2023-11-07

Summary

keywords

TODO

HW

Exercise*

Next time

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Router Architecture

Input port function

• line termination(Physical layer)

  • bit level reception

• link layer protocol recieving

  • Ethernet

• lookup, forwarding table, queueing

  • queueing happens at every port.

  • decentralized switching

    • Destination-based forwarding

    • SDN (more fields to compare)

  • goal is to process at line speed. so, the calculations are excecuted at hardware level.

Longest Prefix Matching

Switching fabric

Switching rate should be $NR$, where $R$ is the speed of each line. fabric designs are 3 types.

• memory, bus, interconnection network.

Switching fabric #1. Memory

past generation.

• memory is the middle point.

• speed is limited by memory bandwidth.

• need two buses(input bus, output bus)

• read&write is needed.

Switching fabric #2. Bus

Bus (64bits of data can flow at a singular clock cycle)

• Popular, but very expensive.

• speed is limited by bus bandwidth (bus contentation)

• cutting-edge buses are 32 Gbps

• bus contentation depends on how many/busy input ports are.

Switching fabric #3. Interconnection network

grid, and each pathways can be open or closed.

• Crossbar methods.

• Initially developed for processor connection methods.

• If bus speed is R, and there are NR capacity (at most, by some sort of parallelism)

• thus can meet the efficiency of bus method.

• Even if we have less bandwidth, NR switching rate can be made because parallel switching is possible.

• up to 60 Gbps.

• limitation : when one pathway blocks other needed pathways.

• Buffer management, scheduling algorithm should be implemented.

Output port contention

HOL Blocking occurs. - when two packet destined to the same output port. The other packet data should wait until one collection of data is passed.

Output buffer comes in. When arrival rate exceeds output line speed. but queueing delay and loss due to output port buffer overflow can happen.

Buffer management is required.

3 tasks on buffer management

1. what should be the buffering size

2. drop policy (which to drop?)

3. scheduling discipline giving priority on each packets

How much buffering?

${RTT*c}/{\sqrt{N}}$

Round Trip time is used. #todo : write about TCP flow and buffer.

If you have too much buffer?

Buffer management

Drop policy

priority

round robin

#todo : fill out.

Network priority

#todo : fill out

Network Neutrality

• technical, social, economic, legal

Three rules made in US, 2015

• No blocking

• No throttling

• no paid prioritization

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Network Layer Protocol

• IP protocol

• ICMP protocol (used in ping service)

IP datagram format

#todo : paste format image.

• source IP address

• destination IP address

• TTL : max hops, thus looping packets are prevented.

• upper layer protocol (TCP, UDP..)

• header checksum (calculation to validate the data is intact). for some reliability

  • header checksum is only for header parts.

• 16bit identifier, flags, fragment offset

  • router also does fragmentation. (actually a link layer process, but since they do not have processing power, network layer does it.)