40:
817:
1367: =2, the receiver will accept and store the final packet 0 (thinking it is packet 8 in the series), while requesting a retransmission of packet 7. However, it is also possible that the transmitter failed to receive any acknowledgments and has retransmitted packet 0. In this latter case, the receiver would accept the wrong packet as packet 8.
193:
total number of packets yet to be acknowledged by the receiver. The receiver informs the transmitter in each acknowledgment packet the current maximum receiver buffer size (window boundary). The TCP header uses a 16 bit field to report the receiver window size to the sender. Therefore, the largest window that can be used is 2 = 64 kilobytes.
928:
A stronger constraint is imposed by the receiver. The operation of the protocol depends on the receiver being able to reliably distinguish new packets (which should be accepted and processed) from retransmissions of old packets (which should be discarded, and the last acknowledgment retransmitted).
1166:
The transmitter alternately sends packets marked "odd" and "even". The acknowledgments likewise say "odd" and "even". Suppose that the transmitter, having sent an odd packet, did not wait for an odd acknowledgment, and instead immediately sent the following even packet. It might then receive an
192:
By placing limits on the number of packets that can be transmitted or received at any given time, a sliding window protocol allows an unlimited number of packets to be communicated using fixed-size sequence numbers. The term "window" on the transmitter side represents the logical boundary of the
188:
Conceptually, each portion of the transmission (packets in most data link layers, but bytes in TCP) is assigned a unique consecutive sequence number, and the receiver uses the numbers to place received packets in the correct order, discarding duplicate packets and identifying missing ones. The
211:
on the network is avoided. The application layer will still be offering data for transmission to TCP without worrying about the network traffic congestion issues as the TCP on sender and receiver side implement sliding windows of packet buffer. The window size may vary dynamically depending on
203:
If the window limit is 10 packets then in slow start mode the transmitter may start transmitting one packet followed by two packets (before transmitting two packets, one packet ack has to be received), followed by three packets and so on until 10 packets. But after reaching 10 packets, further
254:
mechanism reveals corruption, the packet will be ignored by the receiver and a negative or duplicate acknowledgement will be sent by the receiver. The receiver may also be configured to not send any acknowledgement at all. Similarly, the receiver is usually uncertain about whether its
1252:
The advantage, however, is that it is not necessary to discard following correct data for one round-trip time before the transmitter can be informed that a retransmission is required. This is therefore preferred for links with low reliability and/or a high bandwidth-delay product.
1377: ≤6. With this restriction, the receiver knows that if all acknowledgments were lost, the transmitter would have stopped after packet 5. When it receives packet 6, the receiver can infer that the transmitter received the acknowledgment for packet 0 (the transmitter's
204:
transmissions are restricted to one packet transmitted for one ack packet received. In a simulation this appears as if the window is moving by one packet distance for every ack packet received. On the receiver side also the window moves one packet for every packet received.
255:
acknowledgements are being received. It may be that an acknowledgment was sent, but was lost or corrupted in the transmission medium. In this case, the receiver must acknowledge the retransmission to prevent the data being continually resent, but must otherwise ignore it.
1226:≤7. This is because, after transmitting 7 packets, there are 8 possible results: Anywhere from 0 to 7 packets could have been received successfully. This is 8 possibilities, and the transmitter needs enough information in the acknowledgment to distinguish them all.
153:, to be sent without having to wait for an ACK. Each packet receives a sequence number, and the ACKs send back that number. The protocol keeps track of which packets have been ACKed, and when they are received, sends more packets. In this way, the window
1229:
If the transmitter sent 8 packets without waiting for acknowledgment, it could find itself in a quandary similar to the stop-and-wait case: does the acknowledgment mean that all 8 packets were received successfully, or none of them?
