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Network Working Group S. Burleigh
Request for Comments: 5325 NASA/Jet Propulsion Laboratory
Category: Informational M. Ramadas
ISTRAC, ISRO
S. Farrell
Trinity College Dublin
September 2008
Licklider Transmission Protocol - Motivation
Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
IESG Note
This RFC is not a candidate for any level of Internet Standard. It
represents the consensus of the Delay Tolerant Networking (DTN)
Research Group of the Internet Research Task Force (IRTF). See RFC
3932 for more information.
Abstract
This document describes the motivation for the development of the
Licklider Transmission Protocol (LTP) designed to provide
retransmission-based reliability over links characterized by
extremely long message round-trip times (RTTs) and/or frequent
interruptions in connectivity. Since communication across
interplanetary space is the most prominent example of this sort of
environment, LTP is principally aimed at supporting "long-haul"
reliable transmission in interplanetary space, but it has
applications in other environments as well.
In an Interplanetary Internet setting deploying the Bundle protocol,
LTP is intended to serve as a reliable convergence layer over
single-hop deep-space radio frequency (RF) links. LTP does Automatic
Repeat reQuest (ARQ) of data transmissions by soliciting selective-
acknowledgment reception reports. It is stateful and has no
negotiation or handshakes.
This document is a product of the Delay Tolerant Networking Research
Group and has been reviewed by that group. No objections to its
publication as an RFC were raised.
Burleigh, et al. Experimental [Page 1]
RFC 5325 LTP - Motivation September 2008
Table of Contents
1. Introduction ....................................................2
2. Problem .........................................................3
2.1. IPN Operating Environment ..................................3
2.2. Why Not TCP or SCTP? .......................................5
3. Protocol Overview ...............................................6
3.1. Nominal Operation ..........................................6
3.1.1. Link State Cues .....................................9
3.1.2. Deferred Transmission ...............................9
3.1.3. Timers .............................................10
3.2. Retransmission ............................................13
3.3. Accelerated Retransmission ................................16
3.4. Session Cancellation ......................................17
4. Security Considerations ........................................17
5. IANA Considerations ............................................20
6. Acknowledgments ................................................20
7. References .....................................................20
7.1. Informative References ....................................20
1. Introduction
The Licklider Transmission Protocol (LTP) is designed to provide
retransmission-based reliability over links characterized by
extremely long message round-trip times and/or frequent interruptions
in connectivity. Communication in interplanetary space is the most
prominent example of this sort of environment, and LTP is principally
aimed at supporting "long-haul" reliable transmission over deep-space
RF links. Specifically, LTP is intended to serve as a reliable
"convergence layer" protocol, underlying the Delay-Tolerant
Networking (DTN) [DTN] Bundle protocol [BP], in DTN deployments where
data links are characterized by very long round-trip times.
This document describes the motivation for LTP, its features,
functions, and overall design. It is part of a series of documents
describing LTP. Other documents in the series include the main
protocol specification document [LTPSPEC] and the protocol extensions
document [LTPEXT].
The protocol is named in honor of ARPA/Internet pioneer JCR
Licklider.
Burleigh, et al. Experimental [Page 2]
RFC 5325 LTP - Motivation September 2008
2. Problem
2.1. IPN Operating Environment
There are a number of fundamental differences between the environment
for terrestrial communications (such as seen in the Internet) and the
operating environments envisioned for the Interplanetary Internet
(IPN) [IPN].
The most challenging difference between communication among points on
Earth and communication among planets is round-trip delay, of which
there are two main sources, both relatively intractable: physics and
economics.
The more obvious type of delay imposed by nature is signal
propagation time. Round-trip times between Earth and Jupiter's moon
Europa, for example, run between 66 and 100 minutes.
Less obvious and more dynamic is the delay imposed by occultation.
Communication between planets must be by radiant transmission, which
is usually possible only when the communicating entities are in line
of sight of each other. During the time that communication is
impossible, delivery is impaired and messages must wait in a queue
for later transmission.
Round-trip times and occultations can at least be readily computed
given the ephemerides of the communicating entities. Additional
delay that is less easily predictable is introduced by discontinuous
transmission support, which is rooted in economics.
Communicating over interplanetary distances requires expensive
special equipment: large antennas, high-performance receivers, etc.
For most deep-space missions, even non-NASA ones, these are currently
provided by NASA's Deep Space Network (DSN) [DSN]. The communication
resources of the DSN are currently oversubscribed and will probably
remain so for the foreseeable future. Radio contact via the DSN must
therefore be carefully scheduled and is often severely limited.
This over-subscription means that the round-trip times experienced by
packets will be affected not only by the signal propagation delay and
occultation, but also by the scheduling and queuing delays imposed by
the management of Earth-based resources: packets to be sent to a
given destination may have to be queued until the next scheduled
contact period, which may be hours, days, or even weeks away.
