Delay Tolerant Networking B. Sipos
Internet-Draft RKF Engineering
Obsoletes: 7242 (if approved) M. Demmer
Intended status: Standards Track UC Berkeley
Expires: August 1, 2018 J. Ott
Aalto University
S. Perreault
January 28, 2018
Delay-Tolerant Networking TCP Convergence Layer Protocol Version 4
draft-ietf-dtn-tcpclv4-06
Abstract
This document describes a revised protocol for the TCP-based
convergence layer (TCPCL) for Delay-Tolerant Networking (DTN). The
protocol revision is based on implementation issues in the original
TCPCL Version 3 and updates to the Bundle Protocol contents,
encodings, and convergence layer requirements in Bundle Protocol
Version 7. Specifically, the TCPCLv4 uses CBOR-encoded BPv7 bundles
as its service data unit being transported and provides a reliable
transport of such bundles. Several new IANA registries are defined
for TCPCLv4 which define some behaviors inherited from TCPCLv3 but
with updated encodings and/or semantics.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 1, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Convergence Layer Services . . . . . . . . . . . . . . . 4
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 6
2.1. Definitions Specific to the TCPCL Protocol . . . . . . . 6
3. General Protocol Description . . . . . . . . . . . . . . . . 7
3.1. TCPCL Session Overview . . . . . . . . . . . . . . . . . 7
3.2. Example Message Exchange . . . . . . . . . . . . . . . . 8
4. Session Establishment . . . . . . . . . . . . . . . . . . . . 9
4.1. TCP Connection . . . . . . . . . . . . . . . . . . . . . 10
4.2. Contact Header . . . . . . . . . . . . . . . . . . . . . 10
4.2.1. Header Extension Items . . . . . . . . . . . . . . . 13
4.3. Validation and Parameter Negotiation . . . . . . . . . . 14
4.4. Session Security . . . . . . . . . . . . . . . . . . . . 15
4.4.1. TLS Handshake Result . . . . . . . . . . . . . . . . 16
4.4.2. Example TLS Initiation . . . . . . . . . . . . . . . 16
5. Established Session Operation . . . . . . . . . . . . . . . . 17
5.1. Message Type Codes . . . . . . . . . . . . . . . . . . . 17
5.2. Upkeep and Status Messages . . . . . . . . . . . . . . . 18
5.2.1. Session Upkeep (KEEPALIVE) . . . . . . . . . . . . . 18
5.2.2. Message Rejection (MSG_REJECT) . . . . . . . . . . . 19
5.3. Bundle Transfer . . . . . . . . . . . . . . . . . . . . . 20
5.3.1. Bundle Transfer ID . . . . . . . . . . . . . . . . . 21
5.3.2. Transfer Initialization (XFER_INIT) . . . . . . . . . 21
5.3.3. Data Transmission (XFER_SEGMENT) . . . . . . . . . . 22
5.3.4. Data Acknowledgments (XFER_ACK) . . . . . . . . . . . 24
5.3.5. Transfer Refusal (XFER_REFUSE) . . . . . . . . . . . 25
6. Session Termination . . . . . . . . . . . . . . . . . . . . . 27
6.1. Shutdown Message (SHUTDOWN) . . . . . . . . . . . . . . . 27
6.2. Idle Session Shutdown . . . . . . . . . . . . . . . . . . 29
7. Security Considerations . . . . . . . . . . . . . . . . . . . 29
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
8.1. Port Number . . . . . . . . . . . . . . . . . . . . . . . 31
8.2. Protocol Versions . . . . . . . . . . . . . . . . . . . . 31
8.3. Header Extension Types . . . . . . . . . . . . . . . . . 32
8.4. Message Types . . . . . . . . . . . . . . . . . . . . . . 33
8.5. XFER_REFUSE Reason Codes . . . . . . . . . . . . . . . . 33
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8.6. SHUTDOWN Reason Codes . . . . . . . . . . . . . . . . . . 34
8.7. MSG_REJECT Reason Codes . . . . . . . . . . . . . . . . . 35
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 35
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
10.1. Normative References . . . . . . . . . . . . . . . . . . 35
10.2. Informative References . . . . . . . . . . . . . . . . . 36
Appendix A. Significant changes from RFC7242 . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38
1. Introduction
This document describes the TCP-based convergence-layer protocol for
Delay-Tolerant Networking. Delay-Tolerant Networking is an end-to-
end architecture providing communications in and/or through highly
stressed environments, including those with intermittent
connectivity, long and/or variable delays, and high bit error rates.
More detailed descriptions of the rationale and capabilities of these
networks can be found in "Delay-Tolerant Network Architecture"
[RFC4838].
An important goal of the DTN architecture is to accommodate a wide
range of networking technologies and environments. The protocol used
for DTN communications is the Bundle Protocol Version 7 (BPv7)
[I-D.ietf-dtn-bpbis], an application-layer protocol that is used to
construct a store-and-forward overlay network. BPv7 requires the
services of a "convergence-layer adapter" (CLA) to send and receive
bundles using the service of some "native" link, network, or Internet
protocol. This document describes one such convergence-layer adapter
that uses the well-known Transmission Control Protocol (TCP). This
convergence layer is referred to as TCP Convergence Layer Version 4
(TCPCLv4). For the remainder of this document, the abbreviation "BP"
without the version suffix refers to BPv7. For the remainder of this
document, the abbreviation "TCPCL" without the version suffix refers
to TCPCLv4.
The locations of the TCPCL and the BP in the Internet model protocol
stack (described in [RFC1122]) are shown in Figure 1. In particular,
when BP is using TCP as its bearer with TCPCL as its convergence
layer, both BP and TCPCL reside at the application layer of the
Internet model.
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+-------------------------+
| DTN Application | -\
+-------------------------| |
| Bundle Protocol (BP) | -> Application Layer
+-------------------------+ |
| TCP Conv. Layer (TCPCL) | |
+-------------------------+ |
| TLS (optional) | -/
+-------------------------+
| TCP | ---> Transport Layer
+-------------------------+
| IPv4/IPv6 | ---> Network Layer
+-------------------------+
| Link-Layer Protocol | ---> Link Layer
+-------------------------+
Figure 1: The Locations of the Bundle Protocol and the TCP
Convergence-Layer Protocol above the Internet Protocol Stack
This document describes the format of the protocol data units passed
between entities participating in TCPCL communications. This
document does not address:
o The format of protocol data units of the Bundle Protocol, as those
are defined elsewhere in [RFC5050] and [I-D.ietf-dtn-bpbis]. This
includes the concept of bundle fragmentation or bundle
encapsulation. The TCPCL transfers bundles as opaque data blocks.
o Mechanisms for locating or identifying other bundle nodes within
an internet.
1.1. Convergence Layer Services
This version of the TCPCL provides the following services to support
the overlaying Bundle Protocol agent:
Attempt Session The TCPCL allows a BP agent to pre-emptively attempt
to establish a TCPCL session with a peer node. Each session
attempt can send a different set of contact header parameters as
directed by the BP agent.
Session Started The TCPCL supports indication when a new TCP
connection has been started (as either client or server) before
the TCPCL handshake has begun.
Session Established The TCPCL supports indication when a new session
has been fully established and is ready for its first transfer.
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Session Shutdown The TCPCL supports indication when an established
session has been ended by normal exchange of SHUTDOWN messages
with all transfers completed.
Session Failed The TCPCL supports indication when a session fails,
either during contact negotiation, TLS negotiation, or after
establishement for any reason other than normal shutdown.
Transmission Availability Because TCPCL transmits serially over a
TCP connection, it suffers from "head of queue blocking" and
supports indication of when an established session is live-but-
idle (i.e. available for immediate transfer start) or live-and-
not-idle.
