Network Working Group L. Wood
Internet-Draft Cisco Systems
Intended status: Experimental J. McKim
Expires: November 19, 2007 RSIS
W. Eddy
Verizon
W. Ivancic
NASA
C. Jackson
SSTL
May 18, 2007
Saratoga: A Convergence Layer for Delay Tolerant Networking
draft-wood-dtnrg-saratoga-00
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
Saratoga is a simple, lightweight UDP-based transport protocol
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intended for use between immediately neighbouring peers which have
sporadic, intermittent connectivity using dedicated IP links.
Saratoga focuses on link utilization and loss recovery via an ARQ
mechanism. Saratoga is not intended for use over shared paths, so
transport congestion control is unnecessary. Saratoga is in
continuous use by the IP-based Disaster Monitoring Constellation
satellites in Low Earth Orbit (LEO), and is proposed for use as a
convergence layer by Delay Tolerant Networking for deep-space and ad-
hoc networking scenarios.
Table of Contents
1. Background and Introduction . . . . . . . . . . . . . . . . . 3
2. Overview of Saratoga File/Bundle Transfer . . . . . . . . . . 5
2.1. Example File Transfers . . . . . . . . . . . . . . . . . . 5
2.2. Using Saratoga with a DTN Bundle Agent . . . . . . . . . . 8
3. Packet Types . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1. BEACON . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2. REQUEST . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.3. METADATA . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.4. DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.5. HOLETOFILL . . . . . . . . . . . . . . . . . . . . . . . . 20
4. Directory Entry . . . . . . . . . . . . . . . . . . . . . . . 24
5. Behavior of a Saratoga Peer . . . . . . . . . . . . . . . . . 26
5.1. Saratoga Transactions . . . . . . . . . . . . . . . . . . 26
5.2. Beacons . . . . . . . . . . . . . . . . . . . . . . . . . 29
5.3. Upper-Layer Interface . . . . . . . . . . . . . . . . . . 30
5.4. Inactivity Timer . . . . . . . . . . . . . . . . . . . . . 30
6. Security Considerations . . . . . . . . . . . . . . . . . . . 31
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 32
9. A Note on Naming . . . . . . . . . . . . . . . . . . . . . . . 32
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
10.1. Normative References . . . . . . . . . . . . . . . . . . . 32
10.2. Informative References . . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
Intellectual Property and Copyright Statements . . . . . . . . . . 36
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1. Background and Introduction
Saratoga is a rate-based UDP file transfer protocol capable of
efficiently transferring both small and very large files. Saratoga
has been implemented and used since 2004 to move mission imaging data
from the Disaster Monitoring Constellation remote-sensing satellites
to ground stations. These satellites, built by Surrey Satellite
Technology Ltd (SSTL), all use IP for payload communications and
delivery of Earth imagery. Five satellites are already operational
in orbit; three more are under construction. Further details on how
these satellites use IP to communicate with the ground and the
terrestrial Internet are discussed in other documents
[Hogie05][Wood07a].
Saratoga can be used in Delay/Disruption-Tolerant Networking (DTN)
[RFC4838], as a "convergence layer" to exchange DTN bundles
[I-D.irtf-dtnrg-bundle-spec] between peer nodes. The DTN concept is
applicable to networks where ad-hoc, intermittent connectivity is the
norm, connections between peers are intermittent and infrequent, and
end-to-end paths are not present. Saratoga's design presumes that
links are not shared, but rather that the links are dedicated to a
pair of peer DTN nodes that can exchange store-and-forward bundle
messages. Each node runs a single instance of Saratoga; any
multiplexing of simultaneous data transfers takes place in sessions
between the peering instances of Saratoga.
Use of store-and-forward delivery is typical of DTN scenarios in
space exploration for both near-Earth and deep-space missions, and
useful for other DTN scenarios, such as underwater networking, ad-hoc
sensor networks, and some message-ferrying relay scenarios.
High link utilization while a link is established is important in
order to get the most utility out of the available, but limited,
connectivity. Recovery from packets lost due to channel errors is
important. Congestion control, to arbitrate fairly between competing
traffic, is not considered important, as nodes peer without
competition from other nodes, and loss is never due to congestion.
Saratoga's design is intended for dedicated point-to-point links
between peers. In a shared wireless ad-hoc environment, allocation
of channel resources to nodes to establish these links becomes a MAC-
layer function. Forward error coding to get the most reliable
transmission through a channel is best left near the physical layer,
and complements the transport-level negative-acknowledgement approach
to providea reliable ARQ mechanism [RFC3366] that is presented here.
