What Is Serial Line Internet Protocol



The Internet Protocol is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet

The Internet Protocol (IP) is a protocol or set of rules, for routing and addressing packets of data so that they can travel across networks and arrive at the correct destination. Data traversing the Internet is divided into smaller pieces, called packets. IP information is attached to each packet, and this information helps ruters to send packets to the right place. Every device or domain that connects to the Internet is assigned an IP Address, and as packets are directed to the IP address attached to them, data arrives where it is needed.

Once the packets arrive at their destination, they are handled differently depending on which transport protocol is used in combination with IP. The most common transport protocols are TCP and UDP.


The Serial Line Internet Protocol’ is an encapsulation of the Internet Protocol designed to work over serial ports and router connections. It is documented in RFC 1055.

Serial Line Internet Protocol (SLIP) is the protocol which TCP/IP uses when operating through a serial connection.

It is commonly used on dedicated serial links and dial-up connections that operate at speeds between 1200bps and 19.2Kbps or higher.

Note: To use baud rates higher than 38400, specify a baud rate of 50 in the /etc/uucp/Devices file for the desired tty, then change the SMIT configuration for that tty to reflect the actual baud rate desired.

For example. to run the cu command on tty0 with a baud rate of 115200, use the following procedure:

  1. Ensure the hardware supports the baud rate.
  2. Edit /etc/uucp/Devices to include the following line:Direct tty0 - 50 direct
  3. Enter the smit chtty fast path.
  4. Select tty0.
  5. Change the baud rate to 115200.
  6. Exit SMIT.


Serial Line Internet Protocol

  • BGP.
  • DHCP(v6)
  • DNS.
  • FTP.
  • HTTP.
  • HTTPS.
  • IMAP.
  • LDAP.


Configuring the Serial Line Internet Protocol (SLIP)

SCO SLIP is a STREAMS-based computer networking facility that provides for the transmission and reception of IP packets over serial lines. SLIP allows the use of TCP/IP networking applications such as rlogin and telnet over serial lines. SCO SLIP is an implementation of the Serial Line Internet Protocol (SLIP) as defined in RFC 1055 and RFC 1144.

SLIP can be used to connect one host to another via a single, physical serial line connection between serial ports or over longer distances using a modem at each end of a telephone line. A computer that is running SLIP over one or more serial lines and that is also connected to a computer network (such as an Ethernet) can serve as a communication gateway between computers on a network and computers at the far end of each serial line.

SLIP is an old protocol with a limited number of features. If you need any of the following features, you should use ppp rather than SLIP:

  • support for multiple physical conections within a link (multilink)
  • CHAP or PAP authentication
  • packet filtering
  • error detection
  • negotiation of IP addresses
  • bringup on demand of links
  • automatic reacquisition of links
  • timing out of inactive links
  • extensibility to network protocols other than IP

Configuring SLIP links

To configure a SLIP link using the Network Configuration Manager:

  1. Select View -> WAN and then select Software -> Configure SLIP.
  2. When presented with a list of currently configured SLIP link definitions, click on Add to define a new link and choose the desired type of link configuration:
    • outgoing
    • incoming When you have completed entering details for the link, click on OK. If you configure an incoming link, you will then also be prompted to enter the SLIP login account’s user ID and password.To modify the configuration of a link, highlight it in the list and click on Modify. Click on OK when you have finished changing the details of the link’s configuration.To delete a link configuration, highlight it in the list and click on Delete. Click on OK when asked to confirm the deletion.NOTE: If you modify or delete a link configuration and the link is currently established, the changes you have made will only take effect when you next reboot your system.
  3. Click on Advanced Options to configure advanced features such as header compression.
  4. If you need to configure a Systems 4 bnu file entry for an outgoing link, select Call Services -> Outgoing. See `The Dialup systems manager for more information.
  5. If you need to configure a modem to be used for an outgoing or an incoming link, select Hardware -> Add WAN device. For incoming links, configure the serial port to accept incoming connections and turn on logging in and auto-answer.

Configuring an outgoing link

An outgoing link can be configured to be connected over a dedicated serial line, or to use the facilities offered by UUCP to dial up a remote system over a modem and telephone line.

If configured to connect to a system in the UUCP Systems 4bnu file, an outgoing SLIP link allows the physical equipment, such as a modem and telephone line, to be shared with other SLIP connections or other WAN networking services, such as PPP and UUCP. The remote end of the connection must configure an incoming link. It must accept the login and password information forwarded by UUCP to establish the connection.

Provide the following information to configure an outgoing link:
UUCP system nameEnter the name of the entry for the remote system in the Systems file if the link will use this to establish UUCP connectivity.

Do not enter a name in this field if you want the system to establish a SLIP link over a dedicated serial line.
Tty line for SLIP
Baud rate for ttyEnter values in these fields if you want the system to establish an outgoing SLIP link over a dedicated serial line.

Do not enter values in these fields if the SLIP link will be connected using entries in the UUCP Systems file.
Local host nameThe name by which the local SLIP interface is known.
Host IP addressThe address by which the local SLIP interface is known. The remote system must agree to use the specified address or the local system will refuse to establish the link.

