Wireless Networks

Lecture 33
TCP Over Wireless Networks
Dr. Ghalib A. Shah

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Outlines
 Motivation
 TCP Variants
► Slow start
► Fast Retransmit/Recovery (TCP Reno)

 Issues in Heterogeneous Wireless Networks
 TCP Schemes for Wireless
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Pure Link-level Approaches
Soft-state Transport Layer Caching Approaches
Soft-state Cross Layer Signalling Approaches
Hard-state Transport Layer Approaches
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Last Lecture
 MAC Layer

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Scalability
Single Channel
Multi-Channel
Some Ideas
Research Issues

 Network Layer
► Routing
► 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
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Motivation
 Characteristics of wireless networks
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Lack of infrastructure in ad hoc networks
Mobility
Shared channel
Limited bandwidth

 Transport protocols typically designed for
► Fixed end-systems
► Fixed, wired networks
► Characteristics of TCP
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•
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Window-based: not possible to maintain fine-grained timers on a
per-flow basis
Slow –start
Loss-based congestion indication
Dependence on ACKs
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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.
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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
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 Congestion window size is increased when data is


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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
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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 trans m is s ion e rrors
► furthermore, m obility 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
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Schemes
 The various approaches revolve around
distinguishing between the following:
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Congestion loss
Error loss
Delay beyond the retransmission timer threshold
Out of order delivery beyond the three DUPAK
threshold

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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
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 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.

 Hard-state Transport Layer Approaches:
► 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.
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Pure  Link­le ve l Appro ac he s
 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.
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S o ft­s tate  Trans po rt Laye r Cac hing  
Appro ac he s
 Snoop

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Snooping TCP I
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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
correspondent
local retransmission
changes of TCP only within the
foreign agent (+min. MH change)
foreign
host
agent
„wired“ Internet

mobile
host

snooping of ACKs

buffering of data

end-to-end TCP connection

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Snooping TCP II



Data transfer to the mobile host
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Data transfer from the mobile host
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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:
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

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

Maintain end-to-end semantics
No change to correspondent node
No major state transfer during handover


Problems
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Snooping TCP does not isolate the wireless link well
Snooping might be useless depending on encryption schemes
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S o ft­s tate  Cro s s  Laye r S ig naling  
Appro ac he s
 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.

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 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 noncongestion related loss has occurred.

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Hard­s tate  Trans po rt Laye r Appro ac he s
 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)

“wireless” TCP

wired Internet


standard TCP

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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
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Wireless TCP
 preserve the end-to-end semantics.
 This Protocol tries to distinguish Random
losses from Congestion losses by measuring

the packet inter arrival time with the packet
inter departure time.
 WTCP uses rate-based rather than windowbased 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.
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Mobile TCP
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Special handling of lengthy and/or frequent disconnections
with low BER links
M-TCP splits as I-TCP does
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unmodified TCP fixed network to supervisory host (SH)
optimized TCP SH to MH

Supervisory host
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no caching, no retransmission

monitors all packets, if disconnection detected
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old or new SH reopen the window

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

maintains semantics, supports disconnection, no buffer
forwarding

Disadvantages
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loss on wireless link propagated into fixed network
adapted TCP on wireless link
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Ad Hoc Transport Protocol (ATP)
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Layer coordination
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Rate based transmissions
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Congestion control uses feedback from the network; Reliability is
ensured through receiver feedback and selective ACK

Assisted congestion control
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Avoids impact of bursty traffic

Decoupling of congestion control and reliability
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Uses feedback from network nodes for congestion detection,
avoidance, and control


Adapts sending rate based on feedback from intermediate nodes

TCP friendliness and fairness
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Achieved through feedback from intermediate nodes
But fairness yet an issue

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