Chapter 3- Part3 Flashcards
What’s the typical MSS (Maximum segment size) and what does it include?
1460 Bytes
refers to the PAYLOAD of the segment. NOT the entire segment
Key points of TCP?
- Point-to-point: 1 sender,1 receiver
- in order
- bidirectional data flow
- cumulative ACKs
- pipelining (using window size)
- connection oriented
- flow controlled
What does a TCP Segment structure include?
- Source and dest port
- Seq Number
- ACK Number
- Length
- Receiver window
- Checksum
- Urgent data pointer
- Options
- App Data (Variable Length)
What does a UDP Segment structure include?
- Source and dest port
- Length
- Checksum
- App Data
A bit = 0 vs A bit = 1
0: ignore this
1: this segment is a valid ACK number
What is the sequence number in TCP?
Points to the BYTE NUMBER not the segment number.
Number of the first byte in segment’s data
What is the ACK number?
seq # of the next byte expected
How does the receiver handle out-of-order segments for TCP?
TCP spec doesn’t say. It’s up to the implementor
Ideal Timeout value?
Longer than RTT.
How does TCP send ACKs?
Delayed ack. Wait up to 500ms for the next segment.
What is the TCP Fast Retransmit?
If sender receives 3 additional ACKS for the same data (3 DUPLICATES, 4 total). Resend the unACKed segment with the smallest seq. number
How does flow control occur in TCP?
Receiver controls the sender through the receive window (rwnd). Indicates # of bytes receiver is willing to acceptr.
Sender can’t overflow the receiver’s buffer
In TCP this is two-way so they both have a rcvBuffer size.
What are the RST, SYN, and FIN bits?
RST - reset bit
SYN - synchronize bit (for establishing a TCP connection)
FIN - finish bit (for closing a TCP connection) . Set to 1 when closing a connection
3 Way Handshake for TCP?
- Sender sends TCP SYN message (SynBit = 1) and Initial Seq Number [NO DATA SENT]
- Receiver sends TCP SYNACK message and Initial Seq Number [NO DATA SENT]
- Sender received SYNACK
Closing Connection: Both sides set FIN=1
Explain TCP AIMD?
AIMD means: Additive Increase, Multiplicative Decrease
Additive Increase: incease sending rate by 1 MSS every RTT until loss detected
Multiplicative Decrease: cut sending rate in half at each loss event
What kind of congestion control does TCP have?
End-to-end. Only based on observed network behavior seen by sender.
What is the cwnd window?
Congestion Window Size. THe max amount of bytes we can send.
LastByteSent - LastByteAcked <= cwnd
Cwnd can be dynamically adjusted
Explain TCP Slow Start
Increases the rate of sending exponentially until first loss event.
Double the cwnd every RTT by:
- Incrementing the cwnd by 1 for each ACK received. So it doubles the packets sent for each ACK received.
How does the ssthresh work in TCP connections?
ssthresh is used to determine if TCP should send exponential or linear.
The rule is:
1. increase congestion window exponentially until it reaches ssthresh
- increase congestion window linearly when it reaches ssthresh
2 types of loss events?
- TIMEOUT
- TRIPLE Duplicate ACK
What happens to cwnd on loss events?
Timeout: Cut to 1 MSS
For TCP Reno:
Triple Duplicate ACK: Half + 3
How is congestion detected by the TCP
Congestion is determined by the sender
What happens if you increase TCP sending rate with a congested bottleneck
- it won’t increase end-to-end throughput
- it will increase measured RTT
Explain the delay-based TCP congestion control
keep sender-to-receiver “just full enough but no fuller”
= cwnd/RTT
Case 1: throughput close to uncongested throughput: increase cwnd linearly
Case 2: throughput less than uncongested throughput: decrease cwnd linearly
Different from AIMD because it doesn’t induce/force loss
What is explicit congestion notification? (ECN)
- Routers in between source and dest are alerted of congestion.
- Router alerts destination -> destination alerts source
- ECN = 11 to notify congestion