Burleigh, et al. Experimental [Page 3]
RFC 5325 LTP - Motivation September 2008
These operating conditions imply a number of additional constraints
on any protocol designed to ensure reliable communication over deep-
space links.
- Long round-trip times mean substantial delay between the
transmission of a block of data and the reception of an
acknowledgment from the block's destination, signaling arrival of
the block. If LTP postponed transmission of additional blocks of
data until it received acknowledgment of the arrival of all prior
blocks, valuable opportunities to utilize what little deep-space
transmission bandwidth is available would be forever lost.
Multiple parallel data block transmission "sessions" must be in
progress concurrently in order to avoid under-utilization of the
links.
- Like any reliable transport service employing ARQ, LTP is
"stateful". In order to ensure the reception of a block of data it
has sent, LTP must retain for possible retransmission all portions
of that block that might not have been received yet. In order to
do so, it must keep track of which portions of the block are known
to have been received so far and which are not, together with any
additional information needed for purposes of retransmitting part
or all of that block.
- In the IPN, round-trip times may be so long and communication
opportunities so brief that a negotiation exchange, such as an
adjustment of transmission rate, might not be completed before
connectivity is lost. Even if connectivity is uninterrupted,
waiting for negotiation to complete before revising data
transmission parameters might well result in costly under-
utilization of link resources.
- Another respect in which LTP differs from TCP is that, while TCP
connections are bidirectional (blocks of application data may be
flowing in both directions on any single connection), LTP sessions
are unidirectional. This design decision derives from the fact
that the flow of data in deep-space flight missions is usually
unidirectional. (Long round-trip times make interactive spacecraft
operation infeasible, so spacecraft are largely autonomous and
command traffic is very light.) Bidirectional data flow, where
possible, is performed using two unidirectional links in opposite
directions and at different data rates.
Burleigh, et al. Experimental [Page 4]
RFC 5325 LTP - Motivation September 2008
- Finally, the problem of timeout interval computation in the
environment for which LTP is mainly intended is different from the
analogous problem in the Internet. Since multiple sessions can be
conducted in parallel, retardation of transmission on any single
session while awaiting a timeout need not degrade communication
performance on the association as a whole. Timeout intervals that
would be intolerably optimistic in TCP don't necessarily degrade
LTP's bandwidth utilization.
But the reciprocal half-duplex nature of LTP communication makes it
infeasible to use statistical analysis of round-trip history as a
means of predicting round-trip time. The round-trip time for
transmitted segment N could easily be orders of magnitude greater
than that for segment N-1 if there happened to be a transient loss
of connectivity between the segment transmissions. A different
mechanism for timeout interval computation is needed.
2.2. Why Not TCP or SCTP?
These environmental characteristics -- long and highly variable
delays, intermittent connectivity, and relatively high error rates --
make using unmodified TCP for end-to-end communications in the IPN
infeasible. Using the TCP throughput equation from [TFRC] we can
calculate the loss event rate (p) required to achieve a given steady-
state throughput. Assuming the minimum RTT to Mars from planet Earth
is 8 minutes (one-way speed of light delay to Mars at its closest
approach to Earth is 4 minutes), assuming a packet size of 1500
bytes, assuming that the receiver acknowledges every other packet,
and ignoring negligible higher-order terms in p (i.e., ignoring the
second additive term in the denominator of the TCP throughput
equation), we obtain the following table of loss event rates required
to achieve various throughput values.
Throughput Loss event rate (p)
---------- -------------------
10 Mbps 4.68 * 10^(-12)
1 Mbps 4.68 * 10^(-10)
100 Kbps 4.68 * 10^(-8)
10 Kbps 4.68 * 10^(-6)
Note that although multiple losses encountered in a single RTT are
treated as a single loss event in the TCP throughput equation [TFRC],
such loss event rates are still unrealistic on deep-space links.
For the purposes of this discussion, we are not considering the more
aggressive TCP throughput equation that characterizes HighSpeed TCP
[HSTCP].
Burleigh, et al. Experimental [Page 5]
RFC 5325 LTP - Motivation September 2008
The TCP characteristic of an initial three-way handshake for each new
connection, followed by slow-start, is a further obstacle, because
the delay of the three-way handshake and the additional delay of
slow-start could be exorbitant in a long-delay environment.
The Stream Control Transmission Protocol (SCTP) [SCTP] can multiplex
"chunks" (units of application data) for multiple sessions over a
single-layer connection (called an 'association' in SCTP terminology)
as LTP does, but it still requires multiple round trips prior to
transmitting application data for session setup and so clearly does
not suit the needs of the IPN operating environment.
3. Protocol Overview
3.1. Nominal Operation
The nominal sequence of events in an LTP transmission session is as
follows.
Operation begins when a client service instance asks an LTP engine to
transmit a block of data to a remote client service instance.
