Licklider Transmission Protocol (LTP)

1
Licklider Transmission Protocol (LTP)

• A point-to-point protocol for DTNs
• Think of it as somewhere from layer 2 up to maybe
  layer 4!
• LTP is highly stateful
• Needed to avoid negotiation exchanges
• Primarily aimed at deep space DTNs but can also
  (I claim!) be used for many terrestrial cases
• Encoding is terse and binary

2
LTP layering

• LTP runs on top of some MAC layer or deep space
  physical layer
• LTP assumes lower layer cues are provided so
  that some infrastructure (e.g. ephemeris handler
  scheduler) tells the stack when to expect to
  receive or transmit with a given peer.

3
LTP Segment

• Bit 0 1 2 3 4
  5 6 7
• --------------------------
  --------------
• Version number
  Segment Type Flags
• ------------------------------
  ----------------
• 
  
• / Session ID
  \
• \
  /
• Header ------------------------------
  ----------------
• Header Extension Cnt.
  Trailer Extension Cnt.
• ------------------------------
  ----------------
• 
  
• / Header
  Extensions \
• \
  /
• V -------------------------------
  ----------------
• 
  
• 
  
• 
  
• Segment Content
  
• /
  \

4
LTP Encoding

• Many fields use so-called Self-Delimiting Numeric
  Values (SDNV)
• Also used in bundling protocol
• Based on ASN.1 BER encoding of OID arcs
• Might change in future
• But so long as the same thing's used everywhere
  that's ok

5
LTP features

• Sessions/Segments
• A single block is sent per session using
  multiple segments
• Segment size is limited by the underlying MTU
• Session-ID is src-ID number
• Recommended to use a (P)RNG for the number
• Red/green parts
• Data is ACKed (red) or not (green)
• Not ACKing is easier, but doesn't fulfill all
  appn. Requirements
• Red part first (if any), then green (if any)
• Each segment in a session is entirely red or
  entirely green

6
Red/Green motivation

• Lots of science data formats (e.g. images) put
  important information (e.g. codec, timing,
  orientation) at the start, followed by lots of
  less important detail (pixels)
• Loose the codec information and the rest is
  useless
• Losing a pixel or two (hundred) isn't that bad
• Want to have ACKs for the start of the block but
  not the entire block
• Caller or config. determines which, if any,
  segments are red. Others are green.

7
LTP segment types

• Data segment (tx-gtrx)
• Can be Red or Green
• Report segment (rx-gttx)
• Report ACK segment (tx-gtrx)
• Cancel segments (tx/rx differ)
• Required for loopback
• Cancel ACK segments

8
Data segments

• Subset of block's bytes (offset, length)
• Flags can indicate
• End of Red Part
• End of Block
• EORP and EOB require report (ACK)
• Unless zero red bytes

9
Reports

• Each report contains a set of scopes, each of
  which informs the transmitter that some
  (red)bytes have arrived safely
• Report segment has a serial number (RSN)
• Report segments always ACKed using RSN
• Wrinkle reporting the full state might require
  gt1 rx-gttx MTU so a single report might need gt1
  RS
• Spec. has some hyper-efficient ways to handle
  this for deep space reasons
• Also allows for more full reporting where
  bandwidth less constrained

10
Retransmission

• Re-txing based on timeouts and counters
• If EORP/EOB sent but no ACK (RS)
• If RS sent but no RA
• If Cx sent but no CAx
• Timers use punctuated time
• Only decremented during periods when data could
  actually be arriving from the peer
• E.g. Not decremented while peer is turned off

11
Denial of Service

• LTP nodes can be very badly affected by DoS
• Took a couple of weeks to reboot MER Spirit when
  OS had no file handles left
• Anti-DoS features
• Random bits recommended in Session ID
• Recommended random bits in report serial numbers
• Initial version had both being guessable and so
  potential targets for off-path attacks
• Like TCP SYN flooding
• Replay detection algorithm (sample)
• Cookies (next slide)

12
LTP extensions

• Both header and trailer extensions allowed
• LTP authentication extension
• Header ciphersuite
• Trailer MAC/Signature
• LTP cookie extension
• DoS would be very bad for a deep space host
• Cookies aiming to protect against off-path
  attacks using a header extension, once turned on,
  only segments with cookie accepted

13
LTP status

• LTP specified in 3 drafts
• draft-irtf-dtnrg-ltp-03.txt main spec.
• draft-irtf-dtnrg-ltp-motivation-01 reasons for
  stuff
• Draft-irtf-dtnrg-ltp-extensions-01 extensions
• Main spec and motivation are just about ready for
  last call
• Extensions a little later (sync. security stuff)
• At least two implementations I know of