Chapter 4: Network Layer

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1 Chapter 4: Introduction (forwarding and routing) Review of queueing theory Routing algorithms Link state, Distance Vector Router design and operation IP: Internet Protocol IPv4 (datagram format, addressing, ICMP, NAT) Ipv6 Routing in the Internet Autonomous Systems Routing protocols (RIP, OSPF, BGP) Router Architecture Overview Two key router functions: run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing link 1

2 Input Port Functions Physical layer: bit-level reception Data link layer: e.g., Ethernet see chapter 5 Decentralized switching: given datagram destination, lookup output port using forwarding table in input port memory goal: complete input port processing at line speed queueing: if datagrams arrive faster than forwarding rate into switch fabric Three types of switching fabrics (or, more generally, an interconnection network) 2

3 Switching Via Memory First generation routers: were simply computers, with switching under direct control of CPU packet copied to system s memory, routed, then copied out speed limited by memory bandwidth (2 bus crossings per datagram) Input Port Memory Output Port System Bus Switching Via Memory Modern Routers Shared-memory multiprocessors Input port processors switch packets into memory of output port processor Cisco Catalyst 8500 Series - campus and metropolitan network Gbps Gbps EOL Nov 18,

4 Switching Via a Bus datagram from input port memory to output port memory via a shared bus bus contention: switching speed limited by bus bandwidth 32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers Switching Via Interconnection Network overcome bus bandwidth limitations Banyan networks, other interconnection networks initially developed to connect processors in multiprocessor advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric. Cisco 12000: switches 60 Gbps through the interconnection network 4

5 Output port queueing buffering when arrival rate via switch exceeds output line speed queueing (delay) and loss due to output port buffer overflow! Input Port Queueing if the switching fabric is slower than input ports combined speed, queueing may occur at input queues Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward queueing delay and loss due to input buffer overflow! 5

6 Chapter 4: Introduction (forwarding and routing) Review of queueing theory Routing algorithms Link state, Distance Vector Router design and operation IP: Internet Protocol IPv4 (datagram format, addressing, ICMP, NAT) IPv6 Routing in the Internet Autonomous Systems Routing protocols (RIP, OSPF, BGP) Internet Protocol (IP) IP is a DoD standard, designed from the beginning with internetworking in mind IP features unreliable -delivery of packets is not guaranteed connectionless -packets are routed and handled independently, even if part of the same message best-effort delivery -packets are only discarded when underlying components fail or an overload condition occurs Defines basic unit of data transfer, the IP datagram 6

7 The Internet Network layer Host, router network layer functions: Transport layer: TCP, UDP Network layer Routing protocols path selection RIP, OSPF, BGP forwarding table ICMP protocol error reporting router signaling IP protocol addressing conventions datagram format packet handling conventions Link layer physical layer IP (v4) 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 overhead ver head. len 16-bit identifier time to live 32 bits type of service upper layer flgs length fragment offset header checksum 32 bit source IP address 32 bit destination IP address Options (if any) data (variable length, typically a TCP or UDP segment) total datagram length (bytes) for fragmentation/ reassembly E.g. timestamp, record route taken, specify list of routers to visit. 7

8 Internet Addressing To support a universal communication service, a globally accepted addressing method is required. IPv4 address properties unique 32-bit number for each host an IP address comprises two components netid identifies a network (subnet) hostid identifies a host on that network IP Address Allocation Q: How does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers Non-profit organization that: allocates addresses manages DNS assigns domain names, resolves disputes 8

9 Traditional IPv4 Addressing Addressing Conventions Normally, the 32-bit IP addresses are written as four decimal numbers separated by decimal points Is written as A network address with hostid = refers to a network. A network address with hostid = is a (directed) broadcast address, referring to all hosts on a specified network. Where possible (e.g., Ethernet), IP broadcasting takes advantage of a hardware broadcast facility. 9

10 Addressing Conventions A network address with netid = and hostid =1...1 is a broadcast address for the local network. Local broadcast is used in startup before a host knows its IP address or the IP address of the local network is reserved for loopback. Using this network address, a packet will not get to the network. Question: If an IP address identifies a host, how do we assign a single IP address to a gateway/router node, which by definition belongs to multiple networks? IPv4 Addressing IP address: 32-bit identifier for host or 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 =

11 Subnetting (traditional) Some bits in the hostid part of the IP address are actually used to specify a particular physical network, such as a LAN on a campus Subnetting (traditional) All gateways on rest of the Internet route packets as if there were one physical network behind G The partitioning of the local address part is an autonomous decision A subnet mask indicates which bits in the hostid are to be used for network addressing Although not required to do so, managers would usually select contiguous bits for the subnetwork and use the same mask for all physical networks at a particular site. 11

12 Subnetting (generalized) 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 subnet network consisting of 3 subnets Subnets / /24 Procedure To determine the subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet /24 Subnet mask notation: address/#bits 12

