Transport Layer Notes

23.1 TRANSPORT LAYER

The transport layer is responsible for process-to-process delivery, using protocols like UDP, TCP, and SCTP.

Types of Data Deliveries

• Node-to-Node: Data Link Layer

• Host-to-Host: Network Layer

• Process-to-Process: Transport Layer

Responsibilities of the Transport Layer

• Port Addressing

• Segmentation and Reassembly

• Connection Control

• Error Control

• Flow Control

23.2 Port Addressing

Port numbers are used to identify specific processes on a host.

• IP address selects the host.

• Port number selects the process.

IANA Ranges for Port Numbers

• Well-known ports: 0 to 1023. These are assigned to common services.

• Registered ports: 1024 to 49,151. These can be used by applications.

• Dynamic ports: 49,152 to 65,535. These are used for temporary or private ports.

Socket Address

A socket address is a combination of an IP address and a port number. For example, 200.23.56.8:69.

Multiplexing and Demultiplexing

The transport layer uses multiplexing to combine data from multiple processes into a single stream, and demultiplexing to separate the data back into individual streams at the destination.

23.3 Error Control

Error checking occurs at the data link layer for paths within a LAN. For WAN paths, error checking is not performed by the data link layer.

23.4 Protocols in TCP/IP Suite

• Application Layer: SMTP, FTP, TFTP, DNS, SNMP, BOOTP

• Transport Layer: SCTP, TCP, UDP

• Network Layer: IGMP, ICMP, IP, ARP, RARP

• Data Link Layer

• Physical Layer

Underlying LAN or WAN technology.

23-2 USER DATAGRAM PROTOCOL (UDP)

UDP is a connectionless, unreliable transport protocol. It provides process-to-process communication, adding minimal overhead to IP.

Topics

• Well-Known Ports for UDP

• User Datagram Checksum

• UDP Operation

• Use of UDP

Well-Known Ports Used with UDP

In UNIX, well-known ports are stored in /etc/services.

Example 23.1:

FTP can use port 21 with either UDP or TCP. SNMP uses ports 161 and 162 for different purposes.

User Datagram Format

• Source Port Number: 16 bits

• Destination Port Number: 16 bits

• Total Length: 16 bits

• Checksum: 16 bits

• Data

UDP Length = IP Length - IP Header's Length

Pseudoheader for Checksum Calculation

The pseudoheader includes:

• 32-bit Source IP Address

• 32-bit Destination IP Address

• 8-bit Protocol (17 for UDP)

• 16-bit UDP Total Length

Padding is added to make the data a multiple of 16 bits.

Example 23.2: Checksum Calculation

Demonstrates checksum calculation for a small UDP datagram with 7 bytes of data. Padding is added for the calculation.

UDP Header Example

Given a UDP header in hexadecimal format: 0632000DOOlCE217

• Source Port Number: $0632<em>{16} = 1586</em>{10}$

• Destination Port Number: $000D<em>{16} = 13</em>{10}$

• Total Length: $01CE<em>{16} = 462</em>{10}$

• Data Length: Total Length - 8 (UDP header) = $462 - 8 = 454$

Client-to-server or vice versa can be determined by the port numbers. Well-known ports (e.g., 13) indicate a server.

Queues in UDP

UDP uses incoming and outgoing queues for both clients and servers.

23-3 TCP

TCP is a connection-oriented protocol that creates a virtual connection between two TCP endpoints for reliable data transfer. It uses flow and error control mechanisms.

Topics

• TCP Services

• TCP Features

• Segment

• A TCP Connection

• Flow Control

• Error Control

Well-Known Ports Used by TCP

Stream Delivery

TCP provides stream delivery, where data is sent as a continuous stream of bytes.

Sending and Receiving Buffers

TCP uses buffers for sending and receiving data. These buffers manage the flow of data between processes.

TCP Segments

Data is transmitted in segments. Each byte is numbered by TCP, starting with a randomly generated number.

Example 23.3: Sequence Numbers

• Segment 1: Sequence Number: 10,001 (range: 10,001 to 11,000)

• Segment 2: Sequence Number: 11,001 (range: 11,001 to 12,000)

• Segment 3: Sequence Number: 12,001 (range: 12,001 to 13,000)

• Segment 4: Sequence Number: 13,001 (range: 13,001 to 14,000)

• Segment 5: Sequence Number: 14,001 (range: 14,001 to 15,000)

The sequence number field defines the number of the first data byte in that segment. The acknowledgment field defines the number of the next byte a party expects to receive.

TCP Segment Format

• Source Port Address: 16 bits

• Destination Port Address: 16 bits

• Sequence Number: 32 bits

• Acknowledgment Number: 32 bits

• HLEN: 4 bits

• Reserved: 6 bits

• Control Flags: 6 bits (URG, ACK, PSH, RST, SYN, FIN)

• Window Size: 16 bits

• Checksum: 16 bits

• Urgent Pointer: 16 bits

• Options and Padding

Control Field Flags

• URG: Urgent pointer is valid

• ACK: Acknowledgment is valid

• PSH: Request for push

• RST: Reset the connection

• SYN: Synchronize sequence numbers

• FIN: Terminate the connection

Flag Descriptions

Connection Establishment (Three-Way Handshaking)

1. Client (Active open) sends SYN (seq: 8000)

2. Server (Passive open) sends SYN + ACK (seq: 15000, ack: 8001)

3. Client sends ACK (seq: 8000, ack: 15001)

Notes

• A SYN segment cannot carry data but consumes one sequence number.

• A SYN + ACK segment cannot carry data but consumes one sequence number.

• An ACK segment, if carrying no data, consumes no sequence number.

