TCP-wireless

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					TCP over wireless
                 Data Link Layer
• Deliver frames
  between nodes that
  are connected over a
  single link.
     – Frame: a data unit
       delivered in the data
       link layer




8/9/2012                           2
   What should Data Link Layer do?
• Delimit a frame.
     – Framing
• If DLL should provide reliable service to upper layers, it will
  do
     – error control
     – flow control
     – in-order delivery, etc




8/9/2012                                                      3
                     Error control
• Error detection
     – CRC
     – Checksum
• Error correction
     – Automatic Repeat Request
       (ARQ):retransmission
           • Go-Back-N
           • Selective Repeat(Request)
     – Forward Error Correction (FEC)
8/9/2012                                 4
                                Go-back-N
       Selective Repeat &(b) Go-Back-N(GBN)
       (a) Selective Repeat(SR)
                I(0)                                        I(0)
                I(1) ACK(1) 0                               I(1) ACK(1) 0
                I(2)   .     1                              I(2)    .     1
                I(3)   .     2                              I(3)    .     2
                I(4)   .     3
                                Error
                                                            I(4)    .     3Error
                I(5) NAK(4)                 Retransmit
                                                            I(5) NAK(5)    on I(4)
                I(6) ACK(5)  5 on I(4)                      I(6)
                             6              all frames
Retransmit      I(4)                                        I(4)
                                             from I(4)
only I(4)       I(7)         4                              I(5)           4
                I(0)         7                              I(6)           5
                I(1)         0                              I(7)           6
                I(2)    ACK(2) Error                        I(0)    ACK(8) 7 Error
      timeout                                     timeout
                I(3)         on ACK(2)                      I(1)            on ACK(8)
                I(4)                                        I(7)
                                         Retransmit
Retransmit      I(1)                     from I(7)          I(0)
only I(1)       I(5)                                        I(1)
                I(6)                                        I(2
     8/9/2012
                I(7)                                        3)      preserve       5
                            Frames out of order             I(4)    frame order
Is the TCP retransmission GBN or SR?

• The TCP receiver retransmits the packet
  selectively for which the ack has timed out
  or dupacks arrived.
     – Similar to SR
• But unlike SR, the TCP receiver
  acknowledges cumulatively the last packet
  correctly received.


8/9/2012                                        6
           Delay-Bandwidth product
• Congestion window size bounds the amount of
  data that can be sent per round-trip time (RTT).
• Throughput: amount of data sent per time
     – It is equivalent to the bandwidth used for actual
       transmission
                                    W
                     throughput 
                                   RTT

           Ideal window size = max bandwidth(BW) * RTT



8/9/2012                                                   7
              Ideal window size?
• What if W < RTT*BW?
     – Inefficiency (wasted
       bandwidth)
                                  RTT
• What if W > RTT*BW?                   W   Window size
     – Queuing at intermediate              Amount of data
                                            To be transmitted
       routers and increasing
       RTT
     – Potentially, packet loss




8/9/2012                                                8
The problem of TCP in wireless links
• Random errors may cause Fast
  Retransmit.
• The Fast Retransmit results in
     – Retransmission of lost packet
     – And the Fast Recovery reduces congestion
       window.
     – Reducing congestion window decreases
       throughput.
• Reducing congestion window in response
  to errors is unnecessary.
8/9/2012                                          9
                   Continued …
• Burst errors may cause timeouts. If a wireless
  link remains unavailable for extended duration,
  data of the window size will be lost.
     – Example, driving through a tunnel
• Timeout results in Slow Start.
• Slow Start reduces congestion window to 1 MSS
  (maximum segment size) , consequently
  decreasing throughput.
• Again, doing the Congestion Avoidance in
  response to errors is unnecessary.

8/9/2012                                            10
                   Continued …
• Fundamental question: How to distinguish
  loss due to congestion from loss due to
  other wireless/mobility reasons?
• Hard to do: TCP is fundamentally end-to-end.
     – We just know that packet is lost, not why it is lost.
• Existing solutions break the end-to-end
  principle to some extent.
     – Also must be compatible with existing TCP.



