Key SDN technologies-Introduction to segment Routing Protocol

As a new network architecture, SDN has become a hot topic in the industry. The open network it advocates represents a change in the concept of network construction from adapting network capabilities to actively adapting network capabilities to the needs of network applications. Separation of forwarding and control, centralized control, and open network programming interfaces are the main features of SDN network architecture.

As the main SDN protocol, openflow hopes to use a new forwarding protocol to subvert the existing IP and MPLS network architecture. However, this radical evolution method is not supported by equipment manufacturers and telecom operators. by extending and optimizing the existing network protocols, it promotes the smooth evolution of the existing network, the goal of opening up the network is more feasible.

Segment routing is a new routing and forwarding protocol promoted by IETF to support the SDN architecture. Currently, segment routing is still in the draft stage, but it has formed a complete architecture and is supported by most equipment manufacturers.

1. Introduction to segment rouging

Segment routing is a source routing mechanism used to optimize IP and MPLS network capabilities. This allows you to achieve better network scalability and provide features such as Te, FRR, and mpls vpn in a simpler way. In the future SDN network architecture, segment routing will provide the ability for the Network to quickly interact with upper-layer applications.

2. How segment rouging works

Similar to MPLS networks, segmentrouting is also based on tag switching. However, unlike MPLS networks that rely on external protocols such as LDP and RSVP to implement Label Distribution and Te functions, segment routing simply expands the existing IGP protocol, you can implement features such as Te, FRR, and mpls vpn.

In the segment routing network, the network prefix is represented by segment. In the current segment routing draft, two types of segment are defined: Nodal segment and adjacencysegment.

Nodal segment is a global label. Each node is assigned a globally unique nodal segment. Generally, the address of the loopback interface is used.

Adjacency segment is a local label that is valid locally and is used to represent a specific SR node. (Global uniqueness is not required)

Figure 1 Topology

In the topology shown in figure 1, we assume that all routers have enabled the is-is or OSPF protocol, and the link has the same metric. Each Sr node has its own nodal segment and notifies other nodes through the IGP protocol. A sr node automatically creates a group of segments (similar to the MPLS Label Stack) that represent the shortest forwarding paths to other Sr nodes through Is-Is or OSPF ). Adjacency segment is associated with the next hop node to an adjacent device:

(1) Each node uses the loopback address as its own global label (nodal segment), and other nodes on the SR data layer will receive this information. In Figure 1, Node B's nodal segment is 70, and node S and R use 70 to calculate the path to B.

(2) In Figure 1, F allocates adjacency segment10000 (valid locally) for the link between the B-F and notifies you through IGP.
We can understand nodal segment and adjacency segment in this way. The Manager assigns a segment address block to the network. From this address block, we assign a nodal segment for each Sr node to ensure its global uniqueness. The adjacency segment is exclusive to this address block and is allocated by each node.

Through the above example, we can see that by associating nodal segment with the route prefix, we can reach any node through the shortest path, and whether the forwarding path is an equivalent multi-path load balancing, it depends on the IGP topology of the network.

3. segment rouging example

Here are two examples to illustrate how segmentrouging works:

Example 1 (Figure 2): node R advertises its nodal segment 70 to multiple nodes. If S is to reach R, use segment 70 for routing (Source Routing). The forwarding path represented by segment 70 integrates the ecmp attribute of IGP. In the total packet forwarding process shown in figure 2, s marks the segment list {70} For the packet. In section B and D, because R is their direct connection node, segment 70 will pop up from the list, and then send the packet to r according to the IGP forwarding path.

Figure 2 example 1

Example 2 (Figure 3): Use adjacencysegment (Fig. 3) to direct traffic to a specific node and port. In the network topology shown in figure 3, if s wants to reach R through the link between B and D, we use {71,100 00, the list of segments of 70. First, s sends the packet to the B node represented by 71 through the IGP route. When the packet arrives at node A, because B is the next hop of node A, node A will pop up 71 and send the packet to Node B. Node B sends the packet to D according to the adj. segment, and then D uses the IGP path to forward the packet to R.

Figure 3 Example 2

The segment defined by the segment routing is 32bit. In the MPLS environment, the rightmost 20bit label value is used. The MPLS Label Stack represents the segments list. Segment routing can forward IPv4/IPv6 packets and control networks with protocols such as LDP and RSVP.

4. segment rouging application scenarios

(1) Fast re-routing protection: Sr can provide FRR protection independently without relying on the RSVP protocol (RSVP protocol must be deployed for traditional MPLS Te ).

(2) Implementing the traffic engineering (Traffic Engineering), according to bandwidth, latency and other network parameters, the mainstream definition of cos-based service policies.
(3) Sdn network architecture: a programmable open network should be easy to operate and scalable. Compared with LDP and RSVP protocols, segment routing has obvious advantages in ease of operation and scalability.
 
Source: China Mobile Design Institute Technical Communication

 

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Key SDN technologies-Introduction to segment Routing Protocol