192620010 Mobile & Wireless Networking Lecture 10: Mobile - - PowerPoint PPT Presentation

Mobile and Wireless Networking 2013 / 2014

192620010 Mobile & Wireless Networking Lecture 10: Mobile Transport Layer & Ad Hoc Networks [Schiller, Section 8.3 & Section 9] [Reader, Part 8]

Geert Heijenk

Mobile and Wireless Networking 2013 / 2014

Outline of Lecture 10

Mobile transport layer

 Motivation  Approaches for improvement

 Indirect TCP  Snooping TCP  Mobile TCP  Selective retransmission

 Recommended TCP improvements

Ad hoc networks

 Concept  Addressing and forwarding in ad-hoc networks  Routing in ad-hoc networks

 Problem description  DSDV (Destination Sequenced Distance Vector)  Ad-hoc On-demand Distance Vector (AODV)  DSR (Dynamic Source Routing)  Further alternatives

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Motivation I

Transport protocols typically designed for

 Fixed end-systems  Fixed, wired networks

TCP congestion control

 packet loss in fixed networks typically due to (temporary) overload

situations

 router have to discard packets as soon as the buffers are full  TCP recognizes congestion only indirect via missing

acknowledgements, retransmissions unwise, they would only contribute to the congestion and make it even worse

 slow-start algorithm as reaction

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Motivation II

TCP slow-start algorithm

 sender calculates a congestion window for a receiver  start with a congestion window size equal to one segment  exponential increase of the congestion window up to the congestion

threshold, then linear increase

 missing acknowledgement causes the reduction of the congestion

threshold to one half of the current congestion window

 congestion window starts again with one segment

TCP fast retransmit/fast recovery

 TCP sends an acknowledgement only after receiving a packet  if a sender receives several acknowledgements for the same

packet, this is due to a gap in received packets at the receiver

 however, the receiver got all packets up to the gap and is actually

receiving packets

 therefore, packet loss is not due to severe congestion, continue

with (half of) current congestion window (do not use slow-start)

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Influences of mobility on TCP-mechanisms

TCP assumes congestion if packets are dropped

 typically wrong in wireless networks, here we often have packet loss

due to transmission errors

 furthermore, mobility itself can cause packet loss, if e.g. a mobile node

moves from one access point (e.g. foreign agent in Mobile IP) to another while there are still packets in transit and forwarding is not possible

Additional problem:

 it takes a long time to increase the congestion window if the latency of

the wireless link is high

The performance of an unchanged TCP degrades severely

 but TCP cannot be changed due to the large base of installation in the

fixed network (end-to-end protocol)

 therefore TCP for mobility has to remain compatible

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Solutions: Indirect TCP / Performance Enhancing Proxy (RFC 3135)

Indirect TCP or I-TCP segments the connection

 no changes to the TCP protocol for hosts connected to the wired

Internet, millions of computers use (variants of) this protocol

 optimized TCP protocol for mobile hosts  splitting of the TCP connection at, e.g., the foreign agent into 2 TCP

connections, no real end-to-end connection any longer

 hosts in the fixed part of the net do not notice the characteristics of

the wireless part

mobile host performance enhancing proxy: access point / foreign agent „wired“ Internet „wireless“ TCP standard TCP

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mobile host access point2 Internet access point1 socket migration and state transfer

I-TCP socket and state migration

 Socket contains current TCP connection information (seq.num, ports)  During handover, current AP buffers packets  Since these packets have been acknowledged, they must be forwarded to

new AP after handover is executed

 Fixed-side connection must be maintained (socket)

