Xarxes de Computadors – Computer Networks Computer Networks - Xarxes de Computadors Outline Course Syllabus Unit 1: Introduction Unit 2. IP Networks Unit 3. Point to Point Protocols -TCP Unit 4. LANs Unit 5. Data Transmission 1 Llorenç Cerdà-Alabern
Xarxes de Computadors – Computer Networks Unit 3. Point to Point Protocols -TCP Outline Introduction Basic ARQ Protocols UDP Protocol TCP Protocol 2 Llorenç Cerdà-Alabern
Xarxes de Computadors – Computer Networks Unit 3. Point to Point Protocols -TCP Introduction A Point to Point Protocol (PPP) takes place between exactly two endpoints. PPP is usually used to identify protocols that builds up a communication channel between endpoints, adding features of the type: Error detection Error recovery Flow control These are typical data-link layer features, although protocols from other layers can be also regarded as PPPs: Physical: RS-232 Data-link: The PPP protocol used in TCP/IP Network: X.25 Transport: TCP Automatic Repeat reQuest (ARQ) protocols are typically used for PPP. 3 Llorenç Cerdà-Alabern
Xarxes de Computadors – Computer Networks Unit 3. Point to Point Protocols -TCP Introduction - Automatic Repeat reQuest, ARQ ARQ Ingredients Connection oriented Tx/Rx buffers Acknowledgments (ack) Acks can be piggybacked in information PDUs sent in the opposite direction. Retransmission Timeout. Sequence Numbers Layer i+1 Layer i+1 Layer i+1 Layer i+1 Write Read Write Read Write Read Layer i Layer i ARQ Layer i Layer i protocol Tx Rx Sender Receiver Sender Receiver Sender Receiver buffer buffer ack PDUs information PDUs ARQ Protocol Architecture ARQ Protocol Implementation (one way) 4 Llorenç Cerdà-Alabern
Xarxes de Computadors – Computer Networks Unit 3. Point to Point Protocols -TCP Introduction - Automatic Repeat reQuest, ARQ Basic ARQ Protocols: Stop & Wait Go Back N Selective Retransmission 5 Llorenç Cerdà-Alabern
Xarxes de Computadors – Computer Networks Unit 3. Point to Point Protocols -TCP Outline Introduction Basic ARQ Protocols UDP Protocol TCP Protocol 6 Llorenç Cerdà-Alabern
Xarxes de Computadors – Computer Networks Unit 3. Point to Point Protocols -TCP Basic ARQ Protocols - Assumptions We shall focus on the the transmission in one direction. We shall assume a saturated source: There is always information ready to send. We shall assume full duplex links. ppp protocol over a line of D m distance and v t bps bitrate. Propagation speed of v p m/s, thus, propagation delay of D / v p s. We shall refer to a generic layer, where the sender sends Information PDUs ( I k ) and the receiver sends ack PDUs ( A k ). Frames carrying I k respectively A k , are Tx using L I and L A bits, thus the Tx times are respectively: t t = L I / v t and t a = L A / v t s. v t bps bitrate line sender receiver D km distance 7 Llorenç Cerdà-Alabern
Xarxes de Computadors – Computer Networks Unit 3. Point to Point Protocols -TCP Basic ARQ Protocols - Stop & Wait 1.When the sender is ready: (i) allows writing from upper layer, (ii) builds I k , (iii) I k goes down to data-link layer and Tx starts. 2.When I k completely arrives to the receiver: (i) it is read by the upper layer, (ii) A k is generated, A k goes down to data-link layer and Tx starts. 3.When A k completely arrives to the sender, goto 1. I k Transmission (Tx) write write I k I k+ 1 Tx buffer I k process time t t sender sender t t t t t t ... ... ... ... t receiver receiver t t p t a t p t p t a A k A k+ 1 A k T c process time Rx buffer Simplified time diagram I k I k+ 1 read read T c Time diagram 8 Llorenç Cerdà-Alabern
Xarxes de Computadors – Computer Networks Unit 3. Point to Point Protocols -TCP Basic ARQ Protocols - Stop & Wait Retransmission Each time the sender Tx a PDU, a retransmission timeout (TO) is started. If the information PDU do not arrives, or arrives with errors, no ack is sent. When TO expires, the sender ReTx the PDU. TO I k I k I k+ 1 t sender t t ... ... receiver t t a A k 9 Llorenç Cerdà-Alabern
Xarxes de Computadors – Computer Networks Unit 3. Point to Point Protocols -TCP Basic ARQ Protocols – Why sequence numbers are needed? TO I k I k t sender ... receiver t A k I k or I k+ 1 ? Need to number information PDUs TO I k I k I k+ 1 I k+ 2 !! t sender ... receiver t abnormally large A k A k process time Need to number ack PDUs 10 Llorenç Cerdà-Alabern
