TCP/IP Protocol Architecture
TCP/IP protocols map to a four-layer conceptual model known as the DARPA model, named after the U.S. government agency that initially developed TCP/IP. The four layers of the DARPA model are: Application, Transport, Internet, and Network Interface. Each layer in the DARPA model corresponds to one or more layers of the seven-layer Open Systems Interconnection (OSI) model.
The following figure shows the TCP/IP protocol architecture.
Network Interface Layer
The Network Interface layer (also called the Network Access layer) handles placing TCP/IP packets on the network medium and receiving TCP/IP packets off the network medium. TCP/IP was designed to be independent of the network access method, frame format, and medium. In this way, TCP/IP can be used to connect differing network types. These include local area network (LAN) media such as Ethernet and Token Ring and WAN technologies such as X.25 and Frame Relay. Independence from any specific network media allows TCP/IP to be adapted to new media such as asynchronous transfer mode (ATM).
The Network Interface layer encompasses the Data Link and Physical layers of the OSI model. Note that the Internet layer does not take advantage of sequencing and acknowledgment services that might be present in the Network Interface layer. An unreliable Network Interface layer is assumed, and reliable communication through session establishment and the sequencing and acknowledgment of packets is the function of the Transport layer.
The Internet layer handles addressing, packaging, and routing functions. The core protocols of the Internet layer are IP, ARP, ICMP, and IGMP.
The Internet layer is analogous to the Network layer of the OSI model.
The Transport layer (also known as the Host-to-Host Transport layer) handles providing the Application layer with session and datagram communication services. The core protocols of the Transport layer are Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).
The TCP/IP Transport layer encompasses the responsibilities of the OSI Transport layer.
The Application layer lets applications access the services of the other layers and defines the protocols that applications use to exchange data. There are many Application layer protocols and new protocols are always being developed.
The most widely known Application layer protocols are those used for the exchange of user information:
Additionally, the following Application layer protocols help facilitate the use and management of TCP/IP networks:
Examples of Application layer interfaces for TCP/IP applications are Windows Sockets and NetBIOS. Windows Sockets provides a standard application programming interface (API) under Windows Server 2003. NetBIOS is an industry-standard interface for accessing protocol services such as sessions, datagram’s, and name resolution. More information on Windows Sockets and NetBIOS is provided later in this chapter.
The TCP/IP Application layer encompasses the responsibilities of the OSI Session, Presentation, and Application layers.
TCP/IP Core Protocols
The TCP/IP protocol component that is installed in your network operating system is a series of interconnected protocols called the core protocols of TCP/IP. All other applications and other protocols in the TCP/IP protocol suite rely on the basic services provided by the following protocols: IP, ARP, ICMP, IGMP, TCP, and UDP.
IP is a connectionless, unreliable datagram protocol primarily responsible for addressing and routing packets between hosts. Connectionless means that a session is not established before exchanging data. Unreliable means that delivery is not guaranteed. IP always makes a “best effort” attempt to deliver a packet. An IP packet might be lost, delivered out of sequence, duplicated, or delayed. IP does not attempt to recover from these types of errors. The acknowledgment of packets delivered and the recovery of lost packets is the responsibility of a higher-layer protocol, such as TCP. IP is defined in RFC 791.
An IP packet consists of an IP header and an IP payload. The following table describes the key fields in the IP header.
Key Fields in the IP Header
IP Header Field Function
Source Address The IP address of the original source of the IP datagram.
Destination Address The IP address of the final destination of the IP datagram.
Identification Used to identify a specific IP datagram and to identify all fragments of a specific IP datagram if fragmentation occurs.
Protocol Informs IP at the destination host whether to pass the packet up to TCP, UDP, ICMP, or other protocols.
Checksum A simple mathematical computation used to verify the bit-level integrity of the IP header.
Time to Live (TTL) Designates the number of network segments on which the datagram is allowed to travel before being discarded by a router. The TTL is set by the sending host and is used to prevent packets from endlessly circulating on an IP internetwork. When forwarding an IP packet, routers are required to decrease the TTL by at least one.
Fragmentation and reassembly
If a router receives an IP packet that is too large for the network to which the packet is being forwarded, IP fragments the original packet into smaller packets that fit on the downstream network. When the packets arrive at their final destination, IP on the destination host reassembles the fragments into the original payload. This process is referred to as fragmentation and reassembly. Fragmentation can occur in environments that have a mix of networking media, such as Ethernet and Token Ring.
The fragmentation and reassembly works as follows:
When the fragments are received by IP at the remote host, they are identified by the Identification field as belonging together. The Fragment Offset field is then used to reassemble the fragments into the original IP payload.
When IP packets are sent on shared access, broadcast-based networking media — such as Ethernet or Token Ring — the media access control (MAC) address corresponding to a forwarding IP address must be resolved. ARP uses MAC-level broadcasts to resolve a known forwarding or next-hop IP address to its MAC address. ARP is defined in RFC 826.
Internet Control Message Protocol (ICMP) provides troubleshooting facilities and error reporting for packets that are undeliverable. For example, if IP is unable to deliver a packet to the destination host, ICMP sends a Destination Unreachable message to the source host. The following table shows the most common ICMP messages.
Common ICMP Messages
ICMP Message Function
Echo Request Troubleshooting message used to check IP connectivity to a desired host. The ping utility sends ICMP Echo Request messages.
Echo Reply Response to an ICMP Echo Request.
Redirect Sent by a router to inform a sending host of a better route to a destination IP address.
