Explain TCP Operations

Protocol that ensures reliability in a transmission with minimum loss of packets

• Duties
  • Assure packets maintain the right order
  • Error Checking
  • Delay is kept at an acceptable level
  • Prevent possibility of packet duplication
• Ensure data received is consistent, in order, complete and smooth
• OSI – Transport Layer (4)
  • Works before IP
  • Data gets bundled inside of TCP packets before sending to IP which encapsulates into IP packets

TCP Segment Structure

• PDU – Protocol Data Unit
• Consists of Header and Data section
  • Header – 10 mandatory fields
    • Source port – 16 bits – ID’s sending port
    • Destination Port – 16 bits – ID’s receiving port
    • Sequence Number – 32 bits – used for 3 way handshake, SYN flag
    • Acknowledgement Number – 32 bits
    • Data offset – 4 bits –
    • Reserved – 3 bits
    • Flags – 9 bits (Control bits)
    • Window Size – 16 bits – size of the receive window. Specifies number of window size units (bytes) sender on the segment is willing to receive
    • Checksum (16 bits) – error checking of header and data
    • Urgent point – 16 bits
    • Options – Variable 0-320 bit divisible by 32
    • Padding – Ensure TCP header ends and data begins on a 32 bit boundart. All zeros

Connection Establishment

• 3 way handshake – Attempt connection between client and server before sending data
  • SYN – Active open is performed by the client to the server
  • SYN-ACK – Responseto SYN from server to client
  • ACK – Final response from client to server that SYN-ACK was received
• Full duplex communication is established

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IPv4 PTMU

Determine the size of the network path between 2 IP hosts

• Goal is to avoid fragmentation
• Intended for routers. All modern OS’s use it on endpoints

Implementation

• Works by setting DF bit in outgoing IP headers
• Any device along path with smaller MTU sends ICMP – Fragmentation Needed (Type 3, Code 4)
• Process is continued until smallest MTU is found to send packets without fragmentation

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IPv6 PMTU

Determine the size of the network path between 2 IP hosts

• Explicitly delegated to endpoints
• Routers do not support fragmentation
• ICMPv6 – Packet Too Big (Type 2)

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MSS – Maximum Segment Size

• Largest amount of data (in bytes) a host can receive in a single TCP segment
  • Does not count TCP or IP header
• Defined to be the relevant IP datagram minus 40
• Min MTU – 40 = MSS
• IPv4 hosts required to handle MSS of 536 (=576 – 20 – 20)
• IPv6 hosts required to handle 1220 (=1280 – 40 – 20)
• MSS specified as TCP option
  • Send In SYN packet during TCP handshake
  • MSS cannot be changed after connection is established

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Latency

Time interval between point A and point B. Product of delay from physical distance packets traverse a medium

• Measured
  • One-way – Time from source sending a packet to destination receiving the packet
  • Round-trip Delay – Time from Source to Destination and Destination back to Source
• Ping can provide latency, but isn’t fully accurate as ICMP can be treated differently if traffic shaping is applied
• Accurate measurements can come from specific software

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Windowing

TCP Windowing

• Amount of unacknowledged data that can be in transit at any given time
• Referred to as Window Size – 16 bit field in TCP header

Window Scaling

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Bandwidth Delay Product

• Product of data links capacity, Bandwidth (bps) and RTD Time (RTT) (seconds)
• Measured in bits or bytes
• Max amount of data on the network at any given time.
• https://en.wikipedia.org/wiki/Bandwidth-delay_product

 

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Global Synchronization

Can occur during periods of congestion because each sender will reduce their transmission rate at the same time when packet loss occurs

Simplest queuing technique

• Tail Drop – Allow queue to fil to max size and then discard any packets until there is space again
• Problem occurs when there is bursty traffic and the queue is full
  • Full queue results in high latency
  • Introduction of sudden burst of traffic may cause large number of established streams to lose packets simultaneously
• Recovery Mechanism
  • TCP recovers from dropped packets which is interprets as congestion
  • Senders reduce sending rate for period of time
  • Known as slow start algorithm
• Tail drop is leading cause of the problem
  • RED (Random Early Detection) and WRED (Weighted RED) reduce likeliness of global synchronization

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Options

3 Types of TCP options

1. Option-Kind – 1 byte
2. Option-Length – 1 byte
3. Option-Data – variable

Sent in the SYN packet