Review of Operating Systems Networks, and TCP/IP Lesson 2

Computer System Basics Operating Systems Manage the computer’s resources Memory management Scheduling Access Controls Lots of different operating systems Win9X, Win2K, WinME, NT, Unix, Linux, VMS

What is an operating system? Operating system Communications Accounting Input/ Output Command Interpreter Security Scheduling Memory Management Program Development Tools

Networks What is a network? Two or more systems connected together LAN (local area network) WAN (Wide area network) MAN (metropolitan area network)

Network Topologies Topology – the physical arrangement of the network. Completely Connected Star Ring Bus Characteristics # of connections # of “hops” Addition of new nodes Fairness Collisions and points of failure

Network Topologies ?

Network Topologies -- Star Adv: distance = 2 hops connections = n-1 DisAdv: 1 point of failure Fairness? Addition of new nodes? Congestion(collisions)?

Network Topologies -- Ring Adv: avg dist = (n-1)/2 hops connections = n DisAdv: many points of failure Fairness (token)? Addition of new nodes? Congestion(collisions)?

Network Topology -- Bus Adv: distance = 1 “hop” connections = n DisAdv: Collisions Fairness? Addition of new nodes?

Protocols A protocol is an agreed upon format for exchanging information. A protocol will define a number of parameters: Type of error checking Data compression method Mechanisms to signal reception of a transmission There are a number of protocols that have been established in the networking world.

OSI Reference Model ISO standard describing 7 layers of protocols Application: Program-level communication Presentation: Data conversion functions, data format, data encryption Session: Coordinates communication between endpoints. Session state maintained for security. Transport: end-to-end transmission, controls flow of data Network: routes data from one system to the next Data Link: Handles physical passing of data between nodes Physical: Manages the transmission media/HW connections Abstraction is the principle, you only have to communicate with the layer directly above and below

TCP/IP Protocol Suite TCP/IP refers to two network protocols used on the Internet: Transmission Control Protocol (TCP) Internet Protocol (IP) TCP and IP are only two of a large group of protocols that make up the entire “suite” A “real-world” application of the layered concept. There is not a one-to-one relationship between the layers in the TCP/IP suite and the OSI Model.

OSI and TCP/IP comparison OSI Model Application Presentation Session Transport Network Data-link Physical TCP/IP Protocol Suite NFS FTP, Telnet, SSH, SMTPSMB HTTP, NNTP RPC TCP,UDP IPICMP ARP Physical Application-level protocols Network-level protocols

TCP/IP Protocol Suite User Process User Process User Process User Process TCPUDP IP HW Interface RARPARP ICMPIGMP Media

Encapsulation of data User Data Appl header Application data TCP header Application data TCP header IP header Application data TCP header IP header Ethernet header Ethernet trailer Ethernet Frame IP Datagram TCP segment application TCP IP Ethernet driver Ethernet

Demultiplexing of a received Ethernet Frame application UDPTCP ICMPIGMP IP ARPRARP Ethernet driver Incoming Frame

IP Datagram 32 Bits Source Address Destination Address Time-to-live Protocol Header Checksum Version head lngth Type of Service Total Length Identification Flags Frag Offset Options (if any) Data

TCP Packet Structure 32 Bits Source Port Destination Port Sequence Number Acknowledgment Number Options (if any) Data Window Data Offset Reserved Flags Checksum Urgent Pointer TCP establishes a “virtual circuit” between client and server.

Establishment of a TCP connection (“3-way Handshake”) client Server SYN Client sends connection request, Specifying a port to connect to On the server. client Server SYN/ACK Server responds with both an acknowledgement and a queue for the connection. client Server ACK Client returns an acknowledgement and the circuit is opened.

User Datagram Protocol (UDP) UDP is a connectionless transport layer protocol that provides no reliability and has no mechanism for connection establishment or termination. UDP makes no guarantee about packet delivery. This “fire and forget” nature means a packet is placed on the network and you just hope it gets to where it was supposed to go to. UDP is commonly used for network services that are not sensitive to an occasional lost (dropped) packet. Because of the relative overall reliability of the network, it has become less important to have a guaranteed service and thus UDP is a good choice for many streaming services.

Application Protocols HTTP – HyperText Transfer Protocol FTP – File Transfer Protocol Telnet – A terminal communications facility SMTP – Simple Mail Transfer Protocol SSH – Secure SHell

Common Ports Common Ports used to facilitate communication between common protocols HTTP: TCP port 80 Telnet: TCP port 23 FTP: TCP port 20 and 21 SMTP: TCP port 25 SSH: TCP port 22 DNS: UDP and TCP port 53

Routing The overall process of delivering a packet from point A to point B. It is the job of the router to know where to send a packet in order to reach its destination Routing tables are used to determine where to send a packet. Static: entries entered once and not updated Routing Information Protocol (RIP): entire tables shared on a periodic basis. Not very efficient or secure. Open Shortest Path First (OSPF): Table maintained to determine what the shortest path is to specific destinations. Tremendous overhead. Border Gateway Protocol (BGP): Peer routers (neighbors) exchange routing information. Entire table initially loaded but subsequent changes limited to updates.

