What s a protocol? CE80N Introduction to Networks & The Internet. Communication Protocol. Protocol Layers. Dr. Chane L. Fullmer UCSC Winter 2002

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1 E80N Introduction to Networks & The Internet Reading hapter 15 IP: Software To reate A Virtual Network Dr. hane L. Fullmer US Winter 2002 January E80N -- Lecture #6 1 January E80N -- Lecture #6 2 ommunication Protocol A common language computers use to exchange messages. Specifying exact format and meaning of each message Sending and receiving January E80N -- Lecture #6 3 human protocols: what s the time? I have a question Introductions specific msgs sent specific actions taken when msgs received or other events What s a protocol? network protocols: machines rather than humans all communication activity in Internet governed by protocols January E80N -- Lecture #6 4 Human Protocol Q: Other human protocol? What s a protocol? A B? omputer Protocol! " #%$ &'! (+* * * -. / / / :;3 * 7 <;= >?@ January E80N -- Lecture #6 5 Networks are complex! many pieces : hosts routers links of various media applications protocols hardware software Protocol Layers Question: Is there any hope of organizing structure of network? Or at least our discussion of networks? January E80N -- Lecture #6 6 1

2 - '( - '( Organization of air travel $ & $ ; $ # # # " +" ; +$ ; $ ;& #%$ #$ #$ "" a series of steps January E80N -- Lecture #6 7 Organization of air travel: a different view # $ + ; $ & #$ #$ # '# +$ "; $ & #$ "" $ Layers: each layer implements a service via its own internal-layer actions relying on services provided by layer below January E80N -- Lecture #6 8 Layered air travel: services # + # $ & #! ;$ +$ ''$ & # #$ " $ " # $ ; & + &$'$ & #$ #%$ # Distributed implementation of layer functionality * '. + - ( $ # + +$ ";;$ "; $ + * % & & #$ "" & #$ "" < A?@ = A =@ ='= AB' A?B $ * '. + - ( + + / ( :1 5; :1 5; January E80N -- Lecture # :1 5; January Distributed implementation of layer functionality Layering Model ; $ & #$ $ + + / ( ; $ & #$ January * < A?@ = A ='= AEB% A?DB % & Purpose is to divide and conquer complex software and hardware needed to implement services Partition services and functions needed in system into layers Each layer of service is provided by peer protocol entities Extensibility (new protocols and services easily added ommunication can be point-to-point or multipoint NODE A Layer-N Protocol Entity interface Layer-(N - 1 Protocol Entity Layer N packets (virtual communication protocol Layer-N Protocol Entity Layer-(N - 1 Protocol Entity NODE B January E80N -- Lecture #6 12 2

3 Why layering? Dealing with complex systems: explicit structure allows identification relationship of complex system s pieces layered reference model for discussion modularization eases maintenance updating of system change of implementation of layer s service transparent to rest of system e.g. change in gate procedure doesn t affect rest of system Is layering considered harmful? January E80N -- Lecture #6 13 Internet protocol stack application: supporting network applications ftp smtp http transport: host-host data transfer tcp udp network: routing of datagrams from source to destination ip routing protocols link: data transfer between neighboring network elements ppp ethernet physical: bits on the > A@ A < A =@<B?= A <'? A?= January E80N -- Lecture #6 14 > <?B% A@ > Layering: logical communication Each layer: distributed entities implement layer functions at each node entities perform actions exchange messages with peers! Layering: logical communication E.g.: transport take data from application add addressing reliability check info to form datagram send datagram to peer wait for peer to ack receipt analogy: post office " #$%&'($# " " #$%&'($# January E80N -- Lecture #6 15 January E80N -- Lecture #6 16 Layering: physical communication "! " 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 -/. -/ B +?= A'? >?DB A <@ > A@ A < A A <'? A?= > <?B' A@ > A@ A < A A <'? A?= > < *?B% A@ > ''!%!6 5 ' &!# 8 %#%69$% 4 : $% 4/5 January E80N -- Lecture #6 17 January E80N -- Lecture #6 18 3

