Data Link Layer Networks Data Link Layer 1

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Data Link Layer Networks: Data Link Layer 1

Data Link Layer Networks: Data Link Layer 1

Data Link Layer • Provides a well-defined service interface to the network layer. •

Data Link Layer • Provides a well-defined service interface to the network layer. • Determines how the bits of the physical layer are grouped into frames (framing). • Deals with transmission errors (CRC and ARQ). • Regulates the flow of frames. • Performs general link layer management. Networks: Data Link Layer 2

A Packets Data link Layer Frames 2 2 1 (b) 1 2 3 2

A Packets Data link Layer Frames 2 2 1 (b) 1 2 3 2 1 Medium A 1 2 Copyright © 2000 The Mc. Graw Hill Companies B Physical Layer 1 (a) Packets Physical layer entity Data link layer entity 3 1 2 3 B 2 1 Network layer entity Figure 5. 2 Leon-Garcia & Widjaja: Communication Networks: Data Link Layer 3

End to End ACK/NAK 1 2 Data 3 Data 4 Data 5 Data Hop

End to End ACK/NAK 1 2 Data 3 Data 4 Data 5 Data Hop by Hop Data 1 Data 2 ACK/ NAK 3 ACK/ NAK Data 4 ACK/ NAK 5 ACK/ NAK Figure 5. 7 Copyright © 2000 The Mc. Graw Hill Companies Networks: Data Link Layer Leon-Garcia & Widjaja: Communication Networks 4

Tanenbaum’s Data Link Treatment • Concerned with communication between two adjacent nodes in the

Tanenbaum’s Data Link Treatment • Concerned with communication between two adjacent nodes in the subnet (IMP to IMP). • Assumptions: – Bits delivered in the order sent – Rigid interface between the HOST and the node the communications policy and the Host protocol (with OS effects) can evolve separately. – uses a simplified model Networks: Data Link Layer 5

Host A Node 1 Layer 4 4 Layer 2 frame Host B Node 2

Host A Node 1 Layer 4 4 Layer 2 frame Host B Node 2 Data Link Layer Model Assume Host has infinite supply of messages. Node constructs frame from a single packet message. Checksum is automatically appended in the hardware. Protocols are developed in increasing complexity to help understand the data link layer issues. Networks: Data Link Layer 6

Basic Elements of ARQ Error-free packet sequence Information frames Packet sequence Transmitter Receiver Station

Basic Elements of ARQ Error-free packet sequence Information frames Packet sequence Transmitter Receiver Station A Control frames CRC Station B CRC Information packet Header Control frame Information Frame Copyright © 2000 The Mc. Graw Hill Companies Leon-Garcia & Widjaja: Communication Networks: Data Link Layer Figure 5. 8 7

packet network layer buffer frame data link layer info ack seq kind physical layer

packet network layer buffer frame data link layer info ack seq kind physical layer Networks: Data Link Layer 8

Networks: Data Link Layer 9

Networks: Data Link Layer 9

Protocol 3 (PAR) Positive ACK with Retransmission [Old Tanenbaum Version] #define MAX_SEQ 1 typedef

Protocol 3 (PAR) Positive ACK with Retransmission [Old Tanenbaum Version] #define MAX_SEQ 1 typedef enum {frame_arrival, cksum_err, timeout} event_type; include “protocol. h” void sender_par (void) { seq_nr next_frame_to_send; frame s; packet buffer; event_type event; next_frame_to_send = 0; from_network_layer (&buffer); while (true) { s. info = buffer; s. seq = next_frame_to_send; to_physical_layer (&s); start_timer (s. seq); wait_for_event(&event); if (event == frame_arrival) { from_network_layer (&buffer); inc (next_frame_to_send); } } } Networks: Data Link Layer 10

Protocol 3 (PAR) Positive ACK with Retransmission [Old Tanenbaum Version] void receiver_par (void) {

Protocol 3 (PAR) Positive ACK with Retransmission [Old Tanenbaum Version] void receiver_par (void) { seq_nr next_frame_to_send; frame r, s; event_type event; frame_expected = 0; while (true) { wait_for_event (&event); if (event == frame_arrival) { from_physical_layer (&r); if (r. seq == frame_expected) { to_network_layer(&r. info); inc (frame_expected); } to_physical_layer (&s); /* Note – no sequence number on ACK */ } } } Networks: Data Link Layer 11

Ambiguities with Stop-and-Wait [unnumbered frames] (a) Frame 1 lost A Time-out time frame 0

Ambiguities with Stop-and-Wait [unnumbered frames] (a) Frame 1 lost A Time-out time frame 0 frame 1 ACK B (b) ACK lost A frame 1 frame 2 ACK Time-out time frame 0 B frame 1 ACK frame 2 In parts (a) and (b) transmitting station A acts the same way, but part (b) receiving station B accepts frame 1 twice. Copyright © 2000 The Mc. Graw Hill Companies Leon-Garcia & Widjaja: Communication Networks: Data Link Layer Figure 5. 9 12

PAR [OLD] problem Ambiguities when ACKs are not numbered time-out A frame 0 ACK

PAR [OLD] problem Ambiguities when ACKs are not numbered time-out A frame 0 ACK B time frame 0 ACK frame 1 frame 2 Transmitting station A misinterprets duplicate ACKs Copyright © 2000 The Mc. Graw Hill Companies Leon-Garcia & Widjaja: Communication Networks: Data Link Layer Figure 5. 10 13

State Machine for Stop-and-Wait 0 1 0 1 Rnext Slast Timer Slast Transmitter Rnext

State Machine for Stop-and-Wait 0 1 0 1 Rnext Slast Timer Slast Transmitter Rnext Station A (0, 0) Global State: (Slast, Rnext) Copyright © 2000 The Mc. Graw Hill Companies Station B Error-free frame 0 arrives at receiver ACK for frame 1 arrives at transmitter (1, 0) Receiver Error-free frame 1 arrives at receiver Leon-Garcia & Widjaja: Communication Networks: Data Link Layer (0, 1) ACK for frame 0 arrives at transmitter (1, 1) Figure 5. 11 14

Networks: Data Link Layer 15

Networks: Data Link Layer 15

Sliding Window Protocols [Tanenbaum] • Must be able to transmit data in both directions.

