Add acn

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+# Connecting
+
+#### Elasticity: Supply and Demand
+
+This is about **resource management**
+
+- **Supply** - Available link capacity on path
+- **Demand** - Host transmitting and receiving traffic
+- **Elastic** - capacity reduces -> demand is scaled back
+  - Hosts stop sending / send less
+- **Inelastic** - applications can’t handle this
+
+TCP manages resource usage based on observed loss and latency
+
+#### Quality of Service
+
+If capacity > demand, there is no need for quality of service
+
+If capacity < demand, we need to keep queuing minimal
+
+- As queuing directly impacts latency, jitter and loss
+  - In stable networks
+  - **Jitter**: The difference in delays, a measure of stability
+
+#### IP Type of Service
+
+- Single IP header byte
+
+```
+Bits 0-2:  Precedence.
+Bit    3:  0 = Normal Delay,       1 = Low Delay.
+Bits   4:  0 = Normal Throughput,  1 = High Throughput.
+Bits   5:  0 = Normal Reliability, 1 = High Reliability.
+Bit  6-7:  Reserved for Future Use.
+```
+
+- Precedence for *special* traffic
+
+```
+0     1     2     3     4     5     6     7
+-----+-----+-----+-----+-----+-----+-----+-----+
+|                 |     |     |     |     |     |
+|   PRECEDENCE    |  D  |  T  |  R  |  0  |  0  |
+|                 |     |     |     |     |     |
+-----+-----+-----+-----+-----+-----+-----+-----+
+
+Precedence
+
+111 - Network Control
+110 - Internetwork Control
+101 - CRITIC/ECP
+100 - Flash Override
+011 – Flash
+010 – Immediate
+001 – Priority
+000 - Routine
+```
+
+### Differentiated Services (DiffServ)
+
+- Operates on *traffic aggregates*
+  - Label packets with desired class via ToS
+  - Routers apply different queuing as operator sees fit
+- Four service classes, or *per-hop behaviour*
+  - **Default**: best effort
+    - No QoL applied
+  - **Expedited Forwarding**: low delay, loss & jitter
+  - **Assured Forwarding**: low loss if within rate
+  - **Class Selector**: use ToS precedence bits
+
+##### Problems
+
+- End to end semantics
+- Mapping to service level agreement
+  - If an internet company sells a network with a certain speed, this might have legal repercussions if QoS are enacted
+- Mapping to application demands
+
+### Integrated Services (IntServ)
+
+- Operates on explicitly signalled *flows*
+  - Think phone switchboards
+  - The network signals exactly what it can and can’t do to the destination nodes
+- Flow setup specifies some quality of service
+- Routers perform **C**onnection **A**dmission **C**ontrol
+  - CDA can accept and reject traffic based on whether or not the route/path is available
+
+##### Problems
+
+- Complexity
+  - Hard to scale
+- Mapping requirements to parameters
+  - This was easier when ATM did it as they owned all the infrastructure
+  - Whereas now it is difficult to map across all different companies
+- Per-flow state
+  - Extremely difficult
+
+## NAT
+
+### Address Shortages
+
+**IPv4** supports 32 bit addresses
+
+- 95% allocated already (440,000 netblocks)
+
+**IPv6** supports 128 bit address
+
+- Loads of addresses :white_check_mark:
+- Routing protocols need to ported :negative_squared_cross_mark:
+- Associated services needing to move :negative_squared_cross_mark:
+
+### Network Address Translation
+
+Because IPv6 did not magically solve address shortage problem and not all routers are ipv6 aware, we had to rely on NAT.
+
+- Private Addressing, `RFC1918`
+  - `172.16/12`, `192.168/16`, `10/8`
+  - Devices with these local IPs should never be externally routed
+  - Not for security reasons - just for getting more addresses
+- Traditional NAT, `RFC3022` is the standard
+  - Use private addresses internally (within the local network)
+  - Map into a (small) set of routable addresses
+  - Use source ports to distinguish connections
+  - For large scale **carrier grade NAT** [`RFC6598`] on `100.64/10`
+
+#### Implementation
+
+- Requires IP, TCP/UDP header rewriting
+  - Addresses, ports and checksums all need to be recalculated
+- Behaviours
+  - Network Address Translation
+  - Network Address and Port Translation
+
+###### Full Cone
+
+![image-20220124180451508](/home/jay/.config/Typora/typora-user-images/image-20220124180451508.png)
+```
+ea:ep - NAT address : NAT port
+```
+
+When client receives packet from server 1 `da:dp`, the NAT translates the NAT address `ea:ep` to the clients internet address and port `ia:ip`.
+
+###### Address Restricted Cone NAT
+
+![image-20220124181009792](/home/jay/.config/Typora/typora-user-images/image-20220124181009792.png)
+In this case server 2 is not trusted and therefore any request will be dropped.
+
+###### Port Restricted Cone NAT
+
+![image-20220124181127801](/home/jay/.config/Typora/typora-user-images/image-20220124181127801.png)
+
+If the router receives a packet from a bad IP or bad port, it will be dropped.
+
+###### Symmetric NAT
+
+![image-20220124181325935](/home/jay/.config/Typora/typora-user-images/image-20220124181325935.png)
+
+Here the internal address is obfuscated from the external servers, same client can use different ports for different communications.