15-441 Roundup

1. 15-441 Roundup

2. 7-layer or 4-layer dip? • Layering: Reuse, interoperability • OSI 7-layer model Application Application 7 Presentation Presentation 6 Session Session 5 Transport Transport 4 Network Network Network 3 Data link Data link Data link 2 Physical Physical Physical 1

3. OSI Functions • (1) Physical: transmission of a bit stream. • (2) Data link: flow control, framing, error detection. • (3) Network: switching and routing. • (4) Transport: reliable end to end delivery. • (5) Session: managing logical connections. • (6) Presentation: data transformations. • (7) Application: specific uses, e.g. mail, file transfer, telnet, network management. Multiplexing takes place in multiple layers

4. The TCP/IP Model Application Application (plus libraries) Presentation Session TCP/UDP IP/ICMP Transport Network Data link Data link Physical Physical

5. Some layers - particularly in the OSI model - not so well defined Layer “violations” often useful for performance reasons. Buffer management Reduce redundant information between headers Layering and stacks

6. Analog Signal “Digital” Signal 0 0 1 0 1 1 1 0 0 0 1 Bit Stream Packet Transmission 0100010101011100101010101011101110000001111010101110101010101101011010111001 Packets Sender Receiver Header/Body Header/Body Header/Body The lower layers - concepts

7. Limits to Speed and Distance • Noise: “random” energy is added to the signal. • Attenuation: some of the energy in the signal leaks away. • Dispersion: attenuation and propagation speed are frequency dependent. • Changes the shape of the signal • Effects limit the data rate that a channel can sustain. • But affects different technologies in different ways • Effects become worse with distance. • Tradeoff between data rate and distance

8. Why Do We Need Encoding? • Meet certain electrical constraints. • Receiver needs enough “transitions” to keep track of the transmit clock • Avoid receiver saturation • Create control symbols, besides regular data symbols. • E.g. start or end of frame, escape, ... • Error detection or error corrections. • Some codes are illegal so receiver can detect certain classes of errors • Minor errors can be corrected by having multiple adjacent signals mapped to the same data symbol • Encoding can be very complex, e.g. wireless.

9. Encodings • NRZ - “Non-Return to Zero” • Simple: 0 = low, 1 = high • Long runs of 0s and 1s lose synch • NRZI - transition on 1 • Long runs of 0s lose sync • Manchester - low/high = 0, high/low = 1 • Uses 2x as many transitions • 4B/5B, etc - • Encode multiple 0s and 1s. Efficient. Used in Ethernet. • SONET - many observations of flag pattern.

10. Datalink Functions • Framing: encapsulating a network layer datagram into a bit stream. • Add header, mark and detect frame boundaries, … • Media access: controlling which frame should be sent over the link next. • Easy for point-to-point links; half versus full duplex • Harder for multi-access links: who gets to send? • Error control: error detection and correction to deal with bit errors. • May also include other reliability support, e.g. retransmission • Flow control: avoid that the sender outruns the receiver.

11. CSMA/CD Algorithm • Carrier Sense Multiple Access / with Collision Detection • Sense for carrier. • If carrier present, wait until carrier ends. • Send packet and sense for collision. • If no collision detected, done transmitting • Otherwise, abort immediately, perform “exponential back off” and send packet again. • Start to send at a random time picked from an interval • Length of the interval increases with every retransmission

12. Collision Detection: Implications A B C • All nodes must be able to detect the collision. • Any node can be sender • => Must either have short wires, long packets, or both. • Can calculate length/distance based on transmission rate and propagation speed. • Messy: propagation speed is media-dependent, low-level protocol details, .. • Minimum packet size is 64 bytes • Cable length ~256 bit times • Example: maximum coax cable length is 2.5 km

13. Internetworking Options 7 7 7 7 6 6 6 6 5 5 5 5 4 4 4 4 data link 3 3 3 3 physical 2 2 2 2 2 1 1 1 1 1 1 1 repeater Switching/bridging (e.g. 802 MAC) 7 7 7 7 6 6 6 6 5 5 5 5 . . . network 4 4 4 4 3 3 3 3 3 3 3 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 router gateway

14. Internetworking • Repeaters: Physical link. One big collision / transmission domain. • Bridges: Datalink. Can separate broadcast domains and selectively forward traffic. Transparent - preserve MAC addresses. • Routers: Separate addressing domains. Forward through diff. MAC addresses.

15. IP • CIDR - Classless Inter-Domain Routing • 192.4.16/24 == 255.255.255.0 • == 24 bits of network, 8 bits of host • Covers 192.4.16.0 - 192.4.16.255 • 192.4.16./23 == 25.255.254.0 • Covers 192.4.16.0 - 192.4.17.255 • Enables more efficient use of address space through aggregation. • Routing by longest-prefix match • /29 is “longer” (more 1s) than /24.

