Mindmap

203 - Networks

Jarl Jackson Englisch

Inhalt der Mindmap

203 - Networks

Network Structure
- Hosts: Client & Server
  - Sends Packets of data
- Access networks (Physical Layer)
  - Wired Links, Wireless Links
  - Coaxial Cable, Fibre Optic Cable
- Network Core (Interconnected Routers, Network of networks)
- Packet Switching
  - Store and forward (Entire packet must arrive at router before it can be transmitted)
  - Queuing and loss
    - If arrival rate (in bits) to link exceeds transmission rate of link for a period of time: • Packets will queue, wait to be transmitted on link • Packets can be dropped (lost) if memory (buffer) fills up
  - Routing (Determines source-destination of packets)
    - Forwarding (Move packets from routers input to appropriate output)
  - Allows more users to use the network, Great for bursty data
  - Circuit Switching
    - Dedicated resources (No Sharing)
    - End-end resources allocated
- Client-Server
  - Server: Always on host, permanent IP address
    - Clients: May have dynamic IP, Do not communicate
- Peer to peer
  - Peers request service from other peers
  - Self Scalability
  - Arbitrary end systems directly communicate
  - Bit torrent
    - Requesting Chunks - At any given time, different peers have different subsets of file chunks
    - Sending Chunks - Send Chunks to the 4 peers currently sending at highest rate
    - Advantages: Download a bit slow in the beginning but speeds up considerably
    - Disadvantages: Low bandwidth, Rarest first needs large number of chunks
Delay
- Processing Delay
  - Time taken for a device to examine a packets header (Check for bit level errors) (Microseconds long)
- Queuing Delay
  - Time spent waiting in queue to leave the device, dependant on the congestion level of the router
- Transmission Delay
  - Defines the amount of time required to push (Transmit all packets bit into the link)
- Propagation Delay
  - Once transmitted, bits need to propagate and dependant on the physical medium used this will cause delay (From one device to the next)
- Together known as nodal delay
Throughput (Rate at which bits transferred between sender/receiver)
- Measuring throughput via IPERF (Runs in a client-server model)
Packet Loss
- Occurs when a node drops or discards packets
- Reduces the useable throughput of a device
- Can occur when a buffer is full (Has a close relation with Queuing Delay)
Internet Protocol Stack
- Application
  - Transport
    - Network
      - Data Link
        Physical
        Coaxial Cable, Fiber Optics, Twisted Pair, Infrared, Radio
        Ethernet, Wifi, DOCSIS
        Framing (Encapsulate datagrams into frames), Error checking and reliability, media access controls
        CRC (Cyclic redundancy check - Uses field theory to compute a semi-unique value for a given message)
        All Single and duplicate errors, all errors with odd bit numbers, all burst errors up to 16 length
      - IP, IPV6
        IPv4 - 32 Bit addresses (Each number is a byte)
        A Class (2^24)
        B Class (2^16)
        C Class (2^8)
        Datagrams are like a letter (Totally self-contained)
        IPV6 First introduced in 1998, 128 Bit adresses
        8 Groups of 16 bit values
        Double the size of IPV4
        Changing over is a whole internet upgrade
        Negatives, Blacklists wont work on IPv6
      - Logical communication between hosts
      - CIDR (Classless Inter-Domain Routing)
        Key ideas: Flexible division between network and host addresses
        Get rid of IP classes, Network prefix can be any size
        + Subnetting improves routing scalability
        Subnetting (Device interfaces with the same netwokr part of IP adress
    - TCP, UDP
      - Data integrity, Throughput, Timing, Security
        TCP - Reliable Transport, Flow control, Congestion control, Connection-oriented
        Point to points, No message boundaries, pipelined
        Full Duplex data - Connection-oriented (Handshaking) & Flow controlled
        RTT (Round Trip Time)
        Time out interval is RTT plus saftey margin
        Client, Server each close their side of connection
        UDP - Unreliable transfer groups of bytes (Datagrams) between client and server
        Checksum
        Sender: Treat segment contents including header fields as sequence of 16-bit integers, Sender puts checksum into UDP checksum field
        Recevier: Compute checksum of received segment, check if computed checksum equals checksum field value
    - Logical communication between processes
      - Connection Establishment, Logical end-end conncation
      - Error Control
      - End-to-end Flow Control
      - Congestion control
      - Multiplexing/de-multiplexing applications
        Multi - Handle data from multiple sockets (Add transport header)
        De-Multi - User header info to deliver received segments to correct socket
  - FTP, HTTP, SMTP, DNS
  - Presentation
    - Session
      - Sync, Checkpointing, recovery of data
    - Allow applications to interpret meaning of data
  - Secure Socket Layer (SSL)
    - Provides Encrypted TCP Connections
- Benefits: Manage Complexity, Modularity, Extensibility
HTTP - Web application layer protocol (Client browser that requests, receives (USING HTTP), Server SENDS (USING HTTP)
- HTTP is stateless
- Non-persistent HTTP - At most one object sent over TCP, Downloading multiple objects required multiple connections
- Persistent HTTP - Multiple objects can be sent over single TCP connection between client and server
- Method Types
  - HTTP/1.0 - GET, POST, HEAD
  - HTTP/1.1 - GET,POST,HEAD,PUT,DELETE
- Cookies
  - Cookie header line of HTTP response Messages
  - Cookie header line in next HTTP request message
  - Cookie file kept on users host, managed by users browser
  - Used for authorization, Shopping carts, recommendations, User sessions
Web caches
- Satisfy Client Request without involving origin server
- Acts as both client and server
- Reduces response time for client request, reduce traffic on an institution
