Network Layer4-1 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 Layer4-2 IP Addressing: introduction r IP address: 32-bit identifier for host, router interface r interface: connection between host/router and physical link m router’s typically have multiple interfaces m host may have multiple interfaces m IP addresses associated with each interface =

Network Layer4-3 IP Addressing r IP address: m network part (high order bits) m host part (low order bits) r What’s a network ? ( from IP address perspective) m device interfaces with same network part of IP address m can physically reach each other without intervening router network consisting of 3 IP networks (for IP addresses starting with 223, first 24 bits are network address) LAN

Network Layer4-4 IP Addressing How to find the networks? r Detach each interface from router, host r create “islands of isolated networks Interconnected system consisting of six networks

Network Layer4-5 IP Addresses 0 network host 10 network host 110 networkhost 1110 multicast address A B C D class to to to to bits given notion of “network”, let’s re-examine IP addresses: “class-full” addressing:

Network Layer4-6 IP addressing: CIDR r Classful addressing: m inefficient use of address space, address space exhaustion m e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network r CIDR: Classless InterDomain Routing m network portion of address of arbitrary length m address format: a.b.c.d/x, where x is # bits in network portion of address network part host part /23

Network Layer4-7 IP addresses: how to get one? Q: How does host get IP address? r hard-coded by system admin in a file m Wintel: control-panel->network->configuration- >tcp/ip->properties m UNIX: /etc/rc.config r DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server m “plug-and-play” (more shortly)

Network Layer4-8 IP addresses: how to get one? Q: How does network get network 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 Layer4-9 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 Layer4-10 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 Layer4-11 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 addresses m manages DNS m assigns domain names, resolves disputes

Network Layer4-12 Getting a datagram from source to dest. IP datagram: A B E misc fields source IP addr dest IP addr data r datagram remains unchanged, as it travels source to destination r addr fields of interest here Dest. Net. next router Nhops forwarding table in A

Network Layer4-13 Getting a datagram from source to dest. Starting at A, send IP datagram addressed to B: r look up net. address of B in forwarding table r find B is on same net. as A r link layer will send datagram directly to B inside link-layer frame m B and A are directly connected Dest. Net. next router Nhops misc fields data A B E forwarding table in A

Network Layer4-14 Getting a datagram from source to dest. Dest. Net. next router Nhops Starting at A, dest. E: r look up network address of E in forwarding table r E on different network m A, E not directly attached r routing table: next hop router to E is r link layer sends datagram to router inside link- layer frame r datagram arrives at r continued….. misc fields data A B E forwarding table in A

Network Layer4-15 Getting a datagram from source to dest. Arriving at , destined for r look up network address of E in router’s forwarding table r E on same network as router’s interface m router, E directly attached r link layer sends datagram to inside link-layer frame via interface r datagram arrives at !!! (hooray!) misc fields data Dest. Net router Nhops interface A B E forwarding table in router

Network Layer4-16 IP datagram format ver length 32 bits data (variable length, typically a TCP or UDP segment) 16-bit identifier Internet 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. how much overhead with TCP? r 20 bytes of TCP r 20 bytes of IP r = 40 bytes + app layer overhead

Network Layer4-17 IP Fragmentation & Reassembly r network links have MTU (max.transfer size) - largest possible link-level frame. m different link types, different MTUs r large IP datagram divided (“fragmented”) within net m one datagram becomes several datagrams m “reassembled” only at final destination m IP header bits used to identify, order related fragments fragmentation: in: one large datagram out: 3 smaller datagrams reassembly

Network Layer4-18 IP Fragmentation and Reassembly ID =x offset =0 fragflag =0 length =4000 ID =x offset =0 fragflag =1 length =1500 ID =x offset =1480 fragflag =1 length =1500 ID =x offset =2960 fragflag =0 length =1040 One large datagram becomes several smaller datagrams Example r 4000 byte datagram r MTU = 1500 bytes

Network Layer4-19 ICMP: Internet Control Message Protocol r used by hosts, routers, gateways to communication network-level information m error reporting: unreachable host, network, port, protocol m echo request/reply (used by ping) r network-layer “above” IP: m 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 Layer4-20 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 Layer4-21 DHCP client-server scenario A B E DHCP server arriving DHCP client needs address in this network

Network Layer4-22 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 Layer4-23 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 Layer4-24 NAT: Network Address Translation r Motivation: local network uses just one IP address as far as outside word is concerned: m no need to be allocated range of addresses from ISP: - just one IP address is used 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).

Network Layer4-25 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 Layer4-26 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 Layer4-27 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 Layer4-28 Routing in the Internet r The Global Internet consists of Autonomous Systems (AS) interconnected with each other: m Stub AS: small corporation: one connection to other AS’s m Multihomed AS: large corporation (no transit): multiple connections to other AS’s m Transit AS: provider, hooking many AS’s together r Two-level routing: m Intra-AS: administrator responsible for choice of routing algorithm within network m Inter-AS: unique standard for inter-AS routing: BGP

Network Layer4-29 Internet AS Hierarchy Intra-AS border (exterior gateway) routers Inter-AS interior (gateway) routers

Network Layer4-30 Intra-AS Routing r Also known as Interior Gateway Protocols (IGP) r Most common Intra-AS routing protocols: m RIP: Routing Information Protocol m OSPF: Open Shortest Path First m IGRP: Interior Gateway Routing Protocol (Cisco proprietary)

