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)
