Transcript Network Systems

Chapters: All
Final Review
Professor Rick Han
University of Colorado at Boulder
[email protected]
Announcements
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HW #5 solutions posted on May 4
Final May 7, 4:30-7 pm
Tuesday’s lecture now on Web
Office Hours Monday: 3-5 pm
Next, final review…
Prof. Rick Han, University of
Colorado at Boulder
Format of Final
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2 ½ hours
Comprehensive
In class
Closed book
Calculator OK
About 5-6 multi-part questions
About 20-25 minutes for each multi-part
question
Prof. Rick Han, University of
Colorado at Boulder
Format of Final (2)
• Breakdown:
• ~4 questions on topics since last midterm (all
lectures from Feb. 28 onward)
• ~1 each on security, application layer
protocols, TCP, and IP (including BGP and
multicast)
• ~2 questions on topics before the midterm
• ~2 on MAC/link layer and IP
Prof. Rick Han, University of
Colorado at Boulder
Potential Topics for Final
• List is not all-inclusive; some topics may appear
on final not listed here
• How does ___ work? Why is it used?
• Sections in textbook relevant to final:
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Chapter 1: 1.1-1.3
Chapter 2: 2.1-2.8
Chapter 3: 3.1-3.2
Chapter 4: 4.1-4.4
Chapter 5: 5.1-5.2
Chapter 6: 6.2-6.4
Chapter 7: none
Chapter 8: 8.1-8.4
Prof. Rick Han, University of
Chapter 9: 9.1-9.2
Colorado at Boulder
Potential Topics for Final (2)
• All topics in lecture notes are relevant to final
• Relevant topics in lecture notes but not in the
textbook:
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SACK TCP
TCP extensions: window scale & time stamp
Wireless TCP: snoop
Web caching proxies
Load balancing via DNS, HTTP Redirect,
NAT’s for address translation, firewalling, load
balancing
SMB/Samba
Stream ciphers and WEP
Prof. Rick Han, University of
Colorado at Boulder
Potential Topics for Final (3)
• All topics listed in the Midterm Review, plus the
following…
• IP:
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Addressing, Subnets, and CIDR
BGP
IP multicast
• Link-state multicast
• DVMRP
• PIM
UDP
• Unreliable datagram delivery
• Header, Checksum
• Multiplexing/demultiplexing
Prof. Rick Han, University of
Colorado at Boulder
Potential Topics for Final (4)
• TCP:
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Reliable Stream Delivery
Header, Checksum
Connection setup
• How does a 3-way handshake work and why does it
work?
• SYN, SYN/ACK
• How does FIN and FIN/ACK exchange differ from
SYN and SYN/ACK exchange? (half-closed)
State machine
• What states are traversed during connection
setup?
• In a normal termination, how does TIME_WAIT
state differ Prof.
from
CLOSE_WAIT?
Rick Han, University of
Colorado at Boulder
Potential Topics for Final (5)
• TCP:
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Sliding window
• Sequence #’s and segments
• Window-based flow control
• Cumulative ACK’s
• Receiver window advertisements
• Sender-side vs. receiver-side sliding window
flow control
• What is TCP Persist and why is it useful?
• TCP Extensions
• Timestamp address wrap-around with seq. #’s
• Window scaling keeps pipe full over LFN’s
Prof. Rick Han, University of
Colorado at Boulder
Potential Topics for Final (6)
• TCP:
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Adaptive Retransmission
• Under what conditions does TCP retransmit?
• Timeout
• 3 duplicate ACK’s
• How is the RTT originally estimated?
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New RTT estimate = a (old RTT estimate) + (1 - a) (new RTT)
How is the timeout originally computed from RTT?
• RTO = b RTT, where b = 2
What were Karn/Partridge’s refinements to
original approach?
• Recompute RTT only for unambiguous ACK’s
• Backoff the timeout exponentially
Prof. Rick Han, University of
Colorado at Boulder
Potential Topics for Final (7)
• TCP:
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Adaptive Retransmission
• What were Jacobsen/Karels refinements to
timeout algorithm?
• Make the timeout a function of both the
average and deviation – but why?
• RTO = Smoothed Ave + 4 * Smoothed Dev
Prof. Rick Han, University of
Colorado at Boulder
Potential Topics for Final (8)
• TCP:
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Congestion Control
• W = min (CW, FW) – why?
• send no more packets than the network can
handle without loss
• Sawtooth behavior of CW – what’s the basic
principle?
• Probe network by expanding CW until loss, then
reduce CW, then grow CW again, etc.
• Slow Start is actually exponential increase
• How does a sender detect that CW is too large?
