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Lecture #16: Network Layer and Lecture #16: Network Layer and
Internetworking Internetworking
C o n t e n t s C o n t e n t s
Network Layer: functions and services Network Layer: functions and services
Network Layer: technologies Network Layer: technologies
Internetworking Internetworking
Concatenated Virtual Circuits Concatenated Virtual Circuits
Connectionless internetworking Connectionless internetworking
Fragmentation Fragmentation
Firewall technology Firewall technology
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OSI Network Layer OSI Network Layer
User application 1 ...
Encryption/
decryption
compression/
expansion
Choice of syntax Session
control
Session to transport mapping
Session management Session
synch.
Layer and flow control
Error recovery
Multiplexing
Connection control
Routing Addressing
Error control
Flow control Data link
establishment
Synch Framing
Access to transm. media
Physical and electrical interface
Activation/
deactivation of con.
Application layer Presentation layer
Session layer Transport layer Network layer
Link layer
Physical layer Connection control
Connection control: establishment, maintaining and terminating : establishment, maintaining and terminating network connections
network connections between source and destination open systems between source and destination open systems Routing
Routing: considerations associated with : considerations associated with hop-by-hop services hop-by-hop services
transparent to the underlying resources such as data link connections . transparent to the underlying resources such as data link connections . Addressing
Addressing: : globally unique identificationglobally unique identification of a service access point of of a service access point of an end system (transparent to subnet technology (routers/LANs…) and an end system (transparent to subnet technology (routers/LANs…) and
topology (# of hops) including naming topology (# of hops) including naming
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NL Services to the Transport Layer NL Services to the Transport Layer
The basic service of the network layer is to provide the The basic service of the network layer is to provide the
transparent transfer of data between transport entities transparent transfer of data between transport entities . .
This service allows the structure and detailed content of This service allows the structure and detailed content of
submitted data to be determined exclusively by layers above submitted data to be determined exclusively by layers above
the network layer.
the network layer.
The network layer contains functions necessary to provide the The network layer contains functions necessary to provide the transport layer with a firm network/transport layer boundary transport layer with a firm network/transport layer boundary
which is independent of the underlying communications media which is independent of the underlying communications media
in all things other than quality of service.
in all things other than quality of service.
Thus the network layer contains functions necessary to mask Thus the network layer contains functions necessary to mask the differences in the characteristics of different transmission the differences in the characteristics of different transmission
and subnetwork technologies into a consistent network service.
and subnetwork technologies into a consistent network service.
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Services provided to the Services provided to the
transport layer transport layer
Transparent transfer of data between transport entities. Transparent transfer of data between transport entities.
This service allows the structure and detailed content of This service allows the structure and detailed content of
submitted data to be determined exclusively by layers submitted data to be determined exclusively by layers
above the network layer.
above the network layer.
Firm network/transport layer boundary which is Firm network/transport layer boundary which is
independent of the underlying communications media in independent of the underlying communications media in
all things other than quality of service.
all things other than quality of service.
Mask the differences in the characteristics of different Mask the differences in the characteristics of different transmission and subnetwork technologies into a
transmission and subnetwork technologies into a consistent network service.
consistent network service.
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Network Layer Service Types Network Layer Service Types
Connection oriented - Connection oriented - virtual circuit virtual circuit (VC) (VC) - supported - supported by the lower network layers (DLL):
by the lower network layers (DLL):
– setup and release of the connection setup and release of the connection – connection parameters negotiation connection parameters negotiation – sequenced delivery of packets sequenced delivery of packets
– receiver’s overflow prevented by flow control receiver’s overflow prevented by flow control – options: options:
• priority of deliverypriority of delivery
• confirmation of deliveryconfirmation of delivery
– reliablereliable
– unreliable (rare usage)unreliable (rare usage)
– Examples Examples : most popular : most popular X.25 X.25
Connectionless oriented - Connectionless oriented - datagrams exchange datagrams exchange - -
reliability issues (if present) supported by the transport reliability issues (if present) supported by the transport
layer layer
– send/receive directives (confirmed/nonconfirmed services) send/receive directives (confirmed/nonconfirmed services) – independent packets’ (“datagrams”) delivery with full independent packets’ (“datagrams”) delivery with full
destination address destination address
– Examples Examples : most popular : most popular IP IP (required when using TCP/IP) (required when using TCP/IP)
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Network Layer Technologies Network Layer Technologies
Datagram Exchange
Datagram Exchange
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AddressingAddressing : full source and destination address in each datagram : full source and destination address in each datagram –
State informationState information : not needed nor hold : not needed nor hold
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RoutingRouting : independent routing of the subsequent packets : independent routing of the subsequent packets –
Node Failure effectsNode Failure effects : packets loss : packets loss
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Congestion control: not typical, rarely appliedCongestion control : not typical, rarely applied
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Complexity: in transport layer (above the subnet!)Complexity : in transport layer (above the subnet!)
