RFC 5326
Licklider Transmission Protocol - Specification
Networking Working Group M. Ramadas Request for Comments: 5326 ISTRAC, ISRO Category: Experimental S. Burleigh NASA/Jet Propulsion Laboratory S. Farrell Trinity College Dublin September 2008 Licklider Transmission Protocol - Specification 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). It may be considered for standardization by the IETF in the future, but the IETF disclaims any knowledge of the fitness of this RFC for any purpose and in particular notes that the decision to publish is not based on IETF review for such things as security, congestion control, or inappropriate interaction with deployed protocols. See RFC 3932 for more information.Abstract
This document describes 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. 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.
Table of Contents
1. Introduction ....................................................3 2. Terminology .....................................................4 3. Segment Structure ...............................................9 3.1. Segment Header ............................................10 3.1.1. Segment Type Flags .................................11 3.1.2. Segment Type Codes .................................13 3.1.3. Segment Class Masks ................................14 3.1.4. Extensions Field ...................................14 3.2. Segment Content ...........................................16 3.2.1. Data Segment (DS) ..................................16 3.2.2. Report Segment (RS) ................................17 3.2.3. Report Acknowledgment Segment ......................19 3.2.4. Session Management Segments ........................20 3.3. Segment Trailer ...........................................20 4. Requests from Client Service ...................................20 4.1. Transmission Request ......................................21 4.2. Cancellation Request ......................................22 5. Requirements from the Operating Environment ....................23 6. Internal Procedures ............................................24 6.1. Start Transmission ........................................25 6.2. Start Checkpoint Timer ....................................25 6.3. Start RS Timer ............................................25 6.4. Stop Transmission .........................................25 6.5. Suspend Timers ............................................26 6.6. Resume Timers .............................................26 6.7. Retransmit Checkpoint .....................................27 6.8. Retransmit RS .............................................27 6.9. Signify Red-Part Reception ................................28 6.10. Signify Green-Part Segment Arrival .......................28 6.11. Send Reception Report ....................................28 6.12. Signify Transmission Completion ..........................30 6.13. Retransmit Data ..........................................30 6.14. Stop RS Timer ............................................31 6.15. Start Cancel Timer .......................................32 6.16. Retransmit Cancellation Segment ..........................32 6.17. Acknowledge Cancellation .................................32 6.18. Stop Cancel Timer ........................................33 6.19. Cancel Session ...........................................33 6.20. Close Session ............................................33 6.21. Handle Miscolored Segment ................................33 6.22. Handling System Error Conditions .........................34 7. Notices to Client Service ......................................35 7.1. Session Start .............................................35 7.2. Green-Part Segment Arrival ................................36 7.3. Red-Part Reception ........................................36 7.4. Transmission-Session Completion ...........................36
7.5. Transmission-Session Cancellation .........................37
7.6. Reception-Session Cancellation ............................37
7.7. Initial-Transmission Completion ...........................37
8. State Transition Diagrams ......................................38
8.1. Sender ....................................................39
8.2. Receiver ..................................................44
9. Security Considerations ........................................48
9.1. Denial of Service Considerations ..........................48
9.2. Replay Handling ...........................................49
9.3. Implementation Considerations .............................50
10. IANA Considerations ...........................................51
10.1. UDP Port Number for LTP ..................................51
10.2. LTP Extension Tag Registry ...............................51
11. Acknowledgments ...............................................51
12. References ....................................................52
12.1. Normative References .....................................52
12.2. Informative References ...................................52
1. Introduction
This document serves as the main protocol specification of LTP and is
part of a series of documents describing LTP. Other documents in
this series include the motivation document [LTPMTV] and the protocol
extensions document [LTPEXT]. We strongly recommend reading the
protocol motivation document before reading this document, to
establish sufficient background and motivation for the specification.
LTP does Automatic Repeat reQuest (ARQ) of data transmissions by
soliciting selective-acknowledgment reception reports. It is
stateful, and has no negotiation or handshakes.