1192:=1. The receiver refuses to accept any packet but the next one in sequence. If a packet is lost in transit, following packets are ignored until the missing packet is retransmitted, a minimum loss of one
1400:
makes corruption equivalent to loss), but will never appear out of order. The protocol can be extended to support packet reordering, as long as the distance can be bounded; the sequence number modulus
134:. The paradigm is similar to a window sliding sideways to allow entry of fresh packets and reject the ones that have already been acknowledged. When the receiver verifies the data, it sends an
639:
Techniques for defining "reasonable delay" can be extremely elaborate, but they only affect efficiency; the basic reliability of the sliding window protocol does not depend on the details.
196:
In slow-start mode, the transmitter starts with low packet count and increases the number of packets in each transmission after receiving acknowledgment packets from receiver. For every
200:
received, the window slides by one packet (logically) to transmit one new packet. When the window threshold is reached, the transmitter sends one packet for one ack packet received.
1424:
1448:=1 and if a second packet is lost, no more packets are buffered. This gives most of the performance benefit of the full selective-repeat protocol, with a simpler implementation.
1408:
It is possible to not acknowledge every packet, as long as an acknowledgment is sent eventually if there is a pause. For example, TCP normally acknowledges every second packet.
145:
The time that it takes for the ACK signal to be received may represent a significant amount of time compared to the time needed to send the packet. In this case, the overall
142:
protocol (ARQ), the sender stops after every packet and waits for the receiver to ACK. This ensures packets arrive in the correct order, as only one may be sent at a time.
250:
A transmitter that does not get an acknowledgment cannot know if the receiver actually received the packet; it may be that it was lost or damaged in transmission. If the
160:
Sliding windows are a key part of many protocols. It is a key part of the TCP protocol, which inherently allows packets to arrive out of order, and is also found in many
845:
So far, the protocol has been described as if sequence numbers are of unlimited size, ever-increasing. However, rather than transmitting the full sequence number
1420:. Normally, they are each assigned maximum values that respect that limit, but the working value at any given time may be less than the maximum. In particular:
1167:
acknowledgment saying "expecting an odd packet next". This would leave the transmitter in a quandary: has the receiver received both of the packets, or neither?
247:, the receiver must acknowledge received packets. If the transmitter does not receive an acknowledgment within a reasonable time, it re-sends the data.
291:. The window sizes may vary, but in simpler implementations they are fixed. The window size must be greater than zero for any progress to be made.
1411:
It is common to inform the transmitter immediately if a gap in the packet sequence is detected. HDLC has a special REJ (reject) packet for this.
1286:
uses a 3-bit sequence number, and has optional provision for selective repeat. However, if selective repeat is to be used, the requirement that
914:+1 possible sequence numbers that could arrive at any given time. Thus, the transmitter can unambiguously decode the sequence number as long as
1150:
protocol is actually the simplest possible implementation of it. The transmit window is 1 packet, and the receive window is 1 packet. Thus,
697:, the receive sequence number is increased by 1, and possibly more if further consecutive packets were previously received and stored. If
22:
322:
is one more than the sequence number of the highest sequence number received. For simple receivers that only accept packets in order (
608:
In the absence of a communication error, the transmitter soon receives an acknowledgment for all the packets it has sent, leaving
223:(RTT). The limit on the amount of data that it can send before stopping to wait for an acknowledgment should be larger than the
149:
may be much lower than theoretically possible. To address this, sliding window protocols allow a selected number of packets, the
1065:
The additional insight is that the receiver does not need to distinguish between sequence numbers that are too low (less than
1086:). In either case, the receiver ignores the packet except to retransmit an acknowledgment. Thus, it is only necessary that
1416:
The transmit and receive window sizes may be changed during communication, as long as their sum remains within the limit of
1512:
Comer, Douglas E. "Internetworking with TCP/IP, Volume 1: Principles, Protocols, and
Architecture", Prentice Hall, 1995.
