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EE 4272 Spring, 2003
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
EE 4272 Spring, 2003
Internetworking Terms• An internet
Collection of communications networks interconnected by bridges and/or routers
• The Internet - note upper case I 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)
EE 4272 Spring, 2003
Internet Structure
Recent Past (1990)
NSFNET backboneStanford
BARRNET
regional
Berkeley PARC
NCAR
UA
UNM
Westnet
regional
UNL KU
ISU
MidNet
regional…
End user
Service Provider
AS (autonomous system): each with its own idea of routing and metrics defining. An AS is administered independently.
EE 4272 Spring, 2003
Internet Structure
Today
Backbone service provider
Peering
pointPeering
point
Large corporation
Large corporation
Smallcorporation
“Consumer ” ISP
“Consumer ” ISP
“ Consumer ” ISP
Service provider networks
EE 4272 Spring, 2003
Internetworking Protocols in TCP/IP Suite
• Requirements of InternetworkingLink between networks: Minimum physical and link layer
Routing and delivery of data between processes on different networks
Accounting services and status info
Independent of constituting network architectures
EE 4272 Spring, 2003
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
EE 4272 Spring, 2003
Architectural Approaches• Connection oriented: Assume that each network is connection oriented
IS connect two or more networks: IS appear as DTE to each network Logical connection set up between DTEs (Data Terminal Equipment)
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 Corresponds to datagram mechanism in packet switched network Each PDU treated separately Network layer protocol common to all DTEs and routers
Known generically as the internet protocol Internet Protocol (RFC 791 -> IETF)
One such internet protocol developed for ARPANET Lower layer protocol needed to access particular network
EE 4272 Spring, 2003
Connectionless Internetworking
• Advantages Flexibility Robust No unnecessary overhead
• Unreliable 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 Routing Datagram lifetime Fragmentation & re-assembly Error control Flow control
EE 4272 Spring, 2003
Routing
• End systems & routers maintain routing tables to indicate next router to which datagram should be sent Static: May contain alternative routes
Dynamic: Flexible response to congestion and errors
• Source routing Source specifies route as sequential list of routers to be
followed
EE 4272 Spring, 2003
Datagram Lifetime
• Datagrams could loop indefinitely Consumes resources Transport protocol may need upper bound on datagram life
• Datagram marked with lifetime Time-To-Live (TTL) field in IP Once lifetime expires, datagram discarded (not forwarded) Hop count: a simple way to implement TTL
Decrement TTL on passing through at each router
True time count: global clocking mechanism needed Need to know how long since last router
EE 4272 Spring, 2003
Fragmentation and Reassembly
• Each network has some MTU (Maximum Transmission Unit) e.g., Ethernet:1500B; FDDI:4500B, IP: 65,535B
• When to re-assemble At destination (preferred)
Results in packets getting smaller as data traverses internet Intermediate re-assembly
Need large buffers at routers Buffers may fill with fragments All fragments must go through same router
Inhibits dynamic routing
R1
ETH FDDI
IPIP
ETH
TCP R2
FDDI PPP
IP
R3
PPP ETH
IP
H1
IP
ETH
TCP
H8
EE 4272 Spring, 2003
Example
H1 R1 R2 R3 H8
ETH IP (1400) FDDI IP (1400) PPP IP (512)
PPP IP (376)
PPP IP (512)
ETH IP (512)
ETH IP (376)
ETH IP (512)
Ident = x Offset = 0
Start of header
0
Rest of header
1400 data bytes
Ident = x Offset = 0
Start of header
1
Rest of header
512 data bytes
Ident = x Offset = 512
Start of header
1
Rest of header
512 data bytes
Ident = x Offset = 1024
Start of header
0
Rest of header
376 data bytes
Note: Offset field counts 8-byte units of data, not individual bytes
EE 4272 Spring, 2003
Error & Flow Control
• Error Control Not guaranteed delivery Router should attempt to inform source if packet discarded
Source may modify transmission strategy after the discard May inform high layer protocol Datagram identification needed
• Flow Control (? Congestion Control) Allows routers and/or stations to limit rate of incoming data The mechanism is limited in connectionless systems
Send flow control packets: Requesting reduced flow
EE 4272 Spring, 2003
Internet Protocol (IP)
• Part of TCP/IP: Used by the Internet Specifies interface with higher layer: e.g. TCP Specifies protocol format and mechanisms
• IP Services can be described by Primitives to specify functions to be performed: Implementation dependent
Send: Request transmission of data unit Deliver: Notify user of arrival of data unit
Parameters: Used to pass data and control info 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
Uniquely identifies PDU Needed for re-assembly and error reporting
EE 4272 Spring, 2003
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 Carries user data from next layer up Integer multiple of 8 bits long (octet) Max length of datagram (header plus data) 65,535 octets
EE 4272 Spring, 2003
IP Header• Version: Currently 4
IP v6 – next generation• Internet header length (IHL): In 32 bit words
Including options• Type of service (TOS)• Total length : Of datagram, in octets
• Identification: Sequence number Used with addresses and user protocol to identify
datagram uniquely• Flags: More bit
Don’t fragment• Fragmentation offset• Time to live (TTL)• Protocol: Next higher layer to receive data field at destination
• Header checksum 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
EE 4272 Spring, 2003
Global IP Addresses
• Properties globally unique hierarchical: network + host
• Dot Notation 10.3.2.4 128.96.33.81 192.12.69.77
Note: It is more precise to think of IP address
as belonging to interfaces than to hosts
Network Host
7 24
0A:
Network Host
14 16
1 0B:
Network Host
21 8
1 1 0C:
Class D (start 1110) address specify a multicast group
Class E (start 1111): reserved for future use
H5 H6
R2R1
H4
H3H2H1
Network 2 (Ethernet)
Network 1 (Ethernet)
Network 3 (FDDI)
Network 4
(point-to-point)
H7 R3 H8
EE 4272 Spring, 2003
• 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
Subnetting & Subnet Mask
Network number Host number
Class B address
Subnet mask (255.255.255.0)
Subnetted address
111111111111111111111111 00000000
Network number Host IDSubnet ID
Bitwise AND
EE 4272 Spring, 2003
Subnet Example
Subnet mask: 255.255.255.128Subnet number: 128.96.34.0
128.96.34.15 128.96.34.1H1
R1
128.96.34.130Subnet mask: 255.255.255.128Subnet number: 128.96.34.128
128.96.34.129128.96.34.139
R2H2
128.96.33.1128.96.33.14
Subnet mask: 255.255.255.0Subnet number: 128.96.33.0
H3
A host connected to this subnetwork could have an IP address between 128.96.34.1 and 128.96.34.127
A host connected to this subnetwork could have an IP address between 128.96.34.129 and 128.96.34.255
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
Bitwise AND of the host IP address & subnet mask = subnet number
EE 4272 Spring, 2003
IP Versions
• IP v 1-3 defined and replaced• IP v4 - current version• IP v5 - streams protocol• IP v6 - replacement for IP v4
Under development it is called IPng (Next Generation)
• Why IP v6 Address space exhaustion
Two level addressing (network and host) wastes space Growth of networks and the Internet Single address per host
Requirements for new types of service
EE 4272 Spring, 2003
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.
EE 4272 Spring, 2003
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