Studied by 1 person
transport layer services and protocols/ multiplexing and demultiplexing/ TCP and UDP
what is the transport layer?
transport layer provides logical communication between 2 processes on different hosts, the protocols are implemented in the end systems but not network routers
what is the network layer?
network layer provides logical communication between 2 hosts
what is the packet terminology per layer?
application = message
transport = segment
network = datagram
link = frame
how does the transport layer relate to the network layer?
transport layer relies on the network layer and enhances the existing network layer services. [via the IP Service Model]
how does the transport layer work in the IP stack? [sending side- sent TO the transport layer]
transport layer (TL) converts messages from the application layer into segments by breaking down the message into chunks and adding a transport layer header - encapsulation
TL passes the segment to the network layer (NL) where the segment is encapsulated into a datagram and sent to the destination
how does the transport layer work in the IP stack? [receiving side - data received back up from network layer]
NL extracts the TL segment from the datagram and passes it up to the TL
TL processes the segment and makes the data available to receiving application
how do network routers relate to the transport layer?
network routers act on the NL only and do not examine the encapsulated TL segment fields
what are the 2 transport protocols available for the applications on application layer to use?
UDP - unreliable, unordered delivery
TCP - reliable, in-order delivery
these transport protocols from the transport layer provide transport services
what services does the transport layer provide to the application layer?
process-level addressing
multiplexing and demultiplexing
integrity checking
connection management (establishment and termination)
acknowledgements and retransmissions
flow control
congestion control
these services are provided by a combination of UDP and TCP specific services
which of the transport services are specific to TCP services only?
connection management (establishment and termination)
acknowledgements and retransmissions
flow control
congestion control
what is process-level addressing?
TL differentiates between processes on applications, but the network layer identifies the actual hosts
what is multiplexing and demultiplexing?
used to pass data between correct application-level process and network, happens regardless of which protocol is used
what is integrity checking?
error detection for when data is sent
when is connection management (establishment and termination) provided?
provided if using a connection-oriented protocol (TCP)
when is acknowledgments and retransmissions provided?
provided if using a reliable service (TCP)
when is flow control provided?
provided if using an in-order delivery service (TCP)
how does multiplexing and demultiplexing relate to the network layer?
MUX and DMUX at transport layer enhances an existing network layer service
enhaces the existing host-to-host delivery service into a process-to-process delivery service
where does multiplexing take place?
on the sending host
what is multiplexing?
process of gathering data chunks from different sockets, encapsulating each chunk into segments and passing segments to the network layer
data can be sent all in one go
where does demultiplexing take place?
on the destination host
what is demultiplexing?
unwrapping the received segments and delivering them to the correct socket
why do sockets need a unique identfier?
receiving host can have many sockets so each socket has a unique identifier
what impacts the format of the socket identifier?
the type of socket used
UDP socket for connectionless demultiplexing
TCP socket for connection-oriented demultiplexing
what is the UDP socket identifier made up of?
destination ip address
destination port number
what is the TCP socket identifier made up of?
source ip address
source port number
destination ip address
destination port number
what does the host use to demultiplex a segment?
when (UDP or TCP) segment arrives from network to host, the host uses the type of socket, and its identifier to demultiplex the segment to the correct socket
how does demultiplexing occur if there are 2 segments with different source addresses but the same destination address?
using UDP = send both segments to the same destination process using the same socket
using TCP = send the segments to 2 separate sockets that are both made for the same process
how do web servers use port numbers?
well known servers are used by http and have registered socket numbers eg. web server has port number 80
when a client sends segments to a server all the segments (containing the initial connection request and all other requests) have the same destination port number [80]
server differentiates segments from different clients using source socket identifier
what type of correspondence can a client have with a web server?
one to one correspondence
multithreading
what is one on one correspondence?
each connection with a client causes the web server to spawn a new process
each process has its own connection socket that can sent and receive HTTP requests
explain this image of one to one correspondence
P3 is a process, with a corresponding socket underneath
server has 3 connections, therefore 3 processes are spawned, and a socket is assigned to each process
why is multithreading used?
