CIS 725

CIS 725

Lecture 21

Design Techniques for Mobile Networks

• From mobile transmitter to base receivers - maintain timers at base stations - base station sends acks periodically - during handoff, new MSS keeps

receiving by increasing W

• From base sender to mobile receiver - MH sends selective acks - send acks after receiving a block of messages - Base station has a timer to detect loss of acks - no timer at MH

Other Solutions

• Detect handoffs and notify transport layer of the handoff

• Split TCP connections

FH MSS MH

Split Connection Approach : Advantages

• MSS-MH connection can be optimized independent of FH-MSS connection– Different flow / error control on the two connections

• Local recovery of errors– Faster recovery due to relatively shorter RTT on

wireless link • Good performance achievable using appropriate MSS-

MH protocol– Standard TCP on MSS-MH performs poorly when

multiple packet losses occur per window

Split Connection Approach : Disadvantages

• End-to-end semantics violated– ack may be delivered to sender, before data delivered to the

receiver– May not be a problem for applications that do not rely on TCP for

the end-to-end semantics

FH MHMSS

4039

3738

3640

Split Connection Approach : Disadvantages

• MSS retains hard stateMSS failure can result in loss of data (unreliability)– If MSS fails, packet 40 will be lost – Because it is ack’d to sender, the sender does not buffer 40

FH MHMSS

4039

3738

3640

Split Connection Approach : Disadvantages

• MSS retains hard stateHand-off latency increases due to state transfer– Data that has been ack’d to sender, must be moved to new base

station

FH MHMSS

4039

37383640

MH

New MS station

Hand-off

4039

Split Connection Approach : Disadvantages

• Buffer space needed at MSS for each TCP connection– MSS buffers tend to get full, when wireless link

slower (one window worth of data on wired connection could be stored at the base station, for each split connection)

TCP-Aware Link Layer

• Snoop Protocol– observe TCP ACKs at the MSS– discard duplicate ACKs and retransmit

• prevent fast retransmit at TCP sender– end-to-end reliability– soft state at MSS (only buffer packets)

Snoop Protocol• Buffers data packets at the base station BS

– to allow link layer retransmission• When duplicate acks received by MSS from MH, retransmit on

wireless link, if packet present in buffer• Prevents fast retransmit at TCP sender FH by dropping the duplicated

acks at MSS

FH MHMSS

Snoop : Example

FH MHMSS40 39 3738

36

Example assumes delayed ack - every other packet ack’d

363738

35 TCP statemaintained at

link layer

Snoop : Example

41 40 3839

36

363738

35 39

Snoop : Example

40

363636

Duplicate acks

4143 42

373839

4041

Snoop : Example

FH MHMSS41

3636

3744 43

36

373839

404142

Discarddupack

Dupack triggers retransmissionof packet 37 from base station

MSS needs to be TCP-aware to

be able to interpret TCP headers

Snoop Protocol : Disadvantages

• Link layer at base station needs to be TCP-aware

• Not useful if TCP headers are encrypted (IPsec)

• Cannot be used if TCP data and TCP acks traverse different paths (both do not go through the base station)

Routing protocols

• Proactive routing protocols Distance vector, Link state protocols * maintain routing paths at all times• Reactive routing protocols * create paths on demand• Hybrid protocols

Dynamic Source Routing

1

2

3

4

5

6

7

8

1

2

3

4

5

6

7

8

s sd d

Building route record Route reply with route record

<1> <1,2>

<1><1>

<1,3>

<1,4>

<1,3,5>

<1,4,6>

<1,3,5,7>

<1,4,6><1,4,6>

<1,4,6>

• Initially, only next hop information is available. • Send packet to all neighbors• At some point, it will reach the destination and

reverse path can be used to set up path

• Intermediate nodes may send replies if they already know a route

• Cache management• Local repairs

Ad Hoc On Demand Routing (AODV)

• Constructs routes on demand• Nodes maintain routing tables instead of source

routes• Sequence numbers added to handle stale routes• Route discovery• Reverse path setup

Route Discovery

(a) Range of A's broadcast.(b) After B and D have received A's broadcast.(c) After C, F, and G have received A's broadcast.(d) After E, H, and I have received A's broadcast.

RReq

RRep

Route Maintenance

(a) D's routing table before G goes down.

(b) The graph after G has gone down.

D’s routing table before G goes down

Route maintenance

• Route caching timeout used to purge old routes

• Active_timeout period used to determine if neighboring node is active

• If source moves, paths are re-established using RReq