IP Addressing We’re all familiar with the concept of URL’s and IP addresses, we’ve been using them for a while. We normally like to recall the URL, not the IP address but the network requires the address, DNS (Domain Name Service) solves this problem for us. When you use a site’s name, your computer will send a DNS query to your local DNS server, if it knows the address it will return it otherwise it will send a query to a higher-level domain server, which may forward the query further up and so forth until the address is obtained. There are two basic ways to assign an IP address to a computer: Static – an address is assigned to a specific system and stays with it DHCP (Dynamic Host Configuration Protocol) – a dynamic addressing scheme that allows us to allocate IP addresses on a first-come, first-served basis. When a device connects to the network, it queries a DHCP server to obtain an IP address from a range of addresses. If the server runs out of addresses, the requestor is out of luck. more people can access the network without using as many addresses

Network Address Translation (NAT) There are a limited number of IP addresses available and not every system needs one. NAT was developed to provide a means to translate private IP addresses into public IP addresses. A device (typically a router or firewall) will accomplish this translation process. Source: Destination: Source: Destination: Source: Destination: Source: Destination: Firewall performs NAT

Network Address Translation A modern NAT gateway must change the Source address on every outgoing packet to be its single public address. It therefore also renumbers the Source Ports to be unique, so that it can keep track of each client connection. The NAT gateway uses a port mapping table to remember how it renumbered the ports for each client's outgoing packets. The port mapping table relates the client's real local IP address and source port plus its translated source port number to a destination address and port. The NAT gateway can therefore reverse the process for returning packets and route them back to the correct clients. When any remote server responds to an NAT client, incoming packets arriving at the NAT gateway will all have the same Destination address, but the destination port number will be the unique Source Port number that was assigned by the NAT. The NAT gateway looks in its port mapping table to determine which "real“ client address and port number a packet is destined for, and replaces these numbers before passing the packet on to the local client.

Internet Control Message Protocol (ICMP) ICMP uses the IP datagram delivery facility to send its messages. IP Header ICMP message 20 bytes (contents -- depends on type and code) 8-bit type 8-bit code 16-bit checksum The format for the ICMP message is as follows: ICMP performs the following control, error reporting, and informational Functions for TCP/IP: - Flow Control - Detecting unreachable destinations - Redirecting routes - Checking remote hosts (e.g. ping)

Internet Control Message Protocol (ICMP) TypeCodeMessage 00Echo Reply 30Net Unreachable 31Host Unreachable 33Port Unreachable 51Host Redirect 80Echo Request 110Time-to-live exceeded 130Timestamp Request 140Timestamp Reply

DNS Domain Name System (service) – provides services that translate host names to IP addresses and back again. Two modes of operation Communication to clients that need names resolved, accomplished via UDP Transfer of large blocks of DNS records (to distribute the workload of resolving addresses), done using TCP (known as a DNS zone transfer). One of most common DNS servers is the Berkley Internet Name Domain (BIND) DNS server available for most UNIX systems as well as for Microsoft NT. Typically runs on port 53 using UDP and TCP

IPv4 versus IPv6 From the IPv6 FAQ, what are the merits of IPv6? scalability:IPv6 uses 128bit address space. Address length is 4 times longer than IPv4. security:IPv6 basic specification includes security. It includes packet encryption (ESP:Encapsulated Security Payload) and source authentication (AH:Authentication Header). [confidentiality and authentication] real-time:To support real-time traffic such as video conference, IPv6 has "Flow Label". Using flow label, routers can know which end-to-end flow a packet belongs to, and then find out the packet which belongs to real-time traffic. [needed to allow for prioritization of traffic] autoconfiguration:IPv6 basic specification includes address autoconfiguration. So, even a novice user can connect their machine to network. specification optimization:IPv6 keeps good parts and discards old and useless parts of IPv4.

IP Security (IPSEC) Optional in IPv4, supported in IPv6. Two major modes: Tunnel: encrypts both the data and header portions of the packet. Transport: encrypts the data portion of the packet. Requires both ends to be using IPSEC (obviously, since one end will encrypt, the other needs to be able to decrypt).

About IPSEC (from the IPSEC FAQ) IPsec = AH + ESP + IPcomp + IKE IPsec consists of a couple of separate protocols, listed below: Authentication Header (AH): provides authenticity guarantee for packets, by attaching strong crypto checksum to packets. If you receive a packet with AH and the checksum operation was successful, you can be sure about two things : –The packet was originated by the expected peer. The packet was not generated by an impersonator. –The packet was not modified in transit. Encapsulating Security Payload (ESP): provides confidentiality guarantee for packets, by encrypting packets with encryption algorithms. If you receive a packet with ESP and successfully decrypted it, you can be sure that the packet was not wiretapped in the middle. IP payload compression (IPcomp): ESP provides encryption service to the packets. However, encryption tends to negatively impact compression on the wire. IPcomp provides a way to compress packets before encryption by ESP (Of course, you can use IPcomp alone if you wish to). Internet Key Exchange (IKE): AH and ESP need shared secret keys between peers. For communication between distant locations, we need to provide ways to negotiate keys in secrecy. IKE will make this possible.

Ethernet The most common physical layer protocol. A shared media protocol, collisions possible Uses CSMA/CD to control traffic Several different variations of Ethernet exist 10Base-2 : Thinnet (thin coax) 10Base-5: Thicknet (earliest version, thick coax) 10Base-T: Standard twisted pair Ethernet 100Base-T: Fast Ethernet 1000Base-T: Gigabit Ethernet

Summary Computer System Basics Network Topologies Network Addressing IPSEC