Multiprotocol Label Switching (MPLS) uses label-based forwarding to replace traditional route-based forwarding. MPLS has a powerful and flexible routing function and can meet the requirements of various applications for the network. MPLS can be implemented on various physical media, such as the Ethernet, PPP, ATM.
Currently MPLS widely applies to large-scale networks. Therefore, quality of service (QoS) for MPLS networks must be deliberately deployed.
MPLS establishes label switched paths (LSPs) to implement connection-oriented forwarding. QoS for LSP provides QoS guarantee for data flows transmitted over LSPs. Therefore, the DiffServ and IntServ models are applied to MPLS networks. The combination of MPLS and IntServ shapes multiprotocol label switching traffic engineering (MPLS TE), and the combination of MPLS and DiffServ shapes MPLS DiffServ.
IntServ uses the Resource Reservation Protocol (RSVP) to apply for resources over the entire network and maintains a forwarding state for each data flow, hindering the extensibility. Therefore, IntServ does not prevail on networks. Relevant standards, however, extends RSVP by allowing RSVP PATH messages to carry label requests and RSVP RESV messages to support label allocation. The extended RSVP is called Resource Reservation Protocol-Traffic Engineering (RSVP-TE). RSVP-TE allows MPLS to control the path through which traffic traverses and reserve resources during LSP establishment so that traffic can bypass congestion nodes. This method of balancing network traffic is called MPLS TE.
The DiffServ model can distinguish services based on packet contents and allow packets with high priorities to be forwarded preferentially. Therefore, DiffServ widely applies to MPLS networks.
However, DiffServ reserves resources only on a single node and cannot specify the bandwidth for each service in advance. When the traffic rate exceeds the allowed bandwidth, high-priority services are forwarded preferentially at the cost that delays and packet loss of low-priority services deteriorate. In the case of severe traffic congestion, even high-priority services are delayed or lost. Therefore, MPLS DiffServ alone can hardly provide the end-to-end QoS guarantee or allow services to comply with the Service Level Agreement (SLA).
VPN QoS combines MPLS QoS and MPLS VPN to serve for networking that bears services of various priorities. VPN QoS distinguishes services of different priorities and ensures that high-priority services are forwarded preferentially. This guarantees the QoS for important services on VPNs.
DiffServ, RSVP-TE, and MPLS VPN can be jointly used based on actual requirements to isolate services, distinguish services of different priorities, ensure bandwidth resources for important services or important VPNs, and forwards packets on VPNs or MPLS-TE tunnels based on packet priorities. This provides a solid technical basis for carriers to develop voice, video, and SLA-complying VPN services.