Slide #1.

Chapter 4 Network Layer Prof. Hong Liu for ECE369 Adapted from J.F Kurose and K.W. Ross, All Rights Reserved Computer Networking: A Top Down Approach 6th edition Jim Kurose, Keith Ross Addison-Wesley Network Layer 4-1
More slides like this


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
More slides like this


Slide #3.

Chapter 4: outline 4.1 introduction 4.2 virtual circuit and datagram networks 4.3 what’s inside a router 4.4 IP: Internet Protocol     datagram format IPv4 addressing ICMP IPv6 4.5 routing algorithms  link state  distance vector  hierarchical routing 4.6 routing in the Internet  RIP  OSPF  BGP 4.7 broadcast and multicast routing Network Layer 4-3
More slides like this


Slide #4.

Network layer      transport segment from sending to receiving host on sending side encapsulates segments into datagrams on receiving side, delivers segments to transport layer network layer protocols in every host, router router examines header fields in all IP datagrams passing through it application 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 data link physical network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical Network Layer 4-4
More slides like this


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
More slides like this


Slide #6.

nterplay between routing and forwarding routing algorithm routing algorithm determines end-end-path through network local forwarding table header value output link forwarding table determines local forwarding at this router 0100 0101 0111 1001 3 2 2 1 value in arriving packet’s header 0111 1 3 2 Network Layer 4-6
More slides like this


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 (may also involve intervening routers in case of VCs)  transport: between two processes Network Layer 4-7
More slides like this


Slide #8.

Network service model Q: What service model for “channel” transporting datagrams from sender to receiver? example services for example services individual for a flow of datagrams: datagrams:   guaranteed delivery guaranteed delivery with less than 40 msec delay    in-order datagram delivery guaranteed minimum bandwidth to flow restrictions on changes in interpacket spacing Network Layer 4-8
More slides like this


Slide #9.

Network layer service models: Network Architecture Internet Service Model Guarantees ? Congestion Bandwidth Loss Order Timing feedback best effort none ATM CBR ATM VBR ATM ABR ATM UBR constant rate guaranteed rate guaranteed minimum none no no no yes yes yes yes yes yes no yes no no (inferred via loss) no congestion no congestion yes no yes no no Network Layer 4-9
More slides like this


Slide #10.

Datagram networks   no call setup at network layer routers: no state about end-to-end connections  no network-level concept of “connection”  packets forwarded using destination host address application transport network 1. send datagrams data link physical application transport 2. receive datagrams network data link physical Network Layer 4-10
More slides like this


Slide #11.

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 4-11
More slides like this


Slide #12.

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 : but what happens if ranges don’t divide up so nicely? Network Layer 4-12
More slides like this


Slide #13.

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 Range Link interface 11001000 00010111 00010*** ********* 0 11001000 00010111 00011000 ********* 1 11001000 00010111 00011*** ********* 2 otherwise 3 examples: DA: 11001000 00010111 00010110 10100001 which interface? DA: 11001000 00010111 00011000 10101010 which interface? Network Layer 4-13
More slides like this


Slide #14.

Chapter 4: outline 4.1 introduction 4.2 virtual circuit and datagram networks 4.3 what’s inside a router 4.4 IP: Internet Protocol     datagram format IPv4 addressing ICMP IPv6 4.5 routing algorithms  link state  distance vector  hierarchical routing 4.6 routing in the Internet  RIP  OSPF  BGP 4.7 broadcast and multicast routing Network Layer 4-14
More slides like this


Slide #15.

Router architecture overview two key router functions:   run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing link forwarding tables computed, pushed to input ports routing processor routing, management control plane (software) forwarding data plane (hardware) high-seed switching fabric router input ports router output ports Network Layer 4-15
More slides like this


Slide #16.

Input port functions link layer protocol (receive) line termination lookup, forwarding switch fabric queueing physical layer: bit-level reception data link layer: e.g., Ethernet see chapter 5 decentralized switching:    given datagram dest., lookup output port using forwarding table in input port memory (“match plus action”) goal: complete input port processing at ‘line speed’ queuing: if datagrams arrive faster than forwarding rate into switch fabric Network Layer 4-16
More slides like this


Slide #17.