4 Open Systems Interconnect (OSI Stack Proposed by the International Standards Organization Specifies the functions at each layer not the protocols that implement them Open Systems Interconnect APPLIATION PRESENTATION SESSION TRANSPORT NETWORK LINK End-user services (mail file transfer Formatting encryption compression of data Setup and management of end-to-end dialogue End-to-end delivery of messages to processes End-to-end transmission of packets in net Transmission of packets over a link Web access TP IP PPP SMA/D PHYSIAL Transmission of bit over physical media SONET January E80N -- Lecture #6 19 Source: Link 2 Semiconductor January E80N -- Lecture #6 20 Description of OSI layers Application layer: The application layer provides services to a calling computer program. An application layer might take care of all the downloads involved in transferring a web page (that is the text graphics and other files to a browser (the client program or application. Presentation layer: Takes care of any data format translations that might be needed to take the particular bits representing for example a number in the client computer and convert them to a "universal" number representation recognized by the communications system. Session layer: This layer is responsible for controlling exchange of information for example by having the client and server take turns transmitting data. January E80N -- Lecture #6 21 OSI Layers (continued Transport layer: The transport layer is responsible for getting messages from one computer to another. Network layer: The network layer is responsible for getting data across a communications network from one host computer to another. Data Link layer: The data link layer (often abbreviated to "link" layer or DLL gets data from one network node (e.g. a computer or router to another. Physical layer: The physical layer is the set of specifications that describe the actual medium of transmission. For example a physical layer specification might include the type of connectors and wire to be used in a cable linking two machines as well as the function of each wire and voltage levels that specify a "1" or "0" allowed rate of transmission and so on. January E80N -- Lecture #6 22 Basic Functionality: Internet Protocol (IP IP defines computer communication details. Specifying how packets are formed Specifying how routers forward each packet IP Forwarding omputers connecting to the Internet must follow the IP rules. January E80N -- Lecture #6 23 IP Software On Every Machine omputer hardware does not understand IP. onnecting a computer to the Internet does not mean it can use the Internet omputers need IP software before using the Internet. Windows Unix Mac OS January E80N -- Lecture #6 24 4

5 Internet Packets Are alled Datagrams IP Datagrams are packets that follow the IP specifications. Traveling across the Internet independent of sender The Illusion Of A Giant Network Any computer can send IP datagrams to any other computer providing they have IP software installed (Universal Service The Internet operates like a virtual network. January E80N -- Lecture #6 25 January E80N -- Lecture #6 26 The Virtual Network The Reality Of Internal Structure The Internet contains a complex physical structure users never see Interconnecting networks with routers Figure 15.1 The view of the Internet that IP software provides. Users and application programs treat the Internet like a single large network that allow arbitrary numbers of computers to communicate. January E80N -- Lecture #6 28 Internet Internal Structure Datagrams Travel In Packets IP datagram defines a standard for all Internet packets. Routers: Encloses the data before sending out the packet Data Encapsulation Figure 15.2 A small example of the physical structure that remains hidden inside the Internet. Each computer attaches to a single network; routers interconnect the networks. January E80N -- Lecture #6 30 5

