CISCO CH 07

(3) primary purposes of transport layer:

(1) Tracking individual communication between applications on source & destination hosts, (2) Segmenting data for, and reassembling segmented data into, streams of application data at destination, (3) Identifying proper application for each communication stream

Security can be added to data network:

(1) by denying TCP sessions establishment, (2) by only allowing specific services sessions, (3) by only allowing traffic for already-established sessions

2 fields w #s to ensure delivery of all packets in conversation

(fields) SYN & ACK

each connection is actually ___________

(two) one-way sessions

Range of TCP & UDP well-known ports:

0 to 1023

TCP window is 1000 bytes. Server is sending file in 100-byte segments. How many segments will server send before requiring an ACK from PC?

10 segments

Range of Registered Ports used by TCP to identify requested service on destination or as a client source port?

1024 to 49151

Example of a socket:

192.168.1.1:80

each TCP segment has ________ of overhead in header, encapsulating application layer data:

20 bytes

pc1 sent 20 bytes. ack # of returning pc2 will be _____.

21

Range of dynamic, private, ephemeral ports:

49152 to 65535

In every segment *after* established sessions:

ACK Flag

Action performed by client when *establishing communication* with a server via use of UDP at the transport layer?

Client randomly selects a source port number

client sends ISN of 2 to server, & server responds with ISN of 1 to client. What is final stage of TCP 3-way handshake?

Client sends ACK of 2

what is a characteristic of UDP?

Connectionless, *doesn't* re-send datagrams.

Used by *both* TCP & UDP

DHCP & DNS

2 applications that use UDP as transport layer protocol?

DNS Queries, & DHCP

which types of applications use UDP?

DNS, SNMP, DHCP, TFTP, VOIP, RIP, Games, Streaming

ACK # sent back to source for next byte expected:

Expectational Acknowledgement

Which TCP header *control bit* is set *on* to terminate a TCP conversation?

FIN

closes a connection in a 2-way handshake:

FIN flag + ACK

3-way handshake termination:

FIN, FIN+ACK, ACK

A client is pulling large file from FTP server. One of the segments is lost during transit. What happens?

FTP client responds to server w smaller window size in TCP header.

Applications that use TCP:

FTP, HTTP, SMTP, TELNET

Tech uses TFTP to transfer large file from fileserver to remote router. Which statement is correct about scenario?

File is segmented & reassembled in correct order at destination, if needed, by upper-layer protocol. UDP does not reorder data. If necessary, UDP relies on upper-layer protocols for reordering of the data.

____ msgs are exchanged at TCP *session termination* between client & server.

Four (FIN, ACK, FIN, ACK)

initial value for the sequence number:

Initial Sequence Number (ISN) (random)

What does client do when it has UDP datagrams to send?

It just sends the datagrams.

3 fields in a UDP segment header:

Length, Source Port, Checksum

TFTP port #

PORT # 69

numeric identifier in each segment keeps track of specific conversations & services requested:

Port

what is assigned by the transport layer to identify an application or service?

Port

TCP & UDP use ___________ to separate multiple communications on same channel

Port Numbers

During TCP session, _____ flag is used by client to request communication w server.

SYN

2 flags in TCP header in TCP 3-way handshake to establish connectivity between 2 network devices?

SYN & ACK

TCP flags for session establishment:

SYN, SYN+ACK, ACK

TCP PDU breaks up msg into, numbered in sequence, & passed to IP process for assembly into packets:

Segments

To allow destination to acknowledge bytes in DIS-continuous segments & request retransmission of *ONLY* missing data, both hosts must support optional feature: (TCP normally retransmits lost data from last successful acknowledgement)

Selective Acknowledgements (SACK)

3 fields in *all* Transport Layer Headers

Source Port, Destination Port, Checksum

Transport layer feature guarantees session establishment:

TCP 3-way handshake

What happens if 1st packet of TFTP transfer is lost?

TFTP application will retry request if reply is not received.

What happens if part of FTP message is *not* delivered to the destination?

The part of the FTP message that was lost is re-sent

Which scenario describes a function provided by the transport layer?

Transport layer ensures correct web page delivered to correct browser (2 browser windows for 2 web sites)

Advantage of UDP over TCP:

UDP communication requires less overhead

What does UDP do when receiving msgs more than 1 datagram in length?

UDP reassembles data in order received & passes it to application

(6) 1-bit fields containing control info to manage TCP processes:

URG, ACK, PSH, RST, SYN, FIN

Amount of data *destination* can process at once:

Window Size

TCP header field specifies # of segments accepted before ACK is required:

Window Size

2 TCP *header fields* used to confirm receipt of data:

acknowledgment number & sequence number

Value of window size specifies:

amount of data that can be sent *before* ACK is *required*

what happens at app layer if 1st packet of UDP session isn't rec'd?

application would have to resend data

*entities* involved in *temporary* communication established by transport layer?

applications

4 functions of TCP:

connection-oriented, reliable delivery, ordered data-reconstruction, flow control

each set of data flowing between source app & destination app is known as a ________

conversation

pieces of communication in UDP:

datagrams

Host sends data packet to web server by HTTP. What is used by transport layer to pass data stream to proper application on the server?

destination port number

UDP does error _______, *not* correction.

detection

Ports in range: 49152 to 65535

dynamic, ephemeral, or private ports

​beneficial feature of UDP transport protocol?

fewer delays in transmission

TCP is a _________ protocol. Each connection is (two) one-way comm. streams

full-duplex

used for reassembly, added to each block of data

header

What allows different comm. streams to occur at same time, without single data stream using up all bandwidth?

multiplexing

utility to verify connections open & running on networked host:

netstat

ISN in the header of segment is increased by ____ for each byte of data sent

one

Client connects to HTTP server by randomly generated source port #. What destination port # will HTTP server use when responding?

original source port number *randomly generated* by client

TCP sends 1,2,3,4 but only 1,2, and 4 are rec'd. after ACK, device will send:

packets 3 and 4

POP3 port #

port # 110

IMAP port #

port # 143

FTP port #

port # 21

SSH port #

port # 22

TELNET port #

port # 23

SMTP port #

port # 25

HTTPS port #

port # 443

DNS port #

port # 53

DHCP port #

port # 68

HTTP port #

port # 80

What does client application select for TCP or UDP source port number?

random source port value in range of registered ports (1024-49151)

What causes additional network overhead for TCP communication?

re-transmissions & acknowledgements traffice

there is no _________ in UDP header

sequence number

What does Original Source, Original Destination, TCP, + FIN, ACK represent?

session termination

Enables multiple processes on client or server to distinguish themselves from each other?

sockets

what is a socket pair?

source *&* destination IP addresses *+* port numbers for 2 hosts

What is a socket?

source IP address + port #, *or* Destination IP address + port #

found in header of each segment or datagram:

source port & destination port

TCP is considered a:

stateful protocol (keeps track of conversation state)

UDP protocol is:

stateless / connectionless protocol (state of comm session not tracked)

reassembly of packets are handled by:

the application

During TCP session, destination device sends ACK # to source. What does ACK # represent?

the next byte that destination expects to receive

UDP is unreliable because:

there are no ACKs sent. (best-effort delivery)

TCP client responds w segment w ACK-response to TCP-SYN sent by server:

there is no user data in this segment

Purpose of source port number in TCP comms?

track multiple conversations between devices

# of segments that can be accepted at once (bits):

window size (16 bits)