Networks (CS438) Review! High Level Concepts *with suggestions for topics to study in detail

Layers Application Transport Network Link Physical Do things (web browsing, ssh, video chat, …) Clean abstraction for talking to partner Get packets anywhere in the world Deliver packets on local network Medium sharing Use physics to send data bits *How do the layers relate to headers?

application transport network link physical application transport network link physical network link physical link physical Router Switch Application data TCP Seg- ment IP Packet Ethernet Frame Source Destination

End-to-End Principle Keep high-performance parts that everyone uses (the internet) simple. If you want a job done right, do it yourself. *Circuit vs packet switching. Why does End-to-End Principle demand packet switching?

Application Layer: HTTP Just for simple, individual file transfers Fundamentally stateless – (Cookies add state, though) All complexity in HTML, JavaScript, etc. logic *We now optimize for multi-file sessions: exactly how much does parallel/persistent help? *Caching: local, proxy caching *CDNs! Help performance a lot! How exactly do they function?

Application Layer: DNS Maps “illinois.edu”  Distributed, hierarchical database – Root, com, example.com, … each’s nameserver knows about the next’s Resolving a query: old results are cached. We go all the way to root servers rarely. *Iterative vs. recursive resolvers

Transport Layer Multiplexing: the one thing transport must do Other characteristics: – Reliable or not? – Stream or packets? – Congestion control or not? UDP: unreliable packets, no congestion control – “Drops are ok, don’t waste time recovering”: video chat, games; all real-time things *Ports. And, why do we also need to look at IP addr for full multiplexing? *5-tuple, sockets

Transport Layer: TCP Reliable delivery Congestion control *Flow control *SYN and FIN handshakes *TCP states: e.g. waiting after shutdown with FIN/ACK

Reliable Transport Sequence numbers to identify segments ACKs: ACK 7 reaches sender  segment 7 safe. Cumulative ACKs: ACK 7  segments ≤ 7 safe. *Know Go-back-N and Selective Repeat in addition to real-world TCP *Byte- vs segment-based seq. #s *Exact sliding window mechanics. *Sequence numbers, wraparound.

Congestion Control “Slow start”: exponential growth, for rough estimate of channel capacity. “Congestion avoidance”: dynamics of hosts’ behavior automatically converge to fairness! – No coordination or central control needed – The mechanism: AIMD: additive increase, multiplicative decrease *What is “fair”? Max-min fair? Bonus: look up congestion pricing. *Understand how AIMD converges!! *Glance at that 3 state diagram (SS/CA/FR) in the textbook *Why does cwnd+=mss 2 /cwnd work for additive increase?

AIMD Linearly grow while everything is fine Cut in half at congestion event cwnd: TCP sender congestion window size time Real world alert: Linux has used Cubic, not Reno, for a long time. Not on the test, though.

Congestion Events Router gets too many packets; must drop some Dropped packets  duplicate ACKs 3dup ACK  dropped packet  congestion Timeout  everything dropped  broken Single dup ACK means out of order packet. This never happens; forget it can even happen! *How is the timeout timer set? *Explicit congestion notification (ECN), random early drop (RED) *Basic queuing theory: what happens when in rate = out rate?

Network Layer Deliver packets from anywhere, to anywhere. This is the internet!

Routing A distributed system solving all-pairs shortest path Link State: give everyone the same view of the network, run Dijkstra’s algorithm Distance/Path Vector: neighbor tells you it can handle delivery to some node X *DV’s count to infinity; how does PV solve it? *Work through Dijkstra and DV examples!

Autonomous Systems Companies that own groups of routers. – Network infrastructure (Level 3, AT&T) – Consumer ISP (AT&T, Comcast) – Group with big network (universities, companies) AS-level topology – Route at AS level: 1 AS = 1 hop – Border Gateway Protocol (BGP) – Economic routing policy: prefer customers over peers over providers (or: only use a path to or from a customer) The Tier 1 AS clique: global all-to-all connectivity *Why can’t inter-AS routing use link state?

IP Addresses Identify your current location in the internet Nearby addresses grouped into prefixes – Save internet routers from being overwhelmed – Your prefix = your subnet (next slide) IPv6: IPv4’s ~4billion addresses not enough *IPv4/6 tunneling, dual stack *NAT *DHCP *What is a default gateway? How does it relate to subnets and the link layer? *Contrast IP addresses with MAC addresses. Why can’t MAC addresses do the same job as IP?

Data Link Layer Deliver packets locally (within IP subnet) No fancy routing: learning switches *Do a learning switch example! *How exactly does a network interface know if a packet can be sent just as a link level frame? What has to happen if not? *ARP *Spanning Tree Protocol

Shared Medium Access Collision detection: exponential backoff Collision avoidance: RTS/CTS *Hidden terminal problem *Exposed terminal problem

Security Fundamentals Symmetric keys Asymmetric (public/private) keys – How do I know who’s on the other end? (MitM) Signatures  certificates  certificate chains  Certificate Authorities’ root certs