Advertising S-BFD Discriminators in BGP

defines the Seamless Bidirectional Forwarding Detection (S-BFD) mechanism. S-BFD is a simplified mechanism for using BFD with a large proportion of negotiation aspects eliminated, thus providing benefits such as quick provisioning, as well as improved control and flexibility for network nodes initiating path monitoring. Currently, S-BFD can be used to simplify the service deployment.

An important usage for S-BFD is to check the reachability of targets, so that service interruption can be swiftly detected when there is a failure on the service path and services can be switched to a backup path quickly. specifies the Seamless Bidirectional Forwarding Detection (S-BFD) mechanism. Generally, the operators need to manually deploy S-BFD discriminators on the device to generate S-BFD sessions. For the deployment of S-BFD in IPv4 network, the reflector can use the LSR-ID address as the discriminator. This reduces the number of discriminators deployed on the transmit end. This mode cannot be used for IPv6 because the discriminator has only four bytes. defines IS-IS and OSPF to flood BFD discriminators. However, this mode is based on nodes and cannot traverse an IGP area. In addition, without the knowledge of services to be detected, a large number of redundant S-BFD sessions may be generated. It is recommended to use BGP to distribute BFD discriminator information. BGP can transmit routes across domains, and service routes can drive to generate the end-to-end S-BFD sessions on demand.

When the services are across multiple areas or multiple ASes, the BFD discriminator of the remote PE node cannot be obtained through the IGP based S-BFD extensions, thus a BGP based mechanism is required. For SRv6 services, there are two different service types. One is service over SRv6 BE, the other is service over SRv6 Policy. For the service over SRv6 BE, it will use the VPNSID to resolve the forwarding information. Thus we must generate an S-BFD session to detect the VPNSID's reachablity. This is an IP-routed S-BFD. We may use the remote VPNSID's locator as the destination of the S-BFD session. For the service over SRv6 Polcy, it will use <nexthop, color> of the service route to resolve an SRv6 Policy. Then we must generate an S-BFD session to detect the reachablity of the SR Policy.

Figure 1 shows a SRv6 BE based seamless scenario. CE1 is dual-homed to PE1 and PE2, and CE2 is accecssed to PE3. PE1, PE2, and PE3 are cross BGP ASes. CE1 accesses PE1 and PE2 through Layer 3 and advertises its private network routes to PE1. PE1 encapsulates the routes into Type 5 routes in the EVPN format and sends them to PE3. After receiving Type 5 routes advertised by PE1 and PE2, PE3 generates primary and backup entries for the routes to speed up service switchover. In this scenario, the SRv6 BE service mode is used. PE3 will resolve PE1's VPN routes reachbility through the VPNSID. To ensure that PE3 can properly route to PE1, PE1 needs to advertise its own locator route. The advertisement of the locator route is not in the scope of this document. To speed up fault detection, we may configure an S-BFD session on PE3 to detect PE1 or PE2's reachbility. In traditional mode, a discriminator needs to be assigned by PE1 and PE2, and two S-BFD sessions need to be configured on PE3 to detect the VPN SID's reachability of PE1 and PE2. It needs to generate an S-BFD session with the destination set to the VPN SID. To reduce the number of S-BFD sessions, these sessions can be merged into an S-BFD session which target is remote PE's locator route. There are a large number of such PEs that exist on the network. Each PE is configured with several S-BFD sessions to detect PE1 and PE2, which increases the deployment complexity.

Figure 2 shows a SRv6 Policy scenario. CE1 is dual-homed to PE1 and PE2, and CE2 is accessed to PE3. PE1, PE2, and PE3 are cross BGP ASes. CE1 accesses PE1 and PE2 through Layer 3 and advertises its private network routes to PE1. PE1 encapsulates the routes into Type 5 routes in the EVPN format and sends them to PE3. After receiving Type 5 routes advertised by PE1 and PE2, PE3 generates primary and backup entries for the routes, speeding up service switchover. PE3 parses the tunnel based on the <nexthop, color> of the service routes advertised by PE1 and PE2, and matches an SRv6 Policy. After receiving the traffic from CE2 to CE1, PE3 encapsulates and forwards the traffic based on the SRv6 Policy. An S-BFD session needs to be established for these SRv6 Policy-based forwarding paths to swiftly detect the availability of the paths. When detecting a fault on the SRv6 Policy path of the primary service route, services can be swiftly switched to the backup path, providing more reliable protection for services. There are a large number of such PEs that exist on the network. Each PE is configured with several S-BFD sessions to detect PE1 and PE2, which increases the deployment complexity. Certainly, this scenario may also be implemented in other methods. For example, when delivering an SRv6 policy, specify a tunnel to generate an S-BFD session.

