Slide #1.

CSCI 3335: COMPUTER NETWORKS CHAPTER 4 NETWORK LAYER Vamsi Paruchuri University of Central Arkansas http://faculty.uca.edu/vparuchuri/3335.htm Some of the material is adapted from J.F Kurose and K.W. Ross
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Slide #2.

Chapter 4: Network Layer Chapter goals:  understand principles behind network layer services:       network layer service models forwarding versus routing how a router works routing (path selection) broadcast, multicast instantiation, implementation in the Internet Network Layer 4-2
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Slide #3.

Chapter 4: Network Layer 4.51Routing 4. Introduction algorithms Link state 4.2Virtual circuit and datagram networks  Distance Vector 4.3 What’s inside a router  Hierarchical routing 4.4 IP: Internet Protocol 4.6Routing in the Internet Datagram format       RIP IPv4 addressing OSPF ICMP BGP IPv6 4.7 Broadcast and multicast routing Network Layer 4-3
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Slide #4.

Network layer      transport segment from sending to receiving host on sending side encapsulates segments into datagrams on rcving side, delivers segments to transport layer network layer protocols in every host, router router examines header fields in all IP applicatio n transport network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network network data link data link physical physical network data link physical network data link physical network data link physical network data link physical Network Layer applicatio n transport network data link physical 4-4
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Slide #5.

Two Key Network-Layer Functions   forwarding: move packets from router’s input to appropriate router output analogy:  routing: process of planning trip from source to dest routing: determine route taken by packets from source to dest.  forwarding: process of getting through single interchange  routing algorithms Network Layer 4-5
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Slide #6.

Interplay between routing and forwarding routing algorithm local forwarding table header value output link 0100 0101 0111 1001 3 2 2 1 value in arriving packet’s header 0111 1 3 2 Network Layer 4-6
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Slide #7.

Connection setup    3rd important function in some network architectures:  ATM, frame relay, X.25 before datagrams flow, two end hosts and intervening routers establish virtual connection  routers get involved network vs transport layer connection service:  network: between two hosts  transport: between two processes (end-to-end) Network Layer 4-7
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Slide #8.

Chapter 4: Network Layer 4.51Routing 4. Introduction algorithms Link state 4.2Virtual circuit (VC) and datagram networks  Distance Vector 4.3 What’s inside a router  Hierarchical routing 4.4 IP: Internet Protocol 4.6Routing in the Internet Datagram format       RIP IPv4 addressing OSPF ICMP BGP IPv6 4.7 Broadcast and multicast routing Network Layer 4-8
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Slide #9.

Network layer connection and connection-less service  datagram network provides network-layer connectionless service  VC network provides networklayer connection service Network Layer 4-9
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Slide #10.

Datagram Forwarding table 4 billion IP addresses, so rather than list individual destination address list range of addresses (aggregate table entries) routing algorithm local forwarding table dest address output address-range 1 3 link address-range 2 address-range 3 address-range 4 2 2 1 IP destination address in arriving packet’s header 1 3 2 Network Layer 4-10
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Slide #11.

Datagram Forwarding table Destination Address Range Link Interface 11001000 00010111 00010000 00000000 through 11001000 00010111 00010111 11111111 0 11001000 00010111 00011000 00000000 through 11001000 00010111 00011000 11111111 1 11001000 00010111 00011001 00000000 through 11001000 00010111 00011111 11111111 2 otherwise 3 Q: but what happens if ranges don’t divide up so nicely? Network Layer 4-11
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Slide #12.

Longest prefix matching Longest prefix matching when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address. Destination Address (DA) Range Link interface 11001000 00010111 00010*** ********* 0 11001000 00010111 00011000 ********* 1 11001000 00010111 00011*** ********* 2 otherwise 3 Examples: Which interface? DA: 11001000 00010111 00010110 DA:10100001 11001000 00010111 00011000 10101010 Which interface? Network Layer 4-12
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Slide #13.

Chapter 4: Network Layer      4.51Routing 4. Introduction algorithms  Link state circuit and datagram networks 4.2 Virtual  Distance Vector 4.3 What’s inside a router  Hierarchical routing 4.4 IP: Internet Protocol 4.6 Routingformat in the Internet  Datagram        RIP IPv4 addressing OSPF ICMP BGP IPv6 4.7 Broadcast and multicast routing Network Layer 4-13
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Slide #14.