Transmission Success The TCPCL supports positive indication when a
bundle has been fully transferred to a peer node.
Transmission Intermediate Progress The TCPCL supports positive
indication of intermediate progress of transferr to a peer node.
This intermediate progress is at the granularity of each
transferred segment.
Transmission Failure The TCPCL supports positive indication of
certain reasons for bundle transmission failure, notably when the
peer node rejects the bundle or when a TCPCL session ends before
transferr success. The TCPCL itself does not have a notion of
transfer timeout.
Reception Interruption The TCPCL allows a BP agent to interrupt an
individual transfer before it has fully completed (successfully or
not).
Reception Success The TCPCL supports positive indication when a
bundle has been fully transferred from a peer node.
Reception Intermediate Progress The TCPCL supports positive
indication of intermediate progress of transfer from the peer
node. This intermediate progress is at the granularity of each
transferred segment. Intermediate reception indication allows a
BP agent the chance to inspect bundle header contents before the
entire bundle is available, and thus supports the "Reception
Interruption" capability.
Reception Failure The TCPCL supports positive indication of certain
reasons for reception failure, notably when the local node rejects
an attempted transfer for some local policy reason or when a TCPCL
session ends before transfer success. The TCPCL itself does not
have a notion of transfer timeout.
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2. Requirements Language
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 [RFC2119].
2.1. Definitions Specific to the TCPCL Protocol
This section contains definitions specific to the TCPCL protocol.
TCPCL Node: This term refers to either side of a negotiating or in-
service TCPCL Session. For most TCPCL behavior, the two nodes are
symmetric and there is no protocol distinction between them. Some
specific behavior, particularly during negotiation, distinguishes
between the connecting node and the connected-to node. For the
remainder of this document, the term "node" without the prefix
"TCPCL" refers to a TCPCL node.
TCP Connection: This term refers to a transport connection using TCP
as the transport protocol.
TCPCL Session: A TCPCL session (as opposed to a TCP connection) is a
TCPCL communication relationship between two bundle nodes. The
lifetime of a TCPCL session is bound to the lifetime of an
underlying TCP connection. A TCPCL session is terminated when the
TCP connection ends, due either to one or both nodes actively
terminating the TCP connection or due to network errors causing a
failure of the TCP connection. For the remainder of this
document, the term "session" without the prefix "TCPCL" refers to
a TCPCL session.
Session parameters: These are a set of values used to affect the
operation of the TCPCL for a given session. The manner in which
these parameters are conveyed to the bundle node and thereby to
the TCPCL is implementation dependent. However, the mechanism by
which two bundle nodes exchange and negotiate the values to be
used for a given session is described in Section 4.3.
Transfer: This refers to the procedures and mechanisms for
conveyance of an individual bundle from one node to another. Each
transfer within TCPCL is identified by a Transfer ID number which
is unique only to a single direction within a single Session.
Idle Session: A TCPCL session is idle while the only messages being
transmitted or received are KEEPALIVE messages.
Live Session: A TCPCL session is live while any messages are being
transmitted or received.
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Reason Codes: The TCPCL uses numeric codes to encode specific
reasons for individual failure/error message types.
3. General Protocol Description
The service of this protocol is the transmission of DTN bundles via
the Transmission Control Protocol (TCP). This document specifies the
encapsulation of bundles, procedures for TCP setup and teardown, and
a set of messages and node requirements. The general operation of
the protocol is as follows.
3.1. TCPCL Session Overview
First, one node establishes a TCPCL session to the other by
initiating a TCP connection in accordance with [RFC0793]. After
setup of the TCP connection is complete, an initial contact header is
exchanged in both directions to set parameters of the TCPCL session
and exchange a singleton endpoint identifier for each node (not the
singleton Endpoint Identifier (EID) of any application running on the
node) to denote the bundle-layer identity of each DTN node. This is
used to assist in routing and forwarding messages (e.g. to prevent
loops).
Once the TCPCL session is established and configured in this way,
bundles can be transferred in either direction. Each transfer is
performed by an initialization (XFER_INIT) message followed by one or
more logical segments of data within an XFER_SEGMENT message.
Multiple bundles can be transmitted consecutively on a single TCPCL
connection. Segments from different bundles are never interleaved.
Bundle interleaving can be accomplished by fragmentation at the BP
layer or by establishing multiple TCPCL sessions between the same
peers.
A feature of this protocol is for the receiving node to send
acknowledgment (XFER_ACK) messages as bundle data segments arrive .
The rationale behind these acknowledgments is to enable the sender
node to determine how much of the bundle has been received, so that
in case the session is interrupted, it can perform reactive
fragmentation to avoid re-sending the already transmitted part of the
bundle. In addition, there is no explicit flow control on the TCPCL
layer.
A TCPCL receiver can interrupt the transmission of a bundle at any
point in time by replying with a XFER_REFUSE message, which causes
the sender to stop transmission of the associated bundle (if it
hasn't already finished transmission) Note: This enables a cross-
layer optimization in that it allows a receiver that detects that it
already has received a certain bundle to interrupt transmission as
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early as possible and thus save transmission capacity for other
bundles.
For sessions that are idle, a KEEPALIVE message is sent at a
negotiated interval. This is used to convey node live-ness
information during otherwise message-less time intervals.
A SHUTDOWN message is used to start the closing of a TCPCL session
(see Section 6.1). During shutdown sequencing, in-progress transfers
can be completed but no new transfers can be initiated. A SHUTDOWN
message can also be used to refuse a session setup by a peer (see
Section 4.3). It is an implementation matter to determine whether or
not to close a TCPCL session while there are no transfers queued or
in-progress.
TCPCL is a symmetric protocol between the peers of a session. Both
sides can start sending data segments in a session, and one side's
bundle transfer does not have to complete before the other side can
start sending data segments on its own. Hence, the protocol allows
for a bi-directional mode of communication. Note that in the case of
concurrent bidirectional transmission, acknowledgment segments MAY be
interleaved with data segments.
3.2. Example Message Exchange
The following figure depicts the protocol exchange for a simple
session, showing the session establishment and the transmission of a
single bundle split into three data segments (of lengths "L1", "L2",
and "L3") from Node A to Node B.
Note that the sending node MAY transmit multiple XFER_SEGMENT
messages without necessarily waiting for the corresponding XFER_ACK
responses. This enables pipelining of messages on a channel.
Although this example only demonstrates a single bundle transmission,
it is also possible to pipeline multiple XFER_SEGMENT messages for
different bundles without necessarily waiting for XFER_ACK messages
to be returned for each one. However, interleaving data segments
from different bundles is not allowed.
No errors or rejections are shown in this example.
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Node A Node B
====== ======
+-------------------------+ +-------------------------+
| Contact Header | -> <- | Contact Header |
+-------------------------+ +-------------------------+
+-------------------------+
| XFER_INIT | ->
| Transfer ID [I1] |
| Total Length [L1] |
+-------------------------+
+-------------------------+
| XFER_SEGMENT (start) | ->
| Transfer ID [I1] |
| Length [L1] |
| Bundle Data 0..(L1-1) |
+-------------------------+
+-------------------------+ +-------------------------+
| XFER_SEGMENT | -> <- | XFER_ACK (start) |
| Transfer ID [I1] | | Transfer ID [I1] |
| Length [L2] | | Length [L1] |
|Bundle Data L1..(L1+L2-1)| +-------------------------+
+-------------------------+
+-------------------------+ +-------------------------+
| XFER_SEGMENT (end) | -> <- | XFER_ACK |
| Transfer ID [I1] | | Transfer ID [I1] |
| Length [L3] | | Length [L1+L2] |
|Bundle Data | +-------------------------+
| (L1+L2)..(L1+L2+L3-1)|
+-------------------------+
+-------------------------+
<- | XFER_ACK (end) |
| Transfer ID [I1] |
| Length [L1+L2+L3] |
+-------------------------+
+-------------------------+ +-------------------------+
| SHUTDOWN | -> <- | SHUTDOWN |
+-------------------------+ +-------------------------+
Figure 2: An Example of the Flow of Protocol Messages on a Single TCP
Session between Two Nodes (A and B)
4. Session Establishment
For bundle transmissions to occur using the TCPCL, a TCPCL session
MUST first be established between communicating nodes. It is up to
the implementation to decide how and when session setup is triggered.