6 / : ; : 8 D(4.E <7 => 23 3(7? ( @42 ; : : 47-4 IP datagram format 7 -B : 8 '4. 0. * ( / 4.. 3( % : / " / : ; ( 7 4 A 7 3 : F : ; G B B 3< 73 8 H %#% H 8 0.: %$ % 5 5 &6# " # ( % %"!# 4' B $%$'&(# ' 5 6 4/5 &# Every omputer Is Assigned A Unique Address Each computer attached to the Internet must be assigned a unique address. One computer must know the address of another before it can communicate January E80N -- Lecture #6 31 January E80N -- Lecture #6 32 Internet Addresses An Odd IP Address Syntax The unique number assigned to a computer is its Internet (IP address. Each computer (including routers need to have an IP address. omputer stores IP address in four binary units called bytes. one twenty-eight dot one fourteen dot one dot zero January E80N -- Lecture #6 33 January E80N -- Lecture #6 34 IP Addresses Are Not Random IP addresses are not random. omputers on the same network have the same prefix. January E80N -- Lecture #6 35 IP Addressing: introduction IP address: 32-bit identifier for host router interface interface: connection between host router and physical link router s typically have multiple interfaces host may have multiple interfaces IP addresses associated with interface not host router = January E80N -- Lecture #6 36 6

7 / IP Addressing IP Addressing IP address: network part (high order bits host part (low order bits What s a network? (from IP address perspective device interfaces with same network part of IP address can physically reach each other without intervening router /5 4 1'0 4 /5 Ë4.% H. E7? 4 1'7 -- ; How to find the networks? Detach each interface from router host create islands of isolated networks : = January E80N -- Lecture #6 37 January E80N -- Lecture # IP Addresses given notion of network let s re-examine IP addresses: * ** *** / # B 3(2 0 7 "7 -- '(+*-.! " # "# $ # &% $" January E80N -- Lecture #6 39 IP addressing: IDR lassful addressing: inefficient use of address space address space exhaustion e.g. class B net allocated enough addresses for 65K hosts even if only 2K hosts in that network IDR: lassless InterDomain Routing network portion of address of arbitrary length address format: a.b.c.d/x where x is # bits in network portion of address ED;D F ;G F HJIKF D;L7;G January E80N -- Lecture #6 40 IP addresses: how to get one? Q: How does host get IP address? hard-coded by system admin in a file Wintel: control-panel->network->config->tcp/ip->properties (reboot UNIX: /etc/rc.config DHP: Dynamic Host onfiguration Protocol: dynamically get address: plug-and-play host broadcasts DHP discover msg DHP server responds with DHP offer msg host requests IP address: DHP request msg DHP server sends address: DHP ack msg IP addresses: how to get one? Network (network portion: get allocated portion of ISP s address space: ISP s block /20 Organization /23 Organization /23 Organization / Organization /23 January E80N -- Lecture #6 41 January E80N -- Lecture #6 42 7

8 Œ Š Ž IP addressing: the last word... Q: How does an ISP get block of addresses? A: IANN: Internet orporation for Assigned Names and Numbers allocates addresses manages DNS assigns domain names resolves disputes January E80N -- Lecture #6 43 Hierarchical addressing: route aggregation D EGF HJIKLIMONPF M KPQRKLSLSLIHJTTUF V WXKLQ Q Ÿ Z[T\HU] ]PF MPF HJV ^ KOSJ_ HOI^`F TOHJabHOV ^cy]xiyod ^ F VUWeF V]OŸ IafKJ^`F Ÿ Vhg 8 9 :! #"! $%#! '&#" 8 9 :! #"! $(! '&#" 8 9 :! *"! #!&#" + -. / '7 8 9 : ;! *"! "'#!&#" 5 A? # B B 5 < 4 = > < = 4 January E80N -- Lecture #6 44 Hierarchical addressing: more specific routes D ikj[l m no[npqmfrjs mbsutgvow xum`yuxozo{ O{ zuw v }~`x~ v w#ƒ sj O{ s~o{ vo u 8 9 : #! #"#! $%#! '&*" 8 9 :! #"!! '&#" 8 9 : ;! #"! "! '&#" 8 9 :! #"#! $(#! '&*" + -. / '7 # 5 A? # B B : # ˆ 5 < 4 = > < = 4 A Trip Through The Internet A router must choose between two paths that both lead to the destination. hoosing the shortest path January E80N -- Lecture #6 45 January E80N -- Lecture #6 46 Getting a datagram from source to dest. IP datagram: / / datagram remains unchanged as it travels source to destination Œ addr fields of interest here routing table in A Dest. Net. next router Nhops Getting a datagram from source to dest. P * # # š œ œ' œr # #š œ œ š #ž Starting at A given IP datagram addressed to B: look up net. address of B find B is on same net. as A link layer will send datagram directly to B inside link-layer frame B and A are directly connected Dest. Net. next router Nhops January E80N -- Lecture #6 47 January E80N -- Lecture #6 48 8