374:
When the receiver receives a packet, it updates its variables appropriately and transmits an acknowledgment with the new
1242:. This requires a much more capable receiver, which can accept packets with sequence numbers higher than the current
1517:
1502:
1051:
different sequence numbers that the receiver can receive at any one time. It might therefore seem that we must have
788:, because there is no need to worry about receiving a packet that will never be transmitted; the useful range is 1 ≤
219:, it is important that the transmitter is not forced to stop sending by the sliding window protocol earlier than one
83:
61:
54:
1331: =7, as is typically used with the go-back-N variant of HDLC. Further suppose that the receiver begins with
301:
is the next packet to be transmitted, i.e. the sequence number of the first packet not yet transmitted. Likewise,
197:
135:
1423:
It is common to reduce the transmit window size to slow down transmission to match the link's speed, avoiding
1441:=2 and buffers packets following a gap, but only allows a single lost packet; while waiting for that packet,
116:
107:. Sliding window protocols are used where reliable in-order delivery of packets is required, such as in the
1283:
1396:
The above examples assumed that packets are never reordered in transmission; they may be lost in transit (
554:: The highest packet sent is limited by the highest acknowledgement received and the transmit window size.
622:. If this does not happen after a reasonable delay, the transmitter must retransmit the packets between
750:
Whether the packet was accepted or not, the receiver transmits an acknowledgment containing the current
1553:
929:
This can be done given knowledge of the transmitter's window size. After receiving a packet numbered
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If the packet's number is not within the receive window, the receiver discards it and does not modify
189:
problem with this is that there is no limit on the size of the sequence number that can be required.
130:, along with additional data that allows the receiver to ensure it was received correctly, perhaps a
515:: The span of fully received packets cannot extend beyond the end of the partially received packets.
1470:
811:
309:
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139:
48:
224:
1548:
1460:
161:
65:
757:. (The acknowledgment may also include information about additional packets received between
112:
1434:
One common simplification of selective-repeat is so called SREJ-REJ ARQ. This operates with
228:
220:
492:: The highest acknowledgement received by the transmitter cannot be higher than the highest
1239:
315:
The receiver may also keep track of the highest sequence number yet received; the variable
104:
8:
1428:
208:
531:: The highest packet received cannot be higher than the highest packet yet to be sent.
1513:
1498:
1147:
101:
138:, or "ACK", back to the sender to indicate it can send the next packet. In a simple
18:
Type of error-detection protocol at the data link layer, and transport layer for TCP
1348:
Now suppose that the receiver sees the following series of packets (all modulo 8):
119:(TCP). They are also used to improve efficiency when the channel may include high
1397:
1196:. For this reason, it is inefficient on links that suffer frequent packet loss.
1193:
251:
227:
of the communications link. If it is not, the protocol will limit the effective
108:
690:.) If it falls within the window, the receiver accepts it. If it is numbered
1542:
1475:
1175:
381:. The transmitter keeps track of the highest acknowledgment it has received
244:
120:
26:
21:"Sliding window" redirects here. For use in natural language processing, see
872:
For example, the transmitter will only receive acknowledgments in the range
1465:
708:, the packet is stored until all preceding packets have been received. If
651:
is received, the receiver checks to see if it falls in the receive window,
126:
Packet-based systems are based on the idea of sending a batch of data, the
1204:
Suppose that we are using a 3-bit sequence number, such as is typical for
172:
as a way of improving efficiency compared to non-windowed protocols like
1384: ≥1), and thus the following packet numbered 0 must be packet 8.
866:
216:
146:
1146:
Although commonly distinguished from the sliding-window protocol, the
165:
1267:
lost packets that can be tolerated. Thus, small values are popular;
1006:
cannot be higher than the highest acknowledgment ever sent, which is
1495:
1154:= 2 possible sequence numbers (conveniently represented by a single
388:. The transmitter knows that all packets up to, but not including
131:
1532:
816:
563:
Whenever the transmitter has data to send, it may transmit up to
263:
The transmitter and receiver each have a current sequence number
177:
676:. (The simplest receivers only have to keep track of one value
173:
169:
1141:
983:
The lowest sequence number we will ever receive in future is
1205:
1013:. So the highest sequence number we could possibly see is
395:
have been received, but is uncertain about packets between
1155:
1238:
The most general case of the sliding window protocol is
308:
is the first packet not yet received. Both numbers are
1392:
There are many ways that the protocol can be extended:
1324: =2, but an unmodified transmitter is used with
774:
Note that there is no point having the receive window
1405:
must be expanded by the maximum misordering distance.