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For example, some sessions MAY be opened proactively and maintained
for as long as is possible given the network conditions, while other
sessions MAY be opened only when there is a bundle that is queued for
transmission and the routing algorithm selects a certain next-hop
node.
4.1. TCP Connection
To establish a TCPCL session, a node MUST first establish a TCP
connection with the intended peer node, typically by using the
services provided by the operating system. Destination port number
4556 has been assigned by IANA as the Registered Port number for the
TCP convergence layer. Other destination port numbers MAY be used
per local configuration. Determining a peer's destination port
number (if different from the registered TCPCL port number) is up to
the implementation. Any source port number MAY be used for TCPCL
sessions. Typically an operating system assigned number in the TCP
Ephemeral range (49152-65535) is used.
If the node is unable to establish a TCP connection for any reason,
then it is an implementation matter to determine how to handle the
connection failure. A node MAY decide to re-attempt to establish the
connection. If it does so, it MUST NOT overwhelm its target with
repeated connection attempts. Therefore, the node MUST retry the
connection setup no earlier than some delay time from the last
attempt, and it SHOULD use a (binary) exponential backoff mechanism
to increase this delay in case of repeated failures. In case a
SHUTDOWN message specifying a reconnection delay is received, that
delay is used as the initial delay. The default initial re-attempt
delay SHOULD be no shorter than 1 second and SHOULD be configurable
since it will be application and network type dependent.
Once a TCP connection is established, each node MUST immediately
transmit a contact header over the TCP connection. The format of the
contact header is described in Section 4.2.
4.2. Contact Header
Once a TCP connection is established, both parties exchange a contact
header. This section describes the format of the contact header and
the meaning of its fields.
Upon receipt of the contact header, both nodes perform the validation
and negotiation procedures defined in Section 4.3. After receiving
the contact header from the other node, either node MAY refuse the
session by sending a SHUTDOWN message with an appropriate reason
code.
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The format for the Contact Header is as follows:
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| magic='dtn!' |
+---------------+---------------+---------------+---------------+
| Version | Flags | Keepalive Interval |
+---------------+---------------+---------------+---------------+
| Segment MRU... |
+---------------+---------------+---------------+---------------+
| contd. |
+---------------+---------------+---------------+---------------+
| Transfer MRU... |
+---------------+---------------+---------------+---------------+
| contd. |
+---------------+---------------+---------------+---------------+
| EID Length | EID Data... |
+---------------+---------------+---------------+---------------+
| EID Data contd. |
+---------------+---------------+---------------+---------------+
| Header Extension Length... |
+---------------+---------------+---------------+---------------+
| contd. |
+---------------+---------------+---------------+---------------+
| Header Extension Items... |
+---------------+---------------+---------------+---------------+
Figure 3: Contact Header Format
See Section 4.3 for details on the use of each of these contact
header fields. The fields of the contact header are:
magic: A four-octet field that always contains the octet sequence
0x64 0x74 0x6e 0x21, i.e., the text string "dtn!" in US-ASCII (and
UTF-8).
Version: A one-octet field value containing the value 4 (current
version of the protocol).
Flags: A one-octet field of single-bit flags, interpreted according
to the descriptions in Table 1.
Keepalive Interval: A 16-bit unsigned integer indicating the
interval, in seconds, between any subsequent messages being
transmitted by the peer. The peer receiving this contact header
uses this interval to determine how long to wait after any last-
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message transmission and a necessary subsequent KEEPALIVE message
transmission.
Segment MRU: A 64-bit unsigned integer indicating the largest
allowable single-segment data payload size to be received in this
session. Any XFER_SEGMENT sent to this peer SHALL have a data
payload no longer than the peer's Segment MRU. The two nodes of a
single session MAY have different Segment MRUs, and no relation
between the two is required.
Transfer MRU: A 64-bit unsigned integer indicating the largest
allowable total-bundle data size to be received in this session.
Any bundle transfer sent to this peer SHALL have a Total Bundle
Length payload no longer than the peer's Transfer MRU. This value
can be used to perform proactive bundle fragmentation. The two
nodes of a single session MAY have different Transfer MRUs, and no
relation between the two is required.
EID Length and EID Data: Together these fields represent a variable-
length text string. The EID Length is a 16-bit unsigned integer
indicating the number of octets of EID Data to follow. A zero EID
Length SHALL be used to indicate the lack of EID rather than a
truly empty EID. This case allows a node to avoid exposing EID
information on an untrusted network. A non-zero-length EID Data
SHALL contain the UTF-8 encoded EID of some singleton endpoint in
which the sending node is a member, in the canonical format of
<scheme name>:<scheme-specific part>. This EID encoding is
consistent with [I-D.ietf-dtn-bpbis].
Header Extension Length and Header Extension Items: Together these
fields represent protocol extension data not defined by this
specification. The Header Extension Length is the total number of
octets to follow which are used to encode the Header Extension
Item list. The encoding of each Header Extension Item is within a
consistent data container as described in Section 4.2.1.
+----------+--------+-----------------------------------------------+
| Name | Code | Description |
+----------+--------+-----------------------------------------------+
| CAN_TLS | 0x01 | If bit is set, indicates that the sending |
| | | peer is capable of TLS security. |
| | | |
| Reserved | others |
+----------+--------+-----------------------------------------------+
Table 1: Contact Header Flags
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4.2.1. Header Extension Items
Each of the Header Extension Items SHALL be encoded in an identical
Type-Length-Value (TLV) container form as indicated in Figure 4. The
fields of the Header Extension Item are:
Flags: A one-octet field containing generic bit flags about the
Item, which are listed in Table 2. If a TCPCL node receives a
Header Extension Item with an unknown Item Type and the CRITICAL
flag set, the node SHALL close the TCPCL session with SHUTDOWN
reason code of "Contact Failure". If the CRITICAL flag is not
set, a node SHALL skip over and ignore any item with an unknown
Item Type.
Item Type: A 16-bit unsigned integer field containing the type of
the extension item. This specification does not define any
extension types directly, but does allocate an IANA registry for
such codes (see Section 8.3).
Item Length: A 32-bit unsigned integer field containing the number
of Item Value octets to follow.
Item Value: A variable-length data field which is interpreted
according to the associated Item Type. This specification places
no restrictions on an extension's use of available Item Value
data. Extension specification SHOULD avoid the use of large data
exchanges within the TCPCL contact header as no bundle transfers
can begin until the full contact exchange and negotiation has been
completed.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| Item Flags | Item Type | Item Length...|
+---------------+---------------+---------------+---------------+
| length contd. | Item Value... |
+---------------+---------------+---------------+---------------+
| value contd. |
+---------------+---------------+---------------+---------------+
Figure 4: Header Extension Item Format
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+----------+--------+-----------------------------------------------+
| Name | Code | Description |
+----------+--------+-----------------------------------------------+
| CRITICAL | 0x01 | If bit is set, indicates that the receiving |
| | | peer must handle the extension item. |
| | | |
| Reserved | others |
+----------+--------+-----------------------------------------------+
Table 2: Header Extension Item Flags
4.3. Validation and Parameter Negotiation
Upon reception of the contact header, each node follows the following
procedures to ensure the validity of the TCPCL session and to
negotiate values for the session parameters.