Saratoga is capable of transferring small or large files, by choosing
a width of file offset descriptor appropriate for the filesize, and
advertising accepted descriptor sizes. 16-bit, 32-bit, 64-bit and
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128-bit descriptors can be selected, for maximum file sizes of
64KiB-1, 4GiB-1, 2^64-1 and 2^128-1 octets. Earth imaging files
currently transferred by Saratoga are mostly up to a few gigabytes in
size. Some implementations do transfer more than 4GiB in size, and
so require a descriptor larger than 32 bits. We expect that a 128-
bit descriptor will satisfy all future needs, but we expect current
implementations to only support up to 32-bit or 64-bit descriptors,
depending on their application needs. The 16-bit descriptor is
useful for small messages, including messages from 8-bit devices, and
is always supported.
Saratoga can be used with either IPv4 or IPv6. By using an IP-based
convergence layer, compatibility between Saratoga and the wide
variety of links that can already carry IP traffic is assured. Due
to widespread compatibility with a vast variety of existing link
layers, IP is the logical choice of network layer to support DTN
convergence layers, even though the scheduled or intermittent and
point-to-point peering operating paradigm behind this use of IP here
differs from the traditional congestion and competition-oriented
model that has become established in the fixed terrestrial Internet.
Some of the motivations in the design of Saratoga are similar to
those for the Licklider Transmission Protocol (LTP)
[I-D.irtf-dtnrg-ltp-motivation]. Both of these protocols were
designed based on experience gained with using the CCSDS File
Delivery Protocol (CFDP), which was developed for the Consulative
Committee for Space Data Systems (CCSDS). The main design difference
between LTP and Saratoga is that LTP transfers arbitrary un-named
data blobs (binary large objects), while Saratoga transfers named
files including file metadata. LTP can also permit some portion of
trailing bytes of a blob to be transferred unreliably, which is a
feature that Saratoga does not support because its focus on file
transfer requires reliable delivery.
Saratoga was originally implemented as outlined in [Jackson04], but
the specification given here differs substantially, as we have added
a number of features to support DTN bundling, while cleaning up the
initial Saratoga specification. The original Saratoga code uses a
version number of 0, while code that implements this version of the
protocol advertises a version number of 1. Further discussion of the
history and development of Saratoga is given in [Wood07b].
This document contains an overview of the file or bundle transfer
procedure and transactions using Saratoga in Section 2, followed by a
formal definition of the packet types used by Saratoga in Section 3,
and the details of the various protocol mechanisms in Section 5.
Here, Saratoga transaction types are labelled with underscores around
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lowercase names (such as a "_get_" transaction), while Saratoga
packet types are labelled in all capitals (such as a "REQUEST"
packet) in order to distinguish between the two.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119. [RFC2119]
2. Overview of Saratoga File/Bundle Transfer
Saratoga was originally designed for file transfer from LEO imaging
satellites to the ground, prior to the DTNRG's work on the Bundle
Protocol [I-D.irtf-dtnrg-bundle-spec]. It was later recognized that
Saratoga could be used to reliably exchange bundles between DTN
Bundle Agents by using a logical mapping between DTN bundles and
Saratoga files and back. In this section we first describe a typical
file transfer process using Saratoga and then describe the way that
Saratoga has been used to reliably and efficiently transfer bundles.
Saratoga is a peer-to-peer protocol in the sense that between two
Saratoga peers files may be transferred in both directions
simultaneously. Two separate unidirectional file transfers would
need to be established to do simultaneous transfers. The
implementations of Saratoga at each end can be identical equivalent
peers.
2.1. Example File Transfers
Saratoga nodes are simple file servers. Saratoga supports several
types of operations on files including upload, download, directory
listing, and deletion requests. Each request begins a "transaction".
Saratoga nodes MAY advertise their presence, capabilities and desires
by periodically sending BEACON packets. These BEACONs are sent to
either a subnet-directed broadcast address when using IPv4 or a link-
local all-Saratoga-peers multicast address when using IPv6. Saratoga
nodes may discover other Saratoga nodes either through listening for
BEACONs, through pre-configuration, or via some other trigger from a
user, lower-layer protocol, or another process. The BEACON is simply
useful in low-delay ad-hoc networking or as explicit confirmation
that another node is present; it is not required in order to begin a
Saratoga transaction.
