Add explanation of restrictions in constrained environments

draft-ietf-core-dns-over-coap.md

@@ -94,6 +94,13 @@ The application use case of DoC is inspired by DNS over HTTPS {{-doh}}
 (DoH). DoC, however, aims for the deployment in the constrained Internet of
 Things (IoT), which usually conflicts with the requirements introduced by
 HTTPS.
+Constrained IoT devices may be restricted in memory, power consumption,
+link layer frame sizes, throughput, and latency. They may
+only have a handful kilobyte of both RAM and ROM. They may sleep for long
+durations of time, after which they need to refresh the named resources they
+know about. Name resolution in such scenaries must take into account link
+layer frame sizes of only a few hundred bytes, bit rates in the magnitute
+of kBit/s, and latencies of several seconds {{-constr-nodes}}.
 
 To prevent TCP and HTTPS resource requirements, constrained IoT devices
 could use DNS over DTLS {{-dodtls}}. In contrast to DNS over DTLS, DoC