MODULE V. Internetworking: Concepts, Addressing, Architecture, Protocols, Datagram Processing, Transport-Layer Protocols, And End-To-End Services

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1 MODULE V Internetworking: Concepts, Aressing, Architecture, Protocols, Datagram Processing, Transport-Layer Protocols, An En-To-En Services Computer Networks an Internets -- Moule 5 1 Spring, 2014 Copyright 2014 All rights reserve

2 Topics Internet concept an architecture Internet aressing Internet Protocol packets (atagrams) Datagram forwaring Aress resolution Error reporting mechanism Configuration Network aress translation Computer Networks an Internets -- Moule 5 2 Spring, 2014 Copyright 2014 All rights reserve

3 Topics (continue) Transport layer protocol characteristics an techniques Message transport with the User Datagram Protocol (UDP) Stream transport with the Transmission Control Protocol (TCP) Routing algorithms an protocols Internet multicast an multicast routing Computer Networks an Internets -- Moule 5 3 Spring, 2014 Copyright 2014 All rights reserve

4 Internet Concept An Internet Architecture

5 What Is The Internet? Users see it as services an applications Web an e-commerce , texting, instant messenger Social networking an blogs Music an vieo ownloa (an uploa) Voice an vieo teleconferencing Networking professionals see it as infrastructure Platform on which above services run Grows rapily Computer Networks an Internets -- Moule 5 5 Spring, 2014 Copyright 2014 All rights reserve

6 Growth Of The Internet 1000M 900M 800M 700M 600M 500M 400M 300M 200M 100M 0M Plot shows number of computers on the Internet each year Computer Networks an Internets -- Moule 5 6 Spring, 2014 Copyright 2014 All rights reserve

7 Growth Of The Internet (log scale) Plot shows number of computers on the Internet each year Computer Networks an Internets -- Moule 5 7 Spring, 2014 Copyright 2014 All rights reserve

8 Actual Size Of The Internet Previous plots are somewhat misleaing Derive by walking the Domain Name System Only report hosts with IP aresses Since aroun 2000, many Internet evices Do not have a fixe IP aress Connect behin a NAT box (eg, wireless router) Actual size is ifficult to measure Computer Networks an Internets -- Moule 5 8 Spring, 2014 Copyright 2014 All rights reserve

9 Internet Architecture An Design If one were to esign a global communication system from scratch How shoul it be organize? Which technology or technologies shoul be use? The challenges Which applications shoul it support? Which network technologies shoul it use * PANs / LANs / MANs / WANs * Wire / wireless * Terrestrial / satellite Computer Networks an Internets -- Moule 5 9 Spring, 2014 Copyright 2014 All rights reserve

10 Internet Architecture An Design (continue) Key principles Internet is esigne to accommoate extant services plus new services that will be invente Internet is esigne to accommoate any network technology, allowing each technology to be use where appropriate Computer Networks an Internets -- Moule 5 10 Spring, 2014 Copyright 2014 All rights reserve

11 Internet Philosophy Infrastructure Provies a packet communication service Treats all attache enpoints as equal (any enpoint can sen a packet to any other enpoint) Does not restrict or ictate packet contents Does not restrict or ictate unerlying network technologies Attache enpoints Run applications that use the network to communicate with applications on other enpoints Control all content an provie all services Computer Networks an Internets -- Moule 5 11 Spring, 2014 Copyright 2014 All rights reserve

12 Avantages Of The Internet Philosophy Accommoates heterogeneous unerlying networks Accommoates arbitrary applications an services Separates communication from services Computer Networks an Internets -- Moule 5 12 Spring, 2014 Copyright 2014 All rights reserve

13 Internet Follows a network of networks approach Allows arbitrary networks to be inclue Uses IP routers to interconnect iniviual networks Permits each router to connect two or more networks networks routers Computer Networks an Internets -- Moule 5 13 Spring, 2014 Copyright 2014 All rights reserve

14 Internet Architecture: Logical View host computers Computers attache to Internet known as host computers To a host, Internet appears to be one giant network Computer Networks an Internets -- Moule 5 14 Spring, 2014 Copyright 2014 All rights reserve

15 Internet Architecture: Physical View net 1 net 2 net 3 net 4 router net 5 host computers physical net Network of heterogeneous networks connecte by routers Each host attaches to a network Computer Networks an Internets -- Moule 5 15 Spring, 2014 Copyright 2014 All rights reserve

16 Before We Discuss Internet Aressing

17 The Situation Internet aressing is efine by the Internet Protocol (IP) IP is changing Current version is 4 (IPv4) New version is 6 (IPv6) Computer Networks an Internets -- Moule 5 17 Spring, 2014 Copyright 2014 All rights reserve

18 History Of The Internet Protocol IP separate from TCP in 1978 Version 1-3 iscare quickly; version 4 was the first version use by researchers By early 1990s, a movement starte that clamore for a new version of IP because the 32-bit aress space woul run out soon In 1993, the IETF receive proposals, an forme a working group to fin a compromise By 1995, a new version ha been propose an ocuments written Computer Networks an Internets -- Moule 5 18 Spring, 2014 Copyright 2014 All rights reserve

19 Backgroun Of The New Version Of IP Various groups offere opinions about the features Cable companies wante support for broacast elivery Telephone companies argue that everyone woul soon be using a connection-oriente network technology (ATM) Several groups wante mobility The military pushe for better security A compromise was reache: IP version 6 inclues all the above Computer Networks an Internets -- Moule 5 19 Spring, 2014 Copyright 2014 All rights reserve

20 The Uphill Battle To Change IPv4 IP is ifficult to change because IP lies at the heart of the Internet protocols Version 4 of IP has a proven track recor The success of the current version of IP is increible the protocol has accommoate changes in harware technologies, heterogeneous networks, an extremely large scale Computer Networks an Internets -- Moule 5 20 Spring, 2014 Copyright 2014 All rights reserve

21 The Hourglass Moel Appl 1 Appl 2 Appl n IP Net 1 Net 2 Net m IP lies in the mile changing it means changing all hosts an routers in the Internet Computer Networks an Internets -- Moule 5 21 Spring, 2014 Copyright 2014 All rights reserve

22 Our Approach In the current Internet, both IPv4 an IPv6 are relevant an important Throughout the course, we will Discuss general concepts See how IPv4 an IPv6 implement the concepts Computer Networks an Internets -- Moule 5 22 Spring, 2014 Copyright 2014 All rights reserve

