TCP & UDP Flashcards by William Winslade

What is the purpose of L4

Provide transparent transfer of data between hosts

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What does “transparent data transfer between hosts” mean?

Hosts involved are not aware of L1-3 details of the underlying network

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What are the services offered (or not, depending on TCP vs UDP) by L4?

Reliable data transfer
Error recovery
Data sequencing
Flow control

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What is the additional addressing system provided by L4?

Port numbers

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What is an L4 session?

An exchange of data between two or more communicating devices

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What does the L4 destination port identify?

The application layer (L5+) protocol

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Is the L4 source port assigned or randomly selected?

Randomly selected

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What does the L4 source port identify?

The L4 communication session

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L4 addressing helps provide session _____

multiplexing

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Port numbers 0 - 1023 are _____

Well-Known

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Port numbers 1024 - 49151 are _____

Registered

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Port numbers 49152 - 65535 are _____

Ephemeral / private / dynamic

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Well known ports fall between ports ___ - ____

0 - 1023

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Registered ports fall between ports ____ - ____

1024 - 49151

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Ephemeral ports fall between ports ____ - ____

49152 - 65535

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Before TCP can send data to a destination host, the two hosts must do what?

Establish a connection

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Does TCP or UDP provide reliability?

TCP

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Does TCP or UDP provide segment sequencing?

TCP

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Does TCP or UDP provide flow control?

TCP

What is L4 flow control?

A destination being able to dictate the rate that data is sent

What flags in the TCP header are used to establish and terminate connections?

ACK
SYN
FIN

What function does the window size field in the TCP header serve?

Used for flow control, adjusting the rate at which data is sent

By which method does TCP establish connections between hosts?

TCP 3-way handshake

Explain the different parts of the TCP 3-way handshake

Client sends SYN
Server sends SYN ACK
client sends ACK

By which method does TCP terminate connections between hosts?

TCP 4-way handshake

Explain the different parts of the TCP 4-way handshake

Client sends FIN Server sends ACK Server sends FIN Client sends ACK

Describe what TCP forward acknowledgement is

Indicates the sequence number of the next segment the host expects to receive

When a client sends the initial SYN message in the 3-way handshake, what is the initial sequence number?

Randomly chosen by the client

When a server sends the SYN-ACK message in the 3-way handshake, what are the sequence number and acknowledgement number set to?

The server picks its own random sequence number, and increments the sequence number sent by the client by 1, and uses that for the acknowledgement num

When a client sends the final ACK message in the TCP 3-way handshake, what are the sequence number and acknowledgement number set to?

Client flips what it received from the server. Sequence num will be the acknowledgement number the server sent on SYN-ACK, and the acknowledgement number is the sequence number the server sent on SYN-ACK plus one.

Explain what TCP sequencing allows

Allows the client/server to know what segment to expect next.

In layman's terms, what is contained within a TCP header that allows for sequencing?

Here's segment number X, I (client/server), expect segment number Y from you (server/client) next

The TCP header's window size field allows for what?

More data to be sent before an acknowledgement is required

What is the TCP sliding window?

Used to dynamically adjust how large the TCP window size is depending on various factors

Describe the reliability of UDP

There is none. Segments are sent on a best-effort basis

Does UDP establish connections before data is sent?

How does UDP provide segment sequencing?

It doesn't

How does UDP provide flow control?

It doesn't

For downloading a file, what L4 protocol is best used?

TCP

For real time voice and video, what L4 protocol is best used?

UDP

Is it possible for some applications to use both TCP and UDP?

Yes. DNS is an example

Which of the following is a well known port number? a) 1010 b) 2001 c) 4023 d) 65000

a) 1001 Well known are 0-1023

What range of port numbers should hosts select from when randomly selecting a source L4 port number? a) Well known b) Registered c) Ephemeral d) Reserved

c) Ephemeral Ports 49152 - 65535

Which of the following are features of TCP but not UDP (pick 3) a) L4 addressing b) Error recovery c) Session multiplexing d) Flow control e) Sequencing

b) Error recovery d) Flow control e) Sequencing

Which of the following application layer protocols use TCP to provide reliable communications? (pick 3) a) SMTP b) SNMP c) HTTPS d) DHCP e) Syslog f) SSH

a) SMTP c) HTTPS f) SSH

PC1 and SRV1 have an active TCP connection. SRV1 receives a TCP segment from PC1 with a sequence number of 27. When SRV1 acknowledges the segment, what will the value of the Acknowledgement field in the TCP header be? Assume a TCP window size of 1. a) 26 b) 27 c) 28

c) 28 TCP acknowledges that it receives the segment by stating the next segment it expects to receive (forward acknowledgement)

Routers in which states send HSRP hello messages?

Active, standby, and speak states