Network LayerII-1 RSC Part II: Network Layer 3. IP addressing (2nd part) Redes y Servicios de Comunicaciones Universidad Carlos III de Madrid These slides are, mainly, part of the companion slides to the book “Computer Networking: A Top Down Approach” generously made available by their authors (see copyright below). The slides have been adapted, where required, to the teaching needs of the subject above. All material copyright J.F Kurose and K.W. Ross, All Rights Reserved Computer Networking: A Top Down Approach 5 th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009.

Network LayerII-2Network LayerII-2 RSC Part II: Network Layer r II. 1 Basic Network layer concepts r II.2 Introduction to IP m Datagram format m ICMP r II.3 IP addressing m Obtaining addresses, DHCP, NAT r II.4 IP in operation m ARP r II.5 Network routing m Link state m Distance Vector m Hierarchical routing r II.6 Routing in the Internet m RIP m OSPF m BGP m Broadcast and multicast

Network LayerII-3 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 LayerII-4 IP addressing: the last word... Q: How does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers m Allocates and registers addresses m manages DNS m assigns domain names, resolves disputes r The Internet Assigned Numbers Authority (IANA), operated by the ICANN (Internet Corporation for Assigned Names and Numbers) m Manages IP address allocation (and parameters of Internet protocols) m Delegates address assignment to RIRs (Regional Internet Registry): American Registry for Internet Numbers (ARIN) for North America and parts of the Caribbean RIPE Network Coordination Centre (RIPE NCC) for Europe, the Middle East and Central Asia Asia-Pacific Network Information Centre (APNIC) for Asia and the Pacific region Latin American and Caribbean Internet Addresses Registry (LACNIC) for Latin America and parts of the Caribbean region African Network Information Centre (AfriNIC) for Africa m ISPs get IP address blocks from RIRs or Local Internet Registry (LIR)

Network LayerII-5 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: m host broadcasts “DHCP discover” msg m DHCP server responds with “DHCP offer” msg m host requests IP address: “DHCP request” msg m DHCP server sends address: “DHCP ack” msg

Network LayerII-6 DHCP client-server scenario A B E DHCP server arriving DHCP client needs address in this network

Network LayerII-7 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 LayerII-8 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 LayerII-9 NAT: Network Address Translation r Motivation: local network uses just one IP address as far as outside world is concerned: m range of addresses not needed from ISP: just one IP address for all devices m can change addresses of devices in local network without notifying outside world m can change ISP without changing addresses of devices in local network m devices inside local net not explicitly addressable, visible by outside world (a security plus). r Private address space: m (10/8 prefix) m (172.16/12 prefix) m ( /16 prefix)

Network LayerII-10 NAT: Network Address Translation Implementation: NAT router must: m 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. m remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair m 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 LayerII-11 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 LayerII-12 NAT: Network Address Translation r 16-bit port-number field: m 60,000 simultaneous connections with a single LAN-side address! r NAT is controversial: m routers should only process up to layer 3 m violates end-to-end argument NAT possibility must be taken into account by app designers, eg, P2P applications m address shortage should instead be solved by IPv6

Network LayerII-13 NAT traversal problem r client wants to connect to server with address m server address local to LAN (client can’t use it as destination addr) m only one externally visible NATted address: r solution 1: statically configure NAT to forward incoming connection requests at given port to server m e.g., ( , port 2500) always forwarded to port NAT router Client ?

Network LayerII-14 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 LayerII-15 NAT traversal problem r solution 3: relaying (used in Skype) m NATed client establishes connection to relay m External client connects to relay m 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