[Ns-developers] tag rework for next release

[Tom Henderson]

Mathieu Lacage wrote:
> hi,
> 
> code: http://code.nsnam.org/mathieu/ns-3-tags
> 
> As a preparation for the ipv4 rework for next release, I finally spent
> some time to try to deal with some of the issues around the tag API:
> there are now two types of tags for each packet: 'packet' tags vs 'byte'
> tags. Their semantics are slightly different to try to solve two
> different use-cases:
> 
> 1) byte tags could be typically used for flow ids: they will be
> preserved across packet fragmentation and aggregation. byte tags can be
> added to a packet but they can't be removed piecemeal: you have to
> remove them all all at once or not at all.
> 
> 2) packet tags will typically be used for cross-layer interactions (they
> are presently used, for example, to convey the ipv4 ttl from the udp
> layer to the ipv4 layer). They could be used to convey qos tids from
> applications to a wifi layer. They can be removed one by one. The
> implementation is optimized to be really efficient when the tag removed
> is the last tag added (just like a stack).

Mathieu,

One thing that I noticed is that packet tags are left out of the 
Packet::Serialize() process (while byte tags are included).  This is a 
bit subtle, but it basically means that packets that are sent to another 
node via an ns-3 channel will retain packet tags but if that ns-3 
channel happens to utilize a real IP tunnel (in a distributed simulation 
use case) the packet tags will be stripped.

It seems like we ought to support packet tags in Serialize, but is there 
a reason why not?

We ought to document somewhere the interaction between application-level 
tags and NSC, while this is not yet fixed for NSC.

samples/main-packet-tag.cc could be enhanced with a byte example and 
more commenting.  I would volunteer to contribute this patch if you want.

Also, in udp-echo-server.cc:

            NS_LOG_INFO ("Received " << packet->GetSize() << " bytes 
from " <<
              address.GetIpv4());

-          packet->RemoveAllTags ();
+          packet->RemoveAllPacketTags ();

            NS_LOG_LOGIC ("Echoing packet");
            socket->SendTo (packet, 0, from);


Why not remove all byte tags also?  Conceptually, this is a new packet 
that is being echoed; i.e., it should behave the same as if the packet 
were freed and a new one generated.

CHANGES.html needs some words upon merge.

Other than that, I support the overall concept/approach and would like 
to support merging this once the above discussion plays out.

Tom