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Indirect TCP III

Advantages

 no changes in the fixed network necessary, no changes for the hosts

(TCP protocol) necessary, all current optimizations to TCP still work

 transmission errors on the wireless link do not propagate into the fixed

network

 simple to control, mobile TCP is used only for one hop between, e.g.,

a foreign agent and mobile host

 therefore, a very fast retransmission of packets is possible, the delay

• n the mobile hop is known

Disadvantages

 loss of end-to-end semantics, an acknowledgement to a sender does

now not any longer mean that a receiver really got a packet, foreign agents might crash

 higher latency possible due to buffering of data within the foreign

agent and forwarding to a new foreign agent

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Solutions: Snooping TCP I

„Transparent“ extension of TCP within the foreign agent

 buffering of packets sent to the mobile host  lost packets on the wireless link (both directions!) will be

retransmitted immediately by the mobile host or foreign agent, respectively (so called “local” retransmission)

 the foreign agent therefore “snoops” the packet flow and recognizes

acknowledgements in both directions, it also filters ACKs

 changes of TCP only within the foreign agent

„wired“ Internet buffering of data end-to-end TCP connection local retransmission correspondent host foreign agent mobile host snooping of ACKs

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Snooping TCP II

Data transfer to the mobile host

 FA buffers data until it receives ACK of the MH, FA detects packet

loss via duplicated ACKs or time-out

 fast retransmission possible, transparent for the fixed network

Data transfer from the mobile host

 FA detects packet loss on the wireless link via sequence numbers,

FA answers directly with a NACK to the MH

 MH can now retransmit data with only a very short delay

Integration of the MAC layer

 MAC layer often has similar mechanisms to those of TCP  thus, the MAC layer can already detect duplicated packets due to

retransmissions and discard them

Problems

 snooping TCP does not isolate the wireless link as good as I-TCP  snooping might be useless depending on encryption schemes

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Solutions: Mobile TCP

Special handling of lengthy and/or frequent disconnections M-TCP splits as I-TCP does

 unmodified TCP fixed network to supervisory host (SH)  optimized TCP SH to MH

Supervisory host

 no caching, no retransmission  monitors all packets, if disconnection detected

 set sender window size to 0  sender automatically goes into persistent mode

 old or new SH reopen the window

Advantages

 maintains semantics, supports disconnection, no buffer forwarding

Disadvantages

 loss on wireless link propagated into fixed network  adapted TCP on wireless link

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Selective retransmission

TCP acknowledgements are often cumulative

 ACK n acknowledges correct and in-sequence receipt of packets

up to n

 if single packets are missing quite often a whole packet sequence

beginning at the gap has to be retransmitted (go-back-n), thus wasting bandwidth

Selective retransmission as one solution

 RFC2018 allows for acknowledgements of single packets, not only

acknowledgements of in-sequence packet streams without gaps

 sender can now retransmit only the missing packets

Advantage

 much higher efficiency

Disadvantage

 more complex software in a receiver, more buffer needed at the

receiver

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Recommended TCP Improvements (RFC 3481)

• Appropriate Window Size (Sender & Receiver)
• Increased Initial Window (Sender)
• Limited Transmit (Sender)
• IP MTU Larger than Default
• Path MTU Discovery (Sender & Intermediate Routers)
• Selective Acknowledgments (Sender & Receiver)
• Explicit Congestion Notification (Sender, Receiver & Intermediate Routers)
• TCP Timestamps Option (Sender & Receiver)
• Disabling RFC 1144 TCP/IP Header Compression (Wireless Host)

(RObust Header Compression, ROHC, RFC3095, is OK)

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Outline of Lecture 10

Mobile transport layer

 Motivation  Approaches for improvement

 Indirect TCP  Snooping TCP  Mobile TCP  Selective retransmission

 Recommended TCP improvements

Ad hoc networks

 Concept  Addressing and forwarding in ad-hoc networks  Routing in ad-hoc networks

 Problem description  DSDV (Destination Sequenced Distance Vector)  Ad-hoc On-demand Distance Vector (AODV)  DSR (Dynamic Source Routing)  Further alternatives

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Ad hoc network concept

 Networks of wireless terminals that do not necessarily rely on

existing infrastructure

 Although interworking with infrastructure is possible

 Direct communication between terminals when needed  Multi-hop communication  Extended concept of mobility: network mobility (moving routers)

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Mobile ad hoc networks

Standard Mobile IP needs an infrastructure

 Home Agent/Foreign Agent in the fixed network  DNS, routing etc. are not designed for mobility

Sometimes there is no infrastructure!

 remote areas, ad-hoc meetings, disaster areas  cost can also be an argument against an infrastructure!

