Delay Tolerant Networking B. Sipos
Internet-Draft RKF Engineering
Obsoletes: 7242 (if approved) M. Demmer
Intended status: Standards Track UC Berkeley
Expires: March 22, 2020 J. Ott
Aalto University
S. Perreault
September 19, 2019
Delay-Tolerant Networking TCP Convergence Layer Protocol Version 4
draft-ietf-dtn-tcpclv4-14
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 of RFC7242 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 March 22, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5
2.1. Definitions Specific to the TCPCL Protocol . . . . . . . 5
3. General Protocol Description . . . . . . . . . . . . . . . . 8
3.1. Convergence Layer Services . . . . . . . . . . . . . . . 8
3.2. TCPCL Session Overview . . . . . . . . . . . . . . . . . 10
3.3. TCPCL States and Transitions . . . . . . . . . . . . . . 12
3.4. Transfer Segmentation Policies . . . . . . . . . . . . . 18
3.5. Example Message Exchange . . . . . . . . . . . . . . . . 19
4. Session Establishment . . . . . . . . . . . . . . . . . . . . 21
4.1. TCP Connection . . . . . . . . . . . . . . . . . . . . . 21
4.2. Contact Header . . . . . . . . . . . . . . . . . . . . . 22
4.3. Contact Validation and Negotiation . . . . . . . . . . . 23
4.4. Session Security . . . . . . . . . . . . . . . . . . . . 24
4.4.1. TLS Handshake . . . . . . . . . . . . . . . . . . . . 24
4.4.2. TLS Authentication . . . . . . . . . . . . . . . . . 25
4.4.3. Example TLS Initiation . . . . . . . . . . . . . . . 26
4.5. Message Header . . . . . . . . . . . . . . . . . . . . . 27
4.6. Session Initialization Message (SESS_INIT) . . . . . . . 28
4.7. Session Parameter Negotiation . . . . . . . . . . . . . . 30
4.8. Session Extension Items . . . . . . . . . . . . . . . . . 31
5. Established Session Operation . . . . . . . . . . . . . . . . 32
5.1. Upkeep and Status Messages . . . . . . . . . . . . . . . 32
5.1.1. Session Upkeep (KEEPALIVE) . . . . . . . . . . . . . 32
5.1.2. Message Rejection (MSG_REJECT) . . . . . . . . . . . 33
5.2. Bundle Transfer . . . . . . . . . . . . . . . . . . . . . 34
5.2.1. Bundle Transfer ID . . . . . . . . . . . . . . . . . 35
5.2.2. Data Transmission (XFER_SEGMENT) . . . . . . . . . . 35
5.2.3. Data Acknowledgments (XFER_ACK) . . . . . . . . . . . 37
5.2.4. Transfer Refusal (XFER_REFUSE) . . . . . . . . . . . 38
5.2.5. Transfer Extension Items . . . . . . . . . . . . . . 41
6. Session Termination . . . . . . . . . . . . . . . . . . . . . 43
6.1. Session Termination Message (SESS_TERM) . . . . . . . . . 43
6.2. Idle Session Shutdown . . . . . . . . . . . . . . . . . . 45
7. Implementation Status . . . . . . . . . . . . . . . . . . . . 45
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8. Security Considerations . . . . . . . . . . . . . . . . . . . 46
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 47
9.1. Port Number . . . . . . . . . . . . . . . . . . . . . . . 47
9.2. Protocol Versions . . . . . . . . . . . . . . . . . . . . 48
9.3. Session Extension Types . . . . . . . . . . . . . . . . . 48
9.4. Transfer Extension Types . . . . . . . . . . . . . . . . 49
9.5. Message Types . . . . . . . . . . . . . . . . . . . . . . 50
9.6. XFER_REFUSE Reason Codes . . . . . . . . . . . . . . . . 51
9.7. SESS_TERM Reason Codes . . . . . . . . . . . . . . . . . 52
9.8. MSG_REJECT Reason Codes . . . . . . . . . . . . . . . . . 53
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 54
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 54
11.1. Normative References . . . . . . . . . . . . . . . . . . 54
11.2. Informative References . . . . . . . . . . . . . . . . . 55
Appendix A. Significant changes from RFC7242 . . . . . . . . . . 56
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 57
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
1.1. Scope
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 [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 entities
(peers) within a network or across an internet. The mapping of
Node ID to potential CL protocol and network address is left to
implementation and configuration of the BP Agent and its various
potential routing strategies.
o Logic for routing bundles along a path toward a bundle's endpoint.
This CL protocol is involved only in transporting bundles between
adjacent nodes in a routing sequence.
o Policies or mechanisms for assigning X.509 certificates,
provisioning or deploying certificates and private keys, or
configuring security parameters on an individual BP node or across
a network.
Any TCPCL implementation requires a BP agent to perform those above
listed functions in order to perform end-to-end bundle delivery.
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2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2.1. Definitions Specific to the TCPCL Protocol
This section contains definitions specific to the TCPCL protocol.
TCPCL Entity: This is the notional TCPCL application that initiates
TCPCL sessions. This design, implementation, configuration, and
specific behavior of such an entity is outside of the scope of
this document. However, the concept of an entity has utility
within the scope of this document as the container and initiator
of TCPCL sessions. The relationship between a TCPCL entity and
TCPCL sessions is defined as follows:
A TCPCL Entity MAY actively initiate any number of TCPCL
Sessions and should do so whenever the entity is the initial
transmitter of information to another entity in the network.
A TCPCL Entity MAY support zero or more passive listening
elements that listen for connection requests from other TCPCL
Entities operating on other entitys in the network.
A TCPCL Entity MAY passivley initiate any number of TCPCL
Sessions from requests received by its passive listening
element(s) if the entity uses such elements.
These relationships are illustrated in Figure 2. For most TCPCL
behavior within a session, the two entities are symmetric and
there is no protocol distinction between them. Some specific
behavior, particularly during session establishment, distinguishes
between the active entity and the passive entity. For the
remainder of this document, the term "entity" without the prefix
"TCPCL" refers to a TCPCL entity.
TCP Connection: The term Connection in this specification
exclusively refers to a TCP connection and any and all behaviors,
sessions, and other states association with that TCP connection.
TCPCL Session: A TCPCL session (as opposed to a TCP connection) is a
TCPCL communication relationship between two TCPCL entities.
Within a single TCPCL session there are two possible transfer
streams; one in each direction, with one stream from each entity
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being the outbound stream and the other being the inbound stream.
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 entities actively
closing 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 entity and thereby to
the TCPCL is implementation dependent. However, the mechanism by
which two entities exchange and negotiate the values to be used
for a given session is described in Section 4.3.
Transfer Stream: A Transfer stream is a uni-directional user-data
path within a TCPCL Session. Messages sent over a transfer stream
are serialized, meaning that one set of user data must complete
its transmission prior to another set of user data being
transmitted over the same transfer stream. Each uni-directional
stream has a single sender entity and a single receiver entity.
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.
Transfer Segment: A subset of a transfer of user data being
communicated over a trasnfer stream.
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.
Reason Codes: The TCPCL uses numeric codes to encode specific
reasons for individual failure/error message types.
The relationship between connections, sessions, and streams is shown
in Figure 3.
