Transcript inter-AS routing - FSU Computer Science Department

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.
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)
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ARP Example
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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
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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?
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TRY
• tcpdump -ennqti eth0 \( arp or icmp \)
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
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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
gateway routers
• 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
Intra-AS and Inter-AS routing
C.b
Gateways:
B.a
A.a
a
b
A.c
C
B
a
d
A
b
c
a
c
b
•perform inter-AS
routing amongst
themselves
•perform intra-AS
routing with other
routers in their AS
network layer
inter-AS, intra-AS routing
in
gateway A.c
link layer
Intra-AS and Inter-AS routing
C.b
A.a
a
Host
h1
b
C
Inter-AS
routing
between
A and B
A.c
c
b
A
Intra-AS routing
within AS A
Host
h2
c
a
B
a
d
B.a
b
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 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=128.186.0.0/16
NLRI=128.186.0.0/16
ASPATH=[10]
ASPATH=[0]
128.186.0.0/16
NLRI=128.186.0.0/16
ASPATH=[10]
NLRI=128.186.0.0/16
ASPATH=[210] NLRI=128.186.0.0/16
ASPATH=[3210]
NLRI=128.186.0.0/16
NLRI=128.186.0.0/16
ASPATH=[4210]
ASPATH=[210]
NLRI=128.186.0.0/16
ASPATH=[7610]
NLRI=128.186.0.0/16
ASPATH=[610]
NLRI=128.186.0.0/16
ASPATH=[610]
[3210]*
[4210]
[7610]