IP FORWARDING Dr. Rocky K C Chang 1 1 October 2010
IP FORWARDING Dr. Rocky K. C. Chang 11 October 2010 1
Content 口 Switches vs routers a The iP forwarding problem n The IP address lookup problem 口| P tunneling a Forwarding-related ICMP messages
Content 2 Switches vs routers The IP forwarding problem The IP address lookup problem IP tunneling Forwarding-related ICMP messages
Routers vs switches 3 a Price/performance comparison o Besides packet forwarding, routers offer rich functionalities n Support multiple network-layer protocols a block broadcast packets a Provide type-of-service routing(differentiated service) a Perform admission control, per-flow queueing, resource reservation, and fair scheduling n Assist in network congestion control. 口 Support tunneling a Support IP fragmentation Perform nat a etc
Routers vs switches 3 Price/performance comparison Besides packet forwarding, routers offer rich functionalities: Support multiple network-layer protocols. Block broadcast packets. Provide type-of-service routing (differentiated service). Perform admission control, per-flow queueing, resource reservation, and fair scheduling. Assist in network congestion control. Support tunneling Support IP fragmentation Perform NAT etc
Things that a router needs to worry about a Integrity of an incoming packet Checksum for the header a Source address spoofing(limited) a Receiving: queueing, scheduling, detunneling, etc o Dropping or forwarding a Dropping (TTL, broadcasting, congestion, and the integrity issues)and feedback n Forwarding: destination address (and perhaps source addresses and interface), and Tos 口 Forwarding a Fragmentation, tunneling, source address and port translation
Things that a router needs to worry about 4 Integrity of an incoming packet: Checksum for the header Source address spoofing (limited) Receiving: queueing, scheduling, detunneling, etc Dropping or forwarding Dropping (TTL, broadcasting, congestion, and the integrity issues) and feedback Forwarding: destination address (and perhaps source addresses and interface), and TOS. Forwarding Fragmentation, tunneling, source address and port translation
5P forwarding
5 IP forwarding
Forwarding, routing, and switching n Routing: the process by which nodes exchange topological information to build correct forwarding tables 口R。 Uting protocols(○ SPF BGP|S-|S,etc) n Forwarding: the operation of deciding the next-hop address to forward to a Forwarding table vs routing table a Switching: the operation of moving a packet from an input port to an output port. a IP router: one that forwards iP packets for others
Forwarding, routing, and switching 6 Routing: the process by which nodes exchange topological information to build correct forwarding tables. Routing protocols (OSPF, BGP, IS-IS, etc) Forwarding: the operation of deciding the next-hop address to forward to. Forwarding table vs routing table Switching: the operation of moving a packet from an input port to an output port. IP router: one that forwards IP packets for others
IP routing vs IP switching IP routing IP routing protocol protocol Ethernet ATM(cell Token ring Switching FDDI etc table)
IP routing vs IP switching 7 IP routing protocol Ethernet, Token ring, FDDI, etc IP routing protocol ATM (cell switching table)
The IP forwarding problem a Assume that both routers and hosts already have appropriate routing tables in place. a Routing tables for routers are constructed from routing protocols or by hand. a Routing tables for hosts are constructed from other means(to be discussed later). a Problem: Given a forwarding table and an IP packet, how do hosts and routers make forwarding decisions?
The IP forwarding problem 8 Assume that both routers and hosts already have appropriate routing tables in place. Routing tables for routers are constructed from routing protocols or by hand. Routing tables for hosts are constructed from other means (to be discussed later). Problem: Given a forwarding table and an IP packet, how do hosts and routers make forwarding decisions?
IP forwarding mechanisms Routing protocol(router only) ICMP redirect messages(host only) Router discovery protocol(host only) Manual configuration (router and host) IP packets IP Output router only IP forwarding table (compute the next hop) Network interfaces
IP forwarding mechanisms 9 IP Output (compute the next hop) IP forwarding table Routing protocol (router only) ICMP redirect messages (host only) Router discovery protocol (host only) Manual configuration (router and host) IP packets Network interfaces router only
ypes of forwarding entries 10 a Unica st vs multicast destinations n Loopback vs actual routes a Host-specific vs network specific routes n First-hop forwarding vs last-hop forwarding vs in- between forwarding a The last two are for routers only
Types of forwarding entries 10 Unicast vs multicast destinations Loopback vs actual routes Host-specific vs network specific routes First-hop forwarding vs last-hop forwarding vs inbetween forwarding The last two are for routers only