The Internet Network layer Host, router network layer functions: Transport layer: TCP, UDP Network layer Routing protocols • path selection • RIP, OSPF, BGP IP protocol • addressing conventions • datagram format • packet handling conventions forwarding table ICMP protocol • error reporting • router “signaling” Link layer physical layer Network Layer 4-14
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Slide #15.

Chapter 4: Network Layer 4.51Routing 4. Introduction algorithms Link state 4.2Virtual circuit and datagram networks  Distance Vector 4.3 What’s inside a router  Hierarchical routing 4.4 IP: Internet Protocol 4.6Routing in the Internet Datagram format       RIP IPv4 addressing OSPF ICMP BGP IPv6 4.7 Broadcast and multicast routing Network Layer 4-15
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Slide #16.

IP datagram format IP protocol version number header length (bytes) “type” of data max number remaining hops (decremented at each router) upper layer protocol to deliver payload to how much overhead with TCP?  20 bytes of TCP  20 bytes of IP  = 40 bytes + app layer 32 bits ver head. type of len service length fragment 16-bit identifier flgs offset time to upper header layer live checksum total datagram length (bytes) for fragmentation/ reassembly 32 bit source IP address 32 bit destination IP address Options (if any) data (variable length, typically a TCP or UDP segment) E.g. timestamp, record route taken, specify list of routers to visit. Network Layer 4-16
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Slide #17.

IP Fragmentation & Reassembly   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 fragmentation: in: one large datagram out: 3 smaller datagrams reassembly Network Layer 4-17
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Slide #18.

IP Header: Fragmentation  Fields to manage fragmentation  Identification (16 bits) • “Unique ID” for datagram • Original spec said transport layer would set  Flags ( 3 bits) • 1 bit used to say whether there are more fragments following this one in the original datagram • 1 bit used to say “do not fragment” (drop and send error message back to source if need to fragment)  Fragment Offset (13 bits) • Give offset of data in this fragment into original datagram Network Layer 4a-18
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Slide #19.

IP Fragmentation and Reassembly Example  4000 byte datagram  MTU = 1500 bytes 1480 bytes in data field offset = 1480/8 length ID fragflag offset =4000 =x =0 =0 One large datagram becomes several smaller datagrams length ID fragflag offset =1500 =x =1 =0 length ID fragflag offset =1500 =x =1 =185 length ID fragflag offset =1040 =x =0 =370 How would you differentiate a “last fragment” from an unfragmented packet? Network Layer 4-19
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Slide #20.

unfragmented packet Another Example Start of header Ident = x H1 ETH IP (1400) R1 R1 R2 R2 FDDI IP (1400) R3 R3 Rest of header H8 (a) PPP IP (512) ETH IP (512) PPP IP (512) ETH IP (512) PPP IP (376) ETH IP (376) 0 Offset = 0 1400 data bytes Start of header Ident = x 1 Offset = 0 Rest of header 512 data bytes ID of the original/unfragment ed packet “M (more)” bit in the IP “Flag” field * 8 bytes (b) Start of header Ident = x 1 Offset = 64 Rest of header 512 data bytes Start of header Ident = x 0 Offset = 128 Rest of header 376 data bytes fragmented packets
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Slide #21.

Revision - 1  What is the difference between Routing and Forwarding  Why are ranges of IP addresses stored in forwarding table instead of individual IP addresses.  What is longest prefix matching?  What are the key services provided by network layer?  What is the significance of each field in the IP header.  What is the size of IP header. Network Layer 4-21
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Slide #22.

Chapter 4: Network Layer 4.51Routing 4. Introduction algorithms Link state 4.2Virtual circuit and datagram networks  Distance Vector 4.3 What’s inside a router  Hierarchical routing 4.4 IP: Internet Protocol 4.6Routing in the Internet Datagram format       RIP IPv4 addressing OSPF ICMP BGP IPv6 4.7 Broadcast and multicast routing Network Layer 4-22
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Slide #23.

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 typically has one interface  IP addresses associated with each interface 223.1.1.1 223.1.1.2 223.1.1.4 223.1.1.3 223.1.2.1 223.1.2.9 223.1.3.27 223.1.2.2 223.1.3.2 223.1.3.1 223.1.1.1 = 11011111 00000001 00000001 00000001 223 1 1 1 Network Layer 4-23
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Slide #24.