In an Interplanetary Internet setting deploying the Bundle Protocol
that is being developed by the Delay Tolerant Networking Research
Group, LTP is intended to serve as a reliable "convergence layer"
protocol operating in pairwise fashion between adjacent
Interplanetary Internet nodes that are in direct radio frequency (RF)
communication. In that operational scenario, and potentially in some
other deployments of the Bundle Protocol, LTP runs directly over a
data-link layer protocol; when this is the case, forward error
correction coding and/or checksum mechanisms in the underlying data-
link layer protocol must ensure the integrity of the data passed
between the communicating entities.
Since no mechanisms for flow control or congestion control are
included in the design of LTP, this protocol is not intended or
appropriate for ubiquitous deployment in the global Internet.
When LTP is run over UDP, it must only be used for software development or in private local area networks. When LTP is not run over UDP, it must be run directly over a protocol (nominally a link- layer protocol) that meets the requirements specified in Section 5. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [B97].2. Terminology
(1) Engine ID A number that uniquely identifies a given LTP engine, within some closed set of communicating LTP engines. Note that when LTP is operating underneath the Delay-Tolerant Networking (DTN) [DTN] Bundle Protocol [BP], the convergence layer adapter mediating the two will be responsible for translating between DTN endpoint IDs and LTP engine IDs in an implementation-specific manner. (2) Block An array of contiguous octets of application data handed down by the upper layer protocol (typically Bundle Protocol) to be transmitted from one LTP client service instance to another. Any subset of a block comprising contiguous octets beginning at the start of the block is termed a "block prefix", and any such subset of the block ending with the end of the block is termed a "block suffix". (3) Red-Part The block prefix that is to be transmitted reliably, i.e., subject to acknowledgment and retransmission. (4) Green-Part The block suffix that is to be transmitted unreliably, i.e., not subject to acknowledgments or retransmissions. If present, the green-part of a block begins at the octet following the end of the red-part. (5) Session A thread of LTP protocol activity conducted between two peer engines for the purpose of transmitting a block. Data flow in a session is unidirectional: data traffic flows from the sending peer to the
receiving peer, while data-acknowledgment traffic flows from the receiving peer to the sending peer. (6) Sender The data sending peer of a session. (7) Receiver The data receiving peer of a session. (8) Client Service Instance A software entity, such as an application or a higher-layer protocol implementation, that is using LTP to transfer data. (9) Segment The unit of LTP data transmission activity. It is the data structure transmitted from one LTP engine to another in the course of a session. Each LTP segment is of one of the following types: data segment, report segment, report-acknowledgment segment, cancel segment, cancel-acknowledgment segment. (10) Reception Claim An assertion of reception of some number of contiguous octets of application data (a subset of a block) characterized by: the offset of the first received octet, and the number of contiguous octets received (beginning at the offset). (11) Scope Scope identifies a subset of a block and comprises two numbers -- upper bound and lower bound. For a data segment, lower bound is the offset of the segment's application data from the start of the block (in octets), while upper bound is the sum of the offset and length of the segment's application data (in octets). For example, a segment with a block offset of 1000 and length of 500 would have a lower bound of 1000 and upper bound of 1500. For a report segment, upper bound is the end of the block prefix to which the reception claims in the report apply, while lower bound is the end of the (smaller) interior block prefix to which the reception claims in the report do *not* apply. That is, data at any offset equal to or greater than the report's lower bound but less than its
upper bound and not designated as "received" by any of the report's reception claims must be assumed not received, and therefore eligible for retransmission. For example, if a report segment carried a lower bound of 1000 and an upper bound of 5000, and the reception claims indicated reception of data within offsets 1000-1999 and 3000-4999, data within the block offsets 2000-2999 can be considered missing and eligible for retransmission. Reception reports (which may comprise multiple report segments) also have scope, as defined in Section 6.11. (12) End of Block (EOB) The last data segment transmitted as part of the original transmission of a block. This data segment also indicates that the segment's upper bound is the total length of the block (in octets). (13) End of Red-Part (EORP) The segment transmitted as part of the original transmission of a block containing the last octet of the block's red-part. This data segment also indicates that the segment's upper bound is the length of the block's red-part (in octets). (14) Checkpoint A data segment soliciting a reception report from the receiving LTP engine. The EORP segment must be flagged as a checkpoint, as must the last segment of any retransmission; these are "mandatory checkpoints". All other checkpoints are "discretionary checkpoints". (15) Reception Report A sequence of one or more report segments reporting on all block data reception within some scope. (16) Synchronous Reception Report A reception report that is issued in response to a checkpoint. (17) Asynchronous Reception Report A reception report that is issued in response to some implementation- defined event other than the arrival of a checkpoint.
