Outline. Challenges of the Internet. The Internet. CS4233 Network Programming Introduction to TCP/IP

Transcription

1 Outline CS4233 Network Programming Introduction to TCP/IP Chen-Lung Chan Department of Computer Science National Tsing Hua University Introduction to the Internet OSI model TCP/IP model Network s Socket The Internet An inter-net: a of s. A set of s that are connected with each other Networks are connected using routers that support communication in a hierarchical fashion Often need other special devices at the boundaries for security, accounting,.. The Internet: the interconnected set of s of the Internet Service Providers (ISPs) providing data communications services. In order to inter-operate, all participating s have to follow a common set of rules. Challenges of the Internet Scale: 100,000,000s of hosts Heterogeneity: 18,000+ administrative domains Thousands of s Lots of users Fast s, slow s, satellite s, cellular s, carrier pigeons Diversity of technologies Adversarial environment (contention)

2 Packet Switching Source sends information as self-contained packets that have an address. Source may have to break up single message in multiple Each packet travels independently to the destination host. Routers and switches use the address in the packet to determine how to forward the packets Analogy: a letter in surface mail. Implementing Packet-Switched Networks Requirements for packets: Header information: Addresses, etc. Data. What is packet size limit? Everybody must agree on these for interoperability How do packets reach destination? Routing Nodes in forward packets towards destination Routing tells nodes where to send the packets they receive Design questions: What criteria to decide? Destination (essential) Source? Type? IP Addresses (1/2) IP Addresses (2/2) Logical address at layer 32 bit address space Network number, Host number Class A 0 Network ID Network ID 8 16 Host ID 24 Host ID 32 Machines on the same have the same number Class B 10 Class C 110 One address per interface Class D 1110 Multicast Addresses Class E 1111 Reserved for experiments

3 Subnetting Routing Add another layer to hierarchy Could subnet a class B into several chunks How to check an IP is in the same subnet? AND both IPs with the subnet mask Check if the result addresses are identical Ex: { , }, netmask = { , } Who chooses the routes? A human: Static routing Centralized routing (telenet, c.a. 1980s) Distributed routing (Internet, ) Network Host Network Subnet Host Subnet Mask Distributed routing uses a Routing Protocol Many different protocols are in use. Inside an organization: RIP, OSPF, etc Between organizations: BGP Host Routing Table Example Network Service Model Destination Gateway Genmask Iface eth lo eth0 Host is in EECS 8F Ethernet Dest routing to the same Ethernet Dest special loopback address Dest default route to rest of the Internet Main EECS 8F router: To see the routing table of your machine In Windows & UNIX: netstat r To see the routing path to a host UNIX: traceroute Windows: tracert What is the service model? Ethernet/Internet: best-effort packets can get lost, etc. What if you want more? Network can do it Quality of Service Benefits of circuit switching in packet-switched net Hard in the Internet, easy in restricted contexts Hosts can do it end-to-end Transport protocols TCP performs end-to-end retransmission of lost packets to give the illusion of a reliable underlying.

4 Using Networks Using Networks Securely Layering and abstraction Protocol stacks facilitate re-use Hide underlying complexity from the programmer Protocol reuse and code/library reuse Many human-friendly abstractions: Higher-level protocols (e.g., reuse the Web s HTTP instead of writing your own!). Naming ( vs ) The Domain Name System, or DNS The Internet is an unfriendly place Hacking, viruses, denial-of-service, etc. Cryptography to the rescue: Secure Sockets Layer (SSL) Key management, etc. Policy control to the rescue: Firewalls / Denial of Service Network address translation / virtual private s (NAT, VPN) Applications Outline All well and good to have s that deliver packets, but what do we actually do with them? The Web Peer to Peer Voice over IP Introduction to the Internet OSI model TCP/IP model Network s Socket

5 Network Model (OSI) Physical Layer Moves bits from one end to other Media Dependent Copper: Coaxial/Twisted Pair; Typically up to 100 Mbps Fiber: Single/Multi Mode; Can transmit in Gigabits/second Satellite: Channels of 64 kbps, 128 kbps Radio waves: Ranges from Kbps to Mbps Data Link Layer Network Layer Controls a single Medium Access Control (MAC) CSMA/CD, CSMA/CA, Token-Ring, Logical Link Control (LLC): Framing (start and stop) Error Detection Error Correction Optimal Use of Links (Sliding Window Protocol) Examples: HDLC, LAP-B, LAP-D Deliver data from a host to another Why do we need a layer? Can all machines in the world be Ethernet? Other technologies (modems, token ring) Goals and Services: Hide type of subnets (Ethernet, Token Ring, FDDI... ) Hide topology of subnets Choose good routes Uniform addressing scheme OSPF, BGP in Internet

6 Transport Layer Session Layer Provide end-to-end transmission quality to two users Accepts data from session layer and split it into smaller units (if needed) and pass them to layer. Typical layer would provide error-free pointto-point data channel that delivers message/bytes in the order in which they were sent. However, other possible kind could be of isolated messages with no guarantee. It is a true end-to-end layer from source to destination (in lower layers protocols are between each machine and its immediate neighbors). Define the rule of a session Allows users on different machines to establish sessions between them. Allows ordinary data (as does layer) and also provides enhanced services useful for some s (e.g. checkpointing of sessions, dialog control). Presentation Layer Application Layer Deliver meaningful data to users Performs certain functions that are requested sufficiently often to warrant finding a general solution for them. Unlike lower layers, this layer is concerned with syntax and semantics of the information transmitted. Manages abstract data structures and converts from the representation inside the computer (e.g. ASCII, Unicode) to the standard and back. Handles specific issues/protocols e.g. different file naming conventions, different ways of representing text lines Various special and general purpose facilities are taken care of by layer while two remote s communicate with each other.