LTP regards each block of data as comprising two parts: a "red-part",
whose delivery must be assured by acknowledgment and retransmission
as necessary, followed by a "green-part" whose delivery is attempted,
but not assured. The length of either part may be zero; that is, any
given block may be designated entirely red (retransmission continues
until reception of the entire block has been asserted by the
receiver) or entirely green (no part of the block is acknowledged or
retransmitted). Thus, LTP can provide both TCP-like and UDP-like
functionality concurrently on a single session.
Note that in a red-green block transmission, the red-part data does
NOT have any urgency or higher-priority semantics relative to the
block's green-part data. The red-part data is merely data for which
the user has requested reliable transmission, possibly (though not
necessarily) data without which the green-part data may be useless,
such as an application-layer header or other metadata.
The client service instance uses the LTP implementation's application
programming interface to specify to LTP the identity of the remote
client service instance to which the data must be transmitted, the
location of the data to be transmitted, the total length of data to
be transmitted, and the number of leading data bytes that are to be
transmitted reliably as "red" data. The sending engine starts a
transmission session for this block and notifies the client service
instance that the session has been started. Note that
Burleigh, et al. Experimental [Page 6]
RFC 5325 LTP - Motivation September 2008
LTP communication session parameters are not negotiated but are
instead asserted unilaterally, subject to application-level network
management activity; the sending engine does not negotiate the start
of the session with the remote client service instance's engine.
The sending engine then initiates the original transmission: it
queues for transmission as many data segments as are necessary to
transmit the entire block, within the constraints on maximum segment
size imposed by the underlying communication service. The last
segment of the red-part of the block is marked as the end of red-part
(EORP) indicating the end of red-part data for the block, and as a
checkpoint (identified by a unique checkpoint serial number)
indicating that the receiving engine must issue a reception report
upon receiving the segment. The last segment of the block overall is
marked end of block (EOB) indicating that the receiving engine can
calculate the size of the block by summing the offset and length of
the data in the segment.
LTP is designed to run directly over a data-link layer protocol, but
it may instead be deployed directly over UDP in some cases (i.e., for
software development or in private local area networks). In either
case, the protocol layer immediately underlying LTP is here referred
to as the "local data-link layer".
At the next opportunity, subject to allocation of bandwidth to the
queue into which the block data segments were written, the enqueued
segments and their lengths are passed to the local data-link layer
protocol (which might be UDP/IP) via the API supported by that
protocol, for transmission to the LTP engine serving the remote
client service instance.
A timer is started for the EORP, so that it can be retransmitted
automatically if no response is received.
The content of each local data-link layer protocol data unit (link-
layer frame or UDP datagram) is required to be an integral number of
LTP segments, and the local data-link layer protocol is required
never to deliver incomplete LTP segments to the receiving LTP engine.
When the local data-link layer protocol is UDP, the LTP
authentication [LTPEXT] extension should be used to ensure data
integrity unless the end-to-end path is one in which either the
likelihood of datagram content corruption is negligible (as in some
private local area networks) or the consequences of receiving and
processing corrupt LTP segments are insignificant (as perhaps during
software development). When the LTP authentication extension is not
Burleigh, et al. Experimental [Page 7]
RFC 5325 LTP - Motivation September 2008
used, LTP requires the local data-link layer protocol to perform
integrity checking of all segments received; specifically, the local
data-link layer protocol is required to detect any corrupted segments
that are received and to discard them silently.
Received segments that are not discarded are passed up to the
receiving LTP engine via the API supported by the local data-link
layer protocol.
On reception of the first data segment for the block, the receiving
engine starts a reception session for this block and notifies the
local instance of the relevant client service that the session has
been started. In the nominal case, it receives all segments of the
original transmission without error. Therefore, on reception of the
EORP data segment, it responds by (a) queuing for transmission to the
sending engine a report segment indicating complete reception and (b)
delivering the received red-part of the block to the local instance
of the client service: on reception of each data segment of the
green-part, it responds by immediately delivering the received data
to the local instance of the client service.
All delivery of data and protocol event notices to the local client
service instance is performed using the LTP implementation's
application programming interface.
Note that since LTP data flows are unidirectional, LTP's data
acknowledgments -- "reception reports" -- can't be piggybacked on
data segments as in TCP. They are instead carried in a separate
segment type.
At the next opportunity, the enqueued report segment is immediately
transmitted to the sending engine and a timer is started so that the
report segment can be retransmitted automatically if no response is
received.
The sending engine receives the report segment, turns off the timer
for the EORP, enqueues for transmission to the receiving engine a
report-acknowledgment segment, and notifies the local client service
instance that the red-part of the block has been successfully
transmitted. The session's red-part transmission has now ended.
At the next opportunity, the enqueued report-acknowledgment segment
is immediately transmitted to the receiving engine.
The receiving engine receives the report-acknowledgment segment and
turns off the timer for the report segment. The session's red-part
reception has now ended and transmission of the block is complete.
