1. CSE4471: Computer Network Review r Network Layers r TCP/UDP r IP r Ethernet

2. 2 Internet Layers r application: supporting network applications m ftp, smtp, http r transport: host-host data transfer m tcp, udp r network: routing of datagrams from source to destination m ip, routing protocols r link: data transfer between neighboring network elements m ppp, ethernet r physical: bits “on the wire” application transport network link physical

3. OSI Network Layers

4. 4 Layering: logical communication application transport network link physical application transport network link physical application transport network link physical application transport network link physical network link physical Each layer: r distributed r “entities” implement layer functions at each node r entities perform actions, exchange messages with peers

5. 5 Layering: physical communication application transport network link physical application transport network link physical application transport network link physical application transport network link physical network link physical data

6. 6 Protocol layering and data Each layer takes data from above r adds header information to create new data unit r passes new data unit to layer below application transport network link physical application transport network link physical source destination M M M M H t H t H n H t H n H l M M M M H t H t H n H t H n H l message segment datagram frame

7. 7 Internet structure: network of networks r roughly hierarchical r national/international backbone providers (NBPs) m e.g. BBN/GTE, Sprint, AT&T, IBM, UUNet m interconnect (peer) with each other privately, or at public Network Access Point (NAPs) r regional ISPs m connect into NBPs r local ISP, company m connect into regional ISPs NBP A NBP B NAP regional ISP local ISP local ISP

8. 8 National Backbone Provider e.g. Sprint US backbone network

9. TCP r Transport Control Protocol r Flow control and Responds to congestion r Reliable In-order delivery r “Nice” Protocol

10. TCP segment structure source port # dest port # 32 bits application data (variable length) sequence number acknowledgement number rcvr window size ptr urgent data checksum F SR PAU head len not used Options (variable length) URG: urgent data (generally not used) ACK: ACK # valid PSH: push data now (generally not used) RST, SYN, FIN: connection estab (setup, teardown commands) # bytes rcvr willing to accept counting by bytes of data (not segments!) Internet checksum (as in UDP)

11. Reliable Delivery r Sender, Receiver keep track of bytes sent and bytes received. r Acks have an indication of next byte expected. r Three duplicate acks considered a packet loss - sender retransmits

12. TCP seq. #’s and ACKs Seq. #’s: m byte stream “number” of first byte in segment’s data ACKs: m seq # of next byte expected from other side m cumulative ACK Q: how receiver handles out-of-order segments m A: TCP spec doesn’t say, - up to implementer Host A Host B Seq=42, ACK=79, data = ‘C’ Seq=79, ACK=43, data = ‘C’ Seq=43, ACK=80 User types ‘C’ host ACKs receipt of echoed ‘C’ host ACKs receipt of ‘C’, echoes back ‘C’ time simple telnet scenario

13. TCP Flow Control r Window based r Sender cannot send more data than a window without acknowledgements. r Window is a minimum of receiver’s buffer and ‘congestion window’. r After a window of data is transmitted, in steady state, acks control sending rate.

14. Flow Control

15. UDP r No reliability, flow control, congestion control. r Sends data in a burst. r Provides multiplexing and demultiplexing of sources. r Most multimedia applications using UDP

16. UDP: User Datagram Protocol [RFC 768] r “no frills,” “bare bones” Internet transport protocol r “best effort” service, UDP segments may be: m lost m delivered out of order to app r connectionless: m no handshaking between UDP sender, receiver m each UDP segment handled independently of others Why is there a UDP? r no connection establishment (which can add delay) r simple: no connection state at sender, receiver r small segment header r no congestion control: UDP can blast away as fast as desired

17. UDP segment structure r often used for streaming multimedia apps m loss tolerant m rate sensitive r other UDP uses (why?): m DNS m SNMP r reliable transfer over UDP: add reliability at application layer m application-specific error recover! source port #dest port # 32 bits Application data (message) UDP segment format length checksum Length, in bytes of UDP segment, including header

18. IP datagram format ver length 32 bits data (variable length, typically a TCP or UDP segment) 16-bit identifier Internet checksum time to live 32 bit source IP address IP protocol version number header length (bytes) max number remaining hops (decremented at each router) for fragmentation/ reassembly total datagram length (bytes) upper layer protocol to deliver payload to head. len type of service “type” of data flgs fragment offset upper layer 32 bit destination IP address Options (if any) E.g. timestamp, record route taken, pecify list of routers to visit.

