PowerPoint Presentation
Network Layer
All material copyright 1996-2012
J.F Kurose and K.W. Ross, All Rights Reserved
George Parisis
School of Engineering and Informatics
University of Sussex
Network Layer
4-*
introduction
virtual circuit and datagram networks
what’s inside a router
IP: Internet Protocol
datagram format
IPv4 addressing (NAT)
ICMP, IPv6
routing algorithms
link state, distance vector
hierarchical routing
routing in the Internet
RIP, OSPF
BGP
broadcast routing
Outline
Network Layer
Network Layer
4-*
Hierarchical routing
scale: with 600 million destinations:
can’t store all dest’s in routing tables!
routing table exchange would swamp links!
administrative autonomy
internet = network of networks
each network admin may want to control routing in its own network
our routing study thus far – idealization
all routers identical
network “flat”
… not true in practice
Network Layer
*
Network Layer
4-*
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 router:
at “edge” of its own AS
has link to router in another AS
Hierarchical routing
Network Layer
Network Layer
4-*
Interconnected ASes
forwarding table configured by both intra- and inter-AS routing algorithm
intra-AS sets entries for internal dests
inter-AS & intra-AS sets entries for external dests
1d
1c
2c
2b
1b
Intra-AS
Routing
algorithm
Inter-AS
Routing
algorithm
3c
3b
3a
2a
AS3
AS1
AS2
1a
Forwarding
table
Network Layer
Network Layer
4-*
Inter-AS tasks
suppose router in AS1 receives datagram destined outside of AS1:
router should forward packet to gateway router, but which one?
AS1 must:
learn which dests are reachable through AS2, which through AS3
propagate this reachability info to all routers in AS1
job of inter-AS routing!
AS3
AS2
other
networks
other
networks
3b
3c
3a
1c
1a
1d
1b
AS1
2a
2c
2b
Network Layer
Network Layer
4-*
Example: setting forwarding table in router 1d
suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c), but not via AS2
inter-AS protocol propagates reachability info to all internal routers
router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1c
installs forwarding table entry (x,I)
AS3
AS2
other
networks
other
networks
x
…
3b
3c
3a
1c
1a
1d
1b
AS1
2a
2c
2b
Network Layer
Network Layer
4-*
Example: choosing among multiple ASes
now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2.
to configure forwarding table, router 1d must determine which gateway it should forward packets towards for dest x
this is also job of inter-AS routing protocol!
AS3
AS2
other
networks
other
networks
x
……
…
?
3b
3c
3a
1c
1a
1d
1b
AS1
2a
2c
2b
Network Layer
Network Layer
4-*
Example: choosing among multiple ASes
now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2.
to configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest x
this is also job of inter-AS routing protocol!
hot potato routing: send packet towards closest of two routers.
learn from inter-AS
protocol that subnet
x is reachable via
multiple gateways
use routing info
from intra-AS
protocol to determine
costs of least-cost
paths to each
of the gateways
hot potato routing:
choose the gateway
that has the
smallest least cost
determine from
forwarding table the
interface I that leads
to least-cost gateway.
Enter (x,I) in
forwarding table
Network Layer
Network Layer
4-*
introduction
virtual circuit and datagram networks
what’s inside a router
IP: Internet Protocol
datagram format
IPv4 addressing (NAT)
ICMP, IPv6
routing algorithms
link state, distance vector
hierarchical routing
routing in the Internet
RIP, OSPF
BGP
broadcast
Outline
Network Layer
Network Layer
4-*
Intra-AS Routing
also known as interior gateway protocols (IGP)
most common intra-AS routing protocols:
RIP: Routing Information Protocol
OSPF: Open Shortest Path First
IGRP: Interior Gateway Routing Protocol (Cisco proprietary)
Network Layer
Network Layer
4-*
RIP (Routing Information Protocol)
included in BSD-UNIX distribution in 1982
distance vector algorithm
distance metric: # hops (max = 15 hops), each link has cost 1
DVs exchanged with neighbors every 30 sec in response message (aka advertisement)
each advertisement: list of up to 25 destination subnets (in IP addressing sense)
subnet hops
u 1
v 2
w 2
x 3
y 3
z 2
from router A to destination subnets:
D
C
B
A
u
v
w
x
y
z
Network Layer
Network Layer
4-*
RIP: example
destination subnet next router # hops to dest
w A 2
y B 2
z B 7
x — 1
…. …. ….
routing table in router D
w
x
y
z
A
C
D
B
Network Layer
Network Layer
4-*
w
x
y
z
A
C
D
B
destination subnet next router # hops to dest
w A 2
y B 2
z B 7
x — 1
…. …. ….
routing table in router D
RIP: example
A
5
dest next hops
w – 1
x – 1
z C 4
…. … …
A-to-D advertisement
Network Layer
Network Layer
4-*
RIP: link failure, recovery
if no advertisement heard after 180 sec –> neighbor/link declared dead
routes via neighbor invalidated
new advertisements sent to neighbors
neighbors in turn send out new advertisements (if tables changed)
poison reverse used to prevent ping-pong loops (infinite distance = 16 hops)
Network Layer
Network Layer
4-*
RIP table processing
RIP routing tables managed by application-level process called route-d (daemon)
advertisements sent in UDP packets, periodically repeated
physical
link
network forwarding
(IP) table
transport
(UDP)
physical
link
network
(IP)
transprt
(UDP)
forwarding
table
routed
routed
Network Layer
Network Layer
4-*
OSPF (Open Shortest Path First)
“open”: publicly available
uses link state algorithm
LS packet dissemination
topology map at each node
route computation using Dijkstra’s algorithm
OSPF advertisement carries one entry per neighbour
advertisements flooded to entire AS
carried in OSPF messages directly over IP (rather than TCP or UDP
Network Layer
Network Layer
4-*
OSPF “advanced” features (not in RIP)
security: all OSPF messages authenticated (to prevent malicious intrusion)
multiple same-cost paths allowed (only one path in RIP)
for each link, multiple cost metrics for different TOS (e.g., satellite link cost set “low” for best effort ToS; high for real time ToS)
hierarchical OSPF in large domains.
Network Layer
Network Layer
4-*
Hierarchical OSPF
boundary router
backbone router
area 1
area 2
area 3
backbone
area
border
routers
internal
routers
Network Layer
*
Network Layer
4-*
two-level hierarchy: local area, backbone.
link-state advertisements only in area
each node has detailed area topology
area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers.
backbone routers: run OSPF routing limited to backbone.
boundary routers: connect to other AS’s.
Hierarchical OSPF
Network Layer
Network Layer
4-*
Summary
Hierarchical routing
Intra-AS routing in the Internet
RIP
OSPF
Network Layer