ccna 200-301 network simulator tools today

ccna 200-301 network simulator tools today., 28Chapter 2 Network Interconnection and Reference Models*" I02IP does not provide reliable transport services, it does not provide end-to-end or node-to-node acknowledgement, it has no error control over data, it uses only the checksum in the header, and it does not provide retransmission or flow control. If errors can be reported through 1CMP ,ICMP is implemented in the IP module.2. ICMP (InternetControl MessageProtocol, Internet Control Message Protocol) mention ICMP, some people may feel strange, in fact, ICMP is closely related to us. It is a sub-protocol of the TCP/IP protocol family, used to pass control messages between IP hosts, routers. Control messages are messages about the network itself, such as network access, host reachability, and route availability. These control messages do not transmit user data, but they play an important role in the delivery of user data. The control function of the IP layer is the most complex, mainly responsible for error control, congestion control, etc. Any control is based on information, and in the IP datagram-based network system, the IP protocol itself has no built-in mechanism to obtain error information and handle it. In order to handle these errors, TCP/IP designed the 1CMP protocol. When a gateway finds a transmission error, it immediately sends an ICMP message to the source host to report the error information and let the source host take appropriate measures to deal with it, which is an error and control message protocol used to transmit not only error messages but also control messages. We often use ICMP protocol in the network, but we are not aware of it. For example, the Ping command, which is often used to check whether the network is working or not, is actually the process of "Ping", which is how the ICMP protocol works. The sending host first sends an ICMP Echo Request packet, containing 64 bytes of data, which is sent, and the receiver returns an ICMP EchoReply packet, containing a copy of the received data. There are other network commands, such as the Tracert command for traceroute, that are also based on the ICMP protocol. 1CMP packets are contained in IP datagrams and belong to a subscriber of IP. The LP header precedes the ICMP packet, so an ICMP packet includes an IP header, an ICMP header, and an ICMP packet.The Type field in the ICMP header is used to describe the role and format of the ICMP message, and there is also a Code field that details the type of an ICMP message, all of which is found after the ICMP header. The RFC defines 13 ICMP message formats, as shown in Table 2-4-1.Table 241 ICMP Message Formats1 Type Code: Type Description 0 Response Answer (ECHO-REPLY) 3 Unreachable 4 Source suppression 5 Redirect 8 Response Request (ECHO-REQUEST) 11 Timeout 12 Parameter malfunction 13 Timestamp request 14 Timestamp response 15 Message Request (*Validated) 16 Message Answer (*Validated) 17 Address Mask Request 18 Address Mask Answer The message messages with codes 15 and 16 have been voided. The following are a few common ICMP messages. . Response request. The ping we use most everyday is the response request (Type=8) and answer (Type=O), where a host sends an ICMP message with Type=8 to a node, and if there are no exceptions along the way (such as being dropped by the router, the target not responding to the ICMP, or transmission failure), the target returns an ICMP message with Type=0, indicating that this host exists, and more detailed tracert determines the network distance between the host and the target by counting the nodes through which the ICMP message passed. 29 4 CCNA Study and Lab Guide . . Destination unreachable, source suppressed, and timeout messages. The format of these three messages is the same. the target unreachable message (Type=3) is used when the router or host cannot deliver the datagram. such as to connect to a non-existent system port (port number less than 1024) on the other side. an ICMP message with Type=3, Code=3 is returned. it suggests that the target is unreachable. Other common unreachable types are network unreachable (Code=0), host unreachable (Code=l), protocol unreachable (Code=2), etc. Source suppression then acts as a traffic control, which informs the host to reduce datagram traffic, and since ICMP does not resume transmission of the message, the host gradually resumes transmission rate as soon as the message is stopped. Finally, the problem with connectionless method networks is that datagrams can be lost, or wander around the network for long periods of time without finding their destination, or congestion prevents hosts from reconfiguring datagram segments within a specified time, which triggers the generation of an ICMP timeout message. The code field of the timeout message has two values: Code=0 for transmission timeout and Code=l for reassembly segment timeout. . Timestamp. The Timestamp Request message (Type=13) and the Timestamp Answer message (Type=14) are used to test the transmission time of one round trip of datagrams between two hosts. When transmitting, the host fills the original timestamp, the receiver receives the request and fills the receive timestamp and returns it in Type=14 message format, and the sender calculates the time difference. 3. ARP (Address ResolutionProtocol) ARP is responsible for resolving an IP address into a corresponding MAC address. In a LAN, what is actually transmitted in the network is a frame", and the frame contains the MAC address of the target host. In Ethernet, a host has to know the MAC address of the target host in order to communicate directly with another host. But how is this destination MAC address obtained? It is obtained through the Address Resolution Protocol, which is the process by which the host derives the target MAC address from the target IP address before sending the frame. The basic function of ARP protocol is to query the MAC address of the target device through the IP address of the target device to ensure smooth communication.In Figure 2-4-4, the process of ARP is explained as follows.1 computer A wants to send a packet to computer B, computer A to determine the computer B to access the same network with the computer192.168.1.0, Computer A queries the local cache for the MAC address corresponding to Computer B's IP address.