9 Ž B œ A > š œ š? œ Getting a datagram from source to dest. P * # # š œ œ' œh # #š œ # š #'ž Starting at A dest. E: look up network address of E E on different network A E not directly attached routing table: next hop router to E is link layer sends datagram to router inside link-layer frame datagram arrives at continued.. Dest. Net. next router Nhops January E80N -- Lecture #6 49 Getting a datagram from source to dest. L Dest. next # # # #š œ œ œ # #š œ # š #'ž network router Nhops interface Arriving at destined for look up network address of E E on same network as router s interface router E directly attached link layer sends datagram to inside link-layer frame via interface datagram arrives at!!! (hooray! January E80N -- Lecture #6 50 / ;:0 30<;0 =! "#$ %& #!' # (* +%! #-$&!. Graph abstraction for routing algorithms: graph nodes are routers graph edges are physical links link cost: delay $ cost or congestion level Routing good path: typically means minimum cost path other def s possible January E80N -- Lecture #6 51 Animation of Routing Algorithms Dijkstra s algorithm: Distance Vector algorithm: January E80N -- Lecture #6 52 Routing Algorithm classification Global or decentralized information? Global: all routers have complete topology link cost info link state algorithms Decentralized: router knows physicallyconnected neighbors link costs to neighbors iterative process of computation exchange of info with neighbors distance vector algorithms Static or dynamic? Static: routes change slowly over time Dynamic: routes change more quickly periodic update in response to link cost changes A Link-State Routing Algorithm Dijkstra s algorithm net topology link costs known to all nodes accomplished via link state broadcast all nodes have same info computes least cost paths from one node ( source to all other nodes gives routing table for that node iterative: after k iterations know least cost path to k dest. s Notation: c(ij: link cost from node i to j. cost infinite if not direct neighbors D(v: current value of cost of path from source to dest. V p(v: predecessor node along path from source to v that is next v N: set of nodes whose least cost path definitively known January E80N -- Lecture #6 53 January E80N -- Lecture #6 54 9

10 ( Dijkstra s Algorithm 1 Initialization: 2 N = {A} 3 for all nodes v 4 if v adjacent to A 5 then D(v = c(av 6 else D(v = infty 7 8 Loop 9 find w not in N such that D(w is a minimum 10 add w to N 11 update D(v for all v adjacent to w and not in N: 12 D(v = min( D(v D(w + c(wv 13 /* new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v */ 15 until all nodes in N January E80N -- Lecture #6 55 Step Dijkstra s algorithm: example start N A AD ADE ADEB ADEB ADEBF D(Bp(B 2A 2A 2A D(p( D(Dp(D 5A 1A 4D 3E 3E D(Ep(E infinity 2D D(Fp(F infinity infinity 4E 4E 4E January E80N -- Lecture #6 56 Dijkstra s algorithm discussion Algorithm complexity: n nodes each iteration: need to check all nodes w not in N n*(n+1/2 comparisons: O(n**2 more efficient implementations possible: O(nlogn Oscillations possible: e.g. link cost = amount of carried traffic! #"! #" '%! #" % & $" January E80N -- Lecture #6 57 Distance Vector Routing Algorithm iterative: continues until no nodes exchange info. self-terminating: no signal to stop asynchronous: nodes need not exchange info/iterate in lock step! distributed: each node communicates only with directly-attached neighbors Distance Table data structure each node has its own row for each possible destination column for each directly-attached neighbor to node example: in node X for dest. Y via neighbor Z: X D (YZ distance from X to Y via Z as next hop Z c(xz + min {D (Yw} w January E80N -- Lecture #6 58 = = E D (D E D (AD E D (AB Distance Table: example D c(ed + min {D (w} w = = 2+2 = 4 D c(ed + min {D (Aw} w * ++- = = 2+3 = 5 B c(eb + min {D (Aw} w = = 8+6 = 14 * ++- cost to destination via E D ( A B D January E80N -- Lecture #6 59 destination A B D Distance table gives routing table cost to destination via E D ( A B D destination A B D D D4.0/ 1# ;:=< 8 >!?6@ 2;/ 56A243;:=< 8 January E80N -- Lecture #6 60 destination A B Outgoing link to use cost A1 D5 D4 10