Data Transfer

Client sends data with ACK and PSH flags. The server acknowledges the data, and they exchange data segments.

Connection Termination (Three-Way Handshaking)

1. Client sends FIN (seq: x, ack: y)

2. Server sends FIN + ACK (seq: y, ack: x + 1)

3. Client sends ACK (seq: x, ack: y + 1)

Notes

• The FIN segment consumes one sequence number if it does not carry data.

• The FIN + ACK segment consumes one sequence number if it does not carry data.

Half-Close

One side terminates its sending ability while still receiving data.

Sliding Window

Used for efficient transmission and flow control.

Window Size = minimum (rwnd, cwnd)

Notes

• TCP sliding windows are byte-oriented.

• The size of the window is the lesser of rwnd and cwnd.

• The source does not have to send a full window’s worth of data.

• The window can be opened or closed by the receiver but should not be shrunk.

• The destination can send an acknowledgment at any time as long as it does not result in a shrinking window.

• The receiver can temporarily shut down the window; the sender, however, can always send a segment of 1 byte after the window is shut down.

• ACK segments do not consume sequence numbers and are not acknowledged.

Examples

• Example 23.4:
  Host B has a buffer size of 5000 bytes and 1000 bytes of unprocessed data.
  $rwnd = 5000 - 1000 = 4000$ bytes.

• Example 23.5:
  rwnd = 3000 bytes and cwnd = 3500 bytes. The window size is min(3000, 3500) = 3000 bytes.

• Example 23.6:
  cwnd = 20, rwnd = 9. The size of the sender window is min(20, 9) = 9 bytes.

Retransmission

Retransmission occurs if the retransmission timer expires or three duplicate ACK segments have arrived.

Notes

• No retransmission timer is set for an ACK segment.

• Data may arrive out of order, but TCP guarantees that no out-of-order segment is delivered to the process.

Normal Operation

Segments are sent and acknowledged within a reasonable time frame (e.g., 500ms).

Lost Segment

If a segment is lost, the sender's retransmission timer expires, and the segment is resent.

Fast Retransmission

If the sender receives three duplicate ACKs, it resends the lost segment without waiting for the retransmission timer to expire.

Notes

The receiver TCP delivers only ordered data to the process.

23-4 SCTP

SCTP is a new reliable, message-oriented transport layer protocol designed for Internet applications requiring more sophisticated services than TCP can provide.

Topics

• SCTP Services and Features

• Packet Format

• An SCTP Association

• Flow Control and Error Control

Notes

SCTP is a message-oriented, reliable protocol that combines the best features of UDP and TCP.

SCTP Applications

Multiple-Stream Concept

SCTP supports multiple streams within an association, allowing for ordered or unordered delivery of data chunks within each stream.

Notes

An association in SCTP can involve multiple streams.

Multihoming Concept

SCTP supports multihoming, where an association can use multiple IP addresses for each end, providing redundancy and fault tolerance.

Notes

SCTP association allows multiple IP addresses for each end.

Terminology

• TSN: Transmission Sequence Number

• SI: Stream Identifier

• SSN: Stream Sequence Number

Notes

• In SCTP, a data chunk is numbered using a TSN.

• To distinguish between different streams, SCTP uses an SI.

• To distinguish between different data chunks belonging to the same stream, SCTP uses SSNs.

• TCP has segments; SCTP has packets.

SCTP Packet Format

Consists of:

• Data Header and Options

• Control Chunks

• Data Chunks

A comparison with TCP:

• TCP Segment: Source port, destination port, sequence number, acknowledgment number, control flags, window size, checksum, urgent pointer, options, and data.

• SCTP Packet: Source port, destination port, verification tag, checksum, control chunks, and data chunks.

Notes

In SCTP, control information and data information are carried in separate chunks.

Data Chunk Identification

Data chunks are identified by TSN, SI, and SSN.

• TSN: Cumulative number identifying the association.

• SI: Defines the stream.

• SSN: Defines the chunk in a stream.

Notes

Data chunks are identified by three items: TSN, SI, and SSN. TSN is a cumulative number identifying the association; SI defines the stream; SSN defines the chunk in a stream.

Acknowledgment

In SCTP, acknowledgment numbers are used to acknowledge only data chunks; control chunks are acknowledged by other control chunks if necessary.

SCTP Packet Format

• General Header (12 bytes)

• Chunk 1 (variable length)

• Chunk N (variable length)

Notes

In an SCTP packet, control chunks come before data chunks.

General Header

• Source Port Address: 16 bits

• Destination Port Address: 16 bits

• Verification Tag: 32 bits

• Checksum: 32 bits

Chunks

Notes

A connection in SCTP is called an association.

Association Establishment (Four-Way Handshaking)

1. Client sends INIT

2. Server sends INIT ACK with Cookie

3. Client sends COOKIE ECHO with Cookie

4. Server sends COOKIE ACK

Notes

No other chunk is allowed in a packet carrying an INIT or INIT ACK chunk. A COOKIE ECHO or a COOKIE ACK chunk can carry data chunks.

Notes

In SCTP, only DATA chunks consume TSNs; DATA chunks are the only chunks that are acknowledged.

Data Transfer

Data chunks are sent with TSNs. The receiver acknowledges with a SACK chunk containing the cumulative TSN.

Notes

The acknowledgment in SCTP defines the cumulative TSN, the TSN of the last data chunk received in order.

Association Termination

1. Client sends SHUTDOWN

2. Server sends SHUTDOWN ACK

3. Server sends SHUTDOWN COMPLETE

Flow Control

Managed through receiver-side and sender-side parameters.

Error Control

Involves retransmission of lost data chunks and handling out-of-order chunks.