If the magic string is not present or is not valid, the connection
MUST be terminated. The intent of the magic string is to provide
some protection against an inadvertent TCP connection by a different
protocol than the one described in this document. To prevent a flood
of repeated connections from a misconfigured application, a node MAY
elect to hold an invalid connection open and idle for some time
before closing it.
A connecting TCPCL node SHALL send the highest TCPCL protocol version
on a first session attempt for a TCPCL peer. If a connecting node
receives a SHUTDOWN message with reason of "Version Mismatch", that
node MAY attempt further TCPCL sessions with the peer using earlier
protocol version numbers in decreasing order. Managing multi-TCPCL-
session state such as this is an implementation matter.
If a node receives a contact header containing a version that is
greater than the current version of the protocol that the node
implements, then the node SHALL shutdown the session with a reason
code of "Version mismatch". If a node receives a contact header with
a version that is lower than the version of the protocol that the
node implements, the node MAY either terminate the session (with a
reason code of "Version mismatch") or the node MAY adapt its
operation to conform to the older version of the protocol. The
decision of version fall-back is an implementation matter.
A node calculates the parameters for a TCPCL session by negotiating
the values from its own preferences (conveyed by the contact header
it sent to the peer) with the preferences of the peer node (expressed
in the contact header that it received from the peer). The
negotiated parameters defined by this specification are described in
the following paragraphs.
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Transfer MTU and Segment MTU: The maximum transmit unit (MTU) for
whole transfers and individual segments are idententical to the
Transfer MRU and Segment MRU, respectively, of the recevied
contact header. A transmitting peer can send individual segments
with any size smaller than the Segment MTU, depending on local
policy, dynamic network conditions, etc. Determining the size of
each transmitted segment is an implementation matter.
Session Keepalive: Negotiation of the Session Keepalive parameter is
performed by taking the minimum of this two contact headers'
Keepalive Interval. The Session Keepalive interval is a parameter
for the behavior described in Section 5.2.1.
Enable TLS: Negotiation of the Enable TLS parameter is performed by
taking the logical AND of the two contact headers' CAN_TLS flags.
A local security policy is then applied to determine of the
negotated value of Enable TLS is acceptable. If not, the node
SHALL shutdown the session with a reason code of "Contact
Failure". Note that this contact failure is different than a "TLS
Failure" after an agreed-upon and acceptable Enable TLS state. If
the negotiated Enable TLS value is true and acceptable then TLS
negotiation feature (described in Section 4.4) begins immediately
following the contact header exchange.
Once this process of parameter negotiation is completed (which
includes a possible completed TLS handshake of the connection to use
TLS), this protocol defines no additional mechanism to change the
parameters of an established session; to effect such a change, the
TCPCL session MUST be terminated and a new session established.
4.4. Session Security
This version of the TCPCL supports establishing a Transport Layer
Security (TLS) session within an existing TCP connection. When TLS
is used within the TCPCL it affects the entire session. Once
established, there is no mechanism available to downgrade a TCPCL
session to non-TLS operation. If this is desired, the entire TCPCL
session MUST be shutdown and a new non-TLS-negotiated session
established.
The use of TLS is negotated using the Contact Header as described in
Section 4.3. After negotiating an Enable TLS parameter of true, and
before any other TCPCL messages are sent within the session, the
session nodes SHALL begin a TLS handshake in accordance with
[RFC5246]. The parameters within each TLS negotiation are
implementation dependent but any TCPCL node SHOULD follow all
recommended best practices of [RFC7525]. By convention, this
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protocol uses the node which initiated the underlying TCP connection
as the "client" role of the TLS handshake request.
The TLS handshake, if it occurs, is considered to be part of the
contact negotiation before the TCPCL session itself is established.
Specifics about sensitive data exposure are discussed in Section 7.
4.4.1. TLS Handshake Result
If a TLS handshake cannot negotiate a TLS session, both nodes of the
TCPCL session SHALL start a TCPCL shutdown with reason "TLS Failure".
After a TLS session is successfully established, both TCPCL nodes
SHALL re-exchange TCPCL Contact Header messages. Any information
cached from the prior Contact Header exchange SHALL be discarded.
This re-exchange avoids a "man-in-the-middle" attack in identical
fashion to [RFC2595]. Each re-exchange header CAN_TLS flag SHALL be
identical to the original header CAN_TLS flag from the same node.
The CAN_TLS logic (TLS negotiation) SHALL NOT apply during header re-
exchange. This reinforces the fact that there is no TLS downgrade
mechanism.
4.4.2. Example TLS Initiation
A summary of a typical CAN_TLS usage is shown in the sequence in
Figure 5 below.
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Node A Node B
====== ======
+-------------------------+
| Open TCP Connnection | ->
+-------------------------+ +-------------------------+
<- | Accept Connection |
+-------------------------+
+-------------------------+ +-------------------------+
| Contact Header | -> <- | Contact Header |
+-------------------------+ +-------------------------+
+-------------------------+ +-------------------------+
| TLS Negotiation | -> <- | TLS Negotiation |
| (as client) | | (as server) |
+-------------------------+ +-------------------------+
+-------------------------+ +-------------------------+
| Contact Header | -> <- | Contact Header |
+-------------------------+ +-------------------------+
... secured TCPCL messaging ...
+-------------------------+ +-------------------------+
| SHUTDOWN | -> <- | SHUTDOWN |
+-------------------------+ +-------------------------+
Figure 5: A simple visual example of TCPCL TLS Establishment between
two nodes
5. Established Session Operation
This section describes the protocol operation for the duration of an
established session, including the mechanism for transmitting bundles
over the session.
5.1. Message Type Codes
After the initial exchange of a contact header, all messages
transmitted over the session are identified by a one-octet header
with the following structure:
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0 1 2 3 4 5 6 7
+---------------+
| Message Type |
+---------------+
Figure 6: Format of the Message Header
The message header fields are as follows:
Message Type: Indicates the type of the message as per Table 3
below. Encoded values are listed in Section 8.4.
+--------------+----------------------------------------------------+
| Type | Description |
+--------------+----------------------------------------------------+
| XFER_INIT | Contains the length (in octets) of the next |
| | transfer, as described in Section 5.3.2. |
| | |
| XFER_SEGMENT | Indicates the transmission of a segment of bundle |
| | data, as described in Section 5.3.3. |
| | |
| XFER_ACK | Acknowledges reception of a data segment, as |
| | described in Section 5.3.4. |
| | |
| XFER_REFUSE | Indicates that the transmission of the current |
| | bundle SHALL be stopped, as described in Section |
| | 5.3.5. |
| | |
| KEEPALIVE | Used to keep TCPCL session active, as described in |
| | Section 5.2.1. |
| | |
| SHUTDOWN | Indicates that one of the nodes participating in |
| | the session wishes to cleanly terminate the |
| | session, as described in Section 6. |
| | |
| MSG_REJECT | Contains a TCPCL message rejection, as described |
| | in Section 5.2.2. |
+--------------+----------------------------------------------------+
Table 3: TCPCL Message Types
5.2. Upkeep and Status Messages
5.2.1. Session Upkeep (KEEPALIVE)
The protocol includes a provision for transmission of KEEPALIVE
messages over the TCPCL session to help determine if the underlying
TCP connection has been disrupted.