23 Internet Aressing

24 Aressing In The Internet Can we use MAC aresses across an internet? No: heterogeneity means Multiple types of MAC aresses MAC aress meaningful on one network not meaningful on another Solution Create new aressing scheme that is inepenent of MAC aresses Computer Networks an Internets -- Moule 5 24 Spring, 2014 Copyright 2014 All rights reserve

25 The Two Forms Of Aresses Ientity Unique number assigne to each enpoint Analogous to Ethernet aress Locator Enpoint aress encoes location information, such as * Geographic location * Location relative to a service provier * Computer on a given physical network Computer Networks an Internets -- Moule 5 25 Spring, 2014 Copyright 2014 All rights reserve

26 Two Principles To Keep In Min Both ientify an locator forms have avantages in some situations; no form is best in all cases Aressing is inherently linke to routing; the choice of an aressing scheme affects the cost of computing an maintaining routes Computer Networks an Internets -- Moule 5 26 Spring, 2014 Copyright 2014 All rights reserve

27 The IPv4 Aressing Scheme Unique number is assigne to each Internet host 32-bit binary value known as IPv4 aress Virtual aress, not erive from MAC aress Divie into two parts Prefix ientifies physical network (locator) Suffix ientifies a host on the network (ientity) Computer Networks an Internets -- Moule 5 27 Spring, 2014 Copyright 2014 All rights reserve

28 Dotte Decimal Notation (IPv4) Convenient for humans Divies IPv4 aress into octets of eight bits each Represents each octet in ecimal separate by ots Examples 32-bi t Binary Number Equi va l ent Dot te Dec ima l Computer Networks an Internets -- Moule 5 28 Spring, 2014 Copyright 2014 All rights reserve

29 Division Between Prefix An Suffix Original scheme (classful aressing) Each aress ivie on octet (8-bit) bounary Division coul be compute from the aress Current scheme (classless aressing) Formal name Classless Inter-Domain Routing (CIDR) Division permitte at arbitrary bit position Bounary must be specifie external to the aress Computer Networks an Internets -- Moule 5 29 Spring, 2014 Copyright 2014 All rights reserve

30 Classful Aressing Now historic Explains IPv4 multicast range Bits Class A prefix suffix Class B 1 0 prefix suffix Class C prefix suffix Class D multicast aress Class E reserve (not assigne) Computer Networks an Internets -- Moule 5 30 Spring, 2014 Copyright 2014 All rights reserve

31 Aress Mask Require with classless aressing Associate with a network Specifies ivision of aresses into network prefix an host suffix for that network 32-bit binary value 1-bits correspon to prefix 0-bits correspon to suffix Example mask that specifies six bits of prefix Computer Networks an Internets -- Moule 5 31 Spring, 2014 Copyright 2014 All rights reserve

32 CIDR Notation Use by humans to enter aress mask Avois otte ecimal errors Follows aress with slash an integer X, where X is the number of prefix bits Example In otte ecimal, a 26-bit mask is CIDR merely writes /26 Computer Networks an Internets -- Moule 5 32 Spring, 2014 Copyright 2014 All rights reserve

33 Table Of CIDR An Dotte Decimal Equivalences Length (CIDR) Aress Mask Notes / / / / / / / / / / / / / / / / / All 0s (equivalent to no mask) 1-octet bounary 2-octet bounary Computer Networks an Internets -- Moule 5 33 Spring, 2014 Copyright 2014 All rights reserve

34 Table Of CIDR An Dotte Decimal Equivalences Length (CIDR) Aress Mask Notes / / / / / / / / / / / / / / / / octet bounary All 1s (host specific mask) Computer Networks an Internets -- Moule 5 34 Spring, 2014 Copyright 2014 All rights reserve

35 Why CIDR Is Useful ISPs assign IP aresses Corporate customer with N computers nees N aresses CIDR permits ISP to roun to nearest power of two Example Assume ISP owns aress block / 16 Customer has 12 computers ISP assigns 4 bits of suffix to customer Mask use is /28 Example: customer is assigne / 28 Each computer at customer site has unique final 4 bits Computer Networks an Internets -- Moule 5 35 Spring, 2014 Copyright 2014 All rights reserve

36 Example Of A /28 Aress Block Network Prefix / 28 Aress Mask Lowest Host Aress Highest Host Aress Computer Networks an Internets -- Moule 5 36 Spring, 2014 Copyright 2014 All rights reserve

37 Special IPv4 Aresses Some aress forms are reserve Prefix Suffix Type Of Aress Purpose all-0s all-0s this computer use uring bootstrap network all-0s network ientifies a network network all-1s irecte broacast broacast on specifie net all-1s all-1s limite broacast broacast on local net 127 / 8 any loopback testing Loopback aress ( ) use for testing Packets never leave the local host Aresses / 8 an above are multicast Computer Networks an Internets -- Moule 5 37 Spring, 2014 Copyright 2014 All rights reserve

38 Host Aress Count For a given network prefix, the all-0s an all-1s suffixes have special meaning Consequence: if a suffix has N bits, 2 N 2 hosts can be present Computer Networks an Internets -- Moule 5 38 Spring, 2014 Copyright 2014 All rights reserve

39 IP Aressing Principle An IP aress oes not ientify a specific computer Instea, each IP aress ientifies a connection between a computer an a network Consequence A router or a host with multiple network connections must be assigne one IP aress for each connection Note: host with multiple network connections is calle a multi-home host Computer Networks an Internets -- Moule 5 39 Spring, 2014 Copyright 2014 All rights reserve

40 Illustration Of IPv4 Aress Assignment Wire Ethernet / 16 router 1 Wi-Fi Net / router WAN / 8 Each network assigne a unique prefix Each host on a network assigne a unique suffix Computer Networks an Internets -- Moule 5 40 Spring, 2014 Copyright 2014 All rights reserve

41 IPv6 Host Aresses Like IPv4 Binary value Divie into locator prefix an unique ID suffix Ientifies a connection to a network Unlike IPv4 128 bits long Suffix can be erive from MAC aress 3-level aress hierarchy Computer Networks an Internets -- Moule 5 41 Spring, 2014 Copyright 2014 All rights reserve

42 The IPv6 3-Level Hierarchy K bits 64 K bits 64 bits GLOBAL PREFIX SUBNET INTERFACE (COMPUTER) Prefix size chosen by ISP Subnet area allows organization to have multiple networks Computer Networks an Internets -- Moule 5 42 Spring, 2014 Copyright 2014 All rights reserve

