1 Transport Layer Lecture 10 Imran Ahmed University of Management & Technology

2 Agenda Overview of transport layer Connection-less services – UDP Flow control mechanisms Connection-oriented services – TCP TCP congestion control

3 Flow control A technique for assuring that a transmitting entity does not over-whelm a receiving entity with data. –Stop-and-wait flow control –Sliding window flow control Flow Control Stop-and- Wait Sliding Window

4 Model of frame transmission

5 Stop-and-wait operation Sender transmits one packet and waits for receiver’s response. Receiver receives frame and replies with acknowledgement. Source waits for ACK before sending next frame. Receiver can stop flow by not send ACK.

6 Stop-and-wait operation first packet bit transmitted, t = 0 senderreceiver RTT last packet bit transmitted, t = L / R first packet bit arrives last packet bit arrives, send ACK ACK arrives, send next packet, t = RTT + L / R

7 Sliding Window operation Allow multiple frames to be in transit. Receiver has buffer W long. Sender can send up to W frames without ACK. Each frame is numbered. ACK includes number of next frame expected. Sequence number bounded by size of field (k). –Frames are numbered modulo 2 k

8 Sliding Window operation

9 Sliding window example Sender Receiver A A

10 Error control It refers to detect and correct errors that occur in the transmission of frames. Errors can be type of; lost frames or damaged frames. Automatic Repeat request (the effect of ARQ is to turn an un-reliable transmission into a reliable one): –Error detection –+ve ACK –Retransmission after timeout –-ve ACK & retransmission

11 ARQ Stop-and- Wait ARQ Go-Back-n ARQ Selective- Reject ARQ

12 Stop-and-wait ARQ Source transmits single frame & wait for ACK. If received frame damaged, discard it –Transmitter has timeout –If no ACK within timeout, retransmit If ACK damaged, transmitter will not recognize it: –Transmitter will retransmit –Receive gets two copies of frame –Use ACK0 and ACK1

13 Go-back-n ARQ Based on sliding window. If no error, ACK as usual with next frame expected. Use window to control number of outstanding frames. If error, reply with rejection: –Discard that frame and all future frames until error frame received correctly. –Sender must go back and retransmit that frame and all subsequent frames.

14 GBN in action window size = 4

15 Selective Repeat Also called selective retransmission. Only rejected frames are retransmitted. Subsequent frames are accepted by the receiver and buffered. Minimizes retransmission but receiver must maintain large enough buffer. More complex as compared to G-back-n.

16 Selective Repeat: Sender, Receiver Windows w w

17 Selective repeat in action

18 Selective Repeat: Ambiguity Example: seq #’s: 0, 1, 2, 3 window size=3 receiver sees no difference in two scenarios! incorrectly passes duplicate data as new in (a). Q: what relationship between seq # size and window size?