Cs 356: Midterm 2

Front 1

More sophisticated transport protocol then UDP but packet end to end framework is still the same

Back 1

TCP

Front 2

TCP properties

Back 2

1) process to process communication

2) processes are identified using parts

3) congestion control

4) network reliability

5) flow control

6) byte stream oriented -> segmentation into packets

7) connection oriented

8) bidirectional

9) connection between 2 hosts only

Front 3

UDP properties

Back 3

1) process to process communication2) processes are identified using parts3) no congestion control4) "best effort" service, not reliable5) no flow control6) message oriented -> no segmentation7) connectionless8) unicast, broadcast

Front 4

Goal: avoid overwhelming the network with traffic

Back 4

Congestion control

Front 5

Goal: retransmit in case of packet loss on timeout

Back 5

Network reliability

Front 6

Goal: avoid overwhelming the receiving host

Back 6

Flow control

Front 7

__ control & __ control work together complementary

Back 7

Flow & congestion

Front 8

TCP will segment application's data into IP packets that fit into the link MTU

Back 8

Byte-stream oriented

Front 9

Sharing initial data in order to carry out different services; connection setup exchanges initial state between 2 hosts (ex: initial available buffer space, initial sequence numbers, addressing info (IP address and port))

Back 9

Connection oriented

Front 10

Why use UDP?

Back 10

1) Applications that require broadcast (ex: DHCP discover request)

2) service discovery protocols (ex: printer discovery)

3) Applications that need to send a single packet (or small # of packets) may use UDP to avoid overhead of connection setup (ex: DNS & DHCP)

4) applications that value timeliness over reliability (ex: voice, interactive exchange (phone call), games)

5) some services offered by TCP applications don't need, so they won't use TCP

Front 11

Addresses issues related to IP packets since IP is a "best effort" service

Back 11

Network reliability

Front 12

What are the 4 issues with IP packets that Network reliability addresses?

Back 12

1) IP packet dropped due to buffer overflow due to congestion

2) IP packet can be corrupted: link interface of faulty memory

3) IP packet can be delayed

4) IP packets can arrive at destination out of order

Front 13

Ack sequence # matches the packet being acknowledged

Back 13

Selective acks

Front 14

Ack with sequence # N means all packets up to sequence # N have arrived

Back 14

Cumulative acks

Front 15

Case 1: Normal operation mechanism

Back 15

Mechanism for response (acknowledge). Explicit: acknowledgement message, implicit: application response is the acknowledge

Front 16

Case 2: packet doesn't get to receiver (lost packet) mechanism

Back 16

Use timer; retransmit on timeout; base timeout on RTT weighted moving average

Front 17

Case 3: packet sent to receiver, no ack received (lost ack) mechanism

Back 17

Unique message id per packet; see duplicate packet, ignore packet and send ack

Front 18

Case 4: premature timeout

Back 18

Ack will have message is of corresponding packet; see duplicate packet, ignore packet and send ack; sender will receive duplicate ack that contains same id as packet message id

Front 19

Describe TCP implementation of network reliability

Back 19

Supports pipelining; sequence #s are in terms of bytes, not packets

Front 20

The byte stream # of the first byte in the packet

Back 20

Sequence #

Front 21

Byte stream # of the packet expected next cumulative acks

Back 21

Ack #

Front 22

TCP connection set up (3 way handshake)

Back 22

Client: SYN (seq#: 0, len: 0)

Server: SYNACK (seq#: 0, ack#: 1, len: 0)

Client: ACK (seq#: 1, ack#: 1, len: 0)

Front 23

Seq # = ?