(18) Primary Reception Report
A reception report that is issued in response to some event other
than the arrival of a checkpoint segment that was itself issued in
response to a reception report. Primary reception reports include
all asynchronous reception reports and all synchronous reception
reports that are sent in response to discretionary checkpoints or to
the EORP segment for a session.
(19) Secondary Reception Report
A reception report that is issued in response to the arrival of a
checkpoint segment that was itself issued in response to a reception
report.
(20) Self-Delimiting Numeric Value (SDNV)
The design of LTP attempts to reconcile minimal consumption of
transmission bandwidth with
(a) extensibility to satisfy requirements not yet identified, and
(b) scalability across a very wide range of network sizes and
transmission payload sizes.
The SDNV encoding scheme is modeled after the Abstract Syntax
Notation One [ASN1] scheme for encoding Object Identifier values. In
a data field encoded as an SDNV, the most significant bit (MSB) of
each octet of the SDNV serves to indicate whether or not the octet is
the last octet of the SDNV. An octet with an MSB of 1 indicates that
it is either the first or a middle octet of a multi-octet SDNV; the
octet with an MSB of 0 is the last octet of the SDNV. The value
encoded in an SDNV is found by concatenating the 7 least significant
bits of each octet of the SDNV, beginning at the first octet and
ending at the last octet.
The following examples illustrate the encoding scheme for various
hexadecimal values.
0xABC : 1010 1011 1100
is encoded as
{100 1010 1} {0 011 1100}
- -
= 10010101 00111100
0x1234 : 0001 0010 0011 0100
= 1 0010 0011 0100
is encoded as
{10 1 0010 0} {0 011 0100}
- -
= 10100100 00110100
0x4234 : 0100 0010 0011 0100
=100 0010 0011 0100
is encoded as
{1000000 1} {1 00 0010 0} {0 011 0100}
- - -
= 10000001 10000100 00110100
0x7F : 0111 1111
=111 1111
is encoded as
{0 111 1111}
-
= 01111111
Note:
Care must be taken to make sure that the value to be encoded is
padded with zeroes at the most significant bit end (NOT at the least
significant bit end) to make its bitwise length a multiple of 7
before encoding.
While there is no theoretical limit on the size of an SDNV field, we
note that the overhead of the SDNV scheme is 1:7, i.e., 1 bit of
overhead for every 7 bits of actual data to be encoded. Thus, a
7-octet value (a 56-bit quantity with no leading zeroes) would be
encoded in an 8-octet SDNV; an 8-octet value (a 64-bit quantity with
no leading zeroes) would be encoded in a 10-octet SDNV. In general,
an N-bit quantity with no leading zeroes would be encoded in a
ceil(N/7) octet SDNV, where ceil is the integer ceiling function.
Clearly, for fields that typically carry larger values such as RSA
public keys, the SDNV overhead could become unacceptable. Hence,
when adopting the SDNV scheme for other purposes related to this
document, such as any protocol extensions, we RECOMMEND that if the
typical data field value is expected to be larger than 8 octets, then
the data field should be specified as a {LENGTH, VALUE} tuple, with
the LENGTH parameter encoded as an SDNV followed by LENGTH octets
housing the VALUE of the data field.
We also note that SDNV is clearly not the best way to represent every
numeric value. When the maximum possible value of a number is known
without question, the cost of additional bits may not be justified.