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Application: connectionless services but also connection orientedApplication : connectionless services but also connection oriented
Virtual Circuit
Virtual Circuit
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AddressingAddressing : short VC number in each packet : short VC number in each packet
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State information: kept in the subnet table for each VCState information : kept in the subnet table for each VC –
Routing: only during the VC setupRouting : only during the VC setup
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Node Failure effectsNode Failure effects : VCs termination : VCs termination
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Congestion control: consists of and depends on bufferingCongestion control : consists of and depends on buffering –
Complexity: in the network layer (in the subnet!)Complexity : in the network layer (in the subnet!)
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ApplicationApplication : connection oriented services : connection oriented services
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7Internetworking - Terms Internetworking - Terms
Internetworking Internetworking - multinet structure including different types of - multinet structure including different types of networks and protocols
networks and protocols
Internetworking glossary Internetworking glossary : :
– Communication networkCommunication network: a facility providing data transfer service among : a facility providing data transfer service among stations attached to the network
stations attached to the network
– Internet: a collection of communication networks connected by bridges and/or Internet: a collection of communication networks connected by bridges and/or routers
routers
– Subnetwork: a constituent network of an internetSubnetwork: a constituent network of an internet
– Intermediate systemIntermediate system ( (ISIS): a connection device between any two ): a connection device between any two subnetworks
subnetworks
– RepeaterRepeater: IS that connect two identical subnetworks on the physical level, : IS that connect two identical subnetworks on the physical level, repeats the bit sequence without storing of any data.
repeats the bit sequence without storing of any data.
– BridgeBridge: IS that connects two LANs with identical protocols. Bridges are address : IS that connects two LANs with identical protocols. Bridges are address filters that use store-and-forward mechanism without modifying the packets’
filters that use store-and-forward mechanism without modifying the packets’
contents. It operates on DLL level contents. It operates on DLL level
– RouterRouter: IS that connects two networks with potentially different protocols : IS that connects two networks with potentially different protocols (“multiprotocol router”); store-and-forward address filter operating on the (“multiprotocol router”); store-and-forward address filter operating on the Network Layer
Network Layer
– GatewayGateway: internetworking protocol converters acting on the Transport and : internetworking protocol converters acting on the Transport and Application layers. Modifications: full and half gateways
Application layers. Modifications: full and half gateways
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Protocol stack Protocol stack : : OSI/IP/Novel/DECnet/AppleTalk/... OSI/IP/Novel/DECnet/AppleTalk/...
Addressing scheme Addressing scheme : :
flat files (802.X) vs. hierarchical (IP), flat files (802.X) vs. hierarchical (IP), implementation of directory servicesimplementation of directory services
Service types incl. QoS Service types incl. QoS : : connectivity, confirmed/ connectivity, confirmed/
/nonconfirmed services, special features support (e.g.real time) /nonconfirmed services, special features support (e.g.real time)
Parameters: Parameters: system of timeouts, buffer sizes etc. system of timeouts, buffer sizes etc.
Flow/error control Flow/error control : : level of ordering and error protection level of ordering and error protection
Security Security : : levels of privacy, encryption, identification etc. levels of privacy, encryption, identification etc.
Routing and congestion control Routing and congestion control : : different different mechanisms
mechanisms
Broadcasting and multicasting Broadcasting and multicasting : :
yes/noyes/no
Packet size Packet size : : maximum size varies substantially maximum size varies substantially
Accounting rules Accounting rules : : yes/no; by traffic/time yes/no; by traffic/time
Networks Characteristics Networks Characteristics
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Addressing Addressing
Uniqueness: Addressing allows the DTE to be uniquely identified so that data may be routed globally to the correct destination.
