TCP/IP
Concepts
2-12
2.7.6 Serial Line Internet Protocol (SLIP)
The SLIP protocol allows hosts to communicate via TCP/IP over a serial line instead of Ethernet. Due to
the point-to-point nature of serial lines, only two hosts can communicate over a single line. SLIP is most
frequently used for dialup modem lines, where a SLIP-equipped TCP/IP host can dial into the ETS, begin
a SLIP session, and thus gain access to the network attached to the ETS. The remote host can start sessions
to the ETS or any other host on the network, subject to security and other login restrictions. If multiple TCP/
IP hosts are connected to the ETS via SLIP connections, each SLIP host can also access any of the other
SLIP hosts, with the ETS routing packets between the serial lines. An example SLIP setup is shown below.
Figure 2-4: SLIP Example
Note:
See Set/Define SLIP on page 12-95 for more details.
In the example above, assume all hosts have a subnet mask of 255.255.255.0. All four hosts will appear to
be on the same network segment, and all four will be able to start TCP/IP connections with each other.
The ETS in this case “routes” packets between the serial lines and the Ethernet without help from the hosts
involved. Note that since all the hosts are on the same network, the ETS is not routing in the true sense of
the word, and any “real” IP routers on the network will not be affected by the SLIP sessions. The ETS is
only directing network traffic to the proper serial port.
The ETS checks each packet it receives to see if it is destined for the ETS, or if it should send it to one of
the SLIP hosts or out to the Ethernet. If an Ethernet packet is sent to the ETS and there is no SLIP connection
for it, or if a packet is received from a SLIP port but is not from the host that is supposed to be there, the
packet is discarded. This prevents other hosts from connecting to an active SLIP line and accessing the
ETS’s Ethernet. The ETS also provides ARP replies for its SLIP hosts so that hosts on the Ethernet can
resolve their IP addresses.
There are two restrictions on ETS SLIP support. First, there can be only one host at the end of the serial line.
The ETS will only forward packets to and from the host registered on the SLIP line, it will not route packets
to and from a SLIP network (multiple hosts). Second, all of the hosts connected via serial lines must be on
the same network and subnet as the server.
ETS
Server
Ethernet Backbone
Modems
Host 192.73.100.11
Host 192.73.100.23
Host 192.73.100.78
Host 192.73.100.172
Serial Lines
AUI Cable
