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Page 156 / 327 Scroll up to view Page 151 - 155
Chapter 10 Quality of Service (QoS)
VMG3926-B10A User’s Guide
156
IP Precedence
Similar to IEEE 802.1p prioritization at layer-2, you can use IP precedence to prioritize packets in a
layer-3 network. IP precedence uses three bits of the eight-bit ToS (Type of Service) field in the IP
header. There are eight classes of services (ranging from zero to seven) in IP precedence. Zero is
the lowest priority level and seven is the highest.
Automatic Priority Queue Assignment
If you enable QoS on the VMG, the VMG can automatically base on the IEEE 802.1p priority level, IP
precedence and/or packet length to assign priority to traffic which does not match a class.
The following table shows you the internal layer-2 and layer-3 QoS mapping on the VMG. On the
VMG, traffic assigned to higher priority queues gets through faster while traffic in lower index
queues is dropped if the network is congested.
Table 54
Internal Layer2 and Layer3 QoS Mapping
PRIORITY
QUEUE
LAYER 2
LAYER 3
IEEE 802.1P USER
PRIORITY
(ETHERNET
PRIORITY)
TOS (IP
PRECEDENCE)
DSCP
IP PACKET
LENGTH (BYTE)
0
1
0
000000
1
2
2
0
0
000000
>1100
3
3
1
001110
001100
001010
001000
250~1100
4
4
2
010110
010100
010010
010000
5
5
3
011110
011100
011010
011000
<250
6
6
4
100110
100100
100010
100000
5
101110
101000
7
7
6
110000
111000
7
Page 157 / 327
Chapter 10 Quality of Service (QoS)
VMG3926-B10A User’s Guide
157
Token Bucket
The token bucket algorithm uses tokens in a bucket to control when traffic can be transmitted. The
bucket stores tokens, each of which represents one byte. The algorithm allows bursts of up to
b
bytes which is also the bucket size, so the bucket can hold up to
b
tokens. Tokens are generated
and added into the bucket at a constant rate. The following shows how tokens work with packets:
•
A packet can be transmitted if the number of tokens in the bucket is equal to or greater than the
size of the packet (in bytes).
•
After a packet is transmitted, a number of tokens corresponding to the packet size is removed
from the bucket.
•
If there are no tokens in the bucket, the VMG stops transmitting until enough tokens are
generated.
•
If not enough tokens are available, the VMG treats the packet in either one of the following ways:
In traffic shaping:
•
Holds it in the queue until enough tokens are available in the bucket.
In traffic policing:
•
Drops it.
•
Transmits it but adds a DSCP mark. The VMG may drop these marked packets if the network is
overloaded.
Configure the bucket size to be equal to or less than the amount of the bandwidth that the interface
can support. It does not help if you set it to a bucket size over the interface’s capability. The smaller
the bucket size, the lower the data transmission rate and that may cause outgoing packets to be
dropped. A larger transmission rate requires a big bucket size. For example, use a bucket size of 10
kbytes to get the transmission rate up to 10 Mbps.
Single Rate Three Color Marker
The Single Rate Three Color Marker (srTCM, defined in RFC 2697) is a type of traffic policing that
identifies packets by comparing them to one user-defined rate, the Committed Information Rate
(CIR), and two burst sizes: the Committed Burst Size (CBS) and Excess Burst Size (EBS).
The srTCM evaluates incoming packets and marks them with one of three colors which refer to
packet loss priority levels. High packet loss priority level is referred to as red, medium is referred to
as yellow and low is referred to as green.
The srTCM is based on the token bucket filter and has two token buckets (CBS and EBS). Tokens
are generated and added into the bucket at a constant rate, called Committed Information Rate
(CIR). When the first bucket (CBS) is full, new tokens overflow into the second bucket (EBS).
All packets are evaluated against the CBS. If a packet does not exceed the CBS it is marked green.
Otherwise it is evaluated against the EBS. If it is below the EBS then it is marked yellow. If it
exceeds the EBS then it is marked red.
The following shows how tokens work with incoming packets in srTCM:
•
A packet arrives. The packet is marked green and can be transmitted if the number of tokens in
the CBS bucket is equal to or greater than the size of the packet (in bytes).
•
After a packet is transmitted, a number of tokens corresponding to the packet size is removed
from the CBS bucket.
Page 158 / 327
Chapter 10 Quality of Service (QoS)
VMG3926-B10A User’s Guide
158
•
If there are not enough tokens in the CBS bucket, the VMG checks the EBS bucket. The packet is
marked yellow if there are sufficient tokens in the EBS bucket. Otherwise, the packet is marked
red. No tokens are removed if the packet is dropped.
Two Rate Three Color Marker
The Two Rate Three Color Marker (trTCM, defined in RFC 2698) is a type of traffic policing that
identifies packets by comparing them to two user-defined rates: the Committed Information Rate
(CIR) and the Peak Information Rate (PIR). The CIR specifies the average rate at which packets are
admitted to the network. The PIR is greater than or equal to the CIR. CIR and PIR values are based
on the guaranteed and maximum bandwidth respectively as negotiated between a service provider
and client.