43 IPv6 Aress Types Type Purpose unicast The aress correspons to a single computer A atagram sent to the aress is route along a shortest path to the computer multicast The aress correspons to a set of computers, an membership in the set can change at any time IPv6 elivers one copy of the atagram to each member of the set anycast The aress correspons to a set of computers that share a common prefix A atagram sent to the aress is elivere to exactly one of the computers (eg, the computer closest to the sener) Computer Networks an Internets -- Moule 5 43 Spring, 2014 Copyright 2014 All rights reserve

44 Colon Hex Notation Syntactic form use by humans to enter aresses Replacement for IPv4 s otte ecimal Expresses groups of 16 bits in hexaecimal separate by colons Example: becomes 69DC:8864:FFFF:FFFF:0:1280:8C0A:FFFF Computer Networks an Internets -- Moule 5 44 Spring, 2014 Copyright 2014 All rights reserve

45 Colon Compression Many IPv6 aresses contain long strings of zeroes Successive zeros can be replace by two colons Example can be written: FF0C:0:0:0:0:0:0:B1 FF0C::B1 Computer Networks an Internets -- Moule 5 45 Spring, 2014 Copyright 2014 All rights reserve

46 Two Major Reasons To Aopt IPv6 More aresses Eventually, IPv4 aresses will be eplete IPv6 provies more aresses than we will ever nee 340,282,366,920,938,463,463,374,607,431,768,211, aresses per square meter of the Earth s surface! Hype an excitement Researchers view IPv6 as an opportunity to be part of the action Inustries view IPv6 as an opportunity for revenue enhancement Computer Networks an Internets -- Moule 5 46 Spring, 2014 Copyright 2014 All rights reserve

47 Internet Protocol Packets (IP atagrams)

48 Internet Packets Because it inclues incompatible networks, the Internet cannot aopt a particular harware packet format To accommoate heterogeneity, the Internet Protocol efines a harwareinepenent packet format Computer Networks an Internets -- Moule 5 49 Spring, 2014 Copyright 2014 All rights reserve

49 IP Datagram Virtual packet format use in the Internet Same general layout as a network frame Heaer Data Area (known as a payloa area) Format of heaer etermine by protocol version (IPv4 or IPv6) Size of payloa etermine by application Maximum payloa is almost 64K octets Typical atagram size is 1500 octets Computer Networks an Internets -- Moule 5 50 Spring, 2014 Copyright 2014 All rights reserve

50 IPv4 Datagram Heaer Most heaer fiels have fixe size an position Heaer specifies source, estination, an content type VERS H LEN SERVICE TYPE TOTAL LENGTH IDENTIFICATION FLAGS FRAGMENT OFFSET TIME TO LIVE TYPE HEADER CHECKSUM SOURCE IP ADDRESS DESTINATION IP ADDRESS IP OPTIONS (MAY BE OMITTED) PADDING BEGINNING OF PAYLOAD (DATA BEING SENT) Computer Networks an Internets -- Moule 5 51 Spring, 2014 Copyright 2014 All rights reserve

51 A Few Details SOURCE IP ADDRESS fiel gives the IPv4 aress of the original source DESTINATION IP ADDRESS fiel gives the IPv4 aress of the ultimate estination Intermeiate router aresses o not appear in heaer Heaer size Almost no Internet atagrams contain options Therefore heaer length is usually 20 octets Computer Networks an Internets -- Moule 5 52 Spring, 2014 Copyright 2014 All rights reserve

52 IPv6 Heaer Arrangement Multiple heaers use: base plus zero or more extension(s) optional Base Heaer Extension Heaer 1 Extension Heaer N Payloa (Data) The figure is not to scale: extension heaers an/or the payloa can be much larger than the base heaer Computer Networks an Internets -- Moule 5 53 Spring, 2014 Copyright 2014 All rights reserve

53 IPv6 Base Heaer Format VERS TRAFFIC CLASS FLOW LABEL PAYLOAD LENGTH NEXT HEADER HOP LIMIT SOURCE ADDRESS DESTINATION ADDRESS Flow Label fiel allows atagram to be associate with a flow Computer Networks an Internets -- Moule 5 54 Spring, 2014 Copyright 2014 All rights reserve

54 Ientifying Heaers Each heaer contains a NEXT HEADER fiel Value specifies the type of the next item Each layer 4 protocol (UDP, TCP, etc) is also assigne a type Computer Networks an Internets -- Moule 5 55 Spring, 2014 Copyright 2014 All rights reserve

55 Example Use Of Next Heaer Fiel Illustration of heaers when a atagram contains a base heaer an transport protocol Base Heaer NEXT=TCP TCP Data Illustration of heaers when a atagram also contains an optional route heaer Base Heaer NEXT=ROUTE Route Heaer NEXT=TCP TCP Data Computer Networks an Internets -- Moule 5 56 Spring, 2014 Copyright 2014 All rights reserve

56 The Size Of An Extension Heaer Fixe length heaers Size is specifie in the stanars ocument Protocol software contains size constant Variable length heaers Size is etermine by sener Heaer contains an explicit length fiel NEXT HEADER HEADER LEN ONE OR MORE OPTIONS Computer Networks an Internets -- Moule 5 57 Spring, 2014 Copyright 2014 All rights reserve

57 Consequences For Packet Processing Consier a host or router that receives an IPv6 atagram The atagram contains a set of extension heaers Each extension heaer can contain an explicit length fiel To parse the atagram, IP software must iterate through heaers Conclusion: processing IPv6 can entail extra overhea Computer Networks an Internets -- Moule 5 58 Spring, 2014 Copyright 2014 All rights reserve

58 Datagram Forwaring

59 Internet Communication Paraigm Each atagram hanle inepenently Datagram forme on source computer Source sens atagram to nearest router Router forwars atagram to next router along path to estination Final router elivers atagram to estination Datagram passes across a single physical network at each step Computer Networks an Internets -- Moule 5 60 Spring, 2014 Copyright 2014 All rights reserve

60 Datagram Forwaring Performe by initial host an each router along path Selects next hop for the atagram as either Next router along the path Ultimate estination Uses a forwaring table with one entry per network Important point: size of forwaring table proportional to number of networks in the Internet Computer Networks an Internets -- Moule 5 61 Spring, 2014 Copyright 2014 All rights reserve

61 Forwaring Table Entry Uses IP aresses only (no MAC aresses) Contains Destination network IP prefix Aress mask for the estination network IP aress of next hop Computer Networks an Internets -- Moule 5 62 Spring, 2014 Copyright 2014 All rights reserve

