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Chapter 15&16 Internetworking
• Internetwork Structure & Terms
• Internetworking Architecture Features
 Connection/Connectionless Architecture



Fragmentation & Reassembly
Internet Protocol & Services
IP Addressing
Subnetting
• Routing Protocols in IP
Spring, 2003
EE 4272
Internetworking Terms
• An internet

Collection of communications networks interconnected by bridges and/or routers
• The Internet - note upper case I
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•
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The global collection of thousands of individual machines and networks
Intranet: Corporate internet operating within the organization
 Isolated or may have links to Internet
End System (ES): Device attached to one of the networks of an internet
 Supports end-user applications or services
Intermediate System (IS): Device used to connect two networks
 Permits communication between end systems attached to different networks
Bridge: IS used to connect two or more LANs using similar LAN protocols
 Address filter passing on packets to the required network only
 Operated at OSI layer 2 (Data Link)
Router: Connects two or more (possibly dissimilar) networks
 Uses internet protocol present in each router and end system
 Operated at OSI Layer 3 (Network)
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EE 4272
Internet Structure
Recent Past (1990)
End user
NSFNET backbone
Stanford
ISU
BARRNET
MidNet
regional
regional
Westnet
regional
Berkeley
PARC
UNM
NCAR
UNL
KU
UA
Service Provider
AS (autonomous system): each with its own idea of routing
and metrics defining. An AS is administered independently.
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Internet Structure
Today
Service provider networks
Large corporation
“Consumer
” ISP
Peering
point
Backbone service provider
“ Consumer
” ISP
Large corporation
Small
corporation
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Peering
point
EE 4272
“Consumer
”ISP
Internetworking Protocols in TCP/IP Suite
• Requirements of Internetworking
Link
between networks: Minimum physical and link layer
Routing
and delivery of data between processes on different networks
Accounting services and
Independent of
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status info
constituting network architectures
EE 4272
Internetworking Architecture Features
• Accommodate difference among networks

Addressing: global network addressing must be provided
 Packet size -> fragmentation
 Timeouts: longer timeout for delivery across multiple networks
 Error recovery: independent to individual network error rec. cap.
 Status reporting
 Routing
 Connection based or connectionless
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EE 4272
Architectural Approaches
• Connection oriented: Assume that each network is connection oriented
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IS connect two or more networks: IS appear as DTE to each network
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Logical connection set up between DTEs (Data Terminal Equipment)
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Concatenation of logical connections across networks
Individual network virtual circuits joined by IS
May require enhancement of local network services (e.g. 802 or FDDI)
IS performs Relaying & Routing functions
• Connectionless
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

Corresponds to datagram mechanism in packet switched network
Each PDU treated separately
Network layer protocol common to all DTEs and routers
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Internet Protocol (RFC 791 -> IETF)
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
Known generically as the internet protocol
One such internet protocol developed for ARPANET
Lower layer protocol needed to access particular network
Spring, 2003
EE 4272
Connectionless Internetworking
• Advantages

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
Flexibility
Robust
No unnecessary overhead
• Unreliable
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Not guaranteed delivery
Not guaranteed order of delivery: Packets can take different routes
Reliability is responsibility of next layer up (e.g. TCP)
• Design Issues
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Routing
Datagram lifetime
Fragmentation & re-assembly
Error control
Flow control
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EE 4272
Routing
• End systems & routers maintain routing tables to indicate
next router to which datagram should be sent
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
Static: May contain alternative routes
Dynamic: Flexible response to congestion and errors
• Source routing
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Source specifies route as sequential list of routers to be
followed
Spring, 2003
EE 4272
Datagram Lifetime
• Datagrams could loop indefinitely
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Consumes resources
Transport protocol may need upper bound on datagram life
• Datagram marked with lifetime
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Time-To-Live (TTL) field in IP
Once lifetime expires, datagram discarded (not forwarded)
Hop count: a simple way to implement TTL
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Decrement TTL on passing through at each router
True time count: global clocking mechanism needed

Spring, 2003
Need to know how long since last router
EE 4272
Fragmentation and Reassembly
• Each network has some MTU (Maximum Transmission Unit)
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e.g., Ethernet:1500B; FDDI:4500B, IP: 65,535B
• When to re-assemble
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At destination (preferred)
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Results in packets getting smaller as data traverses internet
Intermediate re-assembly
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Need large buffers at routers
 Buffers may fill with fragments
 All fragments must go through same router

