TCP/IP Network Interfaces

The TCP/IP Network Interface layer formats IP datagrams at the Network layer into packets that specific network technologies can understand and transmit. A network interface is the network-specific software that communicates with the network-specific device driver and the IP layer in order to provide the IP layer with a consistent interface to all network adapters that may be present.

The IP layer selects the appropriate network interface based on the destination address of the packet to be transmitted. Each network interface has a network address. The Network Interface layer is responsible for adding or removing any link layer protocol header required to deliver a message to its destination. The network adapter device driver controls the network adapter card.

A network interface is usually associated with a network adapter, but there is no requirement for one. For instance, the loopback interface has no network adapter associated with it. A machine must have one network adapter card for each network (not network type) to which it connects. However, a machine requires only one copy of the network interface software for each network adapter it uses. For instance, if a host attaches to two token-ring networks, it must have two network adapter cards. However, only one copy of the token-ring network interface software and one copy of the token-ring device driver is required.

TCP/IP supports types of network interfaces:

• Standard Ethernet Version 2 (en)
• IEEE 802.3 (et)
• Token-ring (tr)
• Serial Line Internet Protocol, or SLIP (sl)
• Loopback (lo)
• FDDI
• Serial Optical (so)
• ATM (at)
• Point-to-Point Protocol (PPP)

The Ethernet, 802.3, and token-ring interfaces are for use with local area networks (LANs). The SLIP interface is for use with serial connections. The loopback interface is used by a host to send messages back to itself. The Serial Optical interface is for use with optical point-to-point networks using the Serial Optical Link device handler. The ATM interface is for use with 100 Mbits/sec and 155 Mbits/sec ATM connections.

Automatic Configuration of Network Interfaces

When a new network adapter is physically installed in the system, the operating system automatically adds the appropriate network interface for that adapter. For example, if you install a token-ring adapter in your system, the operating system will assign it the name tok0 and add a token-ring network interface named tr0. If you install an Ethernet adapter in your system, the operating system will assign it the name ent0 and add both an Ethernet Version 2 and an IEEE 802.3 interface, named en0 and et0 respectively.

In most cases, there is a one-to-one correspondence between adapter names and network interface names. For example, token-ring adapter tok0 corresponds to interface tr0, adapter tok1 corresponds to interface tr1, and so on. Similarly, Ethernet adapter ent0 corresponds to interface en0 (for Ethernet Version 2) and et0 (for IEEE 802.3), and adapter ent1 corresponds to interface en1 (for Ethernet Version 2) and et1 (for IEEE 802.3).

In the case of ATM, according to RFC1577, it is possible for an ATM station to be part of multiple Logical IP Subnetworks. When this is the case, there will be multiple interfaces associated with a device. This will require that an interface be specifically added and a device name be assigned to it.

Note: Under normal circumstances, you will never need to delete or add a network interface manually. However, some problem determination procedures may require you to do so. In this case, use the Web-based System Manager fast path, wsm network, or the SMIT fast path, smit inet, to delete and re-add the appropriate interface.

At each system startup, the operating system automatically configures the network interface software based upon the information in the ODM database. Initially, the network interface is configured with default values. In order to communicate through a given network interface, the Internet address must be set. This is the only attribute that you need to set. All other necessary attributes can use the default values. The default values for each network interface follow.

Ethernet Default Configuration Values

The following is a list of valid Ethernet network adapter attributes along with their default values, which may be changed using the Web-based System Manager fast path wsm network or the Network Interface Selection menu in SMIT.

The following valid Ethernet network device driver attribute is shown along with its default values, which may be changed using the Web-based System Manager fast path wsm network or the Network Interface Drivers menu in SMIT.

802.3 Default Configuration Values

The following is a list of valid 802.3 network adapter attributes along with their default values, which may be changed using the Web-based System Manager fast path wsm network or the Network Interface Selection menu in SMIT.

The following valid 802.3 network device driver attribute is shown along with its default values, which may be changed using the Web-based System Manager fast path wsm network or the Network Interface Drivers menu in SMIT.

Token-Ring Default Configuration Values

The following is a list of valid token-ring network adapter attributes along with their default values, which may be changed using the Web-based System Manager fast path wsm network or the Network Interface Selection menu in SMIT.

