IPv6 Links

Goals of IPv6

The IPv6 Packet and Header Format

Only eight fields, not fifteen like before.  They are
  1. Version (4 bits):  Always a 6
  2. Priorty (4 bits):  How important is this traffic.  0-7 are transmissions that can slow down for congestion, 8-15 are real time constant speed traffic.  Higher is more important.  But why would anyone be honest?  Wouldn't you get better performance if you always claim priority 15?  Yes, but your network administartor will hunt you down.
  3. Flow label 3 bytes):  Experimental, but probably will be used to make bandwisth and latency reservations.  Zero is no special treatment.  Otherwise numbers will be assigned randomly, and not start at one, for better hashing.  Clever!
  4. Payload length (2 bytes):  Length of everything except the header.  Weirdly, the IPv4 length field DID include the header.
  5. Next Header (1 byte):  Describes which of currently 6 optional headers follows.  The IPv6 header is smaller because the weird cases can have optional extra headers, and the normal case can omit them.  For the last header, this field describes the type of payload (TCP, UDP, etc).
  6. Hop Limit (1 byte):  Just like Time to live in IPv4, except IPv4 claimed that TTL was in seconds (even though no router did that) and IPv6 admits it's hops.
  7. Source Address (16 bytes):  Where from
  8. Destination Address (16 bytes):  Where to.
  9. (Possibly other optional headers)
  10. (The data)
  11. (The CRC)
Interesting is what is omitted
  1. Header Checksum (subsumed by overall checksum)
  2. Protocol (subsumed by Next Header)
  3. Fragement Offset (now an optional header)
  4. Header Length (The header is now a fixed size, so no length field needed)
  5. Don't Fragement (Fragementing not allowed!)
  6. More Fragements

Size of the IPv6 Address Space.

The IPv6 adress space allows for 2^128 addresses.  That's 340,282,366,920,938,463,463,374,607,431,768,211,456 addresses.  Or more concretely, that's 665,570,793,348,866,943,898,599 (~ 665 million quadrillion) addresses per square meter of the whole earth (including the water).  Really a huge number indeed!!  But not all of these addresses will be useable.  In fact LOTS will be wasted by the adress asignment scheme.  If the scheme is as good as the French and US telephone system, IPv4, and IEEE 802 node assignments systems, there will be between 1564 and 3,911,873,538,269,506,102 (3.9 million quadrillion) addresses per square meter.

Currently, IPv4 was scheduled to run out in 2010.  But network address translation has helped that ALOT.

Allocation of Addresses

         Allocation                      Prefix                 Fraction of 
                                         (binary)               Address Space

 Reserved                               0000 0000               1/256
 Unassigned                             0000 0001               1/256
 Reserved for NSAP Allocation           0000 001                1/128
 Reserved for IPX Allocation            0000 010                1/128
 Unassigned                             0000 011                1/128
 Unassigned                             0000 1                  1/32
 Unassigned                             0001                    1/16
 Unassigned                             001                     1/8
 Provider-Based Unicast Address         010                     1/8
 Unassigned                             011                     1/8
 Reserved for Geographic-based          100                     1/8
        Unicast Addresses
 Unassigned                             101                     1/8 
 Unassigned                             110                     1/8
 Unassigned                             1110                    1/16
 Unassigned                             1111 0                  1/32
 Unassigned                             1111 10                 1/64
 Unassigned                             1111 110                1/128
 Unassigned                             1111 1110 0             1/512
 Link Local Use Addresses               1111 1110 10            1/1024
 Site Local Use Addresses               1111 1110 11            1/1024
 Multicast Addresses                    1111 1111               1/256


Provider Based Addresses allow for 32 registries to have their own address space, and hand it out as they see fit.  They could give a chunck out to each telcom company that asks.  But what if you switch companies?
Geographic Based Addresses does the same scheme for location based addresses.  But what if you move?
Multicast addresses have a bit within them that distinguished permenant from temporary groups.  They have 4 bits that distinguish 'scope', incluing link site organization and planet.  The other levels are currently undefined.  Interstingly, planet is level 14, so 15 must be extra-planetary.

The Extra Headers

Interestingly, the 'type' field of every optional header tells the router what to do it it's never heard of it.  The first two bits tell an otherwise clueless router to skip the option, drop the packet, send an ICMP error, or send ICMP error unless it's multicast.
 
Hop-by-hop Support information needed by routers along the way.  The only current one is jumbo-payload (length more than 64K)
Routing Offer a loose or strict source route.  Only 24 addresses can be listed (which takes 24 * 16 bytes.
Fragementation Only the source can fragement.  This describes the fragement offset, etc.
Authentication Verifies between two parties that have already exchanged keys that the contents really came from the source.  Does this by computing a checksome of the payload, non-variable parts of the header, and the secret key.
Encryption Encrypts the payload with an already agreed upon key
Destination Options for the destination.  No ones though of any yet, but their ready when it happens

Encryption and Authentication

Each host has a set of secret keys, and each key is assigned a 24bit number.  At least one key must be secretly given to every person you want to use encryption or authentication with.  IPv6 does not say how to do this, but using RSA seems an obvious idea.

To authenticate, construct a packet (including the authentication header) with the variable portion of the header set to zero, padded to a 16 byte boundrey.  Compute a CRC on this.  Fill this CRC into the authentication header.  Send it.  Receiver verifies they got the same CRC.

Questions:  What are the variable parts of the header?  Why treat them differently?  Why is this different than the normal CRC, which protects against errors?  How do routers in the middle, which don't know the key, deal with this header?

To encrypt:  Encrypt the data with the secret key.  Add to the header the encrytion option, describing the key used.  Send it.

Question:  What algorithm should be used to encrypt and CRC?  The default is MD5, but others can be used.  Would you really trust it, or would you rather each application role it's own?

Controversies

Hop length field:  Do we really want to limit the diameter of the internet to 255.  Do we really want packets roaming for 2^16 hops?
Maximum Packet Length:  Is 64K enough in a world with super-links and huge sound/video/smell files (and supercomputers).  Also, when a 64K packet hits a slow link, won't everyone notice a serious delay?
Encryption:  Some nations (France did and Iraq does) have laws prohibiting their citizens from using serious encryption.  Other nations (US) prhibit the export of such encryption.  What if IPv6 is banned in France and the source cannot be exported from the U.S.
Mobile Hosts:  They still don't know how to do it, but they have a set of addresses (and header options) to support it when it happens.

The Switch

All IPv4 hosts get an IPv6 address of 0:0:0:0:0:0:0:0:0:0:0:0:aaa.bb.ccc.dd
There is a standard way to encode an IPv6 packet within an IPv4 packet.
All routers for now can speak IPv4, even the IPv6 capable ones.
DNS servers need to be upgraded to handle the large addresses.
However, no IPv4 host will be able to see (or speak) to an IPv6 host when the IPv6 host has a non IPv4 address.

Interesting Notes

The Linux version uses a routing deamon using Distance-Vector
They are adding several new tunnels to the 6bone daily (and still routing the 6bone by hand)