Chapter 4
Network Layer
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Computer
Networking: A Top
Down Approach
6th edition
Jim Kurose, Keith Ross
Addison-Wesley
March 2012
Thanks and enjoy! JFK/KWR
All material copyright 1996-2012
J.F Kurose and K.W. Ross, All Rights Reserved
Network Layer 4-1
Chapter 4: network layer
chapter goals:

understand principles behind network layer
services:
 forwarding versus routing
 how a router works
 routing (path selection)

instantiation, implementation in the Internet
Network Layer 4-2
Chapter 4: outline
4.1 introduction
4.2 virtual circuit and
datagram networks
4.3 what’s inside a router
4.4 IP: Internet Protocol




datagram format
IPv4 addressing
ICMP
IPv6
4.5 routing algorithms
 link state
 distance vector
 hierarchical routing
4.6 routing in the Internet
 RIP
 OSPF
 BGP
4.7 broadcast and multicast
routing
Network Layer 4-3
Two key network-layer functions

forwarding: The process of moving packets from
input to output based on:
1. The forwarding table
2. Information in the packet

routing: Process by which the forwarding table is
built and maintained.
1. One or more routing protocols
2. Algorithms to convert routing info to forwarding tables
Network Layer 4-4
Example Routing Table
Image coutesy of http://www.hill2dot0.com/wiki/index.php?title=Routing_table
Network Layer 4-5
Chapter 4: outline
4.1 introduction
4.2 virtual circuit and
datagram networks
4.3 what’s inside a router
4.4 IP: Internet Protocol




datagram format
IPv4 addressing
ICMP
IPv6
4.5 routing algorithms
 link state
 distance vector
 hierarchical routing
4.6 routing in the Internet
 RIP
 OSPF
 BGP
4.7 broadcast and multicast
routing
Network Layer 4-6
Router architecture overview
two key router functions:


run routing algorithms/protocol (RIP, OSPF, BGP)
forwarding datagrams from incoming to outgoing link
forwarding tables computed,
pushed to input ports
routing
processor
routing, management
control plane (software)
forwarding data
plane (hardware)
high-seed
switching
fabric
router input ports
router output ports
Network Layer 4-7
Chapter 4: outline
4.1 introduction
4.2 virtual circuit and
datagram networks
4.3 what’s inside a router
4.4 IP: Internet Protocol




datagram format
IPv4 addressing
ICMP
IPv6
4.5 routing algorithms
 link state
 distance vector
 hierarchical routing
4.6 routing in the Internet
 RIP
 OSPF
 BGP
4.7 broadcast and multicast
routing
Network Layer 4-8
A Link-State Routing Algorithm
Dijkstra’s algorithm


net topology, link costs
known to all nodes
 accomplished via “link state
broadcast”
 all nodes have same info

computes least cost paths
from one node (‘source”)
to all other nodes

 gives forwarding table for
that node

notation:
 c(x,y): link cost from
iterative: after k
iterations, know least cost
path to k dest.’s

