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BITS PilaniPilani Campus

Advanced ComputerNetworks (CS ZG525)

Virendra S ShekhawatDepartment of Computer Science and Information Systems

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BITS PilaniPilani Campus

First Semester 2015-2016Lecture-17 [25 th Oct 2015]

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BITS Pilani, Pilani CampusFirst Sem 2015-16

Agenda

• TCP Performance Over Wireless Links [CH-30] – Reading

• A Comparison of Mechanisms for Improving TCP Performance overWireless Links by H Balakrishnan

– Web Reference• http://daedalus.cs.berkeley.edu/publications/sigcomm96.pdf

• IP Mobility Problem and Solutions [CH-31, Ch-32] – Reading

3Advanced Computer Networks CS G525

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TCP Problems Over Noisy Links

• Wireless links are inherently error-prone – Fades, interference, attenuation – Errors often happen in bursts

•

TCP cannot distinguish between corruption andcongestion – TCP unnecessarily reduces window, resulting in low

throughput and high latency• Burst losses often result in timeouts• Sender retransmission is the only option

– Inefficient use of bandwidth

Advanced Computer Networks CS G525

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Performance Degradation

0.0E+00

5.0E+05

1.0E+06

1.5E+06

2.0E+06

0 10 20 30 40 50 60

Time (s)

S e q u e n c e n u m b e r

( b y t e s

)

TCP Reno(280 Kbps)

Best possibleTCP with no errors(1.30 Mbps)

2 MB wide-area TCP transfer over 2 Mbps Lucent WaveLAN5


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Proposed Solutions

• End-to-End protocols – Selective ACKs, Explicit Loss Notification (ELN),

Selective Acknowledgement•

Split-connection protocols – Separate connections for wired path and wireless hop

• Reliable link-layer protocols (Hide link relatedlosses from the TCP)

– Error-correcting codes – Local retransmission – Snooping


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Approach Styles (End-to-End)•

Improve TCP implementations – Improve loss recovery (SACK, NewReno) i.e. Fast Retransmit – Help it to identify congestion (ELN, ECN)…. How to implement…?

• ACKs include flag indicating wireless loss – Trick TCP into doing right thing e.g. send extra dupacks

Wired link Wireless link


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Approach Styles (Split Connection)

•

Split connections (aka Indirect TCP) – Wireless connection need not be TCP (SRP over UDP) – Hard state at base station

• TCP protocol processing at base station twice for each packet• Complicates mobility• Vulnerable to failures• Violates end-to-end semantics



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Split-Connection CongestionWindow

• Wired connection does not shrink congestion window• But wireless connection times out often, causing sender to stall

0

10000

20000

30000

40000

50000

60000

0 20 40 60 80 100 120Time (sec)

C o n g e s t

i o n W

i n d o w

( b y t e s

)

Wired connectionWireless connection


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Approach Styles (Link Layer)• More aggressive local retransmit than TCP

– Bandwidth not wasted on wired links• Adverse interactions with transport layer

– Timer interactions – Interactions with fast retransmissions – Large end-to-end round-trip time variation

• FEC does not work well with burst losses


ARQ/FEC


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Hybrid Approach: SnoopProtocol

• Shield TCP sender from wireless losses – Eliminate adverse interactions between protocol layers – Congestion control only when congestion occurs

• Modify base station – To cache un-acked TCP packets – … and perform local retransmissions

• Key ideas – No transport level code in base station – When node moves to different base station, state

eventually recreated there• Preserve TCP/IP service model: end-to-end

semantics


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Snoop Protocol: CH to MH

CorrespondentHost

Mobile HostBase Station

5 1

12346

• Snoop agent: active interposition agent – Snoops on TCP segments and ACKs – Detects losses by duplicate ACKs and timers – Suppresses duplicate ACKs from MH

Snoop Agent


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CorrespondentHost


• Transfer of file from CH to MH• Current window = 6 packets

Snoop Agent65

43

2 1


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CorrespondentHost


• Transfer begins

Snoop Agent6 54 3 2 1


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CorrespondentHost


5 1

12346

• Snoop agent caches segments that pass by

Snoop Agent


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CorrespondentHost


51234

6

• Packet 1 is Lost

Snoop Agent23 1

Lost Packets1


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CorrespondentHost


5 12346

• Packet 1 is Lost – Duplicate ACKs generated

Snoop Agent

23

Lost Packets1

4

ack 0


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CorrespondentHost


5 12346

• Packet 1 is Lost – Duplicate ACKs generated

• Packet 1 retransmitted from cache at higher priority

Snoop Agent

23

Lost Packets1

4

ack 0

56 1

ack 0


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CorrespondentHost


5 12346

• Duplicate ACKs suppressed

Snoop Agent

234

ack 4

56

1

ack 0X


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CorrespondentHost


56

• Clean cache on new ACK

Snoop Agent

234

ack 5

6

15

ack 4


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CorrespondentHost


6

• Clean cache on new ACK

Snoop Agent

234

ack 6

156

ack 5

ack 4


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CorrespondentHost


• Active soft state agent at base station• Transport-aware reliable link protocol• Preserves end-to-end semantics

