Web hosting providers - Maximum throughput Used when the volume of data
Maximum throughput Used when the volume of data transmitted in any period of time is important. There are many types of network applications for which latency is not particularly important but the network throughput is; for example, bulk-file transfers. A network provider might choose to route datagrams with this type of service set via high-latency, high-bandwidth routes, such as satellite connections. Maximum reliability Used when it is important that you have some certainty that the data will arrive at the destination without retransmission being required. The IP protocol may be carried over any number of underlying transmission mediums. While SLIP and PPP are adequate datalink protocols, they are not as reliable as carrying IP over some other network, such as an X.25 network. A network provider might make an alternate network available, offering high reliability, to carry IP that would be used if this type of service is selected. Minimum cost Used when it is important to minimize the cost of data transmission. Leasing bandwidth on a satellite for a transpacific crossing is generally less costly than leasing space on a fiber-optical cable over the same distance, so network providers may choose to provide both and charge differently depending on which you use. In this scenario, your “minimum cost” type of service bit may cause your datagrams to be routed via the lower-cost satellite route. Setting the TOS Bits Using ipfwadm or ipchains The ipfwadm and ipchains commands deal with the TOS bits in much the same manner. In both cases you specify a rule that matches the datagrams with particular TOS bits set, and use the -t argument to specify the change you wish to make. The changes are specified using two-bit masks. The first of these bit masks is logically ANDed with the IP options field of the datagram and the second is logically eXclusive-ORd with it. If this sounds complicated, we’ll give you the recipes required to enable each of the types of service in a moment. The bit masks are specified using eight-bit hexadecimal values. Both ipfwadm and ipchains use the same argument syntax: -t andmask xormask Fortunately the same mask arguments can be used each time you wish to set a particular type of service, to save you having to work them out. They are presented with some suggested uses in Table 9.3. Table 9.3: Suggested Uses for TOS Bitmasks TOS ANDmask XORmask Suggested Use Minimum Delay 0×01 0×10 ftp, telnet, ssh Maximum Throughput 0×01 0×08 ftp-data, www Maximum Reliability 0×01 0×04 snmp, dns Minimum Cost 0×01 0×02 nntp, smtp Setting the TOS Bits Using iptables The iptables tool allows you to specify rules that capture only datagrams with TOS bits matching some predetermined value using the -m tos option, and for setting the TOS bits of IP datagrams matching a rule using the -j TOS target. You may set TOS bits only on the FORWARD and OUTPUT chains. The matching and the setting occur quite independently. You can configure all sort of interesting rules. For example, you can configure a rule that discads all datagrams with certain TOS bit combinations, or a rule that sets the TOS bits of datagrams only from certain hosts. Most often you will use rules that contain both matching and setting to perform TOS bit translations, just as you could for ipfwadmor ipchains.