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+--------------------------------------------+
| TCPCL Entity |
| | +----------------+
| +--------------------------------+ | | |-+
| | Actively Inititated Session #1 +------------->| Other | |
| +--------------------------------+ | | TCPCL Entity's | |
| ... | | Passive | |
| +--------------------------------+ | | Listener | |
| | Actively Inititated Session #n +------------->| | |
| +--------------------------------+ | +----------------+ |
| | +-----------------+
| +---------------------------+ |
| +---| +---------------------------+ | +----------------+
| | | | Optional Passive | | | |-+
| | +-| Listener(s) +<-------------+ | |
| | +---------------------------+ | | | |
| | | | Other | |
| | +---------------------------------+ | | TCPCL Entity's | |
| +--->| Passively Inititated Session #1 +-------->| Active | |
| | +---------------------------------+ | | Initiator(s) | |
| | | | | |
| | +---------------------------------+ | | | |
| +--->| Passively Inititated Session #n +-------->| | |
| +---------------------------------+ | +----------------+ |
| | +-----------------+
+--------------------------------------------+
Figure 2: The relationships between TCPCL entities
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+----------------------------+ +--------------------------+
| TCPCL Session | | TCPCL "Other" Session |
| | | |
| +-----------------------+ | | +---------------------+ |
| | TCP Connection | | | | TCP Connection | |
| | | | | | | |
| | +-------------------+ | | | | +-----------------+ | |
| | | Optional Inbound | | | | | | Peer Outbound | | |
| | | Transfer Stream |<-[Seg]--[Seg]--[Seg]-| | Transfer Stream | | |
| | | ----- | | | | | | ----- | | |
| | | RECEIVER | | | | | | SENDER | | |
| | +-------------------+ | | | | +-----------------+ | |
| | | | | | | |
| | +-------------------+ | | | | +-----------------+ | |
| | | Optional Outbound | | | | | | Peer Inbound | | |
| | | Transfer Stream |------[Seg]---[Seg]---->| Transfer Stream | | |
| | | ----- | | | | | | ----- | | |
| | | SENDER | | | | | | RECEIVER | | |
| | +-------------------+ | | | | +-----------------+ | |
| +-----------------------+ | | +---------------------+ |
+----------------------------+ +--------------------------+
Figure 3: The relationship within a TCPCL Session of its two streams
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. Convergence Layer Services
This version of the TCPCL provides the following services to support
the overlaying Bundle Protocol agent. In all cases, this is not an
API defintion but a logical description of how the CL can interact
with the BP agent. Each of these interactions can be associated with
any number of additional metadata items as necessary to support the
operation of the CL or BP agent.
Attempt Session: The TCPCL allows a BP agent to pre-emptively
attempt to establish a TCPCL session with a peer entity. Each
session attempt can send a different set of session negotiation
parameters as directed by the BP agent.
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Terminate Session: The TCPCL allows a BP agent to pre-emptively
terminate an established TCPCL session with a peer entity. The
terminate request is on a per-session basis.
Session State Changed: The TCPCL supports indication when the
session state changes. The top-level session states indicated
are:
Connecting: A TCP connection is being established. This state
only applies to the active entity.
Contact Negotating: A TCP connection has been made (as either
active or passive entity) and contact negotiation has begun.
Session Negotiating: Contact negotation has been completed
(including possible TLS use) and session negotiation has begun.
Established: The session has been fully established and is ready
for its first transfer.
Ending: The entity received a SESS_TERM message and is in the
ending state.
Terminated: The session has finished normal termination
sequencing.
Failed: The session ended without normal termination sequencing.
Session Idle Changed: The TCPCL supports indication when the live/
idle sub-state of the session changes. This occurs only when the
top-level session state is "Established". The session transitions
from Idle to Live at the at the start of a transfer in either
transfer stream; the session transitions from Live to Idle at the
end of a transfer when the other transfer stream does not have an
ongoing transfer. Because TCPCL transmits serially over a TCP
connection, it suffers from "head of queue blocking" this
indication provides information about when a session is available
for immediate transfer start.
Begin Transmission: The principal purpose of the TCPCL is to allow a
BP agent to transmit bundle data over an established TCPCL
session. Transmission request is on a per-session basis, the CL
does not necessarily perform any per-session or inter-session
queueing. Any queueing of transmissions is the obligation of the
BP agent.
Transmission Success: The TCPCL supports positive indication when a
bundle has been fully transferred to a peer entity.
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Transmission Intermediate Progress: The TCPCL supports positive
indication of intermediate progress of transferr to a peer entity.
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 entity rejects the bundle or when a TCPCL session ends before
transferr success. The TCPCL itself does not have a notion of
transfer timeout.
Reception Initialized: The TCPCL supports indication to the reciver
just before any transmssion data is sent. This corresponds to
reception of the XFER_SEGMENT message with the START flag of 1.
Interrupt Reception: The TCPCL allows a BP agent to interrupt an
individual transfer before it has fully completed (successfully or
not). Interruption can occur any time after the reception is
initialized.
Reception Success: The TCPCL supports positive indication when a
bundle has been fully transferred from a peer entity.
Reception Intermediate Progress: The TCPCL supports positive
indication of intermediate progress of transfer from the peer
entity. 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 entity
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.
3.2. 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 establish a shared TCPCL version and
possibly initiate TLS security. Once contact negotiation is
complete, TCPCL messaging is available and the session negotiation is
used to set parameters of the TCPCL session. One of these parameters
is a Node ID of each TCPCL Entity. This is used to assist in routing
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and forwarding messages by the BP Agent and is part of the
authentication capability provided by TLS.
Once negotiated, the parameters of a TCPCL session cannot change and
if there is a desire by either peer to transfer data under different
parameters then a new session must be established. This makes CL
logic simpler but relies on the assumption that establishing a TCP
connection is lightweight enough that TCP connection overhead is
negligable compared to TCPCL data sizes.
Once the TCPCL session is established and configured in this way,
bundles can be transferred in either direction. Each transfer is
performed by an sequence of logical segments of data within
XFER_SEGMENT messages. Multiple bundles can be transmitted
consecutively in a single direction 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.
There is no fundamental limit on the number of TCPCL sessions which a
single node can establish beyond the limit imposed by the number of
available (ephemeral) TCP ports of the passive peer.
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
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 SESS_TERM message is used to start the ending 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 SESS_TERM
message can also be used to refuse a session setup by a peer (see
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Section 4.3). Regardless of the reason, session termination is
initiated by one of the entities and responded-to by the other as
illustrated by Figure 13 and Figure 14. Even when there are no
transfers queued or in-progress, the session termination procedure
allows each entity to distinguish between a clean end to a session
and the TCP connection being closed because of some underlying
network issue.
Once a session is established, TCPCL is a symmetric protocol between
the peers. 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.
3.3. TCPCL States and Transitions
The states of a nominal TCPCL session (i.e. without session failures)
are indicated in Figure 4.
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+-------+
| START |
+-------+
|
TCP Establishment
|
V
+-----------+ +---------------------+
| TCP |----------->| Contact / Session |
| Connected | | Negotiation |
+-----------+ +---------------------+
|
+-----Session Parameters-----+
| Negotiated
V
+-------------+ +-------------+
| Established |----New Transfer---->| Established |
| Session | | Session |
| Idle |<---Transfers Done---| Live |
+-------------+ +-------------+
| |
+------------------------------------+
|
[SESSTERM] Exchange
|
V
+-------------+
| Established | +-------------+
| Session |----Transfers------>| TCP |
| Ending | Done | Terminating |
+-------------+ +-------------+
|
+----------TCP Close Message----------+
|
V
+-------+
| END |
+-------+
Figure 4: Top-level states of a TCPCL session
Notes on Established Session states:
Session "Live" means transmitting or reeiving over a transfer
stream.