8/9/2012                                                       11
       maximum segment size
• TCP and IPv4 headers are 20 bytes long each, whereas an IPv6 header is
  40 bytes long, so the MSS is equal to MTU minus 40 when using IPv4, and
  MTU minus 60 when using IPv6 (typical MTU for IPv4 is 1500 bytes).
• As data is routed over the Internet, it must pass through multiple gateway
  routers.
• Ideally, each TCP segment can pass through every router without being
  fragmented.
• If the data segment size is too large for any of the routers through which the
  data passes, the oversized segments are fragmented.
• This slows down the connection speed as seen by the computer user. In
  some cases the slowdown is dramatic.
• The likelihood of such fragmentation can be minimized by keeping the
  MSS as small as reasonably possible.
• For most computer users, the MSS is set automatically by the operating
  system.


8/9/2012                                                                      12
                    Continued …
• Thus, if the IP datagrams carrying the segments are as large as
  possible, we use the optimum segment size to avoid
  fragmentation.
• Although this may be difficult to achieve, it can be beneficial
  for the system to run smoothly without delayed performances
  or slow connection.
• Thus, in order to achieve this goal, three considerations must
  be made:
     – Make sure the implemented TCP has a mechanism for this.
     – Check the dynamic route changes of the routers to make sure that the
       datagrams does not dynamically change into a size that needs to be
       fragmented.
     – Observe the lower-level protocol headers.


8/9/2012                                                                      13
                    Continued …
• The selection of the MSS is based on the need to balance competing
  performances and implementation issues in the transmission of data being
  passed around on the TCP/IP.
• For example, let us take the definition of overhead management to
  consideration. Since the TCP header takes up 20 bytes, the IP header must
  also use 20 bytes or more, requiring a minimum of 40 bytes to be placed for
  the headers.
• A low MSS setting would lead to an inefficient use of network bandwidth,
  since a larger percentage of each segment would consist of TCP and IP
  header information.
• Another example is IP fragmentation when the TCP is packaged into IP
  packets.
• If the TCP segment is too large, it will lead the IP packet to be too large,
  and then cause fragmentation. Since fragmentation reduces efficiency and
  increases the chance of data loss, such an event can potentially lead to the
  need to retransmit the entire segment.

8/9/2012                                                                    14
           Ideal TCP behavior
• Ideally, TCP sender should only retransmit a
  packet lost due to transmission errors without
  any congestion control actions.
• So, transmission errors should be hidden from
  the TCP sender.
• Therefore we can say that the goal of the work
  for improving TCP performance in wireless links
  is to differentiate packet loss due to transmission
  errors from due to congestion.
•

8/9/2012                                            15
                Broad Approaches

• Two broad approaches to run TCP over wireless
  links.
     – Mask wireless loss from the TCP sender: Then TCP sender will
       not reduce congestion window.
     – Explicitly notify the TCP sender about cause of packet loss: TCP
       sender will not reduce congestion window for wireless losses.
• Some additional approaches designed to explicitly
  handle mobility.
• Solutions may be at the TCP sender, at the TCP
  receiver, or at an intermediate node (typically,
  wireless base station or WLAN access point).


8/9/2012                                                             16
     Techniques to Mask Wireless Losses
              from TCP Sender

• Split connection approach
     – I-TCP [Bakre-Badrinath-ICDCS-95]
• Snoop TCP [Balakrishnan-et-al-ACM-Winet-95].
• These solutions assume that the wireless
  part is just one hop (traditional cellular or
  WLAN network).
• All losses on wireless side assumed not
  connected with congestion.
     – Note that this may not true always; e.g., losses due to
       collision is because of congestion. But such subtleties
       are ignored. Assume that link layer is able to
       overcome congestion losses.

8/9/2012                                                     17
            Indirect TCP (I-TCP)

• Segment the TCP connection into two.
• No changes to the TCP protocol for hosts connected to
  the wired Internet (correspondent host or CH).
• Split the TCP connection at AP into 2 TCP connections,
  one between CH and AP, the other between AP and MH.
  No real end-to-end connection.
• The connection between AP and MH does not need to be
  a real TCP. Can be a custom transport protocol that is
  tuned for the wireless hop. For example, selective repeat
  over UDP.




8/9/2012                                                  18
     I-TCP Socket and State Migration

• On handoff, connection state must be
  migrated.




