How data travels through a network The Internet Packet routing: Journey of a packet in internet

Processing packet at source machine

Processing packet at source machine (Cont.) Application generates a packet to be sent on the network and send it to layer below. transport layer which manages end to end communication between two machines it is sent to network layer which adds source and destination IP in the packet. The packet reaches data link or MAC layer where source and destination MAC address of machines are added Data link layer push this packet to physical layer it is sent as stream of “0” and “1” on physical medium available.

Processing a packet at router Router takes the packet and does three basic operations Routing forwarding encapsulation

routing When router receives packet first of all it strips down the MAC layer header and looks into the IP header which contains destination IP address. Once destination IP is known, router looks into it database in order to find where should this packet be forwarded to make it reach to destination. This databases is known as routing table.

forwarding the packet is switched from the ingress interface to egress interface known as forwarding

Encapsulation Router caches the MAC address of next hops it needs to send packet to, it replaces the source and destination MAC address in it and send it to physical layer.

Processing packet at destination host

Processing packet at destination host (cont.) Packet is received at network card, physical layer, which generates an interrupt to CPU and CPU reads packet in, At data link layer, destination MAC address is checked to see if packet is destined to this machine, If yes, packet is sent up to network layer. At IP layer, packet validation like checksum verification etc is done and then passed on to relevant transport layer. Transport later then passes it on to the appropriate port so that it reaches correct application.

Layer Number Function Example Application 7 Services that are used with end-user application SMTP, HTTP,POP3, IMAP Presentation 6 Formats the data so that it can be viewed by the user Encrypt and Decrypt JPEG, GIF, TIFF, HTTPS, SSL, TLS Session 5 Establishes/ends connections between two hosts RPC, SQL, NetBIOS, PPTP Transport 4 Responsible for the transport protocol and error handling TCP, UDP Network 3 Reads the IP address from the packet IP, ICMP, Routers, Layer 3 switches Data Link 2 Reads the MAC address from the data packet PPP, SLIP, switches Physical 1 Sends data on the physical wire Physical connections, hubs, NICs, cable

WELL-KNOWN PORTS Port Use 20-21 FTP 22 SSH 23 TELNET 25 SMTP 42 WINS 53 DNS 80, 8080 HTTP 88 KERBEROS 110 POP3 111 PORTMAPPER-LINUX 123 NTP 135 RPC-DCOM 143 IMAP 161, 162 SNMP 389 LDAP 445 CIFS 514 SYSLOG 636 SECURE LDAP WELL-KNOWN PORTS

REGISTERED PORTS OF INTEREST USE 1080 SOCKS5 1241 NESSUS SERVER 1433, 1434 SQL SERVER 1494, 2598 CITRIX APPLICATIONS 1521 ORACLE LISTENER 2512, 2513 CIRTIX MANAGEMENT 3389 RDP 6662-6667 IRC