16. Routing Protocols • Intra-domain: • RIP: Routing Information Protocol • Distance-Vector. • Send information about table to neighbors (per-dest cost) • Count to infinity problem. • Split horizon - Don’t advertise routes back to next-hop • Poison reverse: Advertise infinite metric to next-hop • Neither of these solves all loop problems! • OSPF: Open Shortest Path First • Link-state. • Flood neighbor info to entire network • Each node generates own routing table • Fast convergence, but lots of traffic for large nets • Inter-domain: • BGP: Border Gateway Protocol • Path-Vector. Send full AS path along with announcement. • Solves loop problems with DV.

17. BGP • Internet divided into Autonomous Systems. Each has unique #. • Each AS sends routes with BGP • Remember: IBGP full-mesh. Why? • No AS # to distinguish loops. • ASes route internally with an IGP (OSPF, etc). • Some terms: • MED (Multi-Exit Discriminator): Peers send to influence remote peer’s routing. • Localpref: One AS configures to change routing to a peer.

18. AS relationships • Transit: I pay you, you carry my traffic to anyone • Peering: (Often) free, you carry my traffic to your customers and vise-versa. • “Valley-free” routing • A formalization of the above.

19. Multicast • A lot of multicast on project1, so • Won’t be on the final exam. • (Aren’t you glad you came to class today?) • Multicast today • Deployed inside organizations / etc. • Iffy if you want to use across Internet • Concepts useful! E.g., overlay multicast

20. Tunnels, NATs, etc • Things to remember: • NAT - network address translator • Lets you use private addresses inside net • May let you share one external address • (Port-translating NAT) • Can break end-to-end reachability & naming • IPv6: • 128 bit address space • Cleaned up header, no fragmentation, no checksum, fixed option processing. • For faster router processing

21. Cont’d. • Tunnels - wrap packets in an extra IP header • Send indirectly • Implement overlay networks (e.g., overlay multicast, etc.)

22. DNS • The Domain Name System • Distributed name -> IP (and back) database • Addresses returned by “A” records • Hierarchical. Goes from the root (“.”) down. Each level can delegate an “NS” (name server) record. • Recursive resolvers - answer a query completely. Iterative resolvers - give you the next step. • Caching: TTL-based.

23. Transport & TCP • Duties may include: • Reliability, in-order, demultiplexing, message boundaries, congestion control • UDP (User Datagram Protocol): Just demux & checksum. Unreliable, etc. • TCP (Transmission Conrol Protocol): Reliable, in order byte-stream w/congestion control.

24. Transport Demux • TCP & UDP both use “ports” - 16 bit #s - as demux keys

25. ARQ • “Automatic Repeat Request” • (ARR would have endorsed piracy?) • Simplest: Stop-and-Wait • Send packet, wait for response, iterate… • Slow. • Go-back-N • Uses a window. Usually along with… • Sliding window flow control • Use more capacity. • How to size that window? There’s the rub.

26. Sizing Windows • Optimal window size: bw * rtt • Why? Capacity of the pipe, in both directions. • Must keep sending pkts until first ACK gets back to you (one RTT). • BW is available bw. • Must not blast traffic: Congestion Collapse • More work -> more wasted packet retransmissions • In the limit: no useful packets get through! • How do we find a good window size?

27. Congestion Control • Fair and efficient use • Network based (ECN, etc) or end-to-end (TCP) • AIMD: Additive Increase, Multiplicative Decrease • Converges to fair & efficient use. Cool! • What TCP does. MD = cut by half. AI = add one per RTT.

28. TCP • Three-way Handshake: SYN / SYN-ACK / ACK. • ISN - Initial Sequence Number • Each side picks one • TCP is byte-oriented • Tear down with FIN (finshed) • Signal error with RST (reset)

29. TCP 2 • Timeouts: Should be familiar • EWMA = Exponential Weighted Moving Average = Low-pass filter • srtt = (alpha * srtt) + (1 - alpha) * new_sample • Track RTT and linear deviation • Linear deviation always > std. dev • Why? RTT variation is high under high loads because buffers fill, adding queueing delay

30. Pacing • ACK clocking sends pkts out more slowly • Avoid huge bursts (fill buffers -> loss -> bad) • Slow Start: Get up to “operating range” quickly (exponential growth).

31. SACK & Enhancements • Selective ACKnowledgements • Bitmap of received backets • Help recover from multiple losses in window • All TCP variants need large enough window to recover from losses • Nagel’s Algorithm: Delay briefly to coalesce small packets - one outstanding small packet.

