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Network Layer 4-1
Chapter 4Network Layer
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All material copyright 1996-2007J.F Kurose and K.W. Ross, All Rights Reserved
Computer Networking: A Top Down Approach 4th edition. Jim Kurose, Keith RossAddison-Wesley, July 2007.
Network Layer 4-2
Chapter 4: Network Layer
Chapter goals:r understand principles behind network layer
services:m network layer service modelsm forwarding versus routingm how a router worksm routing (path selection)m dealing with scalem advanced topics: IPv6, mobility
r instantiation, implementation in the Internet
Network Layer 4-3
Chapter 4: Network Layer
r 4. 1 Introductionr 4.2 Virtual circuit and
datagram networksr 4.3 What’s inside a
routerr 4.4 IP: Internet
Protocolm Datagram formatm IPv4 addressingm ICMPm IPv6
r 4.5 Routing algorithmsm Link statem Distance Vectorm Hierarchical routing
r 4.6 Routing in the Internetm RIPm OSPFm BGP
r 4.7 Broadcast and multicast routing
Network Layer 4-4
Network layerr transport segment from
sending to receiving host r on sending side
encapsulates segments into datagrams
r on rcving side, delivers segments to transport layer
r network layer protocols in every host, router
r router examines header fields in all IP datagrams passing through it
applicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
networkdata linkphysical network
data linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysicalnetwork
data linkphysical
Network Layer 4-5
Two Key Network-Layer Functions
r forwarding: move packets from router’s input to appropriate router output
r routing: determine route taken by packets from source to dest.
m routing algorithms
analogy:
r routing: process of planning trip from source to dest
r forwarding: process of getting through single interchange
Network Layer 4-6
1
23
0111
value in arrivingpacket’s header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Network Layer 4-7
Connection setup
r 3rd important function in some network architectures:mATM, frame relay, X.25
r before datagrams flow, two end hosts andintervening routers establish virtual connectionmrouters get involved
Network Layer 4-8
Network service modelQ: What service model for “channel” transporting datagrams from sender to receiver?
Example services for individual datagrams:
r guaranteed deliveryr guaranteed delivery
with less than 40 msec delay
Example services for a flow of datagrams:
r in-order datagram delivery
r guaranteed minimum bandwidth to flow
r restrictions on changes in inter-packet spacing
Network Layer 4-9
Chapter 4: Network Layer
r 4. 1 Introductionr 4.2 Virtual circuit and
datagram networksr 4.3 What’s inside a
routerr 4.4 IP: Internet
Protocolm Datagram formatm IPv4 addressingm ICMPm IPv6
r 4.5 Routing algorithmsm Link statem Distance Vectorm Hierarchical routing
r 4.6 Routing in the Internetm RIPm OSPFm BGP
r 4.7 Broadcast and multicast routing
Network Layer 4-10
Network layer connection and connection-less servicer datagram network provides network-layer
connectionless servicer VC network provides network-layer
connection servicerSpecifically:m service: host-to-hostm no choice: network provides one or the otherm implementation: in network core
Network Layer 4-11
Virtual circuits
r call setup, teardown for each call before data can flowr each packet carries VC identifier (not destination host
address)r every router on source-dest path maintains “state” for
each passing connectionr link, router resources (bandwidth, buffers) may be
allocated to VC (dedicated resources = predictable service)
“source-to-dest path behaves much like telephone circuit”m performance-wisem network actions along source-to-dest path
Network Layer 4-12
VC implementation
a VC consists of:1. path from source to destination2. VC numbers, one number for each link along
path3. entries in forwarding tables in routers along
pathr packet belonging to VC carries VC number
(rather than dest address)r VC number can be changed on each link.m New VC number comes from forwarding table
Network Layer 4-13
Forwarding table12 22 32
1 23
VC number
interfacenumber
Incoming interface Incoming VC # Outgoing interface Outgoing VC #
1 12 3 222 63 1 18 3 7 2 171 97 3 87… … … …
Forwarding table innorthwest router:
Routers maintain connection state information!