• A timeout occurs
Prof. Rick Han, University of
Colorado at Boulder
Potential Topics for Final (9)
• TCP:
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Congestion Control
• Additive Increase/Multiplicative Decrease
• After a timeout, divide CW by 2 and store in
ssthresh
• Slow start up to ssthresh, then add a/CW if
CW packets in a RTT are safely ACK’ed
• If 3 duplicate ACKs are received, then infer that
one segment has been lost
• Retransmit immediately, rather than wait for a
timeout : called Fast Retransmit
• Cancel slow start, and drop CW to half its value
(approximately) rather than to one : called Fast
Recovery
Prof. Rick Han, University of
Colorado at Boulder
Potential Topics for Final (10)
• TCP:
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Congestion Avoidance
• Back off before there are packet losses
• Informed by increasing RTT – Source-based
• Informed by routers of congestion – DECbit
(explicit), RED (implicit by dropped packets)
• Queueing Disciplines
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What is a drop-tail policy?
How do Fair Queueing and Weighted Fair Queueing
enforce fairness? How are they work-conserving?
Prof. Rick Han, University of
Colorado at Boulder
Potential Topics for Final (11)
• SACK-TCP
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Why is this an improvement over vanilla TCP?
How are selective ACK’s achieved?
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What is the major problem with TCP over wireless?
Solutions:
• End-to-End approaches: ECN, ELN
• Split connection
• Link-Layer
• Snoop TCP is a hybrid: TCP-aware
• Retransmit locally and suppress duplicate ACK’s
– all without having to terminate TCP connection
• Advantages and disadvantages?
• Wireless TCP
Prof. Rick Han, University of
Colorado at Boulder
Potential Topics for Final (12)
• Application Layer:
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DNS
• Hierarchical naming
• Hierarchical resolution of names: local, root &
authoritative name servers with
iterative/recursive resolution
• Load distribution – DNS round robin
HTTP
• Stateless Request/Response protocol using text
• Persistent HTTP 1.1
• HTTP Caching Proxies – relevant headers?
• HTTP Redirect for load balancing
SMTP, MIME, and
how is email relayed via SMTP mail
Prof. Rick Han, University of
gateways?
Colorado at Boulder
Potential Topics for Final (13)
• Network Address Translation (NAT)
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How does it work?
• Replace sender’s IP addr and source TCP port with
NAT’s IP addr and source TCP port
“Dynamic” NAT serves as firewall
“Static” NAT allows inbound traffic on designated
ports
Load balancing via NAT
IPSec and NAT’s – what’s the conflict?
• TCP ports are encrypted
• Even if TCP ports were visible, can’t modify packet
without causing tampering to be detected via
digital signature
Prof. Rick Han, University of
Colorado at Boulder
Potential Topics for Final (14)
• SMB/Samba
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Protocol for mapping file systems between UNIX and
Windows – how does it work?
What is NETBIOS and how does it relate to SMB?
• Security:
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What are the six major characteristics of concern in
security?
• Confidentiality, Integrity, Authentication, NonRepudiation, Availability, Authorization
What are different types of cryptanalysis attacks?
• Brute force, ciphertext-only, known-plaintext,
chosen-plaintext, adaptive chosen-plaintext
Prof. Rick Han, University of
Colorado at Boulder
Potential Topics for Final (15)
• Security:
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Symmetric Key Cryptography
• Same secret key on both endpoints
• DES uses 16 stages; each employs principles of
confusion and diffusion
• What is a block cipher, how is it vulnerable, and how
does Cipher-Block-Chaining (CBC) address this?
• How do stream ciphers work?
• What are various ways to securely distribute a
shared secret key to both endpoints?
• Diffie-Helman Key Exchange
• Public key encryption of shared symmetric key
• Key Distribution Center (KDC) – Kerberos
Prof. Rick Han, University of
Colorado at Boulder
Potential Topics for Final (16)
• Security:
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Public Key Cryptography
• Asymmetric keys: a public key and a private key
• Helps provide Confidentiality, Authentication,
Integrity
• Based on the difficulty of inverting one-way
functions
• How does RSA work? (see example)
Prof. Rick Han, University of
Colorado at Boulder
Potential Topics for Final (17)
• Security:
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Authentication via
• public-key digital signatures
• 3-way handshakes
• Trusted 3rd party
• Public keys (Fig. 8.11)
How do one-way hashes provide data integrity?
• What are some counterexamples?
• Checksums for IP and WEP
Prof. Rick Han, University of
Colorado at Boulder
Potential Topics for Final (18)
• Security:
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Secure distribution of public keys
• Trusted 3rd party Certificate Authorities (CA)
• What is a digital certificate and how does it certify
the provider of the certificate?
SSL/TLS – how is the secure connection established?
IPSec
• End-to-end encryption at the network layer
• Impact on NAT’s
• How do the two protocols AH and ESP provide
confidentiality, integrity, and/or authentication?
• How is a VPN created using IPSec?
Prof. Rick Han, University of
Colorado at Boulder
Potential Topics for Final (19)
• Security:
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Firewalls
• Packet filters
• Proxies
• What are some sample policies that firewalls could
implement? How are they flawed?
• Good luck on the Final!
Prof. Rick Han, University of
Colorado at Boulder