Burleigh, et al. Experimental [Page 8]
RFC 5325 LTP - Motivation September 2008
3.1.1. Link State Cues
Establishing a communication link across interplanetary distances may
entail hardware configuration changes based on the presumed
operational state of the remote communicating entity, for example:
o orienting a directional antenna correctly;
o tuning a transponder to pre-selected transmission and/or
reception frequencies; and
o diverting precious electrical power to the transponder at the
last possible moment, and for the minimum necessary length of
time.
We therefore assume that the operating environment in which LTP
functions is able to pass information on the link status (termed
"link state cues" in this document) to LTP, telling it which remote
LTP engine(s) should currently be transmitting to the local LTP
engine and vice versa. The operating environment itself must have
this information in order to configure communication link hardware
correctly.
3.1.2. Deferred Transmission
Link state cues also notify LTP when it is and isn't possible to
transmit segments. In deep-space communications, at no moment can
there ever be any expectation of two-way connectivity. It is always
possible for LTP to be generating outbound segments -- in response to
received segments, timeouts, or requests from client services -- that
cannot immediately be transmitted. These segments must be queued for
transmission at a later time when a link has been established, as
signaled by a link state cue.
In concept, every outbound LTP segment is appended to one of two
queues -- forming a queue-set -- of traffic bound for the LTP engine
that is that segment's destination. One such traffic queue is the
"internal operations queue" of that queue set; the other is the
application data queue for the queue set. The de-queuing of a
segment always implies delivering it to the underlying communication
system for immediate transmission. Whenever the internal operations
queue is non-empty, the oldest segment in that queue is the next
segment de-queued for transmission to the destination; at all other
times, the oldest segment in the application data queue is the next
segment de-queued for transmission to the destination.
The production and enqueuing of a segment and the subsequent actual
transmission of that segment are in principle wholly asynchronous.
Burleigh, et al. Experimental [Page 9]
RFC 5325 LTP - Motivation September 2008
In the event that (a) a transmission link to the destination is
currently active and (b) the queue to which a given outbound segment
is appended is otherwise empty and (c) either this queue is the
internal operations queue or else the internal operations queue is
empty, the segment will be transmitted immediately upon production.
Transmission of a newly queued segment is necessarily deferred in all
other circumstances.
Conceptually, the de-queuing of segments from traffic queues bound
for a given destination is initiated upon reception of a link state
cue indicating that the underlying communication system is now
transmitting to that destination; i.e., the link to that destination
is now active. It ceases upon reception of a link state cue
indicating that the underlying communication system is no longer
transmitting to that destination; i.e., the link to that destination
is no longer active.
3.1.3. Timers
LTP relies on accurate calculation of expected arrival times for
report and acknowledgment segments in order to know when proactive
retransmission is required. If a calculated time were even slightly
early, the result would be costly unnecessary retransmission. On the
other hand, calculated arrival times may safely be several seconds
late: the only penalties for late timeout and retransmission are
slightly delayed data delivery and slightly delayed release of
session resources.
Since statistics derived from round-trip history cannot safely be
used as a predictor of LTP round-trip times, we have to assume that
round-trip timing is at least roughly deterministic -- i.e., that
sufficiently accurate RTT estimates can be computed individually in
real time from available information.
This computation is performed in two stages:
- We calculate a first approximation of RTT by simply doubling the
known one-way light time to the destination and adding an
arbitrary margin for any additional anticipated latency, such as
queuing and processing delay at both ends of the transmission.
For deep-space operations, the margin value is typically a small
number of whole seconds. Although such a margin is enormous by
Internet standards, it is insignificant compared to the two-way
Burleigh, et al. Experimental [Page 10]
RFC 5325 LTP - Motivation September 2008
light time component of round-trip time in deep space. We
choose to risk tardy retransmission, which will postpone
delivery of one block by a relatively tiny increment, in
preference to premature retransmission, which will unnecessarily
consume precious bandwidth and thereby degrade overall
performance.
- Then, to account for the additional delay imposed by interrupted
connectivity, we dynamically suspend timers during periods when
the relevant remote LTP engines are known to be unable to
transmit responses. This knowledge of the operational state of
remote entities is assumed to be provided by link state cues
from the operating environment.
The following discussion is the basis for LTP's expected arrival time
calculations.
The total time consumed in a single "round trip" (transmission and
reception of the original segment, followed by transmission and
reception of the acknowledging segment) has the following components:
- Protocol processing time: The time consumed in issuing the
original segment, receiving the original segment, generating and
issuing the acknowledging segment, and receiving the
acknowledging segment.
- Outbound queuing delay: The delay at the sender of the original
segment while that segment is in a queue waiting for
transmission, and delay at the sender of the acknowledging
segment while that segment is in a queue waiting for
transmission.
- Radiation time: The time that passes while all bits of the
original segment are being radiated, and the time that passes
while all bits of the acknowledging segment are being radiated.
(This is significant only at extremely low data transmission
rates.)
- Round-trip light time: The signal propagation delay at the speed
of light, in both directions.
- Inbound queuing delay: Delay at the receiver of the original
segment while that segment is in a reception queue, and delay at
the receiver of the acknowledging segment while that segment is
in a reception queue.