to prevent servers being overloaded with one on one requests servers can use multithreading
what is multithreading?
only 1 process is created and within that process new threads are created and attached to that process. each thread has a connection socket used for a client connection. as usual each socket has its own socket identifier
what is a thread?
a lightweight sub-process
explain this image of multithreading
the server is now a threaded server, and only contains one process [P4] that has 3 threads attached which each have their own socket. for 3 client connections, they each use different sockets that are part of the same process
how does UDP - connectionless transport work?
take messages (payload) from application process - attaches source and destination port # for mux/demux, length, and checksum, and passes segment to network layer
if segment arrives at host, UDP uses destination port # to deliver to correct socket
which services does UDP provide?
process-level addressing
multiplexing and demultiplexing
light error checking
what is the structure of a UDP segment?
width 32 bits is relative to field size, eg. source port # is 16 bits, dest port # is 16 bits
header = 8 bytes
what is the length field?
number of byes in UDP segment (including both header & payload)
needed because size of payload differs from segment to segment
what is the checksum field?
used by receiving host to check if the segment has errors
sender treats segment content as a sequence of 16-bit integers and uses this to calculate the checksum for that segment (one’s complement sum of segment content)
receiver recalculates the checksum and checks if this matches the checksum field
what are the features of TCP - connection oriented transport?
processes must handshake to establish connection
three way handshake procedure
has a logical connection
provides full-duplex service
always point to point
explain the feature: processes must handshake to establish connection
preliminary segments are sent to each other to establish the parameters of the resultant data transfer
both sides will initialise many TCP state variables regarding the TCP connection
explain the feature: three way handshake procedure
client sends special TCP segment to server asking to initiate connection (no payload)
server respond with special TCP segment to initiate or not (no payload)
client sends special TCP segment (may contain payload)
explain the feature: a logical connection
TCP protocol runs on end systems only and not network elements (routers) - network elements are oblivious to TCP connection occurring
explain the feature: provides full-duplex service
application layer data can flow from process A to process B, and vice versa
both hosts allocate a receive buffer and define a receive window variable
explain the feature: always point to point
between one sender and one receiver
what are the different properties of a TCP connection?
A TCP connection consists of the set of properties:
{buffers, variables, and a socket connected to a process}
There is a set of these properties at each host- meaning each host at each side of the TCP connection has it’s own send and receive buffer
what are the steps of how TCP properties are used during a TCP connection?
client process passes a stream of data through the socket
TCP running in the client directs data to the connection’s send buffer
TCP grabs chunks of data from send buffer
Maximum amount of data that can be grabbed and place in segment it defined by the Maximum Segment Size variable (MSS)
max size of the payload (application data), not max size of whole TCP segment (payload & headers)
calculate by getting length of the largest link layer frame that the host can send (Maximum Transmission Unit - MTU) to ensure the TCP segment encapsulated as a datagram + TCP/IP header length (40 bits) will fit in a frame
TCP pairs each chunk of data with a TCP header to form a TCP segment
segments passed to network layer where they are separately encapsulated into an IP datagram
IP datagram is sent into the network
when TCP receives a segment at other host, the segment’s payload is placed into the connection’s receive buffer
the application reads the stream of data from the receive buffer
blue highlight represents encapsulation and multiplexing as mentioned earlier
what is the structure of a TCP segment?
source port #
destination port #
checksum
sequence number
acknowledgement number
receive window
header length
options
flag
urgent data pointer
how does the source port #, destination port # and, checksum differ from TCP to UDP?
they don’t differ, the fields are the same
what TCP service is the sequence and acknowledgement numbers used for and what size are they?
used by TCP sender and receiver for reliable data transfer
both are 32 bits each
what TCP service is the receive window used for and what size is it?
used for flow control
16 bits
what is the header length field used for and what size is it?
