System Management Guide: Communications and Networks

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 might 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.

Although not required, a network interface is usually associated with a network adapter. 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 (SLIP)
• Loopback (lo)
• FDDI
• Serial Optical (so)
• ATM (at)
• Point-to-Point Protocol (PPP)
• Virtual IP Address (vi)

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. Point to Point protocol is most often used when connecting to another computer or network via a modem. The Virtual IP Address interface (also called virtual interface) is not associated with any particular network adapter. Multiple instances of a virtual interface can be configured on a host. When virtual interfaces are configured, the address of the first virtual interface becomes the source address unless an application has chosen a different interface. Processes that use a virtual IP address as their source address can send packets through any network interface that provides the best route for that destination. Incoming packets destined for a virtual IP address are delivered to the process regardless of the interface through which they arrive.

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 assigns 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 assigns 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. In this case, multiple interfaces are associated with a device. This requires that an interface be specifically added and a device name be assigned to it.

Note: Under normal circumstances, you do not need to delete or add a network interface manually. However, some problem determination procedures might require you to do so. In this case, use the Web-based System Manager wsm, 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 can be changed using the Web-based System Manager, wsm or the Network Interface Selection menu in SMIT.

The following valid Ethernet network device driver attribute is shown along with its default values, which can be changed using the Web-based System Manager, wsm, 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 can be changed using the Web-based System Manager, wsm, 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 can be changed using the Web-based System Manager, wsm, 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 can be changed using the Web-based System Manager, wsm, 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, wsm, 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, see Problems with a Token-Ring/Token-Ring Bridge.

When using the IBM 16/4 POWER-based token-ring adapter (ISA), 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, wsm, 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, wsm, 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, wsm, 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, wsm, 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, wsm, 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, wsm, 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 performed through the Network Interface Selection Menu. The PVCs for IP over ATM Network option on this menu is used to list, add, change, or remove PVCs.

Implications of Multiple Network Interfaces on the Same 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 by giving each interface a different IP addresses.

Managing Network Interfaces

To manage network interfaces, use the web-based system manager, WSM Network, FastPath (application) or the tasks in the following table.

Notes:

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

Interface-Specific Network Options

TCP/IP interfaces must be specially tuned to achieve good, high-speed network performance (100 Mb or more). This effort is complicated by the fact that multiple network interfaces and a combination of traditional and high-speed TCP/IP interfaces can be used on a single system. Before AIX 4.3.3 (4330-08) and AIX 5.1, AIX provided a single set of system-wide values for the key IP interface network tuning parameters making it impossible to tune a system that has widely differing network adapter interfaces. Beginning with AIX 4.3.3 (4330-08) and AIX 5.1, Interface Specific Network Options (ISNO) allows system administrators to tune each TCP/IP interface individually for best performance.

There are five ISNO parameters for each supported interface: rfc1323, tcp_nodelay, tcp_sendspace, tcp_recvspace, and tcp_mssdflt. When set, the values for these parameters override the system-wide parameters of the same names that had been set with the no command. When ISNO options are not set for a particular interface, system-wide options are used. When options have been set by an application for a particular socket using the setsockopt subroutine, such options override the ISNOs.

The network option use_isno, set with the no command, must have a value of 1 for the ISNOs to take effect. The default value for use_isno is 1.

Some high-speed adapters have ISNO parameters set by default in the ODM database.

Gigabit Ethernet interfaces, when configured to use an MTU of 9000, use the following ISNO values by default:

Gigabit Ethernet interfaces, when configured to use an MTU of 1500, use the following ISNO values by default:

ATM interfaces, when configured to use an MTU of 1500, use the following ISNO values by default:

ATM interfaces, when configured to use an MTU of 65527, use the following ISNO values by default:

ATM interfaces, when configured to use an MTU of 9180, use the following ISNO values by default:

FDDI interfaces, when configured to use an MTU of 4352, use the following ISNO values by default:

The ISNO parameters cannot be displayed or changed using SMIT. They can be set using the chdev command or the ifconfig command. The ifconfig command changes the values only until the next reboot. The chdev command changes the values in the ODM database so they are used on subsequent reboots. The lsattr or ifconfig commands can be used to display the current values.

Example

The following commands can be used first to verify system and interface support and then to set and verify the new values.

1. Verify general system and interface support using the no and lsattr commands.
   • Ensure the use_isno option is enabled using a command similar to the the following:

     $ no -a | grep isno 
               use_isno=1
     

   • Ensure the interface supports the five new ISNOs using the lsattr -El command, as shown in the following:

     $ lsattr -E -l en0 -H 
               attribute   value  description 
               rfc1323                      N/A 
               tcp_nodelay                  N/A 
               tcp_sendspace                N/A 
               tcp_recvspace                N/A 
               tcp_mssdflt                  N/A
     

2. Set the interface specific values, using either the ifconfig or chdev command. The ifconfig command sets values temporarily, which is recommended for testing. The chdev command alters the ODM, so customized values remain valid after reboot.
   • Set the tcp_recvspace and tcp_sendspace to 64K and enable tcp_nodelay by using one of the following:

     $ ifconfig en0 tcp_recvspace 65536 tcp_sendspace 65536 tcp_nodelay 1 
     $ chdev -l en0 -a tcp_recvspace=65536 -a tcp_sendspace=65536 -a tcp_nodelay=1
     

   • Alternatively, assuming the no command reports an rfc1323=1 global value, the root user can turn rfc1323 off for all connections over en0 with the following commands:

     $ ifconfig en0 rfc1323 0 
     $ chdev -l en0 -a rfc1323=0
     

3. Verify the settings using the ifconfig or lsattr command, as shown in the following example:

   $ ifconfig en0 <UP,BROADCAST,NOTRAILERS,RUNNING,SIMPLEX,MULTICAST,GROUPRT,64BIT>
             en0: flags=e080863  
                 inet 9.19.161.100 netmask 0xffffff00 broadcast 9.19.161.255 
                 tcp_sendspace 65536 tcp_recvspace 65536 tcp_nodelay 1 rfc1323 0 
   $ lsattr -El en0 
             rfc1323         0           N/A               True 
             tcp_nodelay     1           N/A               True 
             tcp_sendspace   65536       N/A               True 
             tcp_recvspace   65536       N/A               True 
             tcp_mssdflt                 N/A               True