callback is placed onto a completion queue. The callback can then be dequeued and separately executed after a call to NA_Trigger().


When using NA, the first step of a program should consist of initializing the NA interface and selecting an underlying plugin that will be used. Initializing the NA interface with a specified info_string results in the creation of a new na_class_t object. Please refer to the available plugins section for more information on the info_string format. Additionally, it is possible to specify whether the na_class_t object will be listening or not—this is the only time where a "server" specific behavior is defined, all subsequent calls do not make any distinction between a "client" and a "server" and instead only use the concept of origin and target. It is worth noting, however, that the listen flag may have an effect on the resources that are allocated and that the address passed through info_string will be used to create an endpoint that remote peers can access.

na_class_t *
NA_Initialize(const char *info_string, bool listen);

If a more specific behavior is required, the following call can also be used to pass specific init options.

Option Description
ip_subnet Preferred IP subnet to use.
auth_key Authorization key that can be used for communication. All processes should use the same key in order to communicate.
max_unexpected_size Max unexpected size hint that can be passed to control the size of unexpected messages.
max_expected_size Max expected size hint that can be passed to control the size of unexpected messages.
progress_mode Progress mode flag. Setting NA_NO_BLOCK will force busy-spin on progress and remove any wait/notification calls.
addr_format Preferred address format. Can be set to NA_ADDR_IPV4, NA_ADDR_IPV6 or NA_ADDR_NATIVE.
max_contexts Maximum number of contexts that are expected to be created.
thread_mode Thread mode flags. Setting NA_THREAD_MODE_SINGLE will relax thread-safety requirements.
request_mem_device Request support for tranfers to/from memory devices (e.g., GPU, etc).
struct na_init_info {
    const char *ip_subnet;
    const char *auth_key;
    size_t max_unexpected_size;
    size_t max_expected_size;
    uint8_t progress_mode;
    enum na_addr_format addr_format;
    uint8_t max_contexts;
    uint8_t thread_mode;
    bool request_mem_device;

na_class_t *
NA_Initialize_opt2(const char *info_string, bool listen, unsigned int version,
    const struct na_init_info *na_init_info);

The na_class_t object created from these initialization calls should later be released with a call to:

NA_Finalize(na_class_t *na_class);

Once the interface has been initialized, a context within this plugin must be created, which internally creates and associates a completion queue for the operations:

na_context_t *
NA_Context_create(na_class_t *na_class);

It can then be destroyed using:

NA_Context_destroy(na_class_t *na_class, na_context_t *context);

Target Address Lookup

To communicate with a target, one must first get its address. The most convenient and safe way of doing that is by calling on the target:

NA_Addr_self(na_class_t *na_class, na_addr_t **addr_p);

And then convert that address to a string using:

NA_Addr_to_string(na_class_t *na_class, char *buf, size_t *buf_size_p, na_addr_t *addr);

The string can then be exchanged to other processes through out-of-band mechanisms (e.g., using a file, etc), which can then look up the target using the function:

NA_Addr_lookup(na_class_t *na_class, const char *name, na_addr_t **addr_p);

All addresses must then be freed using:

NA_Addr_free(na_class_t *na_class, na_addr_t *addr);

Point-to-point Messaging

Point-to-point messaging in NA is always non-blocking with completion callbacks being executed after a call to NA_Trigger() (once the operation has completed and been placed onto the completion queue). NA supports two separates modes for sending and receiving messages: either unexpected or expected. Expected messages should always have their receive pre-posted. Though messages may be dropped without notification if that is not the case, they are usually still queued and later processed. Unexpected messages on the other handle never require receives to be pre-posted and messages are also allowed to be dropped (though once again plugins usually do queue them). Both types of messages are tagged messages that take the same arguments for sends:

NA_Msg_send_unexpected(na_class_t *na_class, na_context_t *context,
    na_cb_t callback, void *arg, const void *buf, size_t buf_size,
    void *plugin_data, na_addr_t *dest_addr, uint8_t dest_id, na_tag_t tag,
    na_op_id_t *op_id);

NA_Msg_send_expected(na_class_t *na_class, na_context_t *context,
    na_cb_t callback, void *arg, const void *buf, size_t buf_size,
    void *plugin_data, na_addr_t *dest_addr, uint8_t dest_id, na_tag_t tag,
    na_op_id_t *op_id);

And only mostly differ in their receive operation:

NA_Msg_recv_unexpected(na_class_t *na_class, na_context_t *context,
    na_cb_t callback, void *arg, void *buf, size_t buf_size,
    void *plugin_data, na_op_id_t *op_id);

NA_Msg_recv_expected(na_class_t *na_class, na_context_t *context,
    na_cb_t callback, void *arg, void *buf, size_t buf_size,
    void *plugin_data, na_addr_t source_addr, uint8_t source_id,
    na_tag_t tag, na_op_id_t *op_id);

One will only match with a specific source_addr and tag while the other will match with any source and tag, which can then later be retrieved from the callback info.

struct na_cb_info_recv_unexpected {
    size_t actual_buf_size;
    na_addr_t *source;
    na_tag_t tag;

Note that for best performance, NA_Msg_buf_alloc() and NA_Msg_buf_free() may be used to allocate send and receive buffers.