shows the length of the TCP header in 32-bit words
4 bits
the TCP header can vary in length depending if the Options field is used or not.
if options empty then header length = 20 bytes
What is the options field used for and what size is it?
optional fields used for negotiating MSS between hosts, window scaling factor for high speed networks, time stamping
has a variable length
what is the flag field used for and what size is it?
contains 6 bits where each bit has a different meaning and use
ACK bit - means the segment contains an acknowledgement (successfully received another segment)
RST, SYN, FIN bits - used for connection setup and teardown
CWR, ECE bits - used in explicit congestion notification
PSH bit - indicates receiver should push data to the upper layer immediately
URG bit - indicates there is data in the segment that is marked by the upper layer as urgent
what is the urgent data pointer field used for and what size is it?
points to the last byte of data that is marked as urgent
16 bits
how is a sequence number used?
⇒ byte-stream number of the first byte in the segment
500KB data (0 - 499,999) over 1KB MSS will be transmitted in 500 segments
The first segment will have sequence number 0, the second segment will have sequence number 1,000, the third segment will have sequence number 2000, etc
how is an acknowledgement number used?
⇒ the sequence number of the next byte the receiving host is expecting from the sender host
when a receiving host is receiving the above data, after receiving the first segment it will send a segment containing acknowledgment number 1,000.
explain this image of seq and ack numbers
the acknowledgement number increment by 1 each time after each segment is sent since they are expecting segments to be sent in order - eg. server expects segment with sequence # 43, then segment with sequence # 44, client expects segment 79, then segment 80
the other host should then send a segment with the sequence number that the current host is expecting
what type of data transfer does IP have?
IP alone has an unreliable best-effort service
what is the TCP service - reliable data transfer?
TCP creates a reliable data transfer service on top of the standalone IP service
ensures the data stream read from the receive buffer is uncorrupted
not gaps or duplication and in-sequence
byte stream sent is the same as the byte stream received
what are the two ways TCP carries out reliable data transfer?
timeout- triggered retransmission
fast retransmit
what is a timeout?
a timer is started to measure the time between when a segment is sent and when an ACK for that segment is expected to be received by the sender → used to identify and recover lost segments.
how does a timeout work?
TCP starts the time when a segment is passed to IP stack as long as the timer is not already running from a previous segment
If the ACK segment is not received in time a timeout occurs.
TCP retransmits the segment that caused the timeout and restarts the timer
When the ACK arrives the TCP compares the ACK value with the sequence number of the oldest unacknowledged byte
If there are issues with the ACK number then TCP restarts the timer if there are any unacknowledged segments
what are the issues with timeout-triggered retransmissions?
if timeout period too long ⇒ when segment lost, the sender has to delay resending the lost packet which increased overall end-to-end delay
if timeout period too short ⇒ the ACK may arrive after the timeout period causing unnecessary retransmission
how does TCP use cumulative acknowledgements?
an ACK acknowledges the receipt of all the bytes before the byte number. based on the ACK number the TCP sender can determine if there are any segments that were not acknowledged
what is fast retransmission?
the TCP sender can use duplicate acknowledgements sent by the receiver to detect packet loss before the timeout occurs
what is a duplicate acknowledgment?
an ACK that acknowledges a segment that the sender has already received an acknowledgment for earlier on
how does fast retransmission work?
TCP receiver receives the segment and checks sequence number
if the sequence number larger that expected it means there’s a gap in the in-order sequence data stream thus, missing segment (eg. sequence num = 99, then 101 when expecting 100)
receiver reacknowlges by sending an ACK for the last in-order byte of data it received (eg. send an ACK for 101 containing ACK number = 100 instead of using ACK number = 101)
if sender is sending segments back to back then multiple duplicate ACKS can occur (eg. 3 segments trying to be send after the missing one, 101, 102, 103. instead of giving an ACK for each of those numbers, duplicates the ACK# = 100 3 times)
if sender received the duplicate ACKS - eg. 3 duplicate ACKs for 100, means that the segment with the same ACK number needs to be sent (eg. need to resend segment 100)
sender performs a fast retransmit of the missing segment before the segment timer expires
what is the TCP service - flow control?
a speed matching service that matches the rate at which sender is sending against rate that receiving application is reading
why is flow control needed?
if application is slow at reading data ten sender can overflow the connection’s receive buffer by sending too much data too quick
how does flow control work?
