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optional.md

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                             **A warm welcome to DNS**
 
-# EDNS, DNAME, TSIG, Dynamic Updates, DNS Cookies
+# EDNS, Dynamic Updates, TSIG, DNAME, DNS Cookies & more
+So far we've focussed on the simplest possible form of DNS that is
+interoperable with today's internet. Over the past 3 decades however, a lot
+has been added to DNS however.
+
+In this document, we will cover:
+
+ * EDNS: Extra fields carried in the additional section of a DNS message,
+   including arbitrary options. The main use of EDNS today is specifying a
+   larger supported UDP packet size, indicating DNSSEC support and carrying
+   Client Subnet information. Defined in [RFC
+   2671](https://tools.ietf.org/html/rfc2671), which also specifies several
+   deprecated innovations, like additional label types.
+ * Dynamic Updates: Transmitting changes to zones to master servers. Mostly
+   used by DHCP servers to publish names of hosts. Defined in [RFC
+   2136](https://tools.ietf.org/html/rfc2136)
+ * TSIG: Secret Key Transaction Authentication for DNS. Ways to sign DNS
+   messages or a list of DNS messages with a secret key. Used to authenticate
+   AXFR requests and to guarantee zone integrity during AXFR. Defined in
+   [RFC 2845](https://tools.ietf.org/html/rfc2136).
+ * EDNS Client Subnet:
+ * DNAME:  ...
+ * DNS Cookies:  ...
+
+Resolvers can safely ignore dynamic updates and TSIG as they are not
+applicable to caches. 
+
+EDNS is very much an enabling technology and it can't really be regarded as
+optional anymore.  It enables DNSSEC, DNS Cookies, EDNS Client Subnet as
+well as larger UDP packets.
+
+
 
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