Transmission Control Protocol
(Part I)
LESSON 15

Connection-Oriented Transport:

• Transmission Control Protocol (TCP):
  • Is an independent, reliable-delivery, general purpose transport protocol used in the TCP/IP protocol suite or for use with other delivery systems.

Reliable Delivery Service:

• Features of the interface between application programs and the TCP/IP reliable delivery service include:
  • Stream Orientation - data is considered as a bitstream divided into bytes.
  • Buffered Transfer - transport mechanisms buffer application data until it can fill a reasonably large datagram, using PUSH for immediate transfer.
  • Virtual Circuit Connection - applications view the delivery service as a dedicated hardware circuit.
  • Unstructured Stream - applications must understand the streams content.
  • Full-Duplex Connection - simultaneous transmission in both directions; without simultaneous connection release (half-duplex).

Providing Reliability:

• Sending positive acknowledgments (ACKs) with retransmission is a fundamental technique used to provide reliable transfer.
• A timer is started during each transmission and if it expires, the message is then retransmitted.
• A combination of several timers are all used to provide a reliable delivery system.

• To avoid confusion over delayed or duplicate ACKs, positive ACK protocols use sequence numbers with ACK messages to correctly associate acknowledgments with packets.
• These sequence numbers can be unique at each side, and are established at connection setup time.

Retransmission Schemes:

• There are three main classes of retransmission schemes:
  • Stop and Wait protocols (previous illustration) which are not bandwidth efficient.
  • Go-Back-N protocols which retransmit all packets that follow a single erred packet.
  • Selective Retransmission protocols which only retransmit erred packets using ACKs and NAKs (TCP).

Sliding Window Protocols:

• Sliding Window Protocols:
  • Used in TCP to make the transmission stream efficient
  • Keep track of many acknowledged and un-acknowledged packets, instead of one at a time.
• Sliding window protocols:
  • Use network bandwidth better because they allow the sender to transmit multiple packets before waiting on each ACK.
  • Transmit all packets in the window and slide it forward for each ACK received.
• Unacknowledged packets are constrained by the windows size (limited to a small fixed number).
• Sliding window performance depends upon the window size and speed the destination processes the packets.
• Well tuned sliding window protocols keep the network saturated with packets (zero idle time).

Transmission Control Protocol:

• Transmission Control Protocol - defines the reliable stream service of the TCP/IP protocol suite.
• TCP is a protocol, not a piece of software! An implementation of TCP is not the standard.
• TCP defines:
  • Format of the data and acknowledgments.
  • Procedures for hosts to ensure reliable transmission.
  • Connection establishment and release.
  • How to distinguish between multiple destinations on a given machine.

Connections, Ports, and Endpoints:

• Unlike UDP, TCP does not use a port number to connect to a single object.
• TCP uses the connection in its entirety as it’s fundamental abstraction.
• Connections are identified by a pair of endpoints.
• Endpoints are a pair of integers (HOST, PORT), where:
  • HOST is the host’s IP address.
  • PORT is the TCP protocol port on that host.
• Ex:
  • MIT (18.26.0.36, 1069) to
  • Purdue (128.10.2.3, 25)
  • CMU (128.2.254.139, 1184) to
  • Purdue (128.10.2.3, 25)
• TCP port numbers can be shared by multiple connections on the same machine because each connection is identified by a pair of end points.

Passive and Active Opens:

• Passive Open - function performed by an application program to inform the operating system it can accept connections.
• Active Open - function performed by an application program to inform the operating system it want to initiate a connection.
• A segment is TCP’s unit of transfer.
• Ideally, each segment travels in a single IP datagram.
• TCP uses a special sliding window to ensure efficient transmission and handle flow control.
• It operates at the octet level, using three pointers for every connection.

• A window for data being sent and data being received exists at each end.
• Four windows exist in total for each connection.
• TCP allows the window size to vary over time.

Window Advertisement:

• Window Advertisement - included with each ACK, specifies the number of additional octets the receiver can accept and is prepared to do so.
• TCP uses the window advertisement feature to handle end-to-end flow control, but it cannot explicitly solve a congestion problem.

