19531 - Telematics - 15th Tutorial - Q&A...19531 - Telematics 15th Tutorial - Q&A Bastian Blywis...

19531 - Telematics15th Tutorial - Q&A

Bastian Blywis

Department of Mathematics and Computer ScienceInstitute of Computer Science17. February, 2011

Institute of Computer Science – Telematics Tutorial – 17. February, 2011 1

Outline

1. Exam

2. Statistics

3. Q&A


Exam

Date of exam: 24.02.11Time: 10:00 - 12:00

Location: Physiology lecture hall, Arnimallee 22, 14195 Berlin

– Entrance starts at 10:00– Leave bags, coats, etc outside of the lecture hall or in front of the chalkboard (not

at your seat!)– Submit dictionaries for review before 23.02.11, 12:00– You are not allowed to use your brought along paper– No aids are allowed, especially no books or calculators– Give concise answers; 1-3 sentences are usually enough


Exam


Statistics

– 165 students in the KVV– 127 students registered for the first exam– Only 4 students submitted questions for this tutorial session

⇒ The lecture and tutorials where adequate and there are not any question left?!


Q&A

“Please repeat how a bit sequence is modulated on acarrier.” ?? ?Institute of Computer Science – Telematics Tutorial – 17. February, 2011 7

Q&A

t

A

1

2

-1

-2

3

-3

00 01 10 01 00 11

S A P

00 1 001 2 010 1 π11 2 π


Q&A

t

A

1

2

-1

-2

3

-3

000 110 010 011

S A P

000 1 0001 2 0010 1 π

2011 2 π

2100 1 π101 2 π

110 1 3π2

111 2 3π2


Q&A

t

A

1

2

-1

-2

3

-3 10 01 01 11 10 01

S F A

00 f 101 f 210 2f 111 2f 2


Q&A

“Please explain flow control and the sliding window algo-rithm again. I do not yet fully understand the format usedin the lecture and tutorial.” ?? ?Institute of Computer Science – Telematics Tutorial – 17. February, 2011 11

Q&A

– M = 8⇒ window size W = 7– Frames with the sequence

numbers 5, 6, 7, 0, 1 have beensent

– Sequence numbers 2 and 3 canbe used

– Received ACK = 2– Acknowledged: 5, 6, 7, 0, 1– Window: 2, 3, 4, 5, 6, 7, 0

0

1

2

34

5

6

7

(a) Previous State

0

1

2

34

5

6

7

(b) Received Acknowledgement


Q&A

– M = 8⇒ window size W = 7– Frames with the sequence

numbers 5, 6, 7, 0, 1 have beensent

– Sequence numbers 2 and 3 canbe used

– Received ACK = 6– Acknowledged: 5– Window: 2, 3, 4

0

1

2

34

5

6

7

(c) Previous State

0

1

2

34

5

6

7

(d) Received Acknowledgement


Q&A

“Which meaningful applications have the presentationand session layer regarding protocols that where dis-cussed in Telematics on layer 4 and 7?” ?? ?Institute of Computer Science – Telematics Tutorial – 17. February, 2011 14

Q&A

– Presentation and session layer do not exist in the TCP/IP model– Layered protocol stack model is most often not even available in this shape in

operating systems– Functions are incorporated in application layer protocols– Application layer protocols can have sub-layers– Web applications would benefit from a session layer (remember the cookie

assignment)– Email would benefit from a presentation layer (remember attachments)– Basically all application layer protocols would benefit from a presentation layer

(host vs. network order, big endian vs. little endian)


Q&A

“On which layers is QoS implemented and on which lay-ers controlled?” ?? ?Institute of Computer Science – Telematics Tutorial – 17. February, 2011 16

Q&A

– How would you define the term QoS?– What properties are required? Throughput, delay, jitter?⇒ depends on the requirements of the application

– QoS functions can be implemented on many layers, example:– Data link: Media access protocol– Network: Virtual switched circuits– Application: Streaming protocol

– Holistic approach is the best, i.e., consider the properties of all layers and provideQoS support

– Problem: You do not have control over all nodes in the Internet


Q&A

“Layer 2 and 4 provide similar functions: connecting twoadjacent stations or end-to-end connection. Is there asplit of functions similar to the MAC and LLC sub-layersin the transport layer?” ?? ?Institute of Computer Science – Telematics Tutorial – 17. February, 2011 18

Q&A

– There is in general no similar split into sub-layers in the transport layer– But a TCP implementation could be split into sub-layers as long as the interface to

the upper layer protocols remains the same– Flow control and congestion control have separate goals but are highly

interdependent– UDP is too simple to be split into sub-layers– SCTP, DCCP, . . . ?