Main topic: routing

 no default router available  every node should be able to forward

A B C

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Addressing & forwarding in ad-hoc networks

• Broadcast (single hop)
• Beaconing
• Broadcast (multihop)
• Geocast
• Unicast

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Beaconing

Beaconing:

• periodic single-hop broadcast

Problems:

• risk of overload
• no feedback from medium because of broadcast

Solutions:

 adapt load to number of users,

e.g., inter-beacon time / power control

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Broadcast (multihop)

Broadcast (multihop) Forwarding approach:

 flooding: all nodes forward all newly received messages

Problems:

 broadcast storm:

 redundant transmissions  synchronization of transmissions  no acks, no feedback from medium

Solutions:

 (random) delay before rebroadcasting,  not all nodes rebroadcast  e.g., distance-based forwarding 19

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Geocast

Geocast:

 like broadcast,

but packet is only forwarded to certain geographic region

Forwarding approach

 flooding to and in region  routing to region

+ flooding in region

Problems:

 trade-off between

accuracy and efficiency

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Unicast

Unicast: single destination with known address Forwarding approach

 based on routing information 21

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Outline of Lecture 10

Mobile transport layer

 Motivation  Approaches for improvement

 Indirect TCP  Snooping TCP  Mobile TCP  Selective retransmission

 Recommended TCP improvements

Ad hoc networks

 Concept  Addressing and forwarding in ad-hoc networks  Routing in ad-hoc networks

 Problem description  DSDV (Destination Sequenced Distance Vector)  Ad-hoc On-demand Distance Vector (AODV)  DSR (Dynamic Source Routing)  Further alternatives

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N1 N4 N2 N5 N3 N1 N4 N2 N5 N3 good link weak link time = t1 time = t2

Routing examples

Routing is a major topic

 in principle, every node should be able to forward  dynamic topology  asymmetric links  redundant links: too many links when terminals are close

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Traditional routing algorithms

 Distance Vector

 periodic exchange of messages with all physical neighbors that contain

information about who can be reached at what distance (monodirectional)

 selection of the shortest path if several paths available

 Link State

 periodic notification of all routers about the current state of all physical

links (bidirectional)

 router get a complete picture of the network

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Problems of traditional routing algorithms

 Dynamic of the topology

 frequent changes of connections, connection quality, participants

 Limited performance of mobile systems

 periodic updates of routing tables need energy without contributing

to the transmission of user data, sleep modes difficult to realize

 limited bandwidth of the system is reduced even more due to the

exchange of routing information

 links can be asymmetric, i.e., they can have a direction dependent

transmission quality

 Problem

 protocols have been designed for fixed networks with infrequent

changes and typically assume symmetric links

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Ad hoc routing algorithms

 Pro-active

Example:

 Destination Sequenced Distance Vector (DSDV)

 Re-active

Example:

 Ad-hoc On-demand Distance Vector (AODV)  Dynamic Source Routing (DSR)

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DSDV (Destination Sequenced Distance Vector)

 Extension of distance vector routing  Sequence numbers for all routing updates

 assures in-order execution of all updates  avoids loops and inconsistencies

 Decrease of update frequency

 store time between first and best announcement of a path  inhibit update if it seems to be unstable (based on the stored time

values) See [Schiller] for details

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Ad-hoc On-demand Distance Vector (AODV)

• Specified in IETF: RFC 3561
• Forms the basis for DYMO

(Dynamic On-demand MANET routing protocol) which is a planned IETF reactive routing protocol.

