When an ICMP destination-unreachable message is sent by a router, it means that the router is unable to send the package to its final destination. The router then discards the original packet.
Note that the sending host, sun, receives an ICMP reply the output line with the comment from ourself shown earliereven though nothing is seen on the wire. This is a general characteristic of broadcasting: Since by definition the term "broadcast" means all the hosts on the local network, it should include the sending host.
Referring to Figure 2. Next, bsdi broadcasts the reply, while svr4 sends the reply only to the requestor. Normally the reply should be unicast unless the source IP address of the request is 0.
The Host Requirements RFC says that a system must not send an address mask reply unless it is an authoritative agent for address masks. To be an authoritative agent it must be specifically configured to send these replies.
As we can see from this example, however, most host implementations send a reply if they get a request. Some hosts even send the wrong reply!
The final point is shown by the following example. We send an address mask request to our own IP address and to the loopback address: Again, referring to Figure 2.
The ICMP address mask reply must correspond to the subnet mask of the interface on which the request was received since a multihomed host can have different subnet masks for each interfaceand in both cases the request is received from the loopback interface.
The nice feature of this ICMP message is that it provides millisecond resolution, whereas some other methods for obtaining the time from another host such as the rdate command provided by some Unix systems provide a resolution of seconds.
The drawback is that only the time since midnight is returned-the caller must know the date from some other means. The requestor fills in the originate timestamp and sends the request.
The replying system fills in the receive timestamp when it receives the request, and the transmit time-stamp when it sends the reply.
In actuality, however, most implementations set the latter two fields to the same value. The reason for providing the three fields is to let the sender compute the time for the request to be sent, and separately compute the time for the reply to be sent.
We can write a simple program named icmptime that sends an ICMP timestamp request to a host and prints the returned reply. We try it first on our small internet: As we can see in this and the following examples, all the hosts set the receive and transmit timestamps to the same value.
We also calculate the round-trip time rttwhich is the time the reply is received minus the time the request was sent.
The difference is the received timestamp minus the originate timestamp.RFC , page 3. The ICMP router discovery messages are called "Router Advertisements" and "Router Solicitations". Each router periodically multicasts a Router Advertisement from each of its multicast interfaces, announcing the IP address(es) of that interface.
One other common ICMP message is the Destination unreachable message.
Here is an example: Host A sends a packet to Host B. Because the Host B is down, the router will send an ICMP Destination host unreachable message to Host A, informing it that the destination host is unreachable, e.g. ICMP Header Checksum. 16 bits. The bit one's complement of the one's complement sum of the ICMP message, starting with the ICMP Type field.
When the checksum is computed, the checksum field should be cleared to 0.
Nov 07, · As Networking begins moving on up the stack toward the layers involved with routing, we must pause for a moment. Some attention needs to be paid to the most misunderstood protocol: ICMP. Figure ICMP message types.. These rules are meant to prevent the broadcast storms that have occurred in the past when ICMP errors were sent in response to broadcast packets.
ICMP Address Mask Request and Reply.
The ICMP address mask request is intended for a diskless system to obtain its subnet mask (Section ) at bootstrap plombier-nemours.com requesting system broadcasts its ICMP request. SIP to PSTN Sequence Diagram In this scenario, Alice is a SIP phone or other SIP-enabled plombier-nemours.com is reachable via the PSTN at global telephone number.
Alice places a call to Bob through a Proxy Server (Proxy 1) and a Network Gateway (NGW 1).