Chapter 5 - Transport Layer Flashcards
(57 cards)
Transport Layer Protocols are implemented in only?
The end systems (“Hosts”)
What does the transport layer provide
Process-to-process communication services
UDP
User Datagram Protocol is a network protocol that allows devices to send messages to each other over the internet for applications that require speed and efficiency
Whats in a UDP Segment
- Fixed 8 byte header that includes source and dest port numbers (16 bytes)
- Checksum field
- Simple Header since it provides a simple service
TCP
Tranmission Control Protocol, that procided reliable ordered and error checked delivery of data between applications
What does the TCP Segment include
- 20 Byte fixed-length header
- Application data
- src and dest port num
- sequence number
- acknowledgement number
- header length 4 bits
- flag field
- flow control window size
- checksum
- poiinter to urgent data
TCP Header Flag Field includes
CWR: Congestion Window Reduced
ECE: Sending rate is causing network congestion
URG: Indicate urgent data
ACK: valid ack number
PSH: deliever data asap
RST: abort connection
SYN,FIN: establish connection
TCP Three-Way Handshake
Client sends a SYN
Server replies with SYN + ACK
Client replies with ACK
Why does TCP use a three-way handshake
Confirm both endpoints are ready and to prohject against forged and duplicate packets
SYN Cookie
Special initial sequence number generated by the servee that encodes necessary state infor. Prevents SYN Flood attacks
Main goals for TCP Connection Release
- Both sides can release state and buffers
- Prevent data loss due to premature release
How does TCP perform graceful connection closure
A sends FIN -> B
B ACKs FIN
B sends FIN -> A
A ACks FIN and waits
After wait A releases connection
What is TCP TIME-WAIT
A waiting period 60-120s where connection is kept hald open to handle delayed segments
TCP Flow Control Mechanism based on?
The receivers advertised window size.
What happens when the flow control window size is 0
Sender stops sending but can resume after recv a window update. Done to prevent deadlock
TCP Window Probe
1-Byte segment sent by the sender towhen the reciever window is 0 it is used to get an updated window size
“Silly window syndrome”
Inefficient and small advertised windows lead to small segment tranmissions.
Nagles Algorithm
Places another contraint on the sender with the goal of avoiding scenarios where the sender floods the network with very small sefments. Prevents previous data from sending unless ACK’s or full MSSS worth of data is ready to be sent
When can delayed ACKs be used in TCP
For data sent in order with no gaps the ACK can be delayed but should acknowlegde every 2nd segement
When must TCP send an ACK immediately
- If a received segment fills a gap
- If a segment is out of order
What is the problem with delayed ACK’s + Nagles algorithm
It can cause deadlock as sender can be waiting for an ack before sending its next segment. Receiver waits to ACK.
When can TCP retransmit segments
- Retransmission timer to control time spent waiting for an ACK.
- Fast Retransmit if 3 Duplicate ACK’s are received
RACK
Recent Acknowledgement uses per packet transmission timestamps. Concludes packet has been losr if some packet sends another after a sufficient period of time
TLP
Tail Loss Probe is if there has not be an ack recently transmit a new segment, retransmit the newest unack’s segment and maybe this will trigger an ACK.
