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Computer Networking - Coggle Diagram

- - - - Non-Persistent connection
        
        a brand-new connection must be established and maintained for each requested object / burden on the Web server
        
        each object suffers a delivery delay of two RTTs—one RTT to establish the TCP connection and one RTT to request and receive an object
      - Persistent connection
      - Round-trip time (RTT): the time it takes for a small packet to travel from client to server and then back to the client
    - - HTTP Request Message
        
        Request line
        
        Method
        
        GET: When the browser requests an object, with the requested object identified in the URL field
        
        POST: If the value of the method field is POST, then the entity body contains what the user entered into the form fields
        
        HEAD: When a server receives a request with the HEAD method, it responds with an HTTP message but it leaves out the requested object. Application developers often use the HEAD method for debugging.
        
        PUT: It allows a user to upload an object to a specific path (directory) on a specific Web server
        
        DELETE: allows a user, or an application, to delete an object on a Web server
        
        URL
        
        HTTP version
        
        Header lines
      - HTTP Response Message
    - - (1) a cookie header line in the HTTP response message;
      - (2) a cookie header line in the HTTP request message;
      - (3) a cookie file kept on the user’s end system and managed by the user’s browser;
      - and (4) a back-end database at the Web site
    - - Web cache / Proxy server: a network entity that satisfies HTTP requests on the behalf of an origin Web server
        
        substantially reduce the response time for a client request
        
        substantially reduce traffic on an institution’s access link to the Internet
        
        both a server and a client at the same time
      - Content Distribution Networks (CDNs)
      - The Conditional GET
        
        An HTTP request message is a so-called conditional GET message if (1) the request message uses the GET method and (2) the request message includes an If-Modified-Since: header line.
    - - reduce perceived latency by enabling request and response multiplexing over a single TCP connection, provide request prioritization and server push, and provide efficient compression of HTTP header fields
      - Head of Line (HOL) blocking
      - HTTP/2 Framing
      - Response Message Prioritization and Server Pushing
      - HTTP/3
  - - - Domain name system (DNS)
        
        (1) a distributed database implemented in a hierarchy of DNS servers, and (2) an application-layer protocol that allows hosts to query the distributed database
        
        Host aliasing
        
        Mail server aliasing
        
        Load distribution
    - - Problem with centralized design of DNS
        
        A single point of failure
        
        Traffic volume
        
        Distant centralized database
        
        Maintenance
      - A Distributed, Hierarchical Database
        
        root DNS servers
        
        top-level domain (TLD) DNS servers
        
        authoritative DNS servers
        
        local DNS server: not strictly belong to the hierarchy of servers but is nevertheless central to the DNS architecture
        
        Recursive queries v.s. Iterative queries
      - DNS Caching: In a query chain, when a DNS server receives a DNS reply (containing, for example, a mapping from a hostname to an IP address), it can cache the mapping in its local memory
    - - resource records (RRs): (Name, Value, Type, TTL)
      - DNS Message
        
        Header section
        
        Question section
        
        Answer section
        
        Authority section
        
        Additional section
        
        nslookup program
      - Inserting Records into the DNS Database
        
        Registrar: a commercial entity that verifies the uniqueness of the domain name, enters the domain name into the DNS databas
  - - - distribution time
    - - rarest first
  - - - Port number
        
        The sending process attaches to the packet a destination address, which consists of the destination host’s IP address and the destination socket’s port number.
    - - This is different from UDP, for which the server must attach a destination address to the packet before dropping it into the socket.
      - The program does not explicitly create a packet and attach the destination address to the packet, as was the case with UDP sockets. Instead the client program simply drops the bytes in the string sentence into the TCP connection
- - - - a network-layer protocol provides logical- communication between host
      - application messages = letters in envelopes
        processes = cousins
        hosts (also called end systems) = houses
        transport-layer protocol = Ann and Bill
        network-layer protocol = postal service (including mail carriers)
    - - The IP service model is a best-effort delivery service, but it makes no guarantees. IP is said to be an unreliable service.
      - process-to-process data delivery and error checking are the only two services that UDP provides
      - TCP provides A protocol that provides reliable data transfer and congestion control
  - - - Since real-time applications often require a minimum sending rate, do not want to overly delay segment transmission, and can tolerate some data loss, TCP’s service model is not particularly well matched to these applications’ needs.
      - TCP uses a three-way handshake before it starts to transfer data. UDP just blasts away without any formal preliminaries. Thus UDP does not introduce any delay to establish a connection.
      - UDP does not maintain connection state and does not track any of these parameters. For this reason, a server devoted to a particular application can typically support many more active clients when the application runs over UDP rather than TCP.
      - The TCP segment has 20 bytes of header over- head in every segment, whereas UDP has only 8 bytes of overhead.
    - - The UDP checksum provides for error detection
  - - - A TCP connection is also always point-to-point
      - the process that is initiating the connection is called the client process, while the other process is called the server process
      - Three way handshake: For now it suffices to know that the client first sends a special TCP segment; the server responds with a second special TCP segment; and finally the client responds again with a third special segment.
      - The maximum segment size (MSS) is typically set by first determining the length of the largest link-layer frame that can be sent by the local sending host (the so-called maximum transmission unit, MTU)
      - A TCP connection provides a full-duplex service
    - - there are three major events related to data transmission and retransmission in the TCP sender: data received from application above; timer timeout; and ACK
        
        Doubling the Timeout Interval
        
        Fast Retransmit
        
        A duplicate ACK is an ACK that reacknowledges a segment for which the sender has already received an earlier acknowledgment.
- - - - Hosts and routers have link-layer addresses. Now you might find this surprising, recalling from Chapter 4 that hosts and routers have network-layer addresses as well.
      - MAC Addresses: A link layer address is variously called a LAN address, a physical address, or a MAC address
        
        sometimes a sending adapter does want all the other adapters on the LAN to receive and process the frame it is about to send. In this case, the sending adapter inserts a special MAC broadcast address into the destination address field of the frame.
      - Address Resolution Protocol (ARP)
        
        An ARP module in the sending host takes any IP address on the same LAN as input, and returns the corresponding MAC address.
        
        one important difference between the two resolvers is that DNS resolves host names for hosts anywhere in the Internet, whereas ARP resolves IP addresses only for hosts and router interfaces on the same subnet.
        