Session "Idle" means no transmission/reception over a transfer
stream.
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Session "Ending" means no new transfers will be allowed.
Contact negotation involves exchanging a Contact Header (CH) in both
directions and deriving a negotiated state from the two headers. The
contact negotiation sequencing is performed either as the active or
passive peer, and is illustrated in Figure 5 and Figure 6
respectively which both share the data validation and analyze final
states of the "[PCH]" activity of Figure 7 and the "[TCPCLOSE]"
activity which indicates TCP connection close. Successful
negotiation results in one of the Session Initiation "[SI]"
activities being performed.
+-------+
| START |
+-------+
|
TCP Connecting
V
+-----------+
| TCP | +---------+
| Connected |--Send CH-->| Waiting |--Timeout-->[TCPCLOSE]
+-----------+ +---------+
|
Recevied CH
V
[PCH]
Figure 5: Contact Initiation as Active peer
+-----------+ +---------+
| TCP |--Wait for-->| Waiting |--Timeout-->[TCPCLOSE]
| Connected | CH +---------+
+-----------+ |
Received CH
V
+-----------------+
| Preparing reply |--Send CH-->[PSI]
+-----------------+
Figure 6: Contact Initiation as Passive peer
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+-----------+
| Peer CH |
| available |
+-----------+
|
Validate and
Negotiate
V
+------------+
| Negotiated |----Failure---->[TCPCLOSE]
+------------+ ^
| | |
No TLS +----Negotiate---+ |
V TLS | Failure
+-----------+ V |
| TCPCL | +---------------+
| Messaging |<--Success--| TLS Finished |
| Available | +---------------+
+-----------+
Figure 7: Processing of Contact Header [PCH]
Session negotation involves exchanging a session initialization
(SESS_INIT) message in both directions and deriving a negotiated
state from the two messages. The session negotiation sequencing is
performed either as the active or passive peer, and is illustrated in
Figure 8 and Figure 9 respectively which both share the data
validation and analyze final states of Figure 10. The validation
here includes certificate validation and authentication when TLS is
used for the session.
+-----------+
| TCPCL | +---------+
| Messaging |--Send SESS_INIT-->| Waiting |--Timeout-->[SESSTERM]
| Available | +---------+
+-----------+ |
Recevied SESS_INIT
|
V
[PSI]
Figure 8: Session Initiation [SI] as Active peer
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+-----------+
| TCPCL | +---------+
| Messaging |----Wait for ---->| Waiting |--Timeout-->[SESSTERM]
| Available | SESS_INIT +---------+
+-----------+ |
Recevied SESS_INIT
|
+-----------------+
| Preparing reply |--Send SESS_INIT-->[PSI]
+-----------------+
Figure 9: Session Initiation [SI] as Passive peer
+----------------+
| Peer SESS_INIT |
| available |
+----------------+
|
Validate and
Negotiate
V
+------------+
| Negotiated |---Failure--->[SESSTERM]
+------------+
|
Success
V
+--------------+
| Established |
| Session Idle |
+--------------+
Figure 10: Processing of Session Initiation [PSI]
Transfers can occur after a session is established and it's not in
the ending state. Each transfer occurs within a single logical
transfer stream between a sender and a receiver, as illustrated in
Figure 11 and Figure 12 respectively.
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+--Send XFER_SEGMENT--+
+--------+ | |
| Stream | +-------------+ |
| Idle |---Send XFER_SEGMENT-->| In Progress |<------------+
+--------+ +-------------+
|
+---------All segments sent-------+
|
V
+---------+ +--------+
| Waiting |---- Receive Final---->| Stream |
| for Ack | XFER_ACK | IDLE |
+---------+ +--------+
Figure 11: Transfer sender states
Notes on transfer sending:
Pipelining of transfers can occur when the sending entity begins a
new transfer while in the "Waiting for Ack" state.
+-Receive XFER_SEGMENT-+
+--------+ | Send XFER_ACK |
| Stream | +-------------+ |
| Idle |--Receive XFER_SEGMENT-->| In Progress |<-------------+
+--------+ +-------------+
|
+--------Sent Final XFER_ACK--------+
|
V
+--------+
| Stream |
| Idle |
+--------+
Figure 12: Transfer receiver states
Session termination involves one entity initiating the termination of
the session and the other entity acknowledging the termination. For
either entity, it is the sending of the SESS_TERM message which
transitions the session to the ending substate. While a session is
being terminated only in-progress transfers can be completed and no
new transfers can be started.
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+-----------+ +---------+
| Session |--Send SESS_TERM-->| Session |
| Live/Idle | | Ending |
+-----------+ +---------+
Figure 13: Session Termination [SESSTERM] from the Initiator
+-----------+ +---------+
| Session |--Send SESS_TERM-->| Session |
| Live/Idle | | Ending |
+-----------+<------+ +---------+
| |
Receive SESS_TERM |
| |
+-------------+
Figure 14: Session Termination [SESSTERM] from the Responder
3.4. Transfer Segmentation Policies
Each TCPCL session allows a negotiated transfer segmentation polcy to
be applied in each transfer direction. A receiving node can set the
Segment MRU in its contact header to determine the largest acceptable
segment size, and a transmitting node can segment a transfer into any
sizes smaller than the receiver's Segment MRU. It is a network
administration matter to determine an appropriate segmentation policy
for entities operating TCPCL, but guidance given here can be used to
steer policy toward performance goals. It is also advised to
consider the Segment MRU in relation to chunking/packetization
performed by TLS, TCP, and any intermediate network-layer nodes.
Minimum Overhead: For a simple network expected to exchange
relatively small bundles, the Segment MRU can be set to be
identical to the Transfer MRU which indicates that all transfers
can be sent with a single data segment (i.e. no actual
segmentation). If the network is closed and all transmitters are
known to follow a single-segment transfer policy, then receivers
can avoid the necessity of segment reassembly. Because this CL
operates over a TCP stream, which suffers from a form of head-of-
queue blocking between messages, while one node is transmitting a
single XFER_SEGMENT message it is not able to transmit any
XFER_ACK or XFER_REFUSE for any associated received transfers.
Predictable Message Sizing: In situations where the maximum message
size is desired to be well-controlled, the Segment MRU can be set
to the largest acceptable size (the message size less XFER_SEGMENT
header size) and transmitters can always segment a transfer into
maximum-size chunks no larger than the Segment MRU. This
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guarantees that any single XFER_SEGMENT will not monopolize the
TCP stream for too long, which would prevent outgoing XFER_ACK and
XFER_REFUSE associated with received transfers.
Dynamic Segmentation: Even after negotiation of a Segment MRU for
each receiving node, the actual transfer segmentation only needs
to guarantee than any individual segment is no larger than that
MRU. In a situation where network "goodput" is dynamic, the
transfer segmentation size can also be dynamic in order to control
message transmission duration.
Many other policies can be established in a TCPCL network between the
two extremes of minimum overhead (large MRU, single-segment) and
predictable message sizing (small MRU, highly segmented). Different
policies can be applied to each transfer stream to and from from any
particular node. Additionally, future header and transfer extension
types can apply further nuance to transfer policies and policy
negotiation.
3.5. 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 Entity A to Entity B.
Note that the sending node can transmit multiple XFER_SEGMENT
messages without waiting for the corresponding XFER_ACK responses.
