Cirrus Logic CobraNet - Manual (Page 17)

CobraNet Programmer’s Reference
Network Stack
DS651PM25
©
Copyright 2006 Cirrus Logic, Inc. 17
down to an approximate 2-per-minute minimum rate. Two conditions must be met before a
CobraNet device will send out BOOTP requests:
• The device must not already have an IP address. Apart from BOOTP, there are
two other means for a CobraNet interface to obtain an IP address - RARP and
the
ipMonitor variables.
• The device must be attached to a switched network. In order to avoid producing
unregulated traffic, BOOTP requests are not transmitted on a repeater network.
Upon receipt of a valid BOOTP response, a CobraNet device will change its IP address to
the IP address indicated by the BOOTP response. It is not necessary for a response to be
paired with a specific request to be considered valid.
3.5 RARP (partial support)
RFC 903 defines reverse address resolution protocol (RARP). Network clients use RARP
to receive an IP address from a central server. RARP differs from BOOTP in that it is
carried at the logical link layer (Layer 2) and thus cannot pass through IP routers.
RARP is comprised of request and response packet types. Upon receipt of a valid RARP
response packet, a CobraNet device will change its IP address to the IP address
indicated by the RARP response. The CobraNet network stack does not transmit RARP
request packets.
RARP is the means used by the CobraNet Discovery application (Disco) and CNDisco
object for IP address assignment.
3.6 ICMP (partial support)
Internet control message protocol (ICMP) is an administrative protocol defined in RFC
972.
CobraNet devices which have been assigned an IP address will respond to ICMP echo
(commonly referred to as ‘ping’) requests. No other ICMP support is implemented in the
CobraNet network stack.
3.7 ARP
Address resolution protocol (ARP) is used by the IP protocol to translate IP addresses to
MAC addresses according to RFC 826.
A host seeking a MAC address associated with an IP address broadcasts an ARP
request. The device using the specified IP address replies with an ARP response packet.
In this way the requesting host obtains the MAC address for the target device.
The CobraNet interface responds to ARP requests when appropriate. CobraNet will not
generate ARP requests. For this reason the CobraNet can only respond to IP messages
and cannot initiate IP communications.
3.8 IP
The internet protocol (IP) is defined in RFC 791. IP is a network protocol (layer 3 of the
OSI 7-layer networking model) responsible for routing of packets and segmentation and
reassembly of packets.