Mobile Transport Layer

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Mobile Transport Layer

• Student Zhi Xu(3723055)
• E-mail zxu061_at_site.uottawa.ca

2
Mobile Transport Layer

• Introduction to Transport Layer
• Issues within Mobile Environment
• Typical Solutions for TCP within Mobile
  Environment
• Challenges Opportunities
• Conclusion

3
Mobile Transport Layer

• Introduction to Transport Layer
• Issues within Mobile Environment
• Typical Solutions for TCP within Mobile
  Environment
• Challenges Opportunities
• Conclusion

4
Transport Layer in OSI

• Position
• Session Layer (upper)
• Transport Layer
• Network Layer (lower)
• Definition
• The transport layer of the OSI reference model is
  intended to provide a more reliable method of
  communication that is not offered by the network
  layer
• -Tanenbaum, Computer Networks, Fourth Edition
• Main Protocols
• Transmission Control Protocol (TCP)
• User Datagram Protocol (UDP)
• Datagram Congestion Control Protocol (DCCP) -
  IETF
• Stream Control Transmission Protocol (SCTP) -
  IETF
• Reliable User Datagram Protocol (RUDP)
  -Bell Labs
• Etc.

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UDP

• User Datagram Protocol (UDP)
• A connectionless protocol that does not offer
  guaranteed packet delivery.
• Using UDP, programs on networked computers can
  send short messages known as datagrams to one
  another.
• UDP does not provide the reliability and ordering
  guarantees that TCP does
• Datagrams may arrive out of order or go missing
  without notice.
• However, as a result, UDP is faster and more
  efficient for many lightweight or time-sensitive
  purposes. Also its stateless nature is useful for
  servers that answer small queries from huge
  numbers of clients.
• Advantage faster, more efficient, lightweight,
  etc.
• Disadvantage reliability not guaranteed, not
  sensitive to order and datagrams missing, etc.

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TCP

• Transmission Control Protocol (TCP)
• An end-to-end protocol that provides reliable,
  in-order transmission over unreliable IP.
• Using TCP, applications on networked hosts can
  create connections to one another, over which
  they can exchange data or packets.
• The protocol guarantees reliable and in-order
  delivery of sender to receiver data.
• TCP also distinguishes data for multiple,
  concurrent applications (e.g. Web server and
  email server) running on the same host
• Advantage reliable, byte stream,
  connection-oriented, in-order delivery, etc.
• Disadvantage time-cost, a little bit complex,
  have to maintain the connection, etc.

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TCP
Client
Server
TCP SYN

• The traditional TCP was designed for fixed
  networks
• TCP gains great success.
• E.g. HTTP (used by web services) typically uses
  TCP

TCP SYN/ACK
Connection setup
TCP ACK
HTTP request
Data transmission
HTTP response
gt15 s no data
We can not apply this traditional TCP to mobile
environment!
Connection release
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Mobile Transport Layer

• Introduction to Transport Layer
• Issues within Mobile Environment
• Typical Solutions for TCP within Mobile
  Environment
• Challenges Opportunities
• Conclusion

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Characters of mobile environment

• Characters of mobile environment
• Mobile, wireless, Ad Hoc networks
• Influence to the Transport Layer (especially,
  TCP)
• Variable link quality
• Different quality of transmitters and receivers,
  distance, conditions, etc. lead to different link
  quality
• Topological changes
• As the nodes communicating may be moving and
  changing. The network is dynamic

5 mins later
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Characters of mobile environment

• Multi-user interference
• In Ad Hoc networks, as the transmitters do not
  have a pinpoint location to send data to, they
  must broadcast the data to every node in range.
  The same, all the nodes received message need to
  decipher and decide whether it should take it
• Power
• Power is always an issue that need to be
  concerned in mobile environment How to find a
  route with the balance between performance and
  power consumption? This wont be so easy as in
  fixed networks.
• Large and varying delay, low bandwidth, path
  asymmetry, etc.

