MPLS QoS is an important part of the deployment of QoS services. In actual MPLS networking solutions, differentiated services model (DiffServ) is often used to implement QoS. MPLS QoS needs to follow up the value of EXP to distinguish different data sources and realize differentiated services. , To ensure low latency and low packet loss rate of voice and video data, and ensure network stability;
1、MPLS DiffServ
The basic mechanism of DiffServ is to map the service to a certain service category according to the service quality requirements of the service at the edge of the network, and use the DS field (ToS) in the IP packet to uniquely mark this type of service, and then each node in the backbone network according to this field Pre-set service strategies are adopted for various services to ensure the corresponding service quality. DiffServ's classification and labeling mechanism for service quality is very similar to MPLS label distribution. In fact, DiffServ based on MPLS is through the distribution of DS Realized by combining with the label distribution process of MPLS;
MPLS DiffServ is implemented by carrying DiffServ PHB (Per-Hop Behavior) in the EXP value in the MPLS packet header. LSR must consider the MPLS EXP value when making forwarding decisions. DiffServ PHB is carried in the EXP field. MPLS DiffServ provides two Program:
E-LSP path: (currently the device only supports E-LSP)
That is, the EXP bit determines the PHB's LSP. This method is suitable for networks that support less than 8 PHBs. A specific DSCP is directly mapped to a specific EXP and identified to a specific PHB. During the forwarding process, the message is forwarded according to the label. The EXP determines the scheduling and drop priority on each hop LSR. Therefore, the same LSP can carry 8 different PHB flows, which are distinguished by the EXP field in the MPLS header. EXP can be directly determined by the operator’s configuration. It can be directly mapped from the DSCP of the message. This method does not require a signaling protocol to transfer PHB information, and the label usage rate is high, and the status is easy to maintain;
L-LSP path:
That is, the label and EXP jointly determine the PHB LSP. This method is suitable for networks that support any number of PHBs. In the forwarding process, the label is not only used to determine the forwarding path but also determines the scheduling behavior on the LSR, and the EXP bit is used to determine The forwarding path also determines the scheduling behavior on the LSR, and the EXP bit is used to determine the discarding priority of the data message. Because the type of service flow is distinguished by the label, it is necessary to establish different LSPs for different flows. This method Need to use more tags, occupy a lot of system resources;
2. The tunneling mode of MPLS DiffServ
The MPLS network essentially provides a tunneling service for the services it carries. The DiffServ mode of MPLS L3VPN includes three modes: Pipe, Short Pipe, and Uniform;
Pipe
The EXP value of the MPLS label pressed into the message on the Ingress is specified by the user. If the message changes the value of the EXP field in the MPLS network, it is only valid in the MPLS network. On the Egress, the message will be selected according to the value of the EXP field. PHB, when the message leaves the MPLS network, the DSCP field carried before the message continues to be valid;
Short Pipe
The EXP value of the MPLS label pressed into the message on the Ingress is specified by the user. If the message changes the value of the EXP field in the MPLS network, it is only valid in the MPLS network. On the Egress, the message will be selected according to the value of the DSCP field. PHB, when the message leaves the MPLS network, the DSCP field carried before the message continues to be valid;
Uniform
The priority identification of the message in the IP network and MPLS network is uniformly defined, that is, the priority identification of the message in both networks is globally effective. On the Ingress, the message is labeled, and the DSCP field is low. The three bits are mapped to the EXP field. If the message changes the value of the EXP field in the MPLS network, it will affect the PHB used after the message leaves the MPLS network, that is, the EXP field will be mapped to the DSCP field at the egress node;
For L2VPN, referring to the Pipe/Uniform model of L3VPN, the MPLS label resides in the outermost layer of the encapsulated message, and the mapping implementation of VLAN 802.1P and MPLS EXP needs to be considered.