Transcript ppt
Computer Networks Lecture 2: Protocols and the TCP/IP Suite Prof. Younghee Lee * Some part of this teaching materials are prepared referencing the lecture note made by F. Kurose, Keith W. Ross(U. of Massachusetts) and Ion Stoica(UC Berkely) Prof. Younghee Lee 1 The need for a Protocol Architecture Object concept with two constraints – Layering » A technique to organize a network system into a succession of logically distinct entities, such that the service provided by one entity is solely based on the service provided by the previous (lower level) entity: 1st constraint » Use abstractions to hide complexity » Abstraction naturally leads to layering Different level of abstraction and services » Can have alternative abstractions at each layer » Advantages Good design principle in general Simple and easy to understand Easy to modify and/or adapt to new situations/technologies Allow for different solution for different situations Vendor competition: => open system ( <=> close system) Sharing, multiplexing, bypassing Easy to test & analysis » Disadvantages – OSI Open System (7 layer) » Only Peer to Peer layer communication for protocol entities: 2nd constraint Prof. Younghee Lee 2 The need for a Protocol Architecture Protocol – Service – says what a layer does – Interface – says how to access the service – Protocol – says how is the service implemented » a set of rules and formats that govern the communication between two peers – Building blocks of a network architecture – Each protocol object has two different interfaces » service interface: defines operations on this protocol » peer-to-peer interface: defines messages exchanged with peer Key feature - Syntax - Semantics - Timing – Term Protocol is overloaded » specification of peer-to-peer interface » module that implements this interface Prof. Younghee Lee 3 The OSI Protocol Architecture Prof. Younghee Lee 4 The OSI Protocol Architecture Prof. Younghee Lee 5 The TCP/IP Protocol Architecture Internet Architecture - Internet Engineering Task Force (IETF) • Application layer • Host-to-Host, or Transport layer • Internet layer • Network access layer • Physical layer – Application vs Application Protocol (FTP, HTTP) – Features » does not imply strict layering » hourglass shape » design and implementation go hand-in-hand Prof. Younghee Lee 6 Protocol layering and data Each layer takes data from above adds header information to create new data unit passes new data unit to layer below source M Ht M Hn Ht M Hl Hn Ht M application transport network link physical destination application Ht transport Hn Ht network Hl Hn Ht link physical Prof. Younghee Lee M message M segment M M datagram frame 7 Physical layer T1/E1 ADSL Cable Modem Modem TDM/FDM/CDM SONET WDM (Optical Internet: Lambda switching, Optical burst switching, Optical Packet switching) Prof. Younghee Lee 8 Physical Media physical link: transmitted data bit propagates across link guided media: – signals propagate in solid media: copper, fiber unguided media: – signals propagate freelye.g., radio Twisted Pair (TP) two insulated copper wires – Category 3: traditional phone wires, 10 Mbps ethernet – Category 5 TP: 100Mbps ethernet Prof. Younghee Lee 9 Physical Media: coax, fiber Coaxial cable: Fiber optic cable: wire (signal carrier) within a wire (shield) – baseband: single channel on cable – broadband: multiple channel on cable bidirectional common use in 10Mbs Ethernet glass fiber carrying light pulses high-speed operation: – 100Mbps Ethernet – high-speed point-to-point transmission (e.g., 5 Gps) low error rate Prof. Younghee Lee 10 Physical media: radio Radio link types: signal carried in electromagnetic spectrum no physical “wire” microwave – e.g. up to 45 Mbps channels LAN (e.g., waveLAN) – 2Mbps, 11Mbps bidirectional propagation environment effects: wide-area (e.g., cellular) – e.g. CDPD, 10’s Kbps – reflection – obstruction by objects – interference satellite – up to 50Mbps channel (or multiple smaller channels) – 270 Msec end-end delay – geosynchronous versus LEOS Prof. Younghee Lee 11 Link layer Point to point Multiple access / shared medium Logical link control Prof. Younghee Lee 12 Switching Switch: moves bits between links – Why do we need switching? – Packet switching » Interleave packets from different sources » Efficient: resources used on demand Statistical multiplexing – rather than arbitrarily assigning a time slot to each signal, each signal is assigned a slot according to priority and need. – 1 Mbps link; users require 0.1 Mbps when transmitting; users active only 10% of the time – Circuit switching: can support 10 users – Packet switching: with 35 users, probability that >=10 are transmitting at the same time < 0.0017 » Multiple types of applications » Accommodates bursty traffic – Circuit switching Prof. Younghee Lee 13 The Network Core mesh of interconnected routers the fundamental question: how is data transferred through net? – circuit