CCIE RS – Written – Network Principles – Explain TCP Operations

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


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


  • 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


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)

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


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


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

Bandwidth Delay Product


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


3 Types of TCP options

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

Sent in the SYN packet

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.