Can't understand uTorrent Transfer Protocol 'extension' option in header - tcp

Question

i'm trying to understand how a uTP header is structured and how it handles traffic and congestion and all that.
While going through the documentation here i couldn't quite wrap my head around what the 'extension' portion of the header worked. Here is the header:
0 4 8 16 24 32
+-------+-------+---------------+---------------+---------------+
| type | ver | extension | connection_id |
+-------+-------+---------------+---------------+---------------+
| timestamp_microseconds |
+---------------+---------------+---------------+---------------+
| timestamp_difference_microseconds |
+---------------+---------------+---------------+---------------+
| wnd_size |
+---------------+---------------+---------------+---------------+
| seq_nr | ack_nr |
+---------------+---------------+---------------+---------------+
And this is the portion talking about the 'extension' field:
extension
The type of the first extension in a linked list of extension headers. 0 means no extension.
There is currently one extension:
Selective acks
Extensions are linked, just like TCP options. If the extension field is non-zero, immediately following the uTP header are two bytes:
0 8 16
+---------------+---------------+
| extension | len |
+---------------+---------------+
where extension specifies the type of the next extension in the linked list, 0 terminates the list. And len specifies the number of bytes of this extension. Unknown extensions can be skipped by simply advancing len bytes.
How do linked lists come into all of this?
It is (to me) implying further available extensions?

Answer 1

The packet header contains a descriptor (1 byte) for the first extension header or a sentinel value of 0.
The first extension in turn would contain an extension header (this time 2 bytes since it includes a length) and the extension body. The header would again point to the next extension or be a sentinel value.
It is a linked list because the Nth extension header links to the N+1th extension.
This is a common pattern to insert a variable amount of extensions between a fixed-layout packet header and the main payload. E.g. IPv6 and TCP use very similar mechanisms.