Subnets  IP address:  subnet part (high order bits)  host part (low order bits)  What’s a subnet ?  device interfaces with same subnet part of IP address  can physically reach each other without intervening router 223.1.1.1 223.1.1.2 223.1.1.4 223.1.1.3 223.1.2.1 223.1.2.9 223.1.3.27 223.1.2.2 subnet 223.1.3.1 223.1.3.2 network consisting of 3 subnets Network Layer 4-24
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Slide #25.

IP Addresses (Classes) “class-full” addressing Unicast A 0 network B 10 C 110 Multicast D 1110 Reserved E 1111 Class Size of  rest bit field Class A     0     8     24 Class B     10     16     16 Class C host class Size Leadin of networ g k bits  bit field network multicast address     8 1.0.0.0 to 127.255.255.255 128.0.0.0 to 191.255.255.255 host network reserved     110     24 host 192.0.0.0 to 223.255.255.255 224.0.0.0 to 239.255.255.255 240.0.0.0 to 255.255.255.255 32 bits 4: Network Layer 4a-25
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Slide #26.

Hosts per Class Class A has ~224 hosts (16777216)  Class B has ~216 hosts (65536)  Class C has ~28 hosts (256)   Class-full 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 4: Network Layer 4a-26
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Slide #27.

IP addressing: CIDR CIDR: Classless InterDomain Routing  subnet portion of address of arbitrary length  address format: a.b.c.d/x, where x is # bits in subnet portion of address subnet part host part 11001000 00010111 00010000 00000000 200.23.16.0/23 Network Layer 4-27
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Slide #28.

IP addresses: how to get one? Q: How does a host get IP address?   hard-coded by system admin in a file  Windows: control-panel->network>configuration ->tcp/ip->properties  UNIX: /etc/rc.config DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server  “plug-and-play” Network Layer 4-28
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Slide #29.

DHCP: Dynamic Host Configuration Protocol Goal: allow host to dynamically obtain its IP address from network server when it joins network Can renew its lease on address in use Allows reuse of addresses (only hold address while connected an “on”) Support for mobile users who want to join network (more shortly) DHCP overview:  host broadcasts “DHCP discover” msg [optional]  DHCP server responds with “DHCP offer” msg [optional]  host requests IP address: “DHCP request” msg  DHCP server sends address: “DHCP ack” msg Network Layer 4-29
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Slide #30.

DHCP client-server scenario A 223.1.1.2 223.1.1.4 223.1.2.9 B 223.1.1.3 223.1.3.1 223.1.2.1 DHCP server 223.1.1.1 223.1.3.27 223.1.2.2 223.1.3.2 E arriving DHCP client needs address in this network Network Layer 4-30
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Slide #31.

DHCP: more than IP address DHCP can return more than just allocated IP address on subnet:  address of first-hop router for client  name and IP address of DNS sever  network mask (indicating network versus host portion of address) Network Layer 4-31
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Slide #32.

DHCP: example DHCP UDP IP Eth Phy DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP UDP IP Eth Phy  connecting laptop needs its IP address, addr of first-hop router, addr of DNS server: use DHCP  DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in 802.1 Ethernet Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server 168.1.1.1 router (runs DHCP)   Ethernet demuxed to IP demuxed, UDP demuxed to DHCP Network Layer 4-32
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Slide #33.

DHCP: example DHCP UDP IP Eth Phy DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP UDP IP Eth Phy router (runs DHCP)  DHCP server formulates DHCP ACK containing client’s IP address, IP address of first-hop router for client, name & IP address of DNS server  encapsulation of DHCP server, frame forwarded to client, demuxing up to DHCP at client client now knows its IP address, name and IP address of DNS server, IP address of its first-hop router  Network Layer 4-33
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Slide #34.

IP addresses: how to get one? Q: How does network get subnet part of IP addr? A: gets allocated portion of its provider ISP’s address space ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20 Organization 0 Organization 1 Organization 2 ... 11001000 00010111 00010000 00000000 11001000 00010111 00010010 00000000 11001000 00010111 00010100 00000000 ….. …. 200.23.16.0/23 200.23.18.0/23 200.23.20.0/23 …. Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23 Network Layer 4-34
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Slide #35.

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 Network Layer 4-35
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Slide #36.

NAT: Network Address Translation rest of Internet local network (e.g., home network) 10.0.0/24 10.0.0.4 10.0.0.1 10.0.0.2 138.76.29.7 10.0.0.3 Datagrams with source or All datagrams leaving local network have same single source destination in this network have 10.0.0/24 address for NAT IP address: 138.76.29.7, different source port numbers source, destination (as usual) Network Layer 4-36
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Slide #37.

