Chapter 4 Network Layer slides are modified from J. Kurose & K. Ross CPE 400 / 600 Computer Communication Networks Lecture 16

Network Layer 2 Chapter 4: Network Layer r 4. 1 Introduction r 4.2 Virtual circuit and datagram networks r 4.3 What’s inside a router r 4.4 IP: Internet Protocol  Datagram format, IPv4 addressing, ICMP, IPv6 r 4.5 Routing algorithms  Link state, Distance Vector, Hierarchical routing r 4.6 Routing in the Internet  RIP, OSPF, BGP r 4.7 Broadcast and multicast routing

Network Layer 3 Router Architecture Overview Two key router functions: r run routing algorithms/protocol (RIP, OSPF, BGP) r forwarding datagrams from incoming to outgoing link

Network Layer 4 Input Port Functions Decentralized switching: r given datagram dest., lookup output port using forwarding table in input port memory r goal: complete input port processing at ‘line speed’ r queuing: if datagrams arrive faster than forwarding rate into switch fabric Physical layer: bit-level reception Data link layer: e.g., Ethernet

Network Layer 5 Three types of switching fabrics

Network Layer 6 Output port queueing r buffering when arrival rate via switch exceeds output line speed r queueing (delay) and loss due to output port buffer overflow!

Network Layer 7 How much buffering? r RFC 3439 rule of thumb: average buffering equal to “typical” RTT (say 250 msec) times link capacity C  e.g., C = 10 Gps link: 2.5 Gbit buffer r Recent recommendation: with N flows, buffering equal to RTT C. N

Network Layer 8 Input Port Queuing r Fabric slower than input ports combined -> queueing may occur at input queues r Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward r queueing delay and loss due to input buffer overflow!

Network Layer 9 Lecture 16: Outline r 4. 1 Introduction r 4.2 Virtual circuit and datagram networks r 4.3 What’s inside a router  Router architecture  Switching fabric  Input/output ports  Queuing r 4.4 Internet Protocol  Datagram format  IPv4 addressing  NAT  ICMP  IPv6

Network Layer 10 The Internet Network layer forwarding table Host, router network layer functions: Routing protocols path selection RIP, OSPF, BGP IP protocol addressing conventions datagram format packet handling conventions ICMP protocol error reporting router “signaling” Transport layer: TCP, UDP Link layer physical layer Network layer

Network Layer 11 IP datagram format ver length 32 bits data (variable length, typically a TCP or UDP segment) 16-bit identifier header checksum time to live 32 bit source IP address IP protocol version number header length (bytes) max number remaining hops (decremented at each router) for fragmentation/ reassembly total datagram length (bytes) upper layer protocol to deliver payload to head. len type of service “type” of data flgs fragment offset upper layer 32 bit destination IP address Options (if any) E.g. timestamp, record route taken, specify list of routers to visit.

Network Layer 12 IP Fragmentation & Reassembly r network links have MTU (max.transfer size) - largest possible link-level frame.  different link types, different MTUs r 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 13 IP Addressing: introduction r IP address: 32-bit identifier for host, router interface r 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 =

Network Layer 14 Subnets r IP address:  subnet part (high order bits)  host part (low order bits) r What’s a subnet ?  device interfaces with same subnet part of IP address  can physically reach each other without intervening router network consisting of 3 subnets subnet

Network Layer 15 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 /23

Network Layer 16 IP addresses: how to get one? Q: How does a host get IP address? r hard-coded by system admin in a file  Windows: control-panel->network->configuration- >tcp/ip->properties  UNIX: /etc/rc.config r DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server  “plug-and-play”

Network Layer 17 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 DHCP overview:  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

Network Layer 18 DHCP client-server scenario A B E DHCP server arriving DHCP client needs address in this network