For example, an SDNV is a poor way to represent an integer whose
value typically falls in the range 128 to 255. In general, though,
we believe that the SDNV representation of various protocol data
fields in LTP segments yields the smallest segment sizes without
sacrificing scalability.
3. Segment Structure
Each LTP segment comprises
(a) a "header" in the format defined below.
(b) zero or more octets of "content".
(c) zero or more octets of "trailer" as indicated by information
in the "Extensions field" of the header.
LTP segments are of four general types depending on the nature of the
content carried:
Data segments flow from the sender to the receiver and carry
client service (application) data.
A report segment flows from the receiver to the sender and carries
data reception claims together with the upper and lower bounds of
the block scope to which the claims pertain.
A report-acknowledgment segment flows from the sender to the
receiver and acknowledges reception of a report segment. It
carries the serial number of the report being acknowledged.
Session management segments may be generated by both the sender
and the receiver and are of two general sub-types: cancellation
and cancellation-acknowledgment. A cancellation segment initiates
session cancellation procedures at the peer and carries a single
byte reason-code to indicate the reason for session cancellation.
Cancellation-acknowledgment segments merely acknowledge reception
of a cancellation segment and have no content.
The overall segment structure is illustrated below:
Bit 0 1 2 3 4 5 6 7
^ +-----+-----+-----+-----+-----+-----+-----+-----+
| | Version number | Segment Type Flags | Control
| +-----------------------+-----------------------+ -byte
| | |
| / Session ID \
| \ /
Header +-----------------------+-----------------------+
| | Header Extension Cnt. | Trailer Extension Cnt.| Extensions
| +-----------------------+-----------------------+
| | |
| / Header Extensions \
| \ /
V +-----------------------------------------------+
| |
| |
| |
| Segment Content |
/ \
\ /
| |
| |
| |
^ +-----------------------------------------------+
| | |
Trailer / Trailer Extensions \
| \ /
V +-----------------------------------------------+
3.1. Segment Header
An LTP segment header comprises three data items: a single-octet
control byte, the session ID, and the Extensions field.
Control byte comprises the following:
Version number (4 bits): MUST be set to the binary value 0000 for
this version of the protocol.
Segment type flags (4 bits): described in Section 3.1.1.
Session ID uniquely identifies, among all transmissions between the
sender and receiver, the session of which the segment is one token.
It comprises the following:
Session originator (SDNV): the engine ID of the sender.
Session number (SDNV): typically a random number (for anti-DoS
reasons), generated by the sender.
The format and resolution of session number are matters that are
private to the LTP sender; the only requirement imposed by LTP is
that every session initiated by an LTP engine MUST be uniquely
identified by the session ID.
The Extensions field is described in Section 3.1.4.
3.1.1. Segment Type Flags
The last 4 bits of the control byte in the segment header are flags
that indicate the nature of the segment. In order (most significant
bit first), these flags are CTRL, EXC, Flag 1, and Flag 0.
A value of 0 in the CTRL (Control) flag identifies the segment as a
data segment, while a value of 1 identifies it as a control segment.
A data segment with the EXC (Exception) flag set to 0 is a red-part
segment; a data segment with EXC set to 1 is a green-part segment.
For a control segment, having the EXC flag set to 1 indicates that
the segment pertains to session cancellation activity. Any data
segment (whether red-part or green-part) with both Flag 1 and Flag 0
set to 1 indicates EOB. Any data segment (whether red-part or
green-part) with both Flag 1 and Flag 0 set to 0 indicates data
without any additional protocol significance. Any red-part data
segment with either flag bit non-zero is a checkpoint. Any red-part
data segment with Flag 1 set to 1 indicates the end of red-part.
