1. Simple Failover Mechanism for Lightweight 4over6
draft-lee-softwire-lw4over6-failover-01
Qi Sun (Presenter)

2. Background lw4over6 is a softwire based IPv6 transition technology
lw4over6 is a stateful technology
lwAFTR maintains a per-subscriber binding table
When the Primary lwAFTR fails, the Backup lwAFTR may not have the necessary state to provide softwire service for the lwB4.
This draft discusses lw4over6 failover design.

3. Use Case Assumptions
A set of lwAFTRs using the same IPv6 Anycast address
IPv4 provisioning used by lwB4 is dynamic (via dhcpv4 over dhcpv6 or pcp)
Primary lwAFTR creates the dynamic binding upon successful lwB4 provisioning
IPv4 address used by the lwB4
either static or dynamic
either full or port-restricted
Shared address cannot be interface addr. ARP
static configuration for address prefix in enterprise case

4. Typical Failover Use Case
Primary lwAFTR fails
lwB4 continues to send packets to the anycast address
Backup lwAFTR receives the packets, then what to do?
Backup lwAFTR
Primary lwAFTR
lwB4
IPv6 anycast

5. We propose two mechanisms
ICMPv6 Trigger Failover
Backup
lwAFTR
Primary
lwB4
Characteristics
Modify lwB4
Original connections would fail
lwAFTRs can advise different v4 prefixes
Comms not needed between lwAFTRs
IPv4-in-IPv6 packet
When lwAFTR receives a packet with an unknown IPv6 source address, it will send an ICMPv6 Error Message which will trigger the lwB4 to re-send IPv4 provisioning message to create the binding in the Backup lwAFTR
Unknown Source IPv6
ICMPv6 Error Message
Restart IPv4 provisioning
Create binding table
IPv4-in-IPv6 packets

6. We propose two mechanisms
Data Packet Trigger Failover
Backup
lwAFTR
Primary
lwB4
Characteristics
Not modify lwB4
Original connections would not fail
lwAFTRs advise the same v4 prefixe
Require additional validation method(s)
IPv4-in-IPv6 packet
When lwAFTR receives a packet with an unknown IPv6 source address, lwAFTR will extract the IPv4 address and validate the IPv4 authentication (e.g., lease query). Upon successful authentication, the Backup lwAFTR will create the binding
Unknown Source IPv6
Validate the IPv4 authentication;
Create binding
IPv4-in-IPv6 packets

7. Comparison with two mechanisms
Control Message Trigger
Data Packet Trigger
Address pool Configuration
Primary and Backup lwAFTR may use the same address pool or different IPv4 address pools
Primary and Backup lwAFTR must use the same IPv4 address pool
Impact on applications
PCP: No guarantee same public v4 addr; can not be used for external-initiated applications.
DHCP: Can use same public v4 addr; also can be used for external-initiated applications.
Use the same public IPv4 address. It can also be used for external-initiated applications.
Implementation requirement
Implement the ICMP mechanism in lwB4 and lwAFTR
Require an interface between lwAFTR and the supporting system to check the validity of the binding.

8. Next Step
Comments?
Any interest to continue the work?

9. Backup: Some Considerations
Backup lwAFTR Discovery
Use Anycast
Recommend to hash the outer v6 header for ECMP
lwAFTR must stop announcing the anycast addr when offline
lwB4 IPv4 Prefix Announcement on lwAFTR Management
Dynamic IPv4 address
No special action. Each lwAFTR advertises its own IPv4 prefix. Different IPv4 prefixes
Static IPv4 address
Primary and Backup lwAFTRs will announce the same IPv4 prefix given that the Backup lwAFTR will use a lower matrix
lwB4 Address Provisioning
DHCPv4 over DHCPv6
DHCP server: Centralized
lwAFTRs: DHCPv6 relay, unicast to DHCP server
PCP
PCP server: Collocate with lwAFTR
Use IPv4-in-IPv6 tunnel; hash on the outer IPv6 header if ECMP is used
IPv6 address is 4 times longer than IPv4 address, it guarantees way better uniqueness for the hash.
lwB4 v4 prefix management (static): run dynamic routing protocol
1. Primary and Backup lwAFTRs are running dynamic routing protocol
2. Primary lwAFTR set the routing matrix higher than the Backup lwAFTR does for the serving IPv4 prefix
3. When Primary detects problem, it stops advertising the IPv4 prefix
4. Routing protocol converges and the Backup lwAFTR is the router announcing the IPv4 prefix