重点问题

Used to start communication between a client and a server.

概念：is the method used to establish TCPsocket connections and tear down TCPsocket connections over the network.

目的：The TCP handshaking mechanism is designed so that two computers attempting to communicate can negotiate the parameters of the network TCP socket connection before beginning communication. This three way handshaking process is also designed so that both ends can initiate and negotiate separate TCPsocket connections at the same time. Being able to negotiate multiple TCP socket connections in both directions at the same time allows a single physical network interface to be multiplexed

A TCP segment consists of a segment header and a data section. 1.The TCP header contains 10 required fields, and an optional extension field .

2.The data section follows the header. Its contents are the payload data carried for the application. The length of the data section is not specified in the TCP segment header. Checksum (校验和)

Checksum is a calculated value that is used to determine the integrity of data. Checksum serves as a unique identifier for the data (a file, a text string, or a hexadecimal string). If the data changes then so does the checksum value. This makes it easy to verify the integrity of the data.

（The 16-bit checksum field is used for error-checking of the header and data, just like hash function.） TCP Fragmentation

The Internet Protocol (IP) implements datagram fragmentation, so that packets may be formed that can pass through a link with a smaller maximum transmission unit (MTU) than the original datagram size.

A sliding window protocol is a feature of packet-based data transmission protocols. Sliding window protocols are used where reliable in-order delivery of packets is required, such as in the Data Link Layer (OSI model) as well as in the Transmission Control Protocol (TCP).

TCP Congestion Control(拥塞控制) can be used to calculate the amount of data the sender can send to the destination on the network.

• Essential strategy: The TCP host sends packets into the network

without a reservation and then the host reacts to observable events. • cwnd is set based on the perceived level of congestion. The Host

receives implicit (packet drop) or explicit (packet mark) indications of internal congestion.

AIMD （Additive Increase / Multiplicative Decrease）is a necessary condition for TCP congestion control to be stable. （Because the simple CC mechanism involves timeouts that cause retransmissions, it is important that hosts have an accurate timeout mechanism.）

• Additive Increase is a reaction to perceived available capacity. • Multiplicate Decrease: TCP reacts to a timeout by halving cwnd.

is used by a gateway NAT （Network Address Translation 网络地址转换）to translate your local IP address to a global one. IP Layers

Application layer: FTP, POP, SMTP, VoIP, IRC, DHCP, P2P, HTTP, SSL, DNS, RPC

Transport: TCP, UDP Internet: IPv4, IPv6, ICMP Link: PPP,SLIP,ISDN,ARP

怎么实现广播？

The ethernet broadcast address in hexadecimal is FF:FF:FF:FF:FF:FF. There are several types of IP broadcasting:

1. The IP limited broadcast address is 255.255.255.255. This broadcast is not forwarded by a router.

2. A broadcast directed to a network has a form of x.255.255.255 where x is the address of a Class A network. This broadcast may be forwarded depending on the router program.

3. A broadcast sent to all subnetworks. If the broadcast is 10.1.255.255 on network 10.1.0.0 and the network is subnetted with multiple networks

10.1.x.0, then the broadcast is a broadcast to all subnetworks.

4. A broadcast sent to a subnet in the form 10.1.1.255 is a subnet broadcast if the subnet mask is 255.255.255.0.

IPv4的主要问题是什么？

The number of users on the Internet is continuely increasing very fast, but the address structure of the IPv4-header is fixed. In its later years, this is leading to problems as the number of free addresses is becoming less and less. Another aspect is that with the development of new applications like Multi-Media and video conferencing, new features of IP are needed. The following points should demonstrate why a new IP is being developed:

Protocols

POP3 (Application Layer) Post Office Protocol Email.

SMTP (Application Layer) Simple Mail Transfer Protocol Email. FTP (Application Layer) File Transport Protocol Send and receives files. HTTP (Application Layer) Hypertext Transport Protocol Web pages. PPP (Link Layer) Point-to-Point Protocol . For serial (bit) stream communication.

SSL (Application Layer) Secure Sockets Layer. provides security, good for tunnels.

RPC (Application Layer) Remote Procedure Calls. calls functions on a server.

IPv4 (Internet Layer) Internet Protocol

IPv6 (Internet Layer) The next generation of IP TCP (Transport Layer) Transport Control Protocol

P2P (Application Layer) Peer-to-Peer. The name given to file sharing programs like Thunder.

UDP (Transport Layer) User Datagram Protocol User Datagram Protocol. Ethernet Protocol (Link or Physical Layer) The protocol your Ethernet card uses to talk to the local network.

ICMP (Internet Layer, Piggyback on IP) Internet Control Message Protocol. Used mainly for errors (NACKs) and for network status between routers.

ARP (Link Layer) Address Resolution Protocol. An older protocol using MAC addresses to assign IP addresses

DHCP (Application Layer) Dynamic Host Configuration Protocol. Used to assign local IP addresses. Uses a Magic Cookie' to identify communication and broadcasts messages.

DNS (Application Layer) Domain Name System hierarchical (tree-like) database system to tie Domain Names (like cqu.edu.cn) to IP addresses. SLIP(Link Layer) Serial Line Internet Protocol An older standard of Windows Remote Access. It is used in UNIX's. Remote Access Server to

connect some ISP