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Problems with traditional TCP in mobile
environment

• Compared with fixed network, mobile environment
  seems unreliable
• Package loss congestion control
• Path change
• Variable performance
• Etc.

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Problems with traditional TCP in mobile
environment

• Package loss congestion control
• Mechanism of congestion control in traditional
  TCP
• missing acknowledgement causes the reduction of
  the congestion threshold to one half of the
  current congestion window

Sender
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Problems with traditional TCP in mobile
environment

• When a corrupted packet is received,
• the receiver discards the packet without sending
  a NACK.
• The sender has no way of discerning this type of
  loss from a loss due to congestion.
• Bit errors are frequent in wireless networks.
• In wired networks, TCP can assume that packet
  loss is due to congestion
• In ad hoc networks, the assumption cannot be made

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Problems with traditional TCP in mobile
environment

• Path change (variable and dynamic)
• A typical end-to-end route

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Mobile Transport Layer

• Introduction to Transport Layer
• Issues within Mobile Environment
• Typical Solutions for TCP within Mobile
  Environment
• Challenges Opportunities
• Conclusion

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Typical Solutions for TCP within Mobile
Environment

• Approaches
• Link layer proposals
• Make special network layer mechanisms or add an
  additional layer between Network Layer and
  Transport Layer over the wireless link. So that
  we can still apply the traditional TCP in
  Transport Layer
• E.g. The Snooping TCP
• Split-connection proposals
• Split the connection between a static host and a
  mobile host at the base station, make a new
  protocol to replace traditional TCP in mobile
  part and still use the traditional TCP in fixed
  part, and two simultaneous connections are
  maintained.
• E.g. The Indirect TCP
• End-to-end proposals
• End-to-end protocols require changing the
  protocol stack at both the sender and the
  receiver. This method provide more efficiency
  than the former ones, but, barring the cost of
  upgrading the protocol stacks.
• E.g. The WTCP

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Protocols for Mobile

• Classic
• Snooping TCP
• Indirect TCP (I-TCP)
• Mobile TCP (M-TCP)
• Fast retransmit/fast recovery
• Transmission/time-out freezing
• Selective retransmission
• Transaction oriented TCP
• Some current methods (Since 2000)
• TCP-BuS
• TCP-F
• ATCP (Ad hoc Trans. Control Protocol)
• ATP ENIC

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Snooping TCP

• Transparent extension of TCP within the foreign
  agent
• - "Improving TCP/IP Performance Over
  Wireless Networks, H. Balakrishnan et al., 1995
• 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

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Indirect TCP (I-TCP)

• Indirect TCP or I-TCP
• - I-TCP indirect TCP for mobile hosts, Bakne
  and Badrinath, 1995
• 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

access point (foreign agent)
wired Internet
mobile host
standard TCP
wireless TCP
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I-TCP socket and state migration
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Fast retransmit/fast recovery

• Change of foreign agent often results in packet
  loss
• TCP reacts with slow-start although there is no
  congestion
• 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
• 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)
• 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

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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
• Disadvantage
• TCP on mobile host has to be changed, mechanism
  depends on MAC layer

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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

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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

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Current Approaches (since 2000)TCP-BuS

• TCP-Bus introduces buffering capabilities in the
  mobile nodes. It also does feedback information
  for detecting route disconnection.
• Dongkyun K., Toh, C.K., and Choi, Y. TCP-BuS
  improving TCP performance in wireless ad hoc
  networks. In Communications, 2000 IEEE
  International Conference on, Volume 3, (18-22
  June 2000), 1707 -1713.
• Modification on traditional TCP
• Explicit notifications for route failures and
  route reestablishment.
• ERDN disconnection notification
• ERSN route successful notification
• Use extended timeout values
• Selective retransmission of lost packets
• Avoid unnecessary requests for fast transmission
• Reliable transmission of control messages
• ERDN_RET_TIMER
• ERSN_RET_TIMER