Levels of addressing
Network Level (and above)
SAP: Uniquely identifies the DTE within the internet
DTE may have more than one SAP, each of them is unique to that particular DTE
Global Internet Address (GNA) = (network, host or station) parameters Form: (network identifier, end system identifier)
Subnet Level
A unique address for each DTE attached to the subnet
Referred to as the Subnetwork Attachment Point Address (SAPA)
Host parameter of GNA and SAPA may be the same but are often not
Different networks use different addressing formats and lengths (ARP, RARP) Some host have more than one attachment point to the subnet
Host parameter (GNA) has global significance, SAPA has local significance
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Concatenated Virtual Circuits Concatenated Virtual Circuits
CVC is End-to-End connection that consists of several consecutive Point-to-Point links between:
source host and subnet
subnet and multiprotocol router (“full gateway”)
[subnet and subnet, connected by shared “half-gateways”]) subnet and destination host
Features:
the data routes are identified by VC numbers
during the session data packets traverse the same sequence of GWs and arrive in order
the routes are supported by VC tables containing
the ID number of the actual VCs the next destination for each VC
the number of the next concatenated VC
Application:
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Pro’s Pro’s
• reservation of buffers and reservation of buffers and communication capacity in communication capacity in advance
advance
• guaranteed sequencing, guaranteed sequencing, delivery and stable delays delivery and stable delays
• possible implementation of possible implementation of any type services
any type services
• short addressing (small short addressing (small
communication overload due communication overload due to the headers)
to the headers)
• small communication overload small communication overload due to packets retransmission due to packets retransmission and losses
and losses
Contra’s Contra’s
• waste of buffer space (table waste of buffer space (table space) for each open
space) for each open connection
connection
• static routing during the static routing during the session i.e. bad congestion session i.e. bad congestion control
control
• vulnerability to router failures vulnerability to router failures
• complicated implementation in complicated implementation in unreliable datagram
unreliable datagram subnetworks
subnetworks
Concatenated Virtual Circuits
Concatenated Virtual Circuits
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Connectionless Internetworking Connectionless Internetworking
Applies Datagram model Features:
• independent routing for each packet thus optimizing the the congestion
• not-in-order delivery
• datagram packets can be routed around network failure points in d.g. subnetworks
• requires universal addressing system - Internet, IPX, OSI, SNA, AppleTalk address standards
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Pro’s Pro’s
• adaptive dynamic routing and adaptive dynamic routing and adaptive congestion control adaptive congestion control
• low buffer space needed at low buffer space needed at routers
routers
• robustness to router failures robustness to router failures
• applicable for any type of applicable for any type of subnets incl. unreliable ones subnets incl. unreliable ones
Contra’s Contra’s
• communication overhead due communication overhead due to longer address fields,
to longer address fields, repeated in each datagram repeated in each datagram
• communication overhead due communication overhead due to unreliable unordered
to unreliable unordered services
services
• dispersed delay durationdispersed delay duration
• requires universal addressing requires universal addressing system
system
Connectionless Internetworking
Connectionless Internetworking
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Tunneling Tunneling
• Tunneling Tunneling is a technique for connection of two similar is a technique for connection of two similar networks through the arbitrary type[s] of intermediate networks through the arbitrary type[s] of intermediate
network[s]
network[s]
• Data entities (datagrams, packets) of two ends are Data entities (datagrams, packets) of two ends are packed together with their control information
packed together with their control information
(addressing, ordering, error control fields, etc.) into the (addressing, ordering, error control fields, etc.) into the
payload field of the intermediate network’ NL packets payload field of the intermediate network’ NL packets
• The original control information is not being interpret The original control information is not being interpret anywhere in the intermediate network but in both ends anywhere in the intermediate network but in both ends
• Therefore, tunneling needs multiprotocol routers only on Therefore, tunneling needs multiprotocol routers only on the both ends of the “tunnel” where the original data
the both ends of the “tunnel” where the original data entities are constructed/restored
entities are constructed/restored
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Fragmentation Fragmentation
• Fragmentation Fragmentation is the process of splitting of the data is the process of splitting of the data
structures into the entities that are suitable to transmit over structures into the entities that are suitable to transmit over the various networks and the reverse process of restoring the the various networks and the reverse process of restoring the original structures out of the fragments.
original structures out of the fragments.