62 Illustration Of An IPv4 Forwaring Table router R 1 router R 2 router R / / / / (a) Destination Mask Next Hop eliver irect eliver irect (b) In practice, table usually contains a efault entry Computer Networks an Internets -- Moule 5 63 Spring, 2014 Copyright 2014 All rights reserve

63 Prefix Extraction Forwaring paraigm Use network prefix when forwaring Use host when elivering Conceptual forwaring step Compare estination in each forwaring table entry with atagram s estination aress, D During comparison, only examine network prefix Note: mask in forwaring table makes comparison efficient if ( (Mask[i] & D) == Destination[i] ) forwar to NextHop[i]; Computer Networks an Internets -- Moule 5 64 Spring, 2014 Copyright 2014 All rights reserve

64 Longest Prefix Match Classless aressing means forwaring table entries can be ambiguous Example: consier estination an a table that inclues the following two entries: / 16 next hop A / 24 next hop B The estination matches both of them! Solution: select the match that has the longest prefix (in the example, take next hop B) Known as longest prefix match Computer Networks an Internets -- Moule 5 65 Spring, 2014 Copyright 2014 All rights reserve

65 Datagram Encapsulation Neee because unerlying network harware oes not unerstan atagrams Entire atagram travels in payloa area of frame IP Heaer IP Payloa Frame Heaer Frame Payloa Frame heaer contains MAC aress of next hop Frame only use for trip across one network: when frame arrives at next hop, atagram is extracte an frame is iscare Datagram remains intact en-to-en Computer Networks an Internets -- Moule 5 66 Spring, 2014 Copyright 2014 All rights reserve

66 Illustration Of Encapsulation Source host atagram Net 1 Frame Hr 1 atagram Router 1 atagram Net 2 Frame Hr 2 atagram Router 2 atagram Net 3 Frame Hr 3 atagram Destination host atagram Computer Networks an Internets -- Moule 5 67 Spring, 2014 Copyright 2014 All rights reserve

67 Semantics Of Internet Communication IP uses best effort elivery semantics IP attempts to eliver each atagram, but specifies that a atagram can be Lost Duplicate Delaye Delivere out-of-orer Delivere with bits scramble Motivation: accommoate any unerlying network Note: in practice, IP works an it works well Computer Networks an Internets -- Moule 5 68 Spring, 2014 Copyright 2014 All rights reserve

68 MTU An Network Heterogeneity Each network technology specifies a Maximum Transfer Unit (MTU) that is the largest amount of ata that can be sent in a packet Example: Ethernet MTU is 1500 octets Datagram can be as large as the network MTU Consier a 1500-octet atagram set from H 1 to H 2 in the following network H 1 R H 2 Net 1 (MTU=1500) Net 2 (MTU=1000) Datagram can reach router R, but cannot traverse Net 2 Computer Networks an Internets -- Moule 5 69 Spring, 2014 Copyright 2014 All rights reserve

69 Datagram Fragmentation Technique for accommoating heterogeneous MTUs Neee if atagram excees MTU Original atagram ivie into smaller atagrams calle fragments Heaer of fragment erive from original atagram heaer Each fragment is forware inepenently IPv4 allows routers to perform fragmentation IPv6 requires sening host to perform fragmentation Important principle for both IPv4 an IPv6: The ultimate estination reassembles fragments Computer Networks an Internets -- Moule 5 70 Spring, 2014 Copyright 2014 All rights reserve

70 The General Iea Of Fragmentation Divie the payloa into a series of atagrams IP Heaer original atagram payloa IP Hr 1 payloa 1 IP Hr 2 payloa 2 IP Hr 3 pay 3 Note: the tail fragment may be smaller than the others Computer Networks an Internets -- Moule 5 71 Spring, 2014 Copyright 2014 All rights reserve

71 IPv4 Fragmentation Details Datagram heaer contains fixe fiels that control fragmentation A bit in FLAGS fiel specifies whether given atagram is a fragment or complete atagram An aitional FLAGS bit specifies whether the fragment carries the tail of the original atagram OFFSET fiel specifies where the payloa belongs in the original atagram Computer Networks an Internets -- Moule 5 72 Spring, 2014 Copyright 2014 All rights reserve

72 IPv6 Fragmentation Details Always performe by source, never by routers Rule: no heaer changes as an IPv6 atagram traverses the Internet Consequences Source must iscover path MTU Separate extension heaer contains fragmentation information (same items as IPv4) Fragmentable part of atagram may inclue some extension heaers Computer Networks an Internets -- Moule 5 73 Spring, 2014 Copyright 2014 All rights reserve

73 Illustration of IPv6 Fragmentation P 1 P 2 P 3 Unfragmentable Part Fragmentable Part (can inclue heaers as well as ata) (a) Unfragmentable Part Frag 1 Heaer P 1 (b) Unfragmentable Part Frag 2 Heaer P 2 (c) Unfragmentable Part Frag 3 Heaer P 3 () A atagram (a) ivie into fragments (b through ) Computer Networks an Internets -- Moule 5 74 Spring, 2014 Copyright 2014 All rights reserve

74 Collecting Fragments Destination collects incoming fragments IDENTIFICATION fiel use to group relate fragments OFFSET fiel allows receiver to recreate the original payloa LAST FRAGMENT bit allows receiver to know when all fragments have arrive If a fragment fails to arrive within a timeout perio, entire atagram is iscare Note: if an IPv4 fragment is ivie into subfragments, reassembly oes not require reassembling subfragments Computer Networks an Internets -- Moule 5 75 Spring, 2014 Copyright 2014 All rights reserve

75 Aress Resolution

76 Review Of Datagram Transmission Host or router has atagram to sen IP uses longest-prefix match to look up atagram s estination aress in forwaring table an obtains IP aress of next hop Network over which to sen (in case there is more than one network connection) IP encapsulates atagram in frame (entire atagram place in payloa area of frame) Is the resulting frame reay to sen to the next hop? No! Computer Networks an Internets -- Moule 5 78 Spring, 2014 Copyright 2014 All rights reserve

77 Harware An Protocol Aressing Unerlying network harware Only unerstans MAC aresses Requires each outgoing frame to contain the MAC aress of the next hop IP forwaring Deals only with (abstract) IP aresses Computes the IP aress of the next hop Conclusion The IP aress of the next hop must be translate to a MAC aress before a frame can be sent Computer Networks an Internets -- Moule 5 79 Spring, 2014 Copyright 2014 All rights reserve

78 Aress Resolution Translates IP aress to equivalent MAC aress that the harware unerstans IP aress is sai to be resolve Restricte to a single physical network at a time Example: consier computer X sening to computer Y X R 1 R 2 B Y A C D A MAC aress is neee at each hop Computer Networks an Internets -- Moule 5 80 Spring, 2014 Copyright 2014 All rights reserve

