TCP/IP Networks

Table of Contents Computer networks, layers, protocols, interfaces; OSI reference model; TCP/IP reference model; Internet Protocol (operations, addresses, classes); Routing; Transmission Control Protocol (TCP); User Datagram Protocol (UDP); Applications; Sockets.

Computer Networks Hosts; Routers - Gateways; Bridges - Repeaters; Data packets networks, ISDN, leased lines;

Computer networks classification Shared media: Bus Ring Backbone network Vs local access network

Switching Techniques Circuit switching; Message switching; Packet switching.

Protocol Hierarchies Physical medium Layer 5 protocol Layer 4 protocol Layer 4/5 interface Layer 3/4 interface Layer 2/3 interface Layer 1/2 interface Layer 5 Layer 1 Layer 2 Layer 3 Layer 4 Host A Layer 3 protocol Layer 2 protocol Layer 1 protocol Layer 5 Layer 1 Layer 2 Layer 3 Layer 4 Host A

Information Flow Layer 5 Layer 5 protocol M Layer 4 protocol H4 M H3 H4 M1H3 M2 H2 H3 H4 M1 T2H2 H3 M2 T2 M H4 M Source machinedestination machine H3 H4 M1H3 M2 H2 H3 H4 M1 T2H2 H3 M2 T2 Layer 2 Layer 1 Layer 4 Layer 3

OSI Reference Model The OSI reference model based on a proposal developed by ISO has seven layers. The principles that were applied to arrive at the seven layers are as follows: A layer should be created where a different level of abstraction is needed; Each layer should perform a well defined function; The function of each layer should be chosen with an eye toward defining internationally standardised protocols; The layer boundaries should be chosen to minimise the information flow across the interfaces; The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity, and small enough that the architecture does not become unwieldy.

OSI Layers Functions Application: provides user access to an OSI environment. Presentation:hides from the application layer differences in representation of information. Session:provides facilities for synchronization. Transport: enables QoS network facilities. Network: establishes, maintains and terminates connections. Data Link: controls data transfer over physical link, including error detection. Physical: provides electrical and mechanical control to transmit data bits onto communication medium. Application Presentation Session Transport Network Data link Physical

TCP/IP Reference Model The protocols came first and model is just a description of existing protocols; The TCP/IP reference model can not describe non-TCP/IP networks; The layers 5 and 6 are not present in this model. Application Transport Internet Host-to- Network

OSI vs TCP/IP Application Presentation Session Transport Network Data link Physical Application TCP IP Host-to- Network UDP

TCP/IP Detailed View IEEE MAC PING TCP IP UDP ARP RARP ICMP FTP, WWW,CMOT Telnet, rlogin, SMTP, TFTP, DNS, SNMP NFS, yp, etc. IEEE MAC IEEE MAC IEEE MAC EthernetToken busToken ringMANWAN IEEE 802.2, HDLC/X.25, PPP, SLIP

Internet Protocol (IP) Connectionless (i.e., each packet it treated independently, with no reference to packets that have long gone before); Cannot guarantee reliable, in-order delivery; PDU: IP datagram, which contains user data, source-destination IP addresses, other inf. (such as its length, time-to-live, etc.); IP main operations: Fragmentation/Reassembly and Routing

Fragmentation/Reassembly

Reassembly Two options: either in host B, or in router G2. It is preferred the first option. Gain: Simpler routers (no buffering of fragments) Loss:decrement of network utilisation and increment of packet loss probability.

IP Addresses l An IP address defines both the network and the host on the particular network; l An IP address has 4 bytes, so there are 4 billion addresses; l There is one-to-one correspondence between IP and physical addresses; l Example of an IP address : ; l An IP address includes two parts: a network identifier (netid) and a host identifier (hostid); l The netid defines the network, while the hostid differentiate a host of the network from the others; l The length of netid depends on the address class: there are three address classes, namely A,B and C;

Address Classes Class Α:0 + 7bits (netid=1byte) + 3bytes (hostid); Class B: bits (netid=2byte) + 2bytes (hostid); Class C: bits (netid=3byte) + 1bytes (hostid); l When a network is separated into subnetworks, the hostid defines both the host and the subnetwork of the host. = l A subnet mask (32-bit) indicates the split of hostid to subnetid and new hostid; l A subnet mask contains 1 for bits of netid and subnetid and 0 for bits of hostid; l Example: The mask defines 14 subnetorks and 4094 hosts for each subnetwork.

Domain Name Service (DNS) l The DNS servers correspond names such as “swpc94.telecom.ece.ntua.gr” in IP addresses like “ ”; l However, the traffic of TCP/IP packets uses IP addresses and not names; l Before an Internet process, there is a dialogue (approx. 1/10 sec) between the source host and the local DNS server for finding the IP address of the target host.

Routing Direct routing: In the same network, usage of the Address Resolution Protocol (ARP) and Reserve Address Resolution Protocol (RARP) Indirect routing: Between different networks, usage of the routers Routers They can manipulate packets from all the interconnected networks; They communicate with all the interconnected networks; They are “multihomed”, i.e., they have multiple IP addresses referring to all the interconnected networks; They perform routing algorithms using the netid of the IP datagrams.

Indirect Routing Example AB C I II III 3 separate physical networks, with their own addresses, packet size and pattern.

Indirect Routing Example AB C I II III D 4 4 IV The networks are connected via two routers. The routers can send/receive packets to/from both networks.

Indirect Routing Example Introduction of the unique IP address for each host and the IP datagram as common transfer unit. AB C I II III D 4 5 IV

Indirect Routing Example AB C I II III D 4 5 IV Each host or router forwards the datagram per one hop towards its destination. For each hop, the datagram is encapsulated into a specific physical layer packet with a local physical address. The datagram keeps the IP address of its destination. The routers firstly exams the netid. Only at the last hop of routing, the hostid is mapped to the physical address. In case of fragmentation, the destination takes over the reassembly. 3.3data D 3.3data 5 3.3data iii

Indirect Routing Example Both hosts and routers keep routing tables for leading the IP datagrams to destinations and physical addresses tables for mapping the IP addresses to corresponding physical addresses. Routing Table: It contains pairs of the form (N,R), where N is the IP address of the destination network and R is the IP address of the next router towards the destination. Examples: Host x 2.x 3.x NR Computation of the physical address 1.4 Router 1.4/2.4 1.x 2.x 3.x NR direct connection 2.5 direct connection A B D C Physical Addresses Table:

Transmission Control Protocol (TCP) Connection-oriented (i.e., a connection is established before the data transmission); Can guarantee reliable stream delivery services; reserved TCP port numbers (16 bits):FTP 21 Telnet 23 Finger79 HTTP80 A B FTP Telnet

Transmission Control Protocol (TCP) l Sliding Window Technique; l Multiplicative Decrease Congestion Avoidance; l Slow Start Recovery; Allowed_window = min (Receiver_Advertisement, Congestion_Window)

User Datagram Protocol (UDP) Connectionless; No confirmations, packets numbering, flow control; No error detection/recovery; Cannot guarantee reliable in order delivery services; reserved UDP port numbers (16 bits):DNS 53 TFTP 69 SNMP161 Mainly, broadcasting applications use UDP.

Applications l FTP; l SMTP; l WWW; l Telnet; l Many others

Sockets A B FTP Telnet l The combination of an IP address with a port number identifies a socket; l A socket defines an application service;