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As described in Section 4.3, a negotiated parameter of each session
is the Session Keepalive interval. If the negotiated Session
Keepalive is zero (i.e. one or both contact headers contains a zero
Keepalive Interval), then the keepalive feature is disabled. There
is no logical minimum value for the keepalive interval, but when used
for many sessions on an open, shared network a short interval could
lead to excessive traffic. For shared network use, nodes SHOULD
choose a keepalive interval no shorter than 30 seconds. There is no
logical maximum value for the keepalive interval, but an idle TCP
connection is liable for closure by the host operating system if the
keepalive time is longer than tens-of-minutes. Nodes SHOULD choose a
keepalive interval no longer than 10 minutes (600 seconds).
Note: The Keepalive Interval SHOULD NOT be chosen too short as TCP
retransmissions MAY occur in case of packet loss. Those will have to
be triggered by a timeout (TCP retransmission timeout (RTO)), which
is dependent on the measured RTT for the TCP connection so that
KEEPALIVE messages MAY experience noticeable latency.
The format of a KEEPALIVE message is a one-octet message type code of
KEEPALIVE (as described in Table 3) with no additional data. Both
sides SHOULD send a KEEPALIVE message whenever the negotiated
interval has elapsed with no transmission of any message (KEEPALIVE
or other).
If no message (KEEPALIVE or other) has been received in a session
after some implementation-defined time duration, then the node MAY
terminate the session by transmitting a one-octet SHUTDOWN message
(as described in Section 6.1) with reason code "Idle Timeout.
5.2.2. Message Rejection (MSG_REJECT)
If a TCPCL node receives a message which is unknown to it (possibly
due to an unhandled protocol mismatch) or is inappropriate for the
current session state (e.g. a KEEPALIVE message received after
contact header negotiation has disabled that feature), there is a
protocol-level message to signal this condition in the form of a
MSG_REJECT reply.
The format of a MSG_REJECT message follows:
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+-----------------------------+
| Message Header |
+-----------------------------+
| Reason Code (U8) |
+-----------------------------+
| Rejected Message Header |
+-----------------------------+
Figure 7: Format of MSG_REJECT Messages
The fields of the MSG_REJECT message are:
Reason Code: A one-octet refusal reason code interpreted according
to the descriptions in Table 4.
Rejected Message Header: The Rejected Message Header is a copy of
the Message Header to which the MSG_REJECT message is sent as a
response.
+-------------+------+----------------------------------------------+
| Name | Code | Description |
+-------------+------+----------------------------------------------+
| Message | 0x01 | A message was received with a Message Type |
| Type | | code unknown to the TCPCL node. |
| Unknown | | |
| | | |
| Message | 0x02 | A message was received but the TCPCL node |
| Unsupported | | cannot comply with the message contents. |
| | | |
| Message | 0x03 | A message was received while the session is |
| Unexpected | | in a state in which the message is not |
| | | expected. |
+-------------+------+----------------------------------------------+
Table 4: MSG_REJECT Reason Codes
5.3. Bundle Transfer
All of the messages in this section are directly associated with
transferring a bundle between TCPCL nodes.
A single TCPCL transfer results in a bundle (handled by the
convergence layer as opaque data) being exchanged from one node to
the other. In TCPCL a transfer is accomplished by dividing a single
bundle up into "segments" based on the receiving-side Segment MRU
(see Section 4.2). The choice of the length to use for segments is
an implementation matter, but each segment MUST be no larger than the
receiving node's maximum receive unit (MRU) (see the field "Segment
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MRU" of Section 4.2). The first segment for a bundle MUST set the
'START' flag, and the last one MUST set the 'end' flag in the
XFER_SEGMENT message flags.
A single transfer (and by extension a single segment) SHALL NOT
contain data of more than a single bundle. This requirement is
imposed on the agent using the TCPCL rather than TCPCL itself.
If multiple bundles are transmitted on a single TCPCL connection,
they MUST be transmitted consecutively without interleaving of
segments from multiple bundles.
5.3.1. Bundle Transfer ID
Each of the bundle transfer messages contains a Transfer ID which is
used to correlate messages (from both sides of a transfer) for each
bundle. A Transfer ID does not attempt to address uniqueness of the
bundle data itself and has no relation to concepts such as bundle
fragmentation. Each invocation of TCPCL by the bundle protocol
agent, requesting transmission of a bundle (fragmentary or
otherwise), results in the initiation of a single TCPCL transfer.
Each transfer entails the sending of a XFER_INIT message and some
number of XFER_SEGMENT and XFER_ACK messages; all are correlated by
the same Transfer ID.
Transfer IDs from each node SHALL be unique within a single TCPCL
session. The initial Transfer ID from each node SHALL have value
zero. Subsequent Transfer ID values SHALL be incremented from the
prior Transfer ID value by one. Upon exhaustion of the entire 64-bit
Transfer ID space, the sending node SHALL terminate the session with
SHUTDOWN reason code "Resource Exhaustion".
For bidirectional bundle transfers, a TCPCL node SHOULD NOT rely on
any relation between Transfer IDs originating from each side of the
TCPCL session.
5.3.2. Transfer Initialization (XFER_INIT)
The XFER_INIT message contains the total length, in octets, of the
bundle data in the associated transfer. The total length is
formatted as a 64-bit unsigned integer.
The purpose of the XFER_INIT message is to allow nodes to
preemptively refuse bundles that would exceed their resources or to
prepare storage on the receiving node for the upcoming bundle data.
See Section 5.3.5 for details on when refusal based on XFER_INIT
content is acceptable.
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The Total Bundle Length field within a XFER_INIT message SHALL be
treated as authoritative by the receiver. If, for whatever reason,
the actual total length of bundle data received differs from the
value indicated by the XFER_INIT message, the receiver SHOULD treat
the transmitted data as invalid.
The format of the XFER_INIT message is as follows:
+-----------------------------+
| Message Header |
+-----------------------------+
| Transfer ID (U64) |
+-----------------------------+
| Total Bundle Length (U64) |
+-----------------------------+
Figure 8: Format of XFER_INIT Messages
The fields of the XFER_INIT message are:
Transfer ID: A 64-bit unsigned integer identifying the transfer
about to begin.
Total Bundle Length: A 64-bit unsigned integer indicating the size
of the data-to-be-transferred.
An XFER_INIT message SHALL be sent as the first message in a transfer
sequence, before transmission of any XFER_SEGMENT messages for the
same Transfer ID. XFER_INIT messages MUST NOT be sent unless the
next XFER_SEGMENT message has the 'START' bit set to "1" (i.e., just
before the start of a new transfer).
5.3.3. Data Transmission (XFER_SEGMENT)
Each bundle is transmitted in one or more data segments. The format
of a XFER_SEGMENT message follows in Figure 9.
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+------------------------------+
| Message Header |
+------------------------------+
| Message Flags (U8) |
+------------------------------+
| Transfer ID (U64) |
+------------------------------+
| Data length (U64) |
+------------------------------+
| Data contents (octet string) |
+------------------------------+
Figure 9: Format of XFER_SEGMENT Messages
The fields of the XFER_SEGMENT message are:
Message Flags: A one-octet field of single-bit flags, interpreted
according to the descriptions in Table 5.
Transfer ID: A 64-bit unsigned integer identifying the transfer
being made.
Data length: A 64-bit unsigned integer indicating the number of
octets in the Data contents to follow.
Data contents: The variable-length data payload of the message.