TCP Segment Format:

• SOURCE PORT (16-bits):
  • Identify source application end-points.
• DESTINATION PORTS (16-bits):
  • Identify destination application end-points.
• SEQUENCE NUMBER (32-bits):
  • Order in the data stream of the first byte in the data field.
• RESERVED (6-bits):
  • Self explanatory.
• ACKNOWLEDGEMENT NUMBER (32-bits):
  • Specifies which octet the sender expects to receive next.
• HLEN (4-bits):
  • The length of segment header in 32-bit words.
• CODE BITS (6-bits):
  • Determines how to interpret the other header fields.
• Code-Bit Bit Position:
  • URG - Urgent pointer field is valid.
  • ACK - Acknowledgement field is valid.
  • PSH - This segment requests a push action.
  • RST - Reset the connection.
  • SYN - Synchronize sequence numbers.
  • FIN - Sender has reached the end of it’s byte stream.

• WINDOW (16-bits):
  • The number of bytes the receiver is willing to accept (remote buffer size available).
• URGENT POINTER (16-bits):
  • The offset to be added with the sequence number field to yield the sequence number of the last byte of urgent data (i.e. interrupts or aborts).
  • This accommodates out-of-band signaling.
• TCP goes into "urgent mode" when important data arrives regardless of it’s position in the octet stream.
• The URG code bit must be set to utilized the urgent pointer and out-of-band signaling.

• OPTIONS (variable length):
  • Each option field begins with an 8-bit (1 byte) field KIND that specifies the option type.
• PADDING (variable length):
  • Header area used to compensate for variable length option fields.
• DATA (variable length):
  • Data passed from application layer protocols.

Maximum Segment Size:

• Maximum Segment Size (MSS):
  • Option is used to allow computers to limit the segment sizes appropriate to the size of their available buffer space.
  • It is important to select a MSS that will fill packets, making good use of available bandwidth.
• Standard MSS = 536
• IP datagram standard = 576
• This allows 20 bytes for the IP header and the TCP header.

Optimum Segment Size:

• Optimum segment size is difficult to find because:
  • TCP doesn’t include a mechanism for doing so.
  • Gateways dynamically change routes which usually have different MTU sizes.
  • Size depends on lower-level protocol headers and must be reduced accordingly.

TCP Header Checksum:

• Checksum - like UDP, TCP also uses a pseudo-header when computing the checksum, verifying data and the TCP header integrity.
• The checksum algorithm uses a 16-bit arithmetic and takes the one’s complement of the one’s complement sum.
• When IP receives a datagram destined for TCP:
  • It must pass to it the source and destination IP addresses, so TCP can recompute the checksum (pseudo-header fields).
  • This scenario also violates our layered abstraction concept and the independent functionality of each layer.

TCP Pseudo-Header:

• TCP Pseudo-Header:
  • Used to verify that the TCP segment has reached it’s correct destination (since TCP headers only use port numbers).
• A checksum is computed over the pseudo-header, TCP-header, and the data area before sending each segment.

• The twelve octets of the pseudo-header include the fields:
  • Source IP address (4-octets).
  • Destination IP address (4-octets).
  • Padding field filled with zeros (1-octet).
  • Protocol Type (1-octet), TCP = 6.
  • TCP Length (2-octets)
    • Length of the TCP segment without the pseudo-header.
• These values are extracted from the IP datagram header, then the checksum is recomputed (using this pseudo-header).
• TCP as well as UDP violates the protocol boundary between it and the IP (Internet) Layer.

TCP Input and Output:

• TCP has complex input and output modules since it is connection-oriented.
• TCP, similar to UDP, has separate modules to handle the processing required for each I/O operation, as well as input and output queues.
• There are two output sub-modules, TCP Process and TCP Timer, which communicate using control messages and provide a reliable communication service.

All work was written, produced, and is copyrighted by Daniel Z. Tabor Jr.
Page created by Daniel Z. Tabor Jr. Copyright ©1995 Illusion Industries Inc.