Put another way:
if (CTRL flag = 0)
segment is a data segment if (EXC flag = 0)
segment contains only red-part data if (Flag 1 = 1)
segment is a checkpoint segment is the last segment in the
red part of the block if (Flag 0 = 1)
segment is the last segment in the block
else // segment is not end of red-part
if (Flag 0 = 1)
segment is a checkpoint
else
segment contains only green-part data if (Flag 1 = 1)
if (Flag 0 = 1)
segment is the last segment in the block
else
segment is a control segment if (EXC flag = 0)
segment pertains to report activity if (flag 0 = 0)
segment is a report segment
else
segment is an acknowledgment of a report segment
else
segment pertains to session cancellation activity if (Flag 1 =
0)
segment pertains to cancellation by block sender if (Flag 0
= 1)
segment is a cancellation by sender
else
segment is an acknowledgment of a cancellation by sender
else
segment pertains to cancellation by block receiver if (Flag
0 = 1)
segment is a cancellation by receiver
else
segment is an acknowledgment of a cancellation by
receiver
3.1.2. Segment Type Codes
Combinations of the settings of the segment type flags CTRL, EXC, Flag 1, and Flag 0 constitute segment type codes, which serve as concise representations of detailed segment nature. CTRL EXC Flag 1 Flag 0 Code Nature of segment ---- --- ------ ------ ---- --------------------------------------- 0 0 0 0 0 Red data, NOT {Checkpoint, EORP or EOB} 0 0 0 1 1 Red data, Checkpoint, NOT {EORP or EOB} 0 0 1 0 2 Red data, Checkpoint, EORP, NOT EOB 0 0 1 1 3 Red data, Checkpoint, EORP, EOB 0 1 0 0 4 Green data, NOT EOB 0 1 0 1 5 Green data, undefined 0 1 1 0 6 Green data, undefined 0 1 1 1 7 Green data, EOB 1 0 0 0 8 Report segment 1 0 0 1 9 Report-acknowledgment segment 1 0 1 0 10 Control segment, undefined 1 0 1 1 11 Control segment, undefined 1 1 0 0 12 Cancel segment from block sender 1 1 0 1 13 Cancel-acknowledgment segment to block sender 1 1 1 0 14 Cancel segment from block receiver 1 1 1 1 15 Cancel-acknowledgment segment to block receiver
3.1.3. Segment Class Masks
For the purposes of this specification, some bit patterns in the segment type flags field correspond to "segment classes" that are designated by mnemonics. The mnemonics are intended to evoke the characteristics shared by all types of segments characterized by these flag bit patterns. CTRL EXC Flag 1 Flag 0 Mnemonic Description ---- --- ------ ------ -------- --------------------------- 0 0 - 1 -- or -- 0 0 1 - CP Checkpoint 0 0 1 - EORP End of red-part; red-part size = offset + length 0 - 1 1 EOB End of block; block size = offset + length 1 0 0 0 RS Report segment; carries reception claims 1 0 0 1 RA Report-acknowledgment segment 1 1 0 0 CS Cancel segment from block sender 1 1 0 1 CAS Cancel-acknowledgment segment to block sender 1 1 1 0 CR Cancel segment from block receiver 1 1 1 1 CAR Cancel-acknowledgment segment to block receiver 1 1 - 0 Cx Cancel segment (generic) 1 1 - 1 CAx Cancel-acknowledgment segment (generic)3.1.4. Extensions Field
The Extensions field enables the inclusion of zero or more functional extensions to the basic LTP segment, each in type-length-value (TLV) representation as explained below.
The first octet of the Extensions field indicates the number of extensions present in the segment: the high-order 4 bits indicate the number of extension TLVs in the header (immediately following the extensions count octet and preceding the segment's content), while the low-order 4 bits indicate the number of extension TLVs in the trailer (immediately following the segment's content). That is, each segment may have from 0 to 15 extension TLVs in its header and from 0 to 15 extension TLVs in its trailer. In the absence of any extension TLVs, all bits of this extensions count octet MUST be set to zero. Note that it is valid for header extensions to be immediately followed by trailer extensions; for example, since a CAx segment has no contents, it may have header extensions immediately followed by trailer extensions. Each extension consists of a one-octet tag identifying the type of the extension, followed by a length parameter in SDNV form, followed by a value of the specified length. The diagram below illustrates the extension TLVs as they may occur in the header or trailer. +--------+----///-----///--+ |ext-tag | length | value | +--------+-------///-------+----------///-------+ |ext-tag | length | value | +--------+-----///-----///-+---------////-------+ |ext-tag | length | value | +--------+----------+----------+ The IANA maintains an LTP Extension Tag registry as shown below. See the IANA considerations section below for details of code point assignment in the Unassigned range. Extension tag Meaning ------------- ------- 0x00 LTP authentication extension [LTPEXT] 0x01 LTP cookie extension [LTPEXT] 0x02-0xAF Unassigned 0xB0-0xBF Reserved 0xC0-0xFF Private / Experimental Use Note that since the last quarter of the extension-tag space is for experimental use, implementations should be aware that collisions for these tags are possible.