8/9/2012                                 19
                      I-TCP Critique
• Advantages
     – No changes in the fixed network necessary.
     – Transmission errors on the wireless link do not propagate into
       the fixed network. Local recovery from errors.
     – Possibility of using custom (optimized) transport protocol for the
       AP-MH hop.
• Disadvantages
     – Loss of end-to-end semantics, an ACK to sender does now not
       any longer mean that a receiver really got a packet. Problem if
       there is a crash at AP.
     – Large buffer space may be needed at AP.
     – AP must maintain per-TCP connection state.
     – State must be forwarded to new AP on handoff. May cause
       higher handoff latency.


8/9/2012                                                                 20
              Disadvantages(1)
• End-to-end semantics violated
     – Ack may be delivered to sender before the packet is
       delivered to the receiver.
• BS failure can result in loss of packets.
                           39

                           40
                                      38        37
FH                           BS                           MH
              40                                     36



8/9/2012                                                     21
                Disadvantages(2)
• Hand-off latency increases due to state
  transfer. Packets that has been acked to
  sender must be moved to new base
  station.        39
                          40       38   37
           FH               BS                MH
                40                       36



                                 39
                                 40           MH   Hand-off


8/9/2012                                              22
                     New base station
             Disadvantages(3)
• Buffers at BS are needed for each TCP
  connection, and buffers tend to become
  full when wireless link is slower.
• Extra copying of packets at BS
     – Copying from FH-BS socket buffer to BS-MH
       socket buffer
• It may not be useful if data and acks
  traverse different paths.

8/9/2012                                           23
  Snoop TCP: Simple LL retransmit
      + TL Delayed dupacks
• Removes the limitation of I-TCP
     – No more split connection.
     – Single end-to-end connection like regular
       TCP.
• Only access-point (AP) modified for a
  base implementation.
     – Modification on MH improves over the base
       implementation. But not mandatory.
• AP “snoops” on all TCP packets. It
  buffers packets for the MH.
8/9/2012                                           24
                   Continued …

• Data transfer to MH
     – AP buffers data until it receives ACK from MH, AP
       detects packet loss via dupacks or time-out, and
       retransmits packet.
     – CH unaware of loss or retransmission. No reduction
       in congestion window.
• Data transfer from MH
     – AP detects packet loss on the wireless link via
       missing sequence numbers, AP answers directly with
       a NACK to the MH.
     – MH can now retransmit data with only a very short
       delay.
     – This requires modification on the MH.
8/9/2012                                                    25
                Continued …
• It is also “Make transmission errors transparent
  to TCP” approach.
• Link layer retransmits frames if errors occur.
• TCP receiver delays third and subsequent
  dupacks for interval D when it receives out-of-
  order packets
• Delayed dupacks gives link layer retransmission
  time to succeed without triggering Fast Transmit
  from the TCP sender.

8/9/2012                                         26
                    Snoop : Example
                                                     Link layer buffer
                      End-to-end TCP connection

      application           application      delay       application
                                             dupacks
      transport             transport                    transport
      network               network                      network
                                              rxmt
      link                  link                         link
                                              OOO
      physical              physical         permitted   physical



                                   BS                           MH
                                          wireless
8/9/2012                                                                 27
           Snoop : Example




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           Snoop : Example




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           Snoop : Example




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           Snoop : Example




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           Snoop : Example




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           Snoop : Example




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           Snoop : Example




8/9/2012                     34
           Snoop : Example




8/9/2012                     35
           Snoop : Example




8/9/2012                     36
            Critique of Snoop TCP
• Advantages:
     – Can work without modification on MH.
     – Preserves end-to-end semantics. Crash does not affect
       correctness, only performance.
     – After handoff, new AP does not need to understand snoop TCP
       for communication to continue. Can automatically fall back on to
       regular TCP.
     – No state needs to be migrated. But if done, this can improve
       performance.
           • Note such “state” is called soft state. Good if available. But can work
             if not available.
• Disadvantages:
     – For the NACK scheme to work MH still needs to be modified.
     – Does not work with encrypted TCP headers.
     – Does not work for asymmetric routes.

8/9/2012                                                                          37
     Explicit Notification-Based Approach

• Send notification to the TCP sender about
  wireless packet loss.
• Upon notification, TCP sender retransmits
  packet, but does not reduce congestion
  window.
• Motivated by the Explicit Congestion
  Notification (ECN) Approach [Floyd-94].
• Many design options: Who sends
  notification? How? How notification is
  interpreted at sender?