Only changed when sending data

Back 23

Seq # + len

Front 24

TCP echo client and server sending 20000 bytes each (link MTU = 16384 bytes, IP = 32 bytes). Assume window size>=2 (skip 3 way handshake)

Back 24

Client: DATA (seq#: 1, ack#: 1, len: 16332)

Client: DATA (seq#: 16333, ack#: 1, len: 3668)Server: ACK (seq#: 1, ack#: 16333, len: 0)Server: ACK (seq#: 1, ack#: 20001, len: 0)

Server: DATA (seq#: 1, ack#: 20001, len: 16332)Server: DATA (seq#: 16333, ack#: 20001, len: 3668)

Client: ACK (seq#: 20001, ack#: 20001, len: 0) <--- delayed ack

Front 25

When host sees that a packet uses up all of its bytes, it will wait before acknowledging since it's likely there's another packet coming

Back 25

Delayed ack

Front 26

What is the web?

Back 26

Interconnected objects residing on hosts distributed across the internet

Front 27

Examples of web objects

Back 27

Web page (text/html) - audio

Images, video

Front 28

How are web objects referenced?

Back 28

Using URLs

Front 29

Url format:

Http(s)://www.example.org:12000/home/intro.html

Back 29

Http(s):// - schema

www.example.org - DNS hostname of host running web server

12000 - port server is running on

Home/intro.html - path to object

Entire thing: absolute URL

Path to object: relative URL

Front 30

Operation: fetch contents of object

HTTP Message:

Back 30

GET

Front 31

Operation: sending data, such as filling in a form

HTTP Message:

Back 31

POST

Front 32

Operation: update the object

HTTP Message:

Back 32

PUT

Front 33

Operation: delete web object

HTTP Message:

Back 33

DELETE

Front 34

Properties of HTTP message

Back 34

Text based, message is an encoded string of the HTTP message format

Front 35

Example of HTTP GET request

Back 35

GET home/intro.html HTTP1.1\r\n

(Called the request line: Operation, relative URL, version, indicate end of line)

Host: www.example.org:12000\r\n

(One or more header lines)

\r\n

(Designate end of header)

Front 36

Example of HTTP GET response

Back 36

HTTP1.1 200 OK\r\n

(Status line: version, return code)

Date: Thu, 7 Mar 2018 10:15:20 GMT\r\n

(Current time when request was processed)

Last-Modified: Wed, 6 Mar 2018 15:16:18 GMT\r\n

(Time when object was last updated)

Content-Length: (length of web object in response)\r\n

Content-Type: text/html; charset = UTF-8\r\n

\r\n

<contents of web object>

Front 37

How do you deal with out of date data in web caches?

Back 37

Supports a mechanism similar to DNS TTL value (absolute expiration time, relative time)

Front 38

Conditional GET

Back 38

Request sent to origin server -> only return the object contents if the object has been modified since the last time object was fetched

Front 39

How does Conditional GET differ from a regular GET request?

Back 39

Same first two lines

If-Modified-Since: Wed, 6 Mar 2018 15:16:18 GMT\r\n

\r\n

Front 40

Example of a not modified response

Back 40

HTTP1.1 304 Not Modified\r\n

Date: <current time>\r\n

\r\n

Front 41

Difference between HTTP 1.0 and HTTP 1.1

Back 41

HTTP 1.0: non-persistent, limit: one object per connection one round trip time overhead per object

HTTP 1.1: persistent, multiple objects per connection one round trip time overhead for all objects

Front 42

HTTP object not found response

Back 42

HTTP1.1 404 Not Found\r\n

Date: <current date>\r\n

\r\n

Front 43

Send window = min( , )

Back 43

Min(receive window, congestion window)

Front 44

Prevent sender from over-running the receiving host (overflowing the receiver's buffer)

Back 44

Flow control

Front 45

Limit sender's transmission rate to the available bandwidth (cwnd)

Back 45

Congestion control

Front 46

Number of packets (bytes) that can be transmitted but not acknowledged ("in-flight")

Back 46

Send window

Front 47

Available buffer space to receive packets in receiver

Back 47

Receive window

Front 48

Which two windows are always changing size?

Back 48

Send and receive

Front 49

What is the goal of TCP congestion control?

Back 49

Do not overrun the network

Front 50

How does TCP congestion control work?

Back 50

No explicit indication;

sender determines there's an issue through loss;

starts off by sending at very slow rate with a small window size; increases sending rate as long as there is no loss;

if there's loss, it'll slow down

Front 51

What are the three states of TCP congestion control?

