Brief introduction: basic principle of srv6

SRv6 Command network functions , Embed instructions that express network functions 128 bit Of IPv6 In the address . stay SRv6 On the Internet , Business requirements can be translated into an ordered list of instructions , By the network equipment along the way , To achieve flexible arrangement and on-demand customization of network services .

1、SRv6 Segment

Usually a computer instruction includes two aspects ：Opcode（ opcode ） and Operand（ Operands ）, among Opcode Decide what to do ,Operand It refers to the data participating in the operation and its unit address . Again , We're doing SRv6 When it comes to network programming , You also need to define network instructions ：SRv6Segment. identification SRv6 Segment Of ID go by the name of SRv6 SID ,SRv6SID It's a 128 bit Value , It is usually made up of 3 Component composition Locator、Function、Arguments.

• Locator
  The function of is to route the message to the network device that executes the instruction , Realize the addressability of network instructions .Locator The location information of an identifier has two important properties ： Routable and aggregatable .Locator The corresponding route will be passed by the node IGP Publish to the web , It is used to help other devices forward messages to the release Locator The node of . Besides ,Locator The corresponding route is also aggregatable .Locator The length of is variable , It is used to adapt to networks of different sizes .

• Function
  Used to express the forwarding action to be executed by the instruction , Equivalent to a computer instruction Opcode. stay SRv6 Network programming , Different forwarding behaviors have different Function To express . Similar to computer instructions , According to different functions Function Defined as different types SID, Express the corresponding forwarding behavior , Such as forwarding the message to the designated link , Or look up and forward in the specified table .

• Arguments
  （Args） Field is an optional field . It is the parameter corresponding to the instruction when it is executed , These parameters may contain a stream 、 Services or any other relevant information .

2、SRv6 node

stay SRv6 There are many types of node roles in the network , Basically, it is divided into the following 3 class ：

• SRv6 Source node
  ： Generate SRv6 The source node of the message , The source node directs the packet to SRv6 Segment List in , If SRv6Segment List Only a single SID, And there is no need to SRv6 Add information or TLV, be SRv6 The destination address field of the message is set to this SID, Can not be encapsulated SRH. The source node can be generated IPv6 Message and support SRv6 The host , It can also be SRv6 Edge device of domain .

• Transit nodes
  ： stay SRv6 Do not participate in the message forwarding path SRv6 To deal with the IPv6 node , That is, the transit node only performs ordinary IPv6 Message forwarding . When the node receives SRv6 After the message , Will parse the message IPv6 DA（Destination Address, Destination address ） Field . If IPv6 The destination address is neither locally configured SRv6SID, Nor is it the local interface address , Then the node will SRv6 Messages are treated as ordinary IPv6 message , Find... According to the longest matching principle IPv6 Routing table , Process and forward , Don't deal with SRH, At this time, this node is the transit node . The transit node can be ordinary IPv6 node , It can also support SRv6 The node of .

• SRv6 Segment endpoint node
  ： stay SRv6 In the process of message forwarding , Of messages received by the node IPv6 The destination address is locally configured SID, Then this node is called the endpoint node . This node needs to handle SRv6 SID and SRH.

3、SRv6 Extended message header

In order to achieve SRv6, according to IPv6 The original routing extension header defines a new type of extension header , Referred to as SRH. The extended message header carries Segment List Such information explicitly specifies a SRv6 route .SRH The format is as follows ：

SRH The description of each field is as follows ：

SRv6 The naming rules of instructions are as follows ：

• End:End Is the most basic Segment Endpoint Execution instruction , Indicates the termination of the current instruction , Start executing the next instruction . The corresponding forwarding action is to forward SL The value of the reduction 1, And will SL Point to the SID Copied to the IPv6 Destination address field of message header .

• X： Specify one or a group of layer 3 interfaces to forward messages . The corresponding forwarding behavior is to forward the message according to the specified interface .

• T： Query the routing forwarding table and forward the message .

• D： decapsulation . remove IPv6 Message header and its associated extended message header .

• V： according to VLAN（Virtual Local Area Network, Virtual LAN ） Table lookup forwarding .

• U： According to unicast MAC Table lookup forwarding .

• M： Query the layer 2 forwarding table , Multicast forwarding .

• B6： Apply the specified SRv6 Policy.

• BM： Apply the specified SR-MPLS Policy.

All instructions are composed of one or more atomic functions mentioned above .

4、SRv6 Message forwarding

SRv6 The node maintains a local SID surface , This table contains all the generated in this node SRv6 SID Information . Local SID The table has the following purposes .

• Store locally generated SID, for example End.X SID.

• Specify binding to these SID Instructions .

• Store forwarding information related to these instructions , for example VPN example 、 Outgoing interface and next hop, etc .

Suppose there are messages that need to be sent from the host H1 Forward to host H2,H1 Send the message to the node A Handle . node A、B、D and F All supported SRv6 The equipment , node C And nodes E For not supporting SRv6 The equipment .

stay SRv6 Source node A Network programming on , I hope the message goes through B-C and D-E These two links , Then it is delivered to the node F, Then pass through the node F Delivery host H2. The following is the message slave node A To the node F Detailed processing steps .

1）SRv6 Source node A To deal with

. node A take SRv6 Path information is encapsulated in SRH in , Appoint B-C Links and D-E Linked End.X SID. in addition , node A You also need to encapsulate nodes F released End.DT4 SID A6::100, This End.DT4 SID Corresponding to node F One of the IPv4 VPN. Press in... In reverse order SID Sequence , Because of 3 individual SID, So node A The initial of the encapsulated message SL=2.SL Point to the operation instructions that currently need to be processed , That is to say Segment List[2] Field , node A Set it to a value A2::23 Copy to outer IPv6 Destination address field of message header , And search according to the longest matching principle IPv6 Routing table , Forward the message to the node B.

2）Endpoint node B To deal with

. node B After receiving the message , according to IPv6 Destination address of message A2::23 Look for local SID surface , hit End.X SID. node B perform End.X SID Command action of , take SL The value of the reduction 1, And will SL Directed SID Update to outer IPv6 Destination address field of message header , At the same time, send the message from End.X SID The bound link is sent .

3） Transit nodes C To deal with

. When the message arrives at the node C after , node C Only processing is supported IPv6 Message header , Can't recognize SRH, Now the node C According to normal IPv6 Message processing flow , Find... According to the longest matching principle IPv6 Routing table , Forward the message to the node represented by the current destination address D.

4）Endpoint node D To deal with

. node D After receiving the message , according to IPv6 Destination address of message A4::45 Look for local SID surface , hit End.X SID. node D perform End.X SID Command action of , take SL The value of the reduction 1, And will SL Directed SID Update to outer IPv6 Destination address field of message header , At the same time, send the message from End.X SID The bound link is sent . If A4::45 yes PSP Flavor Of SID, At this time, it can be based on PSP The instructions of will SRH eject , The message becomes ordinary IPv6 message .

5） Transit nodes E To deal with

. node E Only support IPv6 Header processing , Can't recognize SRH, Now the node E According to normal IPv6 Message processing flow , Find... According to the longest matching principle IPv6 Routing table , Forward the message to the node represented by the current destination address F.

6）Endpoint node F To deal with

. node F After receiving the message , According to the outer layer IPv6 Destination address A6::100 Look for local SID surface , hit End.DT4 SID. node F perform End.DT4 SID Command action of , Unpack message , Remove IPv6 Message header , Then put the inner layer IPv4 The message is in End.DT4 SID The binding of VPN Example of IPv4 Look up and forward in the routing table , Finally, send the message to the host H2.