Per Hop Behaviors Based on Dynamic Packet State
draft-stoica-diffserv-dps-02

DNP Note sent to RFC Editor 2003-02-05:
Sandy, The IESG requests that Per Hop Behaviors Based on Dynamic Packet State
NOT be published as an Experimental Protocol. This document includes the following:
> With Dynamic Packet State, each packet carries in its header, in
> addition to the PHB codepoint, some PHB-specific state. The state
> is initialized by an ingress node, when the packet enters the DS
> domain. Interior nodes process each incoming packet based on the
> state carried in the packet's header, updating both its internal
> state and the state in the packet's header before forwarding it.

This is a potentially serious violation of the end-to-end properties of IP, as it proposes reusing existing IP header fields for other purposes than intended. Use of these techniques can result in incorrect processing of IP packets as a result of information loss occurring when existing IP header fields are reused for alternative purposes.

> By using DPS to coordinate actions of edge and interior nodes
> along the path traversed by a flow, distributed algorithms can
> be designed to approximate the behavior of a broad class of
> "stateful" networks.

This scheme can only be safely used if packet fields are restored properly in all cases. However, it is easy to imagine scenarios where this property will not be honored, and the result will be a less reliable Internet. Deployment of such systems have the potential for causing serious damage to the Internet.

> In this respect, we observe that there is an ip_off field in the IPv4
> header to support packet fragmentation/reassembly which is rarely
> used. For example, by analyzing the traces of over 1.7 million packets
> on an OC-3 link [nlanr], we found that less than 0.22% of all packets
> were fragments. In addition, ther are a relatively small number of
> distinct fragment sizes. Therefore, it is possible to use a fraction
> of ip_off field to encode the fragment sizes, and the remaining bits
> to encode DPS information. The idea can be implemented as follows.
> When a packet arrives at an ingress node, the node checks whether a
> packet is a fragment or needs to be fragmented. If neither of these
> is true, all 13 bits of the ip_off field in the packet header will be
> used to encode DPS values. If the packet is a fragment, the fragment
> size is recoded into a more efficient representation and the rest of
> the bits is used to encode the DPS information. The fragment size field
> will be restored at the egress node.

As a specific example, the above will fail in some circumstances. This would be unacceptable for general or wide deployment and cannot be recommended. Given the potentially serious negative consequences of deployments of the techniques described in the document, the IESG does not believe this document should be published as an RFC.

        Thomas Narten

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State Changes to Pre AD Evaluation                                from AD to write --Don't publish                      by Scott Bradner