Network Layer4-31 RIP ( Routing Information Protocol) r Distance vector algorithm r Included in BSD-UNIX Distribution in 1982 r Distance metric: # of hops (max = 15 hops) m Can you guess why? r Distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement) r Each advertisement: list of up to 25 destination nets within AS

Network Layer4-32 RIP: Example Destination Network Next Router Num. of hops to dest. wA2 yB2 zB7 x--1 ….…..... w xy z A C D B Routing table in D

Network Layer4-33 RIP: Example Destination Network Next Router Num. of hops to dest. wA2 yB2 zB A7 5 x--1 ….…..... Routing table in D w xy z A C D B Dest Next hops w - - x - - z C 4 …. …... Advertisement from A to D

Network Layer4-34 RIP: Link Failure and Recovery If no advertisement heard after 180 sec --> neighbor/link declared dead m routes via neighbor invalidated m new advertisements sent to neighbors m neighbors in turn send out new advertisements (if tables changed) m link failure info quickly propagates to entire net m poison reverse used to prevent ping-pong loops (infinite distance = 16 hops)

Network Layer4-35 RIP Table processing r RIP routing tables managed by application-level process called route-d (daemon) r advertisements sent in UDP packets, periodically repeated physical link network forwarding (IP) table Transprt (UDP) routed physical link network (IP) Transprt (UDP) routed forwarding table

Network Layer4-36 RIP Table example (continued) Router: giroflee.eurocom.fr r Three attached class C networks (LANs) r Router only knows routes to attached LANs r Default router used to “go up” r Route multicast address: r Loopback interface (for debugging) Destination Gateway Flags Ref Use Interface UH lo U 2 13 fa U le U 2 25 qaa U 3 0 le0 default UG

Network Layer4-37 OSPF (Open Shortest Path First) r “open”: publicly available r Uses Link State algorithm m LS packet dissemination m Topology map at each node m Route computation using Dijkstra’s algorithm r OSPF advertisement carries one entry per neighbor router r Advertisements disseminated to entire AS (via flooding) m Carried in OSPF messages directly over IP (rather than TCP or UDP

Network Layer4-38 OSPF “advanced” features (not in RIP) r Security: all OSPF messages authenticated (to prevent malicious intrusion) r Multiple same-cost paths allowed (only one path in RIP) r For each link, multiple cost metrics for different TOS (e.g., satellite link cost set “low” for best effort; high for real time) r Integrated uni- and multicast support: m Multicast OSPF (MOSPF) uses same topology data base as OSPF r Hierarchical OSPF in large domains.

Network Layer4-39 Hierarchical OSPF

Network Layer4-40 Hierarchical OSPF r Two-level hierarchy: local area, backbone. m Link-state advertisements only in area m each nodes has detailed area topology; only know direction (shortest path) to nets in other areas. r Area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers. r Backbone routers: run OSPF routing limited to backbone. r Boundary routers: connect to other AS’s.

Network Layer4-41 Inter-AS routing in the Internet: BGP

Network Layer4-42 Internet inter-AS routing: BGP r BGP (Border Gateway Protocol): the de facto standard r Path Vector protocol: m similar to Distance Vector protocol m each Border Gateway broadcast to neighbors (peers) entire path (i.e., sequence of AS’s) to destination m BGP routes to networks (ASs), not individual hosts m E.g., Gateway X may send its path to dest. Z: Path (X,Z) = X,Y1,Y2,Y3,…,Z

Network Layer4-43 Internet inter-AS routing: BGP Suppose: gateway X send its path to peer gateway W r W may or may not select path offered by X m cost, policy (don’t route via competitors AS), loop prevention reasons. r If W selects path advertised by X, then: Path (W,Z) = w, Path (X,Z) r Note: X can control incoming traffic by controlling it route advertisements to peers: m e.g., don’t want to route traffic to Z -> don’t advertise any routes to Z

Network Layer4-44 BGP: controlling who routes to you r A,B,C are provider networks r X,W,Y are customer (of provider networks) r X is dual-homed: attached to two networks m X does not want to route from B via X to C m.. so X will not advertise to B a route to C

Network Layer4-45 BGP: controlling who routes to you r A advertises to B the path AW r B advertises to X the path BAW r Should B advertise to C the path BAW? m No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers m B wants to force C to route to w via A m B wants to route only to/from its customers!

Network Layer4-46 BGP operation Q: What does a BGP router do? r Receiving and filtering route advertisements from directly attached neighbor(s). r Route selection. m To route to destination X, which path )of several advertised) will be taken? r Sending route advertisements to neighbors.

Network Layer4-47 BGP messages r BGP messages exchanged using TCP. r BGP messages: m OPEN: opens TCP connection to peer and authenticates sender m UPDATE: advertises new path (or withdraws old) m KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request m NOTIFICATION: reports errors in previous msg; also used to close connection

Network Layer4-48 Why different Intra- and Inter-AS routing ? Policy: r Inter-AS: admin wants control over how its traffic routed, who routes through its net. r Intra-AS: single admin, so no policy decisions needed Scale: r hierarchical routing saves table size, reduced update traffic Performance: r Intra-AS: can focus on performance r Inter-AS: policy may dominate over performance