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תזכורת

תזכורת. שיעור השלמה יום שישי הקרוב שעות 9-12 מקום: אורנשטיין 111. Last Week. Basic Routing Schemes Link State: broadcast link information Local computation on global topology Distance vector: Exchange distance information with neighbors Local updates based on neighbors information

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תזכורת

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  1. תזכורת • שיעור השלמה יום שישי הקרוב • שעות 9-12 • מקום: אורנשטיין 111

  2. Last Week • Basic Routing Schemes • Link State: • broadcast link information • Local computation on global topology • Distance vector: • Exchange distance information with neighbors • Local updates based on neighbors information • Hierarchical Routing • Broadcast and multicast • Using a tree topology

  3. This week • Hierarchical routing • IP addresses • Definition of network • Network Address Translation (NAT) • Routing algorithms implementations

  4. Host, router network layer functions: • ICMP protocol • error reporting • router “signaling” • IP protocol • addressing conventions • datagram format • packet handling conventions • Routing protocols • path selection • RIP, OSPF, BGP routing table The Internet Network layer Transport layer: TCP, UDP Network layer Link layer physical layer Lecture 6: Network Layer

  5. scale: with 50 million destinations: can’t store all dest’s in routing tables! routing table exchange would swamp links! administrative autonomy internet = network of networks each network admin may want to control routing in its own network Hierarchical Routing Our routing study thus far - idealization • all routers identical • network “flat” … not true in practice Lecture 6: Network Layer

  6. aggregate routers into regions, “autonomous systems” (AS) routers in same AS run same routing protocol “intra-AS” routing protocol routers in different AS can run different intra-AS routing protocol special routers in AS run intra-AS routing protocol with all other routers in AS also responsible for routing to destinations outside AS run inter-AS routing protocol with other gateway routers gateway routers Hierarchical Routing Lecture 6: Network Layer

  7. c b b c a A.c A.a C.b B.a Intra-AS and Inter-AS routing • Gateways: • perform inter-AS routing amongst themselves • perform intra-AS routers with other routers in their AS b a a C B d A network layer inter-AS, intra-AS routing in gateway A.c link layer physical layer Lecture 6: Network Layer

  8. Inter-AS routing between A and B b c a a C b B b c a d Host h1 A A.a A.c C.b B.a Intra-AS and Inter-AS routing Host h2 Intra-AS routing within AS B Intra-AS routing within AS A • We’ll examine specific inter-AS and intra-AS Internet routing protocols shortly Lecture 6: Network Layer

  9. Routing: Example AS B (OSPF intra AS D AS A (OSPF) routing) d E d->a2: I can reach hosts in D; my path: D Export to E: i->e: I can reach hosts in D; path: IBCD a2 a2->a1: I can reach hosts in D; path: D a1->i: I can reach hosts in D; my path: AD a1 No Exportto F i F AS C choose BCD using i2 i2->i: I can reach hosts in D; path: BCD b->i: I can reach hosts in D; my path: BCD i2 b b->i2: I can reach hosts in D; my path: BCD AS I Lecture 6: Network Layer

  10. Routing: Example AS B (OSPF intra AS D AS A (OSPF) routing) d1 d d2 E a2 a1->i: I can reach hosts in D; my path: AD i F AS C a1 How to specify? b AS I Lecture 6: Network Layer

  11. IP Addressing Scheme We need an address to uniquely identify each destination Routing scalability needs flexibility in aggregation of destination addresses we should be able to aggregate a set of destinations as a single routing unit Preview: the unit of routing in the Internet is a network---the destinations in the routing protocols are networks Lecture 6: Network Layer

  12. 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, or router 223.1.1.2 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 IP Addressing: introduction 223.1.1.1 223.1.2.9 223.1.1.4 223.1.1.3 223.1.1.1 = 11011111 00000001 00000001 00000001 223 1 1 1 Lecture 6: Network Layer

  13. 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 IP Addressing 223.1.1.1 223.1.2.1 223.1.1.2 223.1.2.9 223.1.1.4 223.1.2.2 223.1.3.27 223.1.1.3 LAN 223.1.3.2 223.1.3.1 network consisting of 3 IP networks (for IP addresses starting with 223, first 24 bits are network address) Lecture 6: Network Layer

  14. How to find the networks? Detach each interface from router, host create “islands of isolated networks IP Addressing 223.1.1.2 223.1.1.1 223.1.1.4 223.1.1.3 223.1.7.0 223.1.9.2 223.1.9.1 223.1.7.1 223.1.8.1 223.1.8.0 223.1.2.6 223.1.3.27 Interconnected system consisting of six networks 223.1.2.1 223.1.2.2 223.1.3.1 223.1.3.2 Lecture 6: Network Layer

