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 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 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 address

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

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

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

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, same client can use different ports for different communications.