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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: - PowerPoint PPT Presentation
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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, FSU 3
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, FSU 4
ARP Example
Computer Science, FSU 5
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, FSU 6
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?
TRY
• tcpdump -ennqti eth0 \( arp or icmp \)
Computer Science, FSU 8
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
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
aaC
A
Bd
A.aA.c
C.bB.a
cb
c
Inter-AS routingbetween A and B
Intra-AS and Inter-AS routing
Host h2a
b
b
aaC
A
Bd c
A.aA.c
C.bB.a
cb
Hosth1
Intra-AS routingwithin AS A
Intra-AS routingwithin 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 neededScale:• hierarchical routing saves table size, reduced update
trafficPerformance: • 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 router tell the adjacent routers (the designated router in its LAN) about its link state.
• 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
• From BGP point of view, three types of networks– Stub network– Multiconnected network– Transit network
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=128.186.0.0/16ASPATH=[0]
128.186.0.0/16
NLRI=128.186.0.0/16ASPATH=[10]
NLRI=128.186.0.0/16ASPATH=[10]
NLRI=128.186.0.0/16ASPATH=[210]
NLRI=128.186.0.0/16ASPATH=[610]
NLRI=128.186.0.0/16ASPATH=[610]
NLRI=128.186.0.0/16ASPATH=[210]
NLRI=128.186.0.0/16ASPATH=[7610]
NLRI=128.186.0.0/16ASPATH=[4210]
NLRI=128.186.0.0/16ASPATH=[3210]
[3210]*[4210][7610]