This enables pipelining of messages on a transfer stream. 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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Entity A Entity B
======== ========
+-------------------------+
| Contact Header | -> +-------------------------+
+-------------------------+ <- | Contact Header |
+-------------------------+
+-------------------------+
| SESS_INIT | -> +-------------------------+
+-------------------------+ <- | SESS_INIT |
+-------------------------+
+-------------------------+
| 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] |
+-------------------------+
+-------------------------+
| SESS_TERM | -> +-------------------------+
+-------------------------+ <- | SESS_TERM |
+-------------------------+
Figure 15: An example of the flow of protocol messages on a single
TCP Session between two entities
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4. Session Establishment
For bundle transmissions to occur using the TCPCL, a TCPCL session
MUST first be established between communicating entities. It is up
to the implementation to decide how and when session setup is
triggered. 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, an entity MUST first establish a TCP
connection with the intended peer entity, 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 entity is unable to establish a TCP connection for any reason,
then it is an implementation matter to determine how to handle the
connection failure. An entity 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 entity MUST retry the
connection setup no earlier than some delay time from the last
attempt, and it SHOULD use a (binary) exponential back-off mechanism
to increase this delay in case of repeated failures. The upper limit
on a re-attempt back-off is implementation defined but SHOULD be no
longer than one minute before signaling to the BP agent that a
connection cannot be made.
Once a TCP connection is established, each entity MUST immediately
transmit a contact header over the TCP connection. The format of the
contact header is described in Section 4.2. Because the TCPCL
protocol version in use is part of the initial contact header, nodes
using TCPCL version 4 can coexist on a network with nodes using
earlier TCPCL versions (with some negotiation needed for
interoperation as described in Section 4.3).
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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 entities perform the
validation and negotiation procedures defined in Section 4.3. After
receiving the contact header from the other entity, either entity MAY
refuse the session by sending a SESS_TERM message with an appropriate
reason code.
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 |
+---------------+---------------+
Figure 16: 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 TCPCL).
Flags: A one-octet field of single-bit flags, interpreted according
to the descriptions in Table 1. All reserved header flag bits
SHALL be set to 0 by the sender. All reserved header flag bits
SHALL be ignored by the receiver.
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+----------+--------+-----------------------------------------------+
| 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
4.3. Contact Validation and 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, an entity
MAY elect to hold an invalid connection open and idle for some time
before ending it.
The first negotiation is on the TCPCL protocol version to use. The
active node always sends its Contact Header first and waits for a
response from the passive node. The active node can repeatedly
attempt different protocol versions in descending order until the
passive node accepts one with a corresponding Contact Header reply.
Only upon response of a Contact Header from the passive node is the
TCPCL protocol version established and parameter negotiation begun.
During contact initiation, the active TCPCL node SHALL send the
highest TCPCL protocol version on a first session attempt for a TCPCL
peer. If the active node receives a Contact Header with a different
protocol version than the one sent earlier on the TCP connection, the
TCP connection SHALL be closed. If the active node receives a
SESS_TERM 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 the passive node receives a contact header containing a version
that is greater than the current version of the TCPCL that the node
implements, then the node SHALL shutdown the session with a reason
code of "Version mismatch". If the passive node receives a contact
header with a version that is lower than the version of the protocol
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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.
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 terminated and a new non-TLS-negotiated session
established.
4.4.1. TLS Handshake
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 entities SHALL begin a TLS handshake in accordance with TLS
1.2 [RFC5246] or any successors that are compatible with TLS 1.2. By
convention, this 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 8.
The parameters within each TLS negotiation are implementation
dependent but any TCPCL node SHALL follow all recommended practices
of BCP 195 [RFC7525], or any updates or successors that become part
of BCP 195. When possible, the TLS handshake SHOULD include a Server
Name Indication (SNI) from the active peer in accordance with
[RFC6066]. The SNI SHALL contain the same host name used to
establish the TCP connection. The passive peer MAY use the SNI host
name to choose an appropriate server-side certificate. The TLS
handshake SHALL request a client-side certificate to allow
authentication of the active peer. The passive peer SHOULD supply a
certificate within the TLS handshake to allow authentication of its
side of the session. The active peer SHOULD supply a certificate
within the TLS handshake to allow authentication of its side of the
session. All certificates supplied during TLS handshake SHALL
conform with the profile of [RFC5280], or any updates or successors
to that profile. When a certificate is supplied during TLS
handshake, the full certification chain SHOULD be included unless
security policy indicates that is unnecessary.
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If a TLS handshake cannot negotiate a TLS session, both entities of
the TCPCL session SHALL close the TCP connection. At this point the
TCPCL session has not yet been established so there is no TCPCL
session to terminate. This also avoids any potential security issues
assoicated with further TCP communication with an untrusted peer.
After a TLS session is successfully established, the active peer
SHALL send a SESS_INIT message to begin session negotiation. This
session negotation and all subsequent messaging are secured.
4.4.2. TLS Authentication
Using X.509 certificates exchanged during the TLS handshake, each of
the entities can attempt to authenticate its peer at the network
layer (host name and address) and at the application layer (BP Node
ID). The Node ID exchanged in the Session Initialization is likely
to be used by the BP agent for making transfer and routing decisions,
so attempting host name validation is optional while attempting Node
ID validation is required. The logic for attempting validation is
separate from the logic for handling the result of validation, which
is based on local security policy.
Any certificate received during TLS handshake SHALL be validated up
to one or more trusted certificate authority (CA) certificates. If
certificate validation fails or if security policy disallows a
certificate for any reason, the entity SHOULD terminate the session
(with a reason code of "Contact Failure").
Immediately after the TLS handshake, each side of the TCP connection
SHOULD perform host name validation of its peer in accordance with
[RFC6125] unless it is not needed by security policy. The active
peer SHALL attempt to authenticate the host name (of the passive
peer) used to initiate the TCP connection. The active peer MAY
attempt to authenticate the IP address of the other side of the TCP
connection. The passive peer SHALL attempt to authenticate the IP
address of the other side of the TCP connection. The passive peer
MAY use the IP address to resolve one or more host names of the
active peer and attempt to authenticate those. If host name
validation fails (including failure because the certificate does not
contain any DNS-ID), the entity SHOULD terminate the session (with a
reason code of "Contact Failure") unless security policy allows an
unauthticated host.
Immediately before Session Parameter Negotiation, each side of the
session SHALL perform Node ID validation of its peer as described
below. Node ID validation SHALL succeed if the associated
certificate contains a subjectAltName entry of type
uniformResourceIdentifier whose value matches the Node ID of the
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TCPCL entity. Unless specified otherwise by the definition of the
URI scheme being authenticated, URI matching of Node IDs SHALL use
the URI comparison logic of [RFC3986] and scheme-based normalization
of those schemes specified in [I-D.ietf-dtn-bpbis]. This is similar
to the URI-ID of [RFC6125] but does not require any structure to the
scheme-specific-part of the URI. A URI scheme can refine this "exact
match" logic with rules about how Node IDs within that scheme are to
be compared with the certificate-authenticated URI. If Node ID
validation fails (including failure because the certificate does not
contain any subjectAltName URI), the entity SHOULD terminate the
session (with a reason code of "Contact Failure") unless security
policy allows an unauthticated node.
4.4.3. Example TLS Initiation
A summary of a typical TLS use is shown in the sequence in Figure 17
below.
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Entity A Entity B
active peer passive peer
+-------------------------+
| Open TCP Connnection | ->
+-------------------------+ +-------------------------+
<- | Accept Connection |
+-------------------------+
+-------------------------+
| Contact Header | ->
+-------------------------+ +-------------------------+
<- | Contact Header |
+-------------------------+
+-------------------------+ +-------------------------+
| TLS Negotiation | -> <- | TLS Negotiation |
| (as client) | | (as server) |
+-------------------------+ +-------------------------+
Host name validation occurs.
