2. Contents
Introduction
comparison between tcp and udp
Conjestion strategies
I –TCP
Snooping
Mobile tcp
2.5/3g ether networks
Points from journals
Conclusion
2
3. WHAT IS A TRANSPORT LAYER
• In computer networking, the transport layer is a conceptual
division of methods in the layered architecture of protocols in
the network stack in the Internet Protocol Suite and the Open
Systems Interconnection (OSI). The protocols of the layer
provides host-to-host communication services for
applications.[1] It provides services such as connection-
oriented data stream support, reliability, flow control,
and multiplexing.
(source wikipedia)
3
(
4. TCP UDP
It is a connection oriented
protocol
TCP is used in case of
applications in which fast
transmission of data is not
required
HTTP, FTP, SMTP Telnet etc
It rearranges data packets in the
order specified
Comparatively slow
Reliable (defines that data will
be definitely sent across)
It is a connection less protocol
It is preferred in case of the
applications which have the
priority of sending the data on
time and on faster rates
DHCP, TFTP, SNMP, RIP, VOIP
etc
No inherent ordering, the data
packets of same message may be
ordered differently
Comparatively faster
Unreliable 4
6. Con..
Data is read as a byte stream,
thus no distinguishing
indications are transmitted to
the signal message
It is heavier as it requires
three packets to set up a
socket connection, before any
user data can be sent. TCP
handles reliability and
congestion control.
TCP controls the flow of data
Packets are sent individually
and after the arrival the
packets are rearranged
UDP is lightweight due to no
ordering of messages,
no tracking connections, etc.
UDP does not have an option
for flow control
6
7. Sliding window method
This window covers unacknowledged data and the data it
can send keeping in mind the window size advertised by
the receiver.
Following figure should give you an idea about how a
sliding window looks like :
7
8. Con..
The receiver can send acknowledgement even before its
window fills up completely.
The left edge of the window can move towards right.
Neither left edge should move towards left nor right edge
should move towards left.
8
9. CONJESTION STRATEGY
Slow-start is part of the congestion control strategy used by
TCP, the data transmission protocol used by
many Internet applications. Slow-start is used in conjunction
with other algorithms to avoid sending more data than the
network is capable of transmitting, that is, to avoid
causing network congestion. Slow-start begins initially with
a congestion window size (cwnd) of 1, 2 or 10. The value of the
Congestion Window will be increased by one with,
9
10. Con..
each acknowledgement (ACK) received effectively
doubling the window size each round-trip time . The
transmission rate will be increased with slow-start
algorithm until either a loss is detected, or the receiver's
advertised window (rwnd) is the limiting factor, or the slow
start threshold (ssthresh) is reached. If a loss event occurs,
TCP assumes that it is due to network congestion and takes
steps to reduce the offered load on the network. These
measurements depend on the used TCP congestion
avoidance algorithm.
10
11. Con..
Once ss thresh is reached, TCP changes from slow-start
algorithm to the linear growth (congestion avoidance)
algorithm. At this point, the window is increased by 1
segment for each RTT.
Although the strategy is referred to as "Slow-Start", its
congestion window growth is quite aggressive, more
aggressive than the congestion avoidance phase.[9] Before
slow-start was introduced in TCP, the initial pre-congestion
avoidance phase was even faster.
The behavior upon packet loss depends on the TCP
congestion avoidance algorithm that is used.
11
12. Fast retransmit
Fast Retransmit is an enhancement to TCP that reduces
the time a sender waits before retransmitting a
lost segment.
A TCP sender uses a timer to recognize lost segments. If an
acknowledgement is not received for a particular segment
within a specified time (a function of the estimated round-
trip delay time), the sender will assume the segment was
lost in the network, and will retransmit the segment.
Duplicate acknowledgement is the basis for the fast
retransmit mechanism which works as follows: after
receiving a packet (e.g. with sequence number 1), the
receiver sends an acknowledgement by adding 1 to the
sequence number (i.e., acknowledgement number 2) which
means that the receiver received the packet number 1 and it
expects packet number 2 from the sender 12
13. CON…
Let's assume that three subsequent packets have been lost. In
the meantime the receiver receives packet numbers 5 and
6. After receiving packet number 5, the receiver sends an
acknowledgement, but still only for sequence number 2.
