SL-TCP

ServiceLink Transmission Control Protocol (SL-TCP) is the connection-oriented, stateful transport protocol of the NSLP stack. It is designed specifically for mission-critical payloads where guaranteed, strictly ordered delivery is strictly required (such as dispatching system configuration changes, triggering physical actuators, or sending state-machine commands).

Protocol Identification

When encapsulated within a Layer 2 Network packet, SL-TCP payloads are identified by the Protocol ID 0x01.

Because SL-TCP must ensure absolute data certainty over potentially unreliable mediums, it trades a small amount of bandwidth and latency for explicit delivery tracking and automated retries.

Delivery Mechanics (Stop-and-Wait)

Standard TCP implementations rely on complex "sliding windows" and massive packet buffers that quickly exhaust the RAM of constrained microcontrollers. To maintain a lightweight footprint, SL-TCP implements a highly reliable, synchronous Stop-and-Wait ARQ (Automatic Repeat reQuest) mechanism.

It guarantees delivery through a strict three-step cycle:

1. Transmit and Block: The sender creates an SL-TCP frame containing a specific Sequence Number, transmits it, and immediately blocks any further transmissions for that specific stream. A localized timeout timer is started.
2. Acknowledge: Upon successfully receiving and validating the frame, the receiving device immediately generates a reply frame. Crucially, the receiver places the sender's original sequence number into the reply's Acknowledgement Number field.
3. Resolve:
   • Success: If the sender receives a frame with a matching ACK before its timer expires, the transmission is marked as successful. The block is lifted, and the sender can transmit the next payload.
   • Retry: If the timeout expires before a valid ACK arrives (indicating a dropped packet at Layer 1 or Layer 2), the sender assumes the data was lost and automatically retransmits the exact same frame.

This primitive but bulletproof loop guarantees that the Application Layer (L4) processes state-changes exactly once, and in the exact sequence they were dispatched.

SL-TCP Frame Structure

General structure

0								1
0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15
Source Port								Destination Port
Sequence Number								Acknowledgement Number
Size (Bytes 0 - 1)
Data (Bytes 0 - 63744)

Fields

• Source Port (8 bits): The port identifier of the sending application process.
• Destination Port (8 bits): The port identifier of the target process on the receiving device.
• Sequence Number (8 bits): A rotating ID used to uniquely identify the current outbound payload.
  • Standard Range: The sequence number always starts at 1 and increments with each new packet. Once it reaches 255, it wraps around back to 1.
  • Pure ACKs: A sequence number of 0 is strictly reserved to designate an acknowledgement packet.
    note

    A response packet acting solely as an acknowledgement will have a Sequence Number of 0, its Acknowledgement Number set to the ID of the received frame, and a Size of 0 (it must not contain any data payload).

• Acknowledgement Number (8 bits): Used by the receiver to confirm receipt of a frame, allowing the sender to clear its block.
  • It must perfectly match the Sequence Number of the frame being acknowledged.
  • The acknowledgement value 0 is explicitly unused, as valid standard sequence numbers only range from 1 to 255.
• Size (16 bits): The exact size of the payload being transmitted.
• Data (up to 63,744 bytes): The encapsulated Layer 4 payload (e.g., custom user structs).
  info

  The maximum payload size of 63,744 bytes ensures the entire frame remains within acceptable application limits while accounting for the mandatory 6-byte SL-TCP header.