Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND SYSTEM FOR DATA CONJ~JNICATION
FIELD OF INVENTION
The present invention relates generally to data communication
between computers. More particularly the invention relates to
the problem of efficiently communicating data packets over a
network including both a sub-network, which is substantially
error-immune and has a low latency, and an access link, which
is comparatively error-prone and has a substantial latency.
DESCRIPTION OF THE PRIOR ART
TCP (Transmission Control Protocol) is today the most commonly
used transport layer protocol for communicating data over an
Internet. This protocol is optimised for wired connections,
which, have a high transmission quality and a low latency.
However, TCP is not very efficient for transmitting data over
links that are error prone, have long delays and/or a high
latency. Wireless links constitute typical examples of such
non-optimal links. Mobile communication typically imposes a
wireless link. Thus, two computers, of which at least one is
mobile, cannot communicate efficiently via a standard TCP-
connection, since the transmission algorithms in TCP postulate
a much higher link quality than what a wireless connection
normally can offer. Therefore, the comparatively poor quality
of the wireless connection, in most cases, severely
degenerates the performance of the connection. This is
particularly true if the wireless link is a high-speed link
with a considerable latency.
To ensure a safe transmission of data packets from a sending
to a receiving host, the protocol prescribes that a piece of
information indicating the status of received data packets
must be fed back from the receiving host to the sending host.
A simple positive acknowledgement protocol awaits an
acknowledgement for each particular data packet before sending
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another data packet. Naturally, such a protocol wastes a
substantial amount of network bandwidth while the sending host
waits for acknowledgements. An example of a more efficient
protocol is the so-called sliding window protocol.
Figure 1 illustrates a known method of using a sliding window
protocol to make it possible for a sending host to transmit
multiple data packets DP1 - DP3 before obtaining feed-back
information Ackl - Ack3 on the status of the transmitted data
packets from a receiving host. In figure 1 the sending host is
represented to the left and the receiving host to the right. A
time scale is symbolised vertically, with increasing time
downwards. In this example a congestion window has the size W
of three data packets. This means that three data packets DP1
- DP3 may leave the sending host before a first status message
Ackl from the receiving host arrives. Once such a message Ackl
has come, the congestion window slides one data packet and a
fourth data packet DP4 may be sent.
In order to assure delivery, each data packet is associated
with a retransmission timer. The retransmission timer is
started when a data packet leaves the sending host. At the
expiry of the retransmission timer the sending host
retransmits the data packet. The protocol may also be defined
such that the receiving host returns a negative
acknowledgement message for a data packet, if the data packet
has been received, however incorrectly. The data packet is, of
course, retransmitted also when such a negative
acknowledgement message arrives at the sending host. Thus a
data packet will be retransmitted either at reception of a
negative acknowledgement message or when the retransmission
timer expires, whichever happens first.
The procedure is then repeated for all data packets in the
message until the sending host has obtained positive
acknowledgement messages for each data packet in the message.
The size W of the congestion window thus corresponds to the
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number of data packets that may be sent out unacknowledged
into the network between the sending and the receiving host.
By gradually increasing the congestion window, it is possible
to eliminate the idle time in the network completely. In the
steady state, the sending host can thus transmit data packets
as fast as the network can transfer them. Consequently, a
well-tuned sliding window protocol keeps the network
completely saturated with data packets and obtains
substantially higher throughput than a simple positive
acknowledgement protocol (which is also known under the name
stop-and-wait protocol).
Modern TCP applies four different algorithms for controlling
the transmission of data packets over the Internet. According
to a first algorithm termed Slow Start the congestion window
is gradually increased as described above. Slow Start is
applied whenever a new connection is set up or when packet
loss has been detected by the retransmission timer, e.g. after
a period of congestion. The congestion window is initially set
to one data packet. The congestion~window is then increased to
two data packets at reception of the first acknowledgement
message. The sending host then sends two more data packets and
awaits the corresponding acknowledgement messages. When those
arrive they each increase the congestion window by one, so
that four data packets may be sent unacknowledged, and so on.
The term Slow Start may sometimes be a misnomer, because under
ideal conditions, the transmission rate is ramped up
exponentially.
If capacity limitations of the network do not stop this
exponential increase, the receiving host always has a window
limit, a so-called advertised window, which ultimately
restricts the transmission rate. Once this limit has been
reached the congestion window cannot be increased any further.
To avoid increasing the congestion window too quickly and
thereby causing congestion, TCP includes one additional
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restriction. A third window, usually referred to as the
allowed window, is applied for this purpose. The size of the
allowed window is determined by the following expression:
allowed window = min(advertised window, congestion window).
