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Page 161 / 331 Scroll up to view Page 156 - 160
Chapter 10 Quality of Service (QoS)
VMG1312-B Series User’s Guide
161
DiffServ
QoS is used to prioritize source-to-destination traffic flows. All packets in the flow are given the
same priority. You can use CoS (class of service) to give different priorities to different packet
types.
DiffServ (Differentiated Services) is a class of service (CoS) model that marks packets so that they
receive specific per-hop treatment at DiffServ-compliant network devices along the route based on
the application types and traffic flow. Packets are marked with DiffServ Code Points (DSCPs)
indicating the level of service desired. This allows the intermediary DiffServ-compliant network
devices to handle the packets differently depending on the code points without the need to
negotiate paths or remember state information for every flow. In addition, applications do not have
to request a particular service or give advanced notice of where the traffic is going.
DSCP and Per-Hop Behavior
DiffServ defines a new Differentiated Services (DS) field to replace the Type of Service (TOS) field
in the IP header. The DS field contains a 2-bit unused field and a 6-bit DSCP field which can define
up to 64 service levels. The following figure illustrates the DS field.
DSCP is backward compatible with the three precedence bits in the ToS octet so that non-DiffServ
compliant, ToS-enabled network device will not conflict with the DSCP mapping.
The DSCP value determines the forwarding behavior, the PHB (Per-Hop Behavior), that each packet
gets across the DiffServ network. Based on the marking rule, different kinds of traffic can be
marked for different kinds of forwarding. Resources can then be allocated according to the DSCP
values and the configured policies.
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 Device, the Device 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.
Level 1
This is typically used for non-critical “background” traffic such as bulk transfers that
are allowed but that should not affect other applications and users.
Level 0
Typically used for best-effort traffic.
DSCP (6 bits)
Unused (2 bits)
Table 52
IEEE 802.1p Priority Level and Traffic Type
PRIORITY
LEVEL
TRAFFIC TYPE
Page 162 / 331
Chapter 10 Quality of Service (QoS)
VMG1312-B Series User’s Guide
162
The following table shows you the internal layer-2 and layer-3 QoS mapping on the Device. On the
Device, traffic assigned to higher priority queues gets through faster while traffic in lower index
queues is dropped if the network is congested.
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.
Table 53
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 163 / 331
Chapter 10 Quality of Service (QoS)
VMG1312-B Series User’s Guide
163
•
If there are no tokens in the bucket, the Device stops transmitting until enough tokens are
generated.
•
If not enough tokens are available, the Device 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 Device 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.
•
If there are not enough tokens in the CBS bucket, the Device 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
Page 164 / 331
Chapter 10 Quality of Service (QoS)
VMG1312-B Series User’s Guide
164
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 Device 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 165 / 331
VMG1312-B Series User’s Guide
165
C
HAPTER
11
Network Address Translation (NAT)
11.1
Overview
This chapter discusses how to configure NAT on the Device. 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 166
).
•
Use the
Applications
screen to forward incoming service requests to the server(s) on your local
network (
Section 11.3 on page 169
).
•
Use the
Port Triggering
screen to add and configure the Device’s trigger port settings (
Section
11.4 on page 171
).
•
Use the
DMZ
screen to configure a default server (
Section 11.5 on page 173
).
•
Use the
ALG
screen to enable and disable the NAT and SIP (VoIP) ALG in the Device (
Section
11.6 on page 174
).
•
Use the
Address Mapping
screen to configure the Device's address mapping settings (
Section
11.7 on page 175
).
•
Use the
Sessions
screen to configure the Device's maximum number of NAT sessions (
Section
11.7 on page 175
).
11.1.2
What You Need To Know
Inside/Outside
Inside/outside denotes where a host is located relative to the Device, 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.

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