The document discusses several mechanisms used in TCP for mobile computing. It describes: 1) TCP congestion control mechanisms like slow-start and fast retransmit/fast recovery which are designed to address packet loss. However, these can be inappropriate for wireless networks where packet loss is often due to errors rather than congestion. 2) Approaches like Indirect TCP, Snooping TCP, and Mobile TCP which modify TCP for mobile networks by splitting connections or having a supervisory host monitor the connection to enable local retransmissions and avoid unnecessary window reductions when the mobile host disconnects. 3) Other TCP optimizations for mobile like forced fast retransmit after handovers and transmission timeout freezing to avoid slow-start

1. MOBILE COMPUTING …
MOBILE TRANSPORT LAYER
N.JUNNUBABU ASST PROF.

2. TCP-Mechanisms
 TCP (Transmission Control Protocol) is a standard that defines how to establish and
maintain a network conversation through which application programs can exchange
data. TCP works with the Internet Protocol (IP), which defines how computers send
packets of data to each other.
 congestion control
 packet loss in fixed networks typically due to (temporary) overload situations
 routers have to discard packets as soon as the buffers are full
 TCP recognizes congestion only indirectly via missing (I.e., timed out) acknowledgements
 Immediate retransmissions unwise, they would only contribute to the congestion and make it
even worse
 slow-start algorithm is used as a reactive action to reduce the network load

3. TCP-Mechanisms
 slow-start algorithm
 sender calculates/negotiates a congestion window threshold 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

4. TCP-Mechanisms
 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
 It indicates that the receiver got all packets up to the gap and is actually receiving
packets, but some are missing (hence gap)
 Conclusion: packet loss not due to congestion, retransmit, continue with current
congestion window (do not use slow-start)

5. TCP-Mechanisms
 Implications on mobility
 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 old access point and forwarding from old to new access point is not
possible for some reason.

6. Mobile Network and Transport Layers..
 Classical TCP improvements.
 Indirect TCP
 Snooping TCP
 Mobile TCP
 Mobile IP
 Dynamic Host Configuration Protocol
 Mobile Ad Hoc Routing Protocols
 Multicast routing
 TCP over Wireless Networks
 Fast Retransmit / Fast Recovery
 Transmission/Timeout Freezing-Selective Retransmission
 Transaction Oriented TCP- TCP over 2.5 / 3G wireless Networks

7. Indirect TCP
 Indirect TCP or I-TCP segments:
 no changes to the basic TCP protocol for hosts connected to the wired Internet, millions
of computers use this protocol (or slight variants of it)
 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.

8. I-TCP segments a connection into two parts
.
mobile host
access point
(foreign agent) „wired“ Internet
„wireless“ TCP standard TCP
correspondent
host

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

10. Packet delivering of I-TCP:

11. I-TCP Advantages..
 Advantages
 no changes in the fixed network necessary, no changes for the hosts (TCP protocol)
necessary, all current optimizations to TCP (Reno, Vegas, etc.) 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, very fast retransmission of packets is possible, the short delay on the mobile
hop is known
 It is easy to use different protocols for wired and wireless networks.

12. I-TCP Disadvantages..
 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.
 Security issue:- The foreign agent must be a trusted entity.

13. Snooping TCP:
 The main drawback of I-TCP is the segmentation of the single TCP connection
into two TCP connections, which loses the original end-to-end TCP semantic.
 A new enhancement, which leaves the TCP connection intact and is completely
transparent, is Snooping TCP.
 The main function is to buffer data close to the mobile host to perform fast local
retransmission in case of packet loss.
 the foreign agent buffers all packets with destination mobile host and
additionally ‘snoops’ the packet flow in both directions to recognize
acknowledgements.
 The mobile host can now retransmit the missing packet immediately. Reordering of
packets is done automatically at the correspondent host by TCP.

14. Snooping TCP:
.
„wired“ Internet
buffering of data
end-to-end TCP connection
local retransmission correspondent
hostforeign
agent
mobile
host
snooping of ACKs

15. Snooping TCP: Packet delivery

16. 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

17. Advantages of snooping TCP:
 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.

18. Disadvantages of snooping TCP:
 Snooping TCP does not isolate the behaviour 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.

19. Mobile TCP:
 Both I-TCP and Snooping TCP does not help much, if a mobile host gets
disconnected. The M-TCP (mobile TCP) approach has the same goals as I-TCP
and snooping TCP: to prevent the sender window from shrinking if bit errors
or disconnection but not congestion cause current problems.
 M-TCP splits the TCP connection into two parts as I-TCP does. An
unmodified TCP is used on the standard host-supervisory host (SH) connection,
while an optimized TCP is used on the SH-MH connection.
 The SH monitors all packets sent to the MH and ACKs returned from the MH. If
the SH does not receive an ACK for some time, it assumes that the MH is
disconnected.

20. Advantages of M-TCP:
 It maintains the TCP end-to-end semantics. The SH does not send any ACK itself
but forwards the ACKs from the MH.
 If the MH is disconnected, it avoids useless retransmissions, slow starts or
breaking connections by simply shrinking the sender’s window to 0.
 As no buffering is done as in I-TCP, there is no need to forward buffers to a
new SH. Lost packets will be automatically retransmitted to the SH.

21. 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

22. Disadvantages of M-TCP:
 As the SH does not act as proxy as in I-TCP, packet loss on the wireless link due to
bit errors is propagated to the sender. M-TCP assumes low bit error rates, which
is not always a valid assumption.
 A modified TCP on the wireless link not only requires modifications to the
MH protocol software but also new network elements like the bandwidth
manager.

23. 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.

24. Transmission/time-out freezing
 MAC layer notices connection problems even before the connection is
actually interrupted from a TCP point of view and also knows the real reason for
the interruption. The MAC layer can inform the TCP layer of an upcoming
loss of connection or that the current interruption is not caused by congestion.
 TCP can now stop sending and ‘freezes’ the current state of its congestion
window and further timers. If the MAC layer notices the upcoming
interruption early enough, both the mobile and correspondent host can be
informed.
 the MAC layer detects connectivity again, it signals TCP that it can resume
operation at exactly the same point where it had been forced to stop. For TCP
time simply does not advance, so no timers expire.

25. Advantages:
Advantages:
 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 .
Disadvantages:
 Lots of changes have to be made in software of MH, CH and FA.

26. 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.

27. 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

28. Example of TCP Connection:

29. Overview of classical enhancements to TCP for mobility:
.
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