2.4 Data Encapsulation and Communications Flashcards
Address Resolution Protocol (ARP)
ARP is a system MAC address discovery protocol
Data Encapsulation and Communications
The data to be transferred is encapsulated on the sending host from the top layer of the TCP/IP or OSI model to the bottom. The following events occur:
- The Application layer prepares the data to be sent through the network by encoding it using the appropriate Application layer protocol.
- The Transport layer receives the stream of data from the Application layer and breaks it into smaller chunks called segments. A Transport layer header, which identifies the source port as well as the destination port, is applied to each segment. Sequencing and control information is also added to the header.
- The Internet layer converts the segments into packets by adding an Internet layer header, which specifies source and destination IP addresses for each packet. IP addresses are 32-bit (4-byte) logical addresses that can be assigned, unassigned, and reassigned as needed.
- The Link layer converts the packets into frames by adding a Link layer header, which specifies source and destination MAC addresses for each frame. A MAC address is a 48-bit (6-byte) address that is physically assigned in the firmware of all network interfaces to uniquely identify each interface on the network. MAC addresses are displayed using hexadecimal notation.
- Each frame is converted into bits and transmitted across the network media.
Network Transmission
If necessary, the data is transferred to various routers, which forward the data to the appropriate network. The source and destination network addresses are used to determine whether the hosts reside on the same network or on different networks.
If they reside on the same network, the data can be sent directly to the destination host. The Address Resolution Protocol (ARP) is used to determine the MAC address of the host using the destination IP address:
The sending host checks its ARP cache to see if it already has an IP-to-MAC address mapping for the destination host. If so, it transmits the frames to the destination host’s MAC address. If not, it must use the remaining steps to determine the appropriate MAC address.
The sending host sends out an ARP broadcast frame addressed to all MAC addresses on the subnet to ask for the hardware address of the host with the destination IP address.
The host with the destination IP address responds to the ARP broadcast with a unicast transmission containing its MAC address. All other hosts ignore the broadcast.
The sending host caches the destination host’s MAC address in its ARP cache.
The source MAC address of the frames is set to the MAC address of the sending system and the destination MAC address is set to the MAC address of the receiving system.
The sending host transmits the frames to the destination host’s MAC address.
If the sending and receiving hosts reside on different networks, the packets must be forwarded from router to router until they reach the appropriate destination network and host. The source IP address of each packet in the transmission is the IP address of the sending system and the destination IP address is the IP address of the receiving system. However, the frames can’t be sent directly to the receiving system because it is not on the same network and ARP can only be used on the local subnet. The following occurs in this situation:
If it’s not already cached, the source system uses ARP to determine the MAC address of the first hop router interface that is connected to the same network segment as the source host (usually the default gateway router).
The source MAC address of the frames is set to the MAC address of the sending system, but the destination MAC address is set to the MAC address of the router interface identified with ARP.
The frames are transmitted to the first router.
The router removes the frame header information and examines the packets in the transmission for the source and destination IP addresses. If the destination host is on a network that is directly connected to the router, the router uses ARP to discover its MAC address (if it’s not already cached), re-encapsulates the packets in new frames with the destination host’s MAC address, and transmits the frames directly to the destination host. If the destination host is not on a directly-connected network, the remaining steps occur.
The router uses its routing table to determine the next router the packets should be sent to.
The router re-encapsulates the packets in the transmission in new frames.
The source MAC address of the frames is set to the MAC address of the local router interface and the destination MAC address is set to the MAC address of the next hop router interface.
The router transmits the frames to the MAC address of the next hop router interface.
The routing process repeats until the packets arrive at a router that is directly connected to same network as the destination host.
The router receives the frames and removes the frame headers.
The router examines the packets.
It recognizes that the destination host resides on a network that is directly connected to the router.
If necessary, the router uses ARP to determine the MAC address of the destination system.
The router re-encapsulates the packets in new frames.
The source MAC address of the frames is set to the MAC address of the router interface.
The destination MAC address is set to the MAC address of the destination host.
The frames are transmitted to the destination host.
Destination host de-encapsulation
The data received is de-encapsulated on the destination host by moving from the bottom layer of the TCP/IP or OSI model to the top:
The Link layer converts bits received on the network medium into frames and passes them to the Internet layer.
The Internet layer extracts the packets from the frames and passes them to the Transport layer.
The Transport layer receives packets and uses sequencing and error control information to request retransmission of any missing or damaged packets.
The Transport layer uses sequencing information to convert the packets into segments and passes them to the Application layer.
The Application layer uses the appropriate Application layer protocol to convert the segments back into the original data stream from the application on the source host.
Encapsulation
Encapsulation is the process of breaking a message into packets, adding control and other information, and transmitting the message through the transmission media. You need to know the following four-step data encapsulation process on the sending system using the TCP/IP model:
- The Application layer prepares the data to be sent through the network.
- The Transport layer breaks the data into pieces called segments, adding sequencing and control information.
- The Internet layer converts the segments into packets, adding logical network and device addresses.
- The Link layer converts the packets into frames, adding physical device addressing information. It also converts the frames into bits for transmission across the transmission media.
Encapsulation
Encapsulation is the process of breaking a message into packets, adding control and other information, and then transmitting the message through the transmission medium.
TCP/IP Layers
- The Application layer prepares the data to be sent through the network.
- The Transport layer breaks the data into pieces called segments, adding sequencing and control information.
- The Internet layer converts the segments into packets, adding logical network and device addresses.
- The Link layer converts the packets into frames, adding physical device addressing information and a frame check sequence footer for error detection. It also converts the frames into bits (0s and 1s) for transmission across the transmission media.
Link Layer
The Link layer converts the packets into frames, adding physical device addressing information and a frame check sequence footer for error detection. It also converts the frames into bits (0s and 1s) for transmission across the transmission media.
Control information is added at the Transport layer.
The Transport layer breaks the data into pieces called segments.
The Internet layer converts the segments into packets, adding logical network and device addresses.
What term does the OSI model use that is different from the TCP/IP model uses to refer to frame, packet, and segment?
The OSI model uses the term protocol data unit (PDU) instead of the terms frame, packet and segment.
Presentation and session are layers 5 and 6 of the OSI model respectively and do not correspond to the use of frame, packet, and segment in the TCP/IP model.
IEEE Ethernet standard refers to the standard that defines Ethernet.
Routing data between computers on a network requires several mappings between different addresses. Which of the following statements is true?
ARP lets routers resolve known IP addresses into MAC addresses by broadcasting requests to the network.
DNS is used to map hostnames to IP addresses. ARP is used to map IP addresses to MAC addresses. Diskless workstations use BOOTP to discover their IP address, the server’s IP address, and the boot files they should use. ICMP notifies routers of problems on the network and undeliverable packets.
What is the purpose of a frame check sequence (FCS) footer?
The Link layer converts the packets into frames, adding physical device addressing information and a frame check sequence footer for error detection. It also converts the frames into bits (0s and 1s) for transmission across the transmission media.
What role does ARP play in the routing process?
If a router does not know a destination device’s MAC address, it sends an ARP broadcast containing the destination device’s IP address and requesting its MAC address.
