1 Songwu Lu/UCLACS118/3-14-2002 Computer Networking: the overall picture why packet switching? ...
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1 Songwu Lu/UCLACS118/3-14-2002
Computer Networking:the overall picture
why packet switching?
What’s in a packet: –header: contains all the information needed for data
delivery
layered network protocol architecture: why layering?–Divide and conquer
Switch 2Switch 1
Packet(carriesdestinationaddress)
header data
Efficient resourcesharingFlexible delivery
2 Songwu Lu/UCLACS118/3-14-2002
On Final Exam
TAs will give additional review lectures Office hours next week:
–Monday, 1-2pm–Tuesday, 1-3pm–Thursday, 1-3pm
For your current standing in the class, check with your TA
You can bring two (8x11) sheets into the final exam
3 Songwu Lu/UCLACS118/3-14-2002
Materials to be covered
Main focus (about 80%):–Chapters 4 and 5
Additional chapters (about 20%):–Chapter 6–Chapter 3
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Exam Format
Multiple choices:–Which of the following error detection algorithms
can guarantee 100% error detection?–(A)CRC; (B) Internet Checksum; © both CRC
and checksum; (D) two-dimensional parity; (E) none of the above
Short Q & A:–Does a host know the local DNS server by its
DNS name? Or by its IP address? Or by its Ethernet address? Explain
5 Songwu Lu/UCLACS118/3-14-2002
Exam Format
Standard Problem solving–Given a network topology, compute the minimum
cost path from the source to the destination by applying the distance vector routing algorithm
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Computer Networking:the overall picture
why packet switching?
What’s in a packet: – header: contains all the information needed for data
delivery layered network protocol architecture: why
layering?– Divide and conquer
Switch 2Switch 1
Packet(carriesdestinationaddress)
header data
Efficient resourcesharingFlexible delivery
7 Songwu Lu/UCLACS118/3-14-2002
What are the layers?
Network layer: transmit packets from one host to another host in the Internet–issues: routing, switching, multicast
Internet Protocol (IP):deliver packets hosthost–IP packet format–packet fragmentation and reassembly–delivery by encapsulation–IP addresses: class-based addresses, subnetting,
CIDR
8 Songwu Lu/UCLACS118/3-14-2002
More protocol layers
link layer: send data frames between directly connected nodes–framing–bit error detection (parity check, checksum, CRC)–media-access-control (MAC): –Ethernet, token ring–Hubs, bridges, and switches
Higher layers above network layer–transport protocols:reside in end hosts only–application protocols
9 Songwu Lu/UCLACS118/3-14-2002
Ethernet frame
IP packet
TCP packet
header tail
IP hdr
TCP hdr
DATA
DATA
DATA
DATA
Layered protocol implementation
A protocol defines: the format of message exchanged
between peer entities the actions taken on receipt of the
message
What’s in the header: all the information,and only the information that’s needed for the protocol’s functionality
10 Songwu Lu/UCLACS118/3-14-2002
Network layer design
Network service model Routing IPv4 and IPv6 Multicast routing
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Network Service Model
Datagram vs. virtual circuit–What is the difference?–What kind of service model the Internet is using?–What are the cons and pros of Internet service
model?–Is it appropriate to support data / multimedia?
How can we realize connection-oriented services and connection-less services?–At the network layer–At the transport layer
12 Songwu Lu/UCLACS118/3-14-2002
Network routing
routing protocol: distributed way to compute shortest path to all destinations
distance-vector routing protocol–A node’s update includes a list of [destination,
distance] pairs for all destinations that A knows of
–send routing updates to neighbor nodes only
link-state routing protocol–A node’s update includes a list of [neighbor, link
distance] pairs for all its links–routing updates are flooded to the entire network
13 Songwu Lu/UCLACS118/3-14-2002
What you need to know about routing algorithms?
Given a topology with link cost, how to compute the minimum cost path from a given source to a given destination ?
– apply distance vector routing algorithm– apply link state routing algorithm
On distance vector routing:– What does it mean “good news travels fast”?– What is the “count-to-infinity” problem?– What are the fixes to count-to-infinity problem? Can they
solve the problem completely?
Comparisons between DV and LS routing:– Message complexity, convergence speed, robustness
14 Songwu Lu/UCLACS118/3-14-2002
More on routing
On virtual circuit routing:–What is shortest path first routing?–What is least loaded path routing?–What is maximum free circuit routing?
Why do we need hierarchical routing over the Internet? –two reasons:
15 Songwu Lu/UCLACS118/3-14-2002
An example for Distance Vector routinginitial state at each node
A
G
H
D
F
1
2 3
2
4
1
1
2 3
4
4
B 1 BC ?D ?E ?F ?G ?
Dst Dis Nex
H 2 H
A's routing table
A 1 AC 2 CD ?E ?F ?G ?
Dst Dis Nex
H 3 H
B's routing table
A ?B 2 BD 1 DE 1 EF ?G ?
