Network Layer Fourth Chapter net

4.1 Network layer Service (not end-end)

To transmit a data segment (segment) from the sending host to the receiving host:

Sending a data segment into a datagram

Receiving host, delivering data segments to the transport layer

Each host and router is running the Network layer protocol, where the router examines the head domain of the IP datagram that traverses it, and how the datagram is processed by the decision.

core functions of the network layer-forwarding and routing

Forwarding: Transfers the packet from the input port of the router to the appropriate output port.

each router maintains a forwarding post (pre-established), which determines how packets are forwarded in the router

Routing: Determines the path of grouping from source to destination. The end-to-end path through the network is determined by the routing algorithm (protocol)

core function of network layer-connection establishment

is an important feature of some networks (such as ATM)

Before data packets are transferred, both ends of the host need to establish a virtual connection

Network-layer connectivity : between two hosts (network device participation) (not end-end)

Transport-Layer connectivity : between two application processes (transparent to network devices) (End-end)

Network Layer Service Model:

Different networks provide different service models

No connection service:

Transfer path is not determined in advance for series-grouped transmissions

Each packet independently determines the transmission path

Different groupings may have different transmission paths

Example:

Datagram Network (non-connected service UDP similar to transport layer)

Connection service:

First determine the path from source to destination

Transfer series grouping along this path

Series packet transmission paths are the same, the order is guaranteed

Remove the connection after the transfer is complete

Example:

Virtual Circuit Network (connection-oriented service TCP similar to transport layer)

4.2: Virtual circuit network and datagram network

The essential difference between the network layer service and the Transport Layer service:

The network layer provides host-to-host service, which is the realization of the core of the network, and the transport layer realizes end-to-end service.

Virtual Circuit (virtual circuits): A path from the source host to the destination, similar to the circuit (logical connection)

With packet switching , the transmission of each packet takes advantage of the full bandwidth of the link (the maximum difference from the circuit switching)

The network layer equipment that the source to the destination path passes through together completes the virtual circuit function

The communication process of virtual circuit:

Call setup, data transfer, removal call

Data transfer: Each packet carries a virtual circuit identification (VCID), which transmits on the corresponding virtual circuit, not the destination host address

each network device passed by the virtual circuit needs to maintain the connection state of each virtual circuit passing through it.

Links, network device resources (such as bandwidth, cache, and so on) can be pre-allocated to VCs: pre-allocated resources = = Predictable service performance

Virtual Circuit of the specific implementation:

The composition of the virtual circuit:

1. A path from the source host to the destination host

2. Virtual circuit number (vcid), each link on the virtual circuit has its own number (the same VC, the Vcid on each link is usually different)

3. Each network layer device on the virtual circuit (such as a router), using the forwarding record to pass through each virtual circuit of the device (rewrite/replace the virtual circuit number according to the forwarding post)

Virtual Circuit Signaling Protocol: For the establishment, maintenance and demolition of VC

Build: Path selection (requires routing)

Applied to the virtual Circuit network, the current Internet does not use

Datagram Network:

Network layer is not connected, each packet carries the destination address

Routers are grouped according to the destination address of the packet, and the routing Protocol/algorithm is used to construct the forwarding group.

Each group is selected independently of the path (the datagram path may be different), so it may arrive at the destination host in random order

Data Report Forwarding:

Longest prefix match precedence principle: Select the longest entry with the prefix to match the destination address of the packet

Datagram Network or VC network:

Network Layer Fourth Chapter net