3.2. Segment Content
3.2.1. Data Segment (DS)
The content of a data segment includes client service data and the metadata enabling the receiving client service instance to receive and make use of that data. Client service ID (SDNV) The client service ID number identifies the upper-level service to which the segment is to be delivered by the receiver. It is functionally analogous to a TCP port number. If multiple instances of the client service are present at the destination, multiplexing must be done by the client service itself on the basis of information encoded within the transmitted block. Offset (SDNV) Offset indicates the location of the segment's client service data within the session's transmitted block. It is the number of bytes in the block prior to the byte from which the first octet of the segment's client service data was copied. Length (SDNV) The length of the ensuing client service data, in octets. If the data segment is a checkpoint, the segment MUST additionally include the following two serial numbers (checkpoint serial number and report serial number) to support efficient retransmission. Data segments that are not checkpoints MUST NOT have these two fields in the header and MUST continue on directly with the client service data. Checkpoint serial number (SDNV) The checkpoint serial number uniquely identifies the checkpoint among all checkpoints issued by the block sender in a session. The first checkpoint issued by the sender MUST have this serial number chosen randomly for security reasons, and it is RECOMMENDED that the sender use the guidelines in [ESC05] for this. Any subsequent checkpoints issued by the sender MUST have the serial number value found by incrementing the prior checkpoint serial number by 1. When a checkpoint segment is retransmitted, however, its serial number MUST be the same as when it was originally transmitted. The checkpoint serial number MUST NOT be zero.
Report serial number (SDNV)
If the checkpoint was queued for transmission in response to the
reception of an RS (Section 6.13), then its value MUST be the
report serial number value of the RS that caused the data segment
to be queued for transmission.
Otherwise, the value of report serial number MUST be zero.
Client service data (array of octets)
The client service data carried in the segment is a copy of a
subset of the bytes in the original client service data block,
starting at the indicated offset.
3.2.2. Report Segment (RS)
The content of an RS comprises one or more data reception claims,
together with the upper and lower bounds of the scope within the data
block to which the claims pertain. It also includes two serial
numbers to support efficient retransmission.
Report serial number (SDNV)
The report serial number uniquely identifies the report among all
reports issued by the receiver in a session. The first report
issued by the receiver MUST have this serial number chosen
randomly for security reasons, and it is RECOMMENDED that the
receiver use the guidelines in [ESC05] for this. Any subsequent
RS issued by the receiver MUST have the serial number value found
by incrementing the last report serial number by 1. When an RS is
retransmitted however, its serial number MUST be the same as when
it was originally transmitted. The report serial number MUST NOT
be zero.
Checkpoint serial number (SDNV)
The value of the checkpoint serial number MUST be zero if the
report segment is NOT a response to reception of a checkpoint,
i.e., the reception report is asynchronous; otherwise, it MUST be
the checkpoint serial number of the checkpoint that caused the RS
to be issued.
Upper bound (SDNV)
The upper bound of a report segment is the size of the block
prefix to which the segment's reception claims pertain.
Lower bound (SDNV)
The lower bound of a report segment is the size of the (interior)
block prefix to which the segment's reception claims do NOT
pertain.
Reception claim count (SDNV)
The number of data reception claims in this report segment.
Reception claims
Each reception claim comprises two elements: offset and length.