- Conditional GET, dont send object if cache has up-to-date cached version
- Improved Link Utilisation, Improved Access Delay (Caches located close to users)
DNS
- Distributed Database implemented in hierarchy of many name server
- Application Layer Protocol
- Why not to centralize
  - Single Point of failure, Traffic Volume, Maintenance
- SubDomain (One domain is a subdomain of another if its domain name ends in the others domain name (Lancs.ac.uk), .ac.uk - .uk
- Fully Qualified Domain Names (FQDN) - Implies rooted at top of DNS, not further resolution needed
  - TLD (Top-Level Domain) - Responsible for com, org, net
  - Authoritative DNS servers - Manged by a company, authorise hostname to IP mappings in a company
- 13 Root servers: To addressess for full set fit in a single protocol message (Assuming 512 Byte UDP) - IPV6 Supports more
- Iterative Query (Contacted Server, replies with name)
- Recursive Query (Puts burden of name resolution on contacted name server
- Once a name server learns mapping it caches the mapping (Cache entries timeout after some TTL)
  - Cached entries may be out-of-date
- RR Format (Name, Value, type, ttl)
- Attacking DNS
  - DDOS Attacks (Bombard root server with traffic)
  - Man in the middle
  - Exploit DNS for DDOS (Send Queries with spoofed source:target IP)
  - Solution (DNSSEC)
    - Cryptographically sign critical resource records, TYPE: DNSKEY (Zone Domain name, Public key for the Zone) RRSIG (Type, Name, Cryptographic signature of the query results)
Email
- 3 Major Components
  - Mail Servers
    - Server: Receiving Mail
    - Client: Sending mail
  - User Agents
    - Mail Reader
    - Composing Editing, reading mail
  - SMTP (Simple Mail Transfer Protocol
    - Still in widespread use, Body must be simple 7bit ASCII, OLDER than HTTP
    - Direct Transfer between sending server to receiving server
    - Handshake (Greeting, Transfer of message, Closure
    - SMTP uses persistent connections
      - Requires header and body to be in 7-bit ASCII
      - Uses, CRLF.CRLF to determine end of messages
- Sending does not user intermediate mail servers
- POP (Post office protocol (Authorization, Download
  - POP3 (Authorization Phase, Transaction Phase)
    - "Download and keep", allows copies of message to be kept on different clients
    - Stateless across sessions
- IMAP (Internet Mail Access Protocol (More features, including manipulation of stored messages
  - Keeps all messages in one place (At server) - Allows organisation of messages into folder, Also allows parts of a message to be retrieved
- HTTP (Gmail, Hotmail)
Congestion Controls
- Informally (Too many sources sending too much data too fast for the network to handle
- CWND (Constraint on the rate at which the TCP sender can send
  - Amount of unacknowledged data should not exceed the minimum of CWND or RWND
  - MSS (Maximum Segment Size (TCP header field specifies the largest amount of data in a TCP segment
- Approach (Sender increases transmission rate, probing for usable bandwidth, until loss occurs
  - Additive increase (Increase cwnd by 1 MSS every RTT untill loss detected
    - Multiplicative decrease (Cut cwnd in half after loss)
- Send CWND bytes, wait RTT for ACKS then send more bytes
- Versions
  - TCP Tahoe (Original version, Slow start, Fast retransmit)
    - Initial rate is slow but ramps up exponentially fast
      - Switch to linear once ssthresh has been reached
        Initialised to a large value to start (Set too 1/2 CWND before timeout)
  - TCP Reno (Same as Tahoe, but fast recovery)
    - Fast recovery, tcp retransmits the missing packets
  - TCP Vegas (New, based on delay instead of packet loss)
  - Loss indicated by 3 duplicate ACKS
- Congestion Avoidance
  - TCP begins when reaches a near congestion point
  - Linear rather than exponential growth in this phase
- TCP fair, additive increase gives slope of 1, as throughput increases, Multiplicative decrease decreases throughput proportionally
- ECN, Explicit Congestion Notification (Two bits in IP header (TOS header) marked to indicate congestion
DHCP - NAT
- Dynamic Host Configuration Protocol
  - Can renew its lease on address in use, allows reuse of addresses
  - Assigns Unique IP addresses
  - Server (Assigned to a network, Configuration parameters
    - Databases - 1st database Manual IP, 2nd Database Pool of addresses
  - Returns more than just IP addresses - Name and IP of DNS server, Network Mask, Address Of first hop
  - Manual Lease, Automatic Lease, Dynamic Lease
  - Pros
    - Relieves the network admin of manual config
    - Device can be moved network to network, auto obtain valid config
  - Negatives
    - Server issues (A machine to host DHCP has to be continually
    - Security Problems - Uses UDP
- Basic Idea, Local network uses just one IP address as far as outside world
  - Outgoing Datagram replace every outgoing to NAT IP Address
  - Incoming datagrams, replace all NAT ip adresses with corresponding IP addresses stored in NAT table
  - 16 Bit port number field
  - Routers should only process up to layer 3, what if a client wants to connect to server behind NAT
ARP (Address Resolution Protocol) - Each adapter on LANS has unique address
- MAC - 48 Bit MAC address burned in NIC ROM, also sometimes software settable
  - Used locally to get frame from one interface to another
  - Issued by IEEE
- ARP Table, Each IP node has a table
Ethernet
- Based on ALOHA network
- 802.3 U - 100Mbps, 802.3Z - 1Gbps, 802.3ae - 10Gbps
- Star network, Switch in the center
Switches
- Bridges (Forwarding of frames, Each bridge maintains a forwarding table
- Deletes old entries after a timeout
  - Manual config is possible but time consuming, error prone
- Switches can be connected together

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Beschreibung: Mindmap am 203 - Networks, erstellt von Jarl Jackson am 25/04/2019.
Erstellt von: Jarl Jackson · Apr 2019
Aktualisiert: 7 Mai 2019
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203 - Networks

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