boostudp

Small Windows-focused UDP harness for testing USO (UDP send segmentation) and optional URO-style receive coalescing (UDP_RECV_MAX_COALESCED_SIZE + WSARecvMsg). Useful when comparing guest vNIC / Apollo behavior to a controlled sender/receiver on the PC.

Binaries:

Target	Role
boostudp_client	Sender (USO via WSASendMsg, or plain sendto with --mss 0)
boostudp_server	Receiver (app-level batch reassembly + CRC; optional --uro)

Requirements

• Windows 10 2004+ (build 19041+) for USO / software receive coalescing APIs
• Visual Studio 2019 (or newer) with C++ desktop workload and CMake (3.16+)
• Git (to fetch vendored Boost)

USO behavior varies by OS build, NIC driver, and installed filters. The client prints windows build N at startup. If WSASendMsg fails, the client reports the error and exits — there is no silent fallback to manual multi-datagram send.

Reported in testing:

Build	Behavior
26080	WSASendMsg USO works; multi-homed OK with default --source (egress + IP_PKTINFO ifindex toward --dest).
26100 / Server 2025	WSASendMsg USO works; hardware USO observed in recent testing (driver/filters may still matter). Default IP_PKTINFO path OK.

If routing is ambiguous, set --source to the IP on the subnet that reaches --dest.

Dependencies

Vendored Boost.Asio (header-only subset via Boost superproject submodules).

Dependency	Repository	Commit (verified)
Boost superproject	https://github.com/boostorg/boost	8c3ca159ca9e5ac4b56ced6a6f146d5fef3650bc

After checking out the Boost commit above, initialize only the submodules required by this repo (see script below).

Fetch dependencies

From the repo root in PowerShell:

.\scripts\fetch-deps.ps1

The script clones into deps/ and runs git submodule update --init for the Boost libraries listed in cmake/BoostDeps.cmake.

If deps/boost already exists, the script skips cloning; delete it to re-fetch.

Build

Windows (Visual Studio 2019)

Adjust the VsDevCmd.bat path in scripts/build-vs2019.bat if your VS install differs, then:

scripts\build-vs2019.bat

Outputs:

build\Release\boostudp_client.exe
build\Release\boostudp_server.exe

Release builds link the static MSVC runtime (/MT).

Manual CMake

mkdir build
cd build
cmake ..
cmake --build . --config Release

Run

Replace addresses/ports for your lab (example: guest 10.0.0.7, PC 10.0.0.15, port 5000).

1. Start the receiver (PC)

build\Release\boostudp_server.exe --port 5000 --count 10 -v

Optional receive coalescing:

build\Release\boostudp_server.exe --port 5000 --count 10 -v --uro

Without --count, the server exits after an idle timeout once traffic stops. Use --loop to start another idle session after each summary (not combinable with --count). With --loop, stdout is flushed after each session so > outfile shows progress as rounds complete.

build\Release\boostudp_server.exe --port 5000 -v --loop
build\Release\boostudp_server.exe --port 5000 -v --loop 5

Verbose output:

• Single datagram per batch (--mss 0 on sender): ok seq=N 33792b
• Reassembled from multiple UDP segments: ok seq=N 33792b mss=1408 segs=24 (mss matches sender --mss, not the first post-header slice)

Summary line: batches=… frags=… bad=… lost=… dup=… ooo=… (bad = failed or incomplete logical batches, not resync bytes skipped)

2. Send from the PC (or guest if built there)

USO segmented (many UDP payloads, one logical batch):

build\Release\boostudp_client.exe --dest 10.0.0.15 --port 5000 --size 33792 --mss 1408 --count 10

Guest vNIC / hardware USO (trace large send NBL): bind to the NIC that routes to --dest.

build\Release\boostudp_client.exe --source 10.0.0.7 --dest 10.0.0.15 --port 5000 --size 5120 --mss 1408 --count 10
build\Release\boostudp_client.exe --dest 10.0.0.15 --port 5000 --size 5120 --mss 1408 --count 10

Plain UDP (one datagram per batch; IP may fragment on the wire):

build\Release\boostudp_client.exe --dest 10.0.0.15 --port 5000 --size 33792 --mss 0 --count 10

CLI reference

Server (--help):

Option	Description
--bind <addr>	Bind address (default 0.0.0.0)
--port <port>	Listen port (required)
--count <n>	Exit after n valid batches
--loop [<n>]	Repeat idle sessions (requires no --count); n optional (default: forever)
-v	Verbose per-batch / reassembly logs
--uro	WSARecvMsg + UDP_RECV_MAX_COALESCED_SIZE (64 KiB)

Client (--help):

Option	Description
--source <addr>	Local bind (default 0.0.0.0 = auto egress toward --dest)
--dest <addr>	Destination host (required)
--port <port>	Destination port (required)
--size <bytes>	Logical batch size, 16..65536 (required; includes 16-byte BDUP header)
--count <n>	Batches to send (default 10)
--mss <bytes>	USO segment size (default 1400; 0 = plain sendto, no USO)
--no-pktinfo	WSASendMsg without IP_PKTINFO (bind-only; logged once at startup)
--completion-routine	WSASendMsg with completion routine + alertable SleepEx

Wireshark

Example filter between guest and PC:

ip.addr == 10.0.0.7 && ip.addr == 10.0.0.15 && udp

With --mss 1408 and --size 33792, expect many ~1408-byte UDP frames. With --mss 0, expect one UDP payload per batch (possibly multiple IP fragments).

Protocol sketch

Each batch is one buffer with a fixed 20-byte header and random payload. length equals --size (includes the header). The sender CRCs the full buffer; the receiver reassembles fragments until length bytes arrive, then validates.

struct PacketHeader {   // packed, little-endian on x86
    uint32_t magic;       // 0x42554450 ("BDUP")
    uint32_t seq;         // 0 .. batch_total-1
    uint32_t batch_total; // sender --count (same on every batch in a run)
    uint32_t length;      // total bytes == --size
    uint32_t crc32;       // CRC over full batch with crc32=0
};

batch_total helps the receiver resync after a bad segment: rescan only accepts headers where seq < batch_total, and lost can use batch_total from the wire when the server has no --count.

Limits

Constant	Value	Meaning
Max batch / buffer	64 KiB	--size, recv buffer, USO/URO caps
Default MSS	1400	USO segment hint
Max MSS	1472	Per-segment UDP payload cap
Server SO_RCVBUF	1 MiB	Always after bind (USO/URO burst); --uro adds coalesced recv

Related references

• Sunshine send_batch() — src/platform/windows/misc.cpp (USO send pattern)
• Windows: UDP_SEND_MSG_SIZE, UDP_RECV_MAX_COALESCED_SIZE, WSASendMsg / WSARecvMsg