Wireless networks - Lecture 33: TCP over wireless networks

Wireless networks - Lecture 33: TCP over wireless networks. The main topics covered in this chapter include: motivation; TCP variants; issues in heterogeneous wireless networks; TCP schemes for wireless; hard-state transport layer approaches; soft-state cross layer signalling approaches;...

Wireless Networks

Lecture 33 TCP Over Wireless Networks

Dr Ghalib A Shah

► Fast Retransmit/Recovery (TCP Reno)

► Pure Link-level Approaches

► Soft-state Transport Layer Caching Approaches

► Soft-state Cross Layer Signalling Approaches

► Hard-state Transport Layer Approaches

► Wish List (Scalability, fast route discovery/repair, mobility,

flexibility, QoS, Multicast)

► Route Optimization Criteria

► Routing fairness

► Routing – Cross-layer design

 QoS Support at each layer

 WMN Standards

Motivation

 Characteristics of wireless networks

► Lack of infrastructure in ad hoc networks

TCP congestion control

 Packet loss in fixed networks typically due to overload

and is detected as

► 1) Retransmission timeout (RTO) at source

► 2) Arrival of three duplicate ACKs at source

► 3) Receipt of ICMP source quench message

 Routers discard packets as soon as the buffers are full

 TCP recognizes congestion only indirectly via missing

acknowledgements

 Retransmissions unwise, they would only contribute to

the congestion and make it even worse

 Slow-start algorithm as reaction which slowly

converges to optimal bandwidth.

TCP Slow Start

 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

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TCP Fast Retransmit/Recovery (TCP Reno)

 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 congestion,

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

 When packet loss occurs, congestion window size is

reduced

► Due to timeout: cwnd = 1 and enter slow start

► Due to duplicate ACKs: cwnd = cwnd/2 + 3×segment_size

 Congestion window size is increased when data is

successfully acknowledged

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Issues in Heterogeneous Wireless Networks

 Bit Error Rate (BER):

► 10 or worse are possible upon change in wireless environment

 Bandwidth

► Very less as compared to wired networks

► TCP underestimated bandwidth in wireless networks

 Round Trip Time (RTT):

► The wireless media exhibits longer latencies due to long

distances or NLOS path

► Large variation in RTT in wireless networks

 Mobility:

► Addition of mobile devices introduces huge amount of

indeterminate delay in rather a stationary network

 Power consumption

Influences of BER/mobility on TCP

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

► The performance of an unchanged TCP degrades severely

 however, TCP cannot be changed fundamentally due

to the large base of installation in the fixed network,

TCP for mobility has to remain compatible

 the basic TCP mechanisms keep the whole Internet

together

Schemes

distinguishing between the following:

► Congestion loss

► Error loss

► Delay beyond the retransmission timer threshold

► Out of order delivery beyond the three DUPAK

threshold

Classification

 Pure Link-level Approaches:

► These approaches aim at hiding the unwanted characteristics

of the wireless links from the higher layers

► but a critical factor is the determination of the link-level timeout

value

 Soft-state Transport Layer Caching Approaches:

► not crucial for the end-to-end connection and use caching as a

technique to save the sender from unnecessary invocation of the congestion control mechanism

► but they require changes at the intermediate node (base

station) and optionally at the mobile host and fail in the presence of encryption due to the intermediate node’s dependence

 Soft-state Cross Layer Signaling Approaches:

► These approaches make the transport layer sender

aware of the wireless link and separate the congestion losses from the error losses

► But involve changes at some or all of the

intermediate nodes and at the transport layer of the sender’s protocol.

► These solutions encompass all forms of splitting and

the end-to-end semantics may be sacrificed.

► The advantage of these approaches is that the

wireless link is completely shielded from damage loss.

