adjih/rg-materials

Agenda, Presentations, Minutes

rg-materials

Repository for the NWCRG meeting materials (agenda, presentations, minutes) used during IETF meetings and Interim meetings. Related Internet-drafts are accessible from the NWCRG datatracker web site.

Note that, in addition to this repository:

• meeting material is always uploaded at [https://datatracker.ietf.org/meeting/materials/] prior to each IETF meetings;
• past meeting material for all IETF/IRTF groups is archived at [https://www.ietf.org/how/meetings/past/];
• past meeting material related to NWCRG is easily accessible at [https://datatracker.ietf.org/rg/nwcrg/meetings/].

Eight simple ideas to start understanding FEC and Network Coding for newcomers

Idea 1-

"We focus on networks where a packet either arrives or is lost"

• we're not at PHY-layer, we are above in the protocol stack and potential bit errors have either been fixed or the packet dropped

Idea 2-

"Encoding consists in adding redundancy (i.e., "repair packets") to the flow"

"Decoding consists in using redundancy (i.e., using "repair packets") to recover from packet losses"

Idea 3-

"Math is not an obstacle to understand FEC and NC"

• it's essentially a matter of linear combination and linear system resolution (e.g., via basic Gaussian elimination)
• details (e.g., computations in a certain Finite Field) can be complex, but mastering them is not required

Idea 4-

"There are roughly two categories of FEC codes: block codes and sliding window codes"

• block: segment the packet flow into blocks and apply FEC encoding per block, independently
• sliding window: an encoding window slides progressively over the packet flow, the encoder computes a linear combination of packets in this encoding window

Idea 5-

"With large independant data objects, block codes are great, with streams of real-time data that have time correlation, sliding window codes are preferable"

• ... because splitting the application flow into blocks delays the moment when "repair packets" can be generated!
• code performance also depends on the type of losses (independant, uniform losses versus correlated, bursty losses)

Idea 6-

"Some codes are restricted to a single encoder (e.g., sender) and single decoder (e.g., receiver)"

• usually called FEC

"Other codes can be used within intermediate nodes (i.e., multiple encoders)"

• usually called Network Coding (NC)

Idea 7-

"With NC, network equipments can perform FEC encoding to improve network usage"

• trivial example where a network equipment could reduce traffic (it sends a single "P1 XOR P2" packet (using multicast or broadcast) instead of sending both P1 and P2):

Alice        Wireless router          Bob    
  |    --P1-->      |                 |    
  |                 |     <--P2--     |    
  | <--P1 XOR P2--- | ---P1 XOR P2--> | (multicast or broadcast)
  |                 |                 |
recover P2                     Recover P1

Idea 8-

"One can use FEC and NC in a congestion friendly manner"

• only stupid persons will further overload a congested network with even more redundant traffic in the hope it may help!