DLT and LINKTYPE allocation

===========================

DLT_ types live in pcap/dlt.h.  They can be requested by the community on a

First-Come First-Served basis [i.e. https://tools.ietf.org/html/rfc8126#section-4.4 ]

(Although libpcap is not at this time an IETF specification, there have been

some as yet-incomplete efforts to do this).

The Tcpdump Group prefers to link to an open specification on the new DLT_

type,  but they are available for closed, proprietary projects as well.

In that case, a stable email address suffices so that someone who finds

an unknown DLT_ type can investigate.

We prefer to give out unambiguous numbers, and we try to do it as quickly

as possible, but DLT_USERx is available while you wait.

Note that DLT_ types are, in theory, private to the capture mechanism and can

in some cases be operating system specific, and so a second set of values,

LINKTYPE_ is allocated for actually writing to pcap files.  As much as

possible going forward, the DLT_ and LINKTYPE_ value are identical, however,

this was not always the case.  See pcap-common.c.

The LINKTYPE_ values are not exported, but are in pcap-common.c only.

DEVELOPER NOTES

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When allocating a new DLT_ value, a corresponding value needs to be

added to pcap-common.c.

It is not necessary to copy the comments from dlt.h to pcap-common.c.