Internet Protocols

ICMP ICMP – Internet Control Message Protocol Each ICMP message is encapsulated in an IP packet – Treated like any other datagram, but no error message sent if ICMP message causes error Some interesting messages: – Time exceeded: When an IP packet arrived at a router is dropped because the TTL field becomes 0, the router will send an ICMP TIME EXCEEDED message back to the source. Used in traceroute. – Echo and Echo reply: ping.

Computer Science, FSU3 Address Resolution IP address is virtual – Not understood by underlying the hardware of physical networks IP packets need to be transmitted by the underlying physical network Address resolution – Translating IP address to physical address – Address Resolution Protocol (ARP)

Computer Science, FSU4 ARP Example

Computer Science, FSU5 ARP Cache Each computer maintains a cache table – IP address  hardware address mapping – Only about computers on the same network Exchanges ARP messages – To resolve IP addresses with unknown hardware addresses

Computer Science, FSU6 ARP Protocol When a node sends an IP packet to another node on the same physical network – Look up destination address in the ARP table – If not found Broadcast a request to the local network Whose IP address is this?

Computer Science, FSU7 ARP Response The target node responds to sender (unicast?) – With its physical address – Adds the requester into its ARP table (why?) On receiving the response – Requester updates its table Other nodes upon receiving the request – Refresh the requester entry if already there – No action otherwise (why?) Table entries deleted if not refreshed for a while

DHCP DHCP – Dynamic Host Configuration Protocol A new machine asks for an IP address – Broadcast DHCP DISCOVER packet – A DHCP relay agent got this packet and relay it to the DHCP server – The DHCP server assigns an IP address – Periodically renew

TRY tcpdump -ennqti eth0 \( arp or icmp \)

Hierarchical Routing aggregate routers into regions, “autonomous systems” (AS) routers in same AS run same routing protocol – “intra-AS” routing protocol – routers in different AS can run different intra-AS routing protocol special routers in AS run intra-AS routing protocol with all other routers in AS also responsible for routing to destinations outside AS – run inter-AS routing protocol with other gateway routers gateway routers

Intra-AS and Inter-AS routing Gateways: perform inter-AS routing amongst themselves perform intra-AS routing with other routers in their AS inter-AS, intra-AS routing in gateway A.c network layer link layer a b b a a C A B d A.a A.c C.b B.a c b c

Inter-AS routing between A and B Intra-AS and Inter-AS routing Host h2 a b b a a C A B d c A.a A.c C.b B.a c b Host h1 Intra-AS routing within AS A Intra-AS routing within AS B

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

Intra-AS Routing Also known as Interior Gateway Protocols (IGP) Most common IGPs: – RIP: Routing Information Protocol – OSPF: Open Shortest Path First – IGRP: Interior Gateway Routing Protocol (Cisco proprietary)

OSPF Represents the network as a graph, and runs the shortest path algorithm to find the path to any router. Divide the network into areas for scalability. – The backbone area is called area 0 – Route: local area  backbone  local area

OSPF Each area computes shortest paths. Backbone routers also accept information from area border routers to compute the shortest path to reach other routers. Then advertise this information to the border routers, who tells routers inside the area – To be able to select the best exit router in an area

Inter-AS routing

BGP From BGP point of view, three types of networks – Stub network (only one connection to the network) – Multiconnected network (multiple connection, but refuse to transmit traffic) – Transit network (backbone)

Internet Inter-AS routing: BGP BGP (Border Gateway Protocol): the de facto standard Path Vector protocol: – similar to Distance Vector protocol – each Border Gateway broadcast to neighbors (peers) entire path (i.e, sequence of ASs) to destination – E.g., Gateway X may send its path to dest. Z: Path (X,Z) = X,Y1,Y2,Y3,…,Z

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

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

BGP: an example NLRI= /16 ASPATH=[0] /16 NLRI= /16 ASPATH=[10] NLRI= /16 ASPATH=[10] NLRI= /16 ASPATH=[210] NLRI= /16 ASPATH=[610] NLRI= /16 ASPATH=[610] NLRI= /16 ASPATH=[210] NLRI= /16 ASPATH=[7610] NLRI= /16 ASPATH=[4210] NLRI= /16 ASPATH=[3210] [3210]* [4210] [7610]