+----------+--------+-----------------------------------------------+
| Name | Code | Description |
+----------+--------+-----------------------------------------------+
| END | 0x01 | If bit is set, indicates that this is the |
| | | last segment of the transfer. |
| | | |
| START | 0x02 | If bit is set, indicates that this is the |
| | | first segment of the transfer. |
| | | |
| Reserved | others |
+----------+--------+-----------------------------------------------+
Table 5: XFER_SEGMENT Flags
The flags portion of the message contains two optional values in the
two low-order bits, denoted 'START' and 'END' in Table 5. The
'START' bit MUST be set to one if it precedes the transmission of the
first segment of a transfer. The 'END' bit MUST be set to one when
transmitting the last segment of a transfer. In the case where an
entire transfer is accomplished in a single segment, both the 'START'
and 'END' bits MUST be set to one.
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Once a transfer of a bundle has commenced, the node MUST only send
segments containing sequential portions of that bundle until it sends
a segment with the 'END' bit set. No interleaving of multiple
transfers from the same node is possible within a single TCPCL
session. Simultaneous transfers between two nodes MAY be achieved
using multiple TCPCL sessions.
5.3.4. Data Acknowledgments (XFER_ACK)
Although the TCP transport provides reliable transfer of data between
transport peers, the typical BSD sockets interface provides no means
to inform a sending application of when the receiving application has
processed some amount of transmitted data. Thus, after transmitting
some data, the TCPCL needs an additional mechanism to determine
whether the receiving agent has successfully received the segment.
To this end, the TCPCL protocol provides feedback messaging whereby a
receiving node transmits acknowledgments of reception of data
segments.
The format of an XFER_ACK message follows in Figure 10.
+-----------------------------+
| Message Header |
+-----------------------------+
| Message Flags (U8) |
+-----------------------------+
| Transfer ID (U64) |
+-----------------------------+
| Acknowledged length (U64) |
+-----------------------------+
Figure 10: Format of XFER_ACK Messages
The fields of the XFER_ACK message are:
Message Flags: A one-octet field of single-bit flags, interpreted
according to the descriptions in Table 5.
Transfer ID: A 64-bit unsigned integer identifying the transfer
being acknowledged.
Acknowledged length: A 64-bit unsigned integer indicating the total
number of octets in the transfer which are being acknowledged.
A receiving TCPCL node SHALL send an XFER_ACK message in response to
each received XFER_SEGMENT message. The flags portion of the
XFER_ACK header SHALL be set to match the corresponding DATA_SEGMENT
message being acknowledged. The acknowledged length of each XFER_ACK
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contains the sum of the data length fields of all XFER_SEGMENT
messages received so far in the course of the indicated transfer.
The sending node MAY transmit multiple XFER_SEGMENT messages without
necessarily waiting for the corresponding XFER_ACK responses. This
enables pipelining of messages on a channel.
For example, suppose the sending node transmits four segments of
bundle data with lengths 100, 200, 500, and 1000, respectively.
After receiving the first segment, the node sends an acknowledgment
of length 100. After the second segment is received, the node sends
an acknowledgment of length 300. The third and fourth
acknowledgments are of length 800 and 1800, respectively.
5.3.5. Transfer Refusal (XFER_REFUSE)
The TCPCL supports a mechanism by which a receiving node can indicate
to the sender that it does not want to receive the corresponding
bundle. To do so, upon receiving a XFER_INIT or XFER_SEGMENT
message, the node MAY transmit a XFER_REFUSE message. As data
segments and acknowledgments MAY cross on the wire, the bundle that
is being refused SHALL be identified by the Transfer ID of the
refusal.
There is no required relation between the Transfer MRU of a TCPCL
node (which is supposed to represent a firm limitation of what the
node will accept) and sending of a XFER_REFUSE message. A
XFER_REFUSE can be used in cases where the agent's bundle storage is
temporarily depleted or somehow constrained. A XFER_REFUSE can also
be used after the bundle header or any bundle data is inspected by an
agent and determined to be unacceptable.
A receiver MAY send an XFER_REFUSE message as soon as it receives a
XFER_INIT message without waiting for the next XFER_SEGMENT message.
The sender MUST be prepared for this and MUST associate the refusal
with the correct bundle via the Transfer ID fields.
The format of the XFER_REFUSE message is as follows:
+-----------------------------+
| Message Header |
+-----------------------------+
| Reason Code (U8) |
+-----------------------------+
| Transfer ID (U64) |
+-----------------------------+
Figure 11: Format of XFER_REFUSE Messages
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The fields of the XFER_REFUSE message are:
Reason Code: A one-octet refusal reason code interpreted according
to the descriptions in Table 6.
Transfer ID: A 64-bit unsigned integer identifying the transfer
being refused.
+------------+------------------------------------------------------+
| Name | Semantics |
+------------+------------------------------------------------------+
| Unknown | Reason for refusal is unknown or not specified. |
| | |
| Completed | The receiver already has the complete bundle. The |
| | sender MAY consider the bundle as completely |
| | received. |
| | |
| No | The receiver's resources are exhausted. The sender |
| Resources | SHOULD apply reactive bundle fragmentation before |
| | retrying. |
| | |
| Retransmit | The receiver has encountered a problem that requires |
| | the bundle to be retransmitted in its entirety. |
+------------+------------------------------------------------------+
Table 6: XFER_REFUSE Reason Codes
The receiver MUST, for each transfer preceding the one to be refused,
have either acknowledged all XFER_SEGMENTs or refused the bundle
transfer.
The bundle transfer refusal MAY be sent before an entire data segment
is received. If a sender receives a XFER_REFUSE message, the sender
MUST complete the transmission of any partially sent XFER_SEGMENT
message. There is no way to interrupt an individual TCPCL message
partway through sending it. The sender MUST NOT commence
transmission of any further segments of the refused bundle
subsequently. Note, however, that this requirement does not ensure
that a node will not receive another XFER_SEGMENT for the same bundle
after transmitting a XFER_REFUSE message since messages MAY cross on
the wire; if this happens, subsequent segments of the bundle SHOULD
also be refused with a XFER_REFUSE message.
Note: If a bundle transmission is aborted in this way, the receiver
MAY not receive a segment with the 'END' flag set to '1' for the
aborted bundle. The beginning of the next bundle is identified by
the 'START' bit set to '1', indicating the start of a new transfer,
and with a distinct Transfer ID value.
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6. Session Termination
This section describes the procedures for ending a TCPCL session.
6.1. Shutdown Message (SHUTDOWN)
To cleanly shut down a session, a SHUTDOWN message MUST be
transmitted by either node at any point following complete
transmission of any other message. After sending a SHUTDOWN message,
the sender of the message MAY send further acknowledgments (XFER_ACK
or XFER_REFUSE) but no further data messages (XFER_INIT or
XFER_SEGMENT). A receiving node SHOULD acknowledge all received data
segments before sending a SHUTDOWN message to end the session. A
transmitting node SHALL treat a SHUTDOWN message received mid-
transfer (i.e. before the final acknowledgment) as a failure of the
transfer.
After transmitting a SHUTDOWN message, a node MAY immediately close
the associated TCP connection. Once the SHUTDOWN message is sent,
any further received data on the TCP connection SHOULD be ignored.
Any delay between request to terminate the TCP connection and actual
closing of the connection (a "half-closed" state) MAY be ignored by
the TCPCL node.
The format of the SHUTDOWN message is as follows:
+-----------------------------------+
| Message Header |
+-----------------------------------+
| Message Flags (U8) |
+-----------------------------------+
| Reason Code (optional U8) |
+-----------------------------------+
| Reconnection Delay (optional U16) |
+-----------------------------------+
Figure 12: Format of SHUTDOWN Messages
The fields of the SHUTDOWN message are:
Message Flags: A one-octet field of single-bit flags, interpreted
according to the descriptions in Table 7.
Reason Code: A one-octet refusal reason code interpreted according
to the descriptions in Table 8. The Reason Code is present or
absent as indicated by one of the flags.
