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Chapter 9 Quality of Service (QoS)
VMG8924-B10A and VMG8924-B30A Series User’s Guide
151
9.7
The QoS Monitor Screen
This screen is available only when you set a rate limit for a WAN queue in the
Queue Setup
screen
and the WAN interface is connected. Use this screen to monitor the traffic statistics for both the
WAN and LAN interfaces. To view the Device’s QoS packet statistics, click
Network Setting > QoS
>
Monitor
. The screen appears as shown.
Figure 80
Network Setting > QoS > Monitor
The following table describes the labels in this screen.
Available Class
Selected Class
Select a QoS classifier to apply this QoS policer to traffic that matches the QoS classifier.
Highlight a QoS classifier in the
Available Class
box and use the
>
button to move it to the
Selected Class
box.
To remove a QoS classifier from the
Selected Class
box, select it and use the
<
button.
Apply
Click
Apply
to save your changes.
Cancel
Click
Cancel
to exit this screen without saving.
Table 52
Policer Setup: Add/Edit
LABEL
DESCRIPTION
Table 53
Network Setting > QoS > Monitor
LABEL
DESCRIPTION
Refresh Interval
Enter how often you want the Device to update this screen. Select No Refresh
to stop refreshing statistics.
Interface Monitor
#
This is the index number of the entry.
Name
This shows the name of the interface on the Device.
Pass Rate
This shows how many packets forwarded to this interface are transmitted
successfully.
Drop Rate
This shows how many packets forwarded to this interface are dropped.
Queue Monitor
#
This is the index number of the entry.
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9.8
Technical Reference
The following section contains additional technical information about the Device features described
in this chapter.
IEEE 802.1Q Tag
The IEEE 802.1Q standard defines an explicit VLAN tag in the MAC header to identify the VLAN
membership of a frame across bridges. A VLAN tag includes the 12-bit VLAN ID and 3-bit user
priority. The VLAN ID associates a frame with a specific VLAN and provides the information that
devices need to process the frame across the network.
IEEE 802.1p specifies the user priority field and defines up to eight separate traffic types. The
following table describes the traffic types defined in the IEEE 802.1d standard (which incorporates
the 802.1p).
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)
Name
This shows the name of the queue.
Pass Rate
This shows how many packets assigned to this queue are transmitted
successfully.
Drop Rate
This shows how many packets assigned to this queue are dropped.
Table 53
Network Setting > QoS > Monitor (continued)
LABEL
DESCRIPTION
Table 54
IEEE 802.1p Priority Level and Traffic Type
PRIORITY
LEVEL
TRAFFIC TYPE
Level 7
Typically used for network control traffic such as router configuration messages.
Level 6
Typically used for voice traffic that is especially sensitive to jitter (jitter is the
variations in delay).
Level 5
Typically used for video that consumes high bandwidth and is sensitive to jitter.
Level 4
Typically used for controlled load, latency-sensitive traffic such as SNA (Systems
Network Architecture) transactions.
Level 3
Typically used for “excellent effort” or better than best effort and would include
important business traffic that can tolerate some delay.
Level 2
This is for “spare bandwidth”.
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.
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Chapter 9 Quality of Service (QoS)
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153
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.
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.
DSCP (6 bits)
Unused (2 bits)
Table 55
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
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Chapter 9 Quality of Service (QoS)
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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 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.
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
Table 55
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)
Page 155 / 412
Chapter 9 Quality of Service (QoS)
VMG8924-B10A and VMG8924-B30A Series User’s Guide
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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
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.

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