11 Distance Vector Routing: overview Iterative asynchronous: Each local iteration caused by: local link cost change message from neighbor: its least cost path change from neighbor Distributed: each node notifies neighbors only when its least cost path to any destination changes neighbors then notify their neighbors if necessary 367 5? 8 wait for (change in local link cost of msg from neighbor recompute distance table if least cost path to any dest has changed notify neighbors Distance Vector Algorithm: 2 3 < <45? Initialization: 2 for all adjacent nodes v: 3 D X(*v = infty /* the * operator means "for all rows" */ X 4 D (vv = c(xv 5 for all destinations y X 6 send min D (yw to each neighbor /* w over all X s neighbors */ w January E80N -- Lecture #6 61 January E80N -- Lecture #6 62 Distance Vector Algorithm (cont.: 8 loop 9 wait (until I see a link cost change to neighbor V 10 or until I receive update from neighbor V if (c(xv changes by d 13 /* change cost to all dest s via neighbor v by d */ 14 /* note: d could be positive or negative */ 15 for all destinations y: D X (yv = D X (yv + d else if (update received from V wrt destination Y 18 /* shortest path from V to some Y has changed */ 19 /* V has sent a new value for its min w D V (Yw */ 20 /* call this received new value is "newval" */ 21 for the single destination y: D X (YV = c(xv + newval if we have a new min w D X (Ywfor any destination Y 24 send new value of min w D X (Yw to all neighbors forever E80N -- Lecture #6 63 omparison of LS and DV algorithms Message complexity LS: with n nodes E links O(nE msgs sent each DV: exchange between neighbors only convergence time varies Speed of onvergence LS: O(n 2 algorithm requires O(nE msgs may have oscillations DV: convergence time varies may be routing loops Robustness: what happens if router malfunctions? LS: node can advertise incorrect link cost each node computes only its own table DV: DV node can advertise incorrect path cost each node s table used by others error propagate thru network count-to-infinity problem January E80N -- Lecture #6 64 IMP: Internet ontrol Message Protocol used by hosts routers gateways to communication network-level information error reporting: unreachable host network port protocol echo request/reply (used by ping network-layer above IP: IMP msgs carried in IP datagrams IMP message: type code plus first 8 bytes of IP datagram causing error Type ode description 0 0 echo reply (ping 3 0 dest. network unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used 8 0 echo request (ping 9 0 route advertisement 10 0 router discovery 11 0 TTL expired 12 0 bad IP header Summary Protocols and protocol stack Internet stack OSI stack IP addressing IP forwarding Routing Link State Distance Vector January E80N -- Lecture #6 65 January E80N -- Lecture #

12 Glossary Protocol The rules two or more computers must follow to exchange messages Internet Protocol (IP Specification for the format of packets computers use when communicating across the Internet IP Datagram A packet of data sent across the Internet Glossary Virtual Network Appearance of a single seamless network system Internet Address (IP Address An unique number assigned to a computer attached to the Internet January E80N -- Lecture #6 67 January E80N -- Lecture #