• Uses destination sequence numbers to avoid loops, and ensure

proper updating of routes

• Storage of routes in Route Table
• Uses only symmetric links

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Route Discovery

• Source broadcasts Route Request (RREQ):

<Source_Addr, Source_Seq#, Broadcast_ID, Dest_Addr, Dest_Seq#, Hop_Count>

• A node can reply to the RREQ if
• It is the destination
• It has a “fresh enough” route

to the destination

• Otherwise it rebroadcasts the RREQ
• Nodes keep track of

<Source_Addr, Broadcast_ID> and discard redundant broadcasts

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Source Destination

Route Request Propagation

Source: Perkins & Royer

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Reverse Path Setup

• Nodes update their Route Table with

source node information before forwarding RREQ

• Reverse path entry used to forward

Route Reply (RREP) back to source if one is received

• Expiration time is long enough for

a RREP to be received and forwarded

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with

Source Destination

Reverse Path Formation

ce

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Forward Path Setup

• Destination, or intermediate node with current route to

destination, unicasts Route Reply (RREP) back to source:

<Source_Addr, Dest_Addr, Dest_Seq#, Hop_Count, Lifetime>

• Nodes along path record forward route

in Route Table, use reverse route to forward RREP

• Source can begin sending data

when it receives first RREP

• If it later receives a RREP

with better metric, it updates its route entry

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Source Destination

Forward Path Formation

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Destination A

Next Hop Active Upstream Neighbors

Source Source

Number

Route Table

• Fields:
• Destination IP Address
• Destination Sequence Number
• HopCount
• Next Hop IP Address
• Active Neighbors
• Expiration time
• Each time a route entry is used to transmit data, the expiration

time is updated to current time + active_route_timeout

• Route entries may be updated if a route with greater sequence

number or smaller hopcount is discovered

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Path Maintenance

• Movement of nodes not along active path

does not trigger protocol action

• If source node moves, it can reinitiate route discovery
• When destination or intermediate node moves,

node upstream of break sends unsolicited RREP to all active upstream neighbors

• ∞ metric, incremented Seq#
• Used to flush stale routes
• RREP is propagated to their active neighbors,

and so on back to source

• Source can reinitiate route discovery if route is still needed
• RREQs for reinitiated route discovery contain destination

sequence number of one greater than last known number

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Path Maintenance (example)

• Node 3 moves to new location 3’
• Node 2 notices loss of link, sends link failure to Node 1
• Node1 forwards link failure to Source
• Source reinitiates route discovery,

finds new route through Node 4

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Source Destination 1 2 3 3’ 4 4

The Initial Route After Route Reconstruction

Source Destination 1 2 3’ 4 4

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Local Connectivity Management

• Node must periodically hear from active neighbors to know they

are still within range

• Eavesdrop on neighbor transmissions
• If no other transmissions within hello_interval, broadcast

Hello packet

• Failure to hear from a neighbor for

(1+ allowed_hello_loss) * hello_interval

indicates loss of link

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Optimizations

• Expanding Ring Search
• by limiting TTL at first attempt,

and increasing it at successive attempts.

• RREP generated by intermediate node
• nly if Seq# for route to destination ≥ Dest_Seq# of RREQ
• Maintaining Local Connectivity by means of layer 2 info.
• Local Repair
• node upstream of link failure tries to find

a new route to destination by sending a RREQ (with reduced TTL, and incremented Dest_Seq#)

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DSR (Dynamic Source Routing)

• Similar to AODV
• Big difference:
• DSR uses Source Routing
• AODV relies on storing routing table entries in intermediate nodes

 RREQ and RREP carry addresses of all intermediate nodes

See [Schiller] for details

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A plethora of ad hoc routing protocols

Flat

 proactive  DSDV – Destination Sequenced Distance Vector  FSLS – Fuzzy Sighted Link State  FSR – Fisheye State Routing  OLSR – Optimised Link State Routing Protocol  TBRPF – Topology Broadcast Based on Reverse Path Forwarding

 reactive

 DSR – Dynamic Source Routing  AODV – Ad hoc On demand Distance Vector

Hierarchical

 CGSR – Clusterhead-Gateway Switch Routing  HSR – Hierarchical State Routing  LANMAR – Landmark Ad Hoc Routing  ZRP – Zone Routing Protocol

Geographic position assisted

 DREAM – Distance Routing Effect Algorithm for Mobility  GeoCast – Geographic Addressing and Routing  GPSR – Greedy Perimeter Stateless Routing  LAR – Location-Aided Routing