TCP Protocol Notes

Stream of Bytes and Sequence Numbers

• Host A (source) communicates with Host B (destination) using TCP.
• TCP data is transmitted with a TCP header.
• ISN (Initial Sequence Number): The initial sequence number for the connection.
• Sequence number: Sequence number of the first byte.
• ACK sequence number: The next expected byte.

TCP Byte Service

• Application processes communicate using TCP.
• TCP uses send and receive buffers.
• Write: Bytes are written to the send buffer.
• Read: Bytes are read from the receive buffer.
• Data is transmitted in segments.
• TCP is a full-duplex protocol, meaning data and acknowledgements flow in both directions.

TCP Segment

• TCP segment encapsulated within an IP packet.
• The size of the IP packet must not be bigger than the Maximum Transmission Unit (MTU).
• Example: Up to 1500 bytes on an Ethernet network.
• The TCP header is typically 20 bytes long.

TCP Segment Header Format

• The TCP segment header includes the following fields:
  • SrcPort: Source port.
  • DstPort: Destination port.
  • SequenceNum: Sequence number.
  • Acknowledgment: Acknowledgment number.
  • HdrLen: Header length.
  • Flags: Control flags.
  • AdvertisedWindow: Advertised window size.
  • Checksum: Checksum for error detection.
  • UrgPtr: Urgent pointer.
  • Options: Variable options.
  • Data: The actual data.

Header Fields in a Segment

• <SrcPort, SrcIPAddr, DestPort, DestIPAddr>: Defines a TCP connection.
• SequenceNum: Denotes the location of the first byte in the segment.
• Flags: A 6-bit field containing control flags:
  • SYN: Synchronize, used to establish a connection.
  • FIN: Finish, used to terminate a connection.
  • ACK: Acknowledgment, signifies an acknowledgement.
  • URG: Urgent data, indicates urgent data.
  • UrgPtr: Points to where the urgent data ends.
  • PUSH: Notifies the application to deliver data immediately.
  • RESET: Indicates an error occurred and the connection needs to be aborted.

Simplified TCP Flow

• Data flows toward the receiver (full duplex, with flow in the opposite direction).
• Acknowledgements (Acks) and window size flow in the opposite direction.

Connection Establishment: Three-Way Handshake

• Involves the exchange of three messages.
• Active participant (client) initiates the connection.
• Passive participant (server) listens for connection requests.
  1. Client sends SYN, Sequence Num = x.
  2. Server responds with SYN + ACK, Sequence Num = y, Acknowledgment = x + 1.
  3. Client sends ACK, Acknowledgment = y + 1.

Client's SYN Packet

• Contains the following fields:
  • Client's Port
  • Server's Port
  • Client's ISN (Initial Sequence Number)
  • Acknowledgment
  • SYN flag is set.
  • Other header fields: AdvertisedWindow, Checksum, UrgPtr, Options, Data.

Server's Reply Packet

• Contains the following fields:
  • Server's Port
  • Client's Port
  • Server's ISN
  • Acknowledgment = Client's ISN + 1
  • SYN and ACK flags are set.
  • Other header fields: AdvertisedWindow, Checksum, UrgPtr, Options, Data.

Client's ACK Packet

• Contains the following fields:
  • Client's Port
  • Server's Port
  • Sequence Num
  • Acknowledgment = Server's ISN + 1
  • ACK flag is set.
  • Other header fields: AdvertisedWindow, Checksum, UrgPtr, Options, Data.

Initial Sequence Number (ISN)

• Sequence number for the very first byte of the session.
• Why not a default ISN of 0?
  • Practical issue: IP addresses and port numbers uniquely identify a connection. The same port numbers may get used again, and an old packet might still be in flight, potentially associated with the new connection.
  • Security reasons: To prevent TCP hijacking.
• TCP requires changing the ISN over time.

Tearing Down the Connection

• Closing the connection:
  • FIN (Finish) is sent to close and receive remaining bytes.
  • The other host sends a FIN ACK to acknowledge.
  • RST (Reset) is sent to close and not receive remaining bytes.

Flow Control: Sliding Window in TCP

• Goals of sliding window:
  • Guarantee end-to-end reliable data delivery and in-order delivery.
  • Flow control between sender and receiver to avoid receiver buffer overflow.
• Key variables:
  • LastByteAcked: Last byte acknowledged.
  • LastByteSent: Last byte sent.
  • LastByteWritten: Last byte written.
  • LastByteRead: Last byte read.
  • NextByteExpected: Next byte expected.
  • LastByteRcvd: Last byte received.
• Relationships:
  • $LastByteAcked \leq LastByteSent \leq LastByteWritten$
  • LastByteRead < NextByteExpected \leq LastByteRcvd + 1
• Receive window: MaxRcvBuffer
• Transmit window: advertisedWindow

Flow Control in TCP

• Receiver advertises window no larger than its buffer size:
  • $LastByteRcvd – LastByteRead \leq MaxRcvBuffer$
  • $AdvertisedWindow = MaxRcvBuffer – ((NextByteExpected – 1) – LastByteRead)$
• Example:
  • $LastByteRcvd = 5, LastByteRead = 1, NextByteExpected = 6, MaxRcvBuffer = 10$
  • $5 – 1 \leq 10$
  • $AdvertisedWindow = 10 - ((6 – 1) – 1) = 10 – 4 = 6$
• Sender adheres to the AdvertisedWindow:
  • $LastByteSent – LastByteAcked \leq AdvertisedWindow$
  • $EffectiveWindow = AdvertisedWindow – (LastByteSent – LastByteAcked)$
• Example:
  • $sent = 7, ACK = 5, EffectiveWindow = 6 – (7 - 5) = 4$
• Maximum number of outstanding packets/bytes.

Sequence Number Wraparound

• AdvertisedWindow: 16 bits long, up to $2^{16}$.
• Sequence number field: 32 bits long, up to $2^{32}$.
• SN >> AdvertisedWindow, so there would be no problem in ordering.
• How fast are sequence numbers exhausted:
  • T1 (1.5Mbps): $2^{32} x 8 / 1.5 x 10^6 \approx 6.4hrs$
  • Ethernet (10 Mbps): 57 mins
  • T3 (45Mbps): 13 mins
  • Fast Ethernet (100 Mbps): 6 mins
  • OC-3 (155Mbps): 4 mins
  • OC-12 (622 Mbps): 55 sec
  • OC-48 (2.5 Gbps): 14 sec

Failure due to Seq # Wraparound

• Sequence number wrap-around on the current connection.
• A sequence number may be "wrapped" (cycled) within the time that a segment is delayed in queues.
• Earlier incarnation of the connection.
• A delayed segment from the terminated connection could fall within the current window for the new incarnation and be accepted as valid.
• Solution: Add a timestamp field on every packet.