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Current Approaches (since 2000)ATCP (Ad hoc
Trans. Control Protocol)

• The Ad hoc Transmission Control Protocol acts as
  an intermediary between the network layer and
  standard TCP
• Liu, J., Singh, S. ATCP TCP for mobile ad hoc
  networks. Selected Areas in Communications, IEEE
  Journal on, Volume 19 Issue 7, (July 2001).
• The upper TCP does not change at all. And ATCP is
  invisible to the user and application.
• ATCP detects and reacts to disconnection,
  congestion, and loss due to bit error or
  out-of-order packets.
• ATCP requires the use of information from the
  network layer such as ECN (explicit congestion
  notification) and ICMP Destination Unreachable
  messages.

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Mobile Transport Layer

• Introduction to Transport Layer
• Issues within Mobile Environment
• Typical Solutions for TCP within Mobile
  Environment
• Challenges Opportunities
• Conclusion

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Challenges Opportunities

• TCP over 2.5/3G wireless networks
• Fine tuning todays TCP
• 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, time stamp, no
  header compression
• Already in use
• i-mode running over FOMA
• WAP 2.0 (TCP with wireless profile)

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Challenges Opportunities
Mobile system
wireless

• Performance enhancing proxies (PEP, RFC 3135)
• Transport layer
• Local retransmissions and acknowledgements
• Additionally on the application layer
• Content filtering, compression, picture
  downscaling
• E.g., Internet/WAP gateways
• Web service gateways?
• Big problem breaks end-to-end semantics
• Disables use of IP security
• Choose between PEP and security!
• More open issues
• RFC 3150 (slow links)
• Recommends header compression, no timestamp
• RFC 3155 (links with errors)
• States that explicit congestion notification
  cannot be used
• In contrast to 2.5G/3G recommendations!

PEP
Internet
Comm. partner
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Improve TCP performance over Third Generation
Wireless Networks

• Improving TCP/IP Performance over Third
  Generation Wireless Networks - Mun Choon Chan
  and Ramachandran Ramjee, Bell Labs, 2004
• Approach
• First, it designs a network-based solution called
  the Window Regulator that maximizes TCP
  performance for any given buffer size at the
  congested router.
• Second, it presents a scheduling and buffer
  sharing algorithm that reduces the latency for
  short flows while exploiting user diversity, thus
  allowing the wireless channel to be utilized
  efficiently.

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Conclusion

• The traditional protocols in Transport Layer are
  designed for fixed or wired networks.
• When applied in mobile environment, the
  traditional protocols encounter several problems,
  like disconnection, path reestablishment, etc. To
  solve these problems, several approaches are
  introduced.
• As the mobile communication keeps on booming, new
  technologies related to Transport Layer are
  proposed.
• These new technologies bring opportunities and
  challenges to the Transport Layer.

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References

• Mobile Communications, Chapter 9 Mobile
  Transport Layer 
• Handbook of Wireless Networks and Mobile
  Computing, Chapter 13 Transport over Wireless
  Networks
• Computer Networks, 4th Edition, Chapter 6
  Transport Layer
• On the Current State of Transport Layer
  Protocols in Mobile Ad hoc Networks, John C.,
  Wade C., Brian E. John A. H. ACMSE 04, 2004

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Three Questions

• Question 1
• The typical solutions for TCP within Mobile
  Environment can be categorized into three types.
  Whats the there types?
• Answer
• Link layer proposals, Split-connection
  proposals, End-to-end proposals
• Question 2
• What is the biggest problem with traditional
  TCP in mobile environment?
• Answer
• Traditional TCP consider that the packet loss
  is caused by congestion.
• Question 3
• What should we consider when we want to design
  a new protocol to fulfill the function of TCP in
  mobile environment?
• Answer
• Variable link quality, Topological changes,
  Multi-user interference, Power, Large and varying
  delay, low bandwidth, path asymmetry, etc.

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• Thank you! ?