NAT: Network Address Translation  Motivation: local network uses just one IP address as far as outside world is concerned:  range of addresses not needed from ISP: just one IP address for all devices  can change addresses of devices in local network without notifying outside world  can change ISP without changing addresses of devices in local network  devices inside local net not explicitly addressable, visible by outside world (a security plus). Network Layer 4-37
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Slide #38.

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, Network Layer 4-38
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Slide #39.

NAT: Network Address Translation NAT translation table WAN side addr LAN side addr 1: host 10.0.0.1 2: NAT router sends datagram to changes datagram 138.76.29.7, 5001 10.0.0.1, 3345 128.119.40.186, 80 source addr from …… …… 10.0.0.1, 3345 to 138.76.29.7, 5001, S: 10.0.0.1, 3345 D: 128.119.40.186, updates table 80 2 S: 138.76.29.7, 5001 D: 128.119.40.186, 80 138.76.29.7 S: 128.119.40.186, 80 D: 138.76.29.7, Reply arrives 5001 3 3: dest. address: 138.76.29.7, 5001 10.0.0.1 1 10.0.0.4 S: 128.119.40.186, 80 D: 10.0.0.1, 3345 10.0.0.2 4 10.0.0.3 4: NAT router changes datagram dest addr from 138.76.29.7, 5001 to 10.0.0.1, 3345 Network Layer 4-39
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Slide #40.

NAT: Network Address Translation  16-bit port-number field:  60,000 simultaneous connections with a single LAN-side address!  NAT is controversial:  routers should only process up to 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 IPv6 Network Layer 4-40
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Slide #41.

Chapter 4: Network Layer 4.51Routing 4. Introduction algorithms Link state 4.2Virtual circuit and datagram networks  Distance Vector 4.3 What’s inside a router  Hierarchical routing 4.4 IP: Internet Protocol 4.6Routing in the Internet Datagram format       RIP IPv4 addressing OSPF ICMP BGP IPv6 4.7 Broadcast and multicast routing Network Layer 4-41
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Slide #42.

ICMP: Internet Control Message Protocol    used by hosts & routers to communicate network-level Type Code description information 0 0 echo reply (ping)  error reporting: unreachable host, network, port, protocol 3 0 dest. network unreachable  echo request/reply (used by ping) 3 1 dest host unreachable network-layer “above” IP: 3 2 dest protocol unreachable  ICMP msgs carried in IP datagrams 3 3 dest port unreachable 3 8 6bytes dest unknown ICMP message: type, code plus first of IPnetwork datagram 3 7 dest host unknown causing error 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 Network Layer 4-42
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Slide #43.

Chapter 4: Network Layer 4.51Routing 4. Introduction algorithms Link state 4.2Virtual circuit and datagram networks  Distance Vector 4.3 What’s inside a router  Hierarchical routing 4.4 IP: Internet Protocol 4.6Routing in the Internet Datagram format       RIP IPv4 addressing OSPF ICMP BGP IPv6 4.7 Broadcast and multicast routing Network Layer 4-43
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Slide #44.

IPv6 Initial motivation: 32-bit address space soon to be completely allocated.  Additional motivation:   header format helps speed processing/forwarding  header changes to facilitate QoS IPv6 datagram format:  fixed-length 40 byte header  no fragmentation allowed Network Layer 4-44
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Slide #45.

IPv6 Header (Cont) Priority: identify priority among datagrams in flow Flow Label: identify datagrams in same “flow.” (concept of“flow” not well defined). Next header: identify upper layer protocol for data pri ver payload len flow label next hdr hop limit source address (128 bits) destination address (128 bits) data 32 bits Network Layer 4-45
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Slide #46.

Other Changes from IPv4 Checksum: removed entirely to reduce processing time at each hop  Options: allowed, but outside of header, indicated by “Next Header” field  ICMPv6: new version of ICMP   additional message types, e.g. “Packet Too Big”  multicast group management functions Network Layer 4-46
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Slide #47.

Transition From IPv4 To IPv6  Not all routers can be upgraded simultaneous  no “flag days”  How will the network operate with mixed IPv4 and IPv6 routers?  Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers Network Layer 4-47
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Slide #48.

Tunneling Logical view: Physical view: E F IPv6 IPv6 IPv6 A B E F IPv6 IPv6 IPv6 IPv6 A B IPv6 tunnel IPv4 IPv4 Network Layer 4-48
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Slide #49.

