ISATAP

Packet-in-packet technology
Mostly historical
Used within one site, not an Internet Technology
“v6 islands within a v4 ocean”
Treats the v4 network as NBMA
ISATAP devices must be dual stacked
Adds 20-bytes of overhead
Works as long as v4 routing works
Stateless

Theory

Use v4 connectivity to emulate a NBMA network. Wrap the v6 packet inside v4.

The routers perform ISATAP

         ┌─ v6 only ─┐         ┌────────────  v4 only ───────────────┐         ┌─ v6 only ─┐
         ▼           ▼         ▼                                     ▼         ▼           ▼
┌────────┐           ┌─────────┐            ┌─────────┐              ┌─────────┐           ┌────────┐
│client-1│           │    R1   │            │   R2    │              │    R3   │           │client-2│
│   v6   ├───────────┤  v4/v6  ├────────────┤    v4   ├──────────────│  v4/v6  ├───────────┤   v6   │
└────────┘           └─────────┘            └─────────┘              └─────────┘           └────────┘
                             ISATAP                              ISATAP

                              Interface                           Interface

                           ┌───────────┬───────────┐
                           │ v6-packet │ v4-header │ ───────►
                           └───────────┴───────────┘

The clients perform ISATAP

           ┌──────────────────────────────────  v4 only  ───────────────────────────────────┐
           ▼                                                                                ▼
  ┌────────┐           ┌─────────┐            ┌─────────┐              ┌─────────┐          ┌────────┐
  │client-1│           │   R1    │            │   R2    │              │   R3    │          │client-2│
  │ v4/v6  ├───────────┤    v4   ├────────────┤    v4   ├──────────────┤    v4   │──────────┤  v4/v6 │
  └────────┘           └─────────┘            └─────────┘              └─────────┘          └────────┘
        ISATAP                                                                        ISATAP

         Interface                                                                     Interface

        ┌───────────┬───────────┐
        │ v6-packet │ v4-header │ ────────►
        └───────────┴───────────┘

ISATAP Interface in v6

All ISATAP interfaces look like this. The giveaway is 0000:5EFE in the host portion.

If a router sees a v6 destination that looks like this, it’s an ISATAP packet.

                    64 bits                              32 bits                   32 bits

┌───────────────────────────────────────────────┬───────────────────────────┬──────────────────────┐
│  Global Unicast Prefix                        │        0000:5EFE          │  IPv4 of ISATAP Link │
└───────────────────────────────────────────────┴───────────────────────────┴──────────────────────┘