IBM SC41-5420-04 manual Multiple Routes, TOS Example

The following table shows which type of services your server uses for some of the TCP/IP applications:

Table 6. AS/400 TCP/IP applications and Type of Services

Protocol or Application	Type of Service Used
TELNET	Normal
FTP (control connection)	Minimize delay
FTP (data connection)	Maximize throughput
SMTP (command phase)	Minimize delay
SMTP (data phase)	Maximize throughput
POP (all phases)	Maximize throughput
SNMP	Maximize reliability

Thus, TOS is a suggestion, not a demand, to the interface (if more than one is present in the system) and to the routing algorithms. If a TCP/IP subsystem knows more than one interface and more than one possible route to a given destination, it uses the TOS to select one with characteristics closest to that desired.

TOS Example

For example, suppose the system can select between a low-capacity nonswitched line or a high-bandwidth (but high delay) satellite connection:

vDatagrams carrying keystrokes from a user to a remote computer could have the type of service set to *MINDELAY, requesting that they be delivered as quickly as possible.

vDatagrams carrying a bulk file transfer could have the type of service set to *MAXTHRPUT, requesting that they travel across the high-capacity satellite path.

It is up to the network administrator to define TOS values when defining interfaces and routes in the TCP/IP configuration. Based on the administrator’s knowledge of the hardware technologies available on systems and networks used, TOS values for the routes must also be defined according to the interface’s TOS value. This means that if a *MINDELAY value is defined in the interface definition, at least one route definition must have the *MINDELAY TOS value defined.

Note: A TCP/IP network does not guarantee the TOS requested. However, datagram transmission is never denied.

Multiple Routes

You can have multiple routes in your routing table (by using the ADDTCPRTE command). You can have more than one route for the same destination Internet address with the same type of service or a different type of service. If you have multiple routes with the same types of service, they are used in the order specified. If a particular next hop router is not available, the subsequent specified next hop router is used. This continues until an entry that is active is found or the list of next hop values is exhausted. If you have multiple routes with different TOS, the one with the TOS equal to the one requested by applications with TOS octet in IP datagram is used. If no match is found in any specified routes, the route with the closest TOS or *NORMAL TOS is used.

You can have *DFTROUTE, and specific route destination addresses. Default routes are used only when data is sent to a remote destination system that does not have

Chapter 2. TCP/IP: Operation, Management, and Advanced Topics 61