switching: dedicated circuit per call: telephone net – packet-switching: data sent thru net in discrete “chunks” Prof. Younghee Lee 14 Network Core: Circuit Switching End-end resources reserved for “call” link bandwidth, switch capacity dedicated resources: no sharing circuit-like (guaranteed) performance call setup required Prof. Younghee Lee 15 Network Core: Circuit Switching network resources (e.g., bandwidth) divided into “pieces” pieces allocated to calls resource piece idle if not used by owning call (no sharing) dividing link bandwidth into “pieces” – frequency division – time division Prof. Younghee Lee 16 Network Core: Packet Switching resource contention: each end-end data stream divided into packets aggregate resource demand can exceed user A, B packets share amount available network resources congestion: packets each packet uses full link queue, wait for link use bandwidth resources used as needed, store and forward: packets move one hop at a time Bandwidth division into “pieces” – transmit over link Dedicated allocation – wait turn at next link Resource reservation Prof. Younghee Lee 17 Network Core: Packet Switching 10 Mbs Ethernet A B statistical multiplexing C 1.5 Mbs queue of packets waiting for output link 45 Mbs D E Packet-switching versus circuit switching: analogy Train, cars on highway Any other analogies?: Prof. Younghee Lee 18 Packet-switched networks: routing Goal: move packets among routers from source to destination – we’ll study several path selection algorithms datagram network: – destination address determines next hop – routes may change during session – analogy: driving, asking directions virtual circuit network: – each packet carries tag (virtual circuit ID), tag determines next hop – fixed path determined at call setup time, remains fixed thru call – routers maintain per-call state Advantages and Disadvantages? Prof. Younghee Lee 19 Internetworking Intranet Subnetwork End System(ES) Intermediate System(IS) Bridge Router – – – – Addressing schemes: Max. packet size: fragmentation Interfaces: Reliability Prof. Younghee Lee 20 Internetworking: challenges Many – – – – – differences between networks Address formats Performance – bandwidth/latency Packet size Loss rate/pattern/handling Routing How to translate between various network technologies Prof. Younghee Lee 21 Internetworking Prof. Younghee Lee 22 Internet structure: network of networks roughly hierarchical national/international backbone providers (NBPs) – e.g. BBN/GTE, Sprint, AT&T, IBM, UUNet – interconnect (peer) with each other privately, or at public Network Access Point (NAPs) regional ISPs – connect into NBPs local ISP regional ISP NBP B NAP NAP NBP A regional ISP local ISP local ISP, company – connect into regional ISPs Prof. Younghee Lee 23 Addresses vs. Names How To Find Nodes? Humans use readable host names –Globally unique (can correspond to multiple hosts) Naming system translates to physical address –E.g. DNS translates name to IP Address (e.g. 128.2.11.43) –Address reflects location in network Prof. Younghee Lee 24 Addresses vs. Names globally unique organization length location dependence Address Name Yes Yes (ideally) flat, hierarchical fixed size (usually) Yes flat, hierarchical variable size No Prof. Younghee Lee 25 Packet delivery inside the network Each network technology has different local delivery methods Address resolution provides delivery information within network – E.g., ARP maps IP addresses to Ethernet addresses – Local, works only on a particular network Routing protocol provides path through an internetwork Prof. Younghee Lee 26 Routing Forwarding tables at each router populated by routing protocols. Routing protocols update tables based on “cost” – Exchange tables with neighbors or everyone – Use neighbor leading to shortest path Prof. Younghee Lee 27 Applications, end systems Reliability – Corruption – Lost packets Flow and congestion control – Flow control: end system overloaded – Congestion control: network overloaded Fragmentation In-order delivery Etc… Prof. Younghee Lee 28 The TCP/IP Protocol Architecture Operation of TCP/IP Prof. Younghee Lee 29 The TCP/IP Protocol Architecture Internet Standards – IAB(Internet Architecture Board): » responsible for the development and publication of the standard. (from RFC) » the coordinating committee for Internet design, engineering, and management. – IAB has two principal subsidiary task forces » IETF(Internet Engineering Task Force) responsible for publishing the RFCs which are the working notes of the Internet R&D community. » IRTF(Internet Research Task Force) – To be a standard » Be stable and well-understood. » Be technically competent. » Have multiple, independent, and interoperable implementations with substantial operational experience. » Enjoy significant public support. » Be recognizably useful in some or all parts of the Internet. * Key difference with those of IS: the emphasis on operational experience