There is an assumption that if multiple nodes do hear a BEACON on a
link shared by more than two peers and respond with transaction
requests, the node that sent the BEACON will favour one request and
complete that before turning to other requests. In such cases, a
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cooperative coarse-grained scheduling model, where transactions
between different nodes are completed consecutively rather than
simultaneously, would be desirable. Currently, no protocol mechanism
enforces this. There is no required response to received BEACONs.
Since request packets are generally small, and the number of nodes
sharing a link should be small, the number of simultaneous
transaction requests for a node to deal with, and decide between,
will also be small.
A Saratoga transaction begins with either a _get_, _put_, _getdir_,
or _delete_ transaction request, corresponding to a desired download,
upload, directory listing, or deletion operation. The most common
envisioned transaction is the _get_, which begins with a single
Saratoga REQUEST packet sent from the peer wishing to receive the
file, to the peer who currently has the file. If the transaction is
rejected, then a brief METADATA packet that conveys rejection is
generated. If the file-serving peer accepts the transaction, it
generates and sends a more useful descriptive METADATA packet,
follwed by some number of DATA packets constituting the directory
listing's contents or the requested file.
These DATA packets are finished by (and can intermittently include) a
DATA packet with a flag bit set that demands the file-receiver send a
reception report in the form of a HOLETOFILL packet. The HOLETOFILL
packet lists which octets of the file have not yet been received and
whether or not the METADATA packet was received. From this
HOLETOFILL packet, the file-sender begins a cycle of selective
retransmission of DATA packets, until it sees a HOLETOFILL packet
that acknowledges total reception of all file data.
In the example scenario in Figure 1, a_get_ request is granted and
experiences loss of a single DATA packet due to channel-induced
errors.
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File-Receiver File-Sender
REQUEST -------------------->
(transfer accepted) <--------- METADATA
HOLETOFILL -----------------> (voluntarily sent at start)
<---------------------- DATA #1
(lost) <------ DATA #2
<---------------------- DATA #3 (bit set
requesting HOLETOFILL)
HOLETOFILL ----------------->
(indicating that range in DATA #2 was lost)
<---------------------- DATA #2 (bit set
requesting HOLETOFILL)
HOLETOFILL ----------------->
(complete file and METADATA received)
Figure 1: Example _get_ transaction sequence
A _getdir_ request proceeds similarly.
The HOLETOFILL and DATA packets are allowed to be sent at any time
within the scope of a transaction in order for the file-sending node
to optimize buffer management and transmission order. For example,
if the file-receiver already has the first half of a file from a
previous disrupted transfer, it may send a HOLETOFILL at the
beginning of the transaction indicating that it has the first half of
the file, and so only needs the last half of the file. Thus,
efficient recovery from interrupted sessions between peers becomes
possible.
In deep-space scenarios, the large propagation delays and round-trip
times involved prohibit ping-pong packet exchanges for starting
transactions. The Saratoga _put_ transaction is useful in such
cases. A _put_ is initiated by the file-sender sending a METADATA
packet followed by immediate DATA packets. This is highly desirable
in long-propagation deep-space (and similar) scenarios, without first
waiting for a HOLETOFILL. This can be considered an "optimistic"
mode of protocol operation, as it assumes the transaction request
will be granted. If the sender of a PUT request sees a METADATA
packet indicating that the request was declined, it MUST stop sending
any DATA packets within that transaction immediately.
Figure 2 illustrates the sequence of packets in an example _put_
transaction where the second DATA packet is lost.
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File-Sender File-Receiver
METADATA ---------------->
(transfer accepted) <------- HOLETOFILL
DATA #1 ---------------->
DATA #2 ---> (lost)
DATA #3 (bit set ------------>
requesting HOLETOFILL)
(DATA #2 lost) <---------- HOLETOFILL
DATA #2 (bit set ----------->
requesting HOLETOFILL)
(transfer complete) <------- HOLETOFILL
Figure 2: Example PUT transaction sequence
The _delete_ transactions are simple single packet requests that
trigger a HOLETOFILL packet with a status code that indicates whether
the file was deleted or not. If the file is not able to be deleted
for some reason, this reason can be conveyed in the Status field of
the HOLETOFILL packet.
