Support Scatter Gather Buffers on the client to copy data directly to application provided buffers

[srsubra]

Describe the feature you'd like supported

Currently, MsQuic with XDP provides the application level callbacks using internal buffers. This requires a mem copy from the MsQuic Internal buffers to the target application buffer. For large blob, allocating this memory and performing a memory copy

Proposed solution

Provide an option where the user can specify the Target buffer address where the data needs to be copied to and MsQuic core layer can automatically copy the data to the target buffer.

Additional context

No response

[guhetier]

Hi!

Here is how we see how the implementation would look like:

• Probably a new flag "QUIC_STREAM_OPEN_FLAG_USE_EXTERNAL_RECEIVE_BUFFERS" to opt-in the feature, when opening the stream
• A new API function along the lines of "StreamProvideReceiveBuffers(stream, nb_buff, QUIC_BUFFERS* buff)"
  • The user can call this on an open stream to provide a list of buffers
  • The user would call it before starting the stream and may call it again later (on a receive callback, etc...) if it needs to provide more buffers

MsQuic would write the received data in the buffers in the order they are provided. A buffer would never be internally allocated or re-used: if MsQuic runs out of provided buffer space for receiving data, the stream would be failed.

The user would need to ensure the buffer remain valid until:

• either the stream is closed, and no more data can be received
• or a "receive" notification indicates all the bytes of the buffer
  • the user needs to keep track of the total number of bytes to know this
  • a "receive" notification could indicate only part of a buffer, multiple buffers, etc... depending on how the data is received

Does it sound like this would work for you?

[nibanks]

Probably a new flag "QUIC_STREAM_OPEN_FLAG_USE_EXTERNAL_RECEIVE_BUFFERS" to opt-in the feature, when opening the stream

I'm not convinced we need this for a couple reasons:

1. This doesn't work for peer started streams. You never open them locally.
2. I think it's enough to say calling StreamProvideReceiveBuffers opts into the feature. You don't need two signals for the same thing.

So, I'm leaning towards only the new API.