10. IPv6 Transition II & Coexistence with IPv4

Question 1

IPv6 only scenarios

Answer 1

1. An IPv6 network deployed for a new application area
   - eg large-scale sensor networks
2. An IPv6-only network may consider changing to IPv6
   - unlikely
3. Existing dual-stack network may consider removing support for IPv4 to become IPv6 only
   - whether this reduces complexity is an open question
   - the complexity is pushed to the edge (NAT64)
4. Some IPv6 only devices may be introduced into an existing dual stack network (possible and quite likely)

Question 2

Other translation scenarios

Answer 2

In an IPv6-only network, some elements may not be able to run IPv6 for various reasons:
- legacy applications that cannot be ported to support IPv6
- Legacy IPv4-only operating systems Win98/NT/XP
Legacy IPv4-only hardware - printers

Question 3

Translation

Answer 3

For IPv4-only -> IPv6only comms - translation required
Can be done at various layers:
- network layer (translation/mapping of the fields in the IP headers, somehow)
- Transport layer (using a dual stack TCP relay)
- Application layer ( use a dual-stack application layer gateway (ALG)

Question 4

Translation in the Network layer - NAT64

Answer 4

• Network Address and protocol translation (like IPv4 NAt but with protocol translation)
• uses IP/ICMP Translation algorithm (defines algorithms to translate between the IPv4 and IPv6 header fields, where possible
• Designed for IPv6 client to IPv4 server access (this is the most likely translation scenario)
• Relies on dual stack NAT64 device, on your network edge
  (also requires use of DNS64, it works thanks to a specific DNS proxy trick)

Question 5

DNS64 trick

Answer 5

A prefix from the IPv6 site prefix is reserved for NAT64
A DNS64 proxy is used to look up external sites
- If an IPv6 client uses DNS to look up an IPv4-only site, the DNS proxy returns the result in the form:
:
- the IPv6 client then sends packets to this address, which is routed via the NAT64 device (the NAT64 device recognises the NAT64 prefix, translates the special IPv6 address and handles the reverse mapping of return traffic

Question 6

NAT64 downsides

Answer 6

Similar shortcomings of IPv4 NAT;

• need state to be held in the NAT64 device
• need to handle IP addresses embedded in packet payload eg FTP
• Can’t translate all headers fully
• Stuck if application uses IPv4 address literals not DNS name

No significant deployment experience yet

Question 7

Application layer ALGs

Answer 7

Application Layer Gateways or proxies offer a simple alternative to network layer translation
Many applications already support ALGs
 - Web cache
 - SMPT relay
 - DNS resolver
 - irc server etc
In many cases you can proxy at the application layer
Not all applications support this