The congestion window in its turn is set according to the
following strategy. In steady state, on a non-congested
connection, the congestion window and the advertised window
are of equal size. The congestion~window is reduced by half
(down to a minimum of one data packet) upon loss of data
packets. For those data packets that remain in the
allowed window, the retransmission time is reduced
exponentially.
If the advertised window does not restrain the packet rate to
a level that the network is able to sustain, the
advertised window is larger than a window size, which
corresponds to the entire available capacity in the network,
the congestion window will be increased until a packet is lost
due to congestion. The congestion window will then be reduced
radically to decrease the total load on the network. After
that, the congestion window will once more be increased until
a data packet loss occurs and so on. In this case, a steady
state will never be reached.'
Congestion Avoidance is a second algorithm included in TCP,
which is applied after Slow Start. Whenever a new connection
is set up between two hosts the congestion window is increased
until either (i) a steady state is reached or (ii) a data
packet is lost. The communicating hosts may be informed of the
loss of a data packet in one of two alternative ways. Either
because a retransmission timer expires or because a third
algorithm called Fast Retransmit is activated. This algorithm
will be described after discussing the different methods
applied upon data packet loss detection.
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If a data packet loss is discovered through expiration of a
retransmission timer, the congestion window is immediately
reduced to one data packet. The congestion window is
thereafter increased under the Slow Start algorithm until it
5 reaches one half of the size it had before the retransmission
timer expired. Then, the Congestion Avoidance algorithm is-
activated. During Congestion Avoidance the congestion window
is increased by one data packet only when all the packets in
one window have been positively acknowledged.
If the loss of a data packet is discovered through the Fast
Retransmit algorithm, the congestion window is instantaneously
decreased to half the size it had before the data packet was
lost. The Congestion Avoidance algorithm is then activated and
applied as described above.
The Fast Retransmit algorithm will be illustrated by means of
an example. Suppose a sending host transmits 10 data packets
to a receiving host. All these data packets arrive correctly.
As a consequence of this, the receiving host returns a
positive acknowledgement message for data packet number 10 to
the sending host. This message indicates to the sending host
that all 10 data packets have been received correctly. The
sending host then transmits data packet number 11. This data
packet is however lost somewhere in the network. Later, the
sending host transmits data packet number 12, which reaches
the receiving host correctly. Since the positive
acknowledgement messages are cumulative, the receiving host
cannot now return a positive acknowledgement message for data
packet number 12. Instead, another positive acknowledgement
message for data packet number 10 is sent. The sending host
then transmits data packet number 13. This data packet also
reaches the receiving host correctly. As a response, the
receiving host feeds back yet another positive acknowledgement
message for data packet number 10. When the sending host thus
has received a third positive acknowledgement message for the
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10th data packet, this is interpreted as a loss of data packet
number 11. This data packet is therefore retransmitted and if
it reaches the receiving host correctly, a positive
acknowledgement message for data packet number 13 will be
returned. A general and more detailed description of the Fast
Retransmit algorithm can be found in W. Stevens, "TCP Slow
Start, Congestion Avoidance, Fast Retransmit, and Fast
Recovery Algorithms", Internet RFC 2001, Network Working
Group, NOAO, January 1997.
To sum up, the three algorithms described above, Slow Start
opens up the congestion window exponentially. Congestion
Avoidance, on the other hand, opens up the congestion window
linearly. Fast Retransmit is an algorithm for detecting the
loss of data packets.
Forward Acknowledgement (FACK) is a fourth control mechanism
included in TCP, which actually is a developed version of the
Congestion Avoidance and the Fast Retransmit algorithms.
Through FACK however, the outstanding data packets in the
network may be more accurately controlled. FACK is also less
bursty and can recover from periods of heavy loss better.
Further details regarding FACK can be found in M, Mathis et
al, "Forward Acknowledgement: Refining TCP Congestion
Control", Proceedings of ACM Sigcomm '96, Stanford, USA,
August 1996.
H. Balakrishnan et al describe a fifth mechanism under TCP,
which is called Snoop, in the document "Improving TCP/IP
Performance over Wireless Networks", Proceedings of ACM
Mobicom '95, November 1995. Snoop is a protocol that improves
TCP in wireless networks. The protocol modifies network-layer
software mainly at a base station, while preserving the end-
to-end TCP semantics . The general idea of the protocol is to
cache data packets at the base station and perform local
retransmissions across the wireless link.
