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23.1

Chapter 23

Process-to-Process Delivery:

UDP, TCP, and SCTP

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23-1 PROCESS-TO-PROCESS DELIVERY

The transport layer is responsible for process-to-process delivery—the delivery of a packet, part of a message, from one process to another. Two processes communicate in a client/server relationship, as we will see later.

Client/Server Paradigm�Multiplexing and Demultiplexing

Connectionless Versus Connection-Oriented Service

Reliable Versus Unreliable

Three Protocols

Topics discussed in this section:

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The transport layer is responsible for process-to-process delivery.

Note

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Figure 23.1 Types of data deliveries

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Figure 23.2 Port numbers

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Figure 23.3 IP addresses versus port numbers

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Figure 23.4 IANA ranges

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Figure 23.5 Socket address

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Figure 23.6 Multiplexing and demultiplexing

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Figure 23.7 Error control

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Figure 23.8 Position of UDP, TCP, and SCTP in TCP/IP suite

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23-2 USER DATAGRAM PROTOCOL (UDP)

The User Datagram Protocol (UDP) is called a connectionless, unreliable transport protocol. It does not add anything to the services of IP except to provide process-to-process communication instead of host-to-host communication.

Well-Known Ports for UDP�User Datagram

Checksum

UDP Operation

Use of UDP

Topics discussed in this section:

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Table 23.1 Well-known ports used with UDP

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In UNIX, the well-known ports are stored in a file called /etc/services. Each line in this file gives the name of the server and the well-known port number. We can use the

grep utility to extract the line corresponding to the desired application. The following shows the port for FTP. Note that FTP can use port 21 with either UDP or TCP.

Example 23.1

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Example 23.1 (continued)

SNMP uses two port numbers (161 and 162), each for a different purpose, as we will see in Chapter 28.

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Figure 23.9 User datagram format

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UDP length �= IP length – IP header’s length

Note

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Figure 23.10 Pseudoheader for checksum calculation

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Figure 23.11 shows the checksum calculation for a very small user datagram with only 7 bytes of data. Because the number of bytes of data is odd, padding is added for checksum calculation. The pseudoheader as well as the padding will be dropped when the user datagram is delivered to IP.

Example 23.2

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Figure 23.11 Checksum calculation of a simple UDP user datagram

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Figure 23.12 Queues in UDP

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23-3 TCP

TCP is a connection-oriented protocol; it creates a virtual connection between two TCPs to send data. In addition, TCP uses flow and error control mechanisms at the transport level.

TCP Services�TCP Features

Segment

A TCP Connection

Flow Control

Error Control

Topics discussed in this section:

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Table 23.2 Well-known ports used by TCP

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Figure 23.13 Stream delivery

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Figure 23.14 Sending and receiving buffers

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Figure 23.15 TCP segments

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The bytes of data being transferred in each connection are numbered by TCP.

The numbering starts with a randomly generated number.

Note

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The following shows the sequence number for each segment:

Example 23.3

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The value in the sequence number field of a segment defines the

number of the first data byte �contained in that segment.

Note

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The value of the acknowledgment field in a segment defines

the number of the next byte a party expects to receive.

The acknowledgment number is cumulative.

Note

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Figure 23.16 TCP segment format

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Figure 23.17 Control field

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Table 23.3 Description of flags in the control field

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Figure 23.18 Connection establishment using three-way handshaking

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

Note

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A SYN + ACK segment cannot �carry data, but does consume one �sequence number.

Note

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

Note

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Figure 23.19 Data transfer

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Figure 23.20 Connection termination using three-way handshaking

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

Note

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

Note

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Figure 23.21 Half-close

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Figure 23.22 Sliding window

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A sliding window is used to make transmission more efficient as well as

to control the flow of data so that the destination does not become

overwhelmed with data. �TCP sliding windows are byte-oriented.

Note

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What is the value of the receiver window (rwnd) for host A if the receiver, host B, has a buffer size of 5000 bytes and 1000 bytes of received and unprocessed data?

Example 23.4

Solution

The value of rwnd = 5000 − 1000 = 4000. Host B can receive only 4000 bytes of data before overflowing its buffer. Host B advertises this value in its next segment to A.

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What is the size of the window for host A if the value of rwnd is 3000 bytes and the value of cwnd is 3500 bytes?

Example 23.5

Solution

The size of the window is the smaller of rwnd and cwnd, which is 3000 bytes.

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Figure 23.23 shows an unrealistic example of a sliding window. The sender has sent bytes up to 202. We assume that cwnd is 20 (in reality this value is thousands of bytes). The receiver has sent an acknowledgment number of 200 with an rwnd of 9 bytes (in reality this value is thousands of bytes). The size of the sender window is the minimum of rwnd and cwnd, or 9 bytes. Bytes 200 to 202 are sent, but not acknowledged. Bytes 203 to 208 can be sent without worrying about acknowledgment. Bytes 209 and above cannot be sent.

Example 23.6

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Figure 23.23 Example 23.6

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Some points about TCP sliding windows:

❏ 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.

Note

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

Note

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In modern implementations, a retransmission occurs if the retransmission timer expires or three duplicate ACK segments have arrived.

Note

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No retransmission timer is set for an ACK segment.

Note

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Data may arrive out of order and be temporarily stored by the receiving TCP,

but TCP guarantees that no out-of-order segment is delivered to the process.

Note

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Figure 23.24 Normal operation

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Figure 23.25 Lost segment

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The receiver TCP delivers only ordered data to the process.

Note

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Figure 23.26 Fast retransmission

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23-4 SCTP

Stream Control Transmission Protocol (SCTP) is a new reliable, message-oriented transport layer protocol. SCTP, however, is mostly designed for Internet applications that have recently been introduced. These new applications need a more sophisticated service than TCP can provide.

SCTP Services and Features

Packet Format

An SCTP Association

Flow Control and Error Control

Topics discussed in this section:

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

Note

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Table 23.4 Some SCTP applications

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Figure 23.27 Multiple-stream concept

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An association in SCTP can involve multiple streams.

Note

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Figure 23.28 Multihoming concept

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SCTP association allows multiple IP addresses for each end.

Note

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

Note

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To distinguish between different streams, SCTP uses an SI.

Note

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

Note

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TCP has segments; SCTP has packets.

Note

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Figure 23.29 Comparison between a TCP segment and an SCTP packet

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

Note

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Figure 23.30 Packet, data chunks, and streams

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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.

Note

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In SCTP, acknowledgment numbers are used to acknowledge only data chunks;

control chunks are acknowledged by other control chunks if necessary.

Note

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Figure 23.31 SCTP packet format

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

Note

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Figure 23.32 General header

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Table 23.5 Chunks

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A connection in SCTP is called an association.

Note

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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.

Note

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Figure 23.33 Four-way handshaking

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In SCTP, only DATA chunks �consume TSNs;

DATA chunks are the only chunks �that are acknowledged.

Note

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Figure 23.34 Simple data transfer

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

Note