8/9/2012                                      38
           Explicit Loss Notification (ELN)
                      Approach

• Assume MH is the TCP sender.
• AP keeps track of holes in the packet sequence received
  from the sender
• When a dupack is received from the receiver (CH), AP
  compares the dupack sequence number with the
  recorded holes
   – if there is a match, an ELN bit is set in the dupack
• When sender (MH) receives dupack with ELN set, it
  retransmits packet, but does not reduce congestion
  window.



8/9/2012                                                    39
  Impact of Mobility on TCP Performance

• Handoff can be either at the link layer (IP does not
  know) or at the network layer (IP is aware).
• Link layer handoff may not impact TCP much.
     – Other than a transient increase in RTT.
• Network layer handoff (e.g., Mobile IP) is slow. This
  is because routing must be updated.
     – Packets can be lost.
     – TCP is impacted.
     – We are interested in such handoffs.




8/9/2012                                                  40
           Fast Retransmit-Based Solution

• During the long delay for a mobile-IP handoff
  to complete, a whole window worth of
  packets may be lost.
     – Assuming no buffering/forwarding.
• Sender eventually times out, and retransmits.
• If handoff still not complete, another timeout
  will occur
• Performance penalty
     – Time wasted until timeout occurs.
     – Window shrunk after timeout.


8/9/2012                                       41
           Illustrative Timeline




8/9/2012                           42
  Fast Retransmit-Based Solution
• Assumption: MH is aware of handoff process
• When MH is the TCP receiver: after handoff is
  complete, it sends 3 dupacks to the sender
     – this triggers fast retransmit at the sender.
• When MH is the TCP sender: invoke fast retransmit after
  completion of handoff.
• Advantages
     – no slow start after handoff.
     – Retransmissions immediately after handoff instead of waiting for
       timeout.
     – Very minor change on TCP on MH only.
• Disadvantages
     – Only handles losses due to handoff.
     – Retransmitted packets will still traverse the entire network.
     – Congestion window still reduces upon handoff.

8/9/2012                                                                  43
           Mobile TCP (M-TCP)
• The fast-retransmit based solution can
  start retransmission immediately after
  handoff is complete. But it cannot
  prevent reduction in congestion
  window.
• M-TCP also prevents reduction in
  congestion window.
• How? Using persist mode of TCP.
8/9/2012                                   44
M-TCP Uses TCP’s Persist Mode
• TCP fact: When a new ACK is received with
     receiver’s advertised window = 0 (in TCP
     header), the sender enters persist mode.
• Means receiver does not have space to accept
     more packets.
• Sender does not send any data in persist
     mode.
• When a positive window advertisement is
     received again, sender exits persist mode.
• On exiting persist mode, retransmit
     timeout (RTO) and Congestion
     Avoidance and Control (cwnd) are same
     as
8/9/2012 before the persist mode.                 45
                   M-TCP Details
• Similar to split connection approach (I-TCP).
     – But maintains end-to-end semantics. AP forwards ACK only after
       it receives ACK.
• When the AP detects handoff or disconnection
     – AP advertises zero receiver window to sender.
     – This forces sender into persist mode.
     – After handoff is complete (connectivity is regained) new AP
       advertises correct receive window size.
• How is the zero window advertisement is sent?
     – AP withholds the ACK for the last byte.
     – This ACK carries the zero window advertisement on handoff.



8/9/2012                                                             46
             Critiquing M-TCP

• Some argue that not reducing the
  congestion window may not always be
  a good idea.
     – Level of congestion on new route is unknown!
• M-TCP needs help from AP for zero
  window advertisement.
     – It is possible for the receiver to do this, when it
       is the MH.

8/9/2012                                                47
           Simple Link Layer retransmission:
                       Approach

• Make transmission errors transparent to TCP
• If link layer is reliable,
     – It provides in-order delivery.
     – It retransmits frames whenever errors occur.
• If TCP timeout is large enough to tolerate
  additional delay due to link layer retransmission,
• then transmission errors are transparent to TCP.
  Thus TCP does not need to be modified,
  keeping the end-to-end semantics.

