Mobile Transport layer

1.  Motivation
 TCP-mechanisms
 Classical approaches
 Indirect TCP
 Snooping TCP
 Mobile TCP
 PEPs in general
Mobile Transport Layer
 Additional optimizations
 Fast retransmit/recovery
 Transmission freezing
 Selective retransmission
 Transaction oriented TCP

2. Motivation
 Transport layer responsible for
 Fixed end-end systems
 Fixed, wired networks
 Error recovery
 Flow control
 Congestion control
 Ensuring complete data transfer in TCP
 Efficient retransmissions

3.  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

4.  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
 Linear increase Continues until a time-out at The
sender occurs due to a missing
acknowledgement, or until the sender detects a
gap in transmitted Data because of Continuous
acknowledgements for The same packet.
 missing acknowledgement causes the reduction of
the congestion threshold to one half of the current
congestion window
 congestion window starts again with one segment

6.  TCP fast retransmit/fast recovery
 If a sender receives several acknowledgements for the
same packet, this is due to a gap in received packets
at the receiver or missing acknowledgements
 The sender can now retransmit the missing packet(s)
before the timer expires. This behavior is called fast
retransmit.
 It is an early enhancement for preventing slow-start to
trigger on losses not caused by congestion.
 The receipt of acknowledgements shows that there is
no congestion to justify a slow start.
 The sender can continue with the current congestion
window. The sender performs a fast recovery from
the packet loss.
 therefore, packet loss is not due to congestion,
continue with current congestion window (do not use
slow-start)

7. Influences of mobility on TCP-mechanisms
 Slow Start mechanism in fixed networks decreases the
efficiency of TCP if used with mobile receivers or
senders.
 Error rates on wireless links are orders of magnitude
higher compared to fixed fiber or copper links. This
makes compensation for packet loss by TCP quite
difficult.
 Mobility itself can cause packet loss. There are many
situations where a soft handover from one access point
to another is not possible for a mobile end-system.
 Standard TCP reacts with slow start if
acknowledgements are missing, which does not help in
the case of transmission errors over wireless links and

9. Early approach: Indirect TCP
 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

11. Indirect TCP
 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

13. Snooping TCP
 „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
hostforeign
agent
mobile
host
snooping of ACKs

15. Snooping TCP
 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

16. Advantages
 The end-to-end TCP semantic is preserved.
 Most of the enhancements are done in the foreign
agent itself which keeps correspondent host
unchanged.
 Handover of state is not required as soon as the
mobile host moves to another foreign agent. Even
though packets are present in the buffer, time out
at the CH occurs and the packets are transmitted
to the new COA.
 No problem arises if the new foreign agent uses
the enhancement or not. If not, the approach
automatically falls back to the standard solution.

17. Disadvantages
 Snooping TCP does not isolate the behavior of the
wireless link as well as I-TCP.
 Transmission errors may propagate till CH.
 Using negative acknowledgements between the
foreign agent and the mobile host assumes additional
mechanisms on the mobile host. This approach is no
longer transparent for arbitrary mobile hosts.
 Snooping and buffering data may be useless if certain
encryption schemes are applied end-to-end between
the correspondent host and mobile host. If encryption
is used above the transport layer, (eg. SSL/TLS),
snooping TCP can be used.

18. MOBILE TCP

19. Mobile TCP - Motivation
 Dropping packets due to a handover or higher bit
error rates is not the only phenomenon of wireless
links and mobility – the occurrence of lengthy and/or
frequent disconnections is another problem.
 A TCP sender tries to retransmit data controlled by a
retransmission timer that doubles with each
unsuccessful retransmission attempt, up to a
maximum of one minute
 What happens in the case of I-TCP if the mobile is
disconnected? The proxy has to buffer more and
more data, so the longer the period of disconnection,
the more buffer is needed.
 The snooping approach also suffers from being
disconnected. The mobile will not be able to send
ACKs so, snooping cannot help in this situation.

20. Objective – M-TCP
 To prevent the sender window from shrinking if
bit errors or disconnection but not congestion
cause current problems.
 To improve overall throughput, to lower the delay,
to maintain end-to-end semantics of TCP, and to
provide a more efficient handover.
 Adapted to the problems arising from lengthy or
frequent disconnections

21. Mobile TCP
 The M-TCP splits up the connection into two parts:
 An unmodified TCP is used on the Standard host-
Supervisory Host section
 An optimized TCP is used on the Supervisory Host-
Mobile Host section.
 The Supervisory Host (SH) adorns the same role as
the proxy (Foreign Agent) in I-TCP.
 The SH is responsible for exchanging data to both the
Standard host and the Mobile host.
 Here in this approach, we assume that the error bit
rate is less as compared to other wireless links.
 So if any packet is lost, the retransmission has to
occur from the original sender and not by the SH.
(This also maintains the end-to-end TCP semantic)

22. Mobile TCP
 The SH monitors the ACKs (ACK means
acknowledgement) being sent by the MH. If for a long
period ACKs have not been received, then the SH
assumes that the MH has been disconnected (maybe
due to failure or moved out of range, etc...).
 If so the SH chokes the sender by setting its window
size to 0.
 Because of this the sender goes into persistent mode
i.e. the sender’s state will not change no matter how
long the receiver is disconnected.
 This means that the sender will not try to retransmit
the data.
 Now when the SH detects a connectivity established
again with the MH (the old SH or new SH if
handover), the window of the sender is restored to

23. Advantages:
 Maintains the TCP end-to-end semantics. (No
failed packet retransmission is done by the SH
.All job handled by original sender)
 Does not require the change in the sender’s TCP.
 If MH disconnected, it doesn’t waste time in
useless transmissions and shrinks the window
size to 0.
 No need to send old buffer data to new SH in
case of handover (as in I-TCP).

24. Disadvantages:
 M-TCP assumes low bit error which is not always
true. So, any packet loss due to bit-errors
occurring, then its propagated to the sender.
 Modifications are required for the MH protocol
software.

25. 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, no transparent approach

26. 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
 It offers a way to resume TCP connections even after long
interruptions of the connection.
 It can be used together with encrypted data as it is independent of
other TCP mechanisms such as sequence no or acknowledgements
Disadvantage
 Lots of changes have to be made in software of MH, CH and FA.

27. 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

28. 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!

29.  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

30.  Advantage
 efficiency
 Disadvantage
 requires changed TCP
 mobility not longer transparent

31. 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