Remote Memory Access

Remote memory access requires host memory that is desired to be accessed to be first registered with the NA layer. This is done in two steps, by first creating a handle that describes the memory buffer that is to be registered and calling NA_Mem_register() on it.

NA_Mem_handle_create(na_class_t *na_class, void *buf, size_t buf_size,
    unsigned long flags, na_mem_handle_t **mem_handle_p);

NA_Mem_register(na_class_t *na_class, na_mem_handle_t *mem_handle,
    enum na_mem_type mem_type, uint64_t device);

Similarly, NA_Mem_deregister() and NA_Mem_handle_free() must be called to release resources.

Once memory has been registered, the handle of the target must be serialized and exchanged with the peer that will initiate the RMA operation. This is done by calling:

NA_Mem_handle_serialize(na_class_t *na_class, void *buf, size_t buf_size,
    na_mem_handle_t *mem_handle);

The peer can then deserialize the handle using:

NA_Mem_handle_deserialize(na_class_t *na_class, na_mem_handle_t **mem_handle_p,
    const void *buf, size_t buf_size);

And initiate an RMA operation using both the handle of the target that describes its remote memory and the local handle that describes its local memory:

NA_Put(na_class_t *na_class, na_context_t *context, na_cb_t callback, void *arg,
    na_mem_handle_t *local_mem_handle, na_offset_t local_offset,
    na_mem_handle_t *remote_mem_handle, na_offset_t remote_offset,
    size_t data_size, na_addr_t *remote_addr, uint8_t remote_id, na_op_id_t *op_id);

NA_Get(na_class_t *na_class, na_context_t *context, na_cb_t callback, void *arg,
    na_mem_handle_t *local_mem_handle, na_offset_t local_offset,
    na_mem_handle_t *remote_mem_handle, na_offset_t remote_offset,
    size_t data_size, na_addr_t *remote_addr, uint8_t remote_id, na_op_id_t *op_id);

Similar to point-to-point operations, RMA operations are non-blocking and use a callback-based model that is triggered after a call to NA_Trigger() when the operation completes.

Progress and Cancelation

NA progress model is always explicit and users are expected to call NA_Progress() followed by a call to NA_Trigger():

NA_Progress(na_class_t *na_class, na_context_t *context, unsigned int timeout);

NA_Trigger(na_context_t *context, unsigned int max_count, unsigned int *actual_count);

NA_Trigger() always operates on a single context while NA_Progress() may operate both on a class and a context. When progress is called, it returns as soon as an operation either completes or is already in the completion queue so that a call to NA_Trigger() may be done to empty the queue and execute the user callback.

When an operation must be canceled, users are expected to call NA_Cancel() on that operation:

NA_Cancel(na_class_t *na_class, na_context_t *context, na_op_id_t *op_id);

Cancelation is always asynchronous. When/if the operation is successfully canceled, it will be pushed to the completion queue with an NA_CANCELED error return code.

Available Plugins

NA supports different backend implementations. However, OFI/libfabric is the recommended plugin in most situations for inter-node communication, while SM (shared-memory) is recommended for intra-node communication.


The table below summarizes the current list of plugins along with the transports that we currently support with those plugins.

Plugin / Transport tcp verbs shm opx gni cxi
ofi ✔ ✔ ✔ ✔ ✔ ✔
ucx ✔ ✔ ❔ ❌ ❔ ❌
sm ❌ ❌ ✔ ❌ ❌ ❌


Additional transports may be supported for each plugin but we do not recommend their use unless explicitly mentioned in the above table as they are either unstable or have not been tested. Transports with ❌ are not available for the selected plugin. Transports with ❔ are not supported but may be available in the future. Transports with ❗ have known issues.

Initialization String Format

Below is a table summarizing the protocols and expected format for each plugin ([ ] means optional, in which case the plugin will select default hostnames and ports to use).

Plugin Protocol Initialization format1
ofi tcp
ofi+tcp[://<hostname,IP,interface name>:<port>]
ofi+verbs[://[MLX device/]<hostname,IP,interface name>:<port>]2
ofi+opx[://<HFI device>:<port>]
ofi+gni[://<hostname,IP,interface name>] 4
ofi+cxi[://<CXI device>:<port ID>]
ucx all
rc,ud,dc 5
ucx+all[://[net_device/]<hostname,IP,interface name>:<port>]
na sm na+sm[://<shm_prefix>]


Invalid port numbers that are passed may be silently ignored by the underlying implementation in which case a new port will be automatically picked up.

1 When initialized without listening, the port specification can be elided.

2 The libfabric domain name can also be passed directly to select the right adapter to use. See the output generated by the command hg_info for provider name verbs;ofi_rxm (e.g., mlx5_0).