If both a local name and an IP address are entered, the IP address takes precedence.
Destination nameThe name by which the host at the remote end of the SLIP interface is known.
Destination IP addressThe IP address for the host at the remote end of the SLIP interface. The remote system must agree to use the specified address or the local system will refuse to establish the link.

If both a destination name and an IP address are entered, the IP address takes precedence.
GatewayIf this network interface is not the first on your system, and you want to be able to forward packets between the different interfaces, select Yes.Link configuration information is stored in the file /etc/inet/slip.start referenced by an entry in /etc/inet/config. When TCP/IP is started, the slattach entries in the configuration file are invoked to create outgoing SLIP links.

Configuring an incoming link

An incoming SLIP link permits the local system to accept an incoming SLIP connection request from a remote host. This type of link requires a login account in the /etc/passwd file on the local host. A remote host wishing to initiate a SLIP link with the local host must use an outgoing type of link, and must specify this account name and its associated password in the chat script in the entry for the local system in the remote system’s UUCP Systems 4bnu file.

When you use the Network Configuration Manager to configure this type of link, it creates a SLIP login account which uses a special shell script instead of a standard shell. The shell script must execute an slattach command to establish the SLIP link. You must configure the remote side to use an outgoing link type for its end of the link.

Provide the following information to configure an incoming link:
Login name for SLIPThe name of the account used to service SLIP sessions.
Login shell scriptThe name of the script containing the slattach command that is used to establish the link.You must create the login shell script for the user that you name. The script should be owned by the SLIP user and it must be executable by them. It should contain a line to start slattach similar to the following:

   /usr/sbin/slattach /dev/term/00 38400 &

In this example, SLIP accepts incoming requests from the system via the interface on a modem connected to /dev/term/00. The baud rate in this example is 38400. For more information, see slattach 1mtcp.

Advanced SLIP configuration

Advanced configuration of SLIP may be performed using the Advanced Options available from the Network Configuration Manager.
Proxy ARPIf set to On, add a proxy entry for the IP address of the remote end of the SLIP link in the ARP table. This will allow other hosts on the local network to contact the remote system if its IP address belongs to the local subnet.
TCP/IP header compressionIf set to On, TCP/IP header compression is enabled and the interface sends compressed packets. The remote end of the link must be willing to accept such packets.
Auto detect header compressionIf set to On, the interface will attempt to detect compressed packets. If it does, it will respond with compressed packets.

Auto-detection has no effect if TCP/IP header compression is already configured on.
Hardware flow controlIf set to On, use hardware flow control on the serial line.
ICMP packet suppressionIf set to On, stop ICMP packets (such as those used for redirects, pingtraceroute, and router discovery) from passing through the interface.
Maximum transmission unitEnter the size of the maximum transmission unit (MTU) in bytes.

The default MTU value is 296. Increasing this value may increase throughput as packets can contain more data. However, it may also decrease throughput if packets are routed through systems with lower MTU values, as each packet is split (fragmented) into smaller packets for transmission. You can experiment with this value to obtain the most desirable results.

NOTE: The suggested value for the MTU is 40 plus some power of 2. For example, the default value, 296, is 40 plus 2^8. Other possible values include 552, 1064, and 2088.

DebuggingIf set to On, enable logging of SLIP debug messages about the state of the link to the system console.

Deleting all SLIP links

To delete all SLIP link definitions using the Network Configuration Manager:

  1. Select View rarr“> WAN and then select Software rarr“> Delete SLIP.
  2. When asked if you want to delete all SLIP link definitions, click on OK.

Administering SLIP

The slattarch Mtcp command establishes a connection over a pre-defined SLIP link. Each time slattach is executed, a new slattach process is created to control the additional SLIP link in use. slattach creates a network interface based on the local and remote IP addresses that are passed to it as parameters.

Parameters to the slattach command define:

  • the serial communication path for a link by specifying either a tty device or a UUCP site name (the latter requires the -d option)
  • an IP address for the local side of the communication path and an IP address for the remote side of the communication path
  • other IP and serial line attributes for the link

Multiple applications (or multiple instances of the same application) on a local host can communicate with the same remote host using the same SLIP link. Use of the same SLIP link requires only that the applications use the same IP address to specify the remote host. By using different IP addresses to specify the same remote host, two applications can also use individual SLIP links to the same host, provided a separate SLIP link exists for each remote IP address.

Once a SLIP link is established, it remains active until:

  • you mark the associated SLIP interface down using iftconfigMtcp For example, the command to mark the first SLIP interface down would be:

    ifconfig sl0 downCAUTION: Marking an interface down using ifconfig does not remove the link. Therefore, marking down the interface for an outgoing link that uses a modem and telephone line does not hang up the line. The costs of the line usage continue until the link is removed.
  • you kill the associated slattach process to remove the SLIP interface.

Once a link has been removed, you may need to remove the associated route from the routing table using the route Mftcp command:

route delete destination_address gateway_address

This should only be necessary if the system is not running a routing daemon such as routed Mftcp.

Advantages of SLIP

  • It has a very small overhead. So, it is suitable for usage in microcontrollers.
  • It reuses the existing dial-up connections and telephone lines.
  • It supports the most widely used protocol, Internet Protocol (IP). So, there is ease of deployment.

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