79 An Example With MAC Aresses X MAC: 3A-12-C9 IP: MAC: 04-CF-47 IP: Y R 1 R / / /16 MAC: IP: MAC: D3 IP: MAC: 8E-1A-7F IP: MAC: 54-DB-31 IP: Sener NEXT-HOP SRC MAC DST MAC SRC IP DST IP X A-12-C R D3 8E-1A-7F R DB CF How can a host or router fin the MAC aress of the next hop? Computer Networks an Internets -- Moule 5 81 Spring, 2014 Copyright 2014 All rights reserve

80 Aress Resolution Protocol (ARP) Designe for IPv4 over Ethernet Use by two computers on the same physical network Allows a computer to fin the MAC aress of another computer Operates at layer 2 Uses network to exchange messages Computer seeking an aress sens request to which another replies Computer Networks an Internets -- Moule 5 82 Spring, 2014 Copyright 2014 All rights reserve

81 Example Of ARP Exchange Assume Four computers attache to an Ethernet Computer B has a atagram to sen Computer B Uses forwaring table to fin next-hop aress I C Broacasts an ARP request: I m looking for a computer with IP aress I C Computer C Receives the request an replies; I m the computer with IP aress I C Computer Networks an Internets -- Moule 5 83 Spring, 2014 Copyright 2014 All rights reserve

82 Illustration Of The ARP message Exchange Ethernet switch Ethernet switch W X Y Z W X Y Z (a) (a) Request is broacast to all computers Only the intene recipient replies Reply is sent unicast Computer Networks an Internets -- Moule 5 84 Spring, 2014 Copyright 2014 All rights reserve

83 ARP Message Format Sufficiently general to permit Arbitrary high-level protocol aress Arbitrary harware aress In practice, only use with IP an 48-bit Ethernet aresses HARDWARE ADDRESS TYPE PROTOCOL ADDRESS TYPE HADDR LEN PADDR LEN OPERATION SENDER HADDR (first 4 octets) SENDER HADDR (last 2 octets) SENDER PADDR (first 2 octets) SENDER PADDR (last 2 octets) TARGET HADDR (first 2 octets) TARGET HADDR (last 4 octets) TARGET PADDR (all 4 octets) Computer Networks an Internets -- Moule 5 85 Spring, 2014 Copyright 2014 All rights reserve

84 ARP Encapsulation ARP message is place in payloa area of harware frame When use with Ethernet, type is 0x0806 Source an estination MAC aresses must be ae to frame heaer before sening ARP MESSAGE FRAME HEADER FRAME PAYLOAD CRC Computer Networks an Internets -- Moule 5 86 Spring, 2014 Copyright 2014 All rights reserve

85 ARP Algorithm An Caching Given: An incoming ARP request or response Purpose: Process the message an upate the ARP cache Metho: Extract sener s IP aress, I, an MAC aress, M If ( aress I is alreay in the ARP cache ) { Replace corresponing MAC aress with M; } if ( message is a request an target is me ) { A sener s entry to the ARP cache proviing no entry exists; Generate an sen a response; } Computer Networks an Internets -- Moule 5 87 Spring, 2014 Copyright 2014 All rights reserve

86 Bounary Between Protocol An MAC Aressing ARP isolates harware aresses, allowing layers above to use only IP Application IP aresses use Transport Internet conceptual bounary aress resolution MAC aresses use Network Interface Physical Computer Networks an Internets -- Moule 5 88 Spring, 2014 Copyright 2014 All rights reserve

87 Thought Problem ARP is sometimes cite as a security weakness If someone gains access to a given network, how can they exploit ARP to intercept packets? Computer Networks an Internets -- Moule 5 89 Spring, 2014 Copyright 2014 All rights reserve

88 Aress Bining With IPv6 IPv6 oes not use ARP Instea, IPv6 efines a new aress bining mechanism known as IPv6 Neighbor Discovery (IPv6-ND) Sens a multicast request to fin neighbors Polls neighbors perioically, even if no atagrams are being sent Computer Networks an Internets -- Moule 5 90 Spring, 2014 Copyright 2014 All rights reserve

89 Error Reporting Mechanism

90 IP Error Detection An Reporting Recall that IP allows atagrams to be Lost Duplicate Delaye Delivere out-of-orer Why is error reporting neee? Answer: best-effort oes not mean careless the esign is intene to tolerate errors in the unerlying networks, not to introuce them IP reports problems when they are etecte Computer Networks an Internets -- Moule 5 92 Spring, 2014 Copyright 2014 All rights reserve

91 General Error Detection A variety of basic error etection mechanisms exist Examples Parity bits an other forwar error coes can etect transmission errors A CRC can etect an incorrect frame The IP heaer checksum can etect an incorrect atagram heaer IP s TTL (hop limit) can etect a routing loop A reassembly timer can etect lost fragments Only some types of errors can be reporte Computer Networks an Internets -- Moule 5 93 Spring, 2014 Copyright 2014 All rights reserve

92 Internet Control Message Protocol (ICMP) Require an integral part of IP Reports errors back to the original source Uses IP to carry messages Defines many types of messages, each with a specific format an contents Inclues information messages as well as error reports ICMPv4 an ICMPv6 share many messages Computer Networks an Internets -- Moule 5 94 Spring, 2014 Copyright 2014 All rights reserve

93 Example ICMP Messages Number Type Purpose 0 Echo Reply Use by the ping program 3 Dest Unreachable Datagram coul not be elivere 5 Reirect Host must change a route 8 Echo Request Use by the ping program 11 Time Exceee TTL expire or fragments time out 12 Parameter Problem IP heaer is incorrect 30 Traceroute Use by the traceroute program Most heavily-use ICMP messages are 8 an 0, which are sent an receive by the ping program Computer Networks an Internets -- Moule 5 95 Spring, 2014 Copyright 2014 All rights reserve

94 ICMP Encapsulation Two levels of encapsulation ICMP message encapsulate in an IP atagram IP atagram encapsulate in a network frame ICMP Hr ICMP Payloa IP Heaer IP Payloa Frame Heaer Frame Payloa Computer Networks an Internets -- Moule 5 96 Spring, 2014 Copyright 2014 All rights reserve

95 Example Of An ICMP Error Report Host S creates a atagram for estination D S sets the TTL to 255 an sens the atagram Datagram reaches a loop in the mile of the Internet Datagram circulates aroun the loop until the TTL reaches zero Router that ecrements the TTL to zero Sens a type 11 ICMP message to S Discars the atagram that cause the problem Computer Networks an Internets -- Moule 5 97 Spring, 2014 Copyright 2014 All rights reserve

