Overview of MPLS Basics

MPLS It is between the second and third floors “2.5 layer ” technology , Support IPv4 and IPv6 And other network layer protocols , And compatible ATM And Ethernet and other link layer technologies .MPLS Absorbed ATM Online VCI（Virtual Channel Identifier, Virtual channel identifier ） and VPI （Virtual Path Identifier, Virtual path identifier ） Exchange ideas , There is also IP The flexibility of routing and the simplicity of label switching , For connectionless IP The network adds connection oriented attributes . Through establishment “ Virtual connection ” Methods ,MPLS by IP The network provides better QoS Supportability .

It was first proposed that MPLS Not only because it can be IP The network provides better QoS Supportability , But also because MPLS Based on fixed length 32 bit Tag exchange to forward data , and IP Forward data based on the longest prefix matching principle , Compared with , The former has higher forwarding efficiency . Although with the improvement of hardware capability , MPLS The advantage of high forwarding efficiency is no longer obvious , But its connection oriented tag forwarding is to IP The network provides a good QoS guarantee , It can also support TE、VPN and FRR. These advantages are right IP The continued expansion of the network has played a key role , Accelerated the Internet IP turn .

On the whole ,MPLS Its success is inseparable from the three features it supports ：TE、VPN and FRR.

• TE
  ： be based on RSVP-TE Can achieve MPLS TE Application and distribution of path labels , Resource assurance can be achieved 、 Explicit path forwarding, etc TE characteristic , Make up for IP Network pair TE Short board with poor support ability .

• VPN
  ：MPLS Labels can be used to identify VPN, Realization VPN Business isolation .VPN yes MPLS One of the largest application scenarios at present , It is the key technology to solve enterprise interconnection and multi service bearing , It is also one of the important means of current operators' revenue .

• FRR
  ：：IP The network cannot provide complete FRR Protect , As a result, it cannot meet the needs of carrier level services .MPLS The emergence of IP The Internet FRR The ability of , In most fault scenarios 50 ms The need for carrier level protection switching .

MPLS Although in the network All IP Has played an important role in industrialization , But it also brings the problem of network island , It increases the complexity of network cross domain interworking .

One side ,MPLS Deployed to different network domains , for example IP backbone network 、 Metropolitan area network and mobile bearer network , Formed an independent MPLS Domain , It also brings new network boundaries . But many businesses need end-to-end deployment , Therefore, when deploying the business, it needs to span multiple MPLS Domain , This brings complex MPLS Cross-domain problem . In the history of ,MPLS VPN Yes Option A/B/C And other forms of cross domain solutions  , The complexity of business deployment is relatively high .

On the other hand , With the development of Internet and cloud computing , There are more and more cloud data centers . To meet the needs of multi tenant networking , The industry has put forward a variety of Overlay Technology , Typical is VXLAN. Many people in history have tried to MPLS introduce DC（Data Center, Data Center ） To provide VPN service , But because there are many network boundaries 、 There are many reasons, such as high management complexity and insufficient scalability , MPLS All attempts to enter the data center failed .

Limit MPLS Another major reason for development is lack of scalability . Scalability can be divided into label space scalability and packaging format scalability . Label space ,MPLS Only 20 bit The label space of , When the network becomes larger , There will be insufficient tag resources . And after the network becomes larger , Control plane RSVP-TE The scalability of the protocol is insufficient , The complexity is also too high . Packaging format ,MPLS The packaging format of the label is 32 bit Fixed code ,MPLS Tags provide some scalability , But in the face of more and more new services that need to expand the header to carry data , Such as support SFC Carrying metadata   and IOAM（In-situ Operations,Administration and Maintenance, Follow flow operation 、 Management and maintenance ） when ,MPLS It seems that the heart is more than the strength .