The Bits and Bytes of Computer Networking: Module 3

The Transport Layer allows traffic to be directed to specific network ___.

applications

The Application Layer allows applications to ____ in a way they understand.

communicate

The transport layer is responsible for many important networking functions: ___plexing and demultiplexing traffic, establishing long-running connections and ensuring data ___ through error-checking and data verification.

multi, integrity

_____ in the transport layer means that nodes on a network can direct traffic towards many different receiving services.

multiplexing

___ is: taking traffic that's all aimed at the same node and delivering it to the proper receiving service.

demultiplexing

The transport layer handles multiplexing and demultiplexing through the use of ___.

ports

A port is a __-bit number that's used to direct traffic to a specific ___ running on a networked computer.

16, service

A server or ____ is a program running on a computer, waiting to be asked for data.

service

A ___ is a orogram that is requesting data from a server or service.

client

Different network services run while "listening" for requests on specific ___.

ports

The traditional port for unencrypted web traffic, or ___, is Port ___

HTTP, 80

If you want to request a webpage from a web server running on a computer "listening" on IP 10.1.1.100, the traffic would be directed to Port __ on that computer.

80

Ports are normally denoted with a __ after the full IP address.

colon

A ____ address or ___ number is an IP address followed by a colon, and the port number, like: 10.1.1.100:80 (same word - write it only once)

socket

What does "FTP" mean, as in "FTP server"?

File Transfer Protocol

___ is an older method used for transferring files from one computer to another.

FTP

FTP traditionally "listens" on Port ___.

21

10.1.1.100:21 is the _____ address for an ___ server.

socket, FTP

True or false: in a small company, a web server, file server, printer server, mail server could all be on the same computer.

TRUE

Just as an Ethernet frame encapsulates an __ ___, an IP datagram encapsulates a ___ segment.

IP datagram, TCP

An Ethernet frame has a "payload" section which is really just the entire contents of an __ ___.

IP datagram

An IP datagram has a "payload" section, which is made up of a ____ ___.

TCP segment

A TCP segment is made up of a TCP __ and a TCP ___ section.

header, data

The ___ ___ of a TCP segment is simply the "payload" area where an application places its data.

data section

True or false: A TCP header is simple, with few fields and very basic information.

FALSE

The first two fields of a TCP header are the ___ ___ and ___ ___ fields, which occupy __ bits.

source port, destination port, 32

The Destination port is the ___ of the service the traffic is intended for.

port

Is a Source port a high-numbered or low-numbered port?

high

A Source port is a high-numbered port chosen from a special selection of ___ known as ___ ports.

ports, ephemeral

A source port is necessary to keep lots of outgoing connections separate: true or false?

TRUE

A ____ port is needed so that when the web server replies, the computer making the original request can send this data to the program that was actually requesting it.

source

Which field comes after the source and destination ports fields in a TCP header? The ___ ___ field.

sequence number

How many bits long is the source port field?

16

How many bits long is the sequence number field?

32

The sequence number is used to keep track of where in a sequence of TCP ____ this one is expected to be.

segments

The ___ ___ in a TCP header keeps track of which segment, out of many, a particular segment might be. (It keeps fragmented segments in order, so to speak).

sequence number

What field comes after the sequence number in a TCP segment header? The ____ ___ field.

acknowledgement number

The acknowledgement number is the number of the next expected ___.

segment

In simple terms: A sequence number of "1" and an acknowledgement number of "2" means: "This is segment __; Expect segment __ next."

1, 2

How many bits long is the acknowledgement number field?

32

The ___ ___ field is a 4-bit number that communicates how long the TCP header for this segment is (so it's clear to the receiving device where the payload begins).

data offset

What field comes after the acknowledgement number field? The __ ___ field.

data offset

The data offset field in a TCP header communicates how long the ___ ___ for this segment is, so the receiving network device knows where the payload begins.

TCP header

The data offset field in a TCP header can also be called the ___ ___ field.

header length

What comes right after the 4-bit Data Offset field in a TCP header?

6 reserved bits

What comes right after the 4-bit Data Offset field and the 6 "empty" or "reserved" bits in a TCP header? The TCP ___ ___ (6 bits).

control flags

What comes after the Data Offset field, reserved bits and TCP control flags in a TCP header? The TCP ___. (16 bits)

window

The ___ ___ specifies the range of sequence numbers that might be sent before an acknowledgement is required.

TCP window

True of false: TCP is a protocol that relies on many acknowledgements.

TRUE

___ are important because they help ensure that all expected data is actually being recieved, and that the sending device isn't wasting time sending data that isn't being received.

Acknowledgements

After teh TCP window field in a TCP segment header comes the ___ ___ field.

TCP checksum

True or false: the TCP checksum operates very differently from the checksums at the IP and ethernet levels.