Switching fabrics   transfer packet from input buffer to appropriate output buffer switching rate: rate at which packets can be transfer from inputs to outputs  often measured as multiple of input/output line rate  N inputs: switching rate N times line rate desirable  three types of switching fabrics memory memory bus crossbar Network Layer 4-17
More slides like this


Slide #18.

Output ports switch fabric datagram buffer queueing   link layer protocol (send) line termination buffering required when datagrams arrive from fabric faster than the transmission rate scheduling discipline chooses among queued datagrams for transmission Network Layer 4-18
More slides like this


Slide #19.

Output port queueing switch fabric at t, packets more from input to output   switch fabric one packet time later buffering when arrival rate via switch exceeds output line speed queueing (delay) and loss due to output port buffer overflow! Network Layer 4-19
More slides like this


Slide #20.

How much buffering?  RFC 3439 rule of thumb: average buffering equal to “typical” RTT (say 250 msec) times link capacity C  e.g., C = 10 Gpbs link: 2.5 Gbit buffer  recent recommendation: with N flows, buffering equal to RTT . C N Network Layer 4-20
More slides like this


Slide #21.

Input port queuing   fabric slower than input ports combined -> queueing may occur at input queues  queueing delay and loss due to input buffer overflow! Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward switch fabric output port contention: only one red datagram can be transferred. lower red packet is blocked switch fabric one packet time later: green packet experiences HOL blocking Network Layer 4-21
More slides like this


Slide #22.

Chapter 4: outline 4.1 introduction 4.2 virtual circuit and datagram networks 4.3 what’s inside a router 4.4 IP: Internet Protocol     datagram format IPv4 addressing ICMP IPv6 4.5 routing algorithms  link state  distance vector  hierarchical routing 4.6 routing in the Internet  RIP  OSPF  BGP 4.7 broadcast and multicast routing Network Layer 4-22
More slides like this


Slide #23.

The Internet network layer host, router network layer functions: transport layer: TCP, UDP IP protocol routing protocols network layer • addressing conventions • datagram format • packet handling conventions • path selection • RIP, OSPF, BGP forwarding table link layer ICMP protocol • error reporting • router “signaling” physical layer Network Layer 4-23
More slides like this


Slide #24.

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?  20 bytes of TCP  20 bytes of IP  = 40 bytes + app layer overhead 32 bits ver head. type of len service 16-bit identifier upper time to layer live total datagram length (bytes) length fragment flgs offset header checksum 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 4-24
More slides like this


Slide #25.

IP fragmentation, reassembly  fragmentation: in: one large datagram out: 3 smaller datagrams …  reassembly … network links have MTU (max.transfer size) largest possible linklevel 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 Network Layer 4-25
More slides like this


Slide #26.

IP fragmentation, reassembly example:   4000 byte datagram MTU = 1500 bytes 1480 bytes in data field offset = 1480/8 length ID fragflag =4000 =x =0 offset =0 one large datagram becomes several smaller datagrams length ID fragflag =1500 =x =1 offset =0 length ID fragflag =1500 =x =1 offset =185 length ID fragflag =1040 =x =0 offset =370 Network Layer 4-26
More slides like this


Slide #27.

Chapter 4: outline 4.1 introduction 4.2 virtual circuit and datagram networks 4.3 what’s inside a router 4.4 IP: Internet Protocol     datagram format IPv4 addressing ICMP IPv6 4.5 routing algorithms  link state  distance vector  hierarchical routing 4.6 routing in the Internet  RIP  OSPF  BGP 4.7 broadcast and multicast routing Network Layer 4-27
More slides like this


Slide #28.

