"Invalid PVA header 16" / "Bad request ... CA 01 00 16"

[mdavidsaver]

Log files at SLAC have been filling up with:

Hexdump [Invalid PVA header 16, its payload buffer] size = 16
00 00 00 00 00 00 00 00 00 00 FF FF AC 1B 00 18 .... .... .... ....

Note that "header 16" is really 0x16.

Hexdump [Bad request] size = 65487
CA 01 00 16 10 00 00 00 00 00 00 00 00 00 00 00 .... .... .... ....
00 00 FF FF 86 4F 97 24 CA 01 00 03 38 00 00 00 .... .O.$ .... 8...
01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 .... .... .... ....
00 00 FF FF AC 1B 08 1F 97 B8 01 03 74 63 70 01 .... .... .... tcp.
00 01 00 00 00 12 51 55 41 44 3A 4C 54 55 31 3A .... ..QU AD:L TU1:
38 38 30 3A 52 4D 41 54 3A 4C 49 32 36 3A 33 31 880: RMAT :LI2 6:31
...
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 .... .... .... ....

[mdavidsaver]

It look like somehow CMD_ORIGIN_TAG message have been getting past, or going around, the special case handling for them in BlockingUDPTransport::processBuffer() and triggering warnings in the regular server processing in ServerResponseHandler::handleResponse() and ServerBadResponse::handleResponse().

[mdavidsaver]

As background:

CMD_ORIGIN_TAG is part of the "local multicast hack" (my name for it). A unicast UDP message (according to it's sender) is redirected, by whichever socket happens to receive it (client or server), to 224.0.0.128 on which all local clients and servers are listening. When this happens a CMD_ORIGIN_TAG message is prepended to the packet body.

The idea (as I understand it) is to allow unicast UDP search to "just work" for hosts with multiple PVA servers, without having to manually manage TCP port numbers.

The only (as yet hypothetical) problem which I know of with this idea is that it will cause both sides of a PVA gateway to see unicast searches or beacons from either sides.

redirection on the server side:

pvAccessCPP/src/server/responseHandlers.cpp

Lines 290 to 301 in f84774e

	if ((qosCode & 0x80) == 0x80)
	{
	BlockingUDPTransport::shared_pointer bt = dynamic_pointer_cast<BlockingUDPTransport>(transport);
	if (bt && bt->hasLocalMulticastAddress())
	{
	// RECEIVE_BUFFER_PRE_RESERVE allows to pre-fix message
	size_t newStartPos = (startPosition-PVA_MESSAGE_HEADER_SIZE)-PVA_MESSAGE_HEADER_SIZE-16;
	payloadBuffer->setPosition(newStartPos);
	// copy part of a header, and add: command, payloadSize, NIF address
	payloadBuffer->put(payloadBuffer->getArray(), startPosition-PVA_MESSAGE_HEADER_SIZE, PVA_MESSAGE_HEADER_SIZE-5);
	payloadBuffer->putByte(CMD_ORIGIN_TAG);

redirection on the client side:

pvAccessCPP/src/remoteClient/clientContextImpl.cpp

Lines 2661 to 2677 in 31261ce

	if ((qosCode & 0x80) == 0x80)
	{
	// TODO optimize
	ClientContextImpl::shared_pointer context = _context.lock();
	if (!context)
	return;
	BlockingUDPTransport::shared_pointer bt = dynamic_pointer_cast<BlockingUDPTransport>(transport);
	if (bt && bt->hasLocalMulticastAddress())
	{
	// RECEIVE_BUFFER_PRE_RESERVE allows to pre-fix message
	size_t newStartPos = (startPosition-PVA_MESSAGE_HEADER_SIZE)-PVA_MESSAGE_HEADER_SIZE-16;
	payloadBuffer->setPosition(newStartPos);
	// copy part of a header, and add: command, payloadSize, NIF address
	payloadBuffer->put(payloadBuffer->getArray(), startPosition-PVA_MESSAGE_HEADER_SIZE, PVA_MESSAGE_HEADER_SIZE-5);
	payloadBuffer->putByte(CMD_ORIGIN_TAG);

Handling in UDP receiver common to both client and server:

pvAccessCPP/src/remote/blockingUDPTransport.cpp

Line 354 in 8d2edc0

if (unlikely(command == CMD_ORIGIN_TAG))

[mdavidsaver]

pvAccessCPP/src/server/responseHandlers.cpp

Lines 161 to 164 in f84774e

	// TODO remove debug output
	std::ostringstream name;
	name<<"Invalid PVA header "<<hex<<(int)(0xFF&command);
	name<<", its payload buffer";