32. TCP Performance • Single link, need router buffers • 75% link utilization vs 100% link utilization • How big buffer? Conservatively, BW * RTT • There’s that number again. So common, it can’t help but show up on the final in some form. • Simple model: • (most ignore the constants)

33. Queueing • FIFO: First In, First Out • Scheduling: Who goes out when? • Fairness, etc., entirely up to end hosts • Fair Queueing • Routers decide who gets to go (e.g., round-robin, Weighted Fair Queueing (WFQ), etc.) • Drop-Tail • Drop policy: drop new pkts if queue is full • Can synchronize flows • AQM: Active Queue Management • RED - Random Early Detection • Randomly marks (or drops) pkts before queue full

34. Sharing • Max-Min Fairness • Small demands get what they want; • Large demands compromise • GPS: Generalized Processor Sharing • Fluid model for Max-Min fairness • Accounts for packet sizes • Fair Queueing: Compute virtual completion times, send accordingly • Complex, per-flow state. But nice results.

35. QoS • Quality of Service • Differentiate between flows • Some get “good” service (guarantees, etc) • Some get best effort • Application utility curves • Elastic (file xfer) vs. Inelastic (hard realtime) • Requires admission control • Can’t over-promise! • Token Buckets • Rate: Let average amount of traffic through • Bucket: Accommodate some burstiness • RSVP - Resource reServVtion Protocol • Set up QoS / token bucket state at routers on path

36. Wireless • Mobility • Routing solution: excess global state • Mobile IP: Triangle routing, tunneling via “home agent” that proxies for mobile node • TCP solution: Re-bind connection • Link layer: Learning bridges • Noisy -> losses • Link-layer retransmission (802.11) • End-to-end approach (SACK, ELN - Explicit Loss Notification).

37. Wireless MAC issues • CSMA/CD doesn’t work too well • Hard to listen while transmitting • Hidden terminal - clobber someone else • Exposed terminal - mistakenly think you’ll clobber • Solution: RTS / CTS • Ready To Send / Clear To Send

38. Ad Hoc Networks • Routing harder: No fixed infrastructure • Protocols • DSR - Dynamic Source Routing • AODV - Ad Hoc On-Demand Distance Vector • Sensor Networks • Limited battery life drives everything • Multi-hop can save power (Tx power proportional to distance squared) • Aggregation holds the big promise. Don’t do n^2 communication…

39. HTTP • HyperText Transfer Protocol • Stateless request-response protocol over TCP • Persistent HTTP: Optimizes for fewer TCP connection setups. • Fewer slow starts, 3-way handshakes • Caching • Expires: header, Get-If-Modified-Since request • ETags (“Entity Tags”) help identify version of document when using cookies, etc.

40. Web Caching • Proxy Caches • Client-based. • Content Distribution Networks • Server-driven. • Usually use DNS to send client to replica • Mapping problem • Example: Akamai • Big benefit: Coping with flash crowds • Much content (50%?) uncacheable • Dynamic • Unpopular

41. P2P • Search techniques: Centralized (napster), broadcast (gnutella), superpeers (KaZaA), routing (Chord) • Consistent Hashing • Goal: Don’t move all content around when # of buckets changes slightly • Used in Chord to do routing in log(n) hops using finger table • Points 1/2, 1/4, 1/8, … way around the ring

42. Security • Private Key • E.g., DES (“Data Encryption Standard”), or newer AES (“Advanced Encryption Standard”) • Must have a shared secret. • Public Key • E.g., RSA, Diffie-Hellman • Can encrypt to a public key, and not read • Must have the public key. *really* slow. • Key Distribution - big challenge! • Private: Kerberos (andrew) • Public: Certificiate Authorities (mozilla)

43. Security 2 • Hash functions • One-way. We hope. • Digital signature: Sign a hash of the data • SSL - “Secure Sockets Layer” • Pre-packaged encryption/etc. routines • Now “TLS” (Transport Layer Security) • Used in HTTPS/etc. • IPSEC - ip-layer security

44. Network Security • IP model assumed “much trust” • Spoofing source IPs • DoS - “Denial of Service” attacks • DDoS - “Distributed DoS” • - Hundreds/thousands+ of attack machines • TCP ISN adds some protection • As long as it’s really random. :)

45. Firewalls! • Filter traffic in network • Stateless - match static traffic rules • Stateful - remember more about connections • Basic: Match src, dst, ports, flags • Expect a question about filtering to specific CIDR blocks • Set up rules to do the right things • Create CIDR blocks to match the right ranges of IP addresses… • IDS = “Intrusion Detection System” • Tell you when you’ve been hacked. :) (Or who’s trying to hack you)