application layers

Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Network Layer Electrical and/or Optical Signals Application A Application B Data Link Layer Physical Layer Network Layer Data Link Layer Physical Layer Communication Network Figure 2.6 Review of seven layers

Data Link Control Framing Line Discipline/MAC Flow Control Error Control Addressing WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998

Data Link Layer WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998

WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998

Peer-to-peer protocols Two communicating entities are called peer processes. Communication between layer n+1 peers is virtual and is carried by layer n service Two meanings of peer-to-peer: point-to-point (hop-by-hop): end-to-end (user-to-user): Physical link network

1 2 Physical layer entity Data link layer entity 3 Network layer entity Physical Layer Data link Layer Physical Layer Data link Layer A B A B Packets Packets Frames (a) (b) Peer-to-peer protocol across a single hop 1. take packets 2. form frame 3. transfer through Physical layer 5. deliver to network layer Several pairs of data link & physical entities Only one network layer entity, a router may connect several different networks 4. Pass up 3 2 1 1 2 2 1 3 2 1 1 2 2 1 2 1 Medium

Physical Layer Data link Layer Physical Layer Data link Layer End system  Network Layer Network Layer Physical Layer Data link Layer Network Layer Physical Layer Data link Layer Network Layer Transport Layer Transport Layer Messages Messages Segments End system  Network Peer-to-peer protocol operating end-to-end across network

Medium A B 3 C 3 4 3 4 End System  End System  Network 1 2 Physical layer entity Data link layer entity Network layer entity Network layer entity Transport layer entity Figure 5.4 Peer-to-peer protocol operating end-to-end across network 1. Layer 4 not in middle 2.Data go up and down in router 3. Different paths 4. Out of order, delay, duplicate, lost 3 2 1 1 2 2 1 3 2 1 1 2 2 1 2 1 2 1 1 2 2 1 2 1 2 1 2 1 1 2 3 3 4

Service models Connection-oriented and connectionless Confirmed and unconfirmed A service may transfer in constant bit rate ( CBR) or variant bit rate (VBR)

1 2 3 4 5 Data Data Data Data ACK/NAK ACK/NAK ACK/NAK ACK/NAK End-to-end Hop-by-hop Figure 5.7 Adaptation functions may be implemented end-to-end or hop-by-hop Data are ACK or NAK by the other end Data are ACK or NAK by each hop 1 2 3 4 5 Data Data Data ACK/NAK Data

End-to-end versus hop-by-hop (cont.) Hop-by-hop: faster recovery & more reliable but more burden on middle nodes End-to-end: simpler and only at end-system QUESTIONS: could hop-to-hop waivers end-to-end? NO. it is difficult for all elements in the hop-by-hop chain to operate correctly, furthermore the errors may be introduced in middle nodes --Adaptations are implemented at which layer(s)? Hop-by-hop: End-by-End: Data link & network layer Transport & application layer

End-to-end versus hop-by-hop (cont.) In case of error-detection and recovery: If frequent errors, use hop-by-hop , otherwise end-to-end Flow control and congestion control could be exercised on a hop-by-hop or end-to-end basis or both. Security issue: may be hop-by-hop or end-by-end IPSec ( IP security protocol ) in Internet layer, hop-by-hop/end-to-end? SSL ( Secure Socket Layer ) in transport layer, end-to-end SSH ( Secure Shell ) in application layer, end-to-end

ARQ (Automatic Repeat Request) protocols A technique used to ensure accurate delivery of a data stream despite errors during transmission Form a basis for peer-to-peer protocols Assume that There is a connection between peers The channel is error-prone A sequence of information blocks for transfer

Typical ARQ protocols Assume unidirectional transmission, consider bidirectional transmission later Stop-and-wait ARQ Go-back-N ARQ Based on ARQs, Sliding-window flow control Reliable stream service (TCP preview) Data link layer protocols --HDLC ( High-level Data Link Control ) --PPP ( Point-to-Point protocol )

Stop-and-Wait ARQ Transmitter sends one frame and waits for acknowledgment Receiver acknowledges the receiving of the frame After receiving acknowledgment, transmitter sends the next frame In case the transmitted frame or returned acknowledgment was lost, the transmitter’s timer will time out, the transmitter resends the frame

A B One frame ACK Another frame ACK time Another frame Figure 5.9 Stop-and-Wait ARQ Transmitter A sends one frame and waits for acknowledgment Receiver B acknowledges the receiving of the frame After receiving acknowledgment, transmitter A sends the next frame Any Problem with it? Transmitted frame may lost, the acknowledgment may lost. How to solve? Set up timer, when timer times out, resends the frame

(a) Frame 1 lost or badly garbled A B One frame Another frame ACK The frame ACK time Time-out Another frame (b) ACK lost A B One frame Another frame ACK the frame ACK time Time-out Another frame ACK Figure 5.9 Using timer to retransmit the frame when a frame or acknowledgement lost Any problem? Frame was received twice when ACK lost. How to solve it? Introduce sequence number (SN) into frame and discard duplicate frame

Go-back-N ARQ Sends enough frames to keep channel busy and then waits for ACK ACK to one frame validates all frames ahead of this frame (called accumulated ACK) If ACK for a frame is not received before time out, all outstanding frames are retransmitted.

A B fr 0 time fr 1 fr 2 fr 3 fr 4 fr 5 fr 6 fr 3 ACK1 error Out-of-sequence frames Go-Back-4: 4 frames are outstanding; so go back 4 fr 5 fr 6 fr 4 fr 7 fr 8 fr 9 ACK2 ACK3 ACK4 ACK5 ACK6 ACK7 ACK8 ACK9 Figure 5.13 Basic Go-back-N ARQ A sends 0,1,2,3 frames then waits for ACK ACK1 just comes in time and A sends one more frame: 4 ACK2 and 3 come and A sends frame 5 and 6 Frame 3 lost and no ACK for it B discards out-of-sequence frame 4,5,6 A exhausts its window (4 frames) and does not receive ACK, so resends all outstanding frames 3,4,5,6, called Go-back N

Sliding Window Protocols Bidirectional Protocol Each frame contains a sequence number Sliding window refers to a imaginary boxes at the transmitter and receiver. At any instant of time , the sender maintains a set of sequence numbers corresponding to the frames permitted to send. The receiver also maintains a receiver window corresponding to the set of frames permitted to accept

Sliding Window Figure 10-11 WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998

Sliding window The sequence number within a sender’s window represents the no of frames sent but not yet acknowledged. The receiving window corresponds to the frames the receiver may accept.

WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 Sender Sliding Window

Figure 10-13 WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 Receiver Sliding Window

Figure 10-14 WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 Sliding Window Example

Figure 10-14-continued WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 Sender

WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 Receiver

application layers

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