  15. multicast address 1110 network host 110 network 10 host IP Addresses given notion of “network”, let’s re-examine IP addresses: “class-full” addressing: class 1.0.0.0 to 127.255.255.255 A network 0 host 128.0.0.0 to 191.255.255.255 B 192.0.0.0 to 223.255.255.255 C 224.0.0.0 to 239.255.255.255 D 32 bits Lecture 6: Network Layer

  16. host part network part 11001000 0001011100010000 00000000 200.23.16.0/23 IP addressing: CIDR • classful 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 • CIDR:Classless 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 Lecture 6: Network Layer

  17. CIDR Address Aggregation AS D AS A (OSPF) d1 d a2 130.132.1/24 i->a1: I can reach 130.132/16; my path: I i a1 intradomain routing uses /24 130.132.2/24 130.132.3/24 AS I Lecture 6: Network Layer

  18. CIDR Address Aggregation B x00/24: B x/22: A C A x01/24: C x11/24: GF x10/24: E E x11/24: F G x11/24: F F Lecture 6: Network Layer

  19. IP addresses: how to get one? Hosts (host portion): • hard-coded by system admin in a file • DHCP:Dynamic Host Configuration Protocol: dynamically get address: “plug-and-play” • host broadcasts “DHCP discover” msg • DHCP server responds with “DHCP offer” msg • host requests IP address: “DHCP request” msg • DHCP server sends address: “DHCP ack” msg • The common practice in LAN and home access (why?) Lecture 6: Network Layer

  20. IP addresses: how to get one? Network (network portion): • get allocated portion of ISP’s address space: ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20 Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. …. Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23 Lecture 6: Network Layer

  21. 200.23.16.0/23 200.23.18.0/23 200.23.30.0/23 200.23.20.0/23 . . . . . . Hierarchical addressing: route aggregation Hierarchical addressing allows efficient advertisement of routing information: Organization 0 Organization 1 “Send me anything with addresses beginning 200.23.16.0/20” Organization 2 Fly-By-Night-ISP Internet Organization 7 “Send me anything with addresses beginning 199.31.0.0/16” ISPs-R-Us Lecture 6: Network Layer

  22. 200.23.16.0/23 200.23.18.0/23 200.23.30.0/23 200.23.20.0/23 . . . . . . Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 Organization 0 “Send me anything with addresses beginning 200.23.16.0/20” Organization 2 Fly-By-Night-ISP Internet Organization 7 “Send me anything with addresses beginning 199.31.0.0/16 or 200.23.18.0/23” ISPs-R-Us Organization 1 Lecture 6: Network Layer

  23. Network Address Translation: Motivation • A local network uses just one public IP address as far as outside world is concerned • Each device on the local network is assigned a private IP address rest of Internet local network (e.g., home network) 192.168.1.0/24 192.168.1.2 192.168.1.1 192.168.1.3 138.76.29.7 192.168.1.4 All datagrams leaving local network have same single source NAT IP address: 138.76.29.7, different source port numbers Datagrams with source or destination in this network have 192.168.1/24 address for source, destination (as usual) Lecture 6: Network Layer

  24. NAT: Network Address Translation Implementation: NAT router must: outgoing datagrams:replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #) . . . remote clients/servers will respond using (NAT IP address, new port #) as destination addr. remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair incoming datagrams:replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table Lecture 6: Network Layer

  25. NAT: Network Address Translation 3 1 2 4 S: 192.168.1.2, 3345 D: 128.119.40.186, 80 S: 138.76.29.7, 5001 D: 128.119.40.186, 80 1: host 192.168.1.2 sends datagram to 128.119.40.186, 80 2: NAT router changes datagram source addr from 192.168.1.2, 3345 to 138.76.29.7, 5001, updates table S: 128.119.40.186, 80 D: 192.168.1.2, 3345 S: 128.119.40.186, 80 D: 138.76.29.7, 5001 NAT translation table WAN side addr LAN side addr 138.76.29.7, 5001 192.168.1.2, 3345 …… …… 192.168.1.2 192.168.1.1 192.168.1.3 138.76.29.7 192.168.1.4 4: NAT router changes datagram dest addr from 138.76.29.7, 5001 to 192.168.1.2, 3345 3: Reply arrives dest. address: 138.76.29.7, 5001 Lecture 6: Network Layer

