Computer network Learning Note _5

4-1_network_layer_overview

A point of knowledge: the difference between routing and forwarding. Routing is the process of deciding in which direction to send traffic-network wide (global) and expensive. Forwarding is the process of sending a packet on its way-node process (local) and fast.

　　As the scale of the network expands, it requires a higher level of structure, the Internet layer.

4-2_network_services

Two model: One is datagrams, the typical example is IP. One is virtual circuits. Both use the Store-and-forward packet switching technology.

4-3_internetworking

Talk about the role and IPV4 of IP.

　　

4-4_ip_prefixed

IPv4 with 32-bit address, prefix is divided into 8, 16, 24 bits, the IP address is represented by the remaining bits in the 32-prefix. IPV6 with 128-bit address.

Public/private IP Addresses. Public IP Addresses Resources are scarce, so you need to ipv6! Private IP address requires NAT (Network address translation) to access the public Internet.

4-5_ip_forwarding

The difference between routing and forwarding: forwarding is to handle the received packets sent to the appropriate place, routing is to calculate all the paths in the network, and then packets can forward.

The Network layer is to do the scale-to-large network,using address with hierarchy.

The IP forwarding problem is discussed below: The IP address in a network has the same prefix. Node uses the table to list the next hop and the corresponding IP prefixes. The prefixes in the table may overlap, with the longest matching prefix algorithm.

The difference between host and router. Router do routing,host packets sent to router. Host can access the hosts in the local network with prefix. But sending any external traffic (remote traffic) is sent to the nearest router (router).

　　

Host Forwarding Table, with longest matching prefix.

　　

4-6_ip_helpers_ (ARP_AND_DHCP)

DHCP is used to obtain IP addresses,arp used mapping IP to link addresses.

When a node just opened the IP address, the routing IP address is not known, only know Ethernet address, because Ethernet address is set on the hardware NIC (network interface card). IP depends on which network, a network prefix the same, this is to forward efficiency, so in different places, the IP address is different. DHCP (Dynamic Host configuration Protocol) is a protocol that can automatically configure IP addresses when node wake up. DHCP leases an IP address to node, while providing the network prefix,address of the local Router,dns server (translating the URL into an IP address). DHCP is a applicationg and is not considered a application from a network point of view.

　　

DHCP addressing bootstrap issue, the solution is that node wakes up broadcast. IP (+ bits): 255.255.255.255,ethernet (BITS): Ff.ff.ff.ff.ff.ff. The resulting IP address expires to renew, as long as the request and ACK.

　　

ARP (Address Resolution Protocol) is used to provide destination Ethernet address, which corresponds to the dest.ethernet and Dest.ip addresses.

　　

DHCP requires a server, but ARP does not. Also broadcast, find node with target IP in LAN. The ARP is on the link layer.

　　

4-7_packet_fragmentation

Fragmentation to router and host the burden is relatively large, if lost fragment very troublesome.

Path MTU Discovery is the method that is now used. Because the usual MTU is a few, so find the appropriate MTU is very wide. The path MTU relies on the path, and the paths may change over time, so search is ongoing.

4-8_ip_errors_ (ICMP)

ICMP (Internet Control Message Protocol) is an IP companion Protocol that is used to resolve connectivity problems.

4-9_ip_version_6

This paper introduces IPv6 and a method of coexistence of IPv6 and IPv4, tunneling.

Computer network Learning Note _5