SDN topology discovery principle

Preface

SDN A major feature of the network is that resources are centrally managed by the controller , Controller management network , Of course, the most basic thing is to know the topology of the network , The network topology may change from time to time , So the controller needs to monitor constantly . For traditional switches , Normally they pass LLDP And other similar neighbor discovery protocols to discover each other's network devices , Form the whole network topology .OpenFlow There is no standard topology discovery method specified by the official , current OFDP（OpenFlow Discovery Protocol） The link discovery protocol in the traditional network is still used LLDP. And in the SDN Environment , The device is brainless , At this time, we need to use Openflow and LLDP Work at the same time , To protect Controller Topology discovery of the whole network can be carried out in the environment .

LLDP（Link Layer Discovery Protocol, Link Layer Discovery Protocol ）

LLDP It provides a standard link layer discovery method , The main capabilities of local devices can be 、 Management address 、 Device identifier 、 Information such as interface identification is organized into different groups TLV（Type/Length/Value, type / length / value ）, And it's packaged in LLDPDU（Link Layer Discovery Protocol Data Unit, Link layer discovery protocol data unit ） To your immediate neighbors , After receiving this information, neighbors will regard it as standard MIB（Management Information Base, Management information base ） The form is preserved , For the network management system to query and judge the communication status of the link .
LLDP Is an information discovery and Notification Protocol ,LLDP Two... Are maintained in the entity of MIB library （ Management information base ）, One local system MIB, Used to maintain local related equipment MIB Information , One remote system MIB, Used to maintain remote devices MIB Information .
LLDP adopt PTOPO MIB、Enitity MIB、Interface MIB、Other MIB Wait for the library to maintain local system MIB, And notify relevant local information . When receiving information from other devices, update it to remote system MIB in .

In traditional Ethernet switches , The switch sends from each of its ports LLDP Data packets , This packet will not be forwarded by other switches , Life span only jumps ,LLDP The load is encapsulated in Ethernet frames , The structure is as follows: , The dark gray one is LLDP load ,Chassis ID TLV, Port ID TLV and Time to live TLV Three are mandatory fields , Respectively represents the switch identifier （ It is unique in LAN ）, The port number and TTL. This data is sent out and received by the neighbor node and then parsed , You can know the source destination switch and source destination interface of this link .

OpenFlow in LLDP Processing flow

stay SDN In the network , Because the controller centrally controls the switches in the domain , So the switch is completed by receiving the instructions sent by the controller LLDP Sending and receiving of frame .
Suppose there are two OpenFlow The switch is connected to the controller , Here's the picture , Briefly describe the steps of topology discovery （ With S1 As the subject ,S2 Similar ）：

1. SDN Controller construction PacketOut Message direction S1 The three ports of send the LLDP Data packets , Among them, the Chassis ID TLV and Port ID TLV Respectively set to S1 Of dpid And port number ;
2. Director to switch S1 Distribute the flow table , The flow table rule is ： Will be taken from Controller Port received LLDP The packet is sent out from its corresponding port ;
3. Director to switch S2 Distribute the flow table , The flow table rule is ： Will be from non Controller Received LLDP The packet is sent to the controller ;
4. The controller is analyzed LLDP Data packets , Get the source switch of the link , Source interface , By receiving PacketIn The message knows the destination switch and destination interface ;

As shown in the figure below ,

1. OpenFlow Controller to OpenFlow Switch 2 send out Packet-Out news , To send LLDP frame .
2. OpenFlow Switch 2 adopt OpenFlow Switch 1 The network interface sent on contains packet-Out In the news LLDP frame .
3. OpenFlow Switch 1 Receive from openflow Switch 2 Of LLDP After the frame , take LLDP Frames are added to Packet-In In the news , And send it to OpenFlow controller ,OpenFlow The controller is based on OpenFlow Switch 1 It's from Packet-In The information contained in the message , Build a database for network topology detection .

OpenFlow The switch is wrong LLDP Frames do any processing , But by the OpenFlow The controller performs network topology detection .
LLDP And OpenFlow The relationship between the two is only in OpenFlow Use within the framework LLDP, Even if OpenFlow The specification does not indicate LLDP, The specification also supports LLDP, But that doesn't mean you have to use LLDP To detect the network topology , Again , utilize IS-IS or OSPF It can also realize network topology detection .

There are non OpenFlow Equipment time , How to do link discovery ？

be based on LLDP The message method can only be used for the controller directly connected openflow The switch performs link discovery , If there are non openflow Domain , That's two openflow The switch passes through other non openflow Switch connection , At this time, other link discovery methods are needed . under these circumstances , The controller will still send first packet-out Message to which it is connected openflow Switch , First maintain OpenFlow Topology of switch direct connection scenario . But at the same time, the controller will ask the switch to send broadcast packets , Broadcast packets will be sent to other ports except the ports connected to the switch and controller . Broadcast packets from openflow After the switch sends out , If there are non openflow Domain , Broadcast packets will enter and cross from one end of the network domain , Reach and should not openflow Other domain connections openflow Switch . Because when receiving the broadcast packet openflow There is no corresponding flow table entry in switch for broadcast packet matching , So the broadcast package will be uploaded to the controller , Thus, the controller is informed that there is non openflow Domain .
If the controller does not receive the uploaded broadcast packet , Then we can judge that the whole network has openflow Switch composition .why？ All connected are openflow When switching , Broadcast packets will be discarded ？
（ summary , The controller requires the switch to send broadcast packets , Broadcast packets will pass through non openflow Domain , Finally arrived by openflow Switch handover controller , From this, we know that there is non openflow Domain .）
explain OpenFlow The switch has a stream table for processing broadcasts , And these flow tables are just started LLDP Issued by the controller during detection .

Reference resources

H3C<LLDP Technology is introduced >
LLDP（ Link Layer Discovery Protocol ） and OpenFlow
Improve SDN Topology discovery performance of controller
SDN Switch topology discovery and ARP Handle
get SDN The whole network information is key ： Link discovery technology