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Reconnection Delay: A 16-bit unsigned integer indicating the desired
delay, in seconds, before re-attepmting a TCPCL session to the
sending node. The Reconnection Delay is present or absent as
indicated by one of the flags.
+----------+--------+-----------------------------------------------+
| Name | Code | Description |
+----------+--------+-----------------------------------------------+
| D | 0x01 | If bit is set, indicates that a Reconnection |
| | | Delay field is present. |
| | | |
| R | 0x02 | If bit is set, indicates that a Reason Code |
| | | field is present. |
| | | |
| Reserved | others |
+----------+--------+-----------------------------------------------+
Table 7: SHUTDOWN Flags
It is possible for a node to convey optional information regarding
the reason for session termination. To do so, the node MUST set the
'R' bit in the message flags and transmit a one-octet reason code
immediately following the message header. The specified values of
the reason code are:
+---------------+---------------------------------------------------+
| Name | Description |
+---------------+---------------------------------------------------+
| Idle timeout | The session is being closed due to idleness. |
| | |
| Version | The node cannot conform to the specified TCPCL |
| mismatch | protocol version. |
| | |
| Busy | The node is too busy to handle the current |
| | session. |
| | |
| Contact | The node cannot interpret or negotiate contact |
| Failure | header option. |
| | |
| TLS Failure | The node failed to negotiate TLS session and |
| | cannot continue the session. |
| | |
| Resource | The node has run into some resource limit and |
| Exhaustion | cannot continue the session. |
+---------------+---------------------------------------------------+
Table 8: SHUTDOWN Reason Codes
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If a node does not want its peer to reopen a connection immediately,
it SHALL set the 'D' bit in the flags and include a reconnection
delay to indicate when the peer is allowed to attempt another session
setup. The Reconnection Delay value 0 SHALL be interpreted as an
infinite delay, i.e., that the connecting node MUST NOT re-establish
the session.
A session shutdown MAY occur immediately after transmission of a
contact header (and prior to any further message transmit). This
MAY, for example, be used to notify that the node is currently not
able or willing to communicate. However, a node MUST always send the
contact header to its peer before sending a SHUTDOWN message.
If reception of the contact header itself somehow fails (e.g. an
invalid "magic string" is recevied), a node SHOULD close the TCP
connection without sending a SHUTDOWN message. If the content of the
Header Extension Items data disagrees with the Header Extension
Length (i.e. the last Item claims to use more octets than are present
in the Header Extension Length), the reception of the contact header
is considered to have failed.
If a session is to be terminated before a protocol message has
completed being sent, then the node MUST NOT transmit the SHUTDOWN
message but still SHOULD close the TCP connection. Each TCPCL
message is contiguous in the octet stream and has no ability to be
cut short and/or preempted by an other message. This is particularly
important when large segment sizes are being transmitted; either
entire XFER_SEGMENT is sent before a SHUTDOWN message or the
connection is simply terminated mid-XFER_SEGMENT.
6.2. Idle Session Shutdown
The protocol includes a provision for clean shutdown of idle
sessions. Determining the length of time to wait before closing idle
sessions, if they are to be closed at all, is an implementation and
configuration matter.
If there is a configured time to close idle links and if no TCPCL
messages (other than KEEPALIVE messages) has been received for at
least that amount of time, then either node MAY terminate the session
by transmitting a SHUTDOWN message indicating the reason code of
"Idle timeout" (as described in Table 8).
7. Security Considerations
One security consideration for this protocol relates to the fact that
nodes present their endpoint identifier as part of the contact header
exchange. It would be possible for a node to fake this value and
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present the identity of a singleton endpoint in which the node is not
a member, essentially masquerading as another DTN node. If this
identifier is used outside of a TLS-secured session or without
further verification as a means to determine which bundles are
transmitted over the session, then the node that has falsified its
identity would be able to obtain bundles that it otherwise would not
have. Therefore, a node SHALL NOT use the EID value of an unsecured
contact header to derive a peer node's identity unless it can
corroborate it via other means. When TCPCL session security is
mandated by a TCPCL peer, that peer SHALL transmit initial unsecured
contact header values indicated in Table 9 in order. These values
avoid unnecessarily leaking session parameters and will be ignored
when secure contact header re-exchange occurs.
+--------------------+---------------------------------------------+
| Parameter | Value |
+--------------------+---------------------------------------------+
| Flags | The USE_TLS flag is set. |
| | |
| Keepalive Interval | Zero, indicating no keepalive. |
| | |
| Segment MRU | Zero, indicating all segments are refused. |
| | |
| Transfer MRU | Zero, indicating all transfers are refused. |
| | |
| EID | Empty, indicating lack of EID. |
+--------------------+---------------------------------------------+
Table 9: Recommended Unsecured Contact Header
TCPCL can be used to provide point-to-point transport security, but
does not provide security of data-at-rest and does not guarantee end-
to-end bundle security. The mechanisms defined in [RFC6257] and
[I-D.ietf-dtn-bpsec] are to be used instead.
Even when using TLS to secure the TCPCL session, the actual
ciphersuite negotiated between the TLS peers MAY be insecure. TLS
can be used to perform authentication without data confidentiality,
for example. It is up to security policies within each TCPCL node to
ensure that the negotiated TLS ciphersuite meets transport security
requirements. This is identical behavior to STARTTLS use in
[RFC2595].
Another consideration for this protocol relates to denial-of-service
attacks. A node MAY send a large amount of data over a TCPCL
session, requiring the receiving node to handle the data, attempt to
stop the flood of data by sending a XFER_REFUSE message, or forcibly
terminate the session. This burden could cause denial of service on
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other, well-behaving sessions. There is also nothing to prevent a
malicious node from continually establishing sessions and repeatedly
trying to send copious amounts of bundle data. A listening node MAY
take countermeasures such as ignoring TCP SYN messages, closing TCP
connections as soon as they are established, waiting before sending
the contact header, sending a SHUTDOWN message quickly or with a
delay, etc.
8. IANA Considerations
In this section, registration procedures are as defined in [RFC5226].
Some of the registries below are created new for TCPCLv4 but share
code values with TCPCLv3. This was done to disambiguate the use of
these values between TCPCLv3 and TCPCLv4 while preserving the
semantics of some values.
8.1. Port Number
Port number 4556 has been previously assigned as the default port for
the TCP convergence layer in [RFC7242]. This assignment is unchanged
by protocol version 4. Each TCPCL node identifies its TCPCL protocol
version in its initial contact (see Section 8.2), so there is no
ambiguity about what protocol is being used.
+------------------------+-------------------------------------+
| Parameter | Value |
+------------------------+-------------------------------------+
| Service Name: | dtn-bundle |
| | |
| Transport Protocol(s): | TCP |
| | |
| Assignee: | Simon Perreault <simon@per.reau.lt> |
| | |
| Contact: | Simon Perreault <simon@per.reau.lt> |
| | |
| Description: | DTN Bundle TCP CL Protocol |
| | |
| Reference: | [RFC7242] |
| | |
| Port Number: | 4556 |
+------------------------+-------------------------------------+
8.2. Protocol Versions
IANA has created, under the "Bundle Protocol" registry, a sub-
registry titled "Bundle Protocol TCP Convergence-Layer Version
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Numbers" and initialize it with the following table. The
registration procedure is RFC Required.
+-------+-------------+---------------------+
| Value | Description | Reference |
+-------+-------------+---------------------+
| 0 | Reserved | [RFC7242] |
| | | |
| 1 | Reserved | [RFC7242] |
| | | |
| 2 | Reserved | [RFC7242] |
| | | |
| 3 | TCPCL | [RFC7242] |
| | | |
| 4 | TCPCLbis | This specification. |
| | | |
| 5-255 | Unassigned |
+-------+-------------+---------------------+
8.3. Header Extension Types
EDITOR NOTE: sub-registry to-be-created upon publication of this
specification.