IP addressing: introduction   IP address: 32-bit identifier for host, router interface 223.1.1.2 interface: connection between host/router and physical link  router’s typically have multiple interfaces  host typically has one or two interfaces (e.g., wired Ethernet, wireless 802.11)  223.1.1.1 IP addresses associated with each interface 223.1.2.1 223.1.1.4 223.1.2.9 223.1.3.27 223.1.1.3 223.1.2.2 223.1.3.1 223.1.3.2 223.1.1.1 = 11011111 00000001 00000001 00000001 223 1 1 1 Network Layer 4-28
More slides like this


Slide #29.

IP addressing: introduction 223.1.1.1 Q: how are interfaces actually connected? A: we’ll learn about 223.1.1.2 that in chapter 5, 6. 223.1.2.1 223.1.1.4 223.1.1.3 223.1.2.9 223.1.3.27 223.1.2.2 A: wired Ethernet interfaces connected by Ethernet switches 223.1.3.1 For now: don’t need to worry about how one interface is connected to another (with no intervening router) 223.1.3.2 A: wireless WiFi interfaces connected by WiFi base station Network Layer 4-29
More slides like this


Slide #30.

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 4-30
More slides like this


Slide #31.

Subnets 223.1.1.0/24 recipe  to determine the subnets, detach each interface from its host or router, creating islands of isolated networks  each isolated network is called a subnet 223.1.2.0/24 223.1.1.1 223.1.1.2 223.1.1.4 223.1.1.3 223.1.2.1 223.1.2.9 223.1.3.27 223.1.2.2 subnet 223.1.3.1 223.1.3.2 223.1.3.0/24 subnet mask: /24 Network Layer 4-31
More slides like this


Slide #32.

Subnets 223.1.1.2 how many? 223.1.1.1 223.1.1.4 223.1.1.3 223.1.9.2 223.1.7.0 223.1.9.1 223.1.7.1 223.1.8.1 223.1.8.0 223.1.2.6 223.1.2.1 223.1.3.27 223.1.2.2 223.1.3.1 223.1.3.2 Network Layer 4-32
More slides like this


Slide #33.

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 4-33
More slides like this


Slide #34.

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 4-34
More slides like this


Slide #35.

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/“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 4-35
More slides like this


Slide #36.

DHCP client-server scenario DHCP server 223.1.1.0/24 223.1.2.1 223.1.1.1 223.1.1.2 223.1.1.4 223.1.1.3 223.1.2.9 223.1.3.27 223.1.2.2 arriving DHCP client needs address in this network 223.1.2.0/24 223.1.3.2 223.1.3.1 223.1.3.0/24 Network Layer 4-36
More slides like this


Slide #37.

DHCP client-server scenario DHCP server: 223.1.2.5 DHCP discover src : 0.0.0.0, 68 dest.: 255.255.255.255,67 yiaddr: 0.0.0.0 transaction ID: 654 arriving client DHCP offer src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 654 lifetime: 3600 secs DHCP request src: 0.0.0.0, 68 dest:: 255.255.255.255, 67 yiaddrr: 223.1.2.4 transaction ID: 655 lifetime: 3600 secs DHCP ACK src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 655 lifetime: 3600 secs Network Layer 4-37
More slides like this


Slide #38.

DHCP: more than IP addresses 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 4-38
More slides like this


Slide #39.

DHCP: example DHCP UDP IP Eth Phy DHCP DHCP DHCP DHCP  DHCP DHCP DHCP DHCP DHCP  DHCP UDP IP Eth Phy 168.1.1.1 router with DHCP server built into router   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 frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server Ethernet demuxed to IP demuxed, UDP demuxed to DHCP Network Layer 4-39
More slides like this


Slide #40.

DHCP: example DHCP UDP IP Eth Phy DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP UDP IP Eth Phy router with DHCP server built into router  DCP 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 DSN server, IP address of its firsthop router  Network Layer 4-40
More slides like this


Slide #41.