96 Configuration

97 Protocol Configuration Many items must be set before protocols can be use IP aress of each network interface Aress mask for each network Initial values in the forwaring table Process is known as protocol configuration Usually occurs when operating system boots Two basic approaches Manual Automatic Computer Networks an Internets -- Moule 5 99 Spring, 2014 Copyright 2014 All rights reserve

98 Manual Configuration Use for IP routers or host that has a permanent IP aress Manager Enters configuration once Specifies that the configuration be save in non-volatile storage Interfaces inclue Comman Line Interface (CLI) an web OS Fetches values from non-volatile storage whenever the evice boots Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

99 Automatic Configuration Use primarily for hosts Initially create for iskless workstations Basic iea Use network to obtain configuration information Configure protocol software, an then start to run applications A seeming paraox Automatic configuration requires a computer to be able to use a network before the computer s protocol parameters have been configure Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

100 Ways To Solve The Paraox Use layer 2 protocols to obtain layer 3 parameters, then use layer 3 to obtain higher layers Historic approach Relie on Ethernet broacast One computer on a network respone to requests Use layer 3 to obtain all parameters Current approach Relies on IP broacast (IPv4) or multicast (IPv6) Means routers can forwar requests to a remote server Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

101 Dynamic Host Configuration Protocol (DHCP) The stanar protocol for automatic configuration Popular in private enterprises as well as with service proviers Host broacasts/multicasts a request an receives a reply Single message exchange allows a host to obtain An IP aress an aress mask to use The IP aress of a efault router The aress of a DNS server A DNS name The location of an image to boot (optional) Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

102 DHCP Message Format Same message format use for requests an responses OP HTYPE HLEN HOPS TRANSACTION IDENTIFIER SECONDS ELAPSED FLAGS CLIENT IP ADDRESS YOUR IP ADDRESS SERVER IP ADDRESS ROUTER IP ADDRESS CLIENT HARDWARE ADDRESS (16 OCTETS) SERVER HOST NAME (64 OCTETS) BOOT FILE NAME (128 OCTETS) OPTIONS (VARIABLE) Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

103 DHCP Protocol Significant features of the protocol Recovers from loss or uplication Avois synchronize flooing of requests after a powerfailure an restart Host iscovers DHCP server once an caches server aress for future interaction Derive from BOOTstrap Protocol (BOOTP), but as ynamic aress assignment Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

104 Aress Lease Paraigm DHCP server Owns a set of IP aresses Chooses an aress from the set when a request arrives Issues a lease for the aress for specifie time, T Client Obtains an aress an starts a timer for T time units Uses the aress to communicate When the timer expires, requests the server renew the lease Either receives a renewal an restarts timer or stops using the aress Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

105 Thought Problem Consier how aresses are assigne An ISP using DHCP can choose which IP aress to assign to a customer at a given time There are two approaches The ISP can remember which aress was previously assigne to each customer an use the same aress The ISP can assign aresses at ranom, meaning the customer will not retain the same aress Many ISPs try to change the aress frequently Why? Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

106 IPv6 Configuration DHCPv6 has been efine, but IPv6 prefers a new proceure known as IPv6 autoconfiguration General iea: host can generate an aress without using a server Motivation: allow two hosts to communicate without further infrastructure Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

107 Steps In IPv6 Autoconfiguration Obtain a network prefix Convention is to use a /64 prefix Globally-vali prefix can be obtaine from a router Local-scope prefix create if no router available Generate a unique suffix Verify that no one else on the network is using the resulting aress Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

108 IPv6 Autoconfiguration in Practice Nee a unique host suffix For /64 network, a 64-bit host suffix is neee Recommene approach Start with MAC aress (globally unique, but only 48 bits) Create a 64-bit value IEEE stanar EUI-64 specifies how 48 bits of an IEEE MAC aress are place in a 64-bit host suffix Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

109 Network Aress Translation (NAT)

110 NAT Motivation IPv4 was running out of aresses ISPs only want to limit a customer to one IP aress at any time, but customers want multiple evices to be online Engineers invente Network Aress Translation (NAT) as a way to solve both problems Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

111 NAT Operation Conceptually, NAT evice is locate between computers at a site an the rest of the Internet Site Only nees one globally-vali IP aress Can have multiple local hosts using the Internet Local host has full Internet access Service is transparent No change in protocols on local hosts No change in protocols on Internet servers Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

112 Conceptual Organization Of NAT Internet NAT Device Internet site with multiple computers from the Internet, site appears to be a single host NAT is sai to be in-line From the Internet, site appears to be a single computer From within the site, each computer appears to have an inepenent connection to the Internet Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

113 Aresses Use by NAT NAT evice runs a DHCP server to han out IP aresses to computers at the site Aresses assigne are IPv6 link-local or IPv4 private Block Description 10000/8 Class A private aress block /16 Class B private aress block /12 16 contiguous Class B blocks / contiguous Class C blocks NAT translates source an/or estination aresses in atagrams that pass between the site an the Internet Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

114 NAT Variants Basic NAT Only translates IP aresses Selom use in practice NAPT Translates IP aress an transport-layer port numbers Most wiely-use type of NAT Twice NAT Works with DNS server Provies NAPT plus ability to accept incoming communication Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

115 Example Of Basic NAT Suppose NAT box has globally-vali IP aress of Computer at a site has private aress Computer contacts Internet site Resulting translation is: to the Internet SRC = DST = NAT SRC = DST = SRC = SRC = DST = vali aress DST = host at site with private aress Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

116 Implementation Of NAT NAT evice keeps an internal translation table Table stores translations for both outgoing an incoming atagrams Values fille in automatically when computer at site first sens atagram to the Internet Translation table for previous example Direction Fiel Ol Value New Value IP Source out IP Destination no change -- IP Source no change -- in IP Destination Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

117 Transport-Layer NAT (NAPT) Hanles TCP, UDP, an ICMP Translates TCP/ UDP protocol port numbers as well as IP aresses Permits multiple computers at a site to contact the same Internet service simultaneously without interference Examples: Two computers at a site ownloa songs from itunes Three computers at a site contact Google simultaneously Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

118 Example Of NAPT Translation Suppose Computers at site have private aresses assigne from private aress block / 16 Two computers at the site each contact TCP port on computer NAPT chooses a new port number for each an translates Dir Fiels Ol Value New Value out IP SRC:TCP SRC : :40001 out IP SRC:TCP SRC : :40002 in IP DEST:TCP DEST : :30000 in IP DEST:TCP DEST : :30000 Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

