1 Lecture #16: Network Layer Lecture #16: Network Layer and Internetworking and Internetworking C o n t e n t s C o n t e n t s Network Layer: functions and Network Layer: functions and services services Network Layer: technologies Network Layer: technologies Internetworking Internetworking Concatenated Virtual Circuits Concatenated Virtual Circuits Connectionless internetworking Connectionless internetworking Fragmentation Fragmentation 6 2 7 10 19 15 12
OSI Network LayerOSI Network LayerUser application 1 ...
Encryption/decryption
compression/expansion
Choice of syntax
Sessioncontrol
Session to transportmapping
Session management
Sessionsynch.
Layer and flowcontrol
Error recovery
Multiplexing
Connection control
Routing Addressing
Errorcontrol
Flowcontrol
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 controlConnection control: establishment, maintaining and terminating : establishment, maintaining and terminating network connectionsnetwork connections between source and destination open systems between source and destination open systems
RoutingRouting: considerations associated with : considerations associated with hop-by-hop serviceshop-by-hop services transparent to the underlying resources such as data link connections .transparent to the underlying resources such as data link connections .
AddressingAddressing: : 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 namingtopology (# of hops) including naming
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NL Services to the Transport NL Services to the Transport LayerLayer
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 transparent transfer of data between transport entitiesentities. This service allows the structure and detailed . This service allows the structure and detailed content of submitted data to be determined exclusively content of submitted data to be determined exclusively by layers above the network layer.by layers above the network layer.
The network layer contains functions necessary to The network layer contains functions necessary to provide the transport layer with a firm provide the transport layer with a firm network/transport layer boundary which is independent network/transport layer boundary which is independent of the underlying communications media in all things of the underlying communications media in all things other than quality of service. other than quality of service.
Thus the network layer contains functions necessary to Thus the network layer contains functions necessary to 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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Services provided to the Services provided to the transport layertransport layer
Transparent transfer of data between transport Transparent transfer of data between transport entities. This service allows the structure and entities. This service allows the structure and detailed content of submitted data to be detailed content of submitted data to be determined exclusively by layers above the determined exclusively by layers above the network layer.network layer.
Firm network/transport layer boundary which is Firm network/transport layer boundary which is independent of the underlying communications independent of the underlying communications media in all things other than quality of service. media in all things other than quality of service.
Mask the differences in the characteristics of Mask the differences in the characteristics of different transmission and subnetwork different transmission and subnetwork technologies into a consistent network service.technologies into a consistent network service.
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Network Layer Service Network Layer Service TypesTypes Connection oriented - Connection oriented - virtual circuitvirtual circuit (VC) (VC) - -
supported by the lower network layers (DLL):supported by the lower network layers (DLL):– setup and release of the connectionsetup and release of the connection– connection parameters negotiationconnection parameters negotiation– sequenced delivery of packetssequenced delivery of packets– receiver’s overflow prevented by flow controlreceiver’s overflow prevented by flow control– options:options:
• priority of deliverypriority of delivery• confirmation of deliveryconfirmation of delivery
– ExamplesExamples: most popular : most popular X.25X.25 Connectionless oriented - Connectionless oriented - datagrams exchangedatagrams exchange - -
reliability issues (if present) supported by the reliability issues (if present) supported by the transport layer transport layer – send/receive directives (confirmed/nonconfirmed send/receive directives (confirmed/nonconfirmed
services)services)– independent packets’ (“datagrams”) delivery with full independent packets’ (“datagrams”) delivery with full
destination addressdestination address– ExamplesExamples: most popular : most popular IPIP (required when using TCP/IP) (required when using TCP/IP)
– AddressingAddressing: full source and destination address in each : full source and destination address in each datagramdatagram
– State informationState information: not needed nor hold: not needed nor hold– RoutingRouting: independent routing of the subsequent packets: independent routing of the subsequent packets– Node Failure effectsNode Failure effects: packets loss: packets loss– Congestion controlCongestion control: not typical, rarely applied: not typical, rarely applied– ComplexityComplexity: in transport layer (above the subnet!): in transport layer (above the subnet!)– ApplicationApplication: connectionless services but also connection : connectionless services but also connection
oriented oriented Virtual CircuitVirtual Circuit
– AddressingAddressing: short VC number in each packet: short VC number in each packet– State informationState information: kept in the subnet table for each VC: kept in the subnet table for each VC– RoutingRouting: only during the VC setup: only during the VC setup– Node Failure effectsNode Failure effects: VCs termination: VCs termination– Congestion controlCongestion control: consists of and depends on buffering: consists of and depends on buffering– ComplexityComplexity: in the network layer (in the subnet!): in the network layer (in the subnet!)– ApplicationApplication: connection oriented services: connection oriented services
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Internetworking - TermsInternetworking - Terms InternetworkingInternetworking - multinet structure including different - multinet structure including different
types of networks and protocolstypes of networks and protocols
Internetworking glossaryInternetworking glossary::– Communication networkCommunication network: a facility providing data transfer service : a facility providing data transfer service
among stations attached to the networkamong stations attached to the network– InternetInternet: a collection of communication networks connected by : a collection of communication networks connected by
bridges and/or routersbridges and/or routers– SubnetworkSubnetwork: a constituent network of an internet: a constituent network of an internet– Intermediate systemIntermediate system ( (ISIS): a connection device between any two ): a connection device between any two
