Stream transport layers

Exchange of the data in streams is common abstraction that is provided by most of the transport layers (such as TCP/IP). The important operation to support streams in SHV RPC is to split stream to distinct messages. For this purpose there are two different protocols defined; Block and Serial protocols.

Block

The communication on bidirectional stream where delivery is ensured and checked. The transport layer establishes and maintains point-to-point connection on its own.

Message is transmitted in stream as one complete segment where message length is sent before data. The receiving side first reads size and thus knows how much data it should expect to receive. Message length is encoded as Chainpack unsigned integer.

+--------+------+
| length | data |
+--------+------+

The transport error is detected if there is no byte received from other side for more than 5 seconds during the message transfer.

Transport errors are handled by an immediate disconnect. It is expected that connecting side is able to reestablish connection.

The primary transport layer used with this is TCP/IP, but this also applies to Unix sockets or pair of pipes.

Serial

This is communication over data stream with possible on the line errors (such as data corruption).

The message data is encapsulated in start and stop byte and on link layers without error checking it is also verified with CRC32. The dedicated bytes for this are escaped in the message data.

+-----+------+-----------+---------+
| STX | data | ETX / ATX | *CRC32* |
+-----+------+-----------+---------+

• STX start of message 0xA2
• ETX end of the message 0xA3
• ATX abort the message 0xA4
• ESC escape 0xAA
  • STX in data will be coded as ESC 0x02
  • ETX in data will be coded as ESC 0x03
  • ATX in data will be coded as ESC 0x04
  • ESC in data will be coded as ESC 0x0A
• data - escaped message data
• CRC32 - escaped BigEndian CRC32 of data (same as used in IEEE 802.3 and known as plain CRC-32) only on channels that do not provide data corruption prevention on its own and only after ETX (not after ATX).

The transport error is detected if there is no byte received from other side for more that 5 seconds during the message transfer or when STX or ATX is received before EXT or if CRC32 do not match received data (on channels with possible corruption such as RS232 and not TCP/IP).

Transport errors do not have to be handled explicitly because any subsequent message can still be consistent. The invalid message should be just dropped.

The primary transport layer is RS232 with hardware flow control, but usage with other streams, such as TCP/IP or Unix domain named socket, is also possible.

In some cases the restart of the connection might not be available. That is for example when application just doesn't have rights for it or even when such restart would not be propagated to the target, for what ever reason. To solve this the empty message is reserved (that is message STX ETX 0x0). Once client receives a valid empty message then it must drop any state it keeps for it.