Week 3 Flashcards
What sets the transport layer apart from the other layers?
That multiplexing and demultiplexing take place in it.
What is the medium through which multiplexing and demultiplexing take place?
Ports
Port
A 16 bit number that’s used to direct traffic to specific services running on a networked computer.
Destination port
The port of the service the traffic is intended for
Source port
A high-numbered port chosen from a special section of ports known as ephemeral ports
Sequence number
A 32 bit number that’s used to keep track of where in a sequence of TCP segments this one is expected to be
Acknowledgement number
The number of the next expected segment
Data offset field
A 4 bit number that communicates how long the TCP header for this segment is
TCP window
Specifies the range of sequence numbers that might be sent before an acknowledgement is required
TCP checksum
Operates just like the checksum fields at the IP and the Ethernet level
Urgent pointer field
Used in conjunction with one of the TCP control flags to point out particular segments that might be more important than others.
Options field
Sometimes used for more complicated flow control protocols
Padding
Same as network level padding in datagram header. Series of zeroes to ensure header is correct length so payload begins at right point of the TCP segment
URG (Urgent) flag
A value of one here indicates that the segment is considered urgent and that the urgent pointer field has more data about this
ACK (Acknowledged) flag
A value of one in this field means that the acknowledgement number field should be examined
PSH (Push) flag
The transmitting device wants the receiving device to push currently-buffered data to the application on the receiving end as soon as possible
RST (Reset) flag
One of the sides in a TCP connection hasn’t been able to properly recover from a series of missing or malformed segments
SYN (Synchronized) flag
It’s used when first establishing a TCP connection and makes sure the receiving end knows to examine the sequence number field
FIN (Finish)
When this flag is set to one, it means the transmitting computer doesn’t have any more data to send and the connection can be closed
Handshake
A way for two devices to ensure that they’re speaking the same protocol and will be able to understand each other
Socket
The instantiation of an end point in a potential TCP connection
Instantiation
The actual implementation of something defined elsewhere
LISTEN
A TCP socket is ready and listening for incoming connections
SYN_SENT
A synchronization request has been sent, but the connection hasn’t been established yet
SYN_RECEIVED
A socket previously in a a LISTEN state has received a synchronization request and sent a SYN/ACK back
ESTABLISHED
The TCP connection is in working order and both sides are free to send each other data
FIN_WAIT
A FIN has been sent, but the corresponding ACK from the other end hasn’t been received yet
CLOSE_WAIT
The connection has been closed at the TCP layer, but the application that opened the socket hasn’t released its hold on the socket yet
CLOSED
The connection has been fully terminated and that no further communication is possible
Connection oriented protocol
Establishes a connection, and uses this to ensure that all data has been properly transmitted
Port 0
Not in use for network traffic, but is sometimes used in communications between programs on the same computer
Ports 1 - 1023
System ports, or well-known ports : represent official ports for most well known network services such as HTTP
Ports 1024 - 49151
Registered ppoets: could be used for less common network services that may or may not be registered and acknowledged by IANA : a lot of databases listen on port 3306
Ports 49152 - 65535
Private or ephemeral ports : can’t be registered with IANA and are used for establishing outbound connections : no modern OS will ever use a system port for outbound communication
Firewall
A device that blocks traffic that meets certain criteria : can be used at many of the layers but are primarily used at the transport layer
Session layer in the OSI model
Facilitates the communication between actual applications and the transport layer
It takes application layer data and hands it off to the presentation layer
Presentation layer
Responsible for making sure the unencapsulated application layer data is able to be understood by the application in question