DHCP: Wireshark output (home LAN) Message type: Boot Request (1) Hardware type: Ethernet Hardware address length: 6 Hops: 0 Transaction ID: 0x6b3a11b7 Seconds elapsed: 0 Bootp flags: 0x0000 (Unicast) Client IP address: 0.0.0.0 (0.0.0.0) Your (client) IP address: 0.0.0.0 (0.0.0.0) Next server IP address: 0.0.0.0 (0.0.0.0) Relay agent IP address: 0.0.0.0 (0.0.0.0) Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a) Server host name not given Boot file name not given Magic cookie: (OK) Option: (t=53,l=1) DHCP Message Type = DHCP Request Option: (61) Client identifier Length: 7; Value: 010016D323688A; Hardware type: Ethernet Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a) Option: (t=50,l=4) Requested IP Address = 192.168.1.101 Option: (t=12,l=5) Host Name = "nomad" Option: (55) Parameter Request List Length: 11; Value: 010F03062C2E2F1F21F92B 1 = Subnet Mask; 15 = Domain Name 3 = Router; 6 = Domain Name Server 44 = NetBIOS over TCP/IP Name Server …… request Message type: Boot Reply (2) Hardware type: Ethernet Hardware address length: 6 Hops: 0 Transaction ID: 0x6b3a11b7 Seconds elapsed: 0 Bootp flags: 0x0000 (Unicast) Client IP address: 192.168.1.101 (192.168.1.101) Your (client) IP address: 0.0.0.0 (0.0.0.0) Next server IP address: 192.168.1.1 (192.168.1.1) Relay agent IP address: 0.0.0.0 (0.0.0.0) Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a) Server host name not given Boot file name not given Magic cookie: (OK) Option: (t=53,l=1) DHCP Message Type = DHCP ACK Option: (t=54,l=4) Server Identifier = 192.168.1.1 Option: (t=1,l=4) Subnet Mask = 255.255.255.0 Option: (t=3,l=4) Router = 192.168.1.1 Option: (6) Domain Name Server Length: 12; Value: 445747E2445749F244574092; IP Address: 68.87.71.226; IP Address: 68.87.73.242; IP Address: 68.87.64.146 Option: (t=15,l=20) Domain Name = "hsd1.ma.comcast.net." reply Network Layer 4-41
More slides like this


Slide #42.

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 4-42
More slides like this


Slide #43.

Hierarchical addressing: route aggregation erarchical addressing allows efficient advertisement of routin formation: Organization 0 200.23.16.0/23 Organization 1 200.23.18.0/23 Organization 2 200.23.20.0/23 Organization 7 . . . . . . Fly-By-Night-ISP “Send me anything with addresses beginning 200.23.16.0/20” Internet 200.23.30.0/23 ISPs-R-Us “Send me anything with addresses beginning 199.31.0.0/16” Network Layer 4-43
More slides like this


Slide #44.

Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 Organization 0 200.23.16.0/23 Organization 2 200.23.20.0/23 Organization 7 . . . . . . Fly-By-Night-ISP “Send me anything with addresses beginning 200.23.16.0/20” Internet 200.23.30.0/23 ISPs-R-Us Organization 1 200.23.18.0/23 “Send me anything with addresses beginning 199.31.0.0/16 or 200.23.18.0/23” Network Layer 4-44
More slides like this


Slide #45.

IP addressing: the last word... Q: how does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers http://www.icann.org/  allocates addresses  manages DNS  assigns domain names, resolves disputes Network Layer 4-45
More slides like this


Slide #46.

NAT: network address translation rest of Internet local network (e.g., home network) 10.0.0/24 10.0.0.1 10.0.0.4 10.0.0.2 138.76.29.7 10.0.0.3 all datagrams leaving datagrams with source or local destination in this network network have same have 10.0.0/24 address for single source NAT IP source, destination (as usual) address: 138.76.29.7,different Network Layer 4-46
More slides like this


Slide #47.

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 4-47
More slides like this


Slide #48.

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 Network Layer 4-48
More slides like this


Slide #49.

NAT: network address translation 2: NAT router changes datagram source addr from 10.0.0.1, 3345 to 138.76.29.7, 5001, updates table NAT translation table WAN side addr LAN side addr 1: host 10.0.0.1 sends datagram to 128.119.40.186, 80 138.76.29.7, 5001 10.0.0.1, 3345 …… …… S: 10.0.0.1, 3345 D: 128.119.40.186, 80 1 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, 5001 3: reply arrives dest. address: 138.76.29.7, 5001 3 10.0.0.4 S: 128.119.40.186, 80 D: 10.0.0.1, 3345 10.0.0.1 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 4-49
More slides like this


Slide #50.