Network Layer 19 DHCP client-server scenario DHCP server: arriving client time DHCP discover src : , 68 dest.: ,67 yiaddr: transaction ID: 654 DHCP offer src: , 67 dest: , 68 yiaddrr: transaction ID: 654 Lifetime: 3600 secs DHCP request src: , 68 dest:: , 67 yiaddrr: transaction ID: 655 Lifetime: 3600 secs DHCP ACK src: , 67 dest: , 68 yiaddrr: transaction ID: 655 Lifetime: 3600 secs

Network Layer 20 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 /20 Organization /23 Organization /23 Organization /23... ….. …. …. Organization /23

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

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

Network Layer 23 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 24 NAT: Network Address Translation local network (e.g., home network) /24 rest of Internet Datagrams with source or destination in this network have /24 address for source, destination (as usual) All datagrams leaving local network have same single source NAT IP address: , different source port numbers

Network Layer 25 NAT: Network Address Translation r 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 26 NAT: Network Address Translation Implementation: NAT router must: r outgoing datagrams: replace (source IP, port #) of every outgoing datagram to (NAT IP, new port #)  remote clients/servers will respond using (NAT IP, new port #) as destination addr. r remember (in NAT translation table) every (source IP, port #) to (NAT IP, new port #) translation pair r incoming datagrams: replace (NAT IP, new port #) in destination fields of every incoming datagram with corresponding (source IP, port #) stored in NAT table

Network Layer 27 NAT: Network Address Translation S: , 3345 D: , : host sends datagram to , 80 NAT translation table WAN side addr LAN side addr , , 3345 …… S: , 80 D: , S: , 5001 D: , : NAT router changes datagram source addr from , 3345 to , 5001, updates table S: , 80 D: , : Reply arrives dest. address: , : NAT router changes datagram dest addr from , 5001 to , 3345

Network Layer 28 NAT: Network Address Translation r 16-bit port-number field:  60,000 simultaneous connections with a single LAN- side address! r 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, eg, P2P applications  address shortage should instead be solved by IPv6

Network Layer 29 NAT traversal problem r client wants to connect to server with address  server address local to LAN (client can’t use it as destination addr)  only one externally visible NATted address: r solution 1: statically configure NAT to forward incoming connection requests at given port to server  e.g., ( , port 2500) always forwarded to port NAT router Client ?

Network Layer 30 NAT traversal problem r solution 2: Universal Plug and Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATted host to:  learn public IP address ( )  add/remove port mappings (with lease times) i.e., automate static NAT port map configuration NAT router IGD

Network Layer 31 NAT traversal problem r solution 3: relaying (used in Skype)  NATed client establishes connection to relay  External client connects to relay  relay bridges packets between to connections Client NAT router 1. connection to relay initiated by NATted host 2. connection to relay initiated by client 3. relaying established

Network Layer 32 ICMP: Internet Control Message Protocol r used by hosts & routers to communicate network-level information  error reporting: unreachable host, network, port, protocol  echo request/reply (used by ping) r network-layer “above” IP:  ICMP msgs carried in IP datagrams r ICMP message: type, code plus first 8 bytes of IP datagram causing error Type Code description 0 0 echo reply (ping) 3 0 dest. network unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 route advertisement 10 0 router discovery 11 0 TTL expired 12 0 bad IP header

Network Layer 33 Traceroute and ICMP r Source sends series of UDP segments to dest  First has TTL =1, Second has TTL=2, etc.  Unlikely port number r When nth datagram arrives to nth router:  Router discards datagram  And sends to source an ICMP message (type 11, code 0)  Message includes name of router& IP address r When ICMP message arrives, source calculates RTT r Traceroute does this 3 times Stopping criterion r UDP segment eventually arrives at destination host r Destination returns ICMP “host unreachable” packet (type 3, code 3) r When source gets this ICMP, stops.

Network Layer 34 Lecture 16: Summary r Routers r Internet Protocol  Datagram format  IPv4 addressing  Subnetting  CIDR  DHCP  NAT  ICMP