Snoop Agent

234

ack 6

156978

ack 5


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Performance: FH to MH

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 500 1000 1500 2000 2500

1/Bit-error Rate (1 error every x Kbits)

T h r o u g h p u

t ( M

b p s )

2 MB local-area TCP transfer over 2 Mbps Lucent WaveLAN

• Snoop+SACKand Snoopperform best

• TCP SACKperformancedisappointing

TCP Reno

SPLIT

TCP SACK

SPLIT-SACK

Snoop

Snoop+SACK


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Summary

• End-to-end schemes, while not as effective as localtechniques in handling wireless losses

– But no extra support is required• The split-connection approach, with standard TCP used

for the wireless hop, shields the sender from wirelesslosses

– However the throughput is still slightly less than that for awell-tuned link-layer scheme that does not split the

connection• The TCP-aware link-layer protocol (snoop) with

Selective Acknowledgment performs the best


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Next…

• What is Mobility…? • Why we need Mobility…?

• IP Mobility Solutions (Routing Solutions) – Ethernet – IP Routing – Mobile IP


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Introduction

• Mobility in wireless networks refers to a node, Mobile Node(MN), changing its point of attachment to the network whileits communication to the network remains uninterrupted

•

Host Mobility – Movement of a node

• Network Mobility – Movement of a network

•

Personal Mobility – Movement of users rather than devices

• Session Mobility – Mobility between two terminals


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Why Things Move…?

• Physical Mobility – Devices (Host) move to new attachment point

• Multi-homing – Devices starts using different attachment point – e.g. 3G, WiFi

• Migration – VM migration

• Fail-over – Backup -Primary


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Mobility and Internet

• Since the standard Internet combines the uniquehost identifier with the topology location usingIP addresses, it cannot provide support for

mobility

• IETF Support for Mobility – Mobile IP supports mobility by decoupling the

binding between the host identifier and topologylocation using a fixed indirection point


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Requirements of Mobility

• Efficient Handoff• Location management

– If a mobile host offers services to other nodes, it must be able to be located by these nodes as itmoves as well as keeping the privacy of its topological location.

• Security• Efficient Routing

– Packets should be routed with the latency as low as possible, optimally close to the shortest pathprovided by IP routing.

• Scalability – A mobility scheme is said to be scalable if its performance does not drop as the number of nodes

(MNs and CNs) increases.• Fault Tolerance• Simultaneous Mobility• Compatibility with IP Routing• Link layer Independence• Transparency

– Transparent to the applications

• QoSAdvanced Computer Networks CS G525

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Handling Mobility

• Change Address of Mobile Node – Change the mapping of name to address

• Mobility is a directory problem

• Keeping the Address same – Change the routes to reach the new location

• Mobility is a routing problem


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Internet Design Decisions

• Socket abstraction – Connection between a pair of fixed IP addresses

and port numbers – Leads to more emphasis on routing solutions

• Interface addresses – Addresses refer to interfaces (adaptors) – Not the host, or the service


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IP Mobility

• IP address changes as the mobile device movesand changes its point of attachment

• Problem: – Active TCP connections break – Can DNS solve it?

• Too slow: new IP address might be unknown to clients thatwant to access the mobile node


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Routing Solutions

• Ethernet – MAC learning of the new location

•

IP routing – Inject IP address(es) at new location

• Mobile IP – Stationary home agent directs traffic to new

location


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Ethernet

• Ethernet handles mobility – IP address and MAC address stay the same – Switches learn to route to the new location

• But, larger networks have multiple segments – Cannot retain your IP address as you move

•

Solution: virtual local area networks (VLAN) – Logical Ethernet segment spanning a campus – e.g., interconnecting the WiFi access points


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Pros and Cons

• Advantages – Seamless mobility, no changes to hosts or apps – No changes to MAC or IP addresses