        ARP is probably best considered a protocol that straddles the boundary between the link and network layers
  - - - Blades: 刀鋒伺服器，一種單板型態的伺服器，於2001年由RLX公司提出。
      - Top of Rack (TOR) switch: interconnects the hosts(blades) and other switches in the data center.
      - border routers: connecting the data center network to the public Internet.
      - Load balancer: Inside the data center, the external requests are first directed to a load balancer whose job it is to distribute requests to the hosts, balancing the load across the hosts as a function of their current load.
  - - - the link layer is implemented on a chip called the network adapter, sometimes known as network interface controller (NIC)
        
        Controller
        
        Physical transmission
- - - - End systems are connected together by a network of communication links and packet switches.
        
        routers: used in access networks
        
        link-layer switches: used in the network core
        
        route or path: traversed by a packet from sending to receiving end system.
        
        Transmission rate: bits/second
      - End systems access the Internet through Internet Service Providers (ISPs)
        
        Residential ISPs, corporate ISPs, and cellular data ISPs
      - End systems, packet switches, and the other pieces of Internet run protocols the control the sending and receiving of information within the Internet.
        
        Transmission Control Protocol (TCP)
        
        Internet Protocol (IP)
      - Internet standards are developed by the Internet Engineering Task Force (IETF).
        
        The IETF standards documents are called request for comments (RFCs)
      - Devices like TVs, cars, watches, and more are called hosts or end systems.
    - - an infrastructure that provides services to applications
      - Distributed applications: involve multiple end systems that exchange data with each other.
        
        runs on end system instead of running on packet switches in the network core
      - End systems attached to the Internet provide a socket interface that specifies how a program running on one end system asks the Internet infrastructure to deliver data to a specific destination program running on another end system.
        
        a set of rules that the sending program must follow so that the Internet can deliver the data to the destination program.
    - - A protocol defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event
  - - - packets: smaller chunks of data
      - Store-and-Forward Transmission: the packet switch must receive the entire packet before it can begin to transmit the first bit of the packet onto the outbound link.
      - Queuing Delays and Packet Loss
- - - - set of header field values
      - set of counters
      - set of actions to be taken
    - - Forwarding
      - Dropping
      - Modify-field
  - - - the Internet’s network-layer packet is referred to as a datagram
        
        Version number: By looking at the version number, the router can determine how to interpret the remainder of the IP datagram.
        
        Header length: the typical IP datagram has a 20-byte header
        
        Type of service: included in the IPv4 header
        to allow different types of IP datagrams to be distinguished from each other
        
        Datagram length: This is the total length of the IP datagram (header plus data), measured in bytes. 16 bits can have a maximum value of 1111111111111111 = 65,535 = 2^16
        
        Identifier, flags, fragmentation offset: IP fragmentation, when a large IP datagram is broken into several smaller IP data-
        grams which are then forwarded independently to the destination, where they are reassembled before their payload data (see below) is passed up to the transport layer at the destination host
        
        time-to-live (TTL): ensure that datagrams
        do not circulate forever
        
        Protocol: used only when an IP datagram reaches its final
        destination. The value of this field indicates the specific transport-layer protocol to which the data portion of this IP datagram should be passed
        
        Header checksum: The header checksum aids a router in detecting bit errors in a received IP datagram
        
        why does TCP/IP perform error checking at both the transport and network layers?
        
        Source and destination IP addresses
        
        Options
        
        Data (payload)
    - - The boundary between the host and the physical link is called an interface. An IP address is technically associated with an interface, rather than with the host or router containing that interface
      - dotted-decimal notation
      - A subnet is also called an IP network or simply a network
        
        subnet mask indicates that the leftmost 24 bits of the 32-bit quantity define the subnet address
      - Classless Interdomain Routing (CIDR—pronounced cider): As with subnet addressing, the 32-bit IP address is divided into two parts and again has the dotted-decimal form a.b.c.d/x, where x indicates the number of bits in the first part of the address
      - classful addressing
      - Dynamic Host Configuration Protocol (DHCP)
- - - - Confidentiality
        
        encrypting and decrypting data
      - Message integrity
        
        checksumming
      - End-point authentication
      - Operational security
        
        A firewall sits between the organization’s network and the public network, controlling packet access to and from the network.
        
        An intrusion detection system performs “deep packet inspection,” alerting the network administrators about suspicious activity.
      - Intruder can performe
        
        eavesdropping 竊聽
        
        modification, insertion, or deletion of message 竄改
  - - - Caesar cipher: substituting the letter that is k letters later
      - monoalphabetic cipher: letter can be substituted for any other letter, as long as each letter has a unique substitute letter
      - attack types
        
        When an intruder knows some of the (plaintext, ciphertext) pairings, we refer to this as a known-plaintext attack on the encryption scheme
        
        In a chosen-plaintext attack, the intruder is able to
        choose the plaintext message and obtain its corresponding ciphertext form
        
        statistical analysis can help in a ciphertext-only attack on an encryption scheme
      - Block Ciphers
        
        used in many secure Internet protocols, including PGP (for secure e-mail), TLS (for securing TCP connections), and IPsec (for securing the network-layer transport)
        
        the message to be encrypted is processed in blocks of k bits.