The following valid token-ring network device driver attributes are shown along with its default values, which may be changed using the Web-based System Manager fast path wsm network or the Network Interface Drivers menu in SMIT.

Note: When operating through a bridge, the default value of 1500 for the maximum transmission unit (MTU) should be changed to a value that is 8 less than the maximum information field (maximum I-frame) advertised by the bridge in the routing control field. For example, if the maximum I-frame value is 1500 in the routing control field, the MTU size should be set to 1492. This is for token-ring network interfaces only. For more information, refer to "Problems with a Token-Ring/Token-Ring Bridge".

When using the IBM 16/4 PowerPC token-ring adapter (ISA) or the IBM PCMCIA 16/4 token-ring card, the MTU is restricted to 2000.

SLIP Default Configuration Values

The following is a list of valid SLIP network adapter attributes along with their default values as shown under the Web-based System Manager fast path wsm network or the Network Interface Selection menu in SMIT.

The following valid SLIP network device driver attribute is shown along with its default values as displayed under the Web-based System Manager fast path wsm network or the Network Interface Drivers menu in SMIT.

Serial Optical Default Configuration Values

The following is a list of valid Serial Optical network channel converter attributes along with their default values as shown under the Web-based System Manager fast path wsm network or the Network Interface Selection menu in SMIT.

The following valid serial optical network device handler attribute is shown along with its default values as displayed under the Web-based System Manager fast path wsm network or the Network Interface Drivers menu in SMIT.

ATM Default Configuration Values

The following is a list of valid ATM network adapter attributes along with their default values as shown under the Web-based System Manager fast path wsm network or the Network Interface Selection menu in SMIT.

The following valid ATM network device driver attribute is shown along with its default values as displayed under the Web-based System Manager fast path wsm network or the Network Interface Drivers menu in SMIT.

Note: Network Administrators need to exercise caution while changing MTU size from default to some other value. The MTU size needs to be coordinated with other stations on the network.

If PVCs are to be used on an interface, the VPI:VCIs needs to be defined. This is part of the Network Interface Selection Menu. The last option on this menu is PVCs for IP over ATM Network, which helps you list, add, change, or remove PVCs.

Implications of Multiple Network Interfaces on the Same Network

Sometimes network managers feel the need to provide greater availability and performance by adding a second network adapter to a particular machine. For example, they may want to have two token-ring adapters attached to the same physical network. While it is possible to have more than one interface on the same network, in general this is not recommended for two reasons:

1. Having two interfaces on the same network is a violation of TCP/IP architecture.

   In TCP/IP architecture, a host machine with two network adapters is defined as an IP router. Different network adapters must be attached to different physical networks. In the case of token-ring, TCP/IP addresses multiple rings bridged together as a single logical ring (as if it were a single physical ring).

2. Having two interfaces on the same network can cause broadcast storms.

   Whenever an IP host sees traffic for a network whose IP address is different from its own network, it generates an Internet Control Message Protocol (ICMP) packet announcing this conflict. Since every host on the network sees the traffic that is misaddressed, every host generates ICMP packets. If the amount of misaddressed traffic is significant, the ICMP traffic can grow to the point that network performance degrades.

It is possible to avoid the broadcast storms created when multiple interfaces are connected to the same network. However, doing so is still a violation of TCP/IP architecture. The solution is to give the different interfaces different IP addresses on the same network. For example, you might have two token-ring adapters on the same network named tr0 and tr1. You must assign distinct IP addresses and names to tr0 and tr1. (TCP/IP architecture requires that each interface have a unique IP address and name; otherwise, unpredictable behavior will result.) For instance, you might give interface tr0 the IP address 10.10.10.1 and the name laurel.foo.bar.com, and interface tr1 the IP address 10.10.10.2 and the name hardy.foo.bar.com.

Managing Network Interfaces

To manage network interfaces, use the tasks in the following table.

Notes:

1. Changes from a remotely mounted /usr affect only the Information Database (ODM) until the network is rebooted or until the ifconfig command is used to make the changes take affect right away.
2. When using a remotely mounted /usr, be careful not to modify the interface being used, since that is the location of the libraries, commands, and kernel.