- Context of Network Layer Protocols
- major components of network are the ISP's equipment
- Host directly connected to ISP's routers
- Store and Forward Packet Switching
- Second host
- connected to private router
- private router has line to ISP's equipment
- for purposes of understanding, can still treat private routers as part of the isp network because they run on the same algorithms
- Host with packet to send transmits to nearest router, either to own LAN, or over point to point link to ISP
- packet is stored until fully arrived and checksum verified
- then forward to next router until reach host
Services Provided to the Transport Layer
- Network layer provides services to the transport layer at the network/transport layer interface
- services must be independent of router technology
- transport layer should be shielded from number type and topology of routers
- network addresses should be made available and use a uniform numbering plan
- One camp
- routers job is moving packets and nothing else
- Internet Community
- network is inherently unreliable so hosts should accept this fact
- primitives SEND PACKET and RECEIVE PACKET
- end to end argument
- no packet ordering and flow control should be done because hosts are going to be doing it anyway
- Other Camp
- represented by telephone companies
- quality of service is dominant factor
- controversy, X.25 and Frame Relay are connection oriented
- since ARPANET, connectionless grown in popularity
- IP Protocol shows prominence of connectionless technologies
- however internet is coming back to connection oriented services
- MPLS(MultiProtocol Label Switching)
- VLANS
Implementation of Connectionless Service
- Two different organizations are possible depending on type of service
- packets are
- injected into network individually
- routed indpendently
- no advanced setup
- called datagrams, datagram network
- connection oriented service, path from source router must be established all the way to destination router, VC(virtual circuit) needs to be set up
- called virtual circuit network
- In connectionless
- every router has internal routing table
- use outgoing lines to send packets
- stored briefly at link and checksum verified
- then forward to next router
- packets don't always follow the same path, since routing algorithm can change during transmission
- causes timing of packets to be different at arrival
Implementation of Connection-Oriented Service
- require virtual circuit network
- avoid choosing new routes
- route is part of connection setup and is stored in the router
- when connection released vc also terminated
- Avoiding conflicts is key, label switching
- MPLS(MultiProtocol Label Switching)
- hidden from customers
- MPLS header wrapped around IP packets, used for long term connections
Comparison of Virtual Circuit and Datagram Networks
- trade off is setup time, vc requires significantly more resources
- router is very fast, only forwards info with datagram network, what is currently in use
- table space in router memory an issue
- vc ensures quality of resources and avoids congestion
- vc has vulnerability to crash if any router in path goes down, but datagram just resends packets
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