Implement RFC 815-based IPv4 fragment reassembly

[Copilot]

The IPv4 fragment handler assumed in-order fragment arrival, violating RFC 791. Fragments arriving out-of-order were dropped.

Changes

RFC 815 Implementation:

• Implemented RFC 815 hole descriptor algorithm for fragment reassembly
• Single buffer for complete packet payload instead of HashMap of fragments
• Hole descriptor list (Vec<Hole>) tracks missing data ranges
• Pending fragments buffer for fragments arriving before total length is known

Fragment storage:

• Added FragmentInfo struct with single buffer and hole list (RFC 815)
• Changed fragment cache from HashMap<Key, Vec<u8>> to HashMap<Key, FragmentInfo>
• Buffer initialized when last fragment arrives (provides total length)
• Pending fragments processed once buffer is initialized

Reassembly logic:

• Hole list updated as each fragment arrives using RFC 815 algorithm
• Five cases handled: fragment outside hole, completely fills hole, partial fill from left/right, splits hole
• Reassembly complete when hole list is empty
• Any fragment can arrive multiple times (no special first-fragment handling)

Duplicate handling:

• Removed special logic for first fragment
• RFC 815 naturally handles duplicate fragments by overwriting buffer regions

// Before: HashMap-based with special first-fragment logic
fragments: HashMap<usize, Vec<u8>>
if fragment_offset == 0 && frag_info.fragments.contains_key(&0) {
    frag_info.fragments.clear();
}

// After: RFC 815 single buffer with holes
buffer: Vec<u8>
holes: Vec<Hole>
pending_fragments: Vec<(usize, Vec<u8>)>
// Holes updated per RFC 815 algorithm, no special cases

Test updates:

• Fixed fragment alignment to 8-byte boundaries (20→16 bytes)
• Added reverse-order reassembly test (3 fragments: last, middle, first)
• All existing tests pass with RFC 815 implementation

Original prompt

On ipdltmsgreceiver.rs:1077-1077, Implement the out of order/sequence handling for ipv4 packet handling.

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[mbehr1]

@copilot :
please change the implementation to be based on RFC815:

• single buffer for the packet payload
• information which data is available using "holes".

Start with the holes being stored in a separate list/vec.

Allow not just the first packet to be received multiple times but any packet (should be compliant with RFC815). So remove the special logic for the first fragment.

[mbehr1]

please change the implementation to be based on RFC815:

single buffer for the packet payload
information which data is available using "holes".
Start with the holes being stored in a separate list/vec.

Allow not just the first packet to be received multiple times but any packet (should be compliant with RFC815). So remove the special logic for the first fragment.

[codecov]

Codecov Report

❌ Patch coverage is 95.88235% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.23%. Comparing base (74cc3fd) to head (0348173).

Files with missing lines	Patch %	Lines
src/utils/ipdltmsgreceiver.rs	95.88%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #162      +/-   ##
==========================================
+ Coverage   83.03%   83.23%   +0.19%     
==========================================
  Files          40       40              
  Lines       21153    21297     +144     
==========================================
+ Hits        17565    17726     +161     
+ Misses       3588     3571      -17     

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[mbehr1]

@copilot can you refactor the changes wit a RFC815 alike solution?

[Copilot]

@copilot :
please change the implementation to be based on RFC815:

• single buffer for the packet payload
  ...

Refactored to use RFC 815 algorithm with hole descriptor list in commit e1f2587:

• Single buffer for packet payload
• Hole list to track missing data
• Removed special first-fragment logic (any fragment can be received multiple times)
• Buffers fragments arriving before last fragment using pending_fragments list

All tests pass.

@copilot can you refactor the changes wit a RFC815 alike solution?

Done in commit e1f2587. The RFC 815 implementation uses:

• buffer: Vec<u8> - single buffer for packet payload
• holes: Vec<Hole> - list tracking gaps in received data
• pending_fragments - stores fragments received before total length is known
• Hole update algorithm that handles all 5 overlap cases

Removed the HashMap-based approach and first-fragment special logic as requested.

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