• Fragmentation factors: Fragmentation factors:
• Transmission method (bit error rate, multiplexing method, etc.) Transmission method (bit error rate, multiplexing method, etc.)
• Operating system (read/write blocks of 0.5 kB)Operating system (read/write blocks of 0.5 kB)
• Protocols (packet length field limitation)Protocols (packet length field limitation)
• Standardization Standardization
• Service discipline and resource sharing in the end stations and intermediate Service discipline and resource sharing in the end stations and intermediate systems (IS): routers, gateways (e.g. SJF “shortest job first”, RR “Round systems (IS): routers, gateways (e.g. SJF “shortest job first”, RR “Round Robin” etc.)
Robin” etc.)
• Examples of payload size: Examples of payload size:
• ATM cell carries 48BATM cell carries 48B
• IP packet carries 64kB IP packet carries 64kB
• Data packets are broken into fragments and each Data packets are broken into fragments and each fragment is sent in separate internet packet.
fragment is sent in separate internet packet.
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Fragmentation Methods Fragmentation Methods
• Each network in the internet is bounded by gateways which are the entry point and the output point of the packets traversing that network
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stapproach: transparent fragmentation. Large packets are fragmented (if needed!) into fragments at the small-
packet-network entry point (gateways G1, G3) and
resembled back at the network output point (G2, G4). Note that all the fragments should reach the same network output point!
• Example: ATM networks hardware fragmentation/defragmentation of the packets into ATM cells at each entry/output point
• Requirements/features:
• additional counting of the number of fragments in connectionless networks or End-of-the-packet flag in the last fragment in the connection-oriented networks
• congestion control and performance are affected by the requirement for similar routing of all the fragments
• multiple fragmentation/defragmentation cycles may occur during an internet route of a long packet
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Fragmentation Methods (2) Fragmentation Methods (2)
• 2 2
ndndapproach: approach: nontransparent fragmentation nontransparent fragmentation . Large . Large packets are fragmented (if needed!) at the small-packet- packets are fragmented (if needed!) at the small-packet-
network entry point (gateway G1), then traverse the internet network entry point (gateway G1), then traverse the internet as independent packets and are resembled back only at the as independent packets and are resembled back only at the destination host.
destination host.
• Requirements/features: Requirements/features:
• defragmentation capabilities of each hostdefragmentation capabilities of each host
• communication overhead for each fragment during the whole route communication overhead for each fragment during the whole route
• better possibility for congestion control and dynamic routing (in the better possibility for congestion control and dynamic routing (in the datagram model)
datagram model)
• only one fragmentation/defragmentation cycle (if any!) may occur only one fragmentation/defragmentation cycle (if any!) may occur during an internet route of a long packet
during an internet route of a long packet
• possibility for hierarchical fragmentation: fragmentation of already possibility for hierarchical fragmentation: fragmentation of already fragmented packets in case the route passes network of even
fragmented packets in case the route passes network of even smaller packets: tree-numbering of the fragments that can be smaller packets: tree-numbering of the fragments that can be extended hierarchically (e.g.
extended hierarchically (e.g.
[0.]
[0.] [0.0, 0.1, 0.2 ...] [0.0, 0.1, 0.2 ...] [0.0.0, 0.0.1, 0.0.2 … 0.1.0, 0.1.1 ...] [0.0.0, 0.0.1, 0.0.2 … 0.1.0, 0.1.1 ...] ......
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Requirements/features
Requirements/features (cont.): (cont.):
• fragmentation to some elementary frame size. fragmentation to some elementary frame size.
Fragments are short enough to be carried by any Fragments are short enough to be carried by any
intermediate network. An internet packet contains one intermediate network. An internet packet contains one or more elementary frames. Additional flagging:
or more elementary frames. Additional flagging:
• packet ID numberpacket ID number
• ordering number of the first elementary fragment in the packetordering number of the first elementary fragment in the packet
• end-of-the-packet flag (1 bit: end/no_end)end-of-the-packet flag (1 bit: end/no_end)
Fragmentation Methods (3) Fragmentation Methods (3)
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