Lecture 2: Layering & End-to-End

Transcription

1 Lecture 2: Layering & End-to-End CSE 222A: Computer Communication Networks Alex C. Snoeren Thanks: Mike Freedman & Amin Vahdat

2 Lecture 2 Overview Layering Application interface Transport services Discussion of End-to-End principle CSE 222A Lecture 2: Layering & End-to-End 2

3 OSI Model Function Example Ultimate data destination Application Application Web browser Format conversion Presentation Presentation ASCII/XDR Interaction across presentation Session Session Restartable file transfer Reliable, ordered delivery Transport Transport TCP Routing/ Internetworking Network Network IP Data framing over links Data link Data link Ethernet, ATM Bits on the wire Physical Physical SONET, 100BT CSE 222A Lecture 2: Layering & End-to-End 3

4 OSI Model Function Ultimate data destination Format conversion Interaction across presentation Reliable, ordered delivery Routing/ Internetworking Data framing over links Bits on the wire Application Presentation Session Transport Network Data link Physical Where does security go? What about reliability? CSE 222A Lecture 2: Layering & End-to-End 4

5 OSI Model Discussion OSI standardized before implemented IETF philosophy: We reject kings, presidents and voting. We believe in rough consensus and working code IETF requires two working/interoperable versions before considering a standard Modular design, but some boundaries are arbitrary Why seven layers? What exactly is the session layer? Much basic network functionality at multiple layers Reliability, flow control, security CSE 222A Lecture 2: Layering & End-to-End 5

6 Internet Architecture IP Hourglass: Telecollaboration NFS HTTP rlogin RSVP UDP RPC IP TCP Ethernet modem ATM PPP packet radio 100BT SONET air n Layering not strict Can define new abstractions on any existing protocol CSE 222A Lecture 2: Layering & End-to-End 6

7 Layering in Applications user app Bottlenecks Boundary crossings kernel copy socket task context Copies Context switches TCP revc TCP sw intr Layering nice way to logically consider protocols copy IP May not lead to fastest implementation Packet arrives ether hw intr But! Processors are getting faster people are getting more expensive CSE 222A Lecture 2: Layering & End-to-End 7

8 Socket Abstraction Best-effort packet delivery is a clumsy abstraction Applications typically want higher-level abstractions Messages, uncorrupted data, reliable in-order delivery User process socket Operating System User process socket Operating System Applications communicate using sockets Stream socket: reliable stream of bytes (like a file) Message socket: unreliable message delivery CSE 222A Lecture 2: Layering & End-to-End 8

9 Two Basic Transport Features Demultiplexing: port numbers Client host Client Service request for :80 (i.e., the Web server) Server host OS Web server (port 80) Echo server (port 7) Error detection: checksums IP payload detect corruption CSE 222A Lecture 2: Layering & End-to-End 9

10 Two Main Transport Layers User Datagram Protocol (UDP) Just provides demultiplexing and error detection Header fields: port numbers, checksum, and length Low overhead, good for query/response and multimedia Transmission Control Protocol (TCP) Adds support for a stream of bytes abstraction Retransmitting lost or corrupted data Putting out-of-order data back in order Preventing overflow of the receiver buffer Adapting the sending rate to alleviate congestion Higher overhead, good for most stateful applications CSE 222A Lecture 2: Layering & End-to-End 10

11 Sharing the Net Best-effort network easily becomes overloaded No mechanism to block excess calls Instead excess packets are simply dropped Examples Shared Ethernet medium: frame collisions Ethernet switches and IP routers: full packet buffers Quickly leads to congestion collapse Goodput congestion collapse Increase in load that results in a decrease in useful work done. Load 11

12 Adjusting to Congestion End hosts adapt their sending rates In response to network conditions Learning that the network is congested Shared Ethernet: carrier sense multiple access» Seeing your own frame collide with others IP network: observing your end-to-end performance» Packet delay or loss over the end-to-end path Adapting to congestion Slowing down the sending rate, for the greater good But, host doesn t know how bad things might be CSE 222A Lecture 2: Layering & End-to-End 12

13 Ethernet Back-off Mechanism Carrier sense: wait for link to be idle If idle, start sending; if not, wait until idle Collision detection: listen while transmitting If collision: abort transmission, and send jam signal Exponential back-off: wait before retransmitting Wait random time, exponentially larger on each retry CSE 222A Lecture 2: Layering & End-to-End 13

14 TCP Congestion Control Additive increase, multiplicative decrease On packet loss, divide congestion window in half On success for last window, increase window linearly Window Loss halved CSE 222A Lecture 2: Layering & End-to-End 14 t

15 End-To-End Argument Application Where to Place Functionality? TCP IP Router Link Layer CSE 222A Lecture 2: Layering & End-to-End 15

16 End-to-End Argument Functionality should be implemented at a lower layer if and only if it can be correctly and completely implemented there Should not be implemented at lower level if redundant with higher level Performance optimizations are not a violation Early example ARPANet provided reliable link transfers between switches Packets could still get corrupted on host-to-switch link, or inside switches Want to know if host acted on the request not whether it received it CSE 222A Lecture 2: Layering & End-to-End 16

17 Example: Reliable File Transfer From disk on file (web) server over network to client Disk can introduce bit errors Host I/O buses can introduce bit errors Packets can get garbled, dropped, misordered at any node Solution: integrity check on file, not per packet or per hop CSE 222A Lecture 2: Layering & End-to-End 17

18 Hop by Hop for Performance Does not violate end to end argument to provide reliability at link layer, even if not required for correct operation For file transfer application, consider varying conditions: Prob(corrupted/lost packet per link) = p Prob(packet lost end to end), avg. 15 hops across Internet p = % => Prob(loss) = % p = 1% => Prob(loss) = 14% Chance of file corruption grows with size of file Potentially retransmit entire file for one lost packet? CSE 222A Lecture 2: Layering & End-to-End 18

19 Application-Specific Semantics Example: move reliability into the network communication protocol (such as TCP) Certain computational and bandwidth overheads to implementing reliable, in-order delivery in the network Not all applications want to pay this overhead Real-time voice/audio Better to drop a packet, rather than hold up later packets On-time delivery more important than reliability Applications should be able to pick and choose the semantics they require from underlying system Active Networks, overlay networks CSE 222A Lecture 2: Layering & End-to-End 19

20 Other Examples Distributed transactions Exactly once vs. at most once vs. at least once Security Security only as strong as weakest assumption/link Suppressing duplicate messages Duplicate effort between network and application layer Mis-ordered messages CSE 222A Lecture 2: Layering & End-to-End 20

21 Discussion When should the network support a function? E.g., link-layer retransmission in wireless networks? Who s interests are served by the e2e argument? How does a network operator influence the network without violating the e2e argument? Does the design of IP and TCP make it hard to violate the e2e argument? E.g., middlebox functionality like NATs, firewalls, proxies Should the e2e argument apply to routing? CSE 222A Lecture 2: Layering & End-to-End 21

22 For Next Class NO CLASS MONDAY Read P&D Chapter 3 Read Cerf & Kahn `74 Keep thinking about term project ideas/groups Suggestions available next week CSE 222A Lecture 2: Layering & End-to-End 22