3 Any hostname or port being passed will be ignored.

4 No port information needs to be passed, the most common interface name is ipogif0, which will be used by default if no hostname is passed.

5 Please refer to the UCX documentation for a full list of available transports that can be used.


(as of v1.0.0) The NA OFI/libfabric plugin is available for general purpose use. The plugin currently supports tcp, verbs, opx and cxi transports. The psm2 and gni protocols are deprecated. See this page for additional implementation and performance details.


As of libfabric 1.18.0, tcp no longer uses the RxM layer. To force use of tcp with RxM, tcp;ofi_rxm must be directly passed. However, it is recommended to use tcp for improved stability and performance.

Technical notes:

  • Low CPU consumption (i.e., idles without busy spinning) is supported by all supported libfabric providers.
  • Connection-less and uses reliable datagrams.
  • RMA (for Mercury bulk operations) is implemented natively on transports that support it.
  • ofi/tcp (tcp provider) may use the RxM layer to emulate connection-less endpoints. It also emulates RMA operations.
  • ofi/verbs (verbs provider) uses the RxM layer to emulate connection-less endpoints (the first message being sent may be slower).
  • ofi/opx (opx provider) can be used on legacy Intel® Omni-Path 2 interconnect and Cornelis Omni-Path Express hardware.
  • ofi/gni (gni provider) can be used on Cray® systems with Gemini/Aries interconnects. Note that it requires the use of Cray® DRC to exchange credentials when communication between separate jobs is required (see section on DRC credentials).
  • ofi/cxi (cxi provider) can be used on HPE systems with Slingshot interconnect. For systems with multiple NICs, hwloc can be used to automatically select the closest NIC to the CPU in-use (mercury must be configured with NA_OFI_USE_HWLOC in that case).

Influential variables:

  • RDMAV_HUGEPAGES_SAFE: must be set when using hugepages in combination with verbs provider.
  • FI_UNIVERSE_SIZE: must be set when exceeding 256 peers with tcp or verbs providers.

Please refer to the libfabric manpages for additional details for each transports.


(as of v2.1.0) The UCX plugin is available for general purpose use. By default and as opposed to other plugins, the UCX plugin is able to automatically determine which transport is best to be used. This is achieved by passing the all keyword in lieu of a specific transport. However, note that we are only testing the tcp and rc,ud,dc (verbs) protocols of UCX.

Technical notes:

  • Connection-less is currently emulated on top of connected endpoints. Therefore, it is expected that the first message sent to a target will be slower than subsequent messages.
  • A thread safe enabled UCX library is required to be used unless users explicitly tell NA, using the thread_mode init option (see above), that they will not access classes and contexts with more than one thread.
  • NA_Addr_to_string() cannot be used on non-listening processes to convert a self-address to a string. This is due to the fact that UCX does not expose endpoints prior to their connection.

Influential variables:

  • ucx_info -c -f will display the default configuration. Each of these variables can be overridden by the user. Note, however, that the UCX_TLS and UCX_NET_DEVICES are currently overridden by the NA UCX plugin.


(as of v0.9.0) This is the integrated shared memory NA plugin. Plugin is stable and provides significantly better performance for local node communication. The goal of this plugin is to provide a transparent shortcut for other NA plugins when they connect to local services using the auto_sm initialization option (see the RPC section for more details), but it is also useful as a primary transport for single-node services.

Technical notes:

  • Uses fully connection-less communication.
  • Low CPU consumption (i.e., idles without busy spinning or using threads).
  • RMA (for Mercury bulk operations) is implemented natively through cross-memory attach (CMA) on Linux, and there are fallback methods for other platforms as well. See this page for additional implementation and performance details.

Deprecated Plugins


(deprecated) This NA plugin is no longer available for general purpose use, and is now deprecated as CCI itself is no longer actively maintained.


MPI implementations are widely available for nearly any platform, and the NA MPI plugin provides a convenient option for prototyping and functionality testing. It is not optimized for performance, however, and it has some practical limitations when used for persistent services.

Technical notes:

  • Clients can connect to a server dynamically only if the underlying MPI implementation supports MPI_Comm_connect().
  • RMA (for Mercury bulk operations) is emulated via point-to-point messaging (note: MPI window creation requires collective coordination and is not a good fit for RPC use).
  • Significant CPU consumption (progress function iteratively polls pending operations for completion).


The BMI library itself is no longer under active feature development beyond basic maintenance, but the NA BMI plugin provides a very stable and reasonably performant option for IP networks when used with BMI's TCP method.

Technical notes:

  • Low CPU consumption (i.e., idles without busy spinning or using threads).
  • Supports dynamic client connection and disconnection.
  • RMA (for Mercury bulk operations) is emulated via point-to-point messaging.
  • Does not support initializing multiple instances simultaneously.
  • Other BMI methods besides TCP are not supported.
  • For general BMI information see this paper.

Last update: June 15, 2023