The Type Of Service (TOS) bits are a set of four-bit flags in
the IP header. When any one of these bit flags is set, routers may handle the
datagram differently than datagrams with no TOS bits set. Each of the four bits
has a different purpose and only one of the TOS bits may be set at any time, so
combinations are not allowed. The bit flags are called Type of Service bits
because they enable the application transmitting the data to tell the network
the type of network service it requires.
The classes of network service available are:
- Minimum delay
Used when the time it takes for a datagram to travel from the source host
to destination host (latency) is most important. A network provider might, for
example, use both optical fiber and satellite network connections. Data carried
across satellite connections has farther to travel and their latency is
generally therefore higher than for terrestrial-based network connections
between the same endpoints. A network provider might choose to ensure that
datagrams with this type of service set are not carried by satellite.
- 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.
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:
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 | 0x01 | 0x10 | ftp, telnet, ssh |
| Maximum Throughput | 0x01 | 0x08 | ftp-data, www |
| Maximum Reliability | 0x01 | 0x04 | snmp, dns |
| Minimum Cost | 0x01 | 0x02 | nntp, smtp |
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
ipfwadm or ipchains.
Rather than the complicated two-mask configuration of
ipfwadm and ipchains,
iptables uses the simpler approach of plainly specifying
what the TOS bits should match, or to what the TOS bits should be set.
Additionally, rather than having to remember and use the hexadecimal value,
you may specify the TOS bits using the more friendly mnemonics listed in
the upcoming table.
The general syntax used to match TOS bits looks like:
-m tos --tos mnemonic [other-args] -j target |
The general syntax used to set TOS bits looks like:
[other-args] -j TOS --set mnemonic |
Remember that these would typically be used together, but they can be
used quite independently if you have a configuration that
requires it.