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A further adaptation of TCP to wireless connections is
presented in S. Biaz et al, "TCP over Wireless Networks Using
Multiple Acknowledgements", Department of Computer Science at
Texas A&M University, Technical Report 97-001, January 1997.
Unnecessary retransmissions are here avoided in the network by
feeding back a partial acknowledgement for a data packet that
has reached the base station, if it experiences difficulties
on the wireless link. The base station is responsible for
retransmissions on the wireless link, while it delays timeout
of the retransmission timer via the partial acknowledgement.
The Slow Start-, the Congestion Avoidance-, the Fast
Retransmit- and the Fast Recovery-algorithms are all applied
in most modern implementations of TCP. The FACK- and the Snoop
algorithms however, are still not very frequently used.
The patent document EP, A2, 0 695 053 discloses an asymmetric
protocol for wireless data communications with mobile
terminals, according to which the terminals only transmit
acknowledgement messages and requests for retransmission upon
inquiry or when all data packets Within a data block have been
received. According to the protocol, base stations store
channel information for the wireless links and status
information of received and transmitted data packets. A base
station may also combine acknowledgements for multiple data
packets into a single acknowledgement code, in order to reduce
the power consumption in the mobile terminals.
Generally speaking, wireless connections cause longer round-
trip delays than wired connections. As result of the longer
round-trip delays, the transmission rate increase for a
wireless connection must, according to the prior art transport
protocols, be much slower than for a corresponding wired
connection. This is particularly true for wireless connections
where the bandwidth-delay product is comparatively high. The
prior art protocols always seek to increase the throughput of
a connection as far as the interconnecting networks permit. In
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addition to this, the possible transmission rate increase for
a particular connection is inversely correlated to the round-
trip delay for the connection. Hence, the shorter the round-
trip delay, the faster the increase rate. The combination of
characteristics typical for a wireless connection of high
bandwidth-delay product and long round-trip delay consequently
also constitutes a problem.
StJI~IARY OF T8E INVENTION
The present invention relates generally to data communication
between host computers, and more particularly to the problems
discussed above. The means of solving these problems according
the present invention are summarised below.
As indicated earlier, problems occur when the networks, which
connect a sending host with a receiving host suffer from both
a high random loss of data packets and a high bandwidth-delay
product.
Accordingly, it is an object of the present invention to
unravel the above-mentioned problem.
Particularly, it is an object of the invention to increase the
efficiency of a network with a high a bandwidth-delay product,
being connected to an access link, which causes a high random
loss of data.
Another object of the invention is to minimise the influence
of data packet losses in relatively error-prone access links,
in a substantially less error-prone network.
Qne further object of the present invention is to increase the
efficiency of the less error-prone links of a data
communication system, and thereby admit transmission of a
larger amount of data through the total system.
The proposed method for communicating data packets over a
packet switched network via at least one wireless access link
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includes the following assumptions and steps. The packet
switched network is presumed to offer a connectionless
delivery of data packets. The protocol used by the sending
host and the receiving host is a reliable sliding window
transport protocol, through which data packets, whenever
necessary, may be retransmitted from the sending to the
receiving host. The communicating hosts also take measures to
protect the packet switched network from congestion. The
receiving host generates status messages indicating the
condition of received data packets to the sending host. In
response to the status messages the sending host takes
appropriate data flow control measures. A buffering network
entity interfaces both the wireless access link and the packet
switched network. During communication of data packets the
buffering network entity performs the following steps. First,
receiving data packets from the sending host. Second, either
explicitly or implicitly, notifying the sending host of which
data packets that have been received correctly by the
buffering network entity, and in case of a lost or erroneously
received data packet, indicating whether the data packet was
lost or erroneously received over the access link or in the
packet switched network. Third, storing the correctly received
data packets. Fourth, forwarding the stored data packets to
the receiving host. Finally, the buffering network entity
performs local retransmissions of the stored data packets to
the receiving host, whenever that becomes necessary. Typically
such retransmissions occur after the expiry of a
retransmission timer or at reception of an explicit or
implicit notification of loss from the receiving host.
A method of communicating data packets between two hosts
according to the invention is hereby characterised by what is
apparent from claim 1.
