qMp (Quick Mesh Project) - Packages

h1. Auto-assignment of IPv6 host-oriented prefixes and collision estimation

One of the desired features is qmp nodes are able to offer native IPv6 addressing to final hosts. IPv6 based networks usually assign /64 prefix to each host-oriented interface, subnetted from a bigger /48. This implies the nodes have 2^16 possible /64 prefixes to auto-assign.

If we want to use MAC address numbering and grant the non-coincidence of auto-generated prefixes, we'll need a mapping function from 48 bits of MAC to 48 bits of IPv6 prefix. According to this auto-configure addressing philosophy, we deduce that is impossible to avoid the election of the same prefix /64 (collision) by one or more network interfaces.

Thus the collision probability is greater than 0 if more than one network interface is auto-addressed. We

could calculate this probability and then estimate the maximum number of network interfaces that can be auto-addresses with 

a collision probability less than certain value.

Let the last l LSB of a MAC address, where 1<=l<=24. We left the first 24 bits from OUI. Thus we have a k=2^l possible endings. If 

we have in the network N different OUI, we'll have p=N*2^(24-l) possible MAC addresses for each possible ending. Obviously n=k*p is the 

total space of possible MAC present in that network. If the network randomly includes two or more MAC with the same l-bit ending, we'll have a collision, i.e. the same IPv6 /64 prefix will be auto-assigned on different network interfaces.

If we select randomly m (m < k) interfaces (MAC addresses) from N OUI, we obtain 

  n!

------

m! n-m!

different combinations without replacement. This set of m MAC addresses will contain a subset of combinations where all its MAC addresses have different l-bits ending. These combinations will generate /64 prefixes without collision. The number of such combinations is

  k!

------- * p^m

m! k-m! 

thus the non-collision probability is

          k! n-m!

P_nc(m) = -- ---- * p ^m

          n! k-m!

If only one interface is present in the network P_nc(1) = kp/n = 1, i.e. the collision is impossible.

We can now calculate the maximum number M of interfaces such that the probability of collision is less than certain value. The following table

shows this number for a maximum collision probability of 4%.

|_.Prefix size |_.l |_.M for N=2 |_.M for N=20 |_.M for N=200 |

|/51	|13 	|27		|27		|27|

|/50	|14 	|38		|38		|38|

|/49	|15 	|53		|53		|53|

|/48	|16 	|74		|74		|74|

|/47	|17 	|105		|104		|104|

|/46	|18 	|148		|147		|147|

|/45	|19 	|210		|208		|208|

|/44	|20 	|298		|294		|294|

|/43	|21 	|428		|416		|415|

|/42	|22 	|627		|520		|587|

|/41	|23 	|957		|839		|830|

|/40	|24 	|1656		|1202		|1174|

We note that the variation of M with the number of OUI is very small. Thus the maximum number of interfaces in a /48 network with a collision

probability 4% is 74.