Networks Final exam

What is MSS

Maximum Segment size - maximum TCP payload size per segment (data only)

What does TCP ACK number represent

The sequence number of the next expected byte

CWND

congestion window, sender-side limit on how much data can be in flight

RWND

Receiver advertised window - receiver side buffer limit

Effective TCP Window equals what

min(cwnd, rwnd)

TCP receive window range

[ACK, ACK + rwnd - 1]

When does cumulative ACK advance

only when all bytes starting at the current ACK are received contiguously

Does receiving Out of order data advance an ACK

no

What happens if a packet arrives completely outside the receive window

Packet is dropped; no ACK sent

What happens if a packet partially overlaps the receive window

In-range bytes may be accepted; ACK advances only if the gap at ACK is filled

How does cwnd grow in slow start

+1 MSS per ACK

Why does cwnd double every RTT in slow start

Because cwnd ACKS arrive within one RTT

CWND behavior over one RTT in slow start?

cwnd doubles

Is slow start exponential per ACK?

No - exponential per RTT

How does CWND grow in congestion avoidance

+1 MSS per RTT

Cwnd increase per ACK in congestion avoidance

X/MSS

Why does congestion avoidance grow linearly?

To cautiously probe for available bandwidth

What indicates packet loss via duplicate ACKs

3 duplicate ACKs

What indicates packet loss via timeout

No ACK received before timeout expires

What triggers fast retransmit

Receipt of 3 duplicate ACKs

What does fast retransmit do

Immediately retransmits the missing packet without waiting for timeout

What is fast recovery

A mechanism that avoids resetting CWND to 1 MSS after 3 dup ACKs

ss thresh updates on 3 dup ACKs in fast recovery

ssthresh = cwnd/2C

cwnd during fast recovery

cwnd = ssthresh + 3

When does fast recovery end

When a new Ack acknowledging the retransmitted packet arrives

CWND after fast recovery ends?

cwnd = ssthresh

cwnd behavior on timeout

cwnd = 1 MSS, enter slow start

cwnd behavior on 3 dup ACKs

cwnd halved, enter fast recovery

What is ssthresh

threshold separating slow start and congestion avoidance

when does ssthresh change

only when packet loss is detected

What does ssthresh represent

A memory of the last congestion point

What is BDP

bandwidth * RTT

what does BDP represent

Max data that can be in flight without congestion

How do you compute max cwnd

BDP / MSS

What is TCP Throughput proportional to

inverse square root of packet lossj

Effect of quadrupling loss rate

Throughout put halved

Sequence number for n bits

2^n

Max sender window if ACKs are never lost

equal to sequence number space

Max sender window if ACKS can be lost

half the sequence number spave

Why must sender window be limited

to avoid ambiguity after sequence number wraparound

Virtual finish time

Previous finish time + packet size

Virtual finish time (weighted)

Previous finish time + (packet size / weight)

How does tcp react to congestion

reduce send rate

how does UDP react to congestion

Does not reduce send rate

HTTP

an application layer, request-response protocol for transferring web resources

What transport protocol does HTTP/1.1 use

TCP

What is HTTP/1.1

TCP-based version of HTTP using persistent connections

How many requests can be outstanding on one HTTP/1.1 connection

One at a time!W

What is head-of-line (HOL) blocking in HTTP/1.1

later requests just wait until earlier responses are fully received

How do browsers avoid HOL blocking in HTTP/1.1

By opening multiple parallel TCP connections

Why are multiple TCP connections inefficient

Increase congestion, RTT overhead, slow start penalties

What is HTTPS

HTTP over TLS (encrypted HTTP)

What security properties does HTTPS provide

Confidentiality, integrity, authentication

What is the goal of HTTP/2

Improve performance without changing HTTP semantics

Does HTTP/2 eliminate HOL blocking

No - HOL blocking still exists at the TCP layer

QUIC

A transport protocol builtt on UDP to replace TCP for HTTP

What application protocol runs over QUIC

HTTP/3

Does QUIC suffer from TCP HOL blocking

No - loss in one stream does not effect others

Which protocol avoids TCP

QUIC

Transport Layer

Runs on top of “best effort” network delivery service

A mechanism is reliable if and only if

1. It resends all dropped/corrupted packets

2. It attempts to make progress

Sequence number

monotonically increasing value carried in each sent packetsAC

Individual ACK

Receiver sends ACK for each individual packet it receives

Go Back N

Retransmit all packets starting from the smallest sequence # packet that wasn’t received

selective retransmit

only retransmit unreceived acks

Selective ACK

ACK contains info about ALL received packets thus far

Cumulative ACK

Just first part of selective ACK, ACK only highest continuous received sequence number

Congestion

Overloading the network

Flow

overloading the receiver

3 way handshake for setup

SYN, SYN-ACK, ACK

3 way handshake connection termination

FIN, FIN-ACK, ACK

AIMD

On success, increate CWN 1 MSS

On 3 dup packs, CWND= CWND/2

On timeout, CWND = 1 MSS

Slow start

start small (CWND = 1MSS)

Increase exponentially for short duration AND while Cwnd small

go back to additive increase when meet ssthresh

Throughout

Roughly CWND/RTTF

Blufferboat

high latency caused by excess buffering of packets, small buffers cannot handle traffic bursts

TLS goals

1. Verify identity of server before sending HTTP messages

2. Generate and exchange keys to encrypt HTTP messages