Inhibits dynamic routing
H1
H8
TCP
R1
IP
IP
ETH
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R2
ETH
R3
IP
FDDI
FDDI
IP
PPP
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PPP
TCP
IP
ETH
ETH
Example
Start of header
Ident= x
Offset= 0
0
Rest of header
H1
R1
R2
R3
H8
1400 data bytes
Start of header
ETH IP (1400)
FDDI IP (1400)
PPP IP (512)
ETH IP (512)
PPP IP (512)
ETH IP (512)
PPP IP (376)
ETH IP (376)
Ident= x
1
Offset= 0
Rest of header
512 data bytes
Start of header
Ident= x
1
Offset= 512
Rest of header
Note: Offset field counts 8-byte units of
data, not individual bytes
512 data bytes
Start of header
Ident= x
0 Offset= 1024
Rest of header
376 data bytes
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EE 4272
Error & Flow Control
• Error Control
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Not guaranteed delivery
Router should attempt to inform source if packet discarded
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Source may modify transmission strategy after the discard
 May inform high layer protocol
 Datagram identification needed
• Flow Control (? Congestion Control)
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Allows routers and/or stations to limit rate of incoming data
The mechanism is limited in connectionless systems
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Send flow control packets: Requesting reduced flow
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EE 4272
Internet Protocol (IP)
• Part of TCP/IP: Used by the Internet
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Specifies interface with higher layer: e.g. TCP
Specifies protocol format and mechanisms
• IP Services can be described by
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Primitives to specify functions to be performed: Implementation dependent
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Send: Request transmission of data unit
 Deliver: Notify user of arrival of data unit
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Parameters: Used to pass data and control info
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Source/Destination address
 Protocol: Recipient e.g. TCP
 Type of Service (TOS): Specify QoS of data unit during transmission through networks
 Identification: combined with source, destination address and user protocol
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Uniquely identifies PDU
 Needed for re-assembly and error reporting
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EE 4272
IP Services Parameters (Con’t)
• Time to live (TTL): Send only
• Data length
• Option data : options requested by the IP user

Security
 Source routing
 Route recording
 Stream identification
 Timestamping
• User data
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Carries user data from next layer up
 Integer multiple of 8 bits long (octet)
 Max length of datagram (header plus data) 65,535 octets
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IP Header
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Version: Currently 4
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Internet header length (IHL): In 32 bit words
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•
IP v6 – next generation
Including options
Type of service (TOS)
Total length : Of datagram, in octets
Identification: Sequence number
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Used with addresses and user protocol to identify
datagram uniquely
Flags: More bit
 Don’t fragment
Fragmentation offset
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•
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Time to live (TTL)
Protocol: Next higher layer to receive data field at destination
Header checksum
•
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Reverified and recomputed at each router
16 bit ones complement sum of all 16 bit words
in header
Set to zero during calculation
Source/Destination address
Options
Padding: To fill to multiple of 32 bits long
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EE 4272
Global IP Addresses
• Properties

globally unique
 hierarchical: network + host
A:
B:
• Dot Notation
0
7
24
Network
Host
1 0
14
16
Network
Host
21
8
Network
Host
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10.3.2.4
 128.96.33.81
 192.12.69.77
C:
1 1 0
Class D (start 1110) address specify a multicast group
Class E (start 1111): reserved for future use
Network 1 (Ethernet)
Note: It is more precise to think of IP address
as belonging to interfaces than to hosts
H7
H2
H1
Network 2 (Ethernet)
H3
R1
Network 3 (FDDI)
H5
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EE 4272
H8
Network 4
(point-to-point)
R2
H4
R3
H6
Subnetting & Subnet Mask
•
Problem: Assigning one network # per physical network, not only used up the IP address
space very fast, but also increase the burden of routing.
•
Solution: Add another level to address/routing hierarchy: subnet assign a single IP
network # and allocate the IP addresses with that network # to several physical networks
•
Subnet masks define variable partition of host part
Network number
Host number
Bitwise AND
Class B address
111111111111111111111111
00000000
Subnet mask (255.255.255.0)
Network number
Subnet ID
Subnetted address
Spring, 2003
EE 4272
Host ID
Subnet Example
Subnet mask: 255.255.255.128
Subnet number: 128.96.34.0
128.96.34.15
A host connected to this subnetwork could
have an IP address between 128.96.34.1 and
128.96.34.127
128.96.34.1
H1
R1
Subnet mask: 255.255.255.128
Subnet number: 128.96.34.128
128.96.34.130
128.96.34.139
128.96.34.129
H2
R2
H3
128.96.33.14
A host connected to this subnetwork
could have an IP address between
128.96.34.129 and 128.96.34.255
128.96.33.1
Subnet mask: 255.255.255.0
Subnet number: 128.96.33.0
Bitwise AND of the host IP address & subnet mask = subnet number
A host connected to this subnetwork could
have an IP address between 128.96.33.1 and
128.96.33.255
A single class B (128.96.*.*) address shared by several physical network
Spring, 2003
EE 4272
IP Versions
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IP v 1-3 defined and replaced
IP v4 - current version
IP v5 - streams protocol
IP v6 - replacement for IP v4
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Under development it is called IPng (Next Generation)
• Why IP v6
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Address space exhaustion
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Two level addressing (network and host) wastes space
 Growth of networks and the Internet
 Single address per host
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Requirements for new types of service
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EE 4272
Autonomous Systems (AS)
• Set of routers and networks managed by single
organization
• Group of routers exchange information
• Each AS with its own idea of routing and metrics
defining. An AS is administered independently.
Spring, 2003
EE 4272
Routing Protocols
• Routing Information

About topology and delays in the internet
• Routing Algorithm

Used to make routing decisions based on
information
• Interior Router Protocol: Passes routing
information between routers within AS

Routing algorithms and tables may differ
between different AS
 IRP needs detailed model
 e.g., RIP (using Bellman-Ford algorithm)
 e.g., OSPF ( using Dijkstra’s algorithm)
• Exterior router protocol (ERP): Routers
need some info about networks outside
their AS: e.g. BGP in Internet

supports summary information on
reachability
Spring, 2003
EE 4272