Connection-oriented and Connectionless Protocols

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En provenance du cours de Google

The Bits and Bytes of Computer Networking

2716 notes

Google

The Bits and Bytes of Computer Networking

2716 notes

Cours 2 sur 5 dans Specialization Google IT Support Professional Certificate

This course is designed to provide a full overview of computer networking. In this course, we’ll cover everything from the fundamentals of modern networking technologies and protocols to practical applications and network troubleshooting. By the end of this course, you’ll be able to: - describe computer networks in terms of a five-layer model. - understand all of the standard protocols involved with TCP/IP communications. - grasp powerful network troubleshooting tools and techniques. - learn network services like DNS and DHCP that help make computer networks run. Except as otherwise noted, the contents on this site are licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

À partir de la leçon

The Transport and Application Layers

In the third week of this course, we'll explore the transport and application layers. By the end of this module, you'll be able to describe TCP ports and sockets, identify the different components of a TCP header, show the difference between connection-oriented and connectionless protocols, and explain how TCP is used to ensure data integrity.

The Transport Layer3:31

Dissection of a TCP Segment5:22

TCP Control Flags and the Three-way Handshake5:51

TCP Socket States3:03

Connection-oriented and Connectionless Protocols4:24

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So far, we have mostly focused on TCP which is a connection-oriented protocol.

A connection-oriented protocol is one that establishes a connection,

and uses this to ensure that all data has been properly transmitted.

A connection at the transport layer implies that

every segment of data sent is acknowledged.

This way, both ends of the connection always know which bits of

data have definitely been delivered to the other side and which haven't.

Connection-oriented protocols are important

because the Internet is a vast and busy place,

and lots of things could go wrong while trying to get data from point A to point B.

You might remember from our lesson about

the physical air that even some minor crosstalk from

a neighboring twisted pair in

the same cable can be enough to make a cyclical redundancy check fail.

This could cause the entire frame to be discarded.

Yikes. If even a single bit doesn't get transmitted properly,

the resulting data is often incomprehensible by the receiving end.

And remember that at the lowest level,

a bit is just an electrical signal within a certain voltage range.

But there are plenty of other reasons why traffic might not reach

its destination beyond liners. It could be anything.

Pure congestion might cause a router to drop

your traffic in favor of forwarding more important traffic,

or a construction company could cut a fiber cable connecting two YSPs.

Anything's possible. Connection-oriented protocols like TCP protect

against this by forming connections and through the constant stream of acknowledgements.

Our protocols at lower levels of our network model like IP and

Ethernet do use checksums to ensure that all the data they received was correct.

But did you notice that we never discussed

any attempts to resending data that doesn't pass this check?

That's because that's entirely up to the transport layer protocol.

At the IP or Ethernet level,

if a checksum doesn't compute,

all of that data is just discarded.

It's up to TCP to determine when to resend

this data since TCP expects an ACK for every bit of data it sends,

it's in the best position to know what data successfully got

delivered and it can make the decision to resend a segment if needed.

This is another reason why sequence numbers are so important.

While TCP will generally send all segments in sequential order,

they may not always arrive in that order.

If some of the segments had to be resend due to errors at lower layers,

it doesn't matter if they arrive slightly out of order.

This is because sequence numbers allow for all of the data to

be put back together in the right order. It's pretty handy.

Now, as you might have picked up on,

there's a lot of overhead with connection-oriented protocols like TCP.

You have to establish the connection,

you have to send a stream of constant streams of acknowledgements,

you have to tear the connection down at the end.

That all accounts for a lot of extra traffic.

While this is important traffic,

it's really only useful if you absolutely

positively have to be sure your data reaches its destination.

You can contrast this with connectionless protocols.

The most common of these is known as UDP,

or User Datagram Protocol.

Unlike TCP, UDP doesn't rely on

connections and it doesn't even support the concept of an acknowledgement.

With UDP, you just set a destination port and send the packet.

This is useful for messages that aren't super important.

A great example of UDP is streaming video.

Let's imagine that each UDP datagram is a single frame of a video.

For the best viewing experience,

you might hope that every single frame makes it to the viewer

but it doesn't really matter if a few get lost along the way.

A video will still be pretty watchable unless it's missing a lot of its frames.

By getting rid of all the overhead of TCP,

you might actually be able to send higher quality video with UDP.

That's because you'll be saving more of the available bandwidth for actual data

transfer instead of the overhead of establishing

connections and acknowledging delivered data segments.

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