Secured TCPCL messaging can begin.
+-------------------------+ +-------------------------+
| SESS_INIT | -> <- | SESS_INIT |
+-------------------------+ +-------------------------+
Node ID validation occurs.
Session is established, transfers can begin.
+-------------------------+ +-------------------------+
| SESS_TERM | -> <- | SESS_TERM |
+-------------------------+ +-------------------------+
Figure 17: A simple visual example of TCPCL TLS Establishment between
two entities
4.5. Message Header
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 18: Format of the Message Header
The message header fields are as follows:
Message Type: Indicates the type of the message as per Table 2
below. Encoded values are listed in Section 9.5.
+--------------+------+---------------------------------------------+
| Name | Code | Description |
+--------------+------+---------------------------------------------+
| SESS_INIT | 0x07 | Contains the session parameter inputs from |
| | | one of the entities, as described in |
| | | Section 4.6. |
| | | |
| SESS_TERM | 0x05 | Indicates that one of the entities |
| | | participating in the session wishes to |
| | | cleanly terminate the session, as described |
| | | in Section 6. |
| | | |
| XFER_SEGMENT | 0x01 | Indicates the transmission of a segment of |
| | | bundle data, as described in Section 5.2.2. |
| | | |
| XFER_ACK | 0x02 | Acknowledges reception of a data segment, |
| | | as described in Section 5.2.3. |
| | | |
| XFER_REFUSE | 0x03 | Indicates that the transmission of the |
| | | current bundle SHALL be stopped, as |
| | | described in Section 5.2.4. |
| | | |
| KEEPALIVE | 0x04 | Used to keep TCPCL session active, as |
| | | described in Section 5.1.1. |
| | | |
| MSG_REJECT | 0x06 | Contains a TCPCL message rejection, as |
| | | described in Section 5.1.2. |
+--------------+------+---------------------------------------------+
Table 2: TCPCL Message Types
4.6. Session Initialization Message (SESS_INIT)
Before a session is established and ready to transfer bundles, the
session parameters are negotiated between the connected entities.
The SESS_INIT message is used to convey the per-entity parameters
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which are used together to negotiate the per-session parameters as
described in Section 4.7.
The format of a SESS_INIT message is as follows in Figure 19.
+-----------------------------+
| Message Header |
+-----------------------------+
| Keepalive Interval (U16) |
+-----------------------------+
| Segment MRU (U64) |
+-----------------------------+
| Transfer MRU (U64) |
+-----------------------------+
| Node ID Length (U16) |
+-----------------------------+
| Node ID Data (variable) |
+-----------------------------+
| Session Extension |
| Items Length (U32) |
+-----------------------------+
| Session Extension |
| Items (var.) |
+-----------------------------+
Figure 19: SESS_INIT Format
The fields of the SESS_INIT message are:
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-
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 entities
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
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entities of a single session MAY have different Transfer MRUs, and
no relation between the two is required.
Node ID Length and Node ID Data: Together these fields represent a
variable-length text string. The Node ID Length is a 16-bit
unsigned integer indicating the number of octets of Node ID Data
to follow. A zero-length Node ID SHALL be used to indicate the
lack of Node ID rather than a truly empty Node ID. This case
allows an entity to avoid exposing Node ID information on an
untrusted network. A non-zero-length Node ID Data SHALL contain
the UTF-8 encoded Node ID of the Entity which sent the SESS_INIT
message. Every Node ID SHALL be a URI consistent with the
requirements of [RFC3986] and the URI schemes of
[I-D.ietf-dtn-bpbis]. The Node ID itself can be authenticated as
described in Section 4.4.2.
Session Extension Length and Session Extension Items: Together these
fields represent protocol extension data not defined by this
specification. The Session Extension Length is the total number
of octets to follow which are used to encode the Session Extension
Item list. The encoding of each Session Extension Item is within
a consistent data container as described in Section 4.8. The full
set of Session Extension Items apply for the duration of the TCPCL
session to follow. The order and mulitplicity of these Session
Extension Items MAY be significant, as defined in the associated
type specification(s).
4.7. Session Parameter Negotiation
An entity 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.
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 5.1.1.
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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. It can be a
reasonable security policy to both require or disallow the use of
TLS depending upon the desired network flows. Because this state
is negotiated over an unsecured medium, there is a risk of a TLS
Stripping as described in Section 8. If the Enable TLS state is
unacceptable, the node SHALL terminate the session with a reason
code of "Contact Failure". Note that this contact failure reason
is different than a failure of TLS handshake or TLS authentication
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.
4.8. Session Extension Items
Each of the Session Extension Items SHALL be encoded in an identical
Type-Length-Value (TLV) container form as indicated in Figure 20.
The fields of the Session Extension Item are:
Flags: A one-octet field containing generic bit flags about the
Item, which are listed in Table 3. All reserved header flag bits
SHALL be set to 0 by the sender. All reserved header flag bits
SHALL be ignored by the receiver. If a TCPCL entity receives a
Session Extension Item with an unknown Item Type and the CRITICAL
flag of 1, the entity SHALL close the TCPCL session with SESS_TERM
reason code of "Contact Failure". If the CRITICAL flag is 0, an
entity 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 9.3).
Item Length: A 16-bit unsigned integer field containing the number
of Item Value octets to follow.
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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 specifications SHOULD avoid the use of large data
lengths, as no bundle transfers can begin until the full extension
data is sent.
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... |
+---------------+---------------+---------------+---------------+
Figure 20: Session Extension Item Format
+----------+--------+-----------------------------------------------+
| Name | Code | Description |
+----------+--------+-----------------------------------------------+
| CRITICAL | 0x01 | If bit is set, indicates that the receiving |
| | | peer must handle the extension item. |
| | | |
| Reserved | others |
+----------+--------+-----------------------------------------------+
Table 3: Session Extension Item Flags
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. Upkeep and Status Messages
5.1.1. 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.
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
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lead to excessive traffic. For shared network use, entities 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. Entities 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 2) with no additional data. Both
sides SHALL 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 SHALL
terminate the session by transmitting a SESS_TERM message (as
described in Section 6.1) with reason code "Idle Timeout". If
configurable, the idle timeout duration SHOULD be no shorter than
twice the keepalive interval. If not configurable, the idle timeout
duration SHOULD be exactly twice the keepalive interval.
5.1.2. 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 is as follows in Figure 21.
+-----------------------------+
| Message Header |
+-----------------------------+
| Reason Code (U8) |
+-----------------------------+
| Rejected Message Header |
+-----------------------------+
Figure 21: Format of MSG_REJECT Messages
The fields of the MSG_REJECT message are:
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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
5.2. Bundle Transfer
All of the messages in this section are directly associated with
transferring a bundle between TCPCL entities.
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
MRU" of Section 4.2). The first segment for a bundle is indicated by
the 'START' flag and the last segment is indicated by the 'END' flag.
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.
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5.2.1. 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 sequence of 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
SESS_TERM 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.
5.2.2. Data Transmission (XFER_SEGMENT)
Each bundle is transmitted in one or more data segments. The format
of a XFER_SEGMENT message follows in Figure 22.