Sender has a variable called receive window
TCP is full duplex so both hosts allocate and define a receive window variable
when an application reads data it takes it out from the receive buffer.
the receiver knows the last byte read and the last byte received. the receive window is calculated.
the sender uses the receive window to know how much data to send
what are the equations involved for flow control?
what is a receive buffer?
memory to store the payload of segments
what is a receive window?
a variable showing to the sender how much free buffer space is available at the receiver
the receive window value is sent to the TCP header and because no overflow is permitted the receive window is always ≤ receive buffer
what two parts make up the TCP service - connection management?
connection establishment
connection termination
what is connection establishment?
a three-way handshake procedure to form a connection between 2 hosts before data can be sent
made up of 3 parts:
SYN segment
SYN/ACK segment
ACK segment
what happens in the SYN segment?
client sends special segment to server
segment contains no application layer data
SYNbit in header = 1
client randomly chooses initial sequence number to add to sequence number field (client_isn)
what happens in the SYN/ACK segment?
server receives SYN segments and allocates the TCP buffers and variables to that side of the connection
sends special segment to client
sets SYNbit = 1 in header and ACKbit = 1
sets ACK# to client_isn + 1 and adds to header
chooses random number to be initial sequence number (server_isn) and adds to header
sends connection-granted segment to client where SYN bit = 1
does not contain application layer data in payload
what happens in the ACK segment?
client receives SYN/ACK and allocates TCP buffers and variables to that side of the connection
sends special segment to server acknowledging that connection-granted segment
SYNbit = 0, ACKbit = 1, SEQ# = client_isn + 1 , ACK# = server_isn + 1
can contain application layer data in payload
this is an example of connection establishment with three-way handshake
what is connection termination?
either process can end the connection
when connection ends the resources - [buffers / variables] in the host are deallocated
how does connection termination work?
client application process issues close command
client TCP sends special segment to server
FINbit = 1
server receives segment and sends an ACK segment for it (where ACKbit = 1)
server sends a special shutdown segment
FINbit = 1
client receives the segment and sends and ACK segment for it
resources in the host are deallocated
what is the TCP service - congestion control?
TCP must use end-to-end (between sender and path to receiver) congestion control rather than network-assisted congestion control, since the IP layer provides no explicit feedback to the end systems regarding network congestion
how does congestion control work?
sender limits the rate at which it senders traffic into its connection based on the function of its perceived network congestion
perceives little congestion = increase send rate, high congestion = reduce send rate : inverse relationship
the TCP perceives the network congestion based on the amount of packet loss and/or delayed acknowledgements (high ACK rate = perceived congestion free)
congestion-control mechanism at sender manages a variable called congestion window (cwnd)
CWND imposes constraints on the rate that the sender can send traffic into the network, the sender cannot send an amount of unacknowledged data into the network that is greater than the minimum value of the congestion window or the receive window
what equation is involved for congestion control?
LastByteSent − LastByteAcked ≤ min{cwnd, rwnd}
what is bandwidth probing?
the TCP sender’s rate increases when receiving ACKs until a loss event occurs, then the transmission rate is decreased
what is additive increase multiplicative decrease
the way bandwidth probing occurs
additive increase - cwnd by 1 MSS every RTT until loss detected
loss occurs
multiplicative decreases the bandwidth - cut cwnd in half after loss
when would we use UDP over TCP?
application-level control
minimum sending rate where loss tolerated but not delay
no connection establishment delay
no client-server connection state so can manage more request
smaller size due to smaller 8 byte header vs. 20 byte
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Flashcard (58)