Tunneling Logical view: Physical view: A B IPv6 IPv6 A B C IPv6 IPv6 IPv4 Flow: X Src: A Dest: F data A-to-B: IPv6 E F IPv6 IPv6 D E F IPv4 IPv6 IPv6 tunnel Src:B Dest: E Src:B Dest: E Flow: X Src: A Dest: F Flow: X Src: A Dest: F data data B-to-C: IPv6 inside IPv4 B-to-C: IPv6 inside IPv4 Flow: X Src: A Dest: F data E-to-F: IPv6 Network Layer 4-49
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Slide #50.

Chapter 4: Network Layer 4.51Routing 4. Introduction algorithms Link state 4.2Virtual circuit and datagram networks  Distance Vector 4.3 What’s inside a router  Hierarchical routing 4.4 IP: Internet Protocol 4.6Routing in the Internet Datagram format       RIP IPv4 addressing OSPF ICMP BGP IPv6 4.7 Broadcast and multicast routing Network Layer 4-50
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Slide #51.

Chapter 4: Network Layer 4.51Routing 4. Introduction algorithms Link state 4.2Virtual circuit and datagram networks  Distance Vector 4.3 What’s inside a router  Hierarchical routing 4.4 IP: Internet Protocol 4.6Routing in the Internet Datagram format       RIP IPv4 addressing OSPF ICMP BGP IPv6 4.7 Broadcast and multicast routing Network Layer 4-51
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Slide #52.

Interplay between routing, forwarding routing algorithm local forwarding table header value output link 0100 0101 0111 1001 3 2 2 1 value in arriving packet’s header 0111 1 3 2 Network Layer 4-52
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Slide #53.

Graph abstraction 5 2 u 2 1 Graph: G = (N,E) v x 3 w 3 1 5 z 1 y 2 N = set of routers = { u, v, w, x, y, z } E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) } Remark: Graph abstraction is useful in other network contexts Example: P2P, where N is set of peers and E is set of TCP connections Network Layer 4-53
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Slide #54.

Graph abstraction: costs 5 2 u v 2 1 x • c(x,x’) = cost of link (x,x’) 3 w 3 1 5 z 1 y - e.g., c(w,z) = 5 2 • cost could always be 1, or inversely related to bandwidth, or inversely related to congestion Cost of path (x1, x2, x3,…, xp) = c(x1,x2) + c(x2,x3) + … + c(xp-1,xp) Question: What’s the least-cost path between u and z ? Routing algorithm: algorithm that finds least-cost path Network Layer 4-54
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Slide #55.

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 Network Layer 4-55
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Slide #56.

Comparison of LS and DV algorithms Message complexity   LS: with n nodes, E links, O(nE) msgs sent DV: exchange between neighbors only  convergence time varies Speed of Convergence   LS: O(n2) algorithm requires O(nE) msgs  may have oscillations DV: convergence time varies  may be routing loops  count-to-infinity problem 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 Network Layer 4-56
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Slide #57.

Hierarchical Routing Our routing study thus far - idealization  all routers identical  network “flat” … not true in practice scale: with 200 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 Network Layer 4-57
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Slide #58.

Hierarchical Routing   aggregate routers into regions, “autonomous systems” (AS) routers in same AS run same routing protocol gateway router  at “edge” of its own AS  has link to router in another AS  “intra-AS” routing protocol  routers in different AS can run different intra-AS routing protocol Network Layer 4-58
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Slide #59.

Interconnected ASes 3c 3a 3b AS3 2a 1c 1a 1d 2c AS2 1b AS1 Intra-AS Routing algorithm Inter-AS Routing algorithm Forwarding table  2b forwarding table configured by both intra- and inter-AS routing algorithm  intra-AS sets entries for internal dests  inter-AS & intra-As sets entries for external dests Network Layer 4-59
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Slide #60.

Chapter 4: Network Layer 4.51Routing 4. Introduction algorithms Link state 4.2Virtual circuit and datagram networks  Distance Vector 4.3 What’s inside a router  Hierarchical routing 4.4 IP: Internet Protocol 4.6Routing in the Internet Datagram format       RIP IPv4 addressing OSPF ICMP BGP IPv6 4.7 Broadcast and multicast routing Network Layer 4-60
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Slide #61.

Intra-AS Routing   also known as Interior Gateway Protocols (IGP) most common Intra-AS routing protocols:  RIP: Routing Information Protocol  OSPF: Open Shortest Path First  IGRP: Interior Gateway Routing Protocol (Cisco proprietary) Network Layer 4-61
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Slide #62.