2.2. Using Saratoga with a DTN Bundle Agent
While Saratoga was first developed for efficient file transfer, the
similarity between bundle payloads and files, in that both are
arbitrary blobs of some number of octets, allows Saratoga to be used
as a convergence layer for exchanging bundles between DTN bundle
agents. This section explains the basic concepts involved in mapping
bundle exchange onto the file transfer mechanism.
Routing of bundles is outside the scope of Saratoga and this
document. Once a complete bundle file has been transferred between
peers using Saratoga, it can be forwarded onwards along a next
available hop in any way.
A DTN bundle agent can work alongside a Saragota peer to move bundles
if there is a directory accessible to both the DTN and Saratoga
processes. To send a bundle, the bundle agent places the complete
bundle (the concatenated set of Bundle Protocol blocks) into a file
in the shared directory. The local Saratoga instance is then able to
_put_ this bundle to peers or allow them to _get_ it. A flag bit in
the Saratoga METADATA packet indicates whether a particular file is a
bundle or not, so the receiving Saratoga peer knows whether to write
the received data to the local filesystem as a simple file, or to
pass it to a local bundle agent that it is working with.
Note that the name of a file holding a bundle is actually
unimportant, as long as it can be determined that it does hold a
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bundle. One implementation strategy is to name each bundle file with
a file name constructed from two fields of the Primary Bundle Header:
the DTN Endpoint Identifier (EID) of the destination node and the
bundle's creation time field. In the rare case of file name
collisions in using this scheme, additional octets can be appended to
the filename following some arbitrary local scheme. Bundle files
might be placed in different directories with different Saratoga-peer
access controls depending on the intended next-hop, if this
information is known ahead of time. In any case, Saratoga only
handles reliable transfers, and any forwarding decisions based on
routing intelligence are made within the DTN bundle agents. All of
this detail is considered a matter of implementation for the bundle
agent, and not specified here.
A _get_ REQUEST packet that does not specify a filename (i.e. the
request contains a zero-length File Path field) is specially defined
to be a request for any chosen file that the peer wishes to send it.
This allows a bundle agent coordinating with one Saratoga peer to
blindly request any bundle files that the other Saratoga peer's
bundle agent has ready for it, without prior knowledge of the
directory listing, and without requiring the ability to determine
whether files are bundle files or decode remote file names/paths for
meaningful information such as final destination.
A field in the Saratoga BEACON packet allows a local DTN bundle agent
to advertise its administrative EID via Saratoga, so that other
Saratoga peers that hear that BEACON can notify their local DTN
bundle agents of the contact. These notifications might be used to
integrate contact information into a routing information base, as
they are similar to the "hello" packets used in several routing
protocols. However, this is outside the scope of this document.
If a bundle is larger than Saratoga can be expected to transfer
during a time-limited contact, there are at least two feasible
options:
(1) A bundle agent can use proactive fragmentation to create multiple
smaller-sized bundle files. Saratoga can transfer some number of
these bundles fully during a contact.
(2) To avoid bundle fragmentation, a Saratoga file-receiver can
retain a partially-transferred file and request transfer of the
unreceived bytes during a later contact. This uses a HOLETOFILL
packet to make clear how much of the file has been successfully
received and where transfer should be resumed from. On resumption,
the new METADATA (including file length, timestamps, and possibly MD5
sum) MUST match that of the previous METADATA in order to re-
establish the transfer. Otherwise, the file-receiver MUST assume
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that the file has changed and purge the DATA received during the
first contact.
This version of the Saratoga specification does not specify the
functionality required for reactive fragmentation of bundles as
described in [RFC4838], other than what is needed to support
disrupted delivery and hop-by-hop custody transfer.
However, the status of a transfer that Saratoga provides to a bundle
agent could be used to trigger the reactive fragmentation of bundles
if a bundle file transfer is interrupted part-way through (assuming
at least the bundle protocol headers and some portion of the data was
successfully transferred first). This would allow for efficient
recovery when unplanned interruptions occur. On each end, this
requires some coordination between the Saratoga node and the local
bundle agent, but the local API or coupling between these does not
affect the interoperability between either the Saratoga peers or the
DTN bundle agents, assuming that both sides agree that fragmentation
will occur at the lowest un-acknowledged octet of the bundle file
after the disruption. This reactive fragmentation and any forwarding
of the fragments onwards for reassembly at some downstream node is a
bundle problem. Reactive fragmentation lies outside the scope of
custody transfer, of Saratoga and of this document.
