notes/docs/lectures/acn/11_connecting.md

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 is 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 Connection Admission Control
  • 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 addresses

• Loads of addresses ✅
• Routing protocols need to be ported ❌
• Associated services needing to move ❌

Network Address Translation

Because IPv6 did not magically solve the 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

ea:ep - NAT address : NAT port

When the client receives a packet from server 1 da:dp, the NAT translates the NAT address ea:ep to the client's internet address and port ia:ip.

Address Restricted Cone NAT

In this case server 2 is not trusted and therefore any request will be dropped.

Port Restricted Cone NAT

If the router receives a packet from a bad IP or bad port, it will be dropped.

Symmetric NAT

Here, the internal address is obfuscated from the external servers. The same client can use different ports for different communications.