Recommendation for Prefix Binding in the Softwire DS-Lite Context

IPv6 deployment models assumes IPv6 prefixes are delegated by Service Providers to the connected CPEs (Customer Premise Equipments) or hosts, which in their turn derive IPv6 addresses out of that prefix. In the case of DS-Lite , the B4 element derives an address for the softwire setup purposes. A B4 element might obtain a new external IPv6 address, for a variety of reasons including a reboot of the CPE, power outage, DHCP lease expiry, or other action undertaken by the Service Provider. If this occurs, traffic forwarded to a B4's previous address might be delivered to another B4 that now acquired that address. This affects all mapping types, whether implicit (e.g., by sending a TCP SYN) or explicit (e.g., using PCP ). The main goal of this document is to propose a recommendation to soften the impact of such renumbering issues. Note that in some deployments, CPE renumbering may be require to accommodate some privacy-related requirements to avoid the same prefix be assigned to the same customers. It is out of scope of this document to discuss such contexts. This document complements .

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

Since the network behind B4 can be overlapping across multiple CPEs, B4 address plays a key role in identifying associated resources assigned for each of the connections. These resources maintain state of EIM, EIF, APP, and PCP mappings and flows. However, there can be change in B4 address for any reason, may be because of change in CPE device or may be because of security extensions enabled in generating the IPv6 address. When the address change, the associated mappings created in the AFTR are no more valid. This may result in creation of new set of mappings. ISPs may want to limit the usage of these resources on per subscriber basis for fair usage of resources. To do so, a subscriber is identified by an IPv6 prefix mask (i.e., the length of the prefixes assigned to customers, for example /56 or /48). These policies are used for dimensioning purposes and also to ensure that AFTR resources are not exhausted. However, this policy doesn't resolve stale mappings hanging around in the system, consuming not only system resources, but also reducing the available quota of resources per subscriber. When services are hosted behind B4 element, these services has to advertise about their change, when ever there is a change of the B4 address. Means to discover the change of B4 address are therefore required. Clearing those mappings can be envisaged, but that will causes a lot of churn in the AFTR, and it doesn't address the latency issue where a service has to advertise its new assigned external IP address and port and the clients have to consume and re-initiate connections. PCP-specific failure scenarios are discussed in .

In order to mitigate the issues discussed in , the following recommendations are made: A policy SHOULD enforced at the AFTR level to limit the amount of active softwires per subscriber. The default value MUST be 1. Resource contexts created at the AFTR level SHOULD be based on the prefix mask (or the prefix if it is explicitly configured), and not based on the derived B4 address. Administrators SHOULD configure per prefix limits of resource usage, instead of per tunnel limits. These resources include, number of flows, mappings including PCP, NAT pool resources, etc. In the event a new IPv6 address is assigned to B4, the AFTR SHOULD migrate existing state to be bound to the new B4's IP address. This ensures the traffic destined to the previous IPv6 address will redirected to the new IPv6 address. The destination address for tunneling return traffic SHOULD be the last seen address from the CPE. The source address of the tunnel would remain same as AFTR. In the event of change of the CPE WAN IPv6 prefix, unsolicited PCP ANNOUNCE messages SHOULD be sent by the B4 element to internal hosts to update their mappings. When a new prefix is assigned to the CPE, stale mappings may exist in the AFTR. This will consume both implicit and explicit resources. In order to avoid such issues, stable IPv6 prefix are RECOMMENDED. In case for any reason a prefix has to be reassigned, it is RECOMMENDED to reassign a prefix only when all the resources in use associated with that prefix are cleared from the AFTR.

Security considerations related to DS-Lite are discussed in .

This document does not require any action from IANA.

G. Krishna reviewed document and provided useful comments