  26. Network Address Translation: Advantages No need to be allocated range of addresses from ISP: - just one public IP address is used for all devices 16-bit port-number field allows 60,000 simultaneous connections with a single LAN-side address ! can change ISP without changing addresses of devices in local network can change addresses of devices in local network without notifying outside world Devices inside local net not explicitly addressable, visible by outside world (a security plus) Lecture 6: Network Layer

  27. NAT: Network Address Translation If both hosts are behind different NAT, they will have difficulty establishing connection NAT is controversial: routers should process up to only layer 3 violates end-to-end argument NAT possibility must be taken into account by app designers, e.g., P2P applications address shortage should instead be solved by having more addresses --- IPv6 ! Lecture 6: Network Layer

  28. IP addressing: the last word... Q: How does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers • allocates addresses • manages DNS • assigns domain names, resolves disputes Lecture 6: Network Layer

  29. IP datagram: 223.1.1.1 223.1.2.1 A E B 223.1.1.2 source IP addr 223.1.2.9 misc fields dest IP addr 223.1.1.4 data 223.1.2.2 223.1.3.27 223.1.1.3 223.1.3.2 223.1.3.1 Dest. Net. next router Nhops 223.1.1 1 223.1.2 223.1.1.4 2 223.1.3 223.1.1.4 2 Getting a datagram from source to dest. routing table in A • datagram remains unchanged, as it travels source to destination • addr fields of interest here • mainly dest. IP addr Lecture 6: Network Layer

  30. 223.1.1.1 223.1.2.1 E B A 223.1.1.2 223.1.2.9 223.1.1.4 223.1.2.2 223.1.3.27 223.1.1.3 223.1.3.2 223.1.3.1 Dest. Net. next router Nhops 223.1.1 1 223.1.2 223.1.1.4 2 223.1.3 223.1.1.4 2 Getting a datagram from source to dest. misc fields data 223.1.1.1 223.1.1.3 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 Lecture 6: Network Layer

  31. 223.1.1.1 223.1.2.1 E B A 223.1.1.2 223.1.2.9 223.1.1.4 223.1.2.2 223.1.3.27 223.1.1.3 223.1.3.2 223.1.3.1 Dest. Net. next router Nhops 223.1.1 1 223.1.2 223.1.1.4 2 223.1.3 223.1.1.4 2 Getting a datagram from source to dest. misc fields data 223.1.1.1 223.1.2.2 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 223.1.1.4 • link layer sends datagram to router 223.1.1.4 inside link-layer frame • datagram arrives at 223.1.1.4 • continued….. Lecture 6: Network Layer

  32. Dest. next 223.1.1.1 network router Nhops interface 223.1.2.1 E B A 223.1.1 - 1 223.1.1.4 223.1.1.2 223.1.2 - 1 223.1.2.9 223.1.2.9 223.1.1.4 223.1.3 - 1 223.1.3.27 223.1.2.2 223.1.3.27 223.1.1.3 223.1.3.2 223.1.3.1 Getting a datagram from source to dest. misc fields data 223.1.1.1 223.1.2.2 Arriving at 223.1.4, destined for 223.1.2.2 • look up network address of E • E on same network as router’s interface 223.1.2.9 • router, E directly attached • link layer sends datagram to 223.1.2.2 inside link-layer frame via interface 223.1.2.9 • datagram arrives at 223.1.2.2!!! (hooray!) Lecture 6: Network Layer

  33. IP datagram format IP protocol version number 32 bits total datagram length (bytes) header length (bytes) type of service head. len ver length for fragmentation/ reassembly fragment offset “type” of data flgs 16-bit identifier max number remaining hops (decremented at each router) upper layer time to live Internet checksum 32 bit source IP address 32 bit destination IP address upper layer protocol to deliver payload to E.g. timestamp, record route taken, specify list of routers to visit. Options (if any) data (variable length, typically a TCP or UDP segment) Lecture 6: Network Layer

  34. network links have MTU (max.transfer size) - largest possible link-level frame. different link types, different MTUs large IP datagram divided (“fragmented”) within net one datagram becomes several datagrams “reassembled” only at final destination IP header bits used to identify, order related fragments IP Fragmentation & Reassembly fragmentation: in: one large datagram out: 3 smaller datagrams reassembly Network Layer

  35. length =1500 length =1060 length =1500 length =4000 ID =x ID =x ID =x ID =x fragflag =0 fragflag =0 fragflag =1 fragflag =1 offset =0 offset =185 offset =0 offset =370 One large datagram becomes several smaller datagrams IP Fragmentation and Reassembly Example • 4000 byte datagram • MTU = 1500 bytes 1480 bytes in data field offset = 1480/8 Network Layer