A proposed system includes a buffering network entity, which
interfaces both a wireless access link and a packet switched
network and thus directly or indirectly brings a sending host
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in contact with a receiving host. The sending and the
receiving hosts are assumed to operate according to a reliable
sliding window transport protocol through which lost or.
erroneously received data packets may be retransmitted from
5 the sending host to the receiving host. The packet switched
network is further assumed to offer a connectionless delivery
of data packets. The buffering network entity includes a
receiving means for receiving data packets. This means also
generates and returns to the sending host a first status
10 message, which (i) indicates whether a particular data packet
must be retransmitted or not and (ii) indicates whether a data
packet has been lost or erroneously received in the packet
switched network. The buffering network entity also includes a
means for storing correctly received data packets and a means
for retrieving the stored data packets and transmitting them
to the receiving host. Moreover, the buffering network entity
includes a processing means for receiving the first status
message from the receiving means and for receiving a second
status message from the receiving host. The processing means
generates a third status message in response to the first and
second status messages and returns the third status message to
the sending host. The stored data packets are retransmitted to
the receiving host whenever that proves to be necessary.
Typically, such retransmission is initiated after the expiry
of a retransmission timer or at reception of an explicit or
implicit notification of loss.
The system according to the invention is hereby characterised
by the features set forth in the characterising clause of
claim 9.
The present invention thus prevents congestion control
algorithms from being activated in a substantially error-
immune network when data packets are lost in thereto connected
and comparatively error-prone access links for other reasons
than congestion. This of course, increases the efficiency not
only in the substantially error-immune network per se, but
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also in systems, which include both substantially error-immune
links and comparatively error-prone access links.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 illustrates the known method of using a sliding
window protocol being described above
Figure 2 illustrates the general method according to the
invention by means of a sequence diagram:
Figure 3 depicts a block diagram over a proposed system;
Figures 4a-d illustrate embodiments of the proposed method
for communicating data packets and generating
status information:
Figure S shows a flow diagram over an embodiment of the
proposed method being performed by a buffering
network entity;
Figure 6 depicts a block diagram over a system according
to the invention.
The invention will now be described in more detail with
reference to preferred exemplifying embodiments thereof and
also with reference to the accompanying drawings.
DESCRIPTION OF PREFERRED ~ODIMENTS
A sequence diagram in figure 2 gives a general illustration of
the method according to the invention. A sending host is here
represented to the left in the diagram and a receiving host is
represented to the right. The part of the connection between
the sending host and a buffering network entity IWU is
referred to as a first leg A and the part of the connection
between the buffering network entity and the receiving host is
identified as a second leg B. A time scale is symbolised
vertically, with increasing time downwards.
Data packets DPn up to a number n are assumed to have reached
the receiving host correctly. The receiving host therefore
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returns a positive acknowledgement message Ack n indicating
this to the buffering network entity IWU. The buffering
network entity IWU receives the positive acknowledgement
message Ack n. In this example, a data packet DPm with number
m arrives correctly at the buffering network entity IWU a few
moments later. The buffering network entity IWU subsequently
feeds back to the sending host a first status information
message S(An, Bn) indicating that the receiving host has
received all data packets DPn up to number n correctly, i.e.
no errors or losses have occurred for any of those n data
packets, neither in the first leg A nor the second leg B. The
buffering network entity IWU may also, simultaneously with
this or at any other time after reception of the data packet
DPm, send back a second status information message Ack m,
which indicates to the sending host that all data packets DPm
up to number m have reached the buffering network entity IWU
successfully. The first and second data packet status
information messages S (An, Bn) ; Ack m are preferably
effectuated according to a selective acknowledgement
algorithm, such as SACK. A detailed description of this
algorithm can be found in M. Mathis et al, "TCP Selective
Acknowledgement Options", Internet RFC 2018, Network Working
Group, October 1996.
If a data packet is received erroneously or if a data packet
is lost over any of the legs A or B, the buffering network
entity IWU feeds back data packet status information messages
indicating this fact to the sending host. The buffering
network entity IWU would, in response to a lost or degraded
data packet in the first leg A, return to the sending host
data packet a status information message S(A, B) - [1,-],
which indicates the transmission error on this particular leg
A. In case of a loss or degeneration of a data packet over the
second leg B, the buffering network entity IWU would return at
least a data packet status information message S (A, B) - [O,
1] indicating the transmission error on this leg B.
Optionally, the buffering network entity IWU could already
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have fed back a positive acknowledgement message Ack+, through
which correct reception of the data packet at the buffering
network entity IWU was announced.
A block diagram over a proposed system is depicted in figure
3. A first mobile host 305 is here connected to a first base
station 315 via a first access link 310. The access link 310
is typically constituted by one or more wireless radio links
in a cellular system. However, it may be an arbitrary kind of
connection, which is suitable for the specific application.
