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The Network Simulator ns-3: Frequently Asked Questions (FAQ)

This wiki page is devoted to questions for users of the simulator. There is a similar Developer_FAQ page for ns-3 software developers and maintainers.

What is the difference between ns-2 and ns-3?

ns-2 is a popular discrete-event network simulator developed under several previous research grants and activities. It remains in active use and will continue to be maintained.

ns-3 is a new software development effort focused on improving upon the core architecture, software integration, models, and educational components of ns-2. The project commenced in July 2006 and there have not yet been any releases (pre-alpha stage).

If you are looking for a simulator to currently use for research, please use ns-2.

Will ns-2 scripts run on ns-3?

No. ns-2 uses OTcl as its scripting environment. ns-3 will use C++ programs or python scripts to define simulations.

I found a bug in ns-3. How do I report it?

Please consult this page. Also, please consider fixing the bug, not just reporting it :)


merging a separate repository with a main repository

Q. I want to incorporate some code from a mercurial branch (repository) that is off the main repository.

A. (from Mathieu Lacage)

You can create a local clone of the repository and pull into it from the main repository: you can push these yourself by creating a local clone of that repo and then pushing it back to the main repo. It will keep the original history; e.g.:

hg clone
cd ns-3-ptr
# merge from main ns-3 repository into this other repository
hg pull
hg merge

Pushing this new merged repository into another repository will then keep the original change history:

# push back-- must have privileges to push to the repository
hg push ssh://

working with subsets of repositories

Q. Is there a way with mercurial to pull/merge/push subsets of the repositories? For instance, suppose I wanted only to grab the samples directory from a repo and merge that alone.

A. (from Mathieu Lacage) You cannot cherry-pick individual changesets other than by exporting them to patches first. The reason is that each Changeset contains a single hash which is, among others, based on the hash of the 'parent' changeset hash. So, if you 'reparent' a changeset, you change its hash id because the id of the parent changes. If the hash id of a changeset changes, it is a different changeset so, mercurial cannot really manage these individual changesets.

So, the idea is that merging happens from one repo to another, and cannot be done on a sub-dir basis. The other alternatives are:

1) the latest mercurial has support for in-repo branches: you can maintain multiple branches within a single repository but each branch represents a branch for the whole repository.

2) the latest mercurial contains the 'forest' extension which is a way to make mercurial manage a 'forest' of repos: each repo is independent but they also behave as a single big repo through the forest extension.

Scons (build process)

Library not loaded error

Question: On Apple osX, I built ns-3 successfully, but when I try to run a sample program I get something like:

> ./sample-packet
dyld: Library not loaded: build-dir/dbg-shared/lib/libcommon.dylib
  Referenced from: /tmp/ns-3/build-dir/dbg-shared/bin/./sample-packet
  Reason: image not found
Trace/BPT trap

Answer: This is because the library path variable of your environment does not include the location where the ns-3 modules (libraries) are located.

In OS X, you need to set the DYLD_LIBRARY_PATH variable to include the library directory. These need to include the lib/ directory where these ns-3 libraries are built. On Linux and Solaris systems, you should never warnings of this kind. If you do, please, report a bug.

In this example, setting DYLD_LIBRARY_PATH=/tmp/ns-3/build-dir/dbg-shared/lib/ will work.

How do I do the equivalent of a "make clean"?

scons -c