MPLS DiffServ

Implementation

In the DiffServ model, network edge nodes map a service to a service class based on QoS requirements of the service. A service class is identified by the differentiated service (DS) field (Type of Service (ToS) field) in IP packets or the PRI field (802.1p priority) in VLAN packets. Nodes on a backbone network apply preset policies to the service based on the DS or PRI field to ensure service quality. For details, see Priority Mapping Configuration (CE12800) in CloudEngine 12800 and 12800E Series Switches Configuration Guide - QoS. The service classification and label distribution mechanisms of DiffServ are similar to MPLS label distribution. MPLS DiffServ combines DS or PRI distribution with MPLS label distribution.

Currently, the switches do not support the DiffServ function of the MPLS TE feature.

Figure 3-1 Fields in an MPLS packet

MPLS DiffServ maps the EXP field (as shown in Figure 3-1) to a per-hop behavior (PHB). LSRs forward MPLS packets based on EXP fields in the MPLS packets. MPLS DiffServ provides the following solutions for label switched path (LSP) setup:

• E-LSP: an LSP whose PHB is determined by the EXP field. E-LSP applies to a network with fewer than eight PHBs. A differentiated services code point (DSCP) or 802.1p priority is mapped to a specified EXP value that identifies a PHB. Table 3-1 describes the mapping between PHBs and EXP values. Packets are forwarded based on labels, and the EXP field determines the packet scheduling algorithm and drop priority at each hop. An LSP transmits a maximum of eight PHB flows that are identified by the EXP field in the MPLS packet header. The EXP value can be configured by the Internet service provider (ISP) or mapped from the DSCP or 802.1p priority in a packet. In E-LSP, PHB information does not need to be transmitted by signaling protocols. Additionally, the label efficiency is high, and the label status is easy to maintain.

  Table 3-1 Mapping between DiffServ PHBs and EXP values

  PHB	EXP Value
  BE	0
  AF1	1
  AF2	2
  AF3	3
  AF4	4
  EF	5
  CS6	6
  CS7	7

• L-LSP: an LSP whose PHB is determined by both the label and EXP value. L-LSP applies to a network with any number of PHBs. During packet forwarding, the label of a packet determines the forwarding path and scheduling algorithm, whereas the EXP field determines the drop priority of the packet. Labels differentiate service flows, so service flows of a specified type are transmitted over the same LSP. This solution requires more labels and occupies a large number of system resources.

The device supports only E-LSP.

DiffServ Domain

As shown in Figure 3-2, DiffServ domains include MPLS DiffServ and IP DiffServ domains.

In the E-LSP solution, MPLS DiffServ manages and schedules the two DiffServ domains and implements bidirectional mapping between DSCP or 802.1p priorities and EXP priorities at the MPLS network edge.

Figure 3-2 DiffServ domain

As shown in Figure 3-3, the MPLS DiffServ domain forwards MPLS packets based on EXP values and provides differentiated services.

When MPLS packets enter the P device, the P device classifies packets and maps EXP values in packets to class of service (CoS) values and drop priorities. After traffic classification, QoS implementations including traffic shaping, traffic policing, and congestion avoidance are the same as those on an IP network. When MPLS packets leave the P device, the packets carry EXP values.

Figure 3-3 E-LSP