Source Quench Sent by a router to inform a sending host that its IP datagrams are being dropped due to congestion at the router. The sending host then lowers its transmission rate. Source Quench is an elective ICMP message and is not commonly implemented.
Destination Unreachable Sent by a router or the destination host to inform the sending host that the datagram cannot be delivered.
The following table describes the most common ICMP Destination Unreachable ICMP messages.
Common ICMP Destination Unreachable Messages
Destination Unreachable Message Description
Host Unreachable Sent by an IP router when a route to the destination IP address cannot be found.
Protocol Unreachable Sent by the destination IP node when the Protocol field in the IP header cannot be matched with an IP client protocol currently loaded.
Port Unreachable Sent by the destination IP node when the Destination Port in the UDP header cannot be matched with a process using that port.
Fragmentation Needed and DF Set Sent by an IP router when fragmentation must occur but is not allowed due to the source node setting the Don’t Fragment (DF) flag in the IP header.
Source Route Failed Sent by an IP router when delivery of the IP packet using source route information (stored as source route option headers) fails.
ICMP does not make IP a reliable protocol. ICMP attempts to report errors and provide feedback on specific conditions. ICMP messages are carried as unacknowledged IP datagrams and are themselves unreliable. ICMP is defined in RFC 792.
Internet Group Management Protocol (IGMP) is a protocol that manages host membership in IP multicast groups on a network segment. An IP multicast group, also known as a host group, is a set of hosts that listen for IP traffic destined for a specific IP multicast address. IP multicast traffic is sent to a single MAC address but processed by multiple IP hosts. A specific host listens on a specific IP multicast address and receives all packets to that IP address.
The following are some of the additional aspects of IP multicasting:
For a host to receive IP multicasts, an application must inform IP that it will receive multicasts at a specified IP multicast address. If the network technology supports hardware-based multicasting, the network interface is told to pass up packets for a specific IP multicast address. In the case of Ethernet, the network adapter is programmed to respond to a multicast MAC address corresponding to the specified IP multicast address.
A host supports IP multicast at one of the following levels:
The protocol to register host group information is IGMP, which is required on all hosts that support level 2 IP multicasting. IGMP packets are sent using an IP header.
IGMP messages take three forms.
For IP multicasting to span routers across an internetwork, multicast routing protocols are used by routers to communicate host group information so that each router supporting multicast forwarding is aware of which networks contain members of which host groups. IGMP is defined in RFCs 1112 and 2236.
TCP is a reliable, connection-oriented delivery service. The data is transmitted in segments. Connection-oriented means that a connection must be established before hosts can exchange data. Reliability is achieved by assigning a sequence number to each segment transmitted. An acknowledgment is used to verify that the data is received. For each segment sent, the receiving host must return an acknowledgment (ACK) within a specified period for bytes received. If an ACK is not received, the data is retransmitted. TCP is defined in RFC 793.
TCP uses byte-stream communications, wherein data within the TCP segment is treated as a sequence of bytes with no record or field boundaries. The following table describes the key fields in the TCP header.
Key Fields in the TCP Header
Source Port TCP port of sending host.
Destination Port TCP port of destination host.
Sequence Number Sequence number of the first byte of data in the TCP segment.
Acknowledgment Number Sequence number of the byte the sender expects to receive next from the other side of the connection.
Window Current size of a TCP buffer on the host sending this TCP segment to store incoming segments.
TCP Checksum Verifies the bit-level integrity of the TCP header and the TCP data.
A TCP port provides a specific location for delivery of TCP segments. Port numbers below 1024 are well-known ports and are assigned by the Internet Assigned Numbers Authority (IANA). The following table lists a few well-known TCP ports.
Well-Known TCP Ports
TCP Port Number Description
20 FTP (Data Channel)
21 FTP (Control Channel)
80 HTTP used for the World Wide Web
139 NetBIOS session service
TCP three-way handshake
A TCP connection is initialized through a three-way handshake. The purpose of the three-way handshake is to synchronize the sequence number and acknowledgment numbers of both sides of the connection and exchange TCP window sizes or the use of large window sizes or TCP timestamps. The following steps outline the process:
TCP uses a similar handshake process to end a connection. This guarantees that both hosts have finished transmitting and that all data was received.
UDP provides a connectionless datagram service that offers unreliable, best-effort delivery of data transmitted in messages. This means that neither the arrival of datagrams nor the correct sequencing of delivered packets is guaranteed. UDP does not recover from lost data through retransmission. UDP is defined in RFC 768.
UDP is used by applications that do not require an acknowledgment of receipt of data and that typically transmit small amounts of data at one time. NetBIOS name service, NetBIOS datagram service, and SNMP are examples of services and applications that use UDP. The following table describes the key fields in the UDP header.
Key Fields in the UDP Header
Source Port UDP port of sending host.
Destination Port UDP port of destination host.
UDP Checksum Verifies the bit-level integrity of the UDP header and the UDP data.
To use UDP, an application must supply the IP address and UDP port number of the destination application. A port provides a location for sending messages. A port functions as a multiplexed message queue, meaning that it can receive multiple messages at a time. Each port is identified by a unique number. It is important to note that UDP ports are distinct and separate from TCP ports even though some of them use the same number. The following table lists a few well-known UDP ports.
Well-Known UDP Ports
Well-Known UDP Ports
UDP Port Number Description
53 Domain Name System (DNS) name queries
69 Trivial File Transfer Protocol (TFTP)
137 NetBIOS name service
138 NetBIOS datagram service
So that is about TCP/IP and the protocols we use in TCP/IP.