FALSE

The TCP checksum makes it possible to check that no data was lost or corrupted during transmission: true or false?

TRUE

How many bits long is the TCP checksum field?

16

What comes after the TCP checksum field in a TCP segment? The __ __ field.

urgent pointer

The __ ___ field is used with one of the TCP control flags to point out segments that might be more important than others.

ugernt pointer

The ___ __ field (in a TCP segment) hasn't really be adopted or used - but you need to know it's there.

urgent pointer

What comes after the Urgernt Pointer field in a TCP segment? The __ segment.

options

The ___ field (like the Urgent Pointer field) in a TCP segment is rarely used, although when it is, it's used for more ___ flow control protocols.

options, complicated

What comes after the Options field in a TCP segment?

padding

The ___ in a TCP segment consists of a sequence of 0s to make sure that the data payload section begins at the expected location.

padding

As a protcol, ___ is used to establish connections to send long chains of segments of data.

TCP

The way TCP establishes a connection is through the use of different TCP __ __, used in a very specific order.

control flags

How many TCP control flags are there? (equal to the number of bits)

6

The 6 control flags are:

URG, ACK, PSH, RST, SYN, FIN

The first TCP control flag is URG or "___"

urgent

If the value of the URG control flag (in TCP segment) is "1", it means that the segment is urgent and the urgernt ___ field contains more info. (This isn't used much).

pointer

The second TCP control flag is ACK, for "___".

acknowledged

A value of 1 in the ACK TCP control flag means the ___ ___ field should be examined.

acknowledgement number

The third TCP control is PSH, for "___", which means the transmitting device wants the receiving device to hurry up and send currently-buffered data ASAP.

push

A value of 1 in the PSH TCP control flag means the transmitting device wants the receiving device to hurry up and send currently-____ data to the application ASAP.

buffered

A ___ is a computing technique where a certain amount of data is held before being sent somewhere else.

buffer

Buffering is used in TCP to help send data more efficently: true or false?

TRUE

The fourth TCP control flag is RST, or "___"

reset

A 1 in the ___ TCP control flag field means that one of the sides hasn't recovered from missing or malformed segments. Like, "I can't make sense of this - let's start over!"

RST

The fifth TCP control flag is SYN, or "__".

synchronize

The __ TCP control flag field is set to 1 when first establishing a TCP connection, and makes sure the receiving end examines the Sequence Number field.

SYN

The sixth TCP control flag is FIN, for "___"

finish

A 1 in the ___ TCP control flag field means that the transmitting computer doesn't have anything else to send, so the connection can be closed.

FIN

When computer A wants to start a "conversation" with computer B, it sends a TCP segment with the ___ flag set to 1, which tells the other computer to look at the ___

SYN, sequency number field

When computer B first responds to an initial TCP segment from computer A, it will respond with a segment with the ___ and ___ set to 1.

ACK, SYN

When computer A responds to computer B's TCP segment response, A will respond with the __ flag set to 1 (acknowledging the acknowledgement)

ACK

Which TCP control flags always come into play during the first 3 communications between two computers?

SYN, SYN-ACK, ACK

In TCP communications, SYN, SYN-ACK, ACK is known popularly as "____".

the three-way handshake

A "___" is a way for two devices to know that they're "speaking" the same protocol and will be able to communicate.

handshake

Once the three-way handshake is over, the TCP connection will operate at full-__.

duplex

True or False: In TCP communication, each device will send "ACK" messages so the other one knows what's been received.

TRUE

What is the four-way handshake that helps end TCP communications? (Write the control flag abbreviations)

FIN, ACK, FIN, ACK

A socket is the instantiation of an __-___ in a potential TCP connection.

end-point

A ___ is the instantiation of an end-point in a potential TCP connection.

socket

An ___ is the actual implementation of something defined elsewhere.

instantiation

__ sockets require actual programs to instantiate them. You can contrast this with ___, which are more of a virtual-descriptive thing.

TCP, ports

You can send traffic to any port you want, but you're only going to get a response if a program has opened a ___ on that port.

socket

TCP sockets can exist in many states; understanding these states isn't very important for trouble-shooting: true or false?

FALSE

The TCP socket state "___" means a socket is ready and is monitoring for incoming connections.

LISTEN

Only the (client / server) side will have a TCP socket state of "LISTEN".

server

The TCP socket state "___" means a synchronization request has been sent, but the connection hasn't been established yet.

SYN_SENT

Only the (client/server) side will have a TCP socket state of "SYN_SENT".

client

The TCP socket state "____" means that a socket previously in "LISTEN" has received a synchronization request and has sent back a SYN/ACK.

SYN-RECEIVED

The TCP socket state "SY-RECEIVED" means that a socket previously in "___" has received a synchronization request and has sent back a ____.

LISTEN, SYN/ACK