Fix incorrect layer bindings in LDP contrib module

[mohamedsolaiman]

Found a bug while testing LDP discovery over UDP. The bind_bottom_up calls for UDP were wrong:

# before (broken)
bind_bottom_up(TCP, UDP, sport=646)
bind_bottom_up(TCP, UDP, dport=646)

# after (fixed)
bind_bottom_up(UDP, LDP, sport=646)
bind_bottom_up(UDP, LDP, dport=646)

This meant LDP packets received over UDP on port 646 (used for LDP discovery/hello messages) would not be properly dissected. The old code was binding TCP to UDP which doesnt make any sense. LDP discovery uses UDP port 646, sessions use TCP port 646.

Added a test case to verify UDP-based LDP dissection works.

[gpotter2]

Absolutely, thanks for catching this.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.27%. Comparing base (1e75eb3) to head (49b4913).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4985      +/-   ##
==========================================
- Coverage   80.28%   80.27%   -0.01%     
==========================================
  Files         383      383              
  Lines       94685    94685              
==========================================
- Hits        76015    76010       -5     
- Misses      18670    18675       +5     

Files with missing lines	Coverage Δ
scapy/contrib/ldp.py	86.12% <100.00%> (ø)

... and 7 files with indirect coverage changes

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