Link Layer & Network Layer

Transcription

1 Chapter 7.B and 7.C Link Layer & Network Layer Prof. Dina Kat abi Some slides are from lectures by Nick Mckeown, Ion Stoica, Frans Kaashoek, Hari Balakrishnan, and Sam Madden 1

2 Previous Lecture We learned how to share the network infrastructure between m any connect ions/flows We also learned about the im plications of the sharing schem e (circuit or packet switching) on the service that the traffic receives 2

3 This Lecture An Exam ple: HTTP Layering Link Layer 3

4 Exam ple: HTTP over t he Int ernet Using TCP/ and Et hernet 1 App A MIT B (CNN.Com) 2 0 App OS OS Et hernet R R2 R4 R R5 Et hernet 4

5 In the sending host 1. Applicat ion-program m ing Int erface ( API) Application requests TCP connection with B 2. Transm ission Cont rol Prot ocol (TCP) Creates TCP Connection setup packet TCP Packet TCP requests packet to be sent to B TCP TCP Type = Connection Set up Empty in Connection Setup Packet 5

6 In the sending host (2) 3. Int ernet Prot ocol () Creat es packet wit h correct addresses. requests packet to be sent to router. TCP Packet TCP Encapsulat ion TCP Packet Destination Address: B Source Address: A Prot ocol = TCP 6

7 In the sending host (3) 4. Link ( M AC or Et hernet ) Prot ocol Creates MAC fram e. Wait for access to the line. Send each bit of the fram e. Packet Encapsulat ion Et hernet FCS Ethernet Et hernet Destination Address: MAC R1 Source Address: MAC A Prot ocol = Et hernet Packet 7

8 In Router R1 5. Link ( M AC or Et hernet ) Prot ocol Accept MAC fram e, check address and Fram e Check Sequence (FCS) to ensure no bit errors. Pass data to Protocol. Packet Decapsulat ion Et hernet FCS Ethernet Et hernet Destination Address: MAC R1 Source Address: MAC A Prot ocol = Et hernet Packet 8

9 In Router R1 6. Int ernet Prot ocol () Use destination address to decide where to send packet next ( next-hop routing ). Request Link Protocol to transm it packet. Packet Dest inat ion Address: B Source Address: A Prot ocol = TCP 9

10 In Router R1 7. Link ( M AC or Et hernet ) Prot ocol Creates MAC fram e. Wait for access to the line. Send each bit of the fram e. Packet Encapsulat ion Et hernet FCS Ethernet Et hernet Destination Address: M AC R2 Source Address: M AC R1 Prot ocol = Et hernet Packet 10

11 Steps 8-15 are the same as before In Router R5 16. Link ( M AC or Et hernet ) Prot ocol Creates MAC frame. Wait for access to the line. Send each bit of the frame. Packet Encapsulat ion Et hernet FCS Ethernet Et hernet Destination Address: M AC B Source Address: M AC R5 Prot ocol = Et hernet Packet 11

12 In the receiving host 1 7. Link ( M AC or Et hernet ) Prot ocol Accept MAC fram e, check address and Fram e Check Sequence (FCS) for bit errors. Pass data to Protocol. Packet Decapsulat ion Et hernet FCS Ethernet Et hernet Destination Address: M AC B Source Address: M AC R5 Prot ocol = Et hernet Packet 12

13 In the receiving host (2) 1 8. Int ernet Prot ocol () Verify address. Ext ract/decapsulat e TCP packet from packet. Pass TCP packet t o TCP Prot ocol. TCP Packet TCP Decapsulation TCP Packet Destination Address: B Source Address: A Prot ocol = TCP 13

14 In the receiving host (3) 1 9. Transm ission Cont rol Prot ocol (TCP) Accepts TCP Connection setup packet Establishes connection by sending Ack. 20. Applicat ion-program ming Int erface (API) Application receives request for TCP connection with A. TCP Packet TCP TCP Type = Connection Set up Empty 14

15 This Lecture An Exam ple: HTTP Layering Link Layer 15

16 Network Architecture Problem Networks are com plex (heterogeneity, dist ribut ed, delay, losses, reordering, ) How do we organize a network im plem entation? Solut ion To deal with com plexity use layering 16

17 Layering Layering is a particular form of abst ract ion The system is broken into a vertical hierarchy of logically distinct entities (layers) The service provided by one layer is based solely on the service provided by layer below 17

18 Layering: Our HTTP Exam ple HTTP TCP Applicat ion Transport End-to-End Layer Net work Ethernet Link The 4-layer Internet model 18

19 Link Layer: Who Does What? Delivers data from one end of a link to the other Net work Layer Routes packets and delivers them to their destination Transport Layer Provides useful abstractions: stream, m essage Can provide reliabilit y Can provide congest ion cont rol Applicat ion Layer Your application: HTTP, FTP, etc. 19

20 Where are these layers? Link and network layers are implemented everywhere The end-to-end layer (i.e., transport and application) is im plemented only at hosts Host Application Transport Network link Peer-layer com m unicat ion Rout er Network link Physical medium Host Application Transport Network link 20

21 Encapsulation A layer can use only the service provided by the layer immediate below it Each layer may change and add a header to data packet data data data data data data data data 21

22 Interface Higher layer calls lower layer e.g., Link_Send(this_d ata, this_link) Lower layer uses an up-call function to inform the higher layer of data arrival e.g., Network_Hand le() 22

23 Multiplexing in the Internet Many possible applications, transports, and link layers But they all use at the network layer Application Transport Internet Telnet FTP HTTP TCP UDP Link Ethernet Packet radio 23

24 This Lecture An Exam ple: HTTP Layering Link Layer 24

25 Link Layer Problem: Deliver data from one end of the link to the other Need to address: Bit s Analog Bit s Fram ing Errors Medium Access Cont rol (The Et hernet Paper) 25

26 Sending bits Bit s Analog Bit s Receiver needs to detect the value of the bits Manchester Encoding: each bit is a transition Having a transition in each bit allows the receiver to synchronize to t he sender s clock Tim e 26

27 Fram ing Receiver needs to detect the beginning and the end of a fram e Use special bit-pattern to separate fram es E.g., pattern could be (7 ones) Bit stuffing is used to ensure that a special pattern does not occur in the data If pattern is Whenever the sender sees a sequence of 6 ones in the data, it inserts a zero (reverse t his operat ion at receiver) 27

28 Detection: Error Handling Use error detection codes, which add som e redundancy to allow detecting errors When errors are detected Correction: Som e codes allow for correction Retransm ition: Can have the link layer retransmit the frame (rare) Discard: Most link layers just discard the frame and rely on higher layers to retransmit 28

29 This Lecture To cope with the com plexity, the network architecture is organized into layers The link layer delivers data between two m achines that are directly connected using a link 29