Technical data

you can configure a PPP interface on your system without knowing your own IP
address, and you can obtain the IP address when you connect to a remote system.
Before you establish 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. For more information about ARP,
refer to the Compaq TCP/IP Services for OpenVMS Management guide.
7.2.2 Serial Line Internet Protocol (SLIP)
SLIP sends a datagram across the serial line as a series of bytes. Table 7–1 shows
how SLIP uses the following characters to determine when a series of bytes should
be grouped together.
Table 7–1: SLIP Characters
Character
Function Hexadecimal Value Decimal Value
END Marks the end of the
datagram. When
the receiving SLIP
encounters the END
character, SLIP
knowsthatithasa
complete datagram.
C0
192
ESC Indicates the end
of the SLIP control
characters.
DB
219
SLIP starts by sending an END character. If END is encountered within the
datagram as data, 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 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 for low-speed serial links. Header compression improves packet throughput.
You can enable CSLIP by using the /COMPRESS qualifier when you enter the
SET INTERFACE command.
7.2.3 Point-to-Point Protocol (PPP)
PPP uses a frame format that includes a protocol field. The protocol field identifies
the protocol (for example, IP, DECnet, or OSI) to be used for communication
between the two hosts. The PPP defines the network frame in a 5-byte header and
3-byte trailer. A PPP frame starts and ends with the control byte 7E hexadecimal
(126 decimal). The address and control bytes are constant. The 2-byte protocol field
indicates the contents of the PPP frame.
7–2 Connectivity Services