13 Subnets How many? Traditional Addressing Problem 13

14 IP addressing: CIDR CIDR: Classless InterDomain Routing m subnet portion of address of arbitrary length m address format: a.b.c.d/x, where x is # bits in subnet portion of address subnet part /23 host part IP addresses: how to get one? Q: How does a host get IP address? Static IP: hard-coded by system admin in a file Windows: control-panel->network->configuration->tcp/ip->properties Traditional UNIX: /etc/rc.config Linux: /etc/network/interfaces : Dynamic Host Configuration Protocol: dynamically get address from as server plug-and-play 14

15 : 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 and on ) Support for mobile users who want to join network (more shortly) overview: host broadcasts discover msg [optional] server responds with offer msg [optional] host requests IP address: request msg server sends address: ack msg client-server scenario A server B E arriving client needs address in this network 15

16 client-server scenario server: discover src : , 68 dest.: ,67 yiaddr: transaction ID: 654 arriving client time request offer src: , 68 dest:: , 67 yiaddrr: transaction ID: 655 Lifetime: 3600 secs src: , 67 dest: , 68 yiaddrr: transaction ID: 654 Lifetime: 3600 secs ACK src: , 67 dest: , 68 yiaddrr: transaction ID: 655 Lifetime: 3600 secs Other functionality 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) 16

17 : example UDP IP Eth Phy UDP IP Eth Phy router (runs ) connecting laptop needs its IP address, addr of firsthop router, addr of DNS server: use request encapsulated in UDP, encapsulated in IP, encapsulated in Ethernet Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running server Ethernet demux ed to IP demux ed, UDP demux ed to : example UDP IP Eth Phy DCP server formulates ACK containing client s IP address, IP address of first-hop router for client, name & IP address of DNS server UDP IP Eth Phy router (runs ) encapsulation of server, frame forwarded to client, demux ing up to at client client now knows its IP address, name and IP address of DSN server, IP address of its first-hop router 17

18 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 / Organization /23 Hierarchical addressing: route aggregation Hierarchical addressing allows efficient advertisement of routing information: Organization /23 Organization /23 Organization / Organization /23 Fly-By-Night-ISP Send me anything with addresses beginning /20 Internet ISPs-R-Us Send me anything with addresses beginning /16 18

19 Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 Organization /23 Organization / Organization /23 Organization /23 Fly-By-Night-ISP ISPs-R-Us Send me anything with addresses beginning /20 Send me anything with addresses beginning /16 or /23 Internet NAT: Network Address Translation rest of Internet local network (e.g., home network) / All datagrams leaving local network have same single source NAT IP address: , different source port numbers Datagrams with source or destination in this network have /24 address for source, destination (as usual) 19

20 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). 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 20

21 NAT: Network Address Translation 2: NAT router changes datagram source addr from , 3345 to , 5001, updates table 2 NAT translation table WAN side addr LAN side addr , , 3345 S: , 5001 D: , S: , 80 D: , : Reply arrives dest. address: , S: , 3345 D: , 80 S: , 80 D: , : host sends datagram to , : NAT router changes datagram dest addr from , 5001 to , 3345 NAT: Network Address Translation 16-bit port-number field: 60,000 simultaneous connections with a single LANside address! Problem with this approach??? 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 21

22 NAT traversal problem 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: One solution: statically configure NAT to forward incoming connection requests at given port to server e.g., ( , port 2500) always forwarded to port Client? NAT router NAT traversal problem Another solution: 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

23 NAT traversal problem Yet another: relaying (used in Skype) NATed client establishes connection to relay External client connects to relay relay bridges packets between to connections Client 2. connection to relay initiated by client 3. relaying established 1. connection to relay initiated by NATted host NAT router 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 reassembly fragmentation: in: one large datagram out: 3 smaller datagrams 23

24 IP Fragmentation and Reassembly Example 4000 byte datagram MTU = 1500 bytes 1480 bytes in data field offset = 1480/8 length =4000 ID =x length =1500 fragflag =0 ID =x ID =x offset =0 One large datagram becomes several smaller datagrams length =1500 length =1040 ID =x fragflag =1 fragflag =1 fragflag =0 offset =0 offset =185 offset =370 ICMP: Internet Control Message Protocol IP is actually a large distributed system ICMP issued by hosts & routers to communicate network-level information error reporting: unreachable host, network, port, protocol echo request/reply (used by ping) Technically a network-layer above IP: ICMP msgs carried in IP datagrams 24

25 ICMP Codes 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 ICMP message content ICMP message: type, code plus IP header and first 8 bytes of payload in datagram causing error. Why the payload??? 25

26 Traceroute and ICMP Source sends series of UDP segments to dest First has TTL =1 Second has TTL=2, etc. Unlikely port number 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 When ICMP message arrives, source calculates RTT Traceroute does this 3 times Stopping criterion UDP segment eventually arrives at destination host with bogus port number Destination returns ICMP host unreachable packet (type 3, code 3) When source gets this ICMP, stops. 26

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