node x to y; = ∞ if not
direct neighbors
D(v): current value of
cost of path from source
to dest. v
p(v): predecessor node
along path from source to
v
N': set of nodes whose
least cost path definitively
known
Network Layer 4-9
Dijkstra’s algorithm: building a forwarding table (1)
Step
0
1
2
3
4
5
N'
u
ux
uxy
uxyv
uxyvw
uxyvwz
D(v),p(v) D(w),p(w)
2,u
5,u
2,u
4,x
2,u
3,y
3,y
D(x),p(x)
1,u
D(y),p(y)
∞
2,x
D(z),p(z)
∞
∞
4,y
4,y
4,y
5
v
3
w
2
u
2
1
x
3
5
z
1
y
2
1
Network Layer 4-10
Dijkstra’s algorithm: building a forwarding table (2)
resulting shortest-path tree from u:
v
w
u
z
x
y
resulting forwarding table in u:
destination
link
v
x
(u,v)
(u,x)
y
(u,x)
w
(u,x)
z
(u,x)
Network Layer 4-11
Link-State Walkthrough (1)
1.
2.
3.
Each router sends a “Router Discovery (ICMP)” message to
224.0.0.2. (“All Routers” multicast address)
Each router makes a list of its adjacent neighbors.
Each router sends its list of adjacent neighbors to ALL other
routers in the AS, using 224.0.0.9 (RIP address)
4.
Each router builds a complete map of the network.
5.
Each router uses Dykstra’s to build forwarding table.
6.
Repeat every 30 seconds.
Network Layer 4-12
Link-State Walkthrough (2)
(Step 1)
From: 10.10.1.1
To:
224.0.0.2
Data: What routers can hear this request? (Do not forward.)
(Step 2)
From: 10.10.1.3
To:
10.10.1.1
Data: I am a router
From:
To:
Data:
10.10.1.5
10.10.1.1
I am a router
Network Layer 4-13
Link-State Walkthrough (3)
(Step 3)
From: 10.10.1.1
To:
224.0.0.9
Data: These routers are adjacent to me. (Please forward.)
Origin: 10.10.1.1 Neighbors (10.10.1.3, 10.10.1.5)
Each router receives the complete collection of messages sent by all routers:
Origin: 10.10.1.1 Neighbors (10.10.1.3, 10.10.1.5)
Origin: 10.10.1.3 Neighbors (10.10.1.1, 10.10.1.5, 10.10.1.7)
Origin: 10.10.1.5 Neighbors (10.10.1.1, 10.10.1.3, 10.10.1.9)
Origin: 10.10.1.7 Neighbors (10.10.1.3, 10.10.1.9)
Origin: 10.10.1.9 Neighbors (10.10.1.5, 10.10.1.7)
(Step 4)
Exercise – Draw the network map yourself
Network Layer 4-14
Link-State Walkthrough (4)
(Step 4)
Solution:
10.10.1.3
10.10.1.7
10.10.1.5
10.10.1.9
10.10.1.1
(Step 5)
Each router has the complete network map, can construct
complementary forwarding tables.
Network Layer 4-15
Link-State Walkthrough (4)
10.10.1.3
10.10.1.7
10.10.1.5
10.10.1.9
10.10.1.1
(Step 6)
If a network link goes down, each router will know after 30
seconds when the next update gets sent.
Network Layer 4-16
Link-State Questions

Is this practical on the Internet backbone?
 Q1: Is it scalable to large numbers of routers?
 Q2: Is cost always a function of hop-count or time?
 Q2: Do all router owners have the same incentives?
Network Layer 4-17
Chapter 4: outline
4.1 introduction
4.2 virtual circuit and
datagram networks
4.3 what’s inside a router
4.4 IP: Internet Protocol




datagram format
IPv4 addressing
ICMP
IPv6
4.5 routing algorithms
 link state
 distance vector
 hierarchical routing
4.6 routing in the Internet
 RIP
 OSPF
 BGP
4.7 broadcast and multicast
routing
Network Layer 4-18
Distance vector algorithm
Bellman-Ford equation (dynamic programming)
let
dx(y) := cost of least-cost path from x to y
then
dx(y) = min
{c(x,v)
+
d
(y)
}
v
v
cost from neighbor v to destination y
cost to neighbor v
min taken over all neighbors v of x
Network Layer 4-19
Distance vector: link cost changes
link cost changes:



