Infrastructure Internet perspective Change in point of view Internet standards do not discuss Data Link + Physical Layers Hardware developers define standards Infrastructure Internet Layer Model Internet application Application Layers Expects Internet services from OS Internet Aware Local + remote ports Transport Service requirements Network End-to-end IP routing + forwarding Data Link Layer — hardware management Not Internet Infrastructure Physical Layer — hardware Aware Computer Networks — Hadassah College — Fall 2015 Infrastructure Layers Dr. Martin Land 1 Computer Networks — Hadassah College — Fall 2015 Infrastructure Layers Dr. Martin Land 2 Infrastructure Infrastructure Engineering perspective Economic perspective Infrastructure layers Enormous investment in existing equipment Bottom-up design physical bits Global network of hardware nodes + transmission lines Physical layer (PHY) Developed to provide many services Internet (IP-based unreliable connectionless) just one service Defines physical transmission of bits Exploits a physical technology Most developed before Internet Telegraph — 1794 Data Link layer (DL) defines management of Physical Layer Telephone — 1876 How to make physical technology do what we want Teletype modem — 1943 Infrastructure management Digital telephone — 1962 Delivering data messages — 10% of effort Internet opened to public — 1992 Making hardware work correctly — 90% of effort Hardware updates OAM = Operations+Administration+Maintenance Replacement of manufactured hardware Application assumes infrastructure "just works" Slower than software updates "Just works" ⇒ More expensive than software updates Reliability, availability, stability, serviceability, growth Computer Networks — Hadassah College — Fall 2015 Infrastructure Layers Dr. Martin Land 3 Computer Networks — Hadassah College — Fall 2015 Infrastructure Layers Dr. Martin Land 4 Data Link Functions Data Link Sublayers Similar to transport layer functions Logical Link Control (LLC) sublayer Transport Reliability Multiplexing of data sources / destinations Packet type identification Error correction LLC Data Link Data Link Flow control Sublayer Data Link Reliability Reliability 2 Framing MAC Layer Assemble network PDUs into hardware packets Medium Access (MAC) sublayer Sublayer Attach header + trailer for Data Link and Physical layers Network topology Physical Layer 1 Medium access + flow control + congestion control Medium access management When / how transmitter sends data onto link Sharing medium among nodes Transmitter avoids overflow of receiver buffer Permission to transmit Transmitters avoid interfering with other transmitters Data frame structure Error control Hardware (MAC) addressing Detect / correct transmission bit errors Error detection Local addressing Convert network addresses to hardware local addresses Computer Networks — Hadassah College — Fall 2015 Infrastructure Layers Dr. Martin Land 5 Computer Networks — Hadassah College — Fall 2015 Infrastructure Layers Dr. Martin Land 6 Interface to Data Link Layer Framing Infrastructure layers Data frame format Typically implemented in hardware Header Data Trailer PHY — physical circuits for transmitter / link / receiver DL — embedded program in firmware (ROM) + controller Data link protocol management Host node Header / Trailer format Network interface Card (NIC) Similar to headers at network and transport layers Connection port to medium (link) Addressing, error control, flow control, … Controller Physical layer hardware management Transceiver Transmission parameters Bit rate, Baud rate, modulation method, … Switching node Transmitter / receiver synchronization Switching fabric Clock training bits Controller + multiple transceivers + connection ports to medium (link) 1010101010 … allows receiver clock to sync Frame marking DL layer interface Start / Stop Fields Interface to OS level hardware driver Start field / byte count Network PDU ↔ OS driver ↔ NIC / switch controller ↔ transceiver Lose sync ⇒ drop frame + wait for new Start Field Computer Networks — Hadassah College — Fall 2015 Infrastructure Layers Dr. Martin Land 7 Computer Networks — Hadassah College — Fall 2015 Infrastructure Layers Dr. Martin Land 8 Frame Marking Methods Frame Marking Methods STX / ETX Flags ASCII control codes Start = Stop = 01111110 = 0x7E STX Start of Text 0x02 ETX End of Text 0x03 01111110 Rest of Header Data Rest of Trailer 01111110 DLE Data Link Escape 0x10 7E in data stream DLE STX Rest of Header Data Rest of Trailer DLE ETX Byte / bit stuffing Byte stuffing DLE in data stream Send 7E as 7E 7E — receiver removes extra 7E Byte stuffing Bit stuffing Send as — receiver removes extra Transmitter sends DLE as DLE DLE 11111 111110 0 Receiver removes extra DLE Computer Networks — Hadassah College — Fall 2015 Infrastructure Layers Dr. Martin Land 9 Computer Networks — Hadassah College — Fall 2015 Infrastructure Layers Dr. Martin Land 10 Error Control Error Control Check sequence Parity (even parity) Parity bit = XOR of data bits Transmitter Data + parity = even number of 1 bits Calculates hash of data Includes sequence in transmitted header / trailer Cyclic Redundancy Code (CRC) D = Data field Receiver G = Generator Calculates hash of data Predetermined pattern of r+1 bits Compares received sequence with calculated sequence R = Remainder of (D × 2r) / G (modulo 2 division) = CRC field Generally (D × 2r + R) / G = 0 D = data R = (D × 2r) / G Computer Networks — Hadassah College — Fall 2015 Infrastructure Layers Dr. Martin Land 11 Computer Networks — Hadassah College — Fall 2015 Infrastructure Layers Dr. Martin Land 12 Modulo 2 Polynomial Arithmetic Modulo 2 Long Division Represent data bits as coefficients of polynomial Daa= ... a 5 nn−−12 0 xx+ Dx()=×+×+× a xnn−−120 a x... a x nn−−12 0 x2 +1 Arithmetic modulo 2 in each order (XOR) 00110+ =+= 01101 +=+ = 3 Polynomial addition = subtraction x + x A xBxa±=×++×±×++× xnn−−10... ax b x 10 ... bx 25 () () nn−−1010() xxx+1 + =⊕×++⊕×abxn−10... abx ()nn−−11 () 00 xx53+ Polynomial multiplication / division 3 Multiply / divide as usual x + x Modulo 2 arithmetic in each order xx3 + Examples 32 533535 ()()xxx+ +=+++=++111 xxxxx() xxxx +=+ 523 0 ()()xx+÷ x +=+1 xx Computer Networks — Hadassah College — Fall 2015 Infrastructure Layers Dr. Martin Land 13 Computer Networks — Hadassah College — Fall 2015 Infrastructure Layers Dr. Martin Land 14 Cyclic Redundancy Code (CRC) CRC Example Why it works Data stream Data D = 1101011011 D = data Generator 1 1 0 0 0 0 1 0 1 0 G = 10011 1 0 0 1 1 1 1 0 1 0 1 1 0 1 1 0 0 0 0 Shift left D r bits ⇒ D → D × 2r 1 0 0 1 1 1 0 0 1 1 1 0 0 1 1 D = data 0 …0 0 1 0 1 1 0 1 0 0 1 1 Divide by G ⇒ D × 2r → D × 2r / G = Q + R / G Remainder 0 0 1 0 1 0 0 1 0 0 1 1 Quotient Q R = 1110 1 1 1 0 Remainder R Transmit T = 1101011011 1110 r Transmit T = D × 2 + R CRC check at receiver 1 1 0 0 0 0 1 0 1 0 1 0 0 1 1 1 1 0 1 0 1 1 0 1 1 1 1 1 0 D = data R 10 0 1 1 1 0 0 1 1 1 0 0 1 1 Receiver calculates T / G 0 1 0 1 1 1 1 0 0 1 1 T / G = D × 2r / G + R / G = (Q + R / G) + R / G = Q + (R + R) / G 0 0 1 0 0 1 1 1 0 0 1 1 = Q + 0 / G = Q Zero remainder ⇒ no error 0 0 Computer Networks — Hadassah College — Fall 2015 Infrastructure Layers Dr. Martin Land 15 Computer Networks — Hadassah College — Fall 2015 Infrastructure Layers Dr. Martin Land 16 CRC Standards Flow Control Go Back N (GBN) CRC‐12 Transmit N frames Gx() =+++++ x12 x 11 x 3 x 2 x 1 1 Wait for ACK Piggybacking — transmit ACK signals in data frame CRC‐16 Sliding Window Gx() =+++ x16 x 15 x 2 1 N-bit SEQ number Window size — number of unACKed frames before stopping Source window CRC‐CCITT SEQ numbers of unACKed frames 16 15 5 Gx() =+++ x x x 1 Frames buffered at transmitter until ACKed Destination window CRC‐32 SEQ numbers of frames to be accepted Gxxx() =32 ++ 26 xxxxxxxxxxxx 23 ++++++++++++ 22 16 12 11 10 8 7 5 4 2 1 Frames passed to network layer in SEQ order Out-of-order frames buffered Frames too far ahead of window rejected Timeout — retransmit if no ACK after fixed time Computer Networks — Hadassah College — Fall 2015 Infrastructure Layers Dr. Martin Land 17 Computer Networks — Hadassah College — Fall 2015 Infrastructure Layers Dr. Martin Land 18 High‐Level Data Link Control (HDLC) High‐Level Data Link Control (HDLC) Family of data link protocols Data link attributes in HDLC Based on IBM SDLC Layer 2 protocol in mainframe SNA Framing 0x7E flag with byte / bit stuffing Originally for communication between CPUs and peripherals Link Access Protocol (LAP) Addressing hardware level addressing possible Versions of HDLC used in public network architectures Error control 16‐bit CRC‐CCITT or 32‐bit CRC‐32 X.25 LAPB Frame Relay LAPF Flow control 3‐bit SEQ number ISDN LAPD Ethernet Logical Link Control (LLC) IEEE 802.2 Frame types Information, Supervisory, Unnumbered Internet point‐to‐point SLIP, PPP Computer Networks — Hadassah College — Fall 2015 Infrastructure Layers Dr. Martin Land 19 Computer Networks — Hadassah College — Fall 2015 Infrastructure Layers Dr. Martin Land 20 HDLC Frame Structure HDLC Control Fields General HDLC frame Flow control SEQ — sequence number of data frame 01111110 Address Control data CRC 01111110 NEXT — next expected SEQ (ACK all previous frames) 8 8 8 ≥ 0 16 / 32 8 Type Address 00 — ACK + Receiver Ready (RR) 8 bit address ⇒ 256 hardware addresses 01 — Reject (REJ): retransmit all frames from N(R) Control field 10 — Receiver Not Ready (RNR): ACK N(R) but stop sending Specifies frame type / control 11 — Selective Reject (SREJ): retransmit N(R) 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 Information (data) 0 SEQ N(S) p/f NEXT N(R) Information (data) 0 SEQ N(S) p/f NEXT N(R) 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 Supervisory (flow control) 1 0 type p/f NEXT N(R) Supervisory (flow control) 1 0 type p/f NEXT N(R) 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 Unnumbered (management / connectionless) 1 1 type p/f subtype Unnumbered (management / connectionless) 1 1 type p/f subtype Computer Networks — Hadassah College — Fall 2015 Infrastructure Layers Dr.