Rechnernetze und -Organisation Michael Hutter Karl C. Posch.

Transkript

1 und -Organisation 2010 Michael Hutter Karl C. Posch 1 Overview - Addressing Larger Networks - Domain Name System - Internet Protocol (IP) - Address Resolution Protocol - UDP - TCP - Network Ports and Sockets - Router - Routing Information Protocol (RIP) 2 1

2 Larger Networks - Hubs and switches connect nodes in a network - Routers connect networks The Internet is a network of networks 3 Internet-Protocol Addresses IP addresses are used to address hosts in different networks Composed of a network address and a host address A bitwise AND operation between IP address and subnet mask separates network address from host address The IPv4 address space Class A B C Start Network address End Host address #Networks 2 7 = =16,384 Subnet mask 2 21 =2,097,151 #Hosts =16,777, =65, =254 Reserved addresses /8 (local loopback), /8 (private class A), /12 (private class B), /16 (private class C), X.X.X.255 (broadcasts), Two versions: IPv4 (32-bit) and IPv6 (128-bit) CIDR /8 /16 /24 4 2

3 Domain Name System (DNS) Translates domain names into IP addresses phone book of the Internet A Fully Qualified Domain Name (FQDN) consists of a Hostname, e.g. www or portal portal.tugraz.at Domainname, e.g. tugraz Top-Level Domainname (TLD), e.g. at, com, org, Hierarchical structure TLD-Server Root-DNS-Server Name-Server www Webserver.at TLD-Server tugraz TLD-Server Name-Server portal Webserver Hostadresse Domainname TLD DNS (Reverse) Lookup Distributed database Caching of queries Recursive/iterative resolvers Tools nslookup dig 5 Internet Protocol (IP) IP provides a best effort delivery service No guarantee is given that IP packets are delivered Packets may be corrupted or modified Connectionless protocol Transmission over different network technologies Each network has its own Maximum transmission unit (MTU) size IP therefore provides fragmentation and reassembly Bit 0-7 Version IHL Identification Time to live (TTL) Bit 8-15 Service Protocol Bit Flags Source IP address Destination IP address Options (optional) Bit Length Fragment offset Checksum 6 3

4 The Address Resolution Protocol (ARP) Determines the MAC address by a given IP address Nodes that would like to send an IP packet over Ethernet need to know the MAC address of the receiver to deliver the packet within Ethernet frames Works on the data-link layer (layer 2) of the OSI model The sender sends a broadcast message Target MAC address = FF:FF:FF:FF:FF:FF Ethernet type-field = 0x0806 (ARP) A network device replies with the appropriate MAC address Must not be the target host but can be also a router Operating systems like Windows and Linux store MAC addresses of network devices in the ARP cache 7 Analyzing the Network Internet Control Message Protocol (ICMP) A protocol used to indicate errors in networks It is based on the IP protocol (unreliable transmission) Bit 0-7 Type Bit 8-15 Code Bit Checksum Works on OSI layer 3 (network layer) ID Sequence The Ping Command Test if a network node is reachable or not, measures the RTT Sends an ICMP echo request packet to the target which answers with an echo reply Traceroute/Tracert/PathPing Shows the route taken by the packets Increases the TTL value starting with 1 Hops answer with an ICMP time exceeded reply Unix-based systems use per default UDP instead of ICMP Bit

5 User Datagram Protocol (UDP) Transport layer protocol (OSI layer 4) Connectionless Protocol Fast and simple No Reliability No guarantee about message delivery No acknowledgements of packets Checksum (over header + data) is the only error detection method Addressing of applications using ports Applications Video streaming Voice over IP (VoIP) Bit 0-7 Bit 8-15 Games Source port Protocols: DNS, SNMP, RIP, DHCP, Length UDP flooding Denial of Service Bit Destination port Checksum Bit Network Ports Applications are addressed by different port numbers Sockets are network access points for applications UDP and TCP Port Numbers : well-known ports, universal ports : registered ports, maintained by IANA : dynamic ports, can be used by any process Port number + IP address = Internet socket Stream (TCP) vs. Datagram (UDP) socket Client Browser (TCP) Application layer Transport layer Internet layer Network layer FTP (TCP) 21 Server HTTP SNMP (TCP/UDP) (UDP) Ports IP address 10 5