Network Layer 4-14
Virtual circuits: signaling protocols
r used to setup, maintain teardown VCr used in ATM, frame-relay, X.25r not used in today’s Internet
applicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
1. Initiate call 2. incoming call3. Accept call4. Call connected
5. Data flow begins 6. Receive data
Network Layer 4-15
Datagram networksr no call setup at network layerr routers: no state about end-to-end connections
m no network-level concept of “connection”r packets forwarded using destination host address
m packets between same source-dest pair may take different paths
applicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
1. Send data 2. Receive data
Network Layer 4-16
Forwarding table
Destination Address Range Link Interface
11001000 00010111 00010000 00000000through 0
11001000 00010111 00010111 11111111
11001000 00010111 00011000 00000000through 1
11001000 00010111 00011000 11111111
11001000 00010111 00011001 00000000through 2
11001000 00010111 00011111 11111111
otherwise 3
4 billion possible entries
Network Layer 4-17
Longest prefix matching
Prefix Match Link Interface11001000 00010111 00010 0 11001000 00010111 00011000 111001000 00010111 00011 2
otherwise 3
DA: 11001000 00010111 00011000 10101010
Examples
DA: 11001000 00010111 00010110 10100001 Which interface?
Which interface?
Network Layer 4-18
Datagram or VC network: why?
Internet (datagram)r data exchange among
computersm “elastic” service, no strict
timing req. r “smart” end systems
(computers)m can adapt, perform
control, error recoverym simple inside network,
complexity at “edge”r many link types
m different characteristicsm uniform service difficult
ATM (VC)r evolved from telephonyr human conversation:
m strict timing, reliability requirements
m need for guaranteed service
r “dumb” end systemsm telephonesm complexity inside
network
Network Layer 4-19
Chapter 4: Network Layer
r 4. 1 Introductionr 4.2 Virtual circuit and
datagram networksr 4.3 What’s inside a
routerr 4.4 IP: Internet
Protocolm Datagram formatm IPv4 addressingm ICMPm IPv6
r 4.5 Routing algorithmsm Link statem Distance Vectorm Hierarchical routing
r 4.6 Routing in the Internetm RIPm OSPFm BGP
r 4.7 Broadcast and multicast routing
Network Layer 4-20
The Internet Network layer
forwardingtable
Host, router network layer functions:
Routing protocols•path selection•RIP, OSPF, BGP
IP protocol•addressing conventions•datagram format•packet handling conventions
ICMP protocol•error reporting•router “signaling”
Transport layer: TCP, UDP
Link layer
physical layer
Networklayer
Network Layer 4-21
IP datagram format
ver length
32 bits
data (variable length,typically a TCP
or UDP segment)
16-bit identifierheader
checksumtime to
live
32 bit source IP address
IP protocol versionnumber
header length(bytes)
max numberremaining hops
(decremented at each router)
forfragmentation/reassembly
total datagramlength (bytes)
upper layer protocolto deliver payload to
head.len
type ofservice
“type” of data flgs fragmentoffset
upperlayer
32 bit destination IP address
Options (if any) E.g. timestamp,record routetaken, specifylist of routers to visit.
how much overhead with TCP?
