15441 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 (plus libraries)
Application
Presentation
Session
TCP/UDP IP/ICMP
Transport
Network
Data link
Data link
Physical
Physical
5
Layering and stacks

• 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

6
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
• gt 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 (e.g., Skype supernode)
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 - Dont 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 peers 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

• 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
Contd.

• 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 -gt 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? Theres 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 -gt 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 (finished)
• 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 gt 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 -gt loss -gt 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
• Nagels 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
• Theres that number again. So common, it cant
  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
• Cant 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 -gt losses
• Link-layer retransmission (802.11)
• End-to-end approach (SACK, ELN - Explicit Loss
  Notification).

37
Wireless MAC issues

• CSMA/CD doesnt work too well
• Hard to listen while transmitting
• Hidden terminal - clobber someone else
• Exposed terminal - mistakenly think youll
  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. Dont do n2
  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?) not cacheable
• Dynamic
• Unpopular

41
P2P

• Search techniques Centralized (napster),
  broadcast (gnutella), superpeers (KaZaA), routing
  (Chord)
• Consistent Hashing
• Goal Dont 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 its 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 youve been hacked. ) (Or whos
  trying to hack you)