19. ICMP: Internet Control Message Protocol r used by hosts, routers, gateways to communication network-level information m error reporting: unreachable host, network, port, protocol m echo request/reply (used by ping) r network-layer “above” IP: m ICMP msgs carried in IP datagrams r ICMP message: type, code plus first 8 bytes of IP datagram causing error Type Code description 0 0 echo reply (ping) 3 0 dest. network unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 route advertisement 10 0 router discovery 11 0 TTL expired 12 0 bad IP header

20. Routing in the Internet r The Global Internet consists of Autonomous Systems (AS) interconnected with each other: m Stub AS: small corporation m Multihomed AS: large corporation (no transit) m Transit AS: provider r Two-level routing: m Intra-AS: administrator is responsible for choice: RIP, OSPF m Inter-AS: unique standard: BGP

21. 21 Link Layer

22. 22 Link Layer: setting the context r two physically connected devices: m host-router, router-router, host-host r unit of data: frame application transport network link physical network link physical M M M M H t H t H n H t H n H l M H t H n H l frame phys. link data link protocol adapter card

23. 23 Link Layer Services r Framing, link access: m encapsulate datagram into frame, adding header, trailer m implement channel access if shared medium, m ‘physical addresses’ used in frame headers to identify source, dest different from IP address! r Reliable delivery between two physically connected devices: m we learned how to do this already (chapter 3)! m seldom used on low bit error link (fiber, some twisted pair) m wireless links: high error rates Q: why both link-level and end-end reliability?

24. 24 Link Layer Services (more) r Flow Control: m pacing between sender and receivers r Error Detection: m errors caused by signal attenuation, noise. m receiver detects presence of errors: signals sender for retransmission or drops frame r Error Correction: m receiver identifies and corrects bit error(s) without resorting to retransmission

25. 25 Multiple Access Links and Protocols Three types of “links”: r point-to-point (single wire, e.g. PPP, SLIP) r broadcast (shared wire or medium; e.g, Ethernet, Wavelan, etc.) r switched (e.g., switched Ethernet, ATM etc)

26. 26 Multiple Access protocols r single shared communication channel r two or more simultaneous transmissions by nodes: interference m only one node can send successfully at a time r multiple access protocol: m distributed algorithm that determines how stations share channel, i.e., determine when station can transmit m communication about channel sharing must use channel itself! m what to look for in multiple access protocols: synchronous or asynchronous information needed about other stations robustness (e.g., to channel errors) performance

27. 27 Ethernet: uses CSMA/CD A: sense channel, if idle then { transmit and monitor the channel; If detect another transmission then { abort and send jam signal; update # collisions; delay as required by exponential backoff algorithm; goto A } else {done with the frame; set collisions to zero} } else {wait until ongoing transmission is over and goto A}

28. 28 Network Layer Basic Functions Representative Protocols Security Vulnerability Examples ApplicationProviding services such as WWW to end-users HTTP, SMTP, FTPJavaScript-based malware, Email spams TransportEnd-to-end message transmission independent of the underlying network TCP, UDPTCP SYN attack, UDP flooding attack NetworkRoutingIP, ICMP, RIP, OSPF, BGPIP spoofing, Black hole attack to RIP Data LinkMedia access controlEthernet, Wi-FiEavesdropping attack PhysicalTransmitting raw bit stream Physical attack such as cut to cable A Summary on Network Layers and Their Vulnerabilities

29. 29 Acknowledgement r Part of the slides are from Kurose and Ross’s book “Computer Networking: A Top-Down Approach”.