Xarxes de Computadors – Computer Networks Unit 3. Point to Point Protocols -TCP Basic ARQ Protocols – Notes on computing the efficiency (channel utilization) Line bitrate: v t = 1/ t b , bps Throughput ( velocidad efectiva ) v ef = number of inf. bits / obs. time, bps Efficiency or channel utilization E = v ef / v t (times 100, in percentage) information bits line idle ... ... t t b headers observation time, T #info bits × t b = time Tx information T T E = v ef = #info bits / T = v t 1 / t b #info bits = #info bits T / t b #bits at line bitrate 11 Llorenç Cerdà-Alabern
Xarxes de Computadors – Computer Networks Unit 3. Point to Point Protocols -TCP Basic ARQ Protocols – Stop & Wait efficiency Assuming no errors (maximum efficiency), the Tx is periodic, with period T c . E protocol : We do not take into account headers. I k t sender t t ... ... receiver t t p t p t a A k T c 100 E protocol = t t t t 1 90 = = 1 2 a 80 Efficiency, E (%) t t t a 2 t p T C 70 60 50 t t 1 2 a , where a = t p 1 40 ≃ 30 t t 2 t p t t 20 10 0 0.5 1 1.5 2 2.5 3 a = t p / t t 12 Llorenç Cerdà-Alabern
Xarxes de Computadors – Computer Networks Unit 3. Point to Point Protocols -TCP Basic ARQ Protocols – Continuous Tx Protocols Goal: Allow high efficiency independently of propagation delay. Without errors: E = 100 % I k +2 I k +3 I k +4 I k +5 I k +1 I k +2 I k +3 I k +4 I k +5 I k Tx buffer I k +1 I k +2 I k +3 I k +4 I k I k +1 I k +2 I k +3 I k +4 I k +5 t sender ... ... receiver t A k A k +1 A k +2 A k +3 A k +4 A k +5 13 Llorenç Cerdà-Alabern
Xarxes de Computadors – Computer Networks Unit 3. Point to Point Protocols -TCP Basic ARQ Protocols – Go Back N Cumulative acks: A k confirm I i , i ≤ k If the sender receives an error of out of order PDU: Do not send acks, discards all PDU until the expected PDU arrives. Thus, the receiver does not store out of order PDUs. When a TO occurs, the sender goes back and starts Tx from that PDU. TO I k I k +1 I k +2 I k +3 I k +1 I k +2 t sender ... ... receiver t A k A k +1 A k +2 14 Llorenç Cerdà-Alabern
Xarxes de Computadors – Computer Networks Unit 3. Point to Point Protocols -TCP Basic ARQ Protocols – Selective ReTx. The same as Go Back N, but: The sender only ReTx a PDU when a TO occurs. The receiver stores out of order PDUs, and ack all stored PDUs when missing PDUs arrive. TO I k I k +1 I k +2 I k +3 I k +1 I k +4 t sender ... ... receiver t A k A k +3 A k +4 The receiver has to store and order PDUs 15 Llorenç Cerdà-Alabern
Xarxes de Computadors – Computer Networks Unit 3. Point to Point Protocols -TCP Basic ARQ Protocols – Efficiency with Tx errors: Stop & Wait Assumptions: On average, N t Tx are needed to successfully send a PDU: N t – 1 with Tx errors, and 1 correct. T O T C I k I k I k I k +1 N t 1 2 1 t sender t t ... ... ... t receiver A k T T E protocol = t t t t = T T N t − 1 T O T C To avoid unnecessary ReTx T O > T C . The maximum throughput is when T O ≈ T C : t t 1 E protocol ≃ = N t 1 2 a N t T C 16 Llorenç Cerdà-Alabern
Xarxes de Computadors – Computer Networks Unit 3. Point to Point Protocols -TCP Basic ARQ Protocols – Efficiency with Tx errors: Go Back N Assumptions: On average, N t Tx are needed to successfully send a PDU: N t – 1 with Tx errors, and 1 correct. T O I k I k +2 I k I k +1 I k I k +1 I k +2 I k +1 N t 1 2 1 t sender t t ... ... ... receiver t A k T T E protocol = t t t t = N t − 1 T O t t T T The maximum throughput is when T O ≈ T C : t t 1 1 E protocol ≃ = N t − 1 1 2 a 1 = N t − 1 T C t t N t 1 2 a − 2 a 17 Llorenç Cerdà-Alabern
Xarxes de Computadors – Computer Networks Unit 3. Point to Point Protocols -TCP Basic ARQ Protocols – Efficiency with Tx errors: Select. ReTx Assumptions: On average, N t Tx are needed to successfully send a PDU: N t – 1 with Tx errors, and 1 correct. T O I k I k +2 I k I k +3 I k +i I k I i +1 I i +2 I k +1 N t 1 2 t sender t t ... ... ... receiver t A k E protocol = t t = 1 N t t t N t 18 Llorenç Cerdà-Alabern
Xarxes de Computadors – Computer Networks Unit 3. Point to Point Protocols -TCP Basic ARQ Protocols – Efficiency with Tx errors: Comparison For a << 1, all protocols are similar ( a = t p /t t ) For N t >> 1 all protocols have low E . E in selective ReTx does not depend on a . If a << 1 does not hold: E of Stop & Wait is low. For moderate N t , Sel-ReTx outperforms GoBackN. 100 RSel 90 GBN a = 0.1 80 S&W Efficiency, E (%) 70 a = 1 60 50 40 30 20 10 0 1 1.5 2 2.5 3 3.5 4 4.5 5 N t 19 Llorenç Cerdà-Alabern
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