Offset (SDNV)
The offset indicates the successful reception of data beginning
at the indicated offset from the lower bound of the RS. The
offset within the entire block can be calculated by summing
this offset with the lower bound of the RS.
Length (SDNV)
The length of a reception claim indicates the number of
contiguous octets of block data starting at the indicated
offset that have been successfully received.
Reception claims MUST conform to the following rules:
A reception claim's length shall never be less than 1 and shall
never exceed the difference between the upper and lower bounds
of the report segment.
The offset of a reception claim shall always be greater than
the sum of the offset and length of the prior claim, if any.
The sum of a reception claim's offset and length and the lower
bound of the report segment shall never exceed the upper bound
of the report segment.
Implied requests for retransmission of client service data can be
inferred from an RS's data reception claims. However, *nothing* can
be inferred regarding reception of block data at any offset equal to
or greater than the segment's upper bound or at any offset less than
the segment's lower bound.
For example, if the scope of a report segment has lower bound 0 and upper bound 6000, and the report contains a single data reception claim with offset 0 and length 6000, then the report signifies successful reception of the first 6000 bytes of the block. If the total length of the block is 6000, then the report additionally signifies successful reception of the entire block. If on the other hand, the scope of a report segment has lower bound 1000 and upper bound 6000, and the report contains two data reception claims, one with offset 0 and length 2000 and the other with offset 3000 and length 500, then the report signifies successful reception only of bytes 1000-2999 and 4000-4499 of the block. From this we can infer that bytes 3000-3999 and 4500-5999 of the block need to be retransmitted, but we cannot infer anything about reception of the first 1000 bytes or of any subsequent data beginning at block offset 6000.3.2.3. Report Acknowledgment Segment
The content of an RA is simply the report serial number of the RS in response to which the segment was generated. Report serial number (SDNV) This field returns the report serial number of the RS being acknowledged.
3.2.4. Session Management Segments
Cancel segments (Cx) carry a single byte reason-code with the following semantics: Reason-Code Mnemonic Semantics ----------- -------- --------------------------------------- 00 USR_CNCLD Client service canceled session. 01 UNREACH Unreachable client service. 02 RLEXC Retransmission limit exceeded. 03 MISCOLORED Received either a red-part data segment at block offset above any green-part data segment offset or a green-part data segment at block offset below any red-part data segment offset. 04 SYS_CNCLD A system error condition caused unexpected session termination. 05 RXMTCYCEXC Exceeded the Retransmission-Cycles limit. 06-FF Reserved The Cancel-acknowledgments (CAx) have no content. Note: The reason we use different cancel segment types for the originator and recipient is to allow a loopback mode to work without disturbing any replay protection mechanism in use.3.3. Segment Trailer
The segment trailer consists of a sequence of zero to 15 extension TLVs as described in Section 3.1.4 above.4. Requests from Client Service
In all cases, the representation of request parameters is a local implementation matter, as are validation of parameter values and notification of the client service in the event that a request is found to be invalid.
