networking-notes/Lecture-3.txt at master · tomasero/networking-notes · GitHub

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###Lecture 3

	- How do we evaluate a network?
	- How is communication organized?

---

####Circuits vs Packets
Circuits | Packets
--- | ---
predictable performance | unpredictable performance
inefficient use of network resources | efficient use of network resources
complex (state int he network) | simple (no state in the network)

####How do we evaluate a network?
- Delay
- Loss
- Throughput

####Delay
- How long does it take tos end a packet from its source to destination?
- Consists of four components
	- Transmission delay
		- Due to link properties
	- Propagation delay
		- Due to link properties
	- Queuing delay
		- Due to traffic mix and switch internals
	- Processing delay
		- Due to traffic mix and switch internals

#####A network link
- Link bandwidth
	- Number of bits sent / received per unit time (bits / sec or bps)
- Propagation delay
	- Time for one bit to move through the link (seconds)
- Bandwidth-Delay Product (BDP)
	- Number of bits "in flight" at any time
	- BDP = bandwidth x propagation delay

#####Example
- Same city over a slow link
	- bandwidth: ~100 Mbps
	- propagation delay: ~0.1 msec
	- BDP: 10,000 bits (1.25KBytes)
- Corss-country over fast link
	- bandwidth: ~10Gbps
	- propagation delay: ~10 msec
	- BDP: 10^8 bits (12.5 MBytes)

#####Transmission delay
- Time taken to push all the bits of a packet into the link
- Packet size / link bandwidth

#####Propagation delay
- Time taken to move one bit from one end of a link to the other
- Link length / link propagation delay

---

#####Queuing delay
- Time a packet sits in a buffer before it is processed
- If arrival rate > departure rate (**persistent overload**)
	- approaches infinity (assuming an infinite buffer)
	- in practice, finite buffer --> loss
- If arrival rate < departure rate
	- depends on burst size

#####Basic Queuing Theory Terminology
- Arrival process: how packets arrive
	- Average rate A
- W: average time packets wait in the queue
	- W for "waiting time"
- L: average number of packets waiting int he queue
	- L for "length of queue"

#####Little's Law 1961
	L = A x W
- Compute L: count packets in queue every second
	- How often does a single packet gets counted? W times
- Why do you care?
	- Easy to compute L, harder to compute W

---

#####Processing Delay
- Time taken for the switch to process a packet
	- Typically assume this is negligible

---

####Loss
- What fraction of the packets sent to a destination are dropped?

####Throughput
- At what rate is the destination receiving data from the source?
	- Data size / transfer time
- Transfer time = F / R + propagation delay
	- F = file of size (bits)
	- R = average throughput

---

	Where's my delay coming from?

---

####How is communication organized?
- **Decompose** the problem into tasks
- **Organize** these tasks
- **Assign** tasks to entities (who does that)

#####The path of the letter
	Semantic content -> Identity -> Location

####In the Internet: decomposition
1. **Applications** [L7]
2. **Transport**: Reliable (or unreliable) transport) [L4]
3. **Network**: Best-effort global packet delivery [L3]
4. **Data link**: Best-effort local packet delivery [L2]
5. **Physical**: Physical transfer of bits [L1]

#####Layers
	A part of a system with well-defined interfaces to other parts
- One layer interacts only with layer above and layer below
- Two layers interact only through the interface between them

#####Protocols and Layers
- Communication between peer layers on different systems is defined by **protocols**

#####Protocols
	An agreement between parties on how to communicate
- Defines the syntax for communication
	- **Header**: instructions for how to process the payload
	- Each protocol defined the format of its packet headers
		- e.g. the first 32 bits carry the destination address
	- And semantics
		- First a hello, then a request...
	- Protocols exits at many levels, hardware and software
		- Defined by variety of standard bodies (IETF, IEEE, ITU)

#####Protocols at different layers
<img src="./img/Layers.png" width="80%"/>