When the receiver receives packet number 6, it sends yet
another acknowledgement value of 2. Because the sender
receives more than one acknowledgement with the same
sequence number (2 in this example) this is
called duplicate acknowledgement.
13
14. 14
The fast retransmit enhancement works as follows: if a
TCP sender receives a specified number of
acknowledgements which is usually set to three duplicate
acknowledgements with the same acknowledge number
(that is, a total of four acknowledgements with the same
acknowledgement number), the sender can be reasonably
confident that the segment with the next higher sequence
number was dropped, and will not arrive out of order. The
sender will then retransmit the packet that was presumed
dropped before waiting for its timeout.
15. I -TCP
Indirect TCP or I-TCP segments the connection (figure
1)
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
15
17. 17
If CN sends packet, FA acknowledges packet and forwards
packet to MN
If MN receives packet,it acknowledges
This acknowledgement only used by CN
Similarly if MN sends packet, FA acknowledges packet and
forwards it to CN
I-TCP requires several actions as soon a handover
takes place:
The packets have to be redirected using mobile IP
The access point acts as a proxy bufferring packets for
retransmission
After handover, the old proxy forwards data to new proxy
The sockets(current state of TCP) of old proxy also migrate
to new foreign agent
20. 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
20
21. Snooping TCP
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 with 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 tough if packets are encrypted
21
23. Mobile TCP
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 optimised 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.
.
23
24. Con..
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)
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
24
25. Con..
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 original value.
25
26. 2.5/3g ethernetworks
Tcp over 2.5g/3g ethernetworks
Learn to live with:
• Data rates: 64 kbit/s up, 115-384 kbit/s down
asymmetry: 3-6, but also up to 1000 (broadcast
systems), periodic allocation/release of channels
• High latency, high jitter, packet loss
– Suggestions:
• Large (initial) sending windows, large maximum transfer unit, selective
acknowledgement, explicit congestion notification (ECN), time stamp, no
header compression
– Already in use:
• i-mode running over FOMA
• WAP 2.0 (“TCP with wireless profile)
26
27. Points from journals
[1]The mobile TCP will dramatically improve the
throughput performance under error condition due to
wireless transmission. The mobile TCP solution introduces
a three key technologies, these are, header
conversion,selective retransmission and no slow start
mechanism, while keeping a compatibility against user
interface and inside of the Internet.Thus these network
and transport layers solution in this paper will provide a
chance to create a new type ofcommunication life based on
mobile computing.
27
28. Con..
[2] TIAP for achieving interoperability between TCP and
ATP. TAIP can be implemented as a thin layer between the
network layer and transport layer in the TCP/IP stack and
it is transparen
[3] The employment of DTLS to secure CoAP
communications raises various issues, as addressed
throughout the article, that are also recognized and the
focus of research to both the sender and the receiver
28
29. Con..
[4]
29
LS-SCTP includes path monitoring and
packet assignment mechanisms. These mechanisms monitor
the paths and stripe the packets according to their condition.
Simulation results demonstrated the effectiveness of our
proposed mechanism in providing throughput and
reliability
to mobile applications.
30. Con..
[5] A special code class, which is used for erasure decoding,
are LDPC codes, which can be described via its sparse parity
check matrix H.
30
31. REFERENCES
[1] A NETWORK AND TRANSPORT LAYERS FOR MOBILE
COMPUTING OVER INTERNET byT. OHSAWA*, K OKANOUE and
K.TAKEDA
[2] A Transport Layer Interoperability Model For Mobile
Ad-hoc Environment by Haidar Safa, Marcel Karam, Rawad
Abou
[3] End-to-end transparent transport-layer security for
Internet integrated mobile sensing devices by Jorge Granjal
and Edmundo Monteiro
[4] A Transport Layer Load Sharing Mechanism for Mobile
Wireless Hosts by Ahmed Abd El Al, Tarek Saadawi,
Myung Lee
31
32. Con…
[5] Transport Layer Coding for the Land Mobile Satellite
Channel by Harald Emst, Luca Sartorello and Sandro
Scalise
[6]wikipedia
[7] Raj Kamal, Mobile Computing, Oxford University Press,
2007
32