3. Packet Types
Saratoga is defined for use with UDP over either IPv4 or IPv6
[RFC0768]. UDP checksums MUST be used. Within either version of IP
datagram, a Saratoga packet appears as a typical UDP header followed
by an octet indicating how the remainder of the packet is to be
interpreted:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP source port | UDP destination port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP length | UDP checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver|Packet Type| other Saratoga fields ... //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+//
The two-bit Saratoga version field ("Ver") identifies the version of
the Saratoga protocol that the packet conforms to. The value 01
should be used in this field for implementations conforming to the
specification in this document, which specifies version 1 of
Saratoga. The value 00 was used in earlier implementations, prior to
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the formal specification and public submission of the protocol
design, and is incompatible with version 01 in several respects.
The six-bit Saratoga "Packet Type" field indicates how the remainder
of the packet is intended to be decoded and processed:
+---+------------+-------------------------------------------+
| # | Type | Use |
+---+------------+-------------------------------------------+
| 0 | BEACON | Beacon packet indicating peer status |
| 1 | REQUEST | Commands peer to start a transfer |
| 2 | METADATA | Carries file transfer metadata |
| 3 | DATA | Carries octets of file data |
| 4 | HOLETOFILL | Signals list of unreceived data to sender |
+---+------------+-------------------------------------------+
Several of these packet types include a Flags field, for which only
some of the bits have defined meanings and usages in this document.
Other, undefined, bits may be reserved for future use. Following the
principle of being conservative in what you send and liberal in what
you accept, a packet sender MUST set any undefined bits to zero, and
a packet recipient MUST NOT rely on these undefined bits being zero
on reception.
The specific formats for the different types of packets follow in the
remainder of this section. Some packet types contain file descriptor
fields. The lengths of file descriptors are fixed within a transfer,
but vary between file transfers. The size is set for each particular
transfer, depending on the choice of file descriptor width made in
the METADATA packet, which in turn depends on the size of file being
transferred.
In this document, all of the packet structure figures illustrating a
packet format assume 32-bit lengths for these file descriptor fields,
and indicate the transfer-dependent length of the fields by using a
"(descriptor)" designation within the [field] in all packet diagrams.
That is:
The example 32-bit descriptors shown in all diagrams here
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
[ (descriptor) ]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
are suitable for files of up to 4GiB - 1 octets in length, and may be
replaced in a file transfer by descriptors using a different length,
depending on the size of file to be transferred:
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128-bit descriptor for very long files (optional)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
[ (descriptor) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ (descriptor, continued) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ (descriptor, continued) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ (descriptor, continued) ]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
64-bit descriptor for longer files (optional)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
[ (descriptor) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ (descriptor, continued) ]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
16-bit descriptor for short files (MUST be supported)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
[ (descriptor) ]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Saratoga packets are intended to fit within link MTUs to avoid the
inefficiences and overheads of lower-layer fragmentation. A Saratoga
implementation itself does not perform any form of MTU discovery, but
is assumed to be configured with knowledge of usable maximum IP MTUs
for the link interfaces it uses.
3.1. BEACON
BEACON packets may be broadcast periodically by nodes willing to act
as Saratoga peers. Some implementations have done so every 100
milliseconds, but this rate is arbitrary, and should be chosen to be
appropriate for the environment and implementation.
The main purpose for sending BEACONs is to announce the presence of
the node to potential peers (e.g. satellites, ground stations) to
provide automatic service discovery, and also to confirm the liveness
or presence of the peer.
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Format
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
|0 1| Type | Flags | Bundle Agent EID ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
where
+--------+----------------------------------------------------------+
| Field | Description |
+--------+----------------------------------------------------------+
| Type | 0 |
| Flags | convey whether or not the peer is ready to send/receive |
| | and what the maximum supported file size range and |
| | descriptor is. |
| Bundle | advertises the EID of a bundle agent administrative node |
| Agent | that the BEACON-sender is associated with, or some other |
| EID | peer-identifying string if no bundle agent is present. |
+--------+----------------------------------------------------------+
The Flags field is used to provide some additional information about
the peer. The two highest-order bits (bits 8 and 9 above) indicate
the maximum supported file size parameters that the peer's Saratoga
implementation permits. Other Saratoga packet types contain
variable-length fields that convey file sizes or offsets into a file
-- the file descriptors. These descriptors may be 16-bit, 32-bit,
64-bit, or 128-bit in length, depending on the size of the file being
transferred and/or the integer types supported by the sending peer.