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 4-50
More slides like this


Slide #51.

NAT traversal problem  client wants to connect to server with address 10.0.0.1  server address 10.0.0.1 local to LAN (client can’t use it as destination addr)  only one externally visible NATed address: 138.76.29.7  solution1: statically configure NAT to forward incoming connection requests at given port to server client 10.0.0.1 ? 10.0.0.4 138.76.29.7 NAT router  e.g., (123.76.29.7, port 2500) always forwarded to 10.0.0.1 port 25000 Network Layer 4-51
More slides like this


Slide #52.

NAT traversal problem  solution 2: Universal Plug and Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATed host to:  learn public IP address (138.76.29.7)  add/remove port mappings (with lease times) 10.0.0.1 IGD NAT router i.e., automate static NAT port map configuration Network Layer 4-52
More slides like this


Slide #53.

NAT traversal problem  solution 3: relaying (used in Skype)  NATed client establishes connection to relay  external client connects to relay  relay bridges packets between to connections 2. connection to relay initiated by client client 3. relaying established 1. connection to relay initiated by NATed host 138.76.29.7 10.0.0.1 NAT router Network Layer 4-53
More slides like this


Slide #54.

Chapter 4: outline 4.1 introduction 4.2 virtual circuit and datagram networks 4.3 what’s inside a router 4.4 IP: Internet Protocol     datagram format IPv4 addressing ICMP IPv6 4.5 routing algorithms  link state  distance vector  hierarchical routing 4.6 routing in the Internet  RIP  OSPF  BGP 4.7 broadcast and multicast routing Network Layer 4-54
More slides like this


Slide #55.

ICMP: internet control message protocol  used by hosts & routers to communicate network-level information  error reporting: unreachable host, network, port, protocol  echo request/reply (used by ping)  network-layer “above” IP:  ICMP msgs carried in IP datagrams  ICMP message: type, code plus first 8 bytes of IP datagram causing error Type 0 3 3 3 3 3 3 4 Code 0 0 1 2 3 6 7 0 8 9 10 11 12 0 0 0 0 0 description echo reply (ping) dest. network unreachable dest host unreachable dest protocol unreachable dest port unreachable dest network unknown dest host unknown source quench (congestion control - not used) echo request (ping) route advertisement router discovery TTL expired bad IP header Network Layer 4-55
More slides like this


Slide #56.

Traceroute and ICMP  source sends series of UDP segments to dest  first set has TTL =1  second set has TTL=2, etc.  unlikely port number  when nth set of datagrams arrives to nth router:  router discards datagrams  and sends source ICMP messages (type 11, code 0)  ICMP messages includes name of router & IP address 3 probes 3 probes  when ICMP messages arrives, source records RTTs stopping criteria:  UDP segment eventually arrives at destination host  destination returns ICMP “port unreachable” message (type 3, code 3)  source stops 3 probes Network Layer 4-56
More slides like this


Slide #57.

IPv6: motivation   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 4-57
More slides like this


Slide #58.

Chapter 4: outline 4.1 introduction 4.2 virtual circuit and datagram networks 4.3 what’s inside a router 4.4 IP: Internet Protocol     datagram format IPv4 addressing ICMP IPv6 4.5 routing algorithms  link state  distance vector  hierarchical routing 4.6 routing in the Internet  RIP  OSPF  BGP 4.7 broadcast and multicast routing Network Layer 4-58
More slides like this


Slide #59.

Interplay between routing, forwarding routing algorithm determines end-end-path through network routing algorithm forwarding table determines local forwarding at this router 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 4-59
More slides like this


Slide #60.

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) } aside: graph abstraction is useful in other network contexts, e.g., P2P, where N is set of peers and E is set of TCP connections Network Layer 4-60
More slides like this


Slide #61.