8/9/2012                                              48
                  Continued …
                                                 Link layer state
                  End-to-end TCP connection

    application         application                 application
    transport           transport                   transport
    network             network                     network
                                          rxmt
    link                link                        link
                                         In-order
    physical            physical         enforced   physical




                                      wireless
8/9/2012                                                            49
                   But, the problems…
• Link layer is unaware of TCP connections. So it cannot
  differentiate frames per TCP connection.
• Link layer transmission can cause head-of-line(HOL)
  blocking in the link layer buffer.
• Actually the example below is somewhat exaggerating
  reality, since the seq number are TCP not link layer
  numbers.
                                                    1

           Link layer buffer             bad link
           2   2    2   1

                               base                     2
8/9/2012                       station                      50
                    Continued …
• Furthermore, retransmission can cause frame
  loss due to buffer overflow.
• If the link layer buffer is full, packets may be lost,
  indicating congestion to the TCP sender.

                                                    1

           Link layer buffer             bad link
 22
 2                     1
 2
                               base                     2
8/9/2012                       station                      51
                    In reality
• Most HDLC-like link layer protocols provide
  reliability.
     – Go-Back-N retransmission: in-order delivery
• If the wireless link is single hop which is the
  case in most current wireless application
  environment, the retransmission delay may
  be small fraction of end-to-end TCP RTT
  (TCP Round Trip Time) Round .

8/9/2012                                             52
   TCP-aware link layer: Approach
• If link layer has different buffer per TCP
  connection, then it can avoid the HOL blocking.
• The base station(BS) buffers frames. When it
  receives dupacks from mobile host(MH), it
  retransmit if the frame is still in buffer.
• BS prevents Fast Retransmit at the TCP sender
  by dropping the dupacks.
• Snoop protocol is one that takes this approach.


8/9/2012                                        53
                    Continued …
                                                    Link layer buffer
                                                    per TCP connection
                     End-to-end TCP connection

      application          application               application
      transport            transport                 transport
      network              network                   network
                                             rxmt
      link                 link                      link
      physical             physical                  physical



                                  BS                        MH
                                         wireless
8/9/2012                                                           54
                       Continued …
                                                 TCP state
                                        35
                                                maintained at
                                        36       link layer
                                        37
                                        38


            40            39                  38           37
     FH                                 BS                               MH
                                   34                               36



           Example assumes delayed ack - every other packet ack’d
8/9/2012                                                                      55
                 Continued …
                          35
                          36
                          37
                          38
                          39


           41        40        39        38

                34                  36




8/9/2012                                      56
                      Continued …
                                       37
                                       38
                                       39
                                       40

                 42          41             40   39

                                  36                  36

                                                  dupack
           Duplicate acks are not delayed


8/9/2012                                                   57
                 Continued …
                          37
                          38
                          39
                          40
                          41

           43        42          41        40

                36                    36        36

                               Duplicate acks



8/9/2012                                             58
Continued …                                  37
                                             38
                                             39
                                             40
                                             41
                                             42

                   44            43                     37        41
     FH                                      BS                             MH
                                                             36        36
                                              Discard
           Dupack triggers retransmission     dupack
            of packet 37 from base station
                                             36
        BS needs to be TCP-aware to
      be able to interpret TCP headers
8/9/2012                                                                         59
Continued …           37
                      38
                      39
                      40
                      41
                      42
                      43

           45   44         42        37

                                36        36




                      36
                     36

8/9/2012                                       60
                                37
Continued …                     38
                                39
                                40
                                41
                                42
                                43
                                44

            46        45             43        42

                                          36        41


TCP sender does not
                                36
  fast retransmit
                            36
                           36

8/9/2012                                                 61
                                 37
                                 38
Continued …
                                 39
                                 40
                                 41
                                 42
                                 43
                                 44
                                 45
            47        46              44        43

                                           41


TCP sender does not              36
  fast retransmit
                                36
                            36
8/9/2012                   36                        62
                Continued …
                             42
                             43
                             44
                             45
                             46

           48     47              45   44
     FH                      BS                  MH
                       41                   43


                             36

                            36
                        36
8/9/2012               36                             63
                 Advantages
• It prevents Fast Retransmission despite
  transmission error, and out-of-order
  delivery on the wireless link.
     – Selective Repeat can be used.
• If delay-bandwidth product in the wireless
  link is less than 4 frames, a simple link
  layer retransmission can be sufficient.
• End-to-end TCP semantics is preserved.
• Requires modification to only BS.
8/9/2012                                       64
            Disadvantages
• Link layer at BS needs to be TCP-aware.
• It is not useful if TCP headers are
  encrypted like using IPsec.
• It can be used if TCP data and TCP acks
  traverse different paths, since both of them
  do not go through the base station.