Sliding Window Protocols [Tanenbaum] • Must be able to transmit data in both directions. • Choices for utilization of the reverse channel: – mix DATA frames with ACK frames. – Piggyback the ACK • Receiver waits for DATA traffic in the opposite direction. • Use the ACK field in the frame header to send sequence number of frame being ACKed. – better use of the channel capacity. Networks: Data Link Layer 16

Sliding Window Protocols • ACKs introduce a new issue – how long does receiver

Sliding Window Protocols • ACKs introduce a new issue – how long does receiver wait before sending ONLY an ACK frame. We need an ACKTimer!! sender timeout period needs to set longer. • The protocol must deal with the premature timeout problem and be “robust” under pathological conditions. Networks: Data Link Layer 17

Sliding Window Protocols Each outbound frame must contain a sequence number. With n bits

Sliding Window Protocols Each outbound frame must contain a sequence number. With n bits for the sequence number field, maxseq = 2**n - 1 and the numbers range from 0 to maxseq. Sliding window : : sender has a window of frames and maintains a list of consecutive sequence numbers for frames that it is permitted to send without waiting for ACKs. receiver has a window that is a list of frame sequence numbers it is permitted to accept. Note – sending and receiving windows do NOT have to be the same size. Windows can be fixed size or dynamically growing and shrinking. Networks: Data Link Layer 18

Sliding Window Protocols • Host is oblivious, message order at transport level is maintained.

Sliding Window Protocols • Host is oblivious, message order at transport level is maintained. sender’s window : : frames sent but not yet ACKed. – new packets from the Host cause the upper edge inside sender window to be incremented. – ACKed frames from the receiver cause the lower edge inside window to be incremented. Networks: Data Link Layer 19

Sliding Window Protocols • All frames in the sender’s window must be saved for

Sliding Window Protocols • All frames in the sender’s window must be saved for possible retransmission and we need one timer per frame in the window. • If the maximum sender window size is B, the sender needs B buffers. • If the sender window gets full (i. e. , reaches its maximum window size, the protocol must shut off the Host (the network layer) until buffers become available. Networks: Data Link Layer 20

Sliding Window Protocols receiver window – Frames received with sequence numbers outside the receiver

Sliding Window Protocols receiver window – Frames received with sequence numbers outside the receiver window are not accepted. – The receiver window size is normally static. The set of acceptable sequence numbers is rotated as “acceptable” frames arrive. a receiver window size = 1 the protocol only accepts frames in order. There is referred to as Go Back N. Networks: Data Link Layer 21

Standard Ways to ACK 1. ACK sequence number indicates the last frame successfully received.

Standard Ways to ACK 1. ACK sequence number indicates the last frame successfully received. 2. ACK sequence number indicates the next frame the receiver expects to receive. 3. Both of these can be strictly individual ACKs or represent cumulative ACKing. Cumulative ACKing is the most common technique. Networks: Data Link Layer 22

Go Back N 4 frames are outstanding; so go back 4 Go-Back-4: fr 0

Go Back N 4 frames are outstanding; so go back 4 Go-Back-4: fr 0 A B fr 1 fr 2 A C K 1 fr 3 A C K 2 fr 4 A C K 3 fr 5 fr 6 fr 3 fr 4 fr 5 fr 6 Out-of-sequence frames A C K 4 error fr 7 A C K 5 fr 8 A C K 6 time fr 9 A C K 7 A C K 8 A C K 9 ACKing next frame expected Copyright © 2000 The Mc. Graw Hill Companies Leon-Garcia & Widjaja: Communication Networks: Data Link Layer Figure 5. 13 23

Go Back N with NAK error recovery Transmitter goes back to frame 1 Go-Back-7:

Go Back N with NAK error recovery Transmitter goes back to frame 1 Go-Back-7: A fr 0 fr 1 fr 2 fr 3 fr 4 fr 5 fr 6 fr 7 time fr 0 B A C K 1 N A K 1 Out-of-sequence frames A C K 2 A C K 3 A C K 4 A C K 5 A C K 6 A C K 7 error Copyright © 2000 The Mc. Graw Hill Companies Leon-Garcia & Widjaja: Communication Networks: Data Link Layer Figure 5. 17 24

Selective Repeat with NAK error recovery A B fr 0 fr 1 fr 2

Selective Repeat with NAK error recovery A B fr 0 fr 1 fr 2 A C K 1 fr 3 fr 4 A C K 2 Copyright © 2000 The Mc. Graw Hill Companies fr 5 error fr 6 N A K 2 A C K 2 fr 7 A C K 2 fr 8 fr 9 A C K 7 fr 10 A C K 8 fr 11 A C K 9 Leon-Garcia & Widjaja: Communication Networks: Data Link Layer time fr 12 A C K 1 0 A C K 1 1 A C K 1 2 Figure 5. 21 25