If your computer determines the destination address of a network packet is to a remote network, what does the data link layer do with that packet?
Answers
Explanation:
How Do Packets Get Around?
Up to this point, you have seen how to classify networks; you know that networks have an architectural type, topology, and protocols; and you know that networks are built to share information and resources. You also know that information is split into packets before being shared. This brings us to the next logical question: How do packets get from point A to point B?
To tackle this question, this section looks at two different examples:
A packet destined for a system on the same local network
A packet destined for a remote system on a different network
Before proceeding with these two delivery examples, let's take a minute to talk about the two most common protocols governing packet delivery: TCP and UDP.
TCP Versus UDP
The Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are protocols that run on top of the IP network protocol. Each has its advantages, but the key difference between the two protocols is the method of packet delivery and the guarantee of reliability and transmission.
UDP is a connectionless protocol that has very few error recovery services and no real guarantee of packet delivery. UDP is very much a "fire and forget" protocol: You place packets on the network and hope they get there. For this reason, UDP is commonly used for network services that are not greatly affected by the occasional dropped or lost packet, such as network time requests and responses to name queries. In reality, the overall reliability of networks and data transmission makes UDP a good choice for many streaming network services. Because UDP is rather efficient, requiring no overhead for guaranteed delivery, it is becoming more popular with network application designers.
TCP is a connection-oriented protocol and is designed to allow two hosts to establish a reliable connection and exchange data. Contrary to UDP, TCP guarantees that packets will be delivered and processed in the same order in which they were sent. This guaranteed delivery is extremely important for many network applications, including such popular services as HTTP, FTP, and Telnet. To accomplish this, TCP uses a three-way handshake to establish connections:
The originating host (usually called the client) sends a SYN (synchronize) packet to the destination host (usually called the server). The SYN packet tells the server what port the client wants to connect to and the initial packet sequence number of the client.
The server sends a SYN/ACK packet back to the client. This SYN/ACK (synchronize/acknowledge) tells the client "I received your request" and also contains the server's initial packet sequence number.
The client responds to the server with an ACK packet to complete the connection establishment process.
Local Packet Delivery
Now that you're familiar with the two main methods of packet transmission, assume you want to send a packet from your workstation to another workstation on the same Ethernet segment. The first step is fairly simple: you need to know the address of the remote workstation. From the discussion of IP packets, you know that the remote workstation's address is a 32-bit binary number. Since most people don't routinely remember 32-bit binary numbers, most applications let you use a dotted decimal representation of the destination address (in this case, assume your destination address is 10.10.10.101).
Unfortunately, the IP address alone will not ensure your packet is received at the remote workstation. You have to find out the remote workstation's hardware address, the Media Access Control (MAC) address. MAC addresses are used to uniquely identify each node on the network and are usually hardwired into the physical network device, such as a network interface card (NIC), used to interface with the network media. In fact, at the lower layers of the Ethernet protocol, IP addresses are not even used. All network addresses are translated into MAC addresses.
To find a remote workstation's MAC address, the Address Resolution Protocol (ARP) is used. Basically, this is the computer's way of calling out, "Who owns the tan minivan in the parking lot? (You have your lights on.)" Your workstation sends out an ARP request to find out, "Who is 10.10.10.101?" Every node on the network will examine the packet, but only the workstation with the IP address of 10.10.10.101 will respond. It will send back an ARP reply to tell your workstation, "I'm 10.10.10.101 and here's my MAC address." Your workstation will then format the packet for delivery and drop it on the network media, stamped with the MAC address of the destination workstation. Ultimately, it is the MAC address that causes your packet to be received by the destination workstation.
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