MPLS technology research and application (1)

MPLS Multi-Protocol Label Switching (MPLS) is an exchange technology that provides cost-effective and multi-service capabilities. It solves the limitations of traditional IP group switching and has received wide attention in the industry, it has also been deployed in China Netcom and China tietong's backbone network. MPLS technology can be used to provide flexible traffic engineering, virtual private network, and other services. At the same time, MPLS is also a technology that can complete multi-layer network integrated control and management.
1. MPLS Overview
1.1 Basic Principles of MPLS 
MPLS is a layer-3 routing technology that integrates layer-2 attributes. It introduces a label-based mechanism that separates route selection from data forwarding, the label specifies the path of a group through the network. The MPLS network consists of the core part of the label exchange router LSR) and the edge part of the Label Edge Router LER. The role of LSR can be seen as the combination of an ATM switch and a traditional router. It consists of a control unit and a switching unit. The role of LSR is to analyze IP headers, used to determine the transfer level and label exchange path LSP ). The tag exchange procedure can be summarized into the following three steps:
(1) The LDP Label Distribution Protocol) IS used together with traditional routing protocols such as OSPF and IS-IS to create a route table and label ing table in the LSR;
2) LER receives the IP packet, completes the layer-3 function, and adds a label to the IP packet. On the LER at the MPLS egress, remove the labels in the group and continue forwarding;
3) LSR no longer processes the group on Layer 3, but forwards the Group through the switch unit based on the tags in the group. Procedure 1:

Figure 1

The MPLS packet header defined in the IETF standard document is a 32-bit field inserted between the traditional L2 data link layer header and the L3 IP Address Header, as shown in Figure 2:

Figure 2

Label field: 20 bits, Label field.
EXP: 3 bits, an experiment field.
S field: 1-bit, Stack) field.
TTL field: 8-bit, TTL field.
1.2. MPLS signaling method 
Currently, MPLS signaling methods can be divided into two types: LDP/CR-LDPLabel Dispatch Protocol and Constrain based Routing Label Dispatch Protocol. This is due to the idea of ATM networks. CR-LDP and LDP are the same protocol, and CR-LDP is an extension of LDP, which uses the same messages and mechanisms as LDP, such as peering discovery, session creation and persistence, tag publishing, and error handling. The other type is RSVP, which is based on the traditional IP routing protocol. RSVP and LDP/CR-LDP are two different protocols, they have different protocol characteristics, there are different message sets and signaling processing procedures. In terms of protocol reliability, LDP/CR-LDP is based on TCP. When packet loss occurs, the TCP protocol provides simple Error Indication to achieve rapid response and recovery. RSVP only transmits an IP packet. Due to the lack of reliable transmission mechanisms, RSVP cannot guarantee fast failure notifications. In terms of network scalability, LDP is more advantageous than RSVP. MPLS/LDP should be used in telecom-grade networks, especially ATM networks. ITU-T tends to use CR-LDP in backbone networks. Currently, all routing settings that support the MPLS Function Support both CR-LDP and rsvp mpls signaling protocols.