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TCP

Characteristics

  • Point-to-Point
    • One sender, one receiver
  • Ordered byte stream
  • Pipelined
    • Congestion and flow control
      • Sender and receiver buffers
    • Set window size
  • Connection-oriented
    • TCP handshake (syn, syn-ack, ack)
    • Consistent handshaking/communication
  • Full duplex data
    • Bi-directional data flow in same connection
  • MSS (Maximum segment size)
    • Max size of application layer data
    • Determined by link size (maximum transfer unit)

TCP SEGMENT

  • 20 byte header
  • Source port #, dest port #
  • Sequence number & acknowledgement number (counting by bytes, not segments)
  • RST, SYN, FIN commands
  • Receive window (# bytes receiver willing to accept)
  • Options & data (variable length)

Phases

Handshake

  • Client sends TCP SYN segment
    • Specifies initial seq #
    • No data
  • Server receives SYN, replies with SYNACK segment
    • Server allocates buffers
    • Specifies server initial seq #
  • Client receives SYNACK, replies with ACK
    • May contain data
    • Client allocates

Seq, ACKs (data transfer)

  • Seq # is first byte in a segment's data
    • (nowadays) Randomized and unpredictable to prevent attackers from injecting malicious packets
  • ACKs: seq# of next byte expected
  • ACK packets don't consume seq#
  • SYN, FIN do consume seq# despite having no bytes on payload
  • Data segments consume segment #'s

Closing a connection

  • 0x011 (FIN, ACK) is sent by local
    • Server responds with 0x011 (FIN, ACK)

Timeouts

  • Data received from the app creates a segment + seq #
    • Timer is started
  • Timeout:
    • rtxm segment that caused timeout
    • Timer restarted
  • ACK received
lost ACK ex:
A sends data
B receieves, sends it back, but the packet is lost
A times out, resends data

premature timeout ex:
A sends Seq 92 and seq 100
B sends back ACK 100 and ACK 120
However, takes too long and A resends Seq 92
B throws away duplicate, resends ACK 120

cumulative ACK ex:
A sends Seq 92
B sends ACK 100, but it gets lost
A sends Seq 100 at same time
B sends ACK 120
RECEIVER EVENT TCP ACTION
Arrival of in-order segment w/ expected seq #, previous data already ACKed Delayed ACK 500ms
Arrival of in-order segment w/ expected seq #. One other segment has ACK pending Immediately send cumulative ACK
Arrival of out-of-order segment higher-than-expected seq #. Gap detected Immediately send duplicate ACK (expected byte)