The indicated bounds for the possible values of these bits are
summarized below:
+-------+-------+-------------------------+-------------------+
| Bit 8 | Bit 9 | Supported Field Sizes | Maximum File Size |
+-------+-------+-------------------------+-------------------+
| 0 | 0 | 16 bits | 2^16 - 1 octets. |
| 0 | 1 | 16 or 32 bits | 2^32 - 1 octets. |
| 1 | 0 | 16, 32, or 64 bits | 2^64 - 1 octets. |
| 1 | 1 | 16, 32, 64, or 128 bits | 2^128 - 1 octets. |
+-------+-------+-------------------------+-------------------+
If a Saratoga peer advertises it is capable of receiving a certain
size of file, then it MUST also be capable of receiving files sent
using smaller descriptor values. This avoids overhead on small
files, while increasing interoperability between peers.
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+--------+--------+------------------------------------------------+
| Bit 12 | Bit 14 | Willingness and capability to send files |
+--------+--------+------------------------------------------------+
| 0 | 0 | cannot send files at all. |
| 0 | 1 | capable of sending, but not willing right now. |
| 1 | 0 | invalid. |
| 1 | 1 | capable of and willing to send files. |
+--------+--------+------------------------------------------------+
+--------+--------+-------------------------------------------------+
| Bit 13 | Bit 15 | Willingness and capability to receive files |
+--------+--------+-------------------------------------------------+
| 0 | 0 | cannot receive files at all. |
| 0 | 1 | capable of receiving, but will reject METADATA. |
| 1 | 0 | invalid. |
| 1 | 1 | capable of and willing to receive files. |
+--------+--------+-------------------------------------------------+
Also in the Flags field, bits 14 and 15 act as capabilities bits,
while bits 12 and 13 act as willingness bits. If bit 14 is set, then
the peer has the capability to send files. A peer that is able to
act as a file-sender MUST set this bit in all BEACONs that it sends,
regardless of whether it is willing to send any particular files to a
particular peer at a particular time. Bit 12 indicates the current
presence of data to send and a willingness to send it in general, in
order to augment the capability advertised by bit 14.
If bit 15 is set, then the peer is capable of acting as a receiver,
although it still might not currently be ready or willing to receive
files (for instance, it may be low on free storage). This bit MUST
be set in any BEACON packets sent by nodes capable of acting as file-
receivers. Bit 13 expresses a current general willingness to receive
and accept files.
Although a Saratoga implementation does not need to be working in
conjunction with a DTN bundle agent, the combination will frequently
be the case. Saratoga BEACON packets include the administrative
Endpoint ID of a DTN bundle agent that it is working in conjunction
with. This is not a prerequisite for bundle file transfers, or any
other functionality, but is merely a convenient way to advertise the
presence of a bundle agent along with the advertisement of Saratoga
services. For Saratoga implementations or deployments that do not
involve bundle agent cooperation, some other type of host-identifier
can be used in this field, as long as it is a reasonably unique
string within the range of operational deployment. This field
encompasses the remainder of the packet, and might contain non-UTF-8
and/or null characters.
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3.2. REQUEST
A REQUEST packet is a command to perform either a _get_, _getdir_, or
_delete_ transaction.
Format
0 1 2 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1| Type | Flags | Id ... /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ ... (Id cont.) | File Path ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where
+--------+----------------------------------------------------------+
| Field | Description |
+--------+----------------------------------------------------------+
| Type | 1 |
| Flags | provide additional information about the requested |
| | file/operation; see table below for definition. |
| Id | uniquely identifies the transaction between two peers. |
| File | the path of the requested file/directory following the |
| Path | rules described below. |
+--------+----------------------------------------------------------+
The Id that is used during transactions serves to uniquely associate
a given packet with a particular transaction. This enables multiple
simultaneous data transfer transactions between two peers. The Id
for a transaction is selected by the initiator so as to not conflict
with any other in-progress or recent transactions with the same host.
This Id should be unique and generated using properties of the file,
which will remain constant across a host reboot. The 3-tuple of both
host identifiers and a carefully-generated transaction Id field can
be used to uniquely index a particular transaction's state.