The access link 310 may, for instance, be a satellite link, an
optical link, a sonic link or a hydrophonic link. The first
base station 315 is further connected to a first buffering
network entity 320, generally termed IWU (InterWorking Unit).
The first base station 315 can either be separated from the
first buffering network entity 320 (as shown in the figure 3)
or be co-located with it, whichever is technically and/or
economically the most appropriate. The first buffering network
entity 320 also interfaces a packet switched network 325. The
packet switched network 325 is presupposed to offer a
connectionless delivery of data packets on a best-effort
basis. This means briefly, that every data packet that is
technically feasible to deliver will be delivered as soon as
possible. The Internet is,a well-known example where many
networks together provide a connectionless, best-effort
datagram delivery. Further details as to the definition of the
best-effort datagram can be found in the Internet RFC 1504.
In addition, at least one fixed host 330, at least one second
buffering network entity 335 and a third base station 365 may
be connected to the packet switched network 325. The second
buffering network entity 335 interfaces a second base station
340 and a second mobile host 350 via a second access link 345
in a manner corresponding to what has been described in
connection with the first buffering network entity 320 above.
The third base station 365, which communicates with a third
mobile host 355 over a third access link 360, may either be
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directly connected to the packet switched network 325 or be
connected via a unit, which is not a buffering network entity.
The above-described system makes it possible to exchange data
packets in any direction between any of the hosts 305, 330,
350 and 355 according to a reliable sliding window transport
protocol. Consequently, a sending host may be arbitrary host
305, 330, 350: 355 and receiving hosts) may be one or more of
the other hosts 305, 330, 350; 355. Preferably; the reliable
sliding window transport protocol used by the sending and the
receiving hosts is of a TCP-type (specified in the Internet
RFC793) or of a type specified in the standard document
IS08073. Yet, any alternative sliding window transport
protocol is naturally workable.
The sending host 310, 330, 350; 355 is notified of a data
packet status for each of its transmitted data packets via a
specific status message fed back from the buffering network
unit 320; 335. The status message (which e.g. may be TCP
acknowledgement) is generated at the buffering network unit
being closest to the sending host, i.e. the buffering network
unit 320 or 335 respectively. Thus, when the first mobile host
305 sends data packets the first buffering network unit 320
generates the status information. When the fixed host 330
sends data packets to one, of the mobile hosts 305 or 350
either the first 320 or the second 335 buffering network unit
generates the status information depending on which host 305
or 350 is the receiving host. If however, the fixed host 330
should send data packets to the third mobile host 355, no such
status information would be generated. When the host 350 sends
data packets the buffering network unit 335 generates the
status information. If the mobile host 355 sends data packets
the status information in question will only be generated if
the receiving host is connected to the packet switched network
325 via a buffering network unit such as 320 or 335. The data
packet status information may e.g. be communicated to the
sending host according to a selective acknowledgement
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algorithm (SACK) or according to a so-called TP4 algorithm.
Further description of the TP4 algorithm can be found in the
standard specification ISO 8073.
In order to furthermore illustrate the invention, four
5 different data communication examples will now be described
with reference to figures 4a through 4d.
In a first data communication example, being illustrated in
figure 4a, the fixed host FH is assumed to be the sending host
and the second mobile~host MH2 is assumed to be the receiving
10 host. Data packets DP thus pass from the fixed host FH through
the packet switched network to the second buffering network
entity IWU2. This part of the connection will be referred to
as a first leg B. The second buffering network entity IWU2
then forwards the data packets DP to the second mobile host
15 MH2 via the second base station and the second access link.
This part of the connection will be referred to as a second
leg C.
The second buffering network entity IWU2 here functions as the
end receiver of the data packets DP from the packet switched
network's point-of-view. This means that once a data packet DP
has succeeded in reaching the second buffering network entity
IWU2 correctly it will not be retransmitted from the fixed
host FH. The second buffering network entity IWU2 returns a
status message S(B, C) - [0,-] indicating this fact to the
fixed host FH.
If, on the other hand, a data packet DP is lost on the second
access link between the second base station and the second
mobile host MH2, that data packet DP will be retransmitted
from the second buffering network entity IWU2 until the data
packet DP has been received correctly at the second mobile
host MH2. Such a loss of a data packet is also indicated to
the fixed host FH by the status message S (B, C) - [0, 1] fed
back from the second buffering network entity IWU2.