• Disadvantages – Ethernet does not scale –

Long paths, state per MAC address, flooding, …

• Widely used approach in campus networks


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IP Routing

• Node has a persistent address (e.g., 15.30.40.7)• Injected into routing protocol (e.g., OSPF)


15.30.40.0/24 15.30.40.7

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Pros and Cons

• Advantages – Seamless mobility, no MAC or IP address changes – Traffic follows an efficient path to new location

• Disadvantages – Does not scale to large number of mobile hosts –

More routing-protocol messages – Larger routing tables to store smaller address

blocks

37 37Advanced Computer Networks CS G525

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Mobile IP[1]


HA

FA

MN

MN

Home Subnet

Foreign Subnet

CN

b l [ ]

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Mobile IP[2]: MN to CNCommunication


HA

FA

MN

MN

Home Subnet

Foreign Subnet

CN

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BITS Pilani, Pilani CampusFirst Sem 2015-16 Advanced Computer Networks CS G525 41

Mobile IP: Registration example

visited network: 79.129.13/24home agentHA: 128.119.40.7

foreign agentCOA: 79.129.13.2

COA: 79.129.13.2….

ICMP agent adv. Mobile agentMA: 128.119.40.186

registration req.COA: 79.129.13.2HA: 128.119.40.7MA: 128.119.40.186Lifetime: 9999identification:714….

registration req.COA: 79.129.13.2HA: 128.119.40.7MA: 128.119.40.186Lifetime: 9999identification: 714encapsulation format….

registration replyHA: 128.119.40.7MA: 128.119.40.186Lifetime: 4999Identification: 714encapsulation format….

registration reply

HA: 128.119.40.7MA: 128.119.40.186Lifetime: 4999Identification: 714….

time

P bl MN CN

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Problem: MN to CNCommunication

• The IP address of the MN is topologically incorrectinside the foreign network (remember MN keepsfixed IP)

• Firewall in foreign network may prevent packetsfrom being transmitted with topologically incorrectsource address

• Solution: Reverse tunneling –

Establish a topologically correct reverse tunnel from CoAto HA – Outgoing packets tunneled to the HN and then routed to

the CN


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Mobile IP[3]: Reverse Routing


HA

FA

MN

MN

Home Subnet

Foreign Subnet

CN

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Enhancements-Efficient Routing

• Problem: Triangular Routing in Mobile IPv4

• Solution: – HA informs the CN about the location of MN – CN caches the current COA of the MN for some time – CN directly tunnels packets to COA, instead of using HA – Problem : Security issues


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Smooth Handover

• Problem: MN moves from one FA to anotherCached COA (at CN or HA) is outdated

•

Solution: – The new FA sends a binding update to the old FA – The old FA forwards remaining packets to new FA and releases

resources for the MN – If the old FA receives packets for an MN, but it is not its

current FA, it can send a binding warning to the sender of thepacket


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Mobile IPv4 and NAT Traversal

•

Mobile IP uses IP-in-IP tunneling between HA and FA• Not enough information to allow packets to traverse

NAT (no port information)• Solution:

– Extending the registration and tunnel procedure – Registration message sent using UDP – HA detects presence of NAT by comparing source IP with

COA –

MN indicates UDP tunneling capability by including theUDP tunneling extension in the registration request(includes the NAT tunnel port number)

– HA sets up UDP tunnel using the NAT tunnel port: packetscorrectly forwarded to MN


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Mobile IPv6

• No FA needed• IPv6 has auto configuration (a node can assign itself

a topologically valid IPv6 address composed fromthe network address and its MAC address)

• Use auto configuration to get a topologically correctCOA

• Packets are sent with COA as source address•

COA is the current source address of the MN in theforeign network• No firewall problems


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Need of Mobility

• Increasing demand for seamless IP mobility – E.g., continue a VoIP call while on the train – E.g., virtual machine migration within and between

data centers

• Increasing integration of WiFi and cellular – E.g., multi-homed cell phones that can use both

networks (announced for Samsung Galaxy S5) – E.g., servers with multiple interface cards

• Need better mobility & multi-homing solutions!49


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Mobile IP in practice

•

First RFC 1996• Mobile IPv6, 2011• Proxy Mobile IPv6• Similar to Mobile IP, but implemented entirely inside the network• 3G and UMTS have Mobile IP (conceptually) implemented

Internally• Mobility support for 802.11, WiMAX• As a mechanism to perform IP handover across different wireless

Technologies – LTE->UMTS, WiMAX, 802.11


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Thank You!

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