quicsample not working with mtu < 1500 and SendBufferLength > 1M

[kivu8]

From debugging my program I got to this very reduced and reproducible setup. in sample.c set the SendBufferLength to 10000000 and the MTU on the (loopback) interface to 1400. The Server crashes at this assert:

src/platform/datapath_epoll.c 2183: CXPLAT_DBG_ASSERT(SendData->TotalSize + MaxBufferLength <= sizeof(SendData->Buffer));

outputting these 3 values gives:

SendData->TotalSize: 64288
MaxBufferLength : 1312
sizeof(SendData->Buffer): 65535

meanwhile, when executing with an MTU of 1600, it works and the output of these values are:

SendData->TotalSize: 0
MaxBufferLength : 1200
sizeof(SendData->Buffer): 65535

Am I understanding something wrong? Can the send buffer only be this big?

This answer suggest that I should be able to give in any number (potentially 1G), also after splitting the StreamSend like in the PerfTool I still get the same error. Where am I going wrong?

As Reference, here my custom ServerSend method, where size is just a replacement for SendBufferLength:

ServerSend(
    _In_ HQUIC Stream,
    _In_ uint32_t size
    )
{
    printf("[strm][%p][s] Sending BLOB...\n", Stream);
    uint32_t sentData = 0;
    while (sentData < size) {

        uint64_t BytesLeftToSend = size - sentData;
        uint32_t IoSize = PERF_DEFAULT_IO_SIZE;
        void* SendBufferRaw = malloc(sizeof(QUIC_BUFFER) + IoSize);
        if (SendBufferRaw == NULL) {
            printf("SendBuffer allocation failed!\n");
            MsQuic->StreamShutdown(Stream, QUIC_STREAM_SHUTDOWN_FLAG_ABORT, 0);
            return;
        }

        QUIC_BUFFER* Buffer = (QUIC_BUFFER*)SendBufferRaw;
        Buffer->Buffer = (uint8_t*)SendBufferRaw + sizeof(QUIC_BUFFER);
        Buffer->Length = IoSize;
        QUIC_SEND_FLAGS Flags = QUIC_SEND_FLAG_NONE;

        if ((uint64_t)IoSize >= BytesLeftToSend) {
            IoSize = (uint32_t)BytesLeftToSend;
            Buffer->Length = IoSize;
            Flags = QUIC_SEND_FLAG_FIN;
        }

        sentData += IoSize;

        MsQuic->StreamSend(Stream, Buffer, 1, Flags, Buffer);
        
    }
}