TLV is a very simple communication protocol that can be used to send messages around a complex, distributed system.
The TLV protocol provides only message formatting by definition. How the application handles said messages is up to the application, that is NO assumptions or gaurauntees about losing message, parts of messages, ordering or retransmission.
If any of these more advanced capabilities are to be provided, they must be done so at the application layer (so to speak).
Generic Underlying Transports
The underlying physical and mac layers will have a huge effect on the possibility of packet loss, ordering, delay and possible bandwidth. Each of the communicaiton mediums we use, we will post a comparison of characteristics, bandwidth, delay and reliability amoung other things.
TLV and TLString
Type, Length Vector (Value) is as old as the hills, but still just as useful! Type length Vector is byte oriented, with three types of messages: compact (1 byte), short (2 bytes) and full (up to 0xffff size).
+--------+--------+--------+--------+
| Type | Length | Value ...
+--------+--------+--------+--------+
The first byte is the type of message, the second byte is the length of the message, inclusive, that is the 2 bytes of the Tyte and Length field are INCLUDED in the Length of the message.
Compact Messages
The nature of many types of communication are just quick short messages that only require a single byte, which not only determines the type of message, it also supplies no payload beyond the type.
In this case, TLV supports the message type compact or 1-byte.
Short Messages
In a similar vein, some types my contain only a single value, for example a sensor value between 0 & 0xff can be advertised with only two bytes.
Normally, a two byte message: 1-byte for type and 1 byte for the length would require a 3rd byte for the value. Hence, that makes the two byte message unused.
To that end, we provide a 2nd small optimization: Short messages support the message type and a message value between 0x0 and 0xff in the length field, thus reducing the Short Message from 3 bytes to 2 bytes.
Distinguishing Compact, Short and Normal Packets
The packet types can be identified by the first two bytes of the type field. Compact packets always have the first two bits of the type set:
Compact Packets
+--------+
|11xxxxxx|
+--------+
Compact packets are a single byte in the range of 0xC0 to 0xFF, depending on how the remaining 2^6 bits are set.
Short Packets
+--------+--------+
|10xxxxxx| Value |
+--------+--------+
Reserved Packets
+--------+--------+
|01xxxxxx| Value |
+--------+--------+
The reserved range can be associated with the Short or Regular packet
types is one type is getting used up quicker than another.
Short packets are two bytes, with the type value in the range of 0x80 to 0xBF. Again, short packets have (2^6 = 32) independent types.
Regular Packets
+--------+--------+--------+--------+
|00 Type | Length | Value ...
+--------+--------+--------+--------+
If desired, the compact and short packets can be turned off, altogether allocating the entire 2^8 == 256 types to be regular messages.