LTP protocol definition has two fields that shouldn't be there

[tadkollar]

Brief description

The Scapy LTP protocol implementation should not contain ConditionalFields CancelAckToBlockSender and CancelAckToBlockReceiver.

Scapy version

2.6.1

Python version

3.12

Operating system

Ubuntu 22.04

Additional environment information

RFC 5326, section 3.2.4 states that "The Cancel-acknowledgments (CAx) have no content." However, ConditionalFields CancelAckToBlockSender and CancelAckToBlockReceiver are defined for segments (scapy/contrib/ltp.py lines 176-179) with flags values 13 and 15 (cancel acknowledgement segments).

The result of this is:

• An extra byte is appended when that type of segment is created
• Scapy is unable to decode proper received CAx segments due to that byte missing.

How to reproduce

from scapy.contrib.ltp import LTP
from scapy.all import raw

cancel_segment = LTP(flags = 13, SessionOriginator = 1, SessionNumber = 2)
cancel_segment.show2()
print(f"Length of segment: {len(raw(cancel_segment))}")

Actual result

###[ LTP ]###
  version   = 0
  flags     = 0xD Cancel-acknowledgment segment to block sender
  SessionOriginator= 1
  SessionNumber= 2
  HeaderExtensionCount= 0
  TrailerExtensionCount= 0
  \HeaderExtensions\
  CancelAckToBlockSender= 0
  \TrailerExtensions\

Length of segment: 5

Expected result

###[ LTP ]###
  version   = 0
  flags     = 0xD Cancel-acknowledgment segment to block sender
  SessionOriginator= 1
  SessionNumber= 2
  HeaderExtensionCount= 0
  TrailerExtensionCount= 0
  \HeaderExtensions\
  \TrailerExtensions\

Length of segment: 4

Related resources

https://www.rfc-editor.org/rfc/rfc5326.html#page-20

[gpotter2]

Thanks for the report.
Would you by any chance be able to share a pcap? We sadly generally don't have much tests for LTP
Thanks

[satveerbrar]

If I understood correctly. Removing the Conditional Field for Cancel ACK's(Flag 13 and 15) going to fix this issue.

[tadkollar]

Yes, that's correct.

Sorry, I haven't had a chance to send a pcap. We're developing a test framework for DTN implementations that uses LTP and includes full Scapy-based implementations of BPv6, BPv7, BPSec, and other DTN-related protocols. It will eventually be released as open-source but has to be reviewed and approved first.