Graph abstraction: costs 5 2 u v 2 1 x 3 w 3 1 c(x,x’) = cost of link (x,x’) e.g., c(w,z) = 5 5 z 1 y 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) key question: what is the least-cost path between u and z ? outing algorithm: algorithm that finds that least cost path Network Layer 4-61
More slides like this


Slide #62.

Routing algorithm classification Q: 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 Q: static or dynamic? static:  routes change slowly over time dynamic:  routes change more quickly  periodic update  in response to link cost changes Network Layer 4-62
More slides like this


Slide #63.

Chapter 4: outline 4.1 introduction 4.2 virtual circuit and datagram networks 4.3 what’s inside a router 4.4 IP: Internet Protocol     datagram format IPv4 addressing ICMP IPv6 4.5 routing algorithms  link state  distance vector  hierarchical routing 4.6 routing in the Internet  RIP  OSPF  BGP 4.7 broadcast and multicast routing Network Layer 4-63
More slides like this


Slide #64.

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 forwarding table for that node  iterative: after k iterations, know least cost path to k dest.’s notation:  c(x,y): link cost from    node x to y; = ∞ 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 N': set of nodes whose least cost path definitively known Network Layer 4-64
More slides like this


Slide #65.

Dijsktra’s Algorithm 1 Initialization: 2 N' = {u} 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(u,v) 6 else D(v) = ∞ 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(w,v) ) 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' Network Layer 4-65
More slides like this


Slide #66.

Dijkstra’s algorithm: example Step 0 1 2 3 4 5 N' u uw uwx uwxv uwxvy uwxvyz D(v) D(w) D(x) D(y) D(z) p(v) p(w) p(x) 7,u 6,w 6,w 3,u ∞ ∞ 5,u ∞ 5,u 11,w 11,w 14,x 10,v 14,x 12,y p(y) p(z) x notes:   construct shortest path tree by tracing predecessor nodes ties can exist (can be broken arbitrarily) 5 9 7 4 8 3 u w y 3 7 2 z 4 v Network Layer 4-66
More slides like this


Slide #67.

Dijkstra’s algorithm: another example Step 0 1 2 3 4 5 N' u ux uxy uxyv uxyvw uxyvwz D(v),p(v) D(w),p(w) 2,u 5,u 2,u 4,x 2,u 3,y 3,y D(x),p(x) 1,u D(y),p(y) ∞ 2,x D(z),p(z) ∞ ∞ 4,y 4,y 4,y 5 2 u v 2 1 x 3 w 3 1 5 z 1 y 2 Network Layer 4-67
More slides like this


Slide #68.

Dijkstra’s algorithm: example (2) resulting shortest-path tree from u: v w u z x y resulting forwarding table in u: destination link v x (u,v) (u,x) y (u,x) w (u,x) z (u,x) Network Layer 4-68
More slides like this


Slide #69.

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(n2) more efficient implementations possible: O(nlogn) oscillations possible:  e.g., support link cost equals amount of carried traffic: A 1 D 1 B 0 0 0 1+e C e e initially 2+e D 0 C 0 B 1+e 1 0 1 A 0 D A 0 1 C 2+e B 0 1+e 2+e D A 0 B 1+e 1 0 C 0 given these costs, given these costs, given these costs, find new routing…. find new routing….find new routing…. resulting in new costs resulting in new cost resulting in new costs Network Layer 4-69
More slides like this


Slide #70.

Chapter 4: outline 4.1 introduction 4.2 virtual circuit and datagram networks 4.3 what’s inside a router 4.4 IP: Internet Protocol     datagram format IPv4 addressing ICMP IPv6 4.5 routing algorithms  link state  distance vector  hierarchical routing 4.6 routing in the Internet  RIP  OSPF  BGP 4.7 broadcast and multicast routing Network Layer 4-70
More slides like this


Slide #71.

Distance vector algorithm Bellman-Ford equation (dynamic programming) let dx(y) := cost of least-cost path from x to y v then dx(y) = min {c(x,v) +neighbor dv(y) }v to destination cost from cost to neighbor v min taken over all neighbors v of x Network Layer 4-71
More slides like this


Slide #72.

