Chapter 3: Transport Layer

Four reasons for some applications to prefer UDP over TCP ?

1) Finer application-level control over what data is sent, and when: Under UDP, as soon as an application process passes data to UDP, UDP will package the data inside a UDP segment and immediately pass the segment to the network layer. TCP, on the other hand, has a congestion-control mechanism that throttles the transport-layer TCP sender when one or more links between the source and destination hosts become excessively congested. TCP will also continue to resend a segment until the receipt of the segment has been acknowledged by the destination, regardless of how long reliable delivery takes. Since real-time applications often require a minimum sending rate, do not want to overly delay segment trans- mission, and can tolerate some data loss, TCP's service model is not particularly well matched to these applications' needs. 2) No handshake, means less latency 3) TCP stores connection state includes receive and send buffers, congestion-control parameters, and sequence and acknowledgment number parameters. UDP doesn't maintain connection states and as a result less resources are consumed. For this reason, a server devoted to a particular application can typically support many more active clients when the application runs over UDP rather than TCP. 4) Small packet header overhead. The TCP segment has 20 bytes of header over- head in every segment, whereas UDP has only 8 bytes of overhead.

What are the fields of UDP header?

1) Source port 2) Destination port 3) Length 4) Checksum The UDP segment includes data and header.

A transport layer segment is encapsulated in a layer 3 packet. How do routers interact with the segment ?

Routers act only on the network-layer fields of the packet; that is, they do not examine the fields of the transport-layer segment encapsulated with the packet.

What is a socket (programming) and how does transport layer interact with it ?

Sockets are doors through which data passes from the network to the process and through which data passes from the process to the network. The transport layer in the receiving host does not actually deliver data directly to a process, but instead to an intermediary socket. Because at any given time there can be more than one socket in the receiving host, each socket has a unique identifier.

What does the UDP checksum do ?

The UDP checksum provides for error detection. That is, the checksum is used to determine whether bits within the UDP segment have been altered (for example, by noise in the links or while stored in a router) as it moved from source to destination.

What does the length field of UPD header specifies?

The length field specifies the number of bytes in the UDP segment (header plus data). An explicit length value is needed since the size of the data field may differ from one UDP segment to the next.

What is demultiplexing by transport layer ?

This job of delivering the data in a transport-layer segment to the correct socket is called demultiplexing. This is how it's done in UDP. More subtle in TCP.

True or False: Whereas a transport-layer protocol provides logical communication between processes running on different hosts, a network-layer protocol provides logical communication between hosts. What is meant by logical communication ?

True. process to process - layer 4 host to host - layer 3 logical communication means that processes behave as if they are directly connected and can exchange messages despite being hundreds of miles away, through several routers.

Why does SNMP prefers UDP ?

UDP is preferred to TCP in this case, since network management applications must often run when the network is in a stressed state—precisely when reliable, congestion- controlled data transfer is difficult to achieve.

What are well-known port numbers?

Well-known port numbers are used for fundamental applications such as HTTP or DNS. They have a range from 0 to 1023.

What does it mean by the phrase "transport layer protocols live in the end systems" ?

Within an end system, a transport protocol moves messages from application processes to the network layer and vice versa, but it doesn't have any say about how the messages are moved within the network core. On the other hand, intermediate routers neither act on, nor recognize, any information that the transport layer may have added to the application messages.

Can an application improve it's data transfer reliability while using UDP?

Yes. This can be done if reliability is built into the application itself. QUIC protocol used in Google's Chrome browser implements reliability in an application-layer protocol on top of UDP.

How many fields does UDP header has and size of each field ?

4 fields (2 bytes each) = 8 bytes

How many bits is a port ? How many total ports ?

Each port number is a 16-bit number, ranging from 0 to 65535.

True or False: UDP provides error checking and correction.

False. Only error checking in the form of end-end checksum. It does not do anything to recover from an error. Some implementations of UDP simply discard the damaged segment; others pass the damaged segment to the application with a warning.

What is meant by Internet Protocol (IP) best-effort delivery service ?

IP makes its "best effort" to deliver segments between communicating hosts, but it makes no guarantees. In particular, it does not guarantee segment delivery, it does not guarantee orderly delivery of segments, and it does not guarantee the integrity of the data in the segments. For these reasons, IP is said to be an unreliable service.

What are ports in transport header?

A segment have special fields that indicate the socket to which the segment is to be delivered. These fields are source and destination port number.

What is meant by a port being open or listening ?

A server (device that receives) waits for contact by a remote client - it's in listening mode. If a port is open, we can map the port to a process that is listening.

How does the transport layer extends the function of network layer ?

A critical function of the transport layer is extending the network layer's delivery service between two end systems to a delivery service between two application-layer processes running on the end systems.