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What should be "anchor" base navigational links in the Contents section of these docs are all hardwired to reference docs.opendap.org:

-   [<span class="tocnumber">1</span> <span
    class="toctext">Introduction</span>](https://docs.opendap.org/index.php?title=DAP4:_Specification_Volume_2#Introduction)

We need to fix them so that the point to the actual anchors in the document. I think that they can be rewritten as relative links. Which, if true, would be the best solution IMHO.

The spec allows optional checksumming of variable data but as far as I can tell, we never provided a way for a client to tell a server if checksumming should be used on a request.

So I propose the following change to Volume 2, section 5.1 (Query String Parameters) to add the following new query parameter "dap4.checksum".

The possible cases are defined as follows:

1. "dap4.checksum=true" -- add checksums
2. "dap4.checksum=false" -- suppress checksums
3. "dap4.checksum=" -- same as "dap4.checksum=true"
4. "dap4.checksum" -- same as "dap4.checksum=true"
5. missing (i.e. no "dap4.checksum" specified) -- use the default.

Nathan noted:

"...I think we should also allow for servers that ALWAYS include checksums, i.e. even if the client does not request the checksums the presence of the checksums in the response should not break the clients handling of the response...

I assume that there is no hope for fixing these servers. So I can see three ways of handling this:

1. A short term solution is to make the default be "dap4.checksum=true" when no dap4.checksum query is present (case 5 above). This will work until such time
   as clients start including specific dap4.checksum query parameters. At that point, accessing such a server will fail if the user specifies checksums are off.

2. We can identify the relevant servers and stick a hack in our libraries to force "dap4.checksum=true" when requests are sent to those servers. The DAP2 code we have already does things like this for non-compliant servers like Columbia. Its ugly, but it works. [Nathan, can you create a list of such servers?]

3. The client always computes the checksum as it reads the variable's data off the socket stream. Then it needs to see if there is an extra four bytes of data at the end of the variable's data. I do not know if this is unambiguously possible. And of course, it is a performance hit.