Burleigh, et al. Experimental [Page 11]
RFC 5325 LTP - Motivation September 2008
- Delay in transmission of the original segment or the
acknowledging segment due to loss of connectivity -- that is,
interruption in outbound link activity at the sender of either
segment due to occultation, scheduled end of tracking pass, etc.
In this context, where errors on the order of seconds or even minutes
may be tolerated, protocol processing time at each end of the session
is assumed to be negligible.
Inbound queuing delay is also assumed to be negligible because, even
on small spacecraft, LTP processing speeds are high compared to data
transmission rates.
Two mechanisms are used to make outbound queuing delay negligible:
- The expected arrival time of an acknowledging segment is not
calculated until the moment the underlying communication system
notifies LTP that radiation of the original segment has begun.
All outbound queuing delay for the original segment has already
been incurred at that point.
- LTP's deferred transmission model minimizes latency in the
delivery of acknowledging segments (reports and acknowledgments)
to the underlying communication system. That is, acknowledging
segments are (in concept) appended to the internal operations
queue rather than the application data queue, so they have
higher transmission priority than any other outbound segments,
i.e., they should always be de-queued for transmission first.
This limits outbound queuing delay for a given acknowledging
segment to the time needed to de-queue and radiate all
previously generated acknowledging segments that have not yet
been de-queued for transmission. Since acknowledging segments
are sent infrequently and are normally very small, outbound
queuing delay for a given acknowledging segment is likely to be
minimal.
Deferring calculation of the expected arrival time of the
acknowledging segment until the moment at which the original segment
is radiated has the additional effect of removing from consideration
any original segment transmission delay due to loss of connectivity
at the original segment sender.
Radiation delay at each end of the session is simply segment size
divided by transmission data rate. It is insignificant except when
the data rate is extremely low (for example, 10 bps), in which case
the use of LTP may well be inadvisable for other reasons (LTP header
overhead, for example, could be too much under such data rates).
Therefore, radiation delay is normally assumed to be negligible.
Burleigh, et al. Experimental [Page 12]
RFC 5325 LTP - Motivation September 2008
We assume that one-way light time to the nearest second can always be
known (for example, provided by the operating environment).
So the initial expected arrival time for each acknowledging segment
is typically computed as simply the current time at the moment that
radiation of the original segment begins, plus twice the one-way
light time, plus 2*N seconds of margin to account for processing and
queuing delays and for radiation time at both ends. N is a parameter
set by network management for which 2 seconds seem to be a reasonable
default value.
This leaves only one unknown, the additional round-trip time
introduced by loss of connectivity at the sender of the acknowledging
segment. To account for this, we again rely on external link state
cues. Whenever interruption of transmission at a remote LTP engine
is signaled by a link state cue, we suspend the countdown timers for
all acknowledging segments expected from that engine. Upon a signal
that transmission has resumed at that engine, we resume those timers
after (in effect) adding to each expected arrival time the length of
the timer suspension interval.
3.2. Retransmission
Loss or corruption of transmitted segments may cause the operation of
LTP to deviate from the nominal sequence of events described above.
Loss of one or more red-part data segments other than the EORP
segment triggers data retransmission: the receiving engine returns a
reception report detailing all the contiguous ranges of red-part data
received (assuming no discretionary checkpoints were received, which
are described below). The reception report is normally sent in a
single report segment that carries a unique report serial number and
the scope of red-part data covered. For example, if the red-part
data covered block offsets [0:1000] and all but the segment in range
[500:600] were received, the report segment with a unique serial
number (say 100) and scope [0:1000] would carry two report entries:
(0:500) and (600:1000). The maximum size of a report segment, like
all LTP segments, is constrained by the data-link MTU; if many non-
contiguous segments were lost in a large block transmission and/or
the data-link MTU was relatively small, multiple report segments need
to be generated. In this case, LTP generates as many report segments
as are necessary and splits the scope of red-part data covered across
multiple report segments so that each of them may stand on their own.
For example, if three report segments are to be generated as part of
a reception report covering red-part data in range [0:1,000,000],
they could look like this: RS 19, scope [0:300,000], RS 20, scope
Burleigh, et al. Experimental [Page 13]
RFC 5325 LTP - Motivation September 2008
[300,000:950,000], and RS 21, scope [950,000:1,000,000]. In all
cases, a timer is started upon transmission of each report segment of
the reception report.
On reception of each report segment, the sending engine responds as
follows:
- It turns off the timer for the checkpoint referenced by the
report segment, if any.
- It enqueues a reception-acknowledgment segment acknowledging the
report segment (to turn off the report retransmission timer at
the receiving engine). This segment is sent immediately at the
next transmission opportunity.
