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Chapter 13 Introduction to the Transport Layer. 13.1 Transport-Layer Services. Process-to-Process Communication Addressing : Port Numbers Encapsulation and Decapsulation Multiplexing and Demultiplexing Flow Control Error Control Congestion Control
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13.1 Transport-Layer Services Process-to-Process Communication Addressing : Port Numbers Encapsulation and Decapsulation Multiplexing and Demultiplexing Flow Control Error Control Congestion Control Connectionless and Connection-Oriented Services
Process-to-Process Communication Process is an application-layer entity Responsible for delivery of the message to the appropriate process
Addressing : Port Number • Client-server paradigm : the most common way to achieve process-to-process communication • Process on local host needs service from a process on the remote host • Both processes have the same name : Daytime process • To support several program at the same time, we must define • Local host • Local process • Remote host • Remote process
Addressing : Port Number • To define the hosts, we use IP addresses • To define the processes, we need second identifiers • Port number • In the TCP/IP protocol suite, port numbers are integers between 0 and 65,535 • Ephemeral port number • Recommended to be greater than 1,023 • Well-known port number • Universal port numbers for servers
ICANN Ranges • Well-known ports : 0 ~ 1,023 • Assigned and controlled by ICANN • Registered ports : 1,024 ~ 49,151 • Not assigned and controlled by ICANN • Can be registered with ICANN to prevent duplication • Dynamic ports : 49,152 ~ 65,535 • Can be used as temporary or private port number
Example 13.1 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 TFTP. Note that TFTP can use port 69 on either UDP or TCP. SNMP uses two port numbers (161 and 162), each for a different purpose.
Socket Address The combination of an IP address and a port number
Encapsulation and Decapsulation • Encapsulation • Happen at the sender site • Add the transport-layer header • Decapsulation • Happen at the receiver site • The header is dropped
Mutiplexing and Demultiplexing • Multiplexing • Entity accepts items frommore than one source • Demultiplexing • Entity deliver items to morethan one source
Flow Control • Balance between production and consumption rates • Delivery of items from a producer to a consumer • Pushing : sender delivers items whenever they produced • Pulling : producer delivers the items after the consumer has requested
Flow Control at the Transport Layer One of solutions for flow control is normally to use buffers
Example 13.2 The above discussion requires that the consumers communicate with the producer in two occasions: when the buffer is full and when there are vacancies. If the two parties use a buffer of only one slot, the communication can be easier. Assume that each transport layer uses one single memory location to hold a packet. When this single slot in the sending transport layer is empty, the sending transport layer sends a note the application layer to send its next chunk; when this single slot in the receiving transport layer is empty, it sends an acknowledgment to the sending transport layer to send its next packet. As we will see later, this type of flow control, using a single-slot buffer at the sender and the receiver, is inefficient.
Error Control Detect and discard corrupted packets. Keep track of lost and discarded packets and resend them. Recognize duplicate packets and discard them. Buffer out-of-order packets until the missing packets arrive.
Error Control • Sequence Number • Which packet is to be resent • Which packet is a duplicate • Which packet has arrived out of order • This can be done if the packets are numbered • Acknowledgment • The receiver side can send an acknowledgement for each or a collection of packets that have arrived safe and sound
Combination of Flow and Error Control • Flow control requires the use of two buffers, one at sender site and other at the receiver site. • Error control requires the use of sequence and acknowledgment number by both sides. • Sliding window
Congestion Control • Occur if the load on the network is greater than the capacity of the network • Open-loop congestion control :prevent congestion before it happens • Retransmission policy : using retransmission timer • Window policy : selective repeat and go-back-N window • Acknowledgment policy : using timer, a ACK for N packets • Closed-loop congestion control • alleviating congestion after it happens • Flexible window size
Connectionless and Connection-Oriented Service • Connectionless service • Divide messages into chunks of data • No dependency between the packets • The packets may arrive out of order • Connection-oriented service • Establish a connection between a server and a client. • After data exchange, the connection needs to be teared down
Connection-Oriented Service ⑤ Connection-close request ⑤
13.2 Transport-Layer Protocols • Simple connectionless protocol • No flow control and error control • Connection-oriented protocol • Stop-and-Wait protocol • Provide flow and error control • Go-Back-N protocol • Efficient version of Stop-and-Wait protocol • Selective-Repeat Protocol • Suited to handle packet loss • Piggybacking
Simple Protocol No flow nor error control based connectionless protocol
Example 13.3 Figure 13.18 shows an example of communication using the protocol. It is very simple. The sender sends packets one after another without even thinking about the receiver.
