Classification and Marking at the Data Link Layer Frame RelayATM QoS

Frame Relay Frame

Flag

Frame Relay Header

Information

FCS

Flag

DLCI

C/R

EA

DLCI

FECN

BECN

DE

EA

• Frame Relay DTE devices can set the DE bit of a frame so that if the network becomes congested, Frame Relay devices will discard frames with the DE bit set before discarding those that do not have the DE bit set.

• Frame Relay DTE devices can set the DE bit of a frame so that if the network becomes congested, Frame Relay devices will discard frames with the DE bit set before discarding those that do not have the DE bit set.

• Preserved throughout the Frame Relay network.

ATM UNI cell

• Preserved throughout the Frame Relay network.

ATM UNI cell

G FC

VPI

VCI

HEC

Payload

• The CLP bit indicates that the cell should be discarded if it encounters congestion as it moves through the network.

• Preserved throughout the ATM network.

• The CLP bit indicates that the cell should be discarded if it encounters congestion as it moves through the network.

• Preserved throughout the ATM network.

Before the Internet Engineering Task Force (IETF) defined QoS methods for the network layer, the International Telecommunication Union Telecommunication Standardization Sector (ITU-T), ATM Forum, and the Frame Relay Forum (FRF) had already derived standards for link-layer QoS in ATM and Frame Relay networks.

ATM standards define a very rich QoS infrastructure by supporting traffic contracts, many adjustable QoS knobs (such as peak cell rate [PCR], minimum cell rate [MCR], and so on), signaling, and admission control. Frame Relay provides a simpler set of QoS mechanisms to ensure a committed information rate (CIR), congestion notification, and FRF. 12.

One component of Frame Relay QoS is packet discard when congestion is experienced in the network. Frame Relay will allow network traffic to be sent at a rate exceeding its CIR. Frames sent that exceed the committed rate can be marked as discard eligible (DE). If congestion occurs in the network, frames marked DE will be discarded prior to frames that are not marked.

ATM cells consist of 48 bytes of payload and 5 bytes of header. The ATM header includes the 1-bit cell loss priority (CLP) field, which indicates the drop priority of the cell if that cell encounters extreme congestion as it moves through the ATM network. The CLP bit represents two values: 0 to indicate higher priority and 1 to indicate lower priority. Setting the CLP bit to 1 lowers the priority of the cell, increasing the likelihood that the cell will be dropped when the ATM network experiences congestion.

Three bits used for CoS

Label

S TTL

MPLS uses a 32-bit label field (shim header) which is inserted between Layer 2 and Layer 3 headers (frame mode).

Supports up to 8 classes of service.

The IP precedence or DSCP field is not directly visible to MPLS label switch routers.

By default, Cisco IOS software copies the three most significant bits of the DSCP or the IP precedence of the IP packet to the EXP field.

Preserved throughout the MPLS network.

EXP Application (DSCP)

EXP Application (DSCP)

When a customer transmits IP packets from one site to another, the IP precedence field (the first three bits of the DSCP field in the header of an IP packet) specifies the CoS. Based on the IP precedence marking; the packet is given the desired treatment, such as guaranteed bandwidth or latency. If the service provider network is an MPLS network, the IP precedence bits are copied into the MPLS experimental field at the edge of the network. However, the service provider might want to set an MPLS packet QoS to a different value that is determined by the service offering.

The MPLS experimental field allows the service provider to provide QoS without overwriting the value in the customer IP precedence field. The IP header remains available for customer use, and the IP packet marking is not changed as the packet travels through the MPLS network.

This topic describes the different IP packet classification and marking options that are available at the network layer.

Classification and Marking at the Network Layer IP Precedence and DSCP

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