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Transport Layer (Congestion Control (TCP Approach (Incremental congestion…

- - - - Don't reuse Maximum Segment Lifetime sequence numbers within twice the MSL (2min)
        
        Keeps track of order of segments between two hosts
      - 3-way handshake for establishing connection (diagram)
        
        Extend 2^32 sequence number by using a timestamp
        
        Guard against duplicate connection requests and/or data
      - Sequence number space large enough that it will not wrap, even when sending at full rate
  - - - Either party issue DISCONNECT -> DISCONNECT TPDU -> transmission ends in both directions
      - May have data loss (diagram)
    - - Both parties issue DISCONNECT -> close only one direction at a time -> remain in receive mode
    - - Increasing #handshake does not solve this problem
      - Don't know if the last message was successfully sent
    - - 3-way handshake
      - Finite retry
      - Timeouts
    - - Final ACK lost -> receiving host times out
      - Lost Disconnect Request (DR) causes retransmission
      - Many lost DRs cause both hosts to timeout
  - - - Types
        
        Static
        
        standard allocated TSAPs/ports, embedded in OS
        
        Iana.org
        
        Directory Assistance - port-mapper
        
        A new service must register itself with the portmapper
        
        Give both service name and TSAP
        
        Mediated
        
        Process server intercepts inbound connections and spawns requested server and attaches inbound connection
- - - - Ability to specify ports for source and destination pairs
        
        Both source and destination ports required
        
        Destination allows initial routing for incoming segments
        
        Source allows reply routing for outgoing segments
        
        Header has ports, length and checksum
      - Provides an IP interface with multiplexing/de-multiplexing capabilities -> transmission efficientcies
    - - Does not include support for flow control, error control or retransmission of bad segments
    - - Voice over IP
      - Real-time streaming
    - - Send a message and get a reply
      - Allows local programs to call procedures on remote hosts
      - Client is bound to a small library (client stub) that represents the server procedure in the client's address space
      - Server bound with a procedure (server stub) that hides the fact that the procedure itself is not local
      - UDP with retransmissions is a low-latency transport
  - - - Sender + receiver create "sockets"; has IP address and port number; server binds port with IP address
      - Establish connection
      - One-way special server sockets may be used for multiple connections simultaneously ; multiplexing (diagram)
      - OR connect then give another port number for other clients to connect
    - - Classified into 3 segments
        
        Well-known ports
        
        Registered Ports
        
        Dynamic ports
      - Port numbers regulated by IANA (also IP address)
      - Port numbers range from 0 - 65535
    - - Full duplex: data in both directions simultaneously
      - Virtually point to point: exact pairs of senders and receivers
      - Byte streams: NOT message streams; message boundaries are not preserved
      - Buffer capable: can choose to buffer or not depending on context (PUSH / URGENT)
    - - Data transmitted in segments
      - Segment: fixed 20 byte header + >= 0 data bytes (0 bytes when establishing a connection)
      - Segments constrainted within IP payload limit or Ethernet frame size #
      - Sliding window protocol
        
        Sender transmit and start timer
        
        Receiver sends ACK with next sequence number expected
        
        If sender's timer ends before receiving ACK, resend original segment
      - Header
        
        Source + destination port: identity two end-points in communication
        
        Sequence number (byte stream!)
        
        Acknowledgement #
        
        Header length: how many 32-bits words contained in the header
        
        Grey part for further use
        
        Congestion Window Reduce (CWR) signal congestion when ECN (explicit congestion notification) is used; receiver sends an ECN-Echo signal to sender to slow down when it gets a congestion indication from the network
        
        Explicit Error Control (ECE) signal congestion when ECN (explicit congestion notification) is use; sender sends to receiver to say that it has slow down and can stop sending the ECN_Echo
        
        URG: if 1, use urgent pointer
        
        ACK = 1 if Ack number is valid; 0 to ignore the ACK number
        
        PUSH (PSH) = receiver should deliver the data immediately, don't wait under a full buffer has been received
        
        RESENT (RST) = used to abruptly reset a connection / reject an invalid segment / refuse an attempt to open a connection
        
        SYN = establish connections
        
        Connection request: SYN = 1; ACK = 0
        
        Connection reply: SYN = 1; ACK = 1
        
        Denote both CONNECTION REQUEST and CONNECTION ACCEPTED
        
        FIN = release connection
        
        No more data to transmit
        
        After closing a connection, the closing process may continue to receive data indefinitely (SYMMETRIC) #
        
        Window size: how many bytes that can be sent starting at byte acknowledged
        
        Checksum: extra reliability to ensure header, data is error free
        
        Urgent pointer: indicate the byte offset from the current sequence number at which the urgent data are to be found; pass that data down on a separate channel
        
        Options: provide extra facilities not covered by regular header
        
        Data
      - Connection establishment and release
        
        Three-way handshake
        
        Two simultaneous connection attempts result in only 1 connection; uniquely identified by end points
        
        Connections released asynchronously (symmetric release)
        
        Timers used for lost connection releases (three army problem)
      - Transmission policy (diagram)
- - - - Data link layer: physical point-point connection
      - Transport Layer: virtual connection between two hosts
  - - - Reliabiliity: packet loss
      - Delay
      - Jitter: variation in packet delay (diagram)
      - Bandwidth
    - - Determine expected transit time of a packet -> "shuffled" at each hop where slower packets sent first, faster packets wait in a queue
      - Important for Voice over IP
    - - Over-provisioning: more than adequate buffer, router CPU, bandwidth
      - Buffering: receive flows before delivery; increases delay, but smoothes out jitter; no effect in reliability / bandwidth
      - Traffic shaping: regulate average rate of transmission and burstiness of transmission
        
        Leaky bucket: finite internal queue; ensures outbound flow is always constant
        
        Token bucket: outbound flow depends on number of tokens in the bucket -> variable to maximum outbound flow
      - Resource reservation: reserve bandwidth, buffer space, CPU in advance
      - Admission control: decide based on traffic patterns whether to accept / reject / reroute packets
      - Proportional routing: different traffic types for the same destination is split across multiple routes
      - Packet scheduling: fair queuing, weighted fair queuing
      - Most effective is in Transport layer to ask the sender to slow down
  - - - Network capacity between sender and receiver
      - Receiver capacity
    - - Window described by the receiver
      - Congestion window
      - Max transmission rate is the minimum of the two windows
  - - - At connection establishment, window size is chosen by the receiver based on its buffer size
      - May still have network congestion
    - - Connection establishment -> sender initialises congestion window to the size of the max segment in use -> transmit one segment
      - Segment acknowledged before timer expires -> sender adds another segment's worth of bytes to the congestion window -> transmits two segments
      - Each new segment acknowledged -> congestion window is doubles -> #segments sent increases by power of 2
      - Continue until timeout or receiver's specified window is reached
    - - Same as slow start, but will only increase congestion window when ALL acknowledgements from previous sent has been received
      - num segments will increase with congestion window size