subnetworkssubnetworks– RepeaterRepeater: IS that connect two identical subnetworks on the physical : IS that connect two identical subnetworks on the physical
level, repeats the bit sequence without storing of any data.level, repeats the bit sequence without storing of any data.– BridgeBridge: IS that connects two LANs with identical protocols. Bridges : IS that connects two LANs with identical protocols. Bridges
are address filters that use store-and-forward mechanism without are address filters that use store-and-forward mechanism without modifying the packets’ contents. It operates on DLL levelmodifying the packets’ contents. It operates on DLL level
– RouterRouter: IS that connects two networks with potentially different : IS that connects two networks with potentially different protocols (“multiprotocol router”); store-and-forward address filter protocols (“multiprotocol router”); store-and-forward address filter operating on the Network Layeroperating on the Network Layer
– GatewayGateway: internetworking protocol converters acting on the : internetworking protocol converters acting on the Transport and Application layers. Modifications: full and half gateways Transport and Application layers. Modifications: full and half gateways
implementation of directory servicesimplementation of directory services
Service types incl. QoSService types incl. QoS: : connectivity, confirmed/ connectivity, confirmed/ /nonconfirmed services, special features support (e.g.real /nonconfirmed services, special features support (e.g.real time) time)
Parameters:Parameters: system of timeouts, buffer sizes etc.system of timeouts, buffer sizes etc. Flow/error controlFlow/error control: : level of ordering and error level of ordering and error
protectionprotection SecuritySecurity: : levels of privacy, encryption, identification etc.levels of privacy, encryption, identification etc. Routing and congestion controlRouting and congestion control: : different different
mechanismsmechanisms Broadcasting and multicastingBroadcasting and multicasting: : yes/noyes/no
Packet sizePacket size: : maximum size varies substantiallymaximum size varies substantially Accounting rulesAccounting rules: : yes/no; by traffic/timeyes/no; by traffic/time
Networks Networks CharacteristicsCharacteristics
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AddressingAddressingUniqueness: 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 internetDTE may have more than one SAP, each of them is unique to that
particular DTEGlobal Internet Address (GNA) = (network, host or station)
parametersForm: (network identifier, end system identifier)
Subnet Level A unique address for each DTE attached to the subnetReferred to as the Subnetwork Attachment Point Address (SAPA)Host parameter of GNA and SAPA may be the same but are often
notDifferent networks use different addressing formats and lengths (ARP,
RARP)Some host have more than one attachment point to the subnetHost parameter (GNA) has global significance, SAPA has local
CVC is End-to-End connection that consists of several consecutive Point-to-Point links between:
source host and subnetsubnet 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 numbersduring 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 VCsthe next destination for each VCthe number of the next concatenated VC
Application: internetworking in set of subnets of similar type of services (e.g. either reliable or unreliable). Usually implemented on Transport layer (e.g. TCP - End-to-End transport protocol)
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Pro’sPro’s• reservation of buffers and reservation of buffers and
communication capacity in communication capacity in
advanceadvance
• guaranteed sequencing, guaranteed sequencing,
delivery and stable delays delivery and stable delays
• possible implementation possible implementation
of any type services of any type services
• short addressing (small short addressing (small
communication overload communication overload
due to the headers)due to the headers)
• small communication small communication
overload due to packets overload due to packets
retransmission and lossesretransmission and losses
Contra’sContra’s
• waste of buffer space waste of buffer space
(table space) for each (table space) for each
open connectionopen connection
• static routing during the static routing during the
only on the both ends of the “tunnel” where the only on the both ends of the “tunnel” where the
original data entities are constructed/restoredoriginal data entities are constructed/restored
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FragmentationFragmentation• FragmentationFragmentation is the process of splitting of the is the process of splitting of the data structures into the entities that are suitable to data structures into the entities that are suitable to transmit over the various networks and the reverse transmit over the various networks and the reverse process of restoring the original structures out of the process of restoring the original structures out of the fragments.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 Service discipline and resource sharing in the end stations and
• 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 Data packets are broken into fragments and
each fragment is sent in separate internet each fragment is sent in separate internet
packet.packet.
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Fragmentation Fragmentation MethodsMethods•Each network in the internet is bounded by gateways
which are the entry point and the output point of the packets traversing that network
•1st approach: 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)•22ndnd approach: approach: nontransparent fragmentationnontransparent fragmentation. . Large packets are fragmented (if needed!) at the Large packets are fragmented (if needed!) at the small-packet-network entry point (gateway G1), then small-packet-network entry point (gateway G1), then traverse the internet as independent packets and are traverse the internet as independent packets and are resembled back only at the destination host. resembled back only at the destination host. •Requirements/features:Requirements/features:
• defragmentation capabilities of each hostdefragmentation capabilities of each host• communication overhead for each fragment during the communication overhead for each fragment during the
whole route whole route • better possibility for congestion control and dynamic better possibility for congestion control and dynamic
routing (in the datagram model)routing (in the datagram model)• only one fragmentation/defragmentation cycle (if any!) only one fragmentation/defragmentation cycle (if any!)
may occur during an internet route of a long packetmay occur during an internet route of a long packet• possibility for hierarchical fragmentation: fragmentation of possibility for hierarchical fragmentation: fragmentation of
already fragmented packets in case the route passes already fragmented packets in case the route passes network of even smaller packets: tree-numbering of the network of even smaller packets: tree-numbering of the fragments that can be extended hierarchically (e.g. fragments that can be extended hierarchically (e.g.