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Data packets DP that are lost or degenerated after having left
the fixed host FH, but before reaching the second buffering
network entity IWU2, will be retransmitted from the fixed host
FH. If the second buffering network entity IWU2 registers a
loss or a degeneration of a data packet DP, it notifies the
fixed host FH via the status message S(B, C) - [1,-].
According to the invention, a first part of the status message
S(B, C), here corresponding to the first leg B, indicates if a
data packet DP has been lost or degenerated in the packet
switched network. A second part of the status message S(B, C),
here corresponding to the second leg C, indicates if a data
packet DP has been lost or degenerated over the second access
link. In case the loss or degradation occurred in the packet
switched network, the data packet rate from the fixed host FH
will be reduced via at least one data flow control algorithm,
otherwise not.
In a second data communication example illustrated in figure
4b, the first mobile host MH1 is the sending host and the
fixed host FH is the receiving host. Data packets DP hence
leave the first mobile host MH1 over the first access link and
pass via the first base station to the first buffering network
entity IWU1. This part of the connection will be referred to
as a first leg A. The first buffering network entity IWU1 then
forwards the data packets DP to the fixed host FH through the
packet switched network, which corresponds to a second leg B
of the connection.
The occasionally poor quality of the access link between the
first mobile host MH1 and the first buffering network entity
IWU1 may lead to degeneration or loss of data packets DP. Such
degenerated or lost packets DP must naturally be retransmitted
from the first mobile host MHl to the first buffering network
entity IWU1. Nevertheless, once a data packet DP has reached
the first buffering network entity IWU1 correctly it never has
to be retransmitted from the first mobile host MH1.
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However less likely, the first buffering network entity IWU1
may also have to retransmit data packets DP to the fixed host
FH. This is the case when, for instance, congestion in the
packet switched network has caused the retransmission timer to
expire.
A status message S(A, B) related to each data packet DP is
generated in the first buffering network entity IWU1 and
returned to the first mobile host MH1. A first part of the
status message S(A, B) here corresponding to the second leg B,
indicates if a data packet DP has been lost or degenerated in
the packet switched network, while a second part, here
corresponding to the first leg A, indicates if a data packet
DP has been lost or degenerated in over the first access link.
In case the loss or degradation occurred in the packet
switched network, the data packet rate from the first mobile
host MH1 will be reduced via at least one data flow control
algorithm. The loss or degradation of a data packet DP over
the first access link will, however, not influence the data
packet rate from the first mobile host MH1 in this way.
In a third data communication example, illustrated in figure
4c, the first mobile host MH1 is the sending host and the
second mobile host MH2 is the receiving host. The first mobile
host MH1 now transmits data packets DP over the first access
link, via the first base station to the first buffering
network entity IWU1. This part of the connection will be
referred to as a first leg A. The first buffering network
entity IWU1 subsequently passes the data packets DP via the
packet switched network to the second buffering network entity
IWU2. This part of the connection will be referred to as a
second leg B. Finally, the second buffering network entity
IWU2 sends the data packets DP to the second mobile host MH2
through the second base station and the second access link.
This last part of the connection constitutes a third leg C.
In this case, data packets DP may, if necessary, be
retransmitted over any of the legs A, B and C respectively. A
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data packet DP can either be retransmitted from the first
mobile host MH1 to the first buffering network entity IWU1,
from the first buffering network entity IWU1 to the second
buffering network entity IWU2 or from the second buffering
network entity IWU2 to the second mobile host MH2. Regardless
of what caused the loss or degrading of a particular data
packet DP, retransmission of the data packet will always only
be performed over the leg A, B; C where the data packet DP was
either lost or degenerated.
Status messages S(A, B); S(B, C) indicating the status of each
communicated data packet DP are generated in both the
buffering network entities IWU1; IWU2. The status messages
S(A, B): S(B, C) indicate in which leg A, B; C a loss or
degrading of a particular data packet DP has occurred. If the
status messages S(A, B); S(B, C) announce that a data packet
DP has been lost or degenerated in the packet switched
network, here leg B, at least one data flow control algorithm
will be triggered. The data packet rate from the first mobile
host MH1 will, as a result of such an algorithm, be decreased.
A loss or degradation of a data packet DP in any of the other
legs A or C will, on the other hand, not lead to a reduction
of the data transmission rate from the first mobile host MH1.
A fourth data communication example will now be discussed with
reference to figure 4d. The first mobile host MH1 is here the
sending host and the third mobile host MH3 is the receiving
host. The first mobile host MH1 this time transmits data
packets DP over the first access link, via the first base
station to the first buffering network entity IWU1. This part
of the connection will be referred to as a first leg A. The
first buffering network entity IWU1 then passes the data
packets DP via the packet switched network to the third mobile
host MH3, via the third base station and the third access
link. This part of the connection will be referred to as a
second leg B.