specifies the "BFD Discriminators" (38) attribute, which is an optional transitive BGP attribute that conveys the Discriminators and other optional attributes used to establish BFD sessions. The attribute defined in is used to transmit P2MP BFD session creation information through the BFD Discriminator attribute in MVPN scenarios. For non-multicast services, such as L3VPN services, L2VPN services, and native IP services, BFD discriminators are also required to create an S-BFD session. The S-BFD Discriminator attribute introduced in this document is defined as follows:

o BFD Mode: The BFD Mode field is 1 octet. defines only the P2MP BFD session for MVPN. This document defines two new types of SBFD session types based on the preceding scenarios. As described in the preceding scenario. There are two types of S-BFD sessions for SRv6 services. For service over SRv6 BE, an IP-routed S-BFD session needs to be created to detect the locator route. For service over SRv6 Policy, an S-BFD session for SRv6 Policy path needs to be created to detect the SRv6 Policy path. So two new BFD modes should be introduced here. SBFD for SRv6 Locator Session Mode, which is dedicated to detecting the locator. The temporary type is 176, and is to be allocated by IANA. SBFD for Common Session Mode, which is for general SBFD session. The temporary type is 177, and is to be allocated by IANA. This mode is not only for SRv6, but also can be used for other scenarios. o BFD Discriminators: The field length is 4 octets. Used to specify the discriminator for S-BFD session. o Optional TLVs: Variable-length fields are optional. Indicates the additional information required for creating a S-BFD session. The format is as follows:

If a transit node changes the next hop or reassigns a VPN SID when forwarding a route, the transit node needs to use the locally allocated S-BFD discriminator to advertise the S-BFD discriminator attribute. If the transit node does not recognize the S-BFD Discriminator attribute in the learned route and continues to advertise the route to the remote PE, the receiver may use incorrect information when creating an S-BFD session. Therefore, the advertised S-BFD Discriminator attribute needs to carry the IP address for receiver verification. In this document, S-BFD for SRv6 Locator Session and S-BFD for Common Session must carry IP addresses except discriminators, which reuse the Source IP Address TLV defined in . If the mode is set to SBFD for SRv6 Locator Session, the SRv6 Locator address used for the service is carried. If the mode is set to SBFD for Common Session, the next-hop address used for the service is carried. For details about the error handling, see section "Error Handling".

In BGP address families, such as L3VPN or EVPN, routes can carry the S-BFD Discriminator attribute as required so that S-BFD sessions can be established based on the attribute. The following uses S-BFD for SRv6 Locator as an example. If mode is set to SBFD for Common Session, the processing method is similar.

As shown in figure 1, the S-BFD discriminator is configured on PE1. After obtaining the information, BGP encapsulates the attribute into the EVPN route and sets the BFD Mode to SBFD for Locator Session, when advertising the EVPN route. The Discriminator value is local discriminator value. The optional TLV carries the local PE's locator address used by the VPN.

Here is the end-to-end SRv6 BE scenario. The ASBR does not re-allocate the VPN SID. Thus, the ASBR does not require to modify the VPN SID, and not to alter the BFD discriminator attribute.

After receiving the EVPN Type 5 routes from PE1 and PE2, PE3 imports the routes to the VRF of PE3 based on the route targets. Routes triggers establish the S-BFD sessions based on <S-BFD discriminator, locator ip>. Then, routes with the same prefix from PE1 and PE2 form primary and backup paths. When the primary path or the egress node is in fault, S-BFD detects that fault and forms switch to backup path quickly. To avoid the waste of redundant resources, assume that the ASBR re-assigns the SID in Option B and the ASBR does not recognize the attribute. In this case, the SID and locator carried in the route received by PE3 do not match the Source IP carried in the Optional TLV in the BFD attribute. Therefore, PE3 does not need to establish an S-BFD session to remote PE, which can avoid resource waste.

Error handling complies with . In this document, the BFD discriminator information is used only to establish an S-BFD session. Therefore, if the BFD discriminator information is invalid, the BFD attirbute will be discard and not transmit to other devices. For BFD discriminator attribute, the following case will be processed: o The BFD Discriminator value in receiving BFD Discriminator attribute is 0, the attribute is invalid. For BFD mode type is S-BFD for SRv6 Locator Session, the following case will be processed: o The BFD discriminator attribute doesn't contain optional TLV with type set to 1, the attribute is invalid. o The optional TLV type is 1 but the length is not 16, the attribute is invalid. o The optional TLV type is 1 but the value is all 0, the attribute is invalid. o If multiple Source IP Optional TLVs are carried, the first source IP address should be used as the destination to establish an S-BFD session. For EVPN type 2 MAC-IP routes may use the first and the second IP address because it may carry two SRv6 SIDs with different locators. Other source IP addresses should be ignored. o If a non-Source IP Optional TLV is carried, the Optional TLV will be ignored. For BFD mode type is S-BFD for Common Session, the following case will be processed: o The BFD discriminator attribute doesn't contain optional TLV with type set to 1, the attribute is invalid. o The optional TLV type is 1 but the length is not 4 or 16, the attribute is invalid. o The optional TLV type is 1 but the value is all 0, the attribute is invalid. o If multiple Source IP Optional TLVs are carried, only the first source IP address should be used as the destination to establish an S-BFD session. Other source IP addresses should be ignored. o If a non-Source IP Optional TLV is carried, the Optional TLV will be ignored.

This document defines two new BFD modes in the BFD Discriminator attribute. The following values are recommended to be assigned by IANA:

The new S-BFD Discriminators sub-TLV does not introduce any new security risks for BGP. When creating an S-BFD session, the initiator verifies the S-BFD session based on routing information. This reduces the number of invalid S-BFD sessions and avoid attribute attack.

The authors would like to thank Greg Mirsky for their review and comments.