- If the reception claims in the report segment indicate that not
all data within the scope have been received, it normally
initiates a retransmission by enqueuing all data segments not
yet received. The last such segment is marked as a checkpoint
and contains the report serial number of the report segment to
which the retransmission is a response. These segments are
likewise sent at the next transmission opportunity, but only
after all data segments previously queued for transmission to
the receiving engine have been sent. A timer is started for the
checkpoint, so that it can be retransmitted automatically if no
responsive report segment is received.
- On the other hand, if the reception claims in the report segment
indicate that all data within the scope of the report segment
have been received, and the union of all reception claims
received so far in this session indicates that all data in the
red-part of the block have been received, then the sending
engine notifies the local client service instance that the red-
part of the block has been successfully transmitted; the
session's red-part transmission has ended.
On reception of a report-acknowledgment segment, the receiver turns
off the timer for the referenced report segment.
On reception of a checkpoint segment with a non-zero report serial
number, the receiving engine responds as follows:
- It returns a reception report comprising as many report segments
as are needed in order to report in detail on all data reception
within the scope of the referenced report segment, and a timer
is started for each report segment.
Burleigh, et al. Experimental [Page 14]
RFC 5325 LTP - Motivation September 2008
- If at this point all data in the red-part of the block have been
received, the receiving engine delivers the received block's
red-part to the local instance of the client service and, upon
reception of reception-acknowledgment segments acknowledging all
report segments, the session's red-part reception ends and
transmission of the block is complete. Otherwise, the data
retransmission cycle continues.
Loss of a checkpoint segment or the report segment generated in
response causes timer expiry; when this occurs, the sending engine
normally retransmits the checkpoint segment. Similarly, the loss of
a report segment or the corresponding report-acknowledgment segment
causes the report segment's timer to expire; when this occurs, the
receiving engine normally retransmits the report segment.
Note that the redundant reception of a report segment (i.e., one that
was already received and processed by the sender), retransmitted due
to loss of the corresponding report-acknowledgment segment for
example, causes another report-acknowledgment segment to be
transmitted in response but is otherwise ignored. If any of the data
segments retransmitted in response to the original reception of the
report segment were lost, further retransmission of those data
segments will be requested by the reception report generated in
response to the last retransmitted data segment marked as a
checkpoint. Thus, unnecessary retransmission is suppressed.
Note also that the responsibility for responding to segment loss in
LTP is shared between the sender and receiver of a block: the sender
retransmits checkpoint segments in response to checkpoint timeouts,
and retransmits missing data in response to reception reports
indicating incomplete reception, while the receiver retransmits
report segments in response to timeouts. An alternative design would
have been to make the sender responsible for all retransmission, in
which case the receiver would not expect report-acknowledgment
segments and would not retransmit report segments. There are two
disadvantages to this approach:
First, because of constraints on segment size that might be imposed
by the underlying communication service, it is at least remotely
possible that the response to any single checkpoint might be multiple
report segments. An additional sender-side mechanism for detecting
and appropriately responding to the loss of some proper subset of
those reception reports would be needed. We believe that the current
design is simpler.
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RFC 5325 LTP - Motivation September 2008
Second, an engine that receives a block needs a way to determine when
the session can be closed. In the absence of explicit final report
acknowledgment (which entails retransmission of the report in case of
the loss of the report acknowledgment), the alternatives are (a) to
close the session immediately on transmission of the report segment
that signifies complete reception and (b) to close the session on
receipt of an explicit authorization from the sender. In case (a),
loss of the final report segment would cause retransmission of a
checkpoint by the sender, but the session would no longer exist at
the time the retransmitted checkpoint arrived. The checkpoint could
reasonably be interpreted as the first data segment of a new block,
most of which was lost in transit, and the result would be redundant
retransmission of the entire block. In case (b), the explicit
session termination segment and the responsive acknowledgment by the
receiver (needed in order to turn off the timer for the termination
segment, which in turn would be needed in case of in-transit loss or
corruption of the termination segment) would somewhat complicate the
protocol, increase bandwidth consumption, and retard the release of
session state resources at the sender. Here again we believe that
the current design is simpler and more efficient.
3.3. Accelerated Retransmission
Data segment retransmission occurs only on receipt of a report
segment indicating incomplete reception; report segments are normally
transmitted only at the end of original transmission of the red-part
of a block or at the end of a retransmission. For some applications,
it may be desirable to trigger data segment retransmission
incrementally during the course of red-part original transmission so
that the missing segments are recovered sooner. This can be
accomplished in two ways:
- Any red-part data segment prior to the EORP can additionally be
flagged as a checkpoint. Reception of each such "discretionary"
checkpoint causes the receiving engine to issue a reception
report.
- At any time during the original transmission of a block's red-
part (that is, prior to reception of any data segment of the
block's green-part), the receiving engine can unilaterally issue
additional asynchronous reception reports. Note that the CFDP
protocol's "Immediate" mode is an example of this sort of
asynchronous reception reporting [CFDP]. The reception reports
generated for accelerated retransmission are processed in
exactly the same way as the standard reception reports.
