[Reading notes] Computer networks: top-down approach (Section 1--5)

1 Introduction

What is the Internet

+ = Subnetworks computer network resources subnet (functionally)
= + network edge network core (structure)
network edge : a document of the way
* C / S Mode
* P2P mode
core of the network : data exchange

• Packet switching
  characteristics of a single node transmits to the neighbor node, the forwarding looking down after storage, transmitted to the next node.
  Advantage full use of the network capacity; may convert data between different link rates; format conversion may be implemented; can-many, many-to-access; error control can be achieved.
  Disadvantage of increased resource overhead (ancillary information leading to the processing time and storage resources overhead).
• Circuit switching
  characteristics of the entire message bit stream continuously transmitted from the recipient the sender. Established between the source node and the destination node dedicated to a data transmission link, comprising the step of establishing three links, data transmission, the link is disconnected.
  The advantage of low delay
  disadvantages utilization rate line, the line capacity can not make full use of inconvenient error control.

What is protocol

Protocol is a communication control rule between two peer entities set. Under the control of the protocol, two peer entities may provide services to an upper layer, the lower layer while using the service provided by whom.

Internet performance indicators

• Delay
  $d_{e} = d_{proc} + d_{queue} + d_{trans} + d_{prop}$
• The bandwidth (throughput: since the pipeline processing, the bandwidth that take a small (bottle neck).

Hierarchy

• ISO / OSI seven layer reference model for
  the application layer, presentation layer, session layer, transport layer, network layer, data link layer, physical layer
• TCP / IP four-layer reference model
  Application Transport Internet Interface
• Lower for the upper layer to provide services
• Internet protocol stack
  • The application layer support network applications, as PDUs MESSAGE message, agreement HTTP, FTP
  • Transport layer supports inter-process communication between different hosts, PDUs message segment to segment, protocol is TCP UDP
  • The network layer supports host communication, forwarding data packets from source to routing purposes, PDUs to datagrame datagram (known as packets), the IP protocol
  • Data link layer data transmission in an adjacent network elements, PDUs frame of frame protocol has PPP, Ethernet
  • PDU to the physical layer bitstream

2 Application Layer The application layer

Network edge belong to the application layer.

2-1 edge network organization: C / S & P2P

C / S Mode

feature:

server

• Can not directly communicate;
• Server always open;
• Have a permanent IP address;
• And a central data exchange;

Client

• The sum of the client is not connected;
• Using a dynamic IP address;
• And the client does not necessarily interconnected with each other.

P2P mode

feature

Not always open between the node is a peer and have both upload and download functions shying dynamic IP; complex management; access to services while also providing services
Cons: Gets services would also have to provide services, take up a lot of memory; easily lead to network congestion.
advantage:

• Reducing the pressure on server computing, since the complete elimination of a node, you can assign tasks to nodes, greatly improve system efficiency and resource utilization.
• You can directly share documents among multiple clients
• Good scalability
• Strong network robustness

Reference document distribution time

× Note that u is the upload rate, d represents the download rate, F is the size of a file, there are N nodes.
then:
$d_{C/S} = \max( \frac{NF}{u_{s}} , \frac{F}{d_{min}} )$
Client - server model of document distribution time for the server to upload the maximum size between the N F file with all clients time and download time.

$d_{P2P} = \max( \frac{F}{u_{s}} , \frac{F}{d_{min}},\frac{NF}{u_{s} + \sum_{i}u_{i}} )$
File distribution time for all nodes in the P2P mode to upload a file with the time, all the nodes to download a file with time, and the maximum overall upload time N files using the network.

Pure P2P architecture applications

• Features: the presence of free riding phenomenon, rarest first

HTTP protocol

Stateless protocol - any last visited page on the server used by the same customer and his first visit with the equal time.

HTTP1.0: unsustainable - each TCP connection is closed when the server returns the object, and not continue down to other objects.

HTTP1.1 sustainability

Proxy server proxy server
A network entity, also known as the World Wide Web caching Web cache. The proxy server requests and responses to some recent results temporarily stored in the local disk. effect:

• Significantly reduced response time to client requests
• Means a greatly reduced amount of internal communication network and the Internet] access link
• Reducing Web traffic on the Internet as a whole

Mail protocol

Send Mail --SMTP
accept messages --IMAP not deleted after the download, the download delete POP3 protocol
SMTP and HTTP comparison
the same: a persistent connection C / S architecture
different:

• HTTP为PULL REQUEST，SMTP是PUSH REQUEST
• SMTP requires each packet includes a body 7 are used for the ASCII format, HTTP without
• And further comprising a processing text file containing the image is not the same way: with the HTTP, SMTP band.

DNS protocol

application

• Implement domain name to IP resolution
• Host alias
• Mail server alias
• Load Balancing

Query methods

Recursive queries (depth), iterative query (to carry around a local domain name server)

FTP protocol

FTP and HTTP comparison

Same: TCP-based file transfer protocol
different:

• FTP is stateful, tracking user status. HTTP stateless
• Connected in parallel with the two FTP file transfer, i.e. band transmission, and data protocols into protocol control
• For FTP transmission, control link through to the end, the data link each transfer have to re-establish / release

3 Transport Layer Transport Layer

4 Network Layer The network layer

5 Data-link Layer Data Link Layer

6 Grossary

• network protocol / protocol network protocol, also known as protocol. For network data exchange established rules, standards or conventions. Including the format and timing of data exchange. There are three elements: Syntax (data / control information format), semantics (content control information), the timing (the event order to achieve a detailed description).
• end system end system
• modem modem, cats