Final ReviewCS1652

Jack LangeUniversity of Pittsburgh

Final

Thurs 12th: 12-2PM This room

Not Cumulative One page of notes

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Chapter 4: Network Layer

Network layer service model Forwarding vs. Routing IP addressing, NAT & DHCP Router architecture Routing algorithms Handling scale - CIDR and BGP ICMP, traceroute

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Network layer service model

Unreliable, connectionless data delivery Q: is UDP more reliable than IP? Q: do TCP packets receive more special

treatment than UDP packets? Host-to-host packet delivery

Q: Difference from transport layer? All systems implement network layer

End systems + routers Q: Why not routers implement transport

layer?

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Forwarding vs. Routing

Definition? Switch vs. router? How routing affects forwarding? Routing algorithms

Intradomain routing algorithms• Distance vector, Link state

Interdomain routing algorithms• BGP

Longest prefix match

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IP addressing, DHCP and NAT 32-bit IPv4 address

Subnet part + host part Subnet part is used for forwarding decision

CIDR Subnet portion can be an arbitrary size Why CIDR?

• IP allocation efficiency & supernetting

DHCP How it works? Where is it useful?

NAT Pros. and Cons.

IP datagram format

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifier

header checksum

time tolive

32 bit source IP address

IP protocol versionnumber

header length (bytes)

max numberremaining hops

(decremented at each router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

head.len

type ofservice

“type” of data flgsfragment

offsetupper layer

32 bit destination IP address

Options (if any) E.g. timestamp,record routetaken, specifylist of routers to visit.

how much overhead with TCP?

20 bytes of TCP 20 bytes of IP = 40 bytes + app

layer overhead

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Router Architecture

Two key router functions: Run routing algorithms/protocol (RIP, OSPF, BGP) Forwarding datagrams from incoming to outgoing link

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Routers

Input ports (line cards) Forwarding table lookup – line speed Queuing packets if switching fabric is busy

• Head-of-line blocking?

Switching fabric Via memory, bus, special interconnection

Output ports Buffering & scheduling

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Link state algorithm

Broadcast local link info to all routers Dijkstra’s algorithm

Greedy algorithm Compute the least cost path to every node Each loop finds at least one node whose

least cost path is found Algorithm complexity? O(nlogn) Oscillation problem

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Distance vector algorithm

Distribute one’s view of network to neighbors

Bellman-ford algorithm Dynamic programming Asynchronous update

Problem? Count-to-infinity & routing loops Possible solution?

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Border Gateway Protocol (BGP)

Autonomous System (AS) AS number – 16 bit id

BGP contains full path from src to dest AS AS PATH – list of AS numbers

How to prevent routing loops? Hot potato routing?

One reason for routing path asymmetry How to deliver a packet from one AS to

another? Intradomain (Intra-AS) routing Interdomain routing Forwarding table (FIB)

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Chapter 5: Link Layer

Delivering frames to a direct neighbor Error detection and correction Sharing a broadcast channel Reliable data transfer & flow control

Hop-by-hop vs. end-to-end

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Error detection & correction

Parity checking Single bit vs. two-dimentional bit parity Odd/even parity

Internet checksum – IP/TCP layer Why is error checking needed in the upper

layer? Cyclic Redunancy Check (CRC)

CRC32 is widely used (e.g., Ethernet)

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Multiple Access Protocol

How to share a broadcasting media Medium Acccess Control (MAC) protocol

Channel Partitioning Random Access Taking turns

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Random Access Protocol

Slotted ALOHA Fixed time slot – synchronized If collision, retransmit with a probability of p

for each slot ALOHA

No slot synchronization CSMA, CSMA/CD, CSMA/CA

CSMA – sense carrier before sending CD – detect collision while sending and cancel

it CA – avoid collision by getting permission first

Ethernet and Wi-Fi?

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MAC addresses

Ethernet: 48-bit MAC addresses Burned into hardware Globally uniquely assigned Why not use MAC address instead of IP?

Address Resolution Protocol (ARP) Determining MAC address with IP address ARP table = <IP address, MAC address, TTL> Broadcasting mechanism (make sure to

know!)

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Ethernet & Switch

Access protocol: CSMA/CD Old Ethernet hubs used to share access Understand the exponential back-off algorithm Most current devices are switches Connectionless, unreliable

Frame format Preamble, 2 MAC address, type, data, CRC32

Switch Make sure you understand self-learning

algorithm

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Chapter 6: Wireless

Difference from wired environment Signal attenuation Hidden terminal problem Interference from other sources (phone,

microwave) SNR (Signal-to-Noise ratio)

The larger, the better BER (Bit error rate)

CDMA

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Wi-Fi

Access protocol : CSMA/CA Reserve the channel first before sending No collision detection – why? Understand the sending protocol

Access Point(AP) Link layer device (may run DHCP) Passive/Active scanning for association

802.11 frame format Why we need three MAC addresses?

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Chapter 7: Multimedia

Quality of service guarantee Providing performance guarantee required by

app Current Internet does not directly support it

Delay sensitive, loss tolerant application Video streaming vs. Email? What is jitter?

Multimedia application Stored streaming Live streaming Real-time interactive

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Internet Phone, CDN

Internet phone Network loss vs. delay loss

Content distribution networks (CDNs) Definition? DNS redirection for finding the near server?