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3.4. Session Cancellation
A transmission session may be canceled by either the sending or the
receiving engine in response either to a request from the local
client service instance or to an LTP operational failure as noted
earlier. Session cancellation is accomplished as follows.
The canceling engine deletes all currently queued segments for the
session and notifies the local instance of the affected client
service that the session is canceled. If no segments for this
session have yet been sent to or received from the corresponding LTP
engine, then at this point the canceling engine simply closes its
state record for the session and cancellation is complete.
Otherwise, a session cancellation segment is queued for transmission.
At the next opportunity, the enqueued cancellation segment is
immediately transmitted to the LTP engine serving the remote client
service instance. A timer is started for the segment, so that it can
be retransmitted automatically if no response is received.
The corresponding engine receives the cancellation segment, enqueues
for transmission to the canceling engine a cancellation-
acknowledgment segment, deletes all other currently queued segments
for the indicated session, notifies the local client service instance
that the block has been canceled, and closes its state record for the
session.
At the next opportunity, the enqueued cancellation-acknowledgment
segment is immediately transmitted to the canceling engine.
The canceling engine receives the cancellation-acknowledgment, turns
off the timer for the cancellation segment, and closes its state
record for the session.
Loss of a cancellation segment or of the responsive cancellation-
acknowledgment causes the cancellation segment timer to expire. When
this occurs, the canceling engine retransmits the cancellation
segment.
4. Security Considerations
There is a clear risk that unintended receivers can listen in on LTP
transmissions over satellite and other radio broadcast data links.
Such unintended recipients of LTP transmissions may also be able to
manipulate LTP segments at will.
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Hence, there is a potential requirement for confidentiality,
integrity, and anti-DoS (Denial of Service) security services and
mechanisms.
In particular, DoS problems are more severe for LTP compared to
typical Internet protocols because LTP inherently retains state for
long periods and has very long time-out values. Further, it could be
difficult to reset LTP nodes to recover from an attack. Thus, any
adversary who can actively attack an LTP transmission has the
potential to create severe DoS conditions for the LTP receiver.
To give a terrestrial example: were LTP to be used in a sparse sensor
network, DoS attacks could be mounted resulting in nodes missing
critical information, such as communications schedule updates. In
such cases, a single successful DoS attack could take a node entirely
off the network until the node was physically visited and reset.
Even for deep-space applications of LTP, we need to consider certain
terrestrial attacks, in particular those involving insertion of
messages into an ongoing session (usually without having seen the
exact bytes of the previous messages in the session). Such attacks
are likely in the presence of firewall failures at various nodes in
the network, or due to Trojan software running on an authorized host.
Many message insertion attacks will depend on the attacker correctly
"guessing" something about the state of the LTP peers, but experience
shows that successful guesses are easier than might be thought [DDJ].
We now consider the appropriate layer(s) at which security mechanisms
can be deployed to increase the security properties of LTP, and the
trade-offs entailed in doing so.
The Application layer (above-LTP)
Higher-layer security mechanisms clearly protect LTP payload, but
leave LTP headers open. Such mechanisms provide little or no
protection against DoS type attacks against LTP, but may well
provide sufficient data integrity and ought to be able to provide
data confidentiality.
The LTP layer
An authentication header (similar to IPsec [AH]) can help protect
against replay attacks and other bogus packets. However, an
adversary may still see the LTP header of segments passing by in
the ether. This approach also requires some key management
infrastructure to be in place in order to provide strong
authentication, which may not always be an acceptable overhead.
Such an authentication header could mitigate many DoS attacks.
Burleigh, et al. Experimental [Page 18]
RFC 5325 LTP - Motivation September 2008
Similarly, a confidentiality service could be defined for LTP
payload and (some) header fields. However, this seems less
attractive since (a) confidentiality is arguably better provided
either above or below the LTP layer, (b) key management for such a
service is harder (in a high-delay context) than for an integrity
service, and (c) forcing LTP engines to attempt decryption of
incoming segments can in itself provide a DoS opportunity.
Further, within the LTP layer we can make various design decisions
to reduce the probability of successful DoS attacks. In
particular, we can mandate that values for certain fields in the
header (session numbers, for example) be chosen randomly.
The Data-link layer (below-LTP)
The lower layers can clearly provide confidentiality and integrity
services, although such security may result in unnecessary
overhead if the cryptographic service provided is not required for
all data. For example, it can be harder to manage lower layers so
that only the data that requires encryption is in fact encrypted.
Encrypting all data could represent a significant overhead for
some LTP use cases. However, the lower layers are often the place
where compression and error-correction is done, and so may well
also be the optimal place to do encryption since the same issues
with applying or not applying the service apply to both encryption
and compression.
In light of these considerations, LTP includes the following security
mechanisms:
The optional LTP Authentication mechanism is an LTP segment
extension comprising a ciphersuite identifier and optional key
identifier that precede the segment's content, plus an
authentication value (either a message authentication code or a
digital signature) that follows the segment's content; the
ciphersuite ID is used to indicate the length and format of the
authentication value. The authentication mechanism serves to
assure the segment's integrity and, depending on the ciphersuite
selected and the key management regime, its authenticity.