119 NAT In Practice Many consumer proucts have NAT built in Examples: Cable an DSL moems Wireless routers Note that most wireless routers provie both wire an wireless network connections; they provie NAT on all connections Internet connection wireless router DSL or cable moem Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

120 Transport Layer Protocols: Characteristics An Techniques

121 What Shoul A Network Provie? One possibility: network centric Network offers all services, such as , web, etc Host accesses services Network authenticates user, hanles reliability Know as customer-provier communication Another possibility: network provies communication Network only transfers packets Applications hanle everything else, incluing reliability, flow control, an authentication Known as en-to-en communication Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

122 En-To-En Principle Funamental concept in the Internet Network provies best-effort packet transport Enpoints Control communication Provie all reliability Consequence Some of the most complex protocols in the Internet protocol suite run in hosts rather than in routers Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

123 Transport Layer Layer between applications an IP Application Transport Internet Network Interface Physical LAYER 5 LAYER 4 LAYER 3 LAYER 2 LAYER 1 Allows multiple applications on a given host to communicate with applications on other hosts Uses IP to carry messages Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

124 Problems A Transport Protocol Can Hanle Accommoate spee mismatch between sener an receiver Detect an recover from atagram loss Eliminate uplicate packets Guarantee that messages arrive in orer Respon to congestion in the Internet Prevent elaye packets from being misinterprete Verify that ata was not corrupte uring transit Ensure that each party has agree to communicate Note: a given transport protocol may not hanle all problems Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

125 Techniques Transport Protocols Use Application emultiplexing Sener places a value in each outgoing packet that ientifies an application on the receiving host Receiver uses the value to etermine which application shoul receive the packet Flow-control mechanisms Receiver informs sener of acceptable ata rate Sener limits rate to prevent overrunning the receiver Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

126 Techniques Transport Protocols Use (continue) Congestion control mechanisms Receiver or network informs sener about congestion in the network Sener reuces ata rate (packet rate) until congestion subsies Sequence numbers Sener places a sequence number in each packet Receiver uses the sequence numbers to ensure no packets are missing an that packets are elivere in the correct orer Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

127 Techniques Transport Protocols Use (continue) Positive acknowlegement with retransmission Receiver sens acknowlegement to inform sener when a packet arrives Sener retransmits packet if acknowlegement fails to arrive within a specifie time Sliing winow Instea of transmitting a packet an waiting for an acknowlegement, a sener transmits K packets an each time an acknowlegement arrives, transmits another Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

128 Transport Protocols Use In The Internet Two primary transport protocols use in the Internet User Datagram Protocol (UDP) Transmission Control Protocol (TCP) Choice etermine by application protocol Many applications specify the use of a single transport (eg, transfer uses TCP) Some applications allow the use of either (eg, DNS queries can be sent via UDP or TCP) Recall: each transport protocol has some surprising characteristics Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

129 Message Transport With The User Datagram Protocol

130 User Datagram Protocol (UDP) Use During startup For VoIP an some vieo applications Accounts for less than 10% of Internet traffic Blocke by some ISPs Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

131 UDP Characteristics En-to-en Connectionless communication Message-oriente interface Best-effort semantics Arbitrary interaction Operating system inepenence No congestion or flow control Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

132 En-To-En Communication UDP provies communication among applications Sening UDP Accepts outgoing message from application Places message in a User Datagram Encapsulates User Datagram in an IP atagram an sens Receiving UDP Accepts incoming User Datagram from IP Extracts message an elivers to receiving application Note: message is unchange by the network Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

133 Connectionless Communication An application using UDP can Sen a message to any receiver (universal) Sen at any time (asynchronous) Stop sening at any time (unterminate) That is, a sener oes not Inform the network before sening (ie, oes not establish a communication channel) Inform the other enpoint before sening Inform the network or other enpoint that no more messages will be sent Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

134 Message-Oriente Interface UDP Accepts an elivers messages (blocks of ata) Does not require all messages to be the same size, but oes efine a maximum message size Places each outgoing User Datagram in a single IP atagram for transmission Always elivers a complete message to receiving application Sening application must ivie outgoing ata into messages; UDP sens what it is given (or reports an error if the message is too large) Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

135 UDP Message Size UDP allows up to 64K octet messages As a practical limit, the size of a User Datagram is limite by payloa area in IP atagram Maximum IP payloa is 64K octets minus size of IP heaer Therefore, the maximum UDP payloa is 64K octets minus size of IP an UDP heaers (usually 64K octets minus 28) Application can choose any message size up to the maximum UDP payloa Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

136 Large An Small Messages What happens if an application sens a 10K octet message? The message fits into an IP atagram, but network frames have a smaller MTU (typically 1500 octets) So, the result of sening a large message is IP Fragmentation! What happens if an application chooses a small message size, such as 20 octets? Inefficiency! Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

137 Choosing An Optimal Message Size What size messages shoul an application sen? Optimal UDP message size is S = M H M is the path MTU (ie, minimum MTU on the path) H is the size of IP an UDP heaers Fining M requires an application to Violate layering an obtain forwaring information from IP Note: for IPv4, only the local MTU is known Bottom line: it may be ifficult/ impossible for an application to compute S Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

138 UDP Semantics UDP uses IP for elivery an offers the same semantics! UDP packet can be Lost Duplicate Delaye Delivere out of orer Delivere with ata bits altere Note 1: UDP oes not introuce such errors; the errors arise from the unerlying networks Note 2: UDP oes inclue an optional checksum to protect the ata (but the checksum may be isable) Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

139 Using Best-Effort Semantics Questions Do best-effort semantics make any sense for applications? Why woul a programmer choose UDP? Answers Retransmitting a lost message oes not make sense for real-time auio an vieo applications because a retransmitte packet arrives too late to be use Aitional real-time protocols can be ae to UDP to hanle out-of-orer elivery (we will cover later in the course) Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

140 Arbitrary Interaction UDP permits arbitrary interaction among applications 1-to-1 1-to-many Many-to-1 Many-to-many Application programmer chooses interaction type Ability to sen a single message to multiple recipients can be valuable Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

141 Efficient Implementation Of Interaction Key point: UDP can use IP broacast or multicast to eliver messages Provies efficient elivery to a set of hosts Example: UDP packet sent to IPv4 estination aress is elivere to all hosts on the local network (IPv6 has an all noes multicast aress) No nee for sener to transmit iniviual copies Allows application to fin a server without knowing the computer on which the server runs Broacast is a significant avantage of UDP over TCP for some applications Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