Back 51

1) slow start

2) congestion avoidance

3) fast recovery

Front 52

Describe TCP congestion control: slow start

Back 52

State entered on connection start and when it times out

Initial state: cwnd = 1 packet

Each RTT, cwnd doubles

Front 53

Describe TCP congestion control: congestion avoidance

Back 53

New variable called ssthresh

Ssthresh keeps track of the cwnd size when loss last occurred

Ssthresh = cwnd / 2 when loss occurs

Congestion avoidance entered when cwnd >= ssthresh

Increase sending rate linearly, each RTT -> cwnd = cwnd + 1

Front 54

Describe TCP congestion control: fast retransmit

Back 54

Only lose a single packet, congestion can't be too bad, so retransmit earlier than normal

Front 55

Describe TCP congestion control: fast recovery

Back 55

Fast recovery entered on a triple duplicate ack

Halves the transmission rate

State retransmits the lost packet (fast retransmit) and waits for the ackHalves the transmission rate On entry to fast recovery, ssthresh = cwnd / 2On exit from fast recovery, cwnd = ssthresh

On entry to fast recovery, ssthresh = cwnd / 2

On exit from fast recovery, cwnd = ssthresh

Front 56

The data plane is in charge of

Back 56

Forwarding

Front 57

The control plane is in charge of

Back 57

Routing

Front 58

Which routing processes can routing processes talk to?

Back 58

Only those directly connected to each other

Front 59

What is the call of routing processes?

Back 59

Find the least cost path, which may be the shortest, quickest, or least congested

Front 60

Describe the link state

Back 60

Routers know global topology;

Shortest path algorithm applied (dijkstra algorithm)

Front 61

Describe distance vector

Back 61

Routers do not have global topology;

Communicates distance vectors to neighbors;

Distributed computation

Front 62

Routing processes can only talk to routing processes __ __ to each other

Back 62

Directly connected

Front 63

What are the goals of routing?

Back 63

Least cost path (shortest, quickest, least congested)

Front 64

Two types of routing processes

Back 64

Link state and Distance vector

Front 65

Describe the Link state routing process

Back 65

Routers know global topology; shortest path algorithm applied (Dijkstra's algorithm)

Front 66

Describe the Distance vector routing process

Back 66

Routers do not have global topology; communicates distances vectors to neighbors; distributed computation; bellman Ford equation

Front 67

Describe the host sending rules for Ethernet switching

Back 67

Host determines whether the destination IP address is on the same subnet or not; if it is, the host can communicate with the destination host directly (without going through a router); if not, host sends packet via router

Front 68

Forward packet based on destination IP address; using a forwarding table, populated using a routing protocol; network layer or L3 device

Back 68

Router

Front 69

Forwards (bridges, switches) packets based on destination MAC addresses; uses the MAC/switch table to forward packets out the right interface (MAC, outgoing interface); switch/MAC table is populated by snooping the MAC address in LL header of packets; LL or L2 device

Back 69

Ethernet switch

Front 70

LL or L2 device; supports Wi-Fi & converts between Wi-Fi and Ethernet; wifi support several modes; infrastructure mode is used when connecting to the internet; bridging using destination MAC address

Back 70

Wireless Access Point (AP)

Front 71

How is the MAC table populated for an incoming packet (when a packet arrives on a switch interface)?

Back 71

Record the source MAC address and the incoming interface in the MAC table; update the aging timer

Front 72

How is the MAC table populated for an outgoing packet (which interface to send packet out of)?

Back 72

If destination MAC address is in the MAC table, send packet out of the corresponding interface; if not, send packet out all interfaces except the incoming one

Front 73

What is LAN?

Back 73

Local area network (link) - ethernet, Wi-Fi

Front 74

What is Virtual LAN?

Back 74

Software configured LAN; enable a single physical switch to operate as multiple logical switches

Front 75

Packets carry a VLAN id or VLAN tag in LL header

Back 75

Trunk part