node detects local link cost change
updates routing info, recalculates
distance vector
if DV changes, notify neighbors
“good
news
travels
fast”
1
x
4
y
1
50
z
t0 : y detects link-cost change, updates its DV, informs its
neighbors.
t1 : z receives update from y, updates its table, computes new
least cost to x , sends its neighbors its DV.
t2 : y receives z’s update, updates its distance table. y’s least costs
do not change, so y does not send a message to z.
Network Layer 4-20
Distance-Vector Walkthrough (1)
1.
Each router sends an “Open Shortest Path First (OSPF)”
Hello message to 224.0.0.5. (OSPF multicast address)
2.
Each router finds direct cost to its adjacent neighbors
3.
Each router updates its estimated cost to all known routers
4.
5.
Each router collates its complete list of forwarding costs to
all known routers and then mails the list to its adjacent
neighbors only (no forwarding).
Repeat every 30 seconds.
Network Layer 4-21
Distance-Vector Walkthrough (2)
*** Assume that 10.10.1.1 is entering an established network ***
(Step 1)
From: 10.10.1.1
To:
224.0.0.5
Data: What routers can hear this request? (Do not forward.)
(Step 2)
From: 10.10.1.3
To:
10.10.1.1
Data: I am a router, COST==5
From:
To:
Data:
10.10.1.5
10.10.1.1
I am a router, COST==3
Network Layer 4-22
Distance-Vector Walkthrough(3)
(Step 3) Build forwarding table:
Destination
Cost
NIC
10.10.1.3
5
A
10.10.1.5
3
B
(Step 4)
From: 10.10.1.1
To:
224.0.0.5
Data: These are all of my known routes. (Do not forward.)
10.10.1.1 -> 10.10.1.3 (cost 5)
10.10.1.1 -> 10.10.1.5 (cost 3)
Network Layer 4-23
Distance-Vector Walkthrough(4)
(Step 4 cont) Messages received from the group of routers:
From 10.10.1.3 Known routes:
10.10.1.3 -> 10.10.1.1 (cost 5)
10.10.1.3 -> 10.10.1.5 (cost 1)
10.10.1.3 -> 10.10.1.7 (cost 2)
10.10.1.3 -> 10.10.1.9 (cost 4)
From 10.10.1.5 Known routes:
10.10.1.5 -> 10.10.1.1 (cost 3)
10.10.1.5 -> 10.10.1.3 (cost 1)
10.10.1.5 -> 10.10.1.7 (cost 3)
10.10.1.5 -> 10.10.1.9 (cost 5)
Network Layer 4-24
Distance-Vector Walkthrough(5)
*** SECOND ROUND ***
(Step 3) Build your new forwarding table yourself as an exercise:
Destination
10.10.1.3
10.10.1.5
10.10.1.7
Cost
NIC
Neighbors:
NIC A: 10.10.1.3 (cost 5)
NIC B: 10.10.1.5 (cost 3)
From 10.10.1.3
Known routes:
10.10.1.3 -> 10.10.1.1 (cost 5)
10.10.1.3 -> 10.10.1.5 (cost 1)
10.10.1.3 -> 10.10.1.7 (cost 2)
10.10.1.3 -> 10.10.1.9 (cost 4)
10.10.1.9
From 10.10.1.5
Known routes:
10.10.1.5 -> 10.10.1.1 (cost 3)
10.10.1.5 -> 10.10.1.3 (cost 1)
10.10.1.5 -> 10.10.1.7 (cost 3)
10.10.1.5 -> 10.10.1.9 (cost 5)
Network Layer 4-25
Distance-Vector Walkthrough(6)
*** SECOND ROUND ***
(Step 3) ANSWERS:
Destination
Cost
NIC
10.10.1.3
4
B
10.10.1.5
3
B
10.10.1.7
6
B
10.10.1.9
8
B
Q: Is it possible to draw the network
topology from this information?
Neighbors:
NIC A: 10.10.1.3 (cost 5)
NIC B: 10.10.1.5 (cost 3)
From 10.10.1.3
Known routes:
10.10.1.3 -> 10.10.1.1 (cost 5)
10.10.1.3 -> 10.10.1.5 (cost 1)
10.10.1.3 -> 10.10.1.7 (cost 2)
10.10.1.3 -> 10.10.1.9 (cost 4)
From 10.10.1.5
Known routes:
10.10.1.5 -> 10.10.1.1 (cost 3)
10.10.1.5 -> 10.10.1.3 (cost 1)
10.10.1.5 -> 10.10.1.7 (cost 3)
10.10.1.5 -> 10.10.1.9 (cost 5)
Network Layer 4-26
Distance-Vector Walkthrough(7)
10.10.1.3
2
10.10.1.7
5
1
10.10.1.1
2
3
10.10.1.5
10.10.1.9
10
A: No. This is a drawing of the
actual network with costs. You
cannot reproduce this from what you
know.
Network Layer 4-27
Distance-Vector Walkthrough(8)
10.10.1.3
2
10.10.1.7
5
3
1
10.10.1.1
4
3
10.10.1.5
5
10.10.1.9
You can produce a map that looks like this
• It shows the cost to each node from your neighbors
• The map has a max of two edges from you to any other node
Network Layer 4-28
Comparison of LS and DV algorithms
message complexity


LS: with n nodes, E links, O(nE)
msgs sent
DV: exchange between neighbors
only
 convergence time varies
speed of convergence


