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RFC-0109: Fast UDP sockets

RFC-0109: Fast UDP sockets
  • Networking

Implement network UDP socket data transport using zircon sockets.

Gerrit change
Date submitted (year-month-day)2021-06-17
Date reviewed (year-month-day)2021-06-25


Implement network datagram socket data transport using zircon sockets. Implement client-side argument validation using client-cached network state.


Increase datagram socket throughput and reduce CPU utilization.

Prior to datagram network sockets were implemented using zircon sockets; clients would zx_socket_write to send data and zx_socket_read to receive data. A minimal protocol was used to carry metadata such as the destination address, if one was provided by the calling application.

This approach was abandoned because it did not provide for error signaling to the caller in certain cases. Consider an application that wishes to send a payload to a remote to which the network stack does not have a route; in order to comply with the expectations of third party software targeting Linux, the implementation must return an error to indicate that no action was taken on behalf of the caller.

The current implementation is defined in fuchsia.posix.socket/DatagramSocket.{Recv,Send}Msg. Both sending and receiving data requires multiple context switches due to support FIDL responses. Ignoring FIDL serialization:

| Client                      | Netstack                |
| `zx_channel_call(...)`      |                         |
|                             |                         |
|                             | `zx_channel_read(...)`  |
|                             | `zx_channel_write(...)` |
|                             |                         |
| (`zx_channel_call` returns) |                         |

The reverse (receiving data) looks much the same. Compare this to unacknowledged I/O:

| Client                  | Netstack               |
| `zx_channel_write(...)` |                        |
| `zx_channel_write(...)` | `zx_channel_read(...)` |

While unacknowledged I/O is possible with zircon channels and FIDL, it does not provide backpressure, and can lead to runaway memory growth. Thus, we propose to use zircon sockets:

| Client                 | Netstack              |
| `zx_socket_write(...)` |                       |
| `zx_socket_write(...)` | `zx_socket_read(...)` |


Utilizing unacknowledged I/O presupposes that datagram socket I/O can be entirely validated on the local machine without interaction with the network, and that the results of this validation can be cached across multiple interactions with the socket (so that the validation cost can be amortized).

Note that this assumption notably does not hold for IPPROTO_ICMP sockets - their payloads are checked for validity - so the existing FIDL-based protocol will be retained and used where performance is not critical and deep validation is required.

Extract FIDL from existing types to be reused and rename DatagramSocket for clarity:

protocol BaseDatagramSocket {
  compose BaseSocket;

  /// Retrieves creation information from the socket.
  GetInfo() -> (Domain domain, DatagramSocketProtocol proto) error fuchsia.posix.Errno;

protocol SynchronousDatagramSocket {
  compose BaseDatagramSocket;

  /// Receives a message from the socket.
  RecvMsg(bool want_addr, uint32 data_len, bool want_control, RecvMsgFlags flags) -> ( addr, bytes data, RecvControlData control, uint32 truncated) error fuchsia.posix.Errno;
  /// Sends a message on the socket.
  SendMsg( addr, bytes:MAX data, SendControlData control, SendMsgFlags flags) -> (int64 len) error fuchsia.posix.Errno;

Define the new FIDL protocol with validation functions:

/// Matches the definition in //zircon/system/public/zircon/types.h.
const uint32 ZX_WAIT_MANY_MAX_ITEMS = 64;

/// Describes an intent to send data.
table SendMsgArguments {
  /// The destination address.
  /// If absent, interpreted as the method receiver's connected address and
  /// causes the connected address to be returned in [``].
  /// Required if the method receiver is not connected.
  1: to;

/// Describes a granted approval to send data.
resource table SendMsgBoardingPass {
  /// The validated destination address.
  /// Present only in response to an unset
  /// [``].
  1: to;
  /// Represents the validity of this structure.
  /// The structure is invalid if any of the elements' peer is closed.
  /// Datagrams sent to the associated destination after invalidation will be
  /// silently dropped.
  2: vector<zx.handle:<EVENTPAIR, zx.RIGHTS_BASIC>>:ZX_WAIT_MANY_MAX_ITEMS validity;
  /// The maximum datagram size that can be sent.
  /// Datagrams exceeding this will be silently dropped.
  3: uint32 maximum_size;

protocol DatagramSocket {
  compose BaseDatagramSocket;

  /// Validates that data can be sent.
  /// + request `args` the requested disposition of data to be sent.
  /// - response `pass` the constraints sent data must satisfy.
  /// * error the error code indicating the reason for validation failure.
  SendMsgPreflight(SendMsgArguments args) -> (SendMsgBoardingPass pass) error fuchsia.posix.Errno;

Define FIDL structures to be sent on the zircon socket:

/// A datagram received on a network socket, along with its metadata.
table RecvMsgPayload {
  1: from;
  2: bytes:MAX datagram;
  3: RecvControlData control;

/// A datagram to be sent on a network socket, along with its metadata.
table SendMsgPayload {
  1: SendMsgArguments args;
  2: bytes:MAX datagram;
  3: SendControlData control;

These structures would be encoded and decoded using FIDL-at-rest, which is not yet fully specified (see

Note that this representation does not eliminate the extra data copy incurred in the FIDL deserialization path even in the presence of the vectorized socket I/O operations proposed in This overhead is not addressed in this proposal, and may be the focus of future work, should the need arise.