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A retransmission of a lost or degenerated data packet DP can
here either be carried out over the first leg A or over the
second leg B. A status message S(A, B) indicating the status
of each communicated data packet DP is generated in the first
buffering network entity IWUl and returned to the first mobile
host MH1. The status message thus indicates S(A, B) if a
certain data packet DP has been lost or degenerated over the
first access link, i.e. leg A, or somewhere between the first
buffering network entity IWU1 and the third mobile host MH3,
i.e. leg B. Only the loss or degeneration of a data packet DP
in leg B will trigger data flow control algorithms and thus
decrease the data packet rate from the first mobile host MH1.
Such loss or degrading is most likely to depend on poor
quality of the third access link between the third base
station and the third mobile host MH3, but since this fact is
impossible to verify data flow control algorithms will
nevertheless be activated.
If, regardless of the communication case, a buffering network
entity over a certain period of time receives more data
packets via one of its interfaces than what can be delivered
over its other interface, the superfluous data packets will be
discarded by the buffering network entity. The buffering
network entity will then feed back status messages S(X, Y) to
the sending host indicating that the superfluous data packets
were lost in the packet switched network. This will in its
turn trigger at least one data flow control algorithm, which
directs the sending host to reduce its data packet rate. The
rate will thus be gradually reduced until it meets the
transmission capacity of the limiting interface at the
buffering network entity.
Figure 5 illustrates an embodiment of the inventive method
being carried out in a buffering network entity, when data
packets are transmitted from a sending host to a receiving
host via the buffering network entity. The figure illustrates
the possible fate of a particular data packet DP or a certain
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group of data packets DPs when passing through the buffering
network entity. It is nevertheless important to bear in mind
that since the reliable sliding window transport protocol
allows many data packets to be outstanding in the network
5 between the sending and the receiving host, the buffering
network unit will at any given moment be carrying out many of
the following steps simultaneously and in parallel. The
procedure will thus be in different steps with regard to
different data packets DPs.
10 In a first step 500 the buffering network entity receives one
or more data packets DP(s) from the sending host. A following
step 505 checks whether the data packets) DP(s) is(are)
correct. If so, the procedure continues to step 520.
Otherwise, a request is made in a step 510 for retransmission
15 of the incorrectly received data packets) DP(s). A status
message S(X,-) indicating the erroneous reception of the data
packets) DP(s) is fed back to the sending host in a
subsequent step 515. The procedure then returns to step 505 in
order to determine whether the retransmitted data packets)
20 DP(s) arrives) correctly.
In practice the steps 510 and 515 are most efficiently being
carried out as one joint step, where the status message S(X,-)
per se is interpreted as. a request for retransmission. The
steps 510 and 515 may, of course, also be carried out in
reverse order or in parallel. Their relative order is
nevertheless irrelevant for the result.
In case the buffering network entity in the steps 500 and 505
receives data packets DP(s) in an out-of-sequence order; so
that a loss of earlier data packets) DP(s) likely to have
occurred a retransmission of the assumedly lost data packets)
is requested in the steps 510 and 515.
The step 520 checks whether the connection at the buffering
network entity's output interface has enough bandwidth BW,
i.e. can transport data packets DPs at least as fast as data
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packets DPs arrive to the buffering network entity at the
input interface. In case of insufficient bandwidth BW one or
more data packets DP(s) are discarded in a step 525. The
procedure then returns to the step 510, where retransmission
is requested for the discarded data packet(s). If, on the
other hand, in the step 520 it is found that the output
interface has sufficient bandwidth BW the procedure continues
to a step 530.
In the step 530 a status message S(X,-) indicating correct
reception of the data packet ( s ) DP ( s ) may be fed back to the
sending host. If there is a packet switched network between
the sending host and the buffering network entity, a status
message S(X,-) regarding the result of the transmission is
regularly fed back from the buffering network entity to the
sending host. If, however, there is no packet switched network
between the sending host and the buffering network entity (but
e.g. an access link) the step 530 may be empty. Step 530
namely provides information to the sending host, which is
necessary to control the data flow from the sending host, and
such information need only be communicated if the data
packets) DP(s) has/have been transmitted over a packet
switched network.