Pure Link­level Approaches

 Reliable link-level protocols are implemented on the

wireless link which perform local retransmissions to

improve the reliability of communication independent of the higher-level protocols

► These protocols employ techniques such as forward error

correction (FEC) for error control

► and automatic repeat request (ARQ) for retransmission of lost

packets

 The timeout value for local (link level) retransmissions

is of major concern

► Interaction between the link-level retransmission timeouts and

the transport-level timeouts for TCP can lead to degraded performance if care is not taken while selecting the timeout values

Soft­s tate Trans port Layer Caching 

Approaches

Snooping TCP I

 it involves modification of the network layer (IP) software at the

base station (BS) by adding a module called snoop

 Transparent extension of TCP within the BS/FA

 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 (+min MH change)

„wired“ Internet buffering of data

end-to-end TCP connection

host

foreign agent

mobile

host

snooping of ACKs

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

 Advantages:

► Maintain end-to-end semantics

► No change to correspondent node

► No major state transfer during handover

 Problems

► Snooping TCP does not isolate the wireless link well

► Snooping might be useless depending on encryption schemes

Soft­s tate Cros s  Layer Signaling 

Approaches

 Explicit Congestion Notification (ECN)

► is an extension proposed to Random Early Detection (RED)

► marks a packet instead of dropping in when the average queue

size is between minth and maxth

► Upon receipt of congestion marked packet, the TCP receiver

informs the sender about incipient congestion,

► which in turn will trigger the congestion avoidance algorithm at

the sender

 Explicit Bad State Notification (EBSN)

► proposes a mechanism to update the TCP timer at the source

to prevent source form decreasing its congestion window

► EBSN’s are sent to the source after every unsuccessful

attempt by the base station to transmit packets over the wireless link

► EBSN would cause the previous timeouts to be cancelled and

new timeouts put in place, based on existing estimate of round trip time and variance

 Explicit Loss Notification (ELN)

► Add ELN option to TCP acks When a packet is

dropped on the wireless networks,

► future cumulative acknowledgements corresponding

to the lost packet are marked to identify that a congestion related loss has occurred.

Hard­s tate Trans port Layer Approaches

 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

access point (foreign agent) wired Internet

Indirect TCP II

 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 short delay on the mobile hop is known

 Disadvantages

► loss of end-to-end semantics, an acknowledgement to a

sender does 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

Wireless TCP

 This Protocol tries to distinguish Random

losses from Congestion losses by measuring the packet inter arrival time with the packet

inter departure time

window-based transmission control Hence it shapes its data traffic,

 never allows a burst of packet transmissions,

and is fair when competing connections have different round-trip times.

Mobile TCP

 Special handling of lengthy and/or frequent disconnections

with low BER links

 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

Ad Hoc Transport Protocol (ATP)

 Layer coordination

► Uses feedback from network nodes for congestion detection,

avoidance, and control

 Rate based transmissions

► Avoids impact of bursty traffic

 Decoupling of congestion control and reliability

► Congestion control uses feedback from the network; Reliability is

ensured through receiver feedback and selective ACK

 Assisted congestion control

► Adapts sending rate based on feedback from intermediate nodes

 TCP friendliness and fairness

► Achieved through feedback from intermediate nodes

► But fairness yet an issue

ATCP Approach

 ATCP utilizes network layer feedback (from the

intermediate nodes) to take appropriate actions

► ICMP: The Destination Unreachable ICMP message

indicates route disruption

► ECN: Indicates network congestion With ECN

enabled, time out and 3 dup ACKs are assumed to

no longer be due to congestion

Transport Layer Challenges

 New transport layer protocols need to be developed

that avoids the shortcomings of TCP while being

compatible with it

 Transport layer protocols for supporting real-time traffic

in wireless meshes are desirable

 Integration of transport layer with other layers; or

inferring and reacting with respect to the observations

at other layers

 Impact of mobility on transport layer

► Fast Retransmit/Recovery (TCP Reno)

► Pure Link-level Approaches

► Soft-state Transport Layer Caching Approaches

► Soft-state Cross Layer Signaling Approaches

► Hard-state Transport Layer Approaches