Internet protocol suite
From Wikinfo
The Internet protocol suite' is the set of protocols that
implement the protocol stack on which the Internet runs.
It is sometimes called the TCP/IP protocol suite after two of the
many protocols that make up the suite:
the transport protocol TCP and the internet protocol IP.
The authoritative reference on this subject is RFC 1122, which
can be found at http://www.ietf.org/rfc/rfc1122.txt.
The Internet protocol suite can be described by analogy with the OSI model, which describes the layers of a protocol stack, not all of which correspond well with Internet practice. In a protocol stack, each layer solves a set of problems involving the transmission of data, and provides a well-defined service to the higher layers. Higher layers are logically closer to the user and deal with more abstract data, relying on lower layers to translate data into forms that can eventually be physically manipulated.
The Internet model was designed as the solution to a practical engineering problem. The OSI model, on the other hand, was a more theoretical approach, and was built by committee. Therefore, the OSI model is easier to understand, but the TCP/IP model is more practical. It is helpful to have an understanding of the OSI model before learning TCP/IP, as the same principles apply, but are easier to understand in the OSI model.
Contents |
Layers in the TCP/IP Stack
There is some discussion about where the distinctions between layers are drawn. Since the TCP/IP and OSI protocol suites do not match precisely, there is no one correct answer. The following diagram attempts to show where various TCP/IP and other protocols would reside in the OSI model:
| 7 | Application | e.g. HTTP, SMTP, SNMP, FTP, Telnet, NFS |
| 6 | Presentation | e.g. XDR, ASN.1, SMB, NCP, AFP |
| 5 | Session | e.g. ISO 8327 / CCITT X.225, RPC, NetBIOS, ASP |
| 4 | Transport | e.g. TCP, UDP, RTP, SPX, ATP |
| 3 | Network | e.g. IP, ICMP, IGMP, X.25, CLNP, ARP, OSPF, RIP, IPX, DDP |
| 2 | Data Link | e.g. Ethernet, Token Ring, PPP, HDLC, Frame relay, ISDN, ATM |
| 1 | Physical | e.g. electricity, radio, laser |
Commonly, the top three layers of the OSI model (Application, Presentation and Session) are considered as a single Application Layer in the TCP/IP suite. Because the TCP/IP suite has no unified session layer on which higher layers are built, these functions are typically carried out (or ignored) by individual applications. A simplified TCP/IP interpretation of the stack is shown below:
| Application "layer 7" | e.g. HTTP, FTP, DNS (routing protocols like RIP, which run over UDP, may also be considered part of the network layer) | |
| 4 | Transport | e.g. TCP, UDP, RTP (routing protocols like OSPF, which run over IP, may also be considered part of the Network layer) |
| 3 | Network | For TCP/IP this is the internet protocol (IP) required protocols like ICMP, IGMP and ARP run over IP, but may still be considered part of the network layer |
| 2 | Data Link | e.g. Ethernet, Token Ring, etc. |
| 1 | Physical | e.g. physical media, and encoding techniques |
The Physical Layer
This describes the physical characteristics of the communication, such as conventions about the nature of the medium used for communication (such as wires, fiber optic links or radio links), and all related details such as connectors, channel codes and modulation, signal strengths, wavelengths, low-level sychronization and timing and maximum distances.
The Data-Link Layer
This specifies how packets are transported over the physical layer. Ethernet, for example, includes how to specify which machine or machines on the network a packet is destined for. Examples of Data-link layer protocols are Ethernet, PPP, Wireless Ethernet, SLIP, Token Ring and ATM.
This layer is sometimes further subdivided.
The Network Layer
The Network Layer solves the problem of getting from the source network to the destination network. This likely involves routing the packet across a network of networks, known as an Internet. In the Internet protocol suite, IP performs the basic task of getting packets of data from source to destination, and also supports ICMP (used to transmit diagnostic information about IP transmission) and IGMP (used to manage multicast data). ICMP and IGMP are layered on top of IP but perform network layer functions, illustrating an incompatibility between the Internet and OSI models.
The Network Layer Internet Protocol (IP) can carry data for a number of different higher level protocols. These protocols are each identified by a unique IP Protocol Number. ICMP and IGMP are protocols 1 and 2, respectively.
The Transport Layer
The protocols at this Layer can solve problems like reliability ("did the packet reach the destination?") and ensure that packets arrive in the correct order. It is also at this protocol where it is decided which application to connect to. Dynamic routing protocols typically use protocols at this layer to exchange their data.
TCP (IP protocol number 6) is a very "sturdy" transport mechanism, which makes sure packets arrive in order, are re-transmitted if lost, and eliminates duplicates, as well as handling "emergency" content which must be handled out of order (out-of-band). TCP will attempt to deliver all data correctly in the specified sequence - this is its purpose and main advantage over UDP, but it can be a disadvantage in real-time streaming or routing applications with high layer 3 loss rates. (TCP's IP protocol number is 6).
UDP (IP protocol number 17) is a lower-overhead protocol that is less sturdy. There is no attempt to verify that packets have reached their destination, and no guarantee that they will arrive in order. If the Application requires these guarantees, it must provide them itself, or use TCP. UDP is typically used for applications such as streaming media (audio and video, etc) where the time TCP requires for retransmission and re-ordering might not be available, or for simple query/response applications like DNS lookups, where the overhead of setting up a reliable connection is disproportionately large.
Both TCP and UDP are used to carry a number of higher-level applications. The applications at any given network address are distinguished by their TCP or UDP Port Number. By convention certain well known ports are associated with specific applications.
RTP is an attempt to provide a compromise between TCP and raw UDP.
Although it uses the UDP packet format as a basis, it provides a function that is at the same protocol layer.
OSPF is a routing protocol (IP protocol number 89).
The Application Layer
The application layer is where most common network programs reside.
These programs and their corresponding protocols include HTTP (The World Wide Web), FTP (File Transport), SMTP (Email), SSH (Remote Login), DNS (Name<-->IP Lookups) and many others.
Applications most commonly run on TCP or UDP, and are often associated with a Well Known Port number. Some examples are:
- HTTP on TCP port 80 or 8080.
- SSH on tcp port 22,
- DNS lookups on UDP (or TCP) port 53,
- RIP routing updates on UDP port 520.
These ports were originally allocated by the Internet Assigned Numbers Authority.
Feel free to add to this list: Echo, QOTD, Telnet, NTP, DHCP (Kind-of), Finger, IMAP, SNMP, IRC, POP, IMAPS, HTTPS, POPS, RTSP, NNTP, Gopher.
Implementations
External Links
References
- Adapted from the Wikipedia article, "Internet protocol suite" http://www.wikipedia.org/wiki/Internet_protocol_suite August 3, 2003