Internet draft -> Proposed standard(Min. 6M) -> Draft standard (Min. 4M) -> Internet standard Prof. Younghee Lee 30 Comparison of OSI and TCP/IP OSI TCP/IP – Clean, thought out, explicit OO – Dirty afterthought to design already developed protocol – Not biased towards any protocol – Lower layers unspecified – Good for discussion but bad for implementation(too many layers, – Sloppy but practical options) – unnecessarily complex – mature and well tested at a time when similar OSI protocols were in the development stage – Esperanto – Pascal – Mackintosh – English – C – MSDOS Prof. Younghee Lee 31 A closer look at network structure: network edge: applications and hosts network core: – routers – network of networks access networks, physical media: communication links Prof. Younghee Lee 32 The network edge: end systems (hosts): – run application programs – e.g., WWW, email – at “edge of network” client/server model – client host requests, receives service from server – e.g., WWW client (browser)/ server; email client/server peer-peer model: – host interaction symmetric – e.g.: teleconferencing Prof. Younghee Lee 33 Network edge: connection-oriented service Goal: data transfer between end systems with control for certain purpose such as reliable transfer etc., handshaking: setup (prepare for) data transfer ahead of time – Hello, hello back human protocol – set up “state” in two communicating hosts In case of Telecommunication network service, network node has connection management function reliable, in-order bytestream data transfer – loss: acknowledgements and retransmissions flow control: – sender won’t overwhelm receiver TCP - Transmission Control Protocol – Internet’s connection-oriented service TCP service [RFC 793] congestion control: – senders “slow down sending rate” when network congested – Why? Pros and Cons? Prof. Younghee Lee 34 Network edge: connectionless service Goal: data transfer between end systems – same as before! App’s using TCP: UDP - User Datagram Protocol [RFC 768]: Internet’s connectionless service – unreliable data transfer – no flow control – no congestion control HTTP (WWW), FTP (file transfer), Telnet (remote login), SMTP (email) App’s using UDP: streaming media, teleconferencing, Internet telephony Prof. Younghee Lee 35 Access networks and physical media Q: How to connection end systems to edge router? residential access nets institutional access networks (school, company) mobile access networks Keep in mind: bandwidth (bits per second) of access network? shared or dedicated? Prof. Younghee Lee 36 Residential access: point to point access Dialup via modem – up to 56Kbps direct access to router (conceptually) ISDN: intergrated services digital network: 128Kbps all-digital connect to router ADSL: asymmetric digital subscriber line – up to 1 Mbps home-to-router – up to 8 Mbps router-to-home – ADSL deployment: UPDATE THIS Prof. Younghee Lee 37 Residential access: cable modems HFC: hybrid fiber coax – asymmetric: up to 10Mbps upstream, 1 Mbps downstream network of cable and fiber attaches homes to ISP router – shared access to router among home – issues: congestion, dimensioning deployment: available via cable companies, e.g., MediaOne Prof. Younghee Lee 38 Institutional access: local area networks company/univ local area network (LAN) connects end system to edge router Ethernet: – shared or dedicated cable connects end system and router – 10 Mbs, 100Mbps, Gigabit Ethernet deployment: institutions, home LANs soon LANs: chapter 5 Prof. Younghee Lee 39 Wireless access networks shared wireless access network connects end system to router wireless LANs: – radio spectrum replaces wire – e.g., Lucent Wavelan 10 Mbps router base station wider-area wireless access – CDPD: wireless access to ISP router via cellular network Prof. Younghee Lee mobile hosts 40 Delay in packet-switched networks packets experience delay on end-to-end path four sources of delay at each hop transmission A nodal processing: – check bit errors – determine output link queueing – time waiting at output link for transmission – depends on congestion level of router propagation B nodal processing queueing Prof. Younghee Lee 41 Delay in packet-switched networks Transmission delay: R=link bandwidth (bps) L=packet length (bits) time to send bits into link = L/R Propagation delay: d = length of physical link s = propagation speed in medium (~2x108 m/sec) propagation delay = d/s Note: s and R are very different quantitites! transmission A propagation B nodal processing queueing http://wps.aw.com/aw_kurose_network_ 2/0,7240,227091-,00.html Prof. Younghee Lee 42 Queueing delay R=link bandwidth (bps) L=packet length (bits) a=average packet arrival rate traffic intensity = La/R La/R ~ 0: average queueing delay small La/R -> 1: delays become large La/R > 1: more “work” arriving than can be serviced, average delay infinite! http://wps.aw.com/aw_kurose_network_2/0,724 0,227091-,00.html Prof. Younghee Lee 43