Clients wanting to send data obey the following (horizontally squished) diagram:

+--------------------------------+               +---------------------------+               +----------------+
| cache.getSendMsgBoardingPass() | - Present ->  | checkPeerClosed(validity) |   +- ZX_OK -> | Return success |
+--------------------------------+               +---------------------------+   |           +----------------+
 |    ^                                            ^                     |  |    |
 |    |                                            |                     |  |  +------------------------------+
 |  +------+                  +----------------------------------+       |  |  | socket.write(SendMsgPayload) | - != ZX_OK -----+
 |  | Send |    +- Success -> | cache.storeSendMsgBoardingPass() |       |  |  +------------------------------+                 |
 |  +------+    |             +----------------------------------+       |  |                         ^                         |
 |            +--------------------+                                     |  |                         |                         |
 +- Absent -> | SendMsgPreflight() |  +- (ZX_OK, ZX_SIGNAL_PEER_CLOSED) -+  +- (ZX_ERR_TIMED_OUT) -+  |                         |
              +--------------------+  |                                                            |  +- No -+                  |
                |                ^    |   +-----------------------------------+                    |         |                  |
                |                |    +-> | cache.removeSendMsgBoardingPass() |                    |   +---------------------+  |
                |                |        +-----------------------------------+                    +-> | size > maximum_size |  |
                |                |          |                                                          +---------------------+  |
                |                |          |  +--------------+                                              |                  |
                |                +----------+  |              | <-------------------------------------- Yes -+                  |
                |                              | Return error |                                                                 |
                +- Failure ------------------> |              | <---------------------------------------------------------------+

Where the client's cache "implements" SendMsgPreflight; it is roughly a map from to (vector<zx::eventpair>, maximum_size).

Note that the cache formed by this strategy is eventually-consistent; it is possible for invalidation to interleave a client's validity check and its payload arriving at the network stack. This is acceptable for datagram sockets whose delivery semantics are best-effort.


Add the new implementation to

resource union NodeInfo {
    /// The connection composes [`fuchsia.posix.socket/DatagramSocket`].
    N: DatagramSocket datagram_socket;

/// A [`NodeInfo`] variant.
// TODO( replace with an anonymous struct inline.
resource struct DatagramSocket {
    zx.handle:<SOCKET, zx.RIGHTS_BASIC | zx.RIGHTS_IO> socket;

Change the return type of fuchsia.posix.socket/Provider.DatagramSocket to a variant:

/// Contains a datagram socket implementation.
resource union DatagramSocketImpl {
    1: DatagramSocket datagram_socket;
    2: SynchronousDatagramSocket synchronous_datagram_socket;

...and change the behavior such that a DatagramSocket is returned whenever permitted by the arguments (i.e. the caller did not request an ICMP socket).

The initial implementation is expected to supply two elements in each SendMsgBoardingPass.validity:

  1. Represents a known state of the routing table; shared by all sockets and invalidated on any change to the routing table.
  2. Represents a known state of the particular socket; invalidated on any change to the socket that may change the socket's behavior e.g. calls to bind, connect, setsockopt(..., SO_BROADCAST, ...), setsockopt(..., SO_BINDTODEVICE, ...), etc.


Throughput of SOCK_DGRAM sockets is expected to approximately double; this estimate is based on the performance regression seen after

CPU utilization is expected to decrease by a meaningful but unknown magnitude.


This change does not have meaningful impact on ergonomics as downstream users do not directly consume the interfaces presented here.

Backwards Compatibility

Preserve ABI compatibility by initially leaving fuchsia.posix.socket/Provider.DatagramSocket unchanged and implementing the new functionality as DatagramSocket2. Following necessary ABI transition, rename DatagramSocket2 to DatagramSocket and remove the previous implementation. Following another ABI transition, remove DatagramSocket2.

Security considerations

This proposal has no impact on security.

Privacy considerations

This proposal has no impact on privacy.


Existing unit tests cover the functionality affected.


No documentation is necessary apart from FIDL doc comments presented here.

Drawbacks, alternatives, and unknowns

This proposal addresses the motivation by building machinery in userspace. Another possibility is to build this machinery in the kernel.

A sketch for translating it to kernel:

  1. With each zx::socket endpoint, the kernel would maintain a map from SocketAdddres to max_size.
  2. We'd add some zx_socket_add_route / zx_socket_remove_route system calls for modifying that map on the peer endpoint.
  3. We'd add some zx_socket_send_to / zx_socket_receive_from system calls that would consume/provide addresses.

If userspace called zx_socket_send_to with an address that wasn't in the map, the operation would fail and userspace would need to send a synchronous message to the netstack to request that route be added to the zx::socket. If that request failed, then the address operation fails with an error.


In the kernel approach, sending a UDP packet (in the fast case) is a single syscall (zx_socket_send_to) rather than two syscalls (zx_object_wait_many, zx_socket_write).

This is potentially a non-pro because of possible optimizations to the userspace approach. Realizing that we always zx_object_wait_many with time::infinite_past, we could optimized the operation to do its work without a system call, provided that the necessary state is maintained using atomic operations. This might require the handle table to be in the vDSO as well, which may not be the case and may not be possible.

An alternative for clients with runtimes is to use zx_object_wait_async instead of wait_many to maintain the local cache, allowing the fast path to avoid the extra syscall.

We also avoid the dependency on FIDL-at-rest and the extra data copy inherent in FIDL because the message payload is baked into the system calls, which can copy the payload directly to the final destination.


In the kernel approach, there isn't an obvious way to do O(1) route cancellation when the routing table changes. As described, we could add a flag to zx_socket_remove_route that removes all the routes (probably desirable anyway), but the netstack would need to issue a zx_socket_remove_route on every socket.

We could get super fancy and have zx_socket_add_route take an eventpair for cancellation, but that's getting pretty baroque.

Baking these structures into the kernel is also costly in terms of introducing yet another protocol evolution model to the platform; we'd now have a much tighter coupling between particular FIDL types and system calls, which would not automatically remain in sync.


There's also a question about how to deal with SendControlData. Perhaps that would need to be an additional parameter to zx_socket_send_to or maybe a flag on the operation.