RIP ( Routing Information Protocol)   included in BSD-UNIX distribution in 1982 distance vector algorithm  distance metric: # hops (max = 15 hops), each link has cost 1  DVs exchanged with neighbors every 30 sec in response message (aka advertisement)  each advertisement: list of up to 25 destination subnets (in IP addressing sense) u v A z C B D w x y from router A to destination subnets: subnet hops u 1 v 2 w 2 x 3 y 3 z 2 Network Layer 4-62
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Slide #63.

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)  carried in OSPF messages directly over IP (rather than TCP or UDP Network Layer 4-63
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Slide #64.

OSPF “advanced” features (not in RIP)      security: all OSPF messages authenticated (to prevent malicious intrusion) multiple same-cost paths allowed (only one path in RIP) for each link, multiple cost metrics for different TOS (e.g., satellite link cost set “low” for best effort ToS; high for real time ToS) integrated uni- and multicast support:  Multicast OSPF (MOSPF) uses same topology data base as OSPF hierarchical OSPF in large domains. Network Layer 4-64
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Slide #65.

Hierarchical OSPF boundary router backbone router backbone area border routers Area 3 internal routers Area 1 Area 2 Network Layer 4-65
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Slide #66.

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 AS’s. Network Layer 4-66
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Slide #67.

Internet inter-AS routing: BGP  BGP (Border Gateway Protocol): the de facto inter-domain routing protocol  “glue that holds the Internet together”  BGP provides each AS a means to:  obtain subnet reachability information from neighboring ASs.  propagate reachability information to all AS-internal routers.  determine “good” routes to other networks based on reachability information and policy.  allows subnet to advertise its existence to rest of Internet: “I am here” Network Layer 4-67
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Slide #68.

BGP routing policy legend: B W provider network X A customer network: C Y    A,B,C are provider networks X,W,Y are customer (of provider networks) X is dual-homed: attached to two networks  X does not want to route from B via X to C  .. so X will not advertise to B a route to C Network Layer 4-68
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Slide #69.

BGP routing policy (2) legend: B W provider network X A customer network: C Y    A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C?  No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers  B wants to force C to route to w via A  B wants to route only to/from its customers! Network Layer 4-69
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Slide #70.

Why different Intra- and Inter-AS routing ? Policy:   Inter-AS: admin wants control over how its traffic routed, who routes through its net. Intra-AS: single admin, so no policy decisions needed Scale: hierarchical routing saves table size, reduced update traffic Performance:  Intra-AS: can focus on performance  Inter-AS: policy may dominate over performance  Network Layer 4-70
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Slide #71.

Chapter 4: Network Layer 4. 1Routing Introduction 4.5 algorithms Link state 4.2Virtual circuit and  Distance networks Vector datagram  Hierarchical routing 4.3 What’s inside a 4.6router Routing in the Internet  RIP 4.4 IP: Internet Protocol  OSPF  Datagram format  BGP  IPv4 addressing 4.7Broadcast and multicast ICMP  IPv6 routing Network Layer 4-71
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Slide #72.

Broadcast Routing deliver packets from source to all other nodes  source duplication is inefficient:  duplicate duplicate creation/transmission R1 R1 duplicate R2 R2 R3 R4 source duplication  R3 R4 in-network duplication source duplication: how does source determine recipient addresses? Network Layer 4-72
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Slide #73.

In-network duplication    flooding: when node receives broadcast packet, sends copy to all neighbors  problems: cycles & broadcast storm controlled flooding: node only broadcasts pkt if it hasn’t broadcast same packet before  node keeps track of packet ids already broadcasted  or reverse path forwarding (RPF): only forward packet if it arrived on shortest path between node and source spanning tree  No redundant packets received by any node Network Layer 4-73
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Slide #74.

Multicast Routing: Problem Statement  Goal: find a tree (or trees) connecting routers having local mcast group members  tree: not all paths between routers used  source-based: different tree from each sender to rcvrs  shared-tree: same tree used by all group members Shared tree Source-based trees
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Slide #75.

Chapter 4: summary 4.51Routing 4. Introduction algorithms Link state 4.2Virtual circuit and datagram networks  Distance Vector 4.3 What’s inside a router  Hierarchical routing 4.4 IP: Internet Protocol 4.6Routing in the Internet Datagram format       RIP IPv4 addressing OSPF ICMP BGP IPv6 4.7 Broadcast and multicast routing Network Layer 4-75
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