/*! \page page_rtp Radio Transport Protocols

\tableofcontents

Radio transport protocols are used to exchange samples (or other items) between host and devices.
If one were to sniff Ethernet traffic between a USRP and a PC, the packets would conform to a
radio transport protocol.

For USRP devices, two radio transport protocols are relevant: VRT (the VITA Radio Transport protocol)
and CHDR (compressed header, an Ettus-specific protocol).
Generation-3 devices and the B200 use CHDR, the rest use VRT.

\section rtp_vrt VRT

VRT is an open protocol defined by the VITA-49 standard. It was designed for interoperability,
and to allow different device types to work with different software stacks.

VRT is a very verbose standard, and only a subset is implemented in UHD/USRPs.
The full standard is available from the VITA website: http://www.vita.com .


\section rtp_chdr CHDR

For the third generation of Ettus devices, a new type transport protocol was designed.
It reduces the complexity of the original standard and uses a fixed-length 64-Bit header
for everything except the timestamp. Because it is "compressed" into a 64-bit heaer, it
was dubbed CHDR (pronounced like the cheese "cheddar").

By compressing all information into a 64-bit line, the header can efficiently be parsed
in newer FPGAs, where the common streaming protocol is 64-Bit AXI. The first line in a
packet already provides all necessary information to proceed.

Some CHDR-specific functions can be found in: uhd::transport::vrt::chdr.

The form of a CHDR packet is the following:

Address (Bytes) | Length (Bytes) | Payload
----------------|----------------|----------------------------
0               | 8              | Compressed Header (CHDR)
8               | 8              | Fractional Time (Optional!)
8/16            | -              | Data

If there is no timestamp present, the data starts at address 8, otherwise, it starts at 16.

The 64 Bits in the compressed header have the following meaning:

Bits   | Meaning
-------|--------------------------------------------------
63:62  | Packet Type
61     | Has fractional time stamp (1: Yes)
60     | End-of-burst or error flag
59:48  | 12-bit sequence number
47:32  | Total packet length in Bytes
31:0   | Stream ID (SID)


The packet type is determined mainly by the first two bits, although
the EOB or error flag are also taken into consideration:

Bit 63 | Bit 62 | Bit 60 | Packet Type
-------|--------|--------|--------------
0      | 0      | 0      | Data
0      | 0      | 1      | Data (End-of-burst)
0      | 1      | 0      | Flow Control
1      | 0      | 0      | Command Packet
1      | 1      | 0      | Command Response
1      | 1      | 1      | Command Response (Error)

\section vrt_tools Tools

For CHDR, we provide a Wireshark dissector under tools/chdr_dissector. It can be used
for Ethernet links as well as USB (e.g., for the B210).

\section vrt_code Code

Relevant code sections for the radio transport layer are:
* uhd::transport::vrt - Namespace for radio transport protocol related functions and definitions
* uhd::transport::vrt::chdr - Sub-namespace specifically for CHDR
* uhd::sid_t - Datatype to represent SIDs

*/
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