7 Outline OSI and TCP/IP Models Introduction to the Internet OSI model TCP/IP model Network s Socket OSI Application Presentation Session Transport Network Data Link Physical TCP/IP Application Transport Internet Data Link Physical TCP/IP Layers (1/2) TCP/IP Layers (2/2) Physical: transmitting bits over a communication channel Data Link: transforming the raw layer into a for the higher layer Network: addressing and routing of packets Transport: end-to-end connection characteristics Application: protocols

8 Layering: Physical Communication data Router data Layering: Logical Communication data data ack Router data Overview of TCP/IP (1/2) Some TCP/IP protocols: TCP: Transmission control protocol Connection-oriented, reliable, full duplex, byte stream service UDP: User datagram protocol Connectionless, unreliable IPv4: Internet Protocol, version 4. The protocol that glues the Internet together. Provides (unreliable) packet delivery service for TCP, UDP, ICMP, IGMP IPv6: Internet Protocol, version bits address, newer version of Internet Protocol Overview of TCP/IP (2/2) Some TCP/IP protocols... cont d ICMP: Internet Control Message Protocol Handles errors and control information for IP (ping, traceroute) IGMP: Internet Group Management Protocol Used in IP multicast Socket programming The use of TCP and UDP Can also access the lower layers directly (raw socket)

9 UDP: User Datagram Protocol TCP: Transmission Control Protocol Unreliable, connectionless service Data may be lost, duplicated, or reordered Client and server do not have any long term relation Can perform (unreliable) multicast Useful for Applications such as voice and video, where retransmission should be avoided the loss of a few packets does not greatly affect performance Guaranteed service protocol ensures that a packet has been received by the destination by using acknowledgements and retransmission s need to establish a TCP connection prior to transfer Features Connection Oriented Reliable Byte Stream Flow Control Congestion Control Reliable Communication Outline Implementing reliable communication may require Sequence number, acknowledgement, retransmission, timeout (RTT) Sliding window protocol for efficiency Flow control Connection establishment and release Introduction to the Internet OSI model TCP/IP model Network s Socket

10 Network Applications Client-Server paradigm Domain Name Service (DNS) Simple Mail Transfer Protocol (SMTP) Network File System (NFS) Network Management (SNMP) Network Security (Secured Socket Layer SSL) Typical app has two pieces: client and server Request Reply Actions Client initiates contact with server ( speaks first ) typically requests service from server e.g.: sends request for Web page Server provides requested service to client e.g., sends requested Web page Example: Web Access (HTTP) net.html <html> Some ing companies: <a href= > Cisco</a> <a href= > Motorola</a> </html> HTML rendering of net.html Response: net.html Client Some ing companies: Cisco Motorola Request for resource

11 Some Application Protocols Example: HTTP Application remote terminal access Web file transfer streaming multimedia remote file server Internet telephony Application layer protocol smtp [RFC 821] telnet [RFC 854] http [RFC 2068] ftp [RFC 959] proprietary (e.g. RealNetworks) NFS proprietary (e.g., Vocaltec) Underlying protocol TCP TCP TCP TCP TCP or UDP TCP or UDP typically UDP $ telnet 80 Trying Connected to Escape character is '^]'. GET /index.html <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <TITLE> 國立清華大學資訊工程系 </TITLE> <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=big5"> <META NAME="Generator" CONTENT="NetObjects Fusion 8 for Windows"> <SCRIPT> <!-- function F_loadRollover(){} function F_roll(){} //--> </SCRIPT> <SCRIPT LANGUAGE="JavaScript1.2" SRC="./assets/rollover.js"></SCRIPT> <NOLAYER> Traditional Distributed Applications Middleware (1/2) Application logic Transport interface code: Makes the appropriate calls to send and receive the messages Usually divided into -independent and -dependent parts Middleware provides transparency of the interface code Software between programs and OS/ Provides a set of higher-level distributed computing capabilities and a set of standards-based interfaces

12 Middleware (2/2) Interfaces allow s to be distributed and to take advantage of other services provided over the. Middleware is a set of services that are accessible to programmers through an API. Example: Sockets, RPC, CORBA Middleware & API API Middleware Service 1 Application Program API Middleware Service 2 API Middleware Service 3 Outline Sockets API Introduction to the Internet OSI model TCP/IP model Network s Socket Interface between and layer two processes communicate by sending data into a socket reading data out of a socket Client identifies Server process using <IP address ; port number>

13 Sockets Interface Socket (1/2) controlled by developer controlled by operating system process socket TCP with buffers, variables host or server internet process socket TCP with buffers, variables host or server controlled by developer controlled by operating system host-local, -owned, OScontrolled, communication interface two processes communicate by sending data into socket, reading data out of socket door between process and protocol Socket (2/2) Socket Types Socket identification: IP address of client and server hosts port number of client and server s Socket types: reliable, byte stream-oriented (TCP) Unreliable, connection-less datagram (UDP)

14 Client Actions Server Actions Create a socket (socket()) Map server name to IP address (gethostbyname()) Connect to a given port on the server address (connect()) Client must contact server first! Create a socket (socket()) Bind to port number (bind()) Listen on the socket (listen()) Accept client connections (accept()) Server process must be running! Socket Programming