Bellman-Ford example 5 2 u v 2 1 x 3 w 3 1 clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3 5 z 1 y 2 B-F equation says: du(z) = min { c(u,v) + dv(z), c(u,x) + dx(z), c(u,w) + dw(z) } = min {2 + 5, 1 + 3, 5 + 3} = 4 ode achieving minimum is next op in shortest path, used in forwarding table Network Layer 4-72
More slides like this


Slide #73.

Distance vector algorithm  Dx(y) = estimate of least cost from x to y  x maintains distance vector Dx = [Dx(y): y є N]  node x:  knows cost to each neighbor v: c(x,v)  maintains its neighbors’ distance vectors. For each neighbor v, x maintains Dv = [Dv(y): y є N ] Network Layer 4-73
More slides like this


Slide #74.

Distance vector algorithm key idea:   from time-to-time, each node sends its own distance vector estimate to neighbors when x receives new DV estimate from neighbor, it updates its own DV using B-F equation: Dx(y) ← minv{c(x,v) + Dv(y)} for each node y ∊ N  under minor, natural conditions, the estimate Dx(y) converge to the actual least cost dx(y) Network Layer 4-74
More slides like this


Slide #75.

Distance vector algorithm iterative, asynchronous: each   local iteration caused by: local link cost change DV update message from neighbor distributed:  each node notifies neighbors only when its DV changes  neighbors then notify their neighbors if necessary each node: wait for (change in local link cost or msg from neighbor) recompute estimates if DV to any dest has changed, notify neighbors Network Layer 4-75
More slides like this


Slide #76.

Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2 Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3 from node y table x y z from node z table x y z cost to cost to x y z x y z 0 2 7 x 0 2 3 y 2 0 1 z 7 1 0 ∞∞ ∞ ∞∞ ∞ from from node x table x y z cost to x y z 2 ∞ ∞ ∞ 2 0 1 ∞∞ ∞ x y 7 1 z cost to x y z ∞∞ ∞ ∞∞ ∞ 7 1 0 time Network Layer 4-76
More slides like this


Slide #77.

Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2 Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3 0 2 7 x 0 2 3 y 2 0 1 z 7 1 0 x 0 2 3 y 2 0 1 z 3 1 0 ∞∞ ∞ ∞∞ ∞ from x y z from x y z cost to cost to cost to x y z x y z ∞ ∞ ∞ 2 0 1 x 0 2 7 y 2 0 1 z 7 1 0 x 0 2 3 y 2 0 1 z 3 1 0 ∞∞ ∞ cost to cost to x y z x y z ∞∞ ∞ x 0 2 7 y 2 0 1 z 3 1 0 ∞∞ ∞ 7 1 0 from x y z 2 x y 7 1 z cost to x y z from from node z table x y z cost to x y z from from node y table x y z cost to cost to from from node x table x y z x 0 2 3 y 2 0 1 z 3 1 0 time Network Layer 4-77
More slides like this


Slide #78.

Distance vector: link cost changes link cost changes: 1 y node detects local link cost 4 1 change x z 50  updates routing info, recalculates distance vector  if DV tchanges, change, updates its DV, informs its notify 0 : y detects link-cost “good neighbors. newsneighbors t1 : z receives update from y, updates its table, computes new travels least cost to x , sends its neighbors its DV. fast”  t2 : y receives z’s update, updates its distance table. y’s least costs do not change, so y does not send a message to z. Network Layer 4-78
More slides like this


Slide #79.

Distance vector: link cost changes link cost changes: 60 node detects local link cost change x  bad news travels slow “count to infinity” problem!  44 iterations before algorithm stabilizes: see poisoned reverse: text  If Z routes through Y to get to X :  4 y 1 z 50  Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z)  will this completely solve count to infinity problem? Network Layer 4-79
More slides like this


Slide #80.

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 4-80
More slides like this


Slide #81.

