Chapter 14 Quality of Service (QoS)

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.

DSCP (6 bits)

Unused (2 bits)

 

 

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.

14.8.1 Automatic Priority Queue Assignment

If you enable QoS on the P-870HN-51D, the P-870HN-51D 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 P-870HN-51D. On the P-870HN-51D, traffic assigned to higher priority queues gets through faster while traffic in lower index queues is dropped if the network is congested.

Table 69 Internal Layer2 and Layer3 QoS Mapping

 

LAYER 2

LAYER 3

 

 

PRIORITY

IEEE 802.1P

TOS (IP

 

IP PACKET

QUEUE

USER PRIORITY

DSCP

 

(ETHERNET

PRECEDENCE)

LENGTH (BYTE)

 

PRIORITY)

 

 

 

0

1

0

000000

 

 

 

 

 

 

1

2

 

 

 

 

 

 

 

 

2

0

0

000000

>1100

 

 

 

 

 

 

185

P-870HN-51D User’s Guide