4.1. Transmission Request
In order to request transmission of a block of client service data, the client service MUST provide the following parameters to LTP: Destination client service ID. Destination LTP engine ID. Client service data to send, as an array of bytes. Length of the data to be sent. Length of the red-part of the data. This value MUST be in the range from zero to the total length of data to be sent. On reception of a valid transmission request from a client service, LTP proceeds as follows. First, the array of data to be sent is subdivided as necessary, with each subdivision serving as the client service data of a single new LTP data segment. The algorithm used for subdividing the data is a local implementation matter; it is expected that data size constraints imposed by the underlying communication service, if any, will be accommodated in this algorithm. The last (and only the last) of the resulting data segments must be marked as the EOB (end of block). Note that segment type indicates that the client service data in a given LTP segment either is or is not in the red-part of the block. To prevent segment type ambiguity, each data segment MUST contain either only red-part data or only green-part data. Therefore, when the length of the block's red-part is N, the total length of the block is M, and N is not equal to M, the (N+1)th byte of the block SHOULD be the first byte of client service data in a green-part data segment. Note that this means that at the red-part boundary, LTP may send a segment of size lesser than the link MTU size. For bandwidth efficiency reasons, implementations MAY choose to instead mark the entire segment (within which the red-part boundary falls) as red- part, causing green-part data falling within the segment to also be treated as red-part. If the length of the block's red-part is greater than zero, then the last data segment containing red-part data must be marked as the EORP (end of red-part) segment by setting the appropriate segment type flag bits (Section 3.1.2). Zero or more preceding data segments containing red-part data (selected according to an algorithm that is
a local implementation matter) MAY additionally be marked as a CP (Checkpoint), and serve as additional discretionary checkpoints (Section 3.1.2). All data segments are appended to the (conceptual) application data queue bound for the destination engine, for subsequent transmission. Finally, a session start notice (Section 7.1) is sent back to the client service that requested the transmission.4.2. Cancellation Request
In order to request cancellation of a session, either as the sender or as the receiver of the associated data block, the client service must provide the session ID identifying the session to be canceled. On reception of a valid cancellation request from a client service, LTP proceeds as follows. First, the internal "Cancel Session" procedure (Section 6.19) is invoked. Next, if the session is being canceled by the sender (i.e., the session originator part of the session ID supplied in the cancellation request is the local LTP engine ID): - If none of the data segments previously queued for transmission as part of this session have yet been de-queued and transmitted -- i.e., if the destination engine cannot possibly be aware of this session -- then the session is simply closed; the "Close Session" procedure (Section 6.20) is invoked. - Otherwise, a CS (cancel by block sender) segment with the reason-code USR_CNCLD MUST be queued for transmission to the destination LTP engine specified in the transmission request that started this session. Otherwise (i.e., the session is being canceled by the receiver): - If there is no transmission queue-set bound for the sender (possibly because the local LTP engine is running on a receive- only device), then the session is simply closed; the "Close Session" procedure (Section 6.20) is invoked. - Otherwise, a CR (cancel by block receiver) segment with reason- code USR_CNCLD MUST be queued for transmission to the block sender.
5. Requirements from the Operating Environment
LTP is designed to run directly over a data-link layer protocol. LTP MUST only be deployed directly over UDP, for software development purposes or for use in private local area networks, for example, in a sparse sensor network where the link, when available, is only used for LTP traffic. In either case, the protocol layer immediately underlying LTP is referred to as the "local data-link layer" for the purposes of this specification. When the local data-link layer protocol is UDP, (a) the content of each UDP datagram MUST be an integral number of LTP segments and (b) the LTP authentication [LTPEXT] extension SHOULD be used unless the end-to-end path is one in which either the likelihood of datagram content corruption is negligible or the consequences of receiving and processing corrupt LTP segments are insignificant (as during software development). In addition, 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 local data-link layer protocol is not UDP, the content of each local data-link layer protocol frame MUST be an integral number of LTP segments. The local data-link layer protocol MUST be a protocol that, together with the operating environment in which that protocol is implemented, satisfies the following requirements: - It is required to inform LTP whenever the link to a specific LTP destination is brought up or torn down. Similarly, it is required to inform the local LTP engine whenever it is known that a remote LTP engine is set to begin or stop communication with the local engine based on the engines' operating schedules. - It is required to provide link state cues to LTP upon transmission of the CP, RS (report), EORP, EOB, and Cx (cancel) segments so that timers can be started. - It is required to provide, upon request, the current distance (in light seconds) to any peer engine in order to calculate timeout intervals.
A MIB (Management Information Base) with the above parameters, updated periodically by the local data-link layer and the operating environment, should be made available to the LTP engine for its operations. The details of the MIB are, however, beyond the scope of this document. The underlying data-link layer is required to never deliver incompletely received LTP segments to LTP. In the absence of the use of LTP authentication [LTPEXT], LTP also requires the underlying local data-link layer protocol to perform data integrity checking of the segments received. Specifically, the local data-link layer protocol is required to detect any corrupted segments received and to silently discard them.
(page 24 continued on part 2)