In an ensuing step 535 the correctly received data packets)
DP(s) is/are stored in the buffering network entity. A
thereafter following step 540 forwards the data packets)
DP(s) to the receiving host. A step 545, checks whether a
status message relating to the sent the data packet ( s ) DP ( s )
has been returned from the receiving host. If such a status
message reaches the buffering network entity before the
expiration of a retransmission timer a step 555 checks whether
the status message indicates correct or incorrect reception of
the data packets) DP(s). A step 550 determines if the
retransmission timer has expired, and in case the timer is
still running the procedure is looped back to the step 545.
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If however, no status message has been received when the
retransmission timer expires or if it in step 555 is found
that one or more data packets DP(s) have been received
incorrectly, the data packets) DP(s) in question are
retransmitted in a step 560. A following step 565 returns a
status message S(X, Y) to the sending host indicating the fact
that retransmission of data packets) DP(s) was necessary. If
there is a packet switched network between the buffering
network entity and the receiving host, flow control algorithms
triggered by the status message S(X, Y) will cause the sending
host to reduce its data flow, otherwise the data flow from the
sending host will not be affected.
In alternative embodiments of the inventive method, the steps
560 and 565 are carried out reverse order or in parallel.
Their relative order is irrelevant for the effect of the
steps.
After the step 565 the procedure returns to the step 545,
which checks whether a status message for the retransmitted
data packets) DP(s) has been received. As soon as a status
message indicating correct reception of the data packets)
has/have reached the buffering network entity the procedure
continues for this/these data packets DP(s) from the step 545,
via the step 555, to a final step 570. This final step 570
feeds back a status message S(X, Y) to the sending host, which
indicates that the data packets) has/have been received
correctly by the receiving host.
Figure 6 depicts a block diagram over an arrangement according
to the invention. A sending host, which may be either fixed or
mobile, is here represented by a first general interface 600.
A receiving host, which likewise may be either fixed or
mobile, is correspondingly represented by a second general
interface 650. A buffering network entity IWU interfaces both
the first and the second interface 600: 650.
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The buffering network entity IWU in its turn includes a means
610 for receiving data packets DP from the first interface
600, a storage means 620 for storing data packets DP, a means
630 for transmitting data packets DP over the second interface
650 and a processing means 640 for generating status messages
S(X, Y) and controlling the over all operation of the entity
buffering network IWU in accordance with the method described
in connection with figure 5 above.
Apart from receiving data packets DP the receiving means 610
also determines whether the data packets DP are received
correctly and .generates in response to the condition of the
received data packets DP a first status message S(X,-).
Furthermore, the receiving means 610 may have to discard data
packets DP on instruction from the processing means 640. This
happens if the processing means has found that the bandwidth /
capacity over the interface 600 exceeds the capacity over the
interface 650. The discarded data packets DP are regarded as
lost data packets. A first status message S(X,-) indicating
such a loss is therefore generated and returned after having
discarded data packets in the receiving means 610.
This first status message S(X,-) is returned to the sending
host. The first status message S(X,-) is also forwarded to the
processing means 640. Furthermore, the means 610 generates
requests for retransmission of data packets DP whenever that
becomes necessary. As mentioned earlier the status message
S(X,-) itself may, of course, be interpreted as a requests for
retransmission at the sending host. Data packets DP having
been received correctly by the means 610 are passed on to the
storage means 620 for temporary storage.
The means for transmitting 630 retrieves data packets DP from
the storage means 620 and sends them over the interface 650 to
the receiving host. In response to the sent data packets DP
the receiving host returns a second status message e.g. in the
form of a positive or a negative acknowledgement message Ackt
indicating the condition of the data packets DP at the
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receiving host. The processing means 640 receives the second
status message Ackt and generates a third and combined status
message S (X, Y) , which is determined from the content of the
first status message S(X,-) and the second status message
Ackt. The third status message S(X, Y) thus gives a total
representation of how successfully a certain data packet or a
certain group of data packets was passed over the respective
communication legs before and after the buffering network
entity. The third status message S(X, Y) is transmitted from
the processing means back to the sending host over the
interface 600.
A particular data packet DP having been temporarily stored in
the storage means 620 may be deleted as soon as a second
status message Ack+ has been received for the data packet DP
indicating that the data packet has been received correctly by
the receiving host. The data packet DP may of course also be
deleted at any later, and perhaps more suitable, moment.
The invention being thus described, it will be obvious that
the same may be varied in many ways. Such variations are not
to be regarded as a departure from the scope of the invention,
and all such modifications as would be obvious to one skilled
in the art are intended to be included within the scope of the
following claims.
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