A serial line delivers a stream of bytes. SLIP gives the receiver a way to recognize the end of an IP packet and preserve data bytes that happen to resemble a delimiter. A tiny example makes that job visible.
Follow a delimiter through the stream
RFC 1055 documents SLIP's framing and escape conventions. In hexadecimal, C0 is END and DB is ESC. An END value within packet data becomes DB DC; an ESC value within packet data becomes DB DD.
- The first
C0supplies an initial boundary. Sending a leading END can clear accumulated noise before the packet. - The ordinary byte
45passes through unchanged. - The receiver translates
DB DCback into the data byteC0. - It translates
DB DDback intoDB, then retains01. - The final
C0ends the packet; the recovered data is45 C0 DB 01.
The extra bytes come from two escaped values and two boundaries. A different packet can have different expansion. This example uses the leading-END convention described in the RFC.
Give each part of the system its own job
| Question | Where to look |
|---|---|
| Where does the packet end? | SLIP delimiters and byte escaping. |
| Which serial settings should be used? | The endpoints' configured speed, data format, flow control, and physical interface. |
| Which address and route should the host use? | Configuration outside basic SLIP framing. |
| Who may connect, and who may read the traffic? | Access restrictions and suitable security mechanisms outside SLIP. |
| How are transmission errors detected? | The surrounding protocols and system. Basic SLIP adds no frame checksum. |
PPP adds a negotiated link framework and a protocol field, with network configuration and optional authentication. Its architecture is defined in RFC 1661; HDLC-like serial framing and its frame check sequence are specified separately in RFC 1662. Keep the link protocol and the particular framing arrangement distinct when comparing implementations.
Compressed SLIP, or CSLIP, uses a separate TCP/IP header-compression technique. RFC 1144 explains how repeated header information can be reduced on slow links. Compression addresses overhead; packet boundaries, peer configuration, and access control remain separate concerns.
Use the example before connecting real equipment
For learning, begin with the written byte sequence or a local encoder/decoder. A successful round trip must recover exactly four original bytes. Then test data containing no special values, repeated END values, and repeated ESC values. Record the expected encoded length as well as the decoded result.
For retrocomputing or an inherited device, first identify its documented serial electrical interface, cable wiring, settings, and supported SLIP variant. Keep the experiment on a controlled network segment, configure the intended addresses and routes explicitly, and retain a local recovery method. A compatible plug does not establish electrical or protocol compatibility.
SLIP is a useful small lesson in protocol design: even a minimal packet carrier needs an unambiguous framing rule. For an operational connection, start from the equipment's supported networking options and the required security and management functions.