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+------------------------------+
| Message Header |
+------------------------------+
| Message Flags (U8) |
+------------------------------+
| Transfer ID (U64) |
+------------------------------+
| Transfer Extension |
| Items Length (U32) |
| (only for START segment) |
+------------------------------+
| Transfer Extension |
| Items (var.) |
| (only for START segment) |
+------------------------------+
| Data length (U64) |
+------------------------------+
| Data contents (octet string) |
+------------------------------+
Figure 22: 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. All reserved header
flag bits SHALL be set to 0 by the sender. All reserved header
flag bits SHALL be ignored by the receiver.
Transfer ID: A 64-bit unsigned integer identifying the transfer
being made.
Transfer Extension Length and Transfer Extension Items: Together
these fields represent protocol extension data for this
specification. The Transfer Extension Length and Transfer
Extension Item fields SHALL only be present when the 'START' flag
is set to 1 on the message. The Transfer Extension Length is the
total number of octets to follow which are used to encode the
Transfer Extension Item list. The encoding of each Transfer
Extension Item is within a consistent data container as described
in Section 5.2.5. The full set of transfer extension items apply
only to the assoicated single transfer. The order and
mulitplicity of these transfer extension items MAY be significant,
as defined in the associated type specification(s).
Data length: A 64-bit unsigned integer indicating the number of
octets in the Data contents to follow.
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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' flag SHALL be set to 1 when transmitting the first segment of
a transfer. The 'END' flag SHALL be set to 1 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' flags
SHALL be set to 1.
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' flag set to 1. No interleaving of multiple
transfers from the same node is possible within a single TCPCL
session. Simultaneous transfers between two entities MAY be achieved
using multiple TCPCL sessions.
5.2.3. 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 23.
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+-----------------------------+
| Message Header |
+-----------------------------+
| Message Flags (U8) |
+-----------------------------+
| Transfer ID (U64) |
+-----------------------------+
| Acknowledged length (U64) |
+-----------------------------+
Figure 23: 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. All reserved header
flag bits SHALL be set to 0 by the sender. All reserved header
flag bits SHALL be ignored by the receiver.
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
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 SHOULD transmit multiple XFER_SEGMENT messages
without waiting for the corresponding XFER_ACK responses. This
enables pipelining of messages on a transfer stream.
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.
5.2.4. 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 an XFER_SEGMENT message, the node
MAY transmit a XFER_REFUSE message. As data segments and
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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 any
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 in Figure 24.
+-----------------------------+
| Message Header |
+-----------------------------+
| Reason Code (U8) |
+-----------------------------+
| Transfer ID (U64) |
+-----------------------------+
Figure 24: Format of XFER_REFUSE Messages
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.
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+------------+------+-----------------------------------------------+
| Name | Code | Description |
+------------+------+-----------------------------------------------+
| Unknown | 0x00 | Reason for refusal is unknown or not |
| | | specified. |
| | | |
| Extension | 0x01 | A failure processing the Transfer Extension |
| Failure | | Items ha occurred. |
| | | |
| Completed | 0x02 | The receiver already has the complete bundle. |
| | | The sender MAY consider the bundle as |
| | | completely received. |
| | | |
| No | 0x03 | The receiver's resources are exhausted. The |
| Resources | | sender SHOULD apply reactive bundle |
| | | fragmentation before retrying. |
| | | |
| Retransmit | 0x04 | 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 an entity 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
SHALL 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' flag set to 1, indicating the start of a new transfer,
and with a distinct Transfer ID value.
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5.2.5. Transfer Extension Items
Each of the Transfer Extension Items SHALL be encoded in an identical
Type-Length-Value (TLV) container form as indicated in Figure 25.
The fields of the Transfer Extension Item are:
Flags: A one-octet field containing generic bit flags about the
Item, which are listed in Table 7. All reserved header flag bits
SHALL be set to 0 by the sender. All reserved header flag bits
SHALL be ignored by the receiver. If a TCPCL node receives a
Transfer Extension Item with an unknown Item Type and the CRITICAL
flag is 1, the node SHALL refuse the transfer with an XFER_REFUSE
reason code of "Extension Failure". If the CRITICAL flag is 0, an
entity 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 allocates an IANA registry
for such codes (see Section 9.4).
Item Length: A 16-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 specifications SHOULD avoid the use of large data
lengths, as the associated transfer cannot begin until the full
extension data is sent.
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... |
+---------------+---------------+---------------+---------------+
Figure 25: Transfer 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 7: Transfer Extension Item Flags
5.2.5.1. Transfer Length Extension
The purpose of the Transfer Length extension is to allow entities to
preemptively refuse bundles that would exceed their resources or to
prepare storage on the receiving node for the upcoming bundle data.
Multiple Transfer Length extension items SHALL NOT occur within the
same transfer. The lack of a Transfer Length extension item in any
transfer SHALL NOT imply anything about the potential length of the
transfer. The Transfer Length extension SHALL be assigned transfer
extension type ID 0x0001.
If a transfer occupies exactly one segment (i.e. both START and END
flags are 1) the Transfer Length extension SHOULD NOT be present.
The extension does not provide any additional information for single-
segment transfers.
The format of the Transfer Length data is as follows in Figure 26.
+----------------------+
| Total Length (U64) |
+----------------------+
Figure 26: Format of Transfer Length data
The fields of the Transfer Length extension are:
Total Length: A 64-bit unsigned integer indicating the size of the
data-to-be-transferred. The Total Length field 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 Total Length value, the receiver SHALL treat the
transmitted data as invalid.
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6. Session Termination
This section describes the procedures for ending a TCPCL session.
6.1. Session Termination Message (SESS_TERM)
To cleanly shut down a session, a SESS_TERM message SHALL be
transmitted by either node at any point following complete
transmission of any other message. When sent to initiate a
termination, the REPLY flag of a SESS_TERM message SHALL be 0. Upon
receiving a SESS_TERM message after not sending a SESS_TERM message
in the same session, an entity SHALL send an acknowledging SESS_TERM
message. When sent to acknowledge a termination, a SESS_TERM message
SHALL have identical data content from the message being acknowledged
except for the REPLY flag, which is set to 1 to indicate
acknowledgement.
After sending a SESS_TERM message, an entity MAY continue a possible
in-progress transfer in either direction. After sending a SESS_TERM
message, an entity SHALL NOT begin any new outgoing transfer for the
remainder of the session. After receving a SESS_TERM message, an
entity SHALL NOT accept any new incoming transfer for the remainder
of the session.
Instead of following a clean shutdown sequence, after transmitting a
SESS_TERM message an entity MAY immediately close the associated TCP
connection. When performing an unclean shutdown, a receiving node
SHOULD acknowledge all received data segments before closing the TCP
connection. Not acknowledging received segments can result in
unnecessary retransmission. When performing an unclean shutodwn, a
transmitting node SHALL treat either sending or receiving a SESS_TERM
message (i.e. before the final acknowledgment) as a failure of the
transfer. Any delay between request to close the TCP connection and
actual closing of the connection (a "half-closed" state) MAY be
ignored by the TCPCL node.
The format of the SESS_TERM message is as follows in Figure 27.
+-----------------------------+
| Message Header |
+-----------------------------+
| Message Flags (U8) |
+-----------------------------+
| Reason Code (U8) |
+-----------------------------+
Figure 27: Format of SESS_TERM Messages
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The fields of the SESS_TERM message are:
Message Flags: A one-octet field of single-bit flags, interpreted
according to the descriptions in Table 8. All reserved header
flag bits SHALL be set to 0 by the sender. All reserved header
flag bits SHALL be ignored by the receiver.
