[Ippm-ioam-ix-dt] Proposed change to the draft

[Haoyu song <[e-mail]>]

Hi Team,
Below is my proposed change. I tried to directly update in Github but the format is not correct, so Tal please help to edit the text and merge it into the current draft. Thanks!
Haoyu
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To help correlate and order the exported postcard packets for the same original packet, it is possible to include a 1-byte Hop_Count field in the DEX header (presumably by claiming some space from the Flags field), which starts from 0 and increments its value by one at every IOAM-enabled hop. The Hop_Count field value needs to be included in the exported postcard packet. An issue of this field is that it needs to be updated at every IOAM-enabled hop. Without it, the DEX header is basically read-only except that it will be inserted at a head node and removed at an end node.
An alternative approach is to request to collect the Hop_Lim/Node_ID data by setting the corresponding bit in the IOAM-Trace-Type bitmap.  The Hop_Lim data is acquired from the lower level protocol header such as TTL for IPv4 and Hop Limit for IPv6. In addition to requiring extra packet parsing, Hop_Lim must be coupled with Node_ID which means 4 bytes are needed to be exported. More important, Hop_Lim is not exactly equivalent to Hop_Count, because Hop_Lim is updated at every hop, whether the hop is IOAM-enabled or not. A consequence is, by monitoring Hop_Lim only, if some value is missing, one cannot tell whether or not an exported postcard packet is missing.
Therefore, further discussion is needed to decide if the DEX header should include an explicit Hop_Count field, or by default, set the IOAM-Trace-Type bit to collect the Hop_Lim/Node_ID data field.