r 20 bytes of TCPr 20 bytes of IPr = 40 bytes + app
layer overhead
Network Layer 4-22
IP Fragmentation & Reassemblyr network links have MTU
(max.transfer size) - largest possible link-level frame.m different link types,
different MTUs r large IP datagram divided
(“fragmented”) within netm one datagram becomes
several datagramsm “reassembled” only at final
destinationm IP header bits used to
identify, order related fragments
fragmentation: in: one large datagramout: 3 smaller datagrams
reassembly
Network Layer 4-23
IP Fragmentation and Reassembly
ID=x
offset=0
fragflag=0
length=4000
ID=x
offset=0
fragflag=1
length=1500
ID=x
offset=185
fragflag=1
length=1500
ID=x
offset=370
fragflag=0
length=1040
One large datagram becomesseveral smaller datagrams
Exampler 4000 byte
datagramr MTU = 1500 bytes
1480 bytes in data field
offset =1480/8
Network Layer 4-24
Chapter 4: Network Layer
r 4. 1 Introductionr 4.2 Virtual circuit and
datagram networksr 4.3 What’s inside a
routerr 4.4 IP: Internet
Protocolm Datagram formatm IPv4 addressingm ICMPm IPv6
r 4.5 Routing algorithmsm Link statem Distance Vectorm Hierarchical routing
r 4.6 Routing in the Internetm RIPm OSPFm BGP
r 4.7 Broadcast and multicast routing
Network Layer 4-25
IP Addressing: introductionr IP address: 32-bit
identifier for host, router interface
r interface: connection between host/router and physical linkm router’s typically have
multiple interfacesm host typically has one
interfacem IP addresses
associated with each interface
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
223.1.1.1 = 11011111 00000001 00000001 00000001
223 1 11
Network Layer 4-26
Subnetsr IP address:
m subnet part (high order bits)
m host part (low order bits)
r What’s a subnet ?m device interfaces with
same subnet part of IP address
m can physically reach each other without intervening router
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
network consisting of 3 subnets
subnet
Network Layer 4-27
Subnets 223.1.1.0/24223.1.2.0/24
223.1.3.0/24
Reciper To determine the
subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet.
Subnet mask: /24
Network Layer 4-28
SubnetsHow many? 223.1.1.1
223.1.1.3
223.1.1.4
223.1.2.2223.1.2.1
223.1.2.6
223.1.3.2223.1.3.1
223.1.3.27
223.1.1.2
223.1.7.0
223.1.7.1223.1.8.0223.1.8.1
223.1.9.1
223.1.9.2
Network Layer 4-29
IP addressing: CIDRCIDR: Classless InterDomain Routingm subnet portion of address of arbitrary lengthm address format: a.b.c.d/x, where x is # bits in
subnet portion of address
11001000 00010111 00010000 00000000
subnetpart
hostpart
200.23.16.0/23
Network Layer 4-30
IP addresses: how to get one?
Q: How does host get IP address?
r hard-coded by system admin in a filemWintel: control-panel->network->configuration-
>tcp/ip->propertiesm UNIX: /etc/rc.config
r DHCP: Dynamic Host Configuration Protocol: dynamically get address from as serverm “plug-and-play”
Network Layer 4-31
IP addresses: how to get one?Q: How does network get subnet part of IP
addr?A: gets allocated portion of its provider ISP’s
address space
ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20
Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23
... ….. …. ….Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23
Network Layer 4-32
Hierarchical addressing: route aggregation
“Send me anythingwith addresses beginning 200.23.16.0/20”
200.23.16.0/23
200.23.18.0/23
200.23.30.0/23
Fly-By-Night-ISP
Organization 0
Organization 7Internet
Organization 1
ISPs-R-Us “Send me anythingwith addresses beginning 199.31.0.0/16”
200.23.20.0/23Organization 2
...
...
Hierarchical addressing allows efficient advertisement of routing information:
Network Layer 4-33
Hierarchical addressing: more specific routes
ISPs-R-Us has a more specific route to Organization 1
“Send me anythingwith addresses beginning 200.23.16.0/20”
200.23.16.0/23
200.23.18.0/23
200.23.30.0/23
Fly-By-Night-ISP
Organization 0
Organization 7Internet
Organization 1
ISPs-R-Us “Send me anythingwith addresses beginning 199.31.0.0/16or 200.23.18.0/23”
200.23.20.0/23Organization 2
...
...
Network Layer 4-34
IP addressing: the last word...
Q: How does an ISP get block of addresses?A: ICANN: Internet Corporation for Assigned
Names and Numbersm allocates addressesm manages DNSm assigns domain names, resolves disputes