The trTCM evaluates incoming packets and marks them with one of three colors which refer to
packet loss priority levels. High packet loss priority level is referred to as red, medium is referred to
as yellow and low is referred to as green.
The trTCM is based on the token bucket filter and has two token buckets (Committed Burst Size
(CBS) and Peak Burst Size (PBS)). Tokens are generated and added into the two buckets at the CIR
and PIR respectively.
All packets are evaluated against the PIR. If a packet exceeds the PIR it is marked red. Otherwise it
is evaluated against the CIR. If it exceeds the CIR then it is marked yellow. Finally, if it is below the
CIR then it is marked green.
The following shows how tokens work with incoming packets in trTCM:
•
A packet arrives. If the number of tokens in the PBS bucket is less than the size of the packet (in
bytes), the packet is marked red and may be dropped regardless of the CBS bucket. No tokens
are removed if the packet is dropped.
•
If the PBS bucket has enough tokens, the VMG checks the CBS bucket. The packet is marked
green and can be transmitted if the number of tokens in the CBS bucket is equal to or greater
than the size of the packet (in bytes). Otherwise, the packet is marked yellow.
Page 159 / 327
VMG3926-B10A User’s Guide
159
C
HAPTER
11
Network Address Translation (NAT)
11.1
Overview
This chapter discusses how to configure NAT on the VMG. NAT (Network Address Translation - NAT,
RFC 1631) is the translation of the IP address of a host in a packet, for example, the source address
of an outgoing packet, used within one network to a different IP address known within another
network.
11.1.1
What You Can Do in this Chapter
•
Use the
Port Forwarding
screen to configure forward incoming service requests to the server(s)
on your local network (
Section 11.2 on page 160
).
•
Use the
Applications
screen to forward incoming service requests to the server(s) on your local
network (
Section 11.3 on page 163
).
•
Use the
Port Triggering
screen to add and configure the VMG’s trigger port settings (
Section
11.4 on page 164
).
•
Use the
DMZ
screen to configure a default server (
Section 11.5 on page 167
).
•
Use the
ALG
screen to enable and disable the NAT and SIP (VoIP) ALG in the VMG (
Section 11.6
on page 167
).
•
Use the
Address Mapping
screen to configure the VMG's address mapping settings (
Section
11.7 on page 168
).
•
Use the
Sessions
screen to configure the VMG's maximum number of NAT sessions (
Section
11.7 on page 168
).
11.1.2
What You Need To Know
Inside/Outside
Inside/outside denotes where a host is located relative to the VMG, for example, the computers of
your subscribers are the inside hosts, while the web servers on the Internet are the outside hosts.
Global/Local
Global/local denotes the IP address of a host in a packet as the packet traverses a router, for
example, the local address refers to the IP address of a host when the packet is in the local
network, while the global address refers to the IP address of the host when the same packet is
traveling in the WAN side.
Page 160 / 327
Chapter 11 Network Address Translation (NAT)
VMG3926-B10A User’s Guide
160
NAT
In the simplest form, NAT changes the source IP address in a packet received from a subscriber
(the inside local address) to another (the inside global address) before forwarding the packet to the
WAN side. When the response comes back, NAT translates the destination address (the inside
global address) back to the inside local address before forwarding it to the original inside host.
Port Forwarding
A port forwarding set is a list of inside (behind NAT on the LAN) servers, for example, web or FTP,
that you can make visible to the outside world even though NAT makes your whole inside network
appear as a single computer to the outside world.
Finding Out More
See
Section 11.9 on page 170
for advanced technical information on NAT.
11.2
The Port Forwarding Screen
Use the
Port Forwarding
screen to forward incoming service requests to the server(s) on your
local network.
You may enter a single port number or a range of port numbers to be forwarded, and the local IP
address of the desired server. The port number identifies a service; for example, web service is on
port 80 and FTP on port 21. In some cases, such as for unknown services or where one server can
support more than one service (for example both FTP and web service), it might be better to
specify a range of port numbers. You can allocate a server IP address that corresponds to a port or
a range of ports.
The most often used port numbers and services are shown in
Appendix D on page 309
. Please refer
to RFC 1700 for further information about port numbers.
Note: Many residential broadband ISP accounts do not allow you to run any server
processes (such as a Web or FTP server) from your location. Your ISP may
periodically check for servers and may suspend your account if it discovers any
active services at your location. If you are unsure, refer to your ISP.
Configuring Servers Behind Port Forwarding (Example)
Let's say you want to assign ports 21-25 to one FTP, Telnet and SMTP server (
A
in the example),
port 80 to another (
B
in the example) and assign a default server IP address of 192.168.1.35 to a
third (
C
in the example). You assign the LAN IP addresses and the ISP assigns the WAN IP address.
The NAT network appears as a single host on the Internet.

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