  36. Routing in the Internet • The Global Internet consists of Autonomous Systems (AS) interconnected with each other: • Stub AS: small corporation • Multihomed AS: large corporation (no transit) • Transit AS: provider • Two-level routing: • Intra-AS: administrator is responsible for choice • Inter-AS: unique standard Lecture 6: Network Layer

  37. Internet AS Hierarchy Inter-AS border (exterior gateway) routers Intra-ASinterior (gateway) routers Lecture 6: Network Layer

  38. Intra-AS Routing • Also known as Interior Gateway Protocols (IGP) • Most common IGPs: • RIP: Routing Information Protocol • OSPF: Open Shortest Path First • IGRP: Interior Gateway Routing Protocol (Cisco propr.) Lecture 6: Network Layer

  39. RIP ( Routing Information Protocol) • Distance vector algorithm • Included in BSD-UNIX Distribution in 1982 • Distance metric: # of hops (max = 15 hops) • why? • Distance vectors: exchanged every 30 sec via Response Message (also called advertisement) • Each advertisement: route to up to 25 destination nets Lecture 6: Network Layer

  40. RIP (Routing Information Protocol) z w x y A D B C Destination Network Next Router Num. of hops to dest. w A 2 y B 2 z B 7 x -- 1 …. …. .... Routing table in D Lecture 6: Network Layer

  41. RIP: Link Failure and Recovery If no advertisement heard after 180 sec --> neighbor/link declared dead • routes via neighbor invalidated • new advertisements sent to neighbors • neighbors in turn send out new advertisements (if tables changed) • link failure info quickly propagates to entire net • poison reverse used to prevent ping-pong loops (infinite distance = 16 hops) Lecture 6: Network Layer

  42. OSPF (Open Shortest Path First) • “open”: publicly available • Uses Link State algorithm • LS packet dissemination • Topology map at each node • Route computation using Dijkstra’s algorithm • OSPF advertisement carries one entry per neighbor router • Advertisements disseminated to entire AS (via flooding) Lecture 6: Network Layer

  43. OSPF “advanced” features (not in RIP) • Security: all OSPF messages authenticated (to prevent malicious intrusion); TCP connections used • Multiple same-cost paths allowed • only one path in RIP • For each link, multiple cost metrics for different ToS (eg, satellite link cost set “low” for best effort; high for real time) • Integrated uni- and multicast support: • Multicast OSPF (MOSPF) uses same topology data base as OSPF • Hierarchical OSPF in large domains. Lecture 6: Network Layer

  44. Hierarchical OSPF Lecture 6: Network Layer

  45. Hierarchical OSPF • Two-level hierarchy: local area, backbone. • Link-state advertisements only in area • each nodes has detailed area topology; only know direction (shortest path) to nets in other areas. • Area border routers:“summarize” distances to nets in own area, advertise to other Area Border routers. • Backbone routers: run OSPF routing limited to backbone. • Boundary routers: connect to other ASs. Lecture 6: Network Layer

  46. IGRP (Interior Gateway Routing Protocol) • CISCO proprietary; successor of RIP (mid 80s) • Distance Vector, like RIP • several cost metrics (delay, bandwidth, reliability, load etc) • uses TCP to exchange routing updates • Loop-free routing via Distributed Updating Alg. (DUAL) based on diffused computation Lecture 6: Network Layer

  47. Inter-AS routing Lecture 6: Network Layer

  48. Internet inter-AS routing: BGP • BGP (Border Gateway Protocol):the de facto standard • Path Vector protocol: • similar to Distance Vector protocol • each Border Gateway broadcast to neighbors (peers) entire path (I.e, sequence of ASs) to destination • E.g., Gateway X may send its path to dest. Z: Path (X,Z) = X,Y1,Y2,Y3,…,Z Lecture 6: Network Layer

  49. Internet inter-AS routing: BGP Suppose: gateway X send its path to peer gateway W • W may or may not select path offered by X • cost, policy (don’t route via competitors AS), loop prevention reasons. • If W selects path advertised by X, then: Path (W,Z) = W, Path (X,Z) • Note: X can control incoming traffic by controlling its route advertisements to peers: • e.g., don’t want to route traffic to Z -> don’t advertise any routes to Z Lecture 6: Network Layer

  50. Internet inter-AS routing: BGP • BGP messages exchanged using TCP. • BGP messages: • OPEN: opens TCP connection to peer and authenticates sender • UPDATE: advertises new path (or withdraws old) • KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request • NOTIFICATION: reports errors in previous msg; also used to close connection Lecture 6: Network Layer

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