NETGEAR DGFV338 - ProSafe Wireless ADSL Modem VPN Firewall... User manual

Configuring Type-of-Service (ToS) features on the DGFV338
Scope:
This document details what the packet priority (ToS) feature of the DGFV338 does.
These results were tested on a DGFV338 running firmware 2.1.1-22.
Contents:
1. What is ToS?
2. Setting up ToS on the DGFV338.
3. What changes do the ToS settings make to the packet headers?

What is Type-of-Service (ToS)?
ToS is an IP QoS (quality-of-service) mechanism, which uses certain bits in the IP header to indicate to
network devices how packets should be treated. It is up to each network device that a packet passes
through to know how to respond to the ToS bits, and if a device is not ToS-capable then the bits are
simply ignored and all packets are treated in the same way.
The DGFV338 complies with RFC1349, which defines 5 types of service:
1. Normal –Service
2. Minimize-Cost
3. Maximize-Reliability
4. Maximize-Throughput
5. Minimize-Delay
Earlier versions of ToS only defined 4 types of service, they did not include “Minimize-Cost”. The
“Minimize-Cost” ToS attempts to minimize the financial cost of passing traffic, not to be confused with
normal IP routing protocols which try to route packets based on minimum “Hops” or ”Metrics”, often
referred to as “Costs”.
Most QoS mechanisms, including ToS, essentially consist of a number of queues of different priority that
feed packets out through the physical ports of a networking device. The network device looks at the ToS
bits, and places the packet in the most appropriate queue.

Setting up ToS on the DGFV338
There are 2 places in the DGFV338 where ToS can be set:
1. When adding a Custom Service:
2. When adding an Outbound Firewall Rule:

In both cases, the same 5 ToS options are available, with “Normal-Service” as the default:
1. Normal –Service
2. Minimize-Cost
3. Maximize-Reliability
4. Maximize-Throughput
5. Minimize-Delay
So which setting takes precedence?
Q. If you create a custom service, and use that custom service when creating an outbound rule, which of
the 2 ToS settings will take precedence and determine how the packet is prioritised?
A. If the Custom Service ToS setting is “Normal-Service”, then the Outbound Rule ToS setting takes
precedence.
If the Custom Service ToS is NOT “Normal Service”, then the Custom Service ToS setting takes
precedence.
After the Outbound Rule is created, the ToS setting shown in the Outbound Services “Priority” column is
the ToS setting which has precedence, which may or may not be the same as the ToS setting that was
chosen when the Outbound Rule was being created:

What changes do the ToS settings make to the packet headers?
In the IPv4 packet header, the second byte (bits 9-16) is designated “Type Of Service” (ToS).
The use of this byte has changed as different QoS standards have been established. I shall refer to these
different standards as “Original ToS”, “RFC1349 ToS”, and “DSCP”:
Original ToS:
Precedence
Precedence
Precedence
Minimize
Delay
Maximize
Throughput
Maximize
Reliability
Unused
Unused
RFC 1349 ToS:
Precedence
Precedence
Precedence
Minimize
Delay
Maximize
Throughput
Maximize
Reliability
Minimize
Cost
Unused
Differentiated Services Code Point (DSCP):
DSCP
DSCP
DSCP
DSCP
DSCP
DSCP
ECT
Explicit
Congestion
Notification
Capable
Transport
ECN
Explicit
Congestion
Notification
The DGFV338 complies with RFC1349, and it’s ToS settings cause the following values to be set in the
ToS byte of packets exiting the WAN port:
Precedence
Precedence
Precedence
Minimize
Delay
Maximize
Throughput
Maximize
Reliability
Minimize
Cost
Unused
Decimal
Hex
DSCP
Normal
Service
0
0
0
0
0
0
0
0
0
0
0
Minimize
Delay
0
0
0
1
0
0
0
0
16
Ox10
4
Maximize
Throughput
0
0
0
0
1
0
0
0
8
0x08
2
Maximize
Reliability
0
0
0
0
0
1
0
0
4
0x04
1
Minimize
Cost
0
0
0
0
0
0
1
0
2
0x02
0