Is a 15-hop TTL anywhere near long enough, though? Given that we've just established that it's quite possible for a quick jaunt to TradeMe to take one 21 hops, how can 4-bit TTLs be realistic? On Wed, 2 Mar 2005, Jim Fleming wrote:

The Impact of the Reworked TTL Field on Regional IP Allocations

The TTL Field (or Hop Count) is normally 8-bits wide. Some operating systems start it with a value of 255 (all 1s) and some with 64 and other smaller values.

In order to improve services for broad-band users, there is a benefit in **reducing** the size of the TTL Field from 8-bits to 4-bits. That limits the maximum number of hops to 15. Equipment that auto-encapsulates can work around this by moving a packet a long distance and then removing a wrapper and allowing it to have some of the hops near the source and some near the destination.

Reducing the TTL Field to 4-bits frees up 4-bits for address expansion. With two of those bits for the Source and two bits for the Destination, the entire legacy Internet plus three more equal size address spaces can be supported, without changing the existing protocol header size. One could view that as a planar Internet or a 4-story office building. Existing allocations of address space could all be viewed as from one plane or one could map regions to the 4 planes, allowing a region to expand to space used by another region because the extra bits prevent collisions and provide unique allocations.

Another approach is to use the 4 planes for local networks, VPNs, a wireless extranet, and the legacy Internet (core). In a restricted approach only intra-plane traffic is allowed. A totally open approach would allow any node to address any other node. This would be like any location in the 4-story office being able to connect with any other, rather then be restricted to intra-floor-only connections.

A more conservative approach is simply to zero the left-most 4 bits and reserve (save) the other three planes for future generations, once equipment is better able to route and process the reworked TTL Field.

Current market trials and tests with broad-band-only subscribers will help to determine which approach has the most benefit. IP address allocations are already being red-lined for re-allocation and use in other planar nets because they can not be reached in 15 hops from/to certain regions of the world. Since there may not be any bandwidth to those regions anyway, it does not matter because the laws of physics will prevent broadband services from becoming available in one region with the same level of service observed in another region.

Voice services are of course "the killer apps" and place great demands on the aging best-effort transports. As new equipment is provisioned it can be done with an awareness of the benefits of using a smaller TTL Field. Some might argue that 15 hops is stilll too many and one or two more bits should be removed leaving a 7 hop or a 3 hop limit. The 7 hop approach only frees up 1 more bit and it can be combined with the unused 49th bit to add another entire bit to both the source and destination fields. The 3 hop approach makes one look sooner at the needed equipment to bypass the aging legacy core. Having 6 added bits for addressing and 8 planar levels can have more long-term benefits and there can still be a short-term work-around because the 3-bits of the destination address field can be allowed to be processed by *existing* TTL algorithms and packets then transform their addressing as they pass thru the core network. The address bits can then be used to prioritize delivery by allowing packets that come from a long distance to be processed before those that come from closer locations. They appear to be going to different destinations by looking at the changed TTL value as an address.

Since packets now travel greater distances with less decrements of the hop-count the 7 hop approach may be the best compromise. Eventually, the free-market place will decide. People can not make "governance" decisions based on 20 year old designs and how they are applied in regions of the world that have no bandwidth. That is the lowest-common-denominator approach advocated by The Big Lie Society. As people have seen, that prevents a nation from Running at the speed of light.

Jim Fleming A.Net

_______________________________________________ NZNOG mailing list NZNOG(a)list.waikato.ac.nz http://list.waikato.ac.nz/mailman/listinfo/nznog

-- Matthew Poole "Don't use force. Get a bigger hammer."