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Page 206 / 382 Scroll up to view Page 201 - 205
Chapter 16 Quality of Service (QoS)
P-870HN-51b User’s Guide
206
The following table describes the labels in this screen.
16.8
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).
Table 73
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
No.
This is the index number of the entry.
Name
This shows the name of the WAN interface on the Device.
Pass
This shows how many packets forwarded to this interface are
transmitted successfully.
Drop
This shows how many packets forwarded to this interface are
dropped.
Queue Monitor
No.
This is the index number of the entry.
Name
This shows the name of the queue.
Pass
This shows how many packets assigned to this queue are transmitted
successfully.
Drop
This shows how many packets assigned to this queue are dropped.
Table 74
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).
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Chapter 16 Quality of Service (QoS)
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207
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,
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.
DSCP (6 bits)
Unused (2 bits)
Table 74
IEEE 802.1p Priority Level and Traffic Type
PRIORITY
LEVEL
TRAFFIC TYPE
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Chapter 16 Quality of Service (QoS)
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208
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.
Table 75
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
Page 209 / 382
Chapter 16 Quality of Service (QoS)
P-870HN-51b User’s Guide
209
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.
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
6
6
4
100110
100100
100010
100000
5
101110
101000
7
7
6
110000
111000
7
Table 75
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 210 / 382
Chapter 16 Quality of Service (QoS)
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210
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.

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