[Ndn-interest] NDN Protocol Design Principles

Ignacio.Solis at parc.com Ignacio.Solis at parc.com
Fri Mar 11 00:48:32 PST 2016

>[1] **Universality**:
>    NDN should be a common network protocol for all applications and network environments.

I’m assuming here that you imply a set of protocols. After all, the way you do flow control varies a lot from DTN to non-DTN environments. Or are you limiting this to the basic packet format?

On the web page it calls out for DTN, IoT, Mesh networks, Web, etc.  It seems obvious to me that these require various protocols; potentially layered or interoperating in some fashion.  

If one interprets this principle as “the base protocol will allow…”, then the question becomes how does the base protocol allow this.

The document states that the protocol should be flexible and extensible. It then states that there should be no fixed parts or fixed length fields in the header.  I’m unsure what the full rationale is here. Is it that we will exceed the length of the fields? Or that suddenly one field will become obsolete and we will want to remove it?  Is it because we don’t want to make assumptions about packets coming in?  We want them to be so flexible that anything is possible?

In one way or another, we’re making assumptions about the packet coming in.  If it’s not the static header, it’s the fact that the way we parse the packet has to be consistent.  (As in, the first bit determines if the next byte is a continuation of the first byte, etc).   So, to some degree, you are fixing some formatting.

In one assumption, (fixed header), we are limiting the format and semantics of the first n bytes (8 or 16 or whatever). In the other space, you’re limiting the format (but maybe not the semantics of the first n bytes).  You trade off some processing for some byte inefficiency.

The fact that we want the same protocol to work on IoT, datacenter and interplanetary networks effectively says:
- The benefits we get from optimizing for each of these scenarios are less important than the advantage we get by having a single protocol format that runs in all.

It seems to me that having a single protocol is useful when traffic goes from one to the other.  If we don’t expect traffic to go from one to the other the benefit diminishes.  Here I’m assuming native network compatible traffic. So that one packet, comes from IoT and unmodified goes to the datacenter and unmodified goes to Mars.  As opposed to having gateways that can speak multiple protocols or tunneling and overlays.

To calibrate, I’m in the camp of thinking that PIM-SM and PIM-DM are different protocols. So I would also side with the fact that if you had a situation where packets on one end of the gateway use NDN but are not understood at the other side of the gateway then the packets are effectively from 2 different protocols.

I would also argue that if the purpose of building an extremely flexible format is to save bytes in one scenario where those are not needed at the expense of processing, then I would say that you might as well go all the way and just implement some type of link-layer compression scheme and really save some bytes. Dictionaries can go a long way.

Finally, going back to what’s on the page.  When you say that there should be no fixed parts and no fixed length fields, do you really mean it?
- Can I have a packet with no packet_size field?
- Can I have a packet with no protocol version field?
- Can I have a packet with no name?

- Can I have a packet with an arbitrary length name?
- Can I have a packet with an arbitrary size payload?
- Can I have a packet with 3 payloads?
- Can I have a packet with 2 names?
- Can I have a packet with no signature?
- Must all nodes support all of these packet types?
- Can I have an arbitrary sized T in a TLV?  (I’m assuming you’re proposing TLVs, but maybe you’re planning on using XML or json for flexibility.)
- Can I have an arbitrary sized L?

To preempt the discussion, what part of this is architecture and what part is policy?


Nacho (Ignacio) Solis
Protocol Architect
Principal Scientist
Palo Alto Research Center (PARC)
Ignacio.Solis at parc.com

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