The optional LTP cookie mechanism is an LTP segment extension
comprising a "cookie" -- a randomly chosen numeric value -- that
precedes the segment's content. By increasing the number of bytes
in a segment that cannot be easily predicted by an inauthentic
data source, and by requiring that segments lacking the correct
values of these bytes be silently discarded, the cookie mechanism
increases the difficulty of mounting a successful denial-of-
service attack on an LTP engine.
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RFC 5325 LTP - Motivation September 2008
The above mechanisms are defined in detail in the LTP extensions
document [LTPEXT].
In addition, the serial numbers of LTP checkpoints and reports are
required to be randomly chosen integers, and implementers are
encouraged to choose session numbers randomly as well. This
randomness adds a further increment of protection against DoS
attacks. See [PRNG] for recommendations related to randomness.
5. IANA Considerations
Please see the IANA Considerations sections of [LTPSPEC] and
[LTPEXT].
6. Acknowledgments
Many thanks to Tim Ray, Vint Cerf, Bob Durst, Kevin Fall, Adrian
Hooke, Keith Scott, Leigh Torgerson, Eric Travis, and Howie Weiss for
their thoughts on this protocol and its role in Delay-Tolerant
Networking architecture.
Part of the research described in this document was carried out at
the Jet Propulsion Laboratory, California Institute of Technology,
under a contract with the National Aeronautics and Space
Administration. This work was performed under DOD Contract DAA-B07-
00-CC201, DARPA AO H912; JPL Task Plan No. 80-5045, DARPA AO H870;
and NASA Contract NAS7-1407.
Thanks are also due to Shawn Ostermann, Hans Kruse, and Dovel Myers
at Ohio University for their suggestions and advice in making various
design decisions. This work was done when Manikantan Ramadas was a
graduate student at the EECS Dept., Ohio University, in the
Internetworking Research Group Laboratory.
Part of this work was carried out at Trinity College Dublin as part
of the SeNDT contract funded by Enterprise Ireland's research
innovation fund.
7. References
7.1. Informative References
[LTPSPEC] Ramadas, M., Burleigh, S., and S. Farrell, "Licklider
Transmission Protocol - Specification", RFC 5326, September
2008.
Burleigh, et al. Experimental [Page 20]
RFC 5325 LTP - Motivation September 2008
[LTPEXT] Farrell, S., Ramadas, M., and S. Burleigh, "Licklider
Transmission Protocol - Security Extensions", RFC 5327,
September 2008.
[AH] Kent, S., "IP Authentication Header", RFC 4302, December
2005.
[BP] Scott, K. and S. Burleigh, "Bundle Protocol Specification",
RFC 5050, November 2007.
[CFDP] CCSDS File Delivery Protocol (CFDP). Recommendation for
Space Data System Standards, CCSDS 727.0-B-2 BLUE BOOK
Issue 1, October 2002.
[DDJ] I. Goldberg and E. Wagner, "Randomness and the Netscape
Browser", Dr. Dobb's Journal, 1996, (pages 66-70).
[DSN] Deep Space Mission Systems Telecommunications Link Design
Handbook (810-005) web-page,
"http://eis.jpl.nasa.gov/deepspace/dsndocs/810-005/"
[DTN] K. Fall, "A Delay-Tolerant Network Architecture for
Challenged Internets", In Proceedings of ACM SIGCOMM 2003,
Karlsruhe, Germany, Aug 2003.
[IPN] InterPlanetary Internet Special Interest Group web page,
"http://www.ipnsig.org".
[TFRC] Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification", RFC
3448, January 2003.
[HSTCP] Floyd, S., "HighSpeed TCP for Large Congestion Windows",
RFC 3649, December 2003.
[SCTP] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[PRNG] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
June 2005.
Burleigh, et al. Experimental [Page 21]
RFC 5325 LTP - Motivation September 2008
Authors' Addresses
Scott C. Burleigh
Jet Propulsion Laboratory
4800 Oak Grove Drive
M/S: 301-485B
Pasadena, CA 91109-8099
Telephone: +1 (818) 393-3353
Fax: +1 (818) 354-1075
EMail: Scott.Burleigh@jpl.nasa.gov
Manikantan Ramadas
ISRO Telemetry Tracking and Command Network (ISTRAC)
Indian Space Research Organization (ISRO)
Plot # 12 & 13, 3rd Main, 2nd Phase
Peenya Industrial Area
Bangalore 560097
India
Telephone: +91 80 2364 2602
EMail: mramadas@gmail.com
Stephen Farrell
Computer Science Department
Trinity College Dublin
Ireland
Telephone: +353-1-896-1761
EMail: stephen.farrell@cs.tcd.ie
Burleigh, et al. Experimental [Page 22]
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Burleigh, et al. Experimental [Page 23]