O(n2)
LS:
algorithm requires
O(nE) msgs
 may have oscillations
DV: convergence time varies
 may be routing loops
 count-to-infinity problem
robustness: what happens if
router malfunctions?
LS:
 node can advertise incorrect
link cost
 each node computes only its
own table
DV:
 DV node can advertise
incorrect path cost
 each node’s table used by
others
• error propagate thru
network
Network Layer 4-29
Hierarchical routing
“autonomous systems” (AS)
 aggregate routers into regions
 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
Network Layer 4-30
Chapter 4: outline
4.1 introduction
4.2 virtual circuit and
datagram networks
4.3 what’s inside a router
4.4 IP: Internet Protocol




datagram format
IPv4 addressing
ICMP
IPv6
4.5 routing algorithms
 link state
 distance vector
 hierarchical routing
4.6 routing in the Internet
 RIP
 OSPF
 BGP
4.7 broadcast and multicast
routing
Network Layer 4-31
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
Network Layer 4-32
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)
u
v
A
z
C
B
w
x
D
y
from router A to destination subnets:
subnet hops
u
1
v
2
w
2
x
3
y
3
z
2
Network Layer 4-33
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)
 link failure info quickly (?) propagates to entire net
 poison reverse used to prevent ping-pong loops (infinite
distance = 16 hops)
Network Layer 4-34
OSPF (Open Shortest Path First)


“open”: publicly available
uses link state & distance vector algorithms
 LS packet dissemination
 topology map at each node
 route computation using Dijkstra’s algorithm


OSPF advertisement carries one entry per neighbor
advertisements flooded to entire AS
 carried in OSPF messages directly over IP (rather than
TCP or UDP

IS-IS routing protocol: nearly identical to OSPF
Network Layer 4-35
Hierarchical OSPF
boundary router
backbone router
backbone
area
border
routers
area 3
internal
routers
area 1
area 2
Network Layer 4-36
Summary of Protocols
Protocol
Type of routing
(LS/DV/Other)
Common Usage
RIP
LS
Lower-tier ISPs and enterprise networks
OSPF
LS & DV
Upper-tier ISPs
Hierarchical
• LS for smaller ASs
• DV for collections of ASs
BGP
DV & Policy
Network Layer 4-37
Internet inter-AS routing: BGP

BGP (Border Gateway Protocol): the de facto
inter-domain routing protocol
 “glue that holds the Internet together”

BGP provides each AS a means to:
 eBGP: obtain subnet reachability information from
neighboring ASs.
 iBGP: propagate reachability information to all ASinternal routers.
 determine “good” routes to other networks based on
reachability information and policy.

allows subnet to advertise its existence to rest of
Internet: “I am here”
Network Layer 4-38
BGP basics

BGP session: two BGP routers (“peers”) exchange BGP
messages:
 advertising paths to different destination network prefixes (“path vector”
protocol)
 exchanged over semi-permanent TCP connections

when AS3 advertises a prefix to AS1:
 AS3 promises it will forward datagrams towards that prefix
 AS3 can aggregate prefixes in its advertisement
3c
3b
other
networks
3a
BGP
message
AS3
2c
1c
1a
AS1
1d
2a
1b
2b
other
networks
AS2
Network Layer 4-39
BGP basics: distributing path information

using eBGP session between 3a and 1c, AS3 sends prefix
reachability info to AS1.
 1c can then use iBGP do distribute new prefix info to all routers
in AS1
 1b can then re-advertise new reachability info to AS2 over 1b-to2a eBGP session

when router learns of new prefix, it creates entry for
prefix in its forwarding table.
3c
3b
other
networks
eBGP session
3a
AS3
iBGP session
2c
1c
1a
AS1
1d
2a
1b
2b
other
networks
AS2
Network Layer 4-40
BGP routing policy
legend:
B
W
provider
network
X
A
customer
network:
C
Y



A,B,C are provider networks
X,W,Y are customer (of provider networks)
X is dual-homed: attached to two networks
 X does not want to route from B via X to C
 .. so X will not advertise to B a route to C
Network Layer 4-41
BGP routing policy (2)
legend:
B
W
provider
network
X
A
customer
network:
C
Y



A advertises path AW to B
B advertises path BAW to X
Should B advertise path BAW to C?
 No way! B gets no “revenue” for routing CBAW since neither W nor
C are B’s customers
 B wants to force C to route to w via A
 B wants to route only to/from its customers!
Network Layer 4-42
Why different Intra-, 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

Network Layer 4-43