[ppml] Technical reason why /52,/56,/60,/64 are bad

[michael.dillon at bt.com]

Interesting comment from IETF list which indicates there may be
technical reasons for providers to prefer assigning no longer than /48
to customers at their edge.

> -----Original Message-----
> From: Robin Whittle [mailto:rw at firstpr.com.au] 
> Sent: 18 August 2007 03:47
> To: ietf at ietf.org
> Subject: Re: IPv6 addresses really are scarce after all - /64 
> FIB burden
> 
> I am concerned about DFZ routers needing to have their FIBs look at
> 48 bits of address for every packet sent to one of the new 
> end-user address blocks as allocated by ARIN, AfriNIC and now RIPE.
> 
> But this is up to 16 bits worse:
> 
> > * /64 - Site needing only a single subnet.
> > * /60 - Site with 2-3 subnets initially.
> > * /56 - Site with 4-7 subnets initially.
> > * /52 - Site with 8-15 subnets initially.
> > * /48 - Site with 16+ subnets initially.
> 
> A wide TCAM can handle these address bits in a single 
> clock-cycle, but the high end routers CRS-1, M120 and MX960 
> don't use TCAM in their FIB.  TCAM chews too much power, is 
> too expensive, has long update times and has various other 
> problems.  These high end routers use trie-based algorithms 
> involving lookups into gigabytes of slow, shared, DRAM. This 
> can only be done some number of bits at a time, where that 
> number may by 5 to probably 20 or maybe 24 at the absolute maximum.
> 
>   http://www.firstpr.com.au/ip/sram-ip-forwarding/router-fib/
> 
> Consider a stream of 50 IPv6 VoIP packets a second, each 
> bearing 20 bytes of data.  Let's say it has to go through 16 
> DFZ routers.  With the new /64 allocation size, every packet 
> chews the resources of 20 routers each having to work its way 
> through 64 bits of address, with  RAM lookups.  That's 128 
> bytes of address data to be processed, laboriously, en-route, 
> for a lousy 20 bytes of relatively inconsequential VoIP call 
> - 1/50 second.
> 
> Each VoIP call between hosts in two /64 prefixes with 16 DFZ 
> routers en-route involves those routers collectively working 
> through this many bits:
> 
>   50 * 2 * 64 * 16 = 120,400 bits!!
> 
> This sounds really inefficient, compared to IPv4 in which DFZ 
> routers in practice need to look at 24 bits, at most, of the 
> destination address of IPv4 packets.  Since only about 1.23% 
> of the advertised space is for prefixes of 21 bits or more:
> 
>   http://www.firstpr.com.au/ip/host-density-per-prefix/
> 
> 16 to 20 bits is probably sufficient for most packets.
> 
>  - Robin
> 
>