TCP/UDP/QUIC

Question 1

Name some specialitites of TCP

Answer 1

• Connection oriented
• in-sequence delivery of byte stream
• Reliable (bit error detection, TransportService DU loss detection & retransmit)
• Flow control (do not overwelm receiver)
• Congestion control (does not overwelm network

Question 2

How does the sender know that packets are lost?

Answer 2

• Ack is not received in timeout
• Multiple duplicate ACKs: reordering or packet loss
  3 duplicate ACKs -> fast retransmit

Question 3

Name the formular for estimating a TCP rtt

Answer 3

(1-a) * estimated RTT + a* sampleRTT

Question 4

What two parameters do we need to know for effective congestion control?

Answer 4

• RPprop: round trip propagation delay

- BtlBw: Bottleneck Bandwidth

Question 5

What is the BDP?

Answer 5

Capacity how much data the link can “take”/hold

RTprop * BtlBw

Question 6

How can you measure the fairness of multiple tcp flows?

Answer 6

Jain's fairness index
\+returnval: 0-1
\+scale free
\+ arbitraty num of flows
\+k/n for perfectly fair flows (if rest is 0) 

(sum flows)^2 / n * sum(flows^2)

Question 7

Which congestion control algorithms exist for TCP?

Answer 7

TCP Tahoe (slow start, congest avoid)
TCP Reno (fast retransmit, fast recovery)
TCP Vegas (delay based, keep rtt const) 
TCP Cubic (loss based)
TCP BBR (mix, keep 1 BDP inflight)

Question 8

How do loss based cc algorithms work and what are advantages and disadvantages?algorithms?

Answer 8

no loss -> increase congest windows
loss -> decrease window

+ robust, reliable, efficient
- buffers kept full, high latency and perf drop on lossy links

Question 9

How does TCP reno work?

Answer 9

AIMD: additive increase multiplicative decrease

CWND: congest wdw
sstresh: slow start thres

slow start: increase for every ack by 1 mss
congest avoid: effect. increase by 1mss every RTT

3 dupl acks: sstresh = CWND / 2

Question 10

How does TCP Cubic work?

Answer 10

Loss based; estimate bandwitdth and try to use it. If nothing lost, explore for more
-> congestion window growth modeled after cubic function plateau. tries to recover fast to the bandwidth of last packet loss.

Question 11

What are advantages and disadvantages of TCP Cubic?

Answer 11

+ cwnd growth independent of rtt
+ scalable to high BDP networks
+ more resilient against single packet loss

• buffers are filled faster
• buffers kept filled

Question 12

How do delay based congestion control algorithms work and what are the advantages and disadvantages?

Answer 12

Measure delay to detect congestion. RTT increase: buffer filling somewhere.

+ less restrained by random packet loss
+ early congestion detection
+ high throughput w/ low latency
- one loss-based flow cancels all advantages
- poor performance against loss based flows

Question 13

How does TCP Vegas work?

Answer 13

AIAD: Additivite increase additive decrease

continously measure rtts
delta = CWND * (RTT - RTTmin)/RTT
if delta > beta: decrease by 1mss
if delta < alpha: increase by 1 mss

Question 14

When do you use loss based algorithms?

Answer 14

Background applications like update download - LEDBAT low extra delay background transport

Question 15

What is the base idea of TCP BBR?

Answer 15

measure bottleneck bandwidth and RTT. -> same operation point as delay based algorithms.
max bandwidth is determined by weakest link.

Keep 1 BDP inflight: optimal usage
Send with BtlBw: do not create queues

Question 16

Pacing vs ACK-clocking?

Answer 16

Pacing: control transmissions of packets of a windows across entire rtt

ACK-clocking: arrival rate of acks limits sending
bursts (ack compress, slow start, retransmissions) can create queues even if link is not used

Question 17

Describe the startup phase of TCP BBR

Answer 17

Startup: double sending rate every RTT. Stop after 3 cons RTTs with < 25% delivery rate
-> Measure BtlBW

Question 18

Describe the drain phase of TCP BBR

Answer 18

Drain: remove queue from startup

Question 19

Describe the probe bandwidth phase of TCP BBR

Answer 19

Probe bandwidth: BtlBw * pacing gain
if no bw available: reduce sending rate afterwards
if bw available: BtlBw gets updated to increase sending rate

Question 20

Describe the probe rtt phase of TCP BBR

Answer 20

Probe RTT: every 10s probe for RTprop -> Drain queues and measure. Mult BBR flows have to synchronize

Question 21

What are advantages and disadvantages of BBR?

Answer 21

+robust against rand packet loss
+low delay, high BW usage
+close to optimal operation point
+ no starve against other cc

• alternating probeBW leads to 1.5 BDP
• Shallow buffers -> high amount of retransmissions
• flows w larger RTT receive larger BW than flows with lower RTT (opposite in Reno/Cubic)

Question 22

What is UDP?

Answer 22

• Connection less transport protocol
• bit error correction
• no flow or congest control
• no reordering, loss detect, or recovery

Question 23

What is SCTP?

Answer 23

Stream control transmission protocol
- multi homing
- multiple streams
- advantages of tcp & udp
But: poor adaptation, middleboxes often dropped packets (unknown protocol)

Question 24

What is QUIC?

Answer 24

Quick UDP Internet Connections

substitute of TCP/TLS

• decrease handshake delay
• middlebox resistance
• ip mobility
• fast development cycles

Question 25

What are connection IDs in quic?

Answer 25

Connection identifiers which are used instead of 5-tuple (proto, src ip, src port, dst ip, dst port)
-> allows chaning port and ip

Question 26

What QUIC packet types do exist?

Answer 26

• Version negotiation packet
• initial packet
• retry packet
• handshake packet

Question 27

How does QUIC perform loss detection and re-ordering?

Answer 27

Retransmissions have differen packet numbers -> stream offset allows in-order deliver

selective and negative ACKs

Question 28

How does QUIC cope with handshakes?

Answer 28

0-RTT handshake: reuse old connections

1-RTT handshake: combine TCL & TLS ‘components’

Question 29

How does a QUIC 1-RTT handshake work?

Answer 29

ClientHELO -> Server
Server -> Reject incl Certificate info

ClientHELO enc’ed -> Server (include cert)
ServerHELO enc’ed -> Client

Question 30

What is head-in-line blocking? And how does QUIC fix it?

Answer 30

Lost tcp packet blocks all subsequent packets (including other streams) (in-order property of tcp).
Quick offers multiple streams. Retransmission blocks on stream-level not connection-level.

Question 31

Why can quick be developed faster?

Answer 31

Not implemented in kernel: UDP Is but does not change, QUIC can be updated

Question 32

Name the 5 goals quick wants to fullfil.

Answer 32

1 Minimize connection establishment and transport latency
2 No head of line blocking
3 Requiring only changes to path endpoints to enable deployment (not middleboxes)
4 Multipath & fwd error correction
5 always secure, tls1.3