157:
along the stream of packets making up the transfer.
277:, respectively. They each also have a window size
1072:) or that are too high (greater than or equal to
805:
343:> 1. Note the distinction: all packets below
1540:
999:The receiver also knows that the transmitter's
433:The sequence numbers always obey the rule that
207:The sliding window method ensures that traffic
1529:RFC 1323 - TCP Extensions for High Performance
1370:The solution is for the transmitter to limit
849:in messages, it is possible to transmit only
1249:and store them until the gap is filled in.
577:. That is, it may transmit packet number
570:packets ahead of the latest acknowledgment
23:Sliding window based part-of-speech tagging
1494:Peterson, Larry L. & Davie, Bruce S. "
1142:The simplest sliding window: stop-and-wait
84:Learn how and when to remove this message
815:
558:
47:This article includes a list of general
1263:need only be larger than the number of
1062:. However, the actual limit is lower.
886:, inclusive. Since it guarantees that
239:In any communication protocol based on
1541:
1300: ≤ 8 must be maintained; if
1277:
1199:
25:. For Sliding Window Compression, see
1533:TCP window scaling and broken routers
642:
350:have been received, no packets above
258:
1178:is the sliding window protocol with
1161:
312:with time; they only ever increase.
33:
1233:
980:will never again be retransmitted.
771:, but that only helps efficiency.)
371:, some packets have been received.
13:
53:it lacks sufficient corresponding
14:
1565:
1523:
820:Sequence numbers modulo 4, with
781:larger than the transmit window
357:have been received, and between
183:
38:
1122:below), this can permit larger
1488:
1119:
806:Sequence number range required
1:
1481:
1387:
647:Every time a packet numbered
499:acknowledged by the receiver.
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117:Transmission Control Protocol
100:is a feature of packet-based
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7:
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1104:. As it is common to have
719:, the latter is updated to
10:
1570:
1497:", Morgan Kaufmann, 2000.
969:. Thus, packets numbered
933:, the receiver knows that
809:
329:= 1), this is the same as
294:As typically implemented,
20:
215:For the highest possible
1471:Serial number arithmetic
1314:must be decreased to 6.
812:serial number arithmetic
336:, but can be greater if
310:monotonically increasing
241:automatic repeat request
140:automatic repeat request
225:bandwidth-delay product
162:file transfer protocols
98:sliding window protocol
68:more precise citations.
1461:Federal Standard 1037C
1282:The extremely popular
842:
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559:Transmitter operation
221:round-trip delay time
136:acknowledgment signal
1240:Selective Repeat ARQ
907:, there are at most
115:) as well as in the
1307:is increased to 2,
1185:>1, but a fixed
1219:=1, we must limit
857:, for some finite
843:
643:Receiver operation
259:Protocol operation
1554:Data transmission
1278:Ambiguity example
1200:Ambiguity example
1162:Ambiguity example
1148:stop-and-wait ARQ
1044:Thus, there are 2
212:network traffic.
102:data transmission
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1256:The window size
1234:Selective repeat
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1476:TCP Fast Open
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1176:Go-Back-N ARQ
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113:OSI layer 2
74:August 2010
66:introducing
1543:Categories
1482:References
1429:congestion
1425:saturation
1388:Extensions
1118:(e.g. see
867:power of 2
235:Motivation
217:throughput
209:congestion
198:ack packet
147:throughput
49:references
1171:Go-Back-N
1120:Go-Back-N
615:equal to
229:bandwidth
105:protocols
1455:See also
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132:checksum
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178:SEAlink
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