Implementing IBM System Networking 10Gb Ethernet Switches

102 Implementing IBM System Networking 10Gb Ethernet Switches

򐂰 Re-mark the 802.1p field.

򐂰 Set the COS queue.

2.9.4 ACL metering and re-marking

You can define a profile for the aggregate traffic that flows through the switch by configuring a

QoS meter (if wanted) and assigning ACLs to ports. Actions taken by an ACL are called

In-Profile actions. You can configure additional In-Profile and Out-of-Profile actions on a port.

Data traffic can be metered, and re-marked to ensure that the traffic flow provides certain

levels of service in terms of bandwidth for different types of network traffic.

Metering

QoS metering provides different levels of service to data streams through user-configurable

parameters. A meter is used to measure the traffic stream against a traffic profile that

you create.

Thus, creating meters yields In-Profile and Out-of-Profile traffic for each ACL, as follows:

򐂰 In-Profile: If there is no meter configured or if the packet conforms to the meter, the packet

is classified as In-Profile.

򐂰 Out-of-Profile: If a meter is configured and the packet does not conform to the meter

(exceeds the committed rate or maximum burst rate of the meter), the packet is classified

as Out-of-Profile.

Using meters, you set a Committed Rate in Kbps (in multiples of 64). All traffic within this

Committed Rate is In-Profile. Additionally, you can set a Maximum Burst Size that specifies

an allowed data burst larger than the Committed Rate for a brief period. These parameters

define the In-Profile traffic.

Meters keep the sorted packets within certain parameters. You can configure a meter on an

ACL, and perform actions on metered traffic, such as packet re-marking.

Re-Marking

Re-marking allows the treatment of packets to be reset based on new network specifications

or wanted levels of service. You can configure the ACL to re-mark a packet as follows:

򐂰 Change the DSCP value of a packet, which is used to specify the service level that traffic

should receive.

򐂰 Change the 802.1p priority of a packet.

2.9.5 DiffServ Code Points

The six most significant bits in the TOS byte of the IP header are defined as DiffServ Code

Points (DSCP). Packets are marked with a certain value that depends on the type of

treatment the packet must receive in the network device. DSCP is a measure of the QoS level

of the packet.

Metering: Metering is not supported for IPv6 ACLs. All traffic that matches an IPv6 ACL

is considered in-profile for re-marking purposes.

Chapter 2. IBM System Networking Switch 10Gb Ethernet switch features 103

Differentiated Services concepts

To differentiate between traffic flows, packets can be classified by their DSCP value. The

Differentiated Services (DS) field in the IP header is an octet, and the first 6 bits, called the

DS Code Point (DSCP), can provide QoS functions. Each packet carries its own QoS state in

the DSCP. There are 64 possible DSCP values (0-63).

Figure 2-23 IPv4 packet with DSCP field

The switch can perform the following actions on the DSCP:

򐂰 Read the DSCP value of ingress packets.

򐂰 Re-mark the DSCP value to a new value.

򐂰 Map the DSCP value to an 802.1p priority.

After the DSCP value is marked, the switch can use it to direct traffic prioritization.

Per-Hop Behavior

The DSCP value determines the Per-Hop Behavior (PHB) of each packet. The PHB is the

forwarding treatment given to packets at each hop. QoS policies are built by applying a set of

rules to packets, based on the DSCP value, as they hop through the network.

The default settings are based on the following standard PHBs, as defined in the

IEEE standards:

򐂰 Expedited Forwarding (EF): This PHB has the highest egress priority and lowest drop

precedence level. EF traffic is forwarded ahead of all other traffic. EF PHB is described in

RFC 2598, found at:

http://www.ietf.org/rfc/rfc2598.txt

򐂰 Assured Forwarding (AF): This PHB contains four service levels, each with a different

drop precedence, as shown in Table 2-7. Routers use drop precedence to determine

which packets to discard last when the network becomes congested. AF PHB is described

in RFC 2597, found at:

http://www.ietf.org/rfc/rfc2597.txt

Table 2-7 Assured Forwarding PHB

Drop

precedence

Class 1 Class 2 Class 3 Class 4

Low AF11 (DSCP 10) AF21 (DSCP 18) AF31 (DSCP 26) AF41 (DSCP 34)

Medium AF12 (DSCP 12) AF22 (DSCP 20) AF32 (DSCP 28) AF42 (DSCP 36)

High AF13 (DSCP 14) AF23 (DSCP 22) AF33 (DSCP 30) AF43 (DSCP 38)