Technical data

Configuring Serial Lines
3.1 Key Concepts
3.1.2 Assigning an IP Address to Your PPP or SLIP Interface
Every network interface must have its own unique IP address. Interfaces cannot
share IP addresses.
If you configure PPP interfaces for multiple remote hosts, the remote hosts can
obtain their individual IP addresses from your host when they connect. Similarly,
you can configure a PPP interface on your system without knowing your own IP
address and obtain it when you connect to a remote system.
Before establishing SLIP communication with a remote host, however, you must
obtain the IP address for the host’s serial interface and assign IP addresses for
each interface you configure on the local host.
When using SLIP, consider placing each serial line in a separate subnetwork. You
accomplish this by assigning the same subnet mask for the interfaces at either
end of the link.
If you need to use an address in the same subnetwork as your site LAN, use the
proxy Address Resolution Protocol (ARP) feature (see Section 3.3.4).
3.1.3 Serial Line Internet Protocol
SLIP sends a datagram across the serial line as a series of bytes. It uses the
following characters to determine when a series of bytes should be grouped
together:
Character Function Hex Value Decimal Values
END Marks the end of the datagram.
When the receiving SLIP
encounters the END character,
it knows that it has a complete
datagram.
C0 192
ESC Indicates the end of the SLIP
control characters.
DB 219
The SLIP starts by sending an END character. If END is encountered within
the datagram as data, the SLIP inserts an escape character, sending the two-
character sequence DB DC instead. If the ESC character appears within the
datagram as data, it is replaced with the two-character sequence DB DD. The
datagram ends with the END character after the last byte in the packet is
transmitted.
There is neither a standard SLIP specification nor a defined maximum packet
size for the SLIP. The TCP/IP Services implementation of SLIP accepts 1006-byte
datagrams and does not send more than 1006 bytes in a datagram.
Compressed SLIP provides header compression that is beneficial for small packets
and low-speed serial links. Header compression improves packet throughput. You
can enable the CSLIP by means of the /COMPRESS qualifier when you enter a
SET INTERFACE command. See Table 3–3 for more information.
3–2 Configuring Serial Lines