Wireless Networks: Transport Protocols Lecture

Wireless Networks: Transport Protocols 1
Wireless Networks:
Transport Protocols
 Motivation
 TCP-mechanisms
 Indirect TCP
 Snooping TCP
 Mobile TCP
 Fast retransmit/recovery
 Transmission freezing
 Selective retransmission
 Transaction oriented TCP
Adapted from J. Schiller, “Mobile Communications”, Chapter 10

Motivation I
Transport protocols typically designed for
 Fixed end-systems
 Fixed, wired networks
Research activities
 Performance
 Congestion control
 Efficient retransmissions
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

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

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 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

Indirect TCP I
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
„wireless“ TCP standard TCP

I-TCP socket and state migration
mobile host
access point2
Internet
access point1
socket migration
and state transfer

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
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

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 (+min. MH change)
„wired“ Internet
buffering of data
end-to-end TCP connection
local retransmission correspondent
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
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
10.8.1

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

Fast retransmit/fast recovery
Change of foreign agent often results in packet loss
 TCP reacts with slow-start although there is no congestion
Forced fast retransmit
 as soon as the mobile host has registered with a new foreign agent,
the MH sends duplicated acknowledgements on purpose
 this forces the fast retransmit mode at the communication partners
 additionally, the TCP on the MH is forced to continue sending with
the actual window size and not to go into slow-start after
registration
Advantage
 simple changes result in significant higher performance
Disadvantage
 further mix of IP and TCP (to know when there is a new
registration), no transparent approach
10.10.1

Transmission/time-out freezing
Mobile hosts can be disconnected for a longer time
 no packet exchange possible, e.g., in a tunnel, disconnection due to
overloaded cells or mux. with higher priority traffic
 TCP disconnects after time-out completely
TCP freezing
 MAC layer is often able to detect interruption in advance
 MAC can inform TCP layer of upcoming loss of connection
 TCP stops sending, but does now not assume a congested link
 MAC layer signals again if reconnected
Advantage
 scheme is independent of data and TCP mechanisms (Ack,SN) =>
works even with IPsec
Disadvantage
 TCP on mobile host has to be changed, mechanism depends on
MAC layer

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

Transaction oriented TCP
TCP phases
 connection setup, data transmission, connection release
 using 3-way-handshake needs 3 packets for setup and release,
respectively
 thus, even short messages need a minimum of 7 packets!
Transaction oriented TCP
 RFC1644, T-TCP, describes a TCP version to avoid this overhead
 connection setup, data transfer and connection release can be
combined
 thus, only 2 or 3 packets are needed
Advantage
 efficiency
Disadvantage
 requires changed TCP
 mobility not longer transparent

Comparison of different approaches for a “mobile” TCP
Approach Mechanism Advantages Disadvantages
Indirect TCP splits TCP connection
into two connections
isolation of wireless
link, simple
loss of TCP semantics,
higher latency at
handover
Snooping TCP “snoops” data and
acknowledgements, local
retransmission
transparent for end-to-
end connection, MAC
integration possible
problematic with
encryption, bad isolation
of wireless link
M-TCP splits TCP connection,
chokes sender via
window size
Maintains end-to-end
semantics, handles
long term and frequent
disconnections
Bad isolation of wireless
link, processing
overhead due to
bandwidth management
Fast retransmit/
fast recovery
avoids slow-start after
roaming
simple and efficient mixed layers, not
transparent
Transmission/
time-out freezing
freezes TCP state at
disconnect, resumes
after reconnection
independent of content
or encryption, works for
longer interrupts
changes in TCP
required, MAC
dependant
Selective
retransmission
retransmit only lost data very efficient slightly more complex
receiver software, more
buffer needed
Transaction
oriented TCP
combine connection
setup/release and data
transmission
Efficient for certain
applications
changes in TCP
required, not transparent

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