Reason Code: A one-octet refusal reason code interpreted according
to the descriptions in Table 9.
+----------+--------+-----------------------------------------------+
| Name | Code | Description |
+----------+--------+-----------------------------------------------+
| REPLY | 0x01 | If bit is set, indicates that this message is |
| | | an acknowledgement of an earlier SESS_TERM |
| | | message. |
| | | |
| Reserved | others |
+----------+--------+-----------------------------------------------+
Table 8: SESS_TERM Flags
+--------------+------+---------------------------------------------+
| Name | Code | Description |
+--------------+------+---------------------------------------------+
| Unknown | 0x00 | A termination reason is not available. |
| | | |
| Idle timeout | 0x01 | The session is being closed due to |
| | | idleness. |
| | | |
| Version | 0x02 | The node cannot conform to the specified |
| mismatch | | TCPCL protocol version. |
| | | |
| Busy | 0x03 | The node is too busy to handle the current |
| | | session. |
| | | |
| Contact | 0x04 | The node cannot interpret or negotiate |
| Failure | | contact header option. |
| | | |
| Resource | 0x05 | The node has run into some resource limit |
| Exhaustion | | and cannot continue the session. |
+--------------+------+---------------------------------------------+
Table 9: SESS_TERM Reason Codes
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
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able or willing to communicate. However, an entity MUST always send
the contact header to its peer before sending a SESS_TERM message.
If reception of the contact header itself somehow fails (e.g. an
invalid "magic string" is recevied), an entity SHALL close the TCP
connection without sending a SESS_TERM message. If the content of
the Session Extension Items data disagrees with the Session Extension
Length (i.e. the last Item claims to use more octets than are present
in the Session 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 SESS_TERM
message but still SHALL 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 SESS_TERM 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 ending 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 SESS_TERM message indicating the reason code of
"Idle timeout" (as described in Table 9).
7. Implementation Status
[NOTE to the RFC Editor: please remove this section before
publication, as well as the reference to [RFC7942] and
[github-dtn-bpbis-tcpcl].]
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
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be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations can
exist.
An example implementation of the this draft of TCPCLv4 has been
created as a GitHub project [github-dtn-bpbis-tcpcl] and is intented
to use as a proof-of-concept and as a possible source of
interoperability testing. This example implementation uses D-Bus as
the CL-BP Agent interface, so it only runs on hosts which provide the
Python "dbus" library.
8. Security Considerations
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 bundle security mechanisms defined in
[I-D.ietf-dtn-bpsec] are to be used instead.
When negotating whether to use TLS security as part of the contact
header exchange, it is possible for a man-in-the-middle attacker to
set the CAN_TLS flag to 0 on either side of the exchange. This leads
to the "SSL Stripping" attack described in [RFC7457]. If TLS is
desired for use on any TCPCL network, it is strongly encouraged that
the security policy disallow use of TCPCL when "Enable TLS" is
negotiated to false. This requires that the TLS handshake occurs,
regardless of the policy-driven parameters of the handshake and
policy-driven handling of the handshake outcome.
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].
The certificates exchanged by TLS enable authentication of peer host
name and Node ID, but it is possible that a peer either not provide a
valid certificate or that the certificate does not validate either
the host name or Node ID of the peer. Having a CA-validated
certificate does not alone guarantee the identity of the network host
or BP node from which the certificate is provided; additional
validation procedures bind the host name or node ID based on the
contents of the certificate. The host name validation is a weaker
form of authentication, because even if a peer is operating on an
authenticated network host name it can provide an invalid Node ID and
cause bundles to be "leaked" to an invalid node. Especially in DTN
environments, network names and addresses of nodes can be time-
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variable so binding a certificate to a Node ID is a more stable
identity. Node ID validation ensures that the peer to which a bundle
is transferred is in fact the node which the BP Agent expects it to
be. It is a reasonable policy to skip host name validation if
certificates can be guaranteed to validate the peer's Node ID. In
circumstances where certificates can only be issued to network host
names, Node ID validation is not possible but it could be reasonable
to assume that a trusted host is not going to present an invalid Node
ID. Trusting an authenticated host name can be feasable on a network
secured by a private CA but is not advisable on the Internet when
using a variety of public CAs.
Another consideration for this protocol relates to denial-of-service
attacks. An entity MAY send a large amount of data over a TCPCL
session, requiring the receiving entity 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 other, well-behaving sessions. There is also nothing to
prevent a malicious entity from continually establishing sessions and
repeatedly trying to send copious amounts of bundle data. A
listening entity 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 SESS_TERM
message quickly or with a delay, etc.
9. IANA Considerations
Registration procedures referred to in this section are defined in
[RFC8126].
Some of the registries have been defined as version specific to
TCPCLv4, and imports some or all codepoints from TCPCLv3. This was
done to disambiguate the use of these codepoints between TCPCLv3 and
TCPCLv4 while preserving the semantics of some of the codepoints.
9.1. 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 entity identifies its TCPCL
protocol version in its initial contact (see Section 9.2), so there
is no ambiguity about what protocol is being used.
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+------------------------+-------------------------------------+
| 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 |
+------------------------+-------------------------------------+
9.2. Protocol Versions
IANA has created, under the "Bundle Protocol" registry, a sub-
registry titled "Bundle Protocol TCP Convergence-Layer Version
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 | TCPCLv4 | This specification. |
| | | |
| 5-255 | Unassigned |
+-------+-------------+---------------------+
9.3. Session 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
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Session Extension Types" and initialize it with the contents of
Table 10. The registration procedure is Expert Review within the
lower range 0x0001--0x7FFF. Values in the range 0x8000--0xFFFF are
reserved for use on private networks for functions not published to
the IANA.
Specifications of new session extension types need to define the
encoding of the Item Value data as well as any meaning or restriction
on the number of or order of instances of the type within an
extension item list. Specifications need to define how the extension
functions when no instance of the new extension type is received
during session negotiation.
Expert(s) are encouraged to be biased towards approving registrations
unless they are abusive, frivolous, or actively harmful (not merely
aesthetically displeasing, or architecturally dubious).
+----------------+--------------------------+
| Code | Session Extension Type |
+----------------+--------------------------+
| 0x0000 | Reserved |
| | |
| 0x0001--0x7FFF | Unassigned |
| | |
| 0x8000--0xFFFF | Private/Experimental Use |
+----------------+--------------------------+
Table 10: Session Extension Type Codes
9.4. Transfer 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
Transfer Extension Types" and initialize it with the contents of
Table 11. The registration procedure is Expert Review within the
lower range 0x0001--0x7FFF. Values in the range 0x8000--0xFFFF are
reserved for use on private networks for functions not published to
the IANA.
Specifications of new transfer extension types need to define the
encoding of the Item Value data as well as any meaning or restriction
on the number of or order of instances of the type within an
extension item list. Specifications need to define how the extension
functions when no instance of the new extension type is received in a
transfer.
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Expert(s) are encouraged to be biased towards approving registrations
unless they are abusive, frivolous, or actively harmful (not merely
aesthetically displeasing, or architecturally dubious).
+----------------+---------------------------+
| Code | Transfer Extension Type |
+----------------+---------------------------+
| 0x0000 | Reserved |
| | |
| 0x0001 | Transfer Length Extension |
| | |
| 0x0002--0x7FFF | Unassigned |
| | |
| 0x8000--0xFFFF | Private/Experimental Use |
+----------------+---------------------------+
Table 11: Transfer Extension Type Codes
9.5. 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 12. The
registration procedure is RFC Required within the lower range 0x01--
0xEF. Values in the range 0xF0--0xFF are reserved for use on private
networks for functions not published to the IANA.