Chapter 4: outline 4.1 introduction 4.2 virtual circuit and datagram networks 4.3 what’s inside a router 4.4 IP: Internet Protocol     datagram format IPv4 addressing ICMP IPv6 4.5 routing algorithms  link state  distance vector  hierarchical routing 4.6 routing in the Internet  RIP  OSPF  BGP 4.7 broadcast and multicast routing Network Layer 4-81
More slides like this


Slide #82.

Hierarchical routing our routing study thus far idealization  all routers identical  network “flat” … not true in practice administrative scale: with 600 autonomy million destinations:   can’t store all dest’s in routing tables! routing table exchange would swamp links!   internet = network of networks each network admin may want to control routing in its own network Network Layer 4-82
More slides like this


Slide #83.

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 4-83
More slides like this


Slide #84.

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 4-84
More slides like this


Slide #85.

Inter-AS tasks  suppose router in AS1 receives datagram destined outside of AS1:  router should forward packet to gateway router, but which one? AS1 must: 1. learn which dests are reachable through AS2, which through AS3 2. propagate this reachability info to all routers in AS1 job of inter-AS routing! 3c 3b other networks 3a AS3 1c 1a AS1 1d 2a 1b 2c 2b other networks AS2 Network Layer 4-85
More slides like this


Slide #86.

Example: setting forwarding table in router 1d   suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c), but not via AS2  inter-AS protocol propagates reachability info to all internal routers router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1c  installs forwarding table entry (x,I) … 3c 3b other networks x 3a AS3 1c 1a AS1 1d 2a 1b 2c 2b other networks AS2 Network Layer 4-86
More slides like this


Slide #87.

Example: choosing among multiple ASes   now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2. to configure forwarding table, router 1d must determine which gateway it should forward packets towards for dest x  this is also job of inter-AS routing protocol! … 3c 3b other networks x 3a AS3 … … 1c 1a AS1 1d 2a 1b 2c 2b other networks AS2 ? Network Layer 4-87
More slides like this


Slide #88.

Example: choosing among multiple ASes    now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2. to configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest x  this is also job of inter-AS routing protocol! hot potato routing: send packet towards closest of two routers. learn from inter-AS protocol that subnet x is reachable via multiple gateways use routing info from intra-AS protocol to determine costs of least-cost paths to each of the gateways hot potato routing: choose the gateway that has the smallest least cost determine from forwarding table the interface I that leads to least-cost gateway. Enter (x,I) in forwarding table Network Layer 4-88
More slides like this


Slide #89.

Chapter 4: outline 4.1 introduction 4.2 virtual circuit and datagram networks 4.3 what’s inside a router 4.4 IP: Internet Protocol     datagram format IPv4 addressing ICMP IPv6 4.5 routing algorithms  link state  distance vector  hierarchical routing 4.6 routing in the Internet  RIP  OSPF  BGP 4.7 broadcast and multicast routing Network Layer 4-89
More slides like this


Slide #90.

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 4-90
More slides like this


Slide #91.

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 broadacsted  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 4-91
More slides like this


Slide #92.

Multicast routing: problem statement goal: find a tree (or trees) connecting routers having local mcast group members legend    tree: not all paths between routers used group shared-tree: same tree used by all group members member not group source-based: different tree from each member sender to rcvrs router with a group member router without group member shared tree source-based trees Network Layer 4-92
More slides like this


Slide #93.

Approaches for building mcast trees approaches:  source-based tree: one tree per source  shortest path trees  reverse path forwarding  group-shared tree: group uses one tree  minimal spanning (Steiner)  center-based trees …we first look at basic approaches, then specific protocols adopting these approaches Network Layer 4-93
More slides like this


Slide #94.

Chapter 4: done! 4.1 introduction 4.2 virtual circuit and datagram networks 4.3 what’s inside a router 4.4 IP: Internet Protocol  datagram format, IPv4 addressing, ICMP, IPv6   4.5 routing algorithms  link state, distance vector, hierarchical routing 4.6 routing in the Internet  RIP, OSPF, BGP 4.7 broadcast and multicast routing 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 4-94
More slides like this