Dst Dis Nex
H ?
C's routing table
A ?B ?C 1 CE ?F 4 FG ?
Dst Dis Nex
H ?
D's routing table
B CA ?B 2 BC 1 CD ?F 3 FG 2 GH ?
E's routing table
E
16 Songwu Lu/UCLACS118/3-14-2002
Routing table after one update
B 1 BC 3 BD ?E ?F ?G ?H 2 H
A's routing table after receiving an update from B
B's routing table after receiving an update from C
A 1 AC 2 CD 3 CE 3 CF ?G ?H 3 H
B 1 BC ?D ?E ?F ?G ?
Dst Dis Nex
H 2 H
A's routing table
A 1 AC 2 CD ?E ?F ?G ?
Dst Dis Nex
H 3 H
B's routing table
A ?B 2 BD 1 DE 1 EF ?G ?
Dst Dis Nex
H ?
C's routing table
A ?B 2 BC 1 CD ?F 3 FG 2 GH ?
E's routing table
B 3 BD 1 DE 1 EF 4 EG 3 EH 2 H
C's routing table after receiving an update from E
A ?
17 Songwu Lu/UCLACS118/3-14-2002
An example for link state protocol:A's topology table
A
G
H
D
F
1
2 3
2
4
1
1
2 3
4
4
B C
E
0 1 2 0 0 0 0 0
A B C D E F G H
0
ABCDEFGH 0
0 1 31 0 2 3 0 0 0 0
A B C D E F G H
0
ABCDEFGH 0
After update from B
0 1 21 0 2 3
0 0
1 0 3 2 0
A B C D E F G H
0
ABCDEFGH 0
After updatefrom E
18 Songwu Lu/UCLACS118/3-14-2002
A
G
HF
1
2 3
2
4
1
1
2 3
4
4
B C
E
0 1 21 0 2 3 2 0 1 1 1 0 4 1 0 3 2 4 3 0 4
A B C D E F G H
2 4 0 4
ABCDEFGH 2 3 4 0
After A has received an update from every other node
D
19 Songwu Lu/UCLACS118/3-14-2002
An example for Dijkstra Algorithm
G
HF
1
2 3
2
4
1
1
2 3
4
4
B C
E
DA
(0) {A}, AB=1, AC=, AD=, AE=, AF=, AG=, AH=2
(2) {A, B, H}, Nh=H, update: AC=3, AD=, AE=, AF=, AG=AH+HG=6
(1) {A, B}, Nb= B, update: AC=AB+BC=3, AD=, AE=, AF=,
AG=, AH=2
(3) {A, B, H, C}, Nc=B, update: AD=AC+CD=4, AE=AC+CE=4,
AF=, AG=6(4) {A, B, H, C, D}, Nd=B, update: AE=4, AF=AD+DF=8, AG=6
(5) {A, B, H, C, D, E}, Ne=B, update: AF=AE+EF=7, AG=6
(6) {A, B, H, C, D, E, G}, Ng=B, update: AF=7
(7) {A, B, H, C, D, E, G, F} Nf=B
0 1 3 4 4 7 6 2A B C D E F G H
A
20 Songwu Lu/UCLACS118/3-14-2002
Internet Protocol (IP)
Relation between IP & network layer–IP is responsible for host -to-host packet
delivery, normally through a chain of IP routers–physical networks do the real work of getting
packets from one IP node to the next
understand IPv4 addressing–IP address structure
» two-level hierarchy• Network ID + host ID
» class-based address: Class A, B, C, D
» subnetting
21 Songwu Lu/UCLACS118/3-14-2002
CIDR: Classless InterDomain Routing
assign network addresses by blocks of contiguous IP addresses, in a form of
<IP address, mask>–mask identifies block size, must be power of 2–example: allocation of 4 class-C address blocks 192.4.16.0192.4.19.255, <192.4.16.0, 255.255.252.0>, or 192.4.16/22
11000000000001000001000000000000 11000000000001000001001111111111
192 4 16 0 192 4 19 255
11111111111111111111110000000000
255 255 252 0
22 Songwu Lu/UCLACS118/3-14-2002
Forwarding IP Packets
hosts decide whether the destination is on the same network, if not, send packet to a default router
routing: IP router looks up the forwarding table to determine the next hop to forward the packet to–routers may also use a default router for far-
away destinations address translation: mapping an IP
address to physical network address: ARP packet encapsulation and decapsulation
when crossing each physical network
23 Songwu Lu/UCLACS118/3-14-2002
IPv4 Header
What is the purpose for each header field?–For example, why do we need TTL? Why do we
need Options?
IP Fragmentation and reassembly:–Given a large IP packet passing through a small-
packet network, how does fragmentation work? The reassembly process?
–Where do we reassembly the fragments? Why?
On ICMP: how does traceroute work?
24 Songwu Lu/UCLACS118/3-14-2002
Internet routing
What is the main difference between RIP and BGP?