Specifications of new message types need to define the encoding of
the message data as well as the purpose and relationship of the new
message to existing session/transfer state within the baseline
message sequencing.
Expert(s) are encouraged to favor new session/transfer extension
types over new message types. TCPCL messages are not self-
delimiting, so care must be taken in introducing new message types.
If an entity receives an unknown message type the only thing that can
be done is to send a MSG_REJECT and close the TCP connection; not
even a clean termination can be done at that point.
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+------------+--------------------------+
| Code | Message Type |
+------------+--------------------------+
| 0x00 | Reserved |
| | |
| 0x01 | XFER_SEGMENT |
| | |
| 0x02 | XFER_ACK |
| | |
| 0x03 | XFER_REFUSE |
| | |
| 0x04 | KEEPALIVE |
| | |
| 0x05 | SESS_TERM |
| | |
| 0x06 | MSG_REJECT |
| | |
| 0x07 | SESS_INIT |
| | |
| 0x08--0xEF | Unassigned |
| | |
| 0xF0--0xFF | Private/Experimental Use |
+------------+--------------------------+
Table 12: Message Type Codes
9.6. 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 13. The registration procedure is Specification Required
within the lower range 0x00--0xEF. Values in the range 0xF0--0xFF
are reserved for use on private networks for functions not published
to the IANA.
Specifications of new XFER_REFUSE reason codes need to define the
meaning of the reason and disambiguate it with pre-exisiting reasons.
Each refusal reason needs to be usable by the receving BP Agent to
make retransmission or re-routing decisions.
Expert(s) are encouraged to be biased towards approving registrations
unless they are abusive, frivolous, or actively harmful (not merely
aesthetically displeasing, or architecturally dubious).
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+------------+--------------------------+
| Code | Refusal Reason |
+------------+--------------------------+
| 0x00 | Unknown |
| | |
| 0x01 | Extension Failure |
| | |
| 0x02 | Completed |
| | |
| 0x03 | No Resources |
| | |
| 0x04 | Retransmit |
| | |
| 0x05--0xEF | Unassigned |
| | |
| 0xF0--0xFF | Private/Experimental Use |
+------------+--------------------------+
Table 13: XFER_REFUSE Reason Codes
9.7. SESS_TERM 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
SESS_TERM Reason Codes" and initialize it with the contents of
Table 14. The registration procedure is Specification Required
within the lower range 0x00--0xEF. Values in the range 0xF0--0xFF
are reserved for use on private networks for functions not published
to the IANA.
Specifications of new SESS_TERM reason codes need to define the
meaning of the reason and disambiguate it with pre-exisiting reasons.
Each termination reason needs to be usable by the receving BP Agent
to make re-connection decisions.
Expert(s) are encouraged to be biased towards approving registrations
unless they are abusive, frivolous, or actively harmful (not merely
aesthetically displeasing, or architecturally dubious).
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+------------+--------------------------+
| Code | Termination Reason |
+------------+--------------------------+
| 0x00 | Unknown |
| | |
| 0x01 | Idle timeout |
| | |
| 0x02 | Version mismatch |
| | |
| 0x03 | Busy |
| | |
| 0x04 | Contact Failure |
| | |
| 0x05 | Resource Exhaustion |
| | |
| 0x06--0xEF | Unassigned |
| | |
| 0xF0--0xFF | Private/Experimental Use |
+------------+--------------------------+
Table 14: SESS_TERM Reason Codes
9.8. 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 15. The registration procedure is Specification Required
within the lower range 0x01--0xEF. Values in the range 0xF0--0xFF
are reserved for use on private networks for functions not published
to the IANA.
Specifications of new MSG_REJECT reason codes need to define the
meaning of the reason and disambiguate it with pre-exisiting reasons.
Each rejection reason needs to be usable by the receving TCPCL Entity
to make message sequencing and/or session termination decisions.
Expert(s) are encouraged to be biased towards approving registrations
unless they are abusive, frivolous, or actively harmful (not merely
aesthetically displeasing, or architecturally dubious).
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+------------+--------------------------+
| Code | Rejection Reason |
+------------+--------------------------+
| 0x00 | reserved |
| | |
| 0x01 | Message Type Unknown |
| | |
| 0x02 | Message Unsupported |
| | |
| 0x03 | Message Unexpected |
| | |
| 0x04--0xEF | Unassigned |
| | |
| 0xF0--0xFF | Private/Experimental Use |
+------------+--------------------------+
Table 15: MSG_REJECT Reason Codes
10. Acknowledgments
This specification is based on comments on implementation of
[RFC7242] provided from Scott Burleigh.
11. References
11.1. Normative References
[I-D.ietf-dtn-bpbis]
Burleigh, S., Fall, K., and E. Birrane, "Bundle Protocol
Version 7", draft-ietf-dtn-bpbis-14 (work in progress),
August 2019.
[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>.
[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>.
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[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[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>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>.
[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>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
11.2. Informative References
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[github-dtn-bpbis-tcpcl]
Sipos, B., "TCPCL Example Implementation",
<https://github.com/BSipos-RKF/dtn-bpbis-tcpcl/tree/
develop>.
[I-D.ietf-dtn-bpsec]
Birrane, E. and K. McKeever, "Bundle Protocol Security
Specification", draft-ietf-dtn-bpsec-12 (work in
progress), September 2019.
[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>.
[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>.
[RFC7457] Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
Known Attacks on Transport Layer Security (TLS) and
Datagram TLS (DTLS)", RFC 7457, DOI 10.17487/RFC7457,
February 2015, <https://www.rfc-editor.org/info/rfc7457>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
Appendix A. Significant changes from RFC7242
The areas in which changes from [RFC7242] have been made to existing
headers and messages are:
o Split contact header into pre-TLS protocol negotiation and
SESS_INIT parameter negotiation. The contact header is now fixed-
length.
o Changed contact header content to limit number of negotiated
options.
o Added session option to negotiate maximum segment size (per each
direction).
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o Renamed "Endpoint ID" to "Node ID" to conform with BPv7
terminology.
o Added session extension capability.
o Added transfer extension capability. Moved transfer total length
into an extension item.
o Defined new IANA registries for message / type / reason codes to
allow renaming some codes for clarity.
o Segments of all new IANA registries are reserved for private/
experimental use.
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_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.
o Renamed SHUTDOWN to SESS_TERM to deconflict term "shutdown"
related to TCP connections.
o Removed the notion of a re-connection delay parameter.
The areas in which extensions from [RFC7242] have been made as new
messages and codes are:
o Added contact negotiation failure SESS_TERM reason code.
o Added MSG_REJECT message to indicate an unknown or unhandled
message was received.
o Added TLS session security mechanism.
o Added Resource Exhaustion SESS_TERM reason code.
Authors' Addresses
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Brian Sipos
RKF Engineering Solutions, LLC
7500 Old Georgetown Road
Suite 1275
Bethesda, MD 20814-6198
United States of America
Email: BSipos@rkf-eng.com
Michael Demmer
University of California, Berkeley
Computer Science Division
445 Soda Hall
Berkeley, CA 94720-1776
United States of America
Email: demmer@cs.berkeley.edu
Joerg Ott
Aalto University
Department of Communications and Networking
PO Box 13000
Aalto 02015
Finland
Email: ott@in.tum.de
Simon Perreault
Quebec, QC
Canada
Email: simon@per.reau.lt
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