Dedicated Micros DV-IP NV8, Руководство по установке

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NV1
Installation Guide
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Appendix G - mDNS and DNS S-D
What is...
Multicast DNS and DNS S-D (Service Discovery) provide a convenient ways for devices and
services to register themselves, and to discover other network-based services without relying
on centrally administered services. Multicast DNS is a way of using familiar DNS programming
interfaces, packet formats and operating semantics, in a small network where no conventional DNS
server has been installed. DNS S-D uses mDNS to browse the network for services.
Standard DNS (Domain Name System) provides a mechanism for registering a name against an IP
address, allowing devices to be referred to by a humanly recognisable text string instead of a set of
octets. When the text string is typed into a browser, the browser requests the IP address from the
DNS server, then navigates to the returned IP address.
mDNS together with DNS Service Discovery part of the zeroconf technology utilised by Dedicated
Micros.
Zeroconf has three main functions
● Address allocation without DHCP
● Name-to-Address Translation (NAT).
● Service Discovery.
Each device on an IP network requires a unique IP address. The Internet Protocol (IP) already has a
mechanism for determining which addresses are in use; Zeroconf adds conflict resolution capability
for when two devices try for the same address.
IP networks utilise either ;
a) static addresses to allocate IP addresses to devices on the network.
b) a DHCP server dedicated to assigning addresses to devices as they apply to join the
network.
Both these methods use a Unicast DNS server which registers each IP address and
the associated name, and then passes the IP address to any device searching for that
name. A single request is sent to a single point of contact to obtain the information.
c) the zeroconf methodology described above.
By contrast, a device joining a zeroconf network announces its intended IP address
across the network and waits for a conflict report. If there is no conflict, it assumes that
address. Multicast DNS broadcasts the name request across the network on IP address
224.0.0.251:5353, asking the IP address of the device.
Since zeroconf is normally limited to the local network, this volume of traffic is rarely a
problem.
Service Discovery works on the same principle. The device broadcasts across the local
network domain for a particular service type (http, tcp, etc) and the suitable devices
reply. Once all sources are discovered, the unit can then search for specific service
providers. Replies are cached to speed up future requests.