How do we further reduce the routing table size?–Default route
25 Songwu Lu/UCLACS118/3-14-2002
IPv6
What header fields are available in both IPv4 and IPv6?
What are the new fields in IPv6 header? Why do we need them?
Comparison btw IPv4 and IPv6 Does IPv6 allow fragmentation or not? If
not, how does IPv6 handle the case of small-packet-network?
26 Songwu Lu/UCLACS118/3-14-2002
Transition from IPv4 to IPv6
What is dual-stack solution? On tunneling:
how does tunneling work?What is the advantage? Incrementally deploy new
protocols: IPv6, IP multicast, etc….
27 Songwu Lu/UCLACS118/3-14-2002
IP Multicasting
IP multicast service model–each group identified by an IP mcast address–members can be anywhere–members may join and leave any time
How to map IP mcast addr to a link-layer mcast addr (not in the exam)–place the low-order 23 bits of IP mcast addr to
the lower 23 bits of Ethernet mcast addr: 01.00.5E.00.00.00
IGMP protocol–hosts report group membership to a local router
28 Songwu Lu/UCLACS118/3-14-2002
IGMP A query router in each link querier periodically polls the link on receipt of query message, hosts set a
random timer for each mcast group it belongs to
when timer expires, send a report to group G others in G hear the report and stop the timers joining: send reports immediately leaving: send a leave msg, and querier sends
group-specific queries again
29 Songwu Lu/UCLACS118/3-14-2002
More on IGMP
Why do we need a timer in IGMP? Does IGMP know how many active
receivers in each multicast group? Why? How does IGMP handle:
–An existing member’s departure–A new member join–A failing receiver
30 Songwu Lu/UCLACS118/3-14-2002
Multicast routing
distance vector mcast routing (DVMRP)–reverse path broadcast: only broadcast over
output interfaces if the input interface is on the shortest path to the source S
» You have to know this !
–pruning and grafting–Given a topology, know how to apply RPF to
solve the problem
Mbone:–use IP tunneling to connect mcast routers
31 Songwu Lu/UCLACS118/3-14-2002
Link layer design
Error detection Framing: byte stuffing MAC protocols Bridges, hubs
32 Songwu Lu/UCLACS118/3-14-2002
Error detection
Parity bit, 2D parity bit–Can they detect all error cases?
CRC–What is the main idea of CRC?–What is the advantage to use CRC?
33 Songwu Lu/UCLACS118/3-14-2002
MAC protocols
What are the three MAC protocol families?
What protocols to use for the following scenarios?– wired voice, wireless data, ftp over wired
network
Compare TDMA and token-based protocol Compare Aloha, slotted Aloha, and
CSMA/CD
34 Songwu Lu/UCLACS118/3-14-2002
More on MAC
CSMA/CD:– What is carrier sensing?– How to do collision detection?– Can carrier sensing avoid collisions completely? Why?
Wireless MAC (802.11)– What is hidden terminal problem?– Why do we need RTS-CTS?– Why do we need ACK?
Taking turns– How can we design a protocol to provide the best of both
worlds: channel partitioning and random access?
35 Songwu Lu/UCLACS118/3-14-2002
Ethernet Address and ARP
Compare ethernet address with IP address
The detailed operations of ARP If the underlying network does not have a
broadcast medium, can we still use ARP? Design a solution so that ARP can work in
point-to-point link scenarios. The steps to send a datagram to a node
off the LAN
36 Songwu Lu/UCLACS118/3-14-2002
Ethernet
Ethernet MAC:–What is BEB? Why use it?–What is the capture effect?
37 Songwu Lu/UCLACS118/3-14-2002
Connecting LANs
Compare hubs, bridges and routers The exact operations of bridge learning
algorithm (bridge filtering and forwarding) Why do we need to build spanning trees?
What failure happens, what to do? Wireless Ethernet MAC:
–What is hidden terminal problem?–Does wireless Ethernet MAC implement collision
detection? Why?–The steps in 802.11 MAC, how RTS-CTS helps?
38 Songwu Lu/UCLACS118/3-14-2002
Multimedia networking
What is the new requirement to support multimedia?
On client-side buffering:–How does it solve the jitter problem?
How do the FEC and the interleaving work? What is the difference between guaranteed
service and controlled load service?
39 Songwu Lu/UCLACS118/3-14-2002
ON TCP
How does TCP handle reliable transfer?–What are the fundamental mechanisms?–How do they work?
How does TCP perform flow control? How does TCP perform congestion
control? How does TCP estimate its RTT and
retransmission timeout (RTO)?
40 Songwu Lu/UCLACS118/3-14-2002
What you can do in the future? Networking will be (already is in some
sense) the next big wave of computer and information technology It is just the end of the beginning merging key industry sectors: PC &
handheld devices, communications, and even system control (automobile etc.)
It is time for you to think about making your personal landmark for the emerging technology
How to do it: doing the right thing at the right time in a right way