8/9/2012                                     65
Can TCP sender know the cause of
      dupacks? Approach
• If the TCP sender can know that dupacks
  are due to errors, it does not need to
  reduce congestion window.
• Then is there any way of letting the sender
  know the cause?
• Who guess the cause?
     – Receiver-based
     – Sender-based
     – Wireless node-based
8/9/2012                                    66
           Receiver-based approach
• The TCP receiver uses a heuristics to
  guess the cause of packet loss.
• If the receiver guess that packet loss is
  due to errors, it sends a notification to the
  sender.
• The sender, on receiving the notification,
  retransmits the lost packet, not reducing
  congestion window.
8/9/2012                                          67
 How can receiver know the cause?
• Receiver uses the inter-arrival time
  between consecutively received packets to
  guess the cause of packet loss.




8/9/2012                                  68
           Packet loss due to Congestion
                                            T

                                            12           10
     FH                           BS                          MH


                                 11

           Packet loss due to transmission error

                                        2T

                                       12           11        10
                                                 Error loss
    FH                           BS                           MH


8/9/2012                                                           69
               Advantages
• It does not need any modification to TCP
  at base station.
• But, obviously it has limited applicability,
  since queuing delay for packets is not so
  uniform.




8/9/2012                                         70
           Sender-based approach
• The TCP sender guess the cause, using
  statistics based on the information he can
  gather.
     – RTT
     – Congestion Window size(W)
     – Loss pattern, etc
• Define condition C=f(RTT, W)
• If (C==True), reduces congestion window.
• C is calculated heuristically.
     – Several proposals for condition C
8/9/2012                                       71
            Disadvantages
• Unfortunately, correlation between RTT
  and W is often weak.
• Statistics collected by the sender can be
  garbled by other traffic on the network.
• The condition C should be changed
  depending on the network states.



8/9/2012                                      72
           Wireless node-based
• Wireless nodes determines that packets
  are lost due to errors and informs the
  sender using an explicit notification.
• The question is how the nodes can
  determine that packets are lost due to
  errors or congestion.



8/9/2012                                   73
       Space Communication Protocol Standards-
             Transport Protocol(SCPS-TP)

• The receiving ground station keeps track of how
  many packets with errors are received.
• When error rate exceeds a threshold, the ground
  station sends corruption-experienced messages
  to the destinations.


      Satellite      wireless



                                     TCP destinations
                  Ground station
8/9/2012                                                74
            Continued …
• Then the TCP destination tag acks with
  corruption-experienced bit.
• TCP sender receiving this bit does not
  back off until it receives and ack without
  this bit even if timeout or fast retransmit
  occurs.



8/9/2012                                        75
           When MH is TCP sender
• The base station keeps track of holes in the packet
  sequence received from the sender.
• When a dupack is received from the receiver, BS
  compares the dupack sequence number with the
  recorded holes. If there is a match, an ELN bit is et in the
  dupack.
• When sender receives dupack with ELN bit set, it
  retransmits packet, but does not reduce congestion
  window.

                                   Record
                                  hole at 2
             4   3    2   1                           4    3       1
 MH                                    BS                              FH
           wireless   1       1                                1   1
8/9/2012                             Dupack with ELN set                    76
           When MH is TCP receiver
• If packet cannot be delivered using a small
  number of retransmission, BS sends a
  Explicit Bad State Notification(EBSN)
  message to TCP sender.
     – Ex, MH is passing through a tunnel.
• When TCP sender receives EBSN, it
  resets its timer.
     – Timeout delayed when wireless channel is in
       bad state.

8/9/2012                                             77
                         Partial ACK
• Send two types of acks.
• A partial ack informs the sender that a packet was
  received by BS.
• When a packet for which a partial ack is received is
  detected to be lost, the sender does not reduce its
  congestion window.


           37                    37
                                 BS
                            37                      37



           Partial ack
8/9/2012   Cumulative ack                                78
           Concluding Remarks

• Need extra knowledge on wireless side to detect loss
  due to wireless/mobility effects that is unconnected
  to congestion.
• MH and/or AP may know such information.
• Approaches modify TCP on MH or introduce a
  support protocol on AP (or do both).
     – Doing anything on AP contradicts end-to-end principle.
• Some approaches only provide specific help. For
  example, improvements only when MH is TCP
  receiver or sender, but not both.

•
8/9/2012                                                        79

				
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Description: mobile computing