IANA will create, under the "Bundle Protocol" registry, a sub-
registry titled "Bundle Protocol TCP Convergence-Layer Version 4
Header Extension Types" and initialize it with the contents of
Table 10. The registration procedure is RFC Required within the
lower range 0x0001--0x3fff. Values in the range 0x8000--0xffff are
reserved for use on private networks for functions not published to
the IANA.
+----------------+--------------------------+
| Code | Message Type |
+----------------+--------------------------+
| 0x0000 | Reserved |
| | |
| 0x0001--0x3fff | Unassigned |
| | |
| 0x8000--0xffff | Private/Experimental Use |
+----------------+--------------------------+
Table 10: Header Extension Type Codes
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8.4. Message Types
EDITOR NOTE: sub-registry to-be-created upon publication of this
specification.
IANA will create, under the "Bundle Protocol" registry, a sub-
registry titled "Bundle Protocol TCP Convergence-Layer Version 4
Message Types" and initialize it with the contents of Table 11. The
registration procedure is RFC Required.
+-----------+--------------+
| Code | Message Type |
+-----------+--------------+
| 0x00 | Reserved |
| | |
| 0x01 | XFER_SEGMENT |
| | |
| 0x02 | XFER_ACK |
| | |
| 0x03 | XFER_REFUSE |
| | |
| 0x04 | KEEPALIVE |
| | |
| 0x05 | SHUTDOWN |
| | |
| 0x06 | XFER_INIT |
| | |
| 0x07 | MSG_REJECT |
| | |
| 0x08--0xf | Unassigned |
+-----------+--------------+
Table 11: Message Type Codes
8.5. XFER_REFUSE Reason Codes
EDITOR NOTE: sub-registry to-be-created upon publication of this
specification.
IANA will create, under the "Bundle Protocol" registry, a sub-
registry titled "Bundle Protocol TCP Convergence-Layer Version 4
XFER_REFUSE Reason Codes" and initialize it with the contents of
Table 12. The registration procedure is RFC Required.
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+----------+---------------------------+
| Code | Refusal Reason |
+----------+---------------------------+
| 0x0 | Unknown |
| | |
| 0x1 | Completed |
| | |
| 0x2 | No Resources |
| | |
| 0x3 | Retransmit |
| | |
| 0x4--0x7 | Unassigned |
| | |
| 0x8--0xf | Reserved for future usage |
+----------+---------------------------+
Table 12: XFER_REFUSE Reason Codes
8.6. SHUTDOWN Reason Codes
EDITOR NOTE: sub-registry to-be-created upon publication of this
specification.
IANA will create, under the "Bundle Protocol" registry, a sub-
registry titled "Bundle Protocol TCP Convergence-Layer Version 4
SHUTDOWN Reason Codes" and initialize it with the contents of
Table 13. The registration procedure is RFC Required.
+------------+---------------------+
| Code | Shutdown Reason |
+------------+---------------------+
| 0x00 | Idle timeout |
| | |
| 0x01 | Version mismatch |
| | |
| 0x02 | Busy |
| | |
| 0x03 | Contact Failure |
| | |
| 0x04 | TLS failure |
| | |
| 0x05 | Resource Exhaustion |
| | |
| 0x06--0xFF | Unassigned |
+------------+---------------------+
Table 13: SHUTDOWN Reason Codes
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8.7. MSG_REJECT Reason Codes
EDITOR NOTE: sub-registry to-be-created upon publication of this
specification.
IANA will create, under the "Bundle Protocol" registry, a sub-
registry titled "Bundle Protocol TCP Convergence-Layer Version 4
MSG_REJECT Reason Codes" and initialize it with the contents of
Table 14. The registration procedure is RFC Required.
+-----------+----------------------+
| Code | Rejection Reason |
+-----------+----------------------+
| 0x00 | reserved |
| | |
| 0x01 | Message Type Unknown |
| | |
| 0x02 | Message Unsupported |
| | |
| 0x03 | Message Unexpected |
| | |
| 0x04-0xFF | Unassigned |
+-----------+----------------------+
Table 14: REJECT Reason Codes
9. Acknowledgments
This specification is based on comments on implementation of
[RFC7242] provided from Scott Burleigh.
10. References
10.1. Normative References
[I-D.ietf-dtn-bpbis]
Burleigh, S., Fall, K., and E. Birrane, "Bundle Protocol
Version 7", draft-ietf-dtn-bpbis-10 (work in progress),
November 2017.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5050] Scott, K. and S. Burleigh, "Bundle Protocol
Specification", RFC 5050, DOI 10.17487/RFC5050, November
2007, <https://www.rfc-editor.org/info/rfc5050>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<https://www.rfc-editor.org/info/rfc5226>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>.
10.2. Informative References
[I-D.ietf-dtn-bpsec]
Birrane, E. and K. McKeever, "Bundle Protocol Security
Specification", draft-ietf-dtn-bpsec-06 (work in
progress), October 2017.
[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP",
RFC 2595, DOI 10.17487/RFC2595, June 1999,
<https://www.rfc-editor.org/info/rfc2595>.
[RFC4838] Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst,
R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant
Networking Architecture", RFC 4838, DOI 10.17487/RFC4838,
April 2007, <https://www.rfc-editor.org/info/rfc4838>.
[RFC6257] Symington, S., Farrell, S., Weiss, H., and P. Lovell,
"Bundle Security Protocol Specification", RFC 6257,
DOI 10.17487/RFC6257, May 2011,
<https://www.rfc-editor.org/info/rfc6257>.
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[RFC7242] Demmer, M., Ott, J., and S. Perreault, "Delay-Tolerant
Networking TCP Convergence-Layer Protocol", RFC 7242,
DOI 10.17487/RFC7242, June 2014,
<https://www.rfc-editor.org/info/rfc7242>.
Appendix A. Significant changes from RFC7242
The areas in which changes from [RFC7242] have been made to existing
headers and messages are:
o Changed contact header content to limit number of negotiated
options.
o Added contact option to negotiate maximum segment size (per each
direction).
o Added contact header extension capability.
o Defined new IANA registries for message / type / reason codes to
allow renaming some codes for clarity.
o Expanded Message Header to octet-aligned fields instead of bit-
packing.
o Added a bundle transfer identification number to all bundle-
related messages (XFER_INIT, XFER_SEGMENT, XFER_ACK, XFER_REFUSE).
o Use flags in XFER_ACK to mirror flags from XFER_SEGMENT.
o Removed all uses of SDNV fields and replaced with fixed-bit-length
fields.
The areas in which extensions from [RFC7242] have been made as new
messages and codes are:
o Added contact negotiation failure SHUTDOWN reason code.
o Added MSG_REJECT message to indicate an unknown or unhandled
message was received.
o Added TLS session security mechanism.
o Added TLS failure and Resource Exhaustion SHUTDOWN reason code.
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Authors' Addresses
Brian Sipos
RKF Engineering Solutions, LLC
7500 Old Georgetown Road
Suite 1275
Bethesda, MD 20814-6198
US
Email: BSipos@rkf-eng.com
Michael Demmer
University of California, Berkeley
Computer Science Division
445 Soda Hall
Berkeley, CA 94720-1776
US
Email: demmer@cs.berkeley.edu
Joerg Ott
Aalto University
Department of Communications and Networking
PO Box 13000
Aalto 02015
Finland
Email: jo@netlab.tkk.fi
Simon Perreault
Quebec, QC
Canada
Email: simon@per.reau.lt
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