Stop-and-Wait Protocol • Use both flow and error control • Both the sender and the receiver use a sliding window of size 1. • The sender sends one packet and waits for an ACK before sending the next one. • Add a checksum to detect corrupted packets • When sender sends a packet, it starts a timer • If an ACK arrives, timer is stopped • Either the packet was lost or corrupted, the sender resends the previous packet
Stop-and-Wait Protocol • Sequence Number • To prevent duplicate packets • A field is added to the packet header to hold the sequence number • Use x as a sequence number, x+1 as a acknowledgment number • Acknowledgement number • Sequence number of the next packet expected by the receiver
Example 13.4 Following figure shows an example of Sop-and Wait protocol. Packet 0 is sent and acknowledged. Packet 1 is lost and resent after the time-out. The resent packet 1 is acknowledged and the timer stops. Packet 0 is sent and acknowledged, but the acknowledgment is lost. The sender has no idea if the packet or the acknowledgment is lost, so after the time-out, it resends packet 0, which is acknowledged.
Example 13.5 Assume that, in a Stop-and-Wait system, the bandwidth of the line is 1 Mbps, and 1 bit takes 20 milliseconds to make a round trip. What is the bandwidth-delay product? If the system data packets are 1,000 bit in length, what is the utilization percentage of the link? Solution The bandwidth-delay product is (1 x 106) x (20 x 10-3) = 20,000 bits. The system can send 20,000 bits during the time it takes for the data to go from the sender to the receiver and the acknowledgment to come back. However, the system sends only 1,000 bits. We can say that the link utilization is only 1,000/20,000, or 5 percent. For this reason, for a link with a high bandwidth or long delay, the use of Stop-and Wait wastes the capacity of the link.
Example 13.6 What is the utilization percentage of the link in Example 13.5 if we have a protocol that can send up to 15 packets before stopping and worrying about the acknowledgement Solution The bandwidth-delay product is still 20,000 bits. The system can send up to 15 packets or 15,000 bits during a round trip. This means the utilization is 15,000/20,000, or 75 percent. Of course, if there are damaged packets, the utilization percentage is much less because packets have to be resent.
Go-Back-N Protocol To improve the efficiency of transmission, we can send several packets before receiving acknowledgments.
Send Window for Go-Back-N The send window is an abstract concept defining an imaginary box of maximum size = 2m-1 with three variable : sf, sn, and ssize. The send window can slide one or more slot when an error-free ACK with ackNo between Sf and Sn arrives.
Receive Window for Go-Back-N The receive window is an abstract concept defining an imaginary box of size 1 with one single variable Rn. The window slides when a correct packet has arrived: sliding occurs one slot at a time.
Send Window Size for Go-Back-N In the Go-Back-N protocol, the size of the send window must be less than 2m; the size of the receive window is always 1.
Example 13.7 Following figure shows an example of Go-Back-N. This is an example of a case where the forward channel is reliable, but the reverse is not. No data packets are lost, but some ACKs are delayed and one is lost. The example also shows how cumulative acknowledgements can help if acknowledgements are delayed of lost.
Example 13.8 Following figure shows what happen when a packet is lost. Packets 0, 1, 2, and 3 are sent. However, packet 1 is lost. The receiver receives packets 2 and 3, but they are discarded because they are received out of order. When the receiver receives packets 2 and 3, it send ACK 1 to show that it expects to receive packet 1. However, these ACKs are nor useful for the sender because the ackNo is equal Sf, not greater that Sf. So the sender discards them. When the time-out occur, the sender resends packets 1, 2, and 3, which are acknowledged.
Selective-Repeat Protocol • The Go-Back-N protocol simplifies the process of receiver • No need to buffer out-of-order packets • Inefficient when the underlying network protocol loses a lot of packets • SR(Selective-Repeat) protocol devised above. • Resend only selective packets
Example 13.9 Assume a sender sends 6 packets: packets 0, 1, 2, 3, 4, and 5. The sender receives an ACK with ackNo=3. What is the interpretation if the system is using GBN or SR ? Solution If the system is using GBN, it means that packet 0, 1, and 2 have been received uncorrupted and the receiver is expecting packet 3. in using SR, it means that packet 3 has been received uncorrupted; the ACK does not say anything about other pakcets.
Example 13.10 This example is similar to Example 3.8 in which packet 1 is lost. We show how Selective-Repeat behave in this case. Figure 13.34 shows the situation. At the sender, packet 0 is transmitted and acknowledged. Packet 1 is lost. Packet 2 and 3 arrive out of order and are acknowledged. When the timer times out, packet 1 is resent and is acknowledged. The send window then slides.