6 Transmission Control Protocol (TCP) Transport layer protocol (OSI layer 4) Connection-oriented Three-way handshake before transmission of data Connection termination after transmission of data Reliable data transfer Acknowledgement of packets (selective/cumulative) Use of sequence numbers for packet ordering Error detection through mandatory checksum Error correction through retransmission of lost packets Flow control (sliding window) Congestion control (slow start, congestion window, ) Segmentation of packets Flags: SYN, ACK, PSH, RST, FIN, URG Bit 0-7 Bit 8-15 Bit Bit Source port Destination port Sequence number Acknowledgement number Offset Reserved Flags Window size Checksum Options Data Urgent pointer 11 TCP Three-Way Handshake Client SYN, SEQ=x SYN, ACK, SEQ=y, ACK=x+1 ACK, SEQ=x+1, ACK=y+1 Server Half-Open Connections What if the client does not send the final ACK? The server waits for the acknowledgement (taking up resources) SYN flooding: all resources of the server are reserved for half-open connections, no connection can be made up anymore Denial of Service 12 6

7 TCP Data Transfer (1) Client Server Send packet 1 Acknowledge packet 1 Time Send packet 2 Stop and wait algorithm Receiver acknowledges every packet The sender employs a retransmission timer (based on the RTT) If no packet was received within the time, it retransmits the packet Disadvantage: low throughput 13 window Client TCP Data Transfer (2) Send packet 1 Send packet 2 Send packet 3 Send packet 4 Send packet 5 ACK 1 ACK 2 ACK 3 ACK 4 ACK 5 Server Sliding-Window Method Try to keep the pipe full Sender transmits several packets without waiting for an acknowledgement Sender and receiver have a buffer for sent/received packets (window) Window is moved after acknowledgement of packets Selective acknowledgment: acknowledge individual packets Cumulative acknowledgement: one packet acknowledges all received packets window 14 7

8 Closing the TCP Connection Client Server FIN, SEQ=x, ACK=y FIN, ACK, SEQ=y, ACK=x+1 ACK, SEQ=x+1, ACK=y+1 15 LISTEN SYN-SENT SYN-RECEIVED ESTABLISHED FIN-WAIT-1 FIN-WAIT-2 CLOSE-WAIT CLOSING LAST-ACK TIME-WAIT CLOSED States of the Internet Socket 16 8

9 The Router (1) - Connects different networks - The Internet is composed of millions of routers (backbone router) - A router can have different interfaces (Ethernet, ATM, Token Ring, Wireless, DSL, ) / / /24 17 The Router (2) - Routers work at the network layer (OSI layer 3) - They evaluate the IP address to deliver the packets - They have an own MAC and IP address - Network routes are stored in a Routing Table - It stores routes to other networks - Network address and subnet mask - Next hop (gateway) - Interface (eth0, eth1, ) - Metric (number of hops) - Static vs. dynamic routing entries - Show routing table entries - netstat r, route 18 9

10 Routing Information Protocol (RIP) - Routers exchange information with other routers - Sharing routing-table information with neighbors - RIP is a dynamic routing protocol - Transmits routing updates every 30 seconds - UDP broadcast is used for the update no reliable transmission - Employs the hop count as routing metric - Allows only 15 hops maximum - Slow information exchange (7 minutes for 15 hops) - Only direct neighbors are known by RIP routers - 3 versions available: RIPv1, RIPv2, RIPng. - Other routing protocols: - BGP, OSPF, IGRP, EIGRP, 19 What should you know by now? Understand the terms; understand the connections between them; be able to operate with them: Differences between Hubs, Switches, and Routers IP Best Effort Delivery IP vs. MAC IP Fragmentation Domain Name System DNS (Reverse) Lookup IP Address Resolution ICMP Ping and traceroute UDP (Packet Structure) Network Ports and Internet Sockets TCP (Packet Structure) Retransmission Timer Sliding Window Selective/Cumulative Acknowledgements TCP Segmentation 3-way Handshake TCP Closing Connections Router (Routing Table) Routing Information Protocol 20 10