142 Operating System Inepenence Goal is to allow applications on heterogeneous computers to interact Must avoi OS-specific ientifiers, such as Process IDs Task names Instea, create application ientifiers that are not erive from any OS Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

143 UDP Application Ientifiers 16-bit integer known as UDP protocol port number Each application using UDP must obtain a port number Sening UDP Places a port number in UDP heaer to ientify estination application on receiving host Also inclues port number of sening application Receiving UDP Uses value in heaer to select appropriate application UDP protocol port numbers are universal across all computers, an o not epen on the operating system Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

144 Ientifying An Application Both sening an receiving applications nee a port number Assignment of port numbers epens on the type of application Application that offers a stanarize service (server) Uses a well-known port number for the service Value is less than 1024 Example: TFTP service uses UDP port 69 Other applications (client) Request a port number from the local operating system Value is greater than Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

145 Steps Taken To Contact A Service Request an unuse local port number from the local operating system Obtain the IP aress of the local computer from the operating system Look up the port number of the service to be contacte Obtain the omain name of a computer that runs the service an map to an IP aress Form a UDP atagram with a source port fiel set to the local port number an the estination port fiel set to the port number of the service Request that the UDP atagram be encapsulate in an IP atagram an sent using the source an estination IP aresses obtaine above Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

146 Examples Of Well-Known UDP Ports Port Number Description 0 Reserve (never assigne) 7 Echo 9 Discar 11 Active Users 13 Daytime 15 Network Status Program 17 Quote of the Day 19 Character Generator 37 Time 42 Host Name Server 43 Who Is 53 Domain Name Server 67 BOOTP or DHCP Server 68 BOOTP or DHCP Client 69 Trivial File Transfer 88 Kerberos Security Service 111 Sun Remote Proceure Call 123 Network Time Protocol 161 Simple Network Management Protocol 162 SNMP Traps 514 System Log Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

147 UDP Datagram Format Extremely thin layer User Datagram is ivie into heaer an payloa Heaer contains only 8 octets: UDP SOURCE PORT UDP DESTINATION PORT UDP MESSAGE LENGTH UDP CHECKSUM PAYLOAD (MESSAGE DATA) Question: why is length neee? Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

148 UDP Checksum 16-bit 1s-complement checksum Covers entire UDP packet, incluing ata (recall: IP oes not checksum the payloa) Is optional: value of zero means sener i not compute a checksum Inclues extra pseuo heaer that contains IP aresses Example of IPv4 pseuo heaer: IP SOURCE ADDRESS IP DESTINATION ADDRESS ZERO PROTO UDP LENGTH Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

149 Purpose Of A Pseuo Heaer Receiver can verify that message arrive at correct computer as well as correct application on that computer Consequence for NAT: if it changes the IP source or estination aress, NAT must recompute UDP checksum Note: pseuo heaers provie another example of layering violations Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

150 UDP Encapsulation User Datagram travels in IP atagram Two levels of encapsulation occur UDP Hr UDP Payloa IP Heaer IP Payloa Frame Heaer Frame Payloa Note: the message the application places in the UDP Payloa fiel may also have heaer an payloa fiels Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

151 Transmission Control Protocol (Stream Transport)

152 Transmission Control Protocol (TCP) The primary transport-layer protocol use in the Internet Accounts for about 90% of all Internet traffic (some estimates are higher) Provies reliability Appeals to programmers Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

153 TCP Characteristics En-to-en communication Connection-oriente paraigm Point-to-point connections Complete reliability Full-uplex communication Stream interface Reliable connection startup Graceful connection shutown Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

154 En-To-En Communication TCP provies communication among pairs of applications Allows an application on one host to communicate with an application on another host Permits multiple applications on a given computer to communicate simultaneously without interference Uses protocol port numbers to istinguish among applications Note: TCP ports are completely inepenent of UDP ports Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

155 En-To-En Principle An Transport Protocols Transport protocols operate in en systems, an view the unerlying Internet as a virtual network Host A appl TCP IP net iface communication system as viewe by TCP router IP net iface net 1 net 2 Host B appl TCP IP net iface IP oes not rea or interpret TCP packets When forwaring atagrams, router only processes layers 1 through 3 Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

156 TCP Protocol Port Numbers 16-bit integers use to ientify applications Each application nees a port number TCP well-known port assignments are inepenent of UDP assignments However, to help humans, the same value chosen if service available via either transport Examples Both UDP an TCP assign port 53 to the Domain Name System (DNS) Both UDP an TCP assign port 7 to the echo service Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

157 Protocol Ports, The Four-Tuple, An Flows Key concept: because a TCP connection correspons to a pair of enpoints, the connection is ientifie by four items IP source aress TCP source port IP estination aress TCP estination port Commonly calle the four-tuple Explains how an application such as a web server can communicate with multiple clients at the same time Interestingly, more than four values must be extracte from a frame to ientify a TCP flow Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

158 TCP s Connection-Oriente Paraigm Analogous to a telephone call Pair of applications must Establish a TCP connection before communicating Terminate the connection when finishe Important insights A TCP connection is virtual because only the two enpoints know a connection is in place TCP oes not have keep-alive messages: no packets are exchange unless applications are sening ata Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

159 Limite Interaction A TCP connection only provies communication between a pair of applications Known as a point-to-point communication TCP connection oes not support Reception from an arbitrary set seners Multi-point connections with more than two enpoints Broacast or multicast elivery Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

160 The TCP Reliability Guarantee TCP provies full reliability Compensates for Loss Duplication Delivery out of orer Does so without overloaing the unerlying networks an routers TCP makes the following guarantee Data will be elivere or sener will (eventually) be notifie Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

161 TCP Reliability Uses timeout-an-retransmission Receiver returns an acknowlegement (ACK) to sener when ata arrives Sener waits for acknowlegement an retransmits ata if no acknowlegement arrives Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

162 Illustration Of TCP Retransmission Events at Host 1 Events at Host 2 sen message 1 receive ack 1 sen message 2 receive ack 2 sen message 3 receive message 1 sen ack 1 receive message 2 sen ack 2 retransmission timer expires retransmit message 3 packet lost receive message 3 sen ack 3 Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

163 Why TCP Retransmission Is Har TCP esigne for Internet Roun-trip elays iffer among connections Roun-trip elays vary over time Waiting too long introuces unnecessary elay Not waiting long enough sens unnecessary copies Key to TCP s success: aaptive retransmission Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

164 How Ba Is The Internet? In the ol ays: elays in secons, high variability Now: elays in secons, high variability Example roun-trip measurements from Irelan to California, 2009 Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

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