This layer combines functionalities of the three top layers of the OSI model and may also be called Process/Application layer. Some of the most popular applications (email, file transport, and so on) interface with this layer to communicate with other applications on the network. If you’ll remember, the description of the Application layer of the OSI model included a list of application protocols and their primary functions. (Refer to Table 1.3.) These applications are also relative to the Application layer of the TCP/IP model. Table 1.9 provides a quick list of the protocols at their respective layers of the TCP/IP model.
The Transport layer corresponds with the Transport layer of the OSI model and is also known as the Host-to-Host layer. Not only is this layer responsible for reliable data delivery, but it can also make certain that data arrives in the proper order. You will see two transport layer protocols on the CCNA exam. These protocols are TCP and UDP. The following sections cover each protocol and its related applications.
TCP is a reliable connection-oriented protocol. TCP uses acknowledgments, sequencing, and flow control to ensure reliability (please refer back to the “Transport Layer” section of the OSI model for definitions of these terms). A TCP segment contains fields for the Sequence, Acknowledgment, and Windowing numbers. These fields help make sure that datagrams arrive undamaged. This is considered to be reliable delivery.
TCP uses Positive Acknowledgment and Retransmission (PAR):
The source device begins a timer when a segment is sent and retransmits if the timer runs out before an acknowledgment is received. The source device keeps track of segments that are sent and requires an acknowledgment for each segment. The destination device acknowledges when a segment is received by sending a packet to the source that iterates the next sequence number it is looking for from the source.
UDP is much simpler than TCP because it is aconnectionless protocol. UDP headers contain
only the source and destination ports, a length field, and a checksum. Because of the lack of a sequence, acknowledgment, and windowing field, UDP cannot guarantee delivery.Because there are no delivery guarantees, UDP is considered unreliable. With this protocol,it is up to the application to provide reliability.
Internet Control Messaging Protocol is used by ping and traceroute utilities.
Ping (Packet Internet Groper) enables you to validate that an IP address exists and can accept requests. The following transmissions are used by the Ping utility:
Ping sends an echo request packet to receive the echo response.
Routers send Destination Unreachable messages when they can’t reach the destination network and they are forced to drop the packet. The router that drops the packet sends the ICMP DU message. Traceroute traces the route or path taken from a client to a remote host. Traceroute also reports the IP addresses of the routers at each next hop on the way to the destination. This is especially useful when you suspect that a router on the route to an unreachable network is responsible for dropping the packet.
The Address Resolution Protocol (ARP), Reverse Address Resolution Protocol (RARP), and Proxy Address Resolution Protocol (Proxy ARP) are all protocols used at the TCP/IP model’s Internet layer.
ARP maps a known IP address to a MAC address by sending a broadcast ARP. When the destination IP address is on another subnet, the sender broadcasts ARP for the router’s ethernet port or default gateway, so the MAC address sent back is that of the router’s ethernet port.
RARP maps a known MAC address to an IP address. Proxy ARP enables a router to respond to an ARP request that has been sent to a remote host. Some Unix machines (especially Solaris) rely on Proxy ARP versus default gateways.