DS-Lite

Stands for: Dual-Stack Lite

A broadband design that carries subscribers' IPv4 inside IPv6 to a carrier NAT, so the access network can be IPv6-only.

Dual-Stack Lite (RFC 6333) gives each home native IPv6 plus IPv4 tunnelled to the provider's AFTR and translated there. RFC 9313 reports it as the most common of the IPv4-as-a-service methods in broadband, and it brings all the logging and attribution issues of carrier-grade NAT.

The customer's B4 element wraps each IPv4 packet in IPv6, IP protocol 4, without changing its address family; the AFTR, an IPv4-in-IPv6 tunnel endpoint and an IPv4-to-IPv4 NAT on one node, unwraps it and translates it to a shared public address. Because no family translation happens, applications that carry IPv4 literals in their payloads are much simpler to support. RFC 6333 numbers the tunnel ends from 192.0.0.0/29: 192.0.0.1 for the AFTR, 192.0.0.2 for the B4.

Since many subscribers share the same inside address, the AFTR tells them apart by the B4's IPv6 address. RFC 6888 applies paired pooling per subscriber rather than per inside address, and names that IPv6 address as the subscriber identifier in the CGN's logs; RFC 7422's deterministic mapping recovers it but loses the inner IPv4 address.

Encapsulation costs 40 bytes: on a 1500-byte path the inner IPv4 MTU is 1460 and the IPv4 MSS 1420, and RFC 6333 asks operators to raise link MTUs, since path MTU discovery is not reliable here. On Linux a B4 tunnel uses mode ipip6. Lightweight 4over6 moves the NAT into the customer's equipment to remove per-flow state from the provider.

Also known as: ds lite, dual-stack lite, dslite, aftr, b4, rfc 6333

Sources

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