Q&A

“A P2P network is an overlay network. Are all operationsof the P2P network based in the application layer alone?” ?? ?Institute of Computer Science – Telematics Tutorial – 17. February, 2011 20

Q&A

– Of course you could implement a P2P network on many layers but it is acommonly accepted application layer approach

– Addressing, content distribution and querying are normally only application layerfunctions

– To reduce the overlay stretch, network layer information could be considered foroverlay address assignment or to improve the locating of content


Q&A

“Is a network with a central database, e.g., Napster still aP2P network?” ?? ?Institute of Computer Science – Telematics Tutorial – 17. February, 2011 22

Q&A

– Depends on your definition of an P2P network– Pure P2P networks have no central entity– Several P2P networks have super nodes or other special entities to improve the

performance– P2P networks should have no single point of failure– Several function can be either fully distributed or centralized, e.g., the database or

the content distribution


Q&A

“Is there a minimum IP size?” ?? ?Institute of Computer Science – Telematics Tutorial – 17. February, 2011 24

Q&A

– An IP datagram can have any size that fits the length field– An IP datagram could have no payload but the header is of course always there– Minimum size depends on the length of the header: IPv4 vs. IPv6– From RFC 791:

“All hosts must be prepared to accept datagrams of up to 576 octets”– IP implementations must be able to handle datagrams of up to 576 bytes– IP datagram of this size may arrive in fragments– There is some confusing and inconsistent information available in literature, RFCs,

and implementations– From RFC 879:

“The default IP Maximum Datagram Size is 576.”– Conclusion: If your datagram has a size of ≤ 576 bytes (and the don’t fragment

flag is not set), it should arrive at the destination


Q&A

“I do not understand the following statement fromslides 08, problem 05:Layer 1-2 → protocol header is not preserved but re-placed ” ?? ?Institute of Computer Science – Telematics Tutorial – 17. February, 2011 26

Q&A

– The Internet is is not a huge Ethernet LAN– Datagrams are routed– Network layer packet is placed as payload in data link layer packet (frame) for

transmission to next router– But frames can also be tunneled⇒ frame arrives at the destination host in a

different LAN– The particular question was only intended to highlight the general encapsulation,

decapsulation of packets


Q&A

“How can there be contention when TDMA is applied?” ?? ?Institute of Computer Science – Telematics Tutorial – 17. February, 2011 28

Q&A

– TDMA schemes can be contention-free or with contention– Static assignment or assignment by master⇒ contention-free– All dynamic/on-demand approaches require contention and can lead to collisions,

e.g., reservation TDMA


Q&A

“Please explain the relationship between HDLC and thedata link layer. Is HDLC a concrete implementation [ofthe DLL]? Is HDLC actually used? Or is it an extension/e-quivalent of LLC? Please answer the same questions forPPP.” ?? ?Institute of Computer Science – Telematics Tutorial – 17. February, 2011 30

Q&A

– HDLC and PPP do not fit intuitively into the hierarchical model– Both are protocols found on the data link layer that provide particular services

typical for the DLL (not further discussed here, see slides)– HDLC specifies a frame format, PPP doesn’t (see PPP in HDLC-like framing)– HDLC can be used over many point-to-point, multi-point links, circuit-switched and

packet-switched networks, . . .– HDLC is synchronous⇒ requires clocking and synchronizing on the physical layer– Today HDLC is mostly used in WANs– PPP is used to transport multi-protocol datagrams over point-to-point links, e.g.,

router-to-router and host-to-network– PPP originally was an encapsulation protocol to transport IP datagrams between

peers– Physical layers for PPP: ISDN, EIA/TIA-232-C, V.24, V.35, Ethernet, ATM– LLC provide HDLC style services mainly for LANs


Q&A

“What is the difference between Ethernet andIEEE 802.3?” ?? ?Institute of Computer Science – Telematics Tutorial – 17. February, 2011 32

Q&A

– See tutorial slides 6.26 and 6.34– Type field in Ethernet, length field in IEEE 802.3– 1-bit difference in the preamble/start frame delimiter– IEEE 802.3 format generally used with LLC (and SNAP)– IEEE 802.3+LLC mostly used for STP and other protocols to provide services to

the network layer regardless of media type


Q&A

7 1 6 6 2 0-1500 0-64 4

Preamble SFD DA SA L/T Data Padding FCS


Q&A

“How does MPLS fit into the ISO/OSI model? What doesit provide? Where is MPLS applied? Under which condi-tions will my station receive a packet that is encapsulatedby MPLS?” ?? ?Institute of Computer Science – Telematics Tutorial – 17. February, 2011 35

Q&A

– MPLS is used between the data link layer and network layer– It provides virtual switched circuits⇒ traffic engineering– Forwarding on layer uses less resources: classified once, only a single value is

evaluated by LSRs– Datagrams are classified at ingress router and are assigned a label; most often

contained in the 2.5 layer shim-header (compare the approach for ATM and MPLS)– All forwarding is based on this label; labels can be stacked– MPLS labels are either statically assigned for particular forwarding equivalence

classes (FEC) or based on a label distribution protocol– The mapping of particular label values to FECs is specific for routers on the same

level– There are 220 − 1 label values, values 0-15 are reserved, and values 4-15 are

reserved for future– Egress router removes shim-header and datagram is forwarded based on layer 3

information– MPLS is only applied in an MPLS domain, e.g., an autonomous system– While you can run MPLS in you network you will probably never receive an MPLS

packet (shim-header removed by egress)


The Last SlideTM

Thank you for your attention.Questions?


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