Announcement r Recitation tomorrow on Project 2 r Midterm Survey at the end of this class

Review r Hierarchical Routing r The Internet (IP) Protocol m IPv4 addressing m Moving a datagram from source to destination Some slides are in courtesy of J. Kurose and K. Ross

Overview r The Internet (IP) Protocol m Datagram format m IP fragmentation m ICMP: Internet Control Message Protocol m NAT: Network Address Translation r Routing in the Internet m Intra-AS routing: RIP and OSPF m Inter-AS routing: BGP r Multicast Routing Some slides are in courtesy of J. Kurose and K. Ross

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

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:

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

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

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

IP Fragmentation and Reassembly ID =x offset =0 fragflag =0 length =4000 ID =x offset =0 fragflag =1 length =1500 ID =x offset =185 fragflag =1 length =1500 ID =x offset =370 fragflag =0 length =1040 One large datagram becomes several smaller datagrams Example r 4000 byte datagram r MTU = 1500 bytes 1480 bytes in data field offset = 1480/8

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 Ping, traceroute uses ICMP

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

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

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

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

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

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).

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

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

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

Overview r The Internet (IP) Protocol m Datagram format m IP fragmentation m ICMP: Internet Control Message Protocol m NAT: Network Address Translation r Routing in the Internet m Intra-AS routing: RIP and OSPF m Inter-AS routing: BGP r Multicast Routing Some slides are in courtesy of J. Kurose and K. Ross

Architecture of Dynamic Routing AS 1 AS 2 EGP (= BGP) EGP = Exterior Gateway Protocol IGP = Interior Gateway Protocol Metric based: OSPF, IS-IS, RIP, EIGRP (cisco) Policy based: BGP The Routing Domain of BGP is the entire Internet IGP

The Gang of Four Link StateVectoring EGP IGP BGP RIP IS-IS OSPF Used in upper-tier ISPs Lower-tier ISPs and enterprise networks

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 (per link) r Advertisements disseminated to entire AS (via flooding) m Periodically broadcast even when there is no link cost change m Carried in OSPF messages directly over IP (rather than TCP or UDP

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 Integrated uni- and multicast support: m Multicast OSPF (MOSPF) uses same topology data base as OSPF r Hierarchical OSPF in large domains.

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.