In the Flags field, the bits labelled 8 and 9 in the figure above
indicate the maximum supported file length fields that the peer can
handle, and are interpreted exactly as the bits 8 and 9 in the BEACON
packet described above. The remaining defined bits are:
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+---------+-------+-------------------------------------------------+
| Bit | Value | Meaning |
| Number | | |
+---------+-------+-------------------------------------------------+
| 14 | 0 | a _get_ or _getdir_ transaction is requested |
| 14 | 1 | a _delete_ transaction is requested |
| 15 | 0 | the File Path field holds a file for a _get_ or |
| | | _delete_ |
| 15 | 1 | the File Path field specifies a directory name |
| | | for a _getdir_ or _delete_ |
+---------+-------+-------------------------------------------------+
The File Path portion of a _get_ packet is a null-terminated UTF-8
encoded string [RFC3629] that represents the path and base file name
on the file-sender of the file (or directory) that the file-receiver
wishes to perform the _get_, _getdir_, or _delete_ operation on.
Implementations SHOULD only send as many octets of File Path as are
needed for carrying this string, although some implementations MAY
choose to send a fixed-size File Path field in all REQUEST packets
that is filled with null octets after the last UTF-8 encoded octet of
the path. A maximum of 1024 octets for this field, and for the File
Path fields in other Saratoga packet types, is used to limit the
total packet size to within a single IPv6 minimum MTU (minus some
padding for network layer headers), and thus avoid the need for
fragmentation. The 1024-octet maximum applies after UTF-8 encoding
and null termination.
As in the standard Internet File Transfer Protocol (FTP) [RFC0959],
for path separators, Saratoga allows whatever the local conventions
on the peers are to be used. There are security implications to
processing these strings without some intelligent filtering and
checking on the filesystem items they refer to, as discussed in the
Security Considerations section later within this document.
If the length of the packet indicates that the File Path field is
empty (zero-length), then this indicates that the file-receiver is
ready to receive any file that the file-sender would like to send it,
rather than requesting a particular file. This is useful in bundle
transfer, as it permits the file-sender to determine the order and
selection of bundle files that it would like to forward through the
file-receiving host's bundle agent in more of a "push" manner. Of
course, bundle file retrieval could also follow a "pull" manner, with
the file-receiving host requesting specific bundle files from the
file-sender. This may be desirable at times if the file-receiver is
low on storage space, or other resources. The file-receiver could
also use the Saratoga _getdir_ transaction results in order to select
small-sized bundles, or make other optimizations, such as using its
local knowledge of contacts to pick small bundles that it has the
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best odds of being able to deliver completely. Bundle file transfer
through pushing sender-selected files implements delivery
prioritization decisions made solely at the Saratoga file-sending
node. Bundle file transfer through pulling specific receiver-
selected files implements prioritization involving more participation
from the Saratoga file-receiver. This is how Saratoga implements
Quality of Service (QoS).
3.3. METADATA
METADATA packets are sent as part of a data transfer transaction
(_get_, _getfile_, and _put_). A METADATA packet says how large the
file is and what its name is, as well as what size of file descriptor
is chosen for the session.
Format
0 1 2 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1| Type | Flags | Id ... /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ ... (Id cont.) | MD5 Sum ... /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ /
/ /
/ ... (MD5 Sum cont. middle 96-bits) ... /
/ /
/ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ ... (MD5 Sum cont.) | /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ /
/ /
/ single Directory Entry describing file /
/ (variable length) /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where
+-----------+-------------------------------------------------------+
| Field | Description |
+-----------+-------------------------------------------------------+
| Type | 2 |
| Flags | indicate additional boolean metadata about a file |
| Id | identifies the transaction that this packet describes |
| MD5 Sum | full 128-bit MD5 checksum over file contents |
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| Directory | describes file system information about the file, |
| Entry | including length, timestamps, etc.; the format is |
| | specified in Section 4 |
+-----------+-------------------------------------------------------+
In the Flags field, the bits labelled 8 and 9 in the figure above
indicate the exact size of the file length fields used in this
particular packet and are interpreted exactly as the bits 8 and 9 in
the BEACON packet described above. The value of these bits
determines the size of the File Length field in the current packet,
as well as indicating the size of the offset fields used in DATA and
HOLETOFILL packets within the session that will follow this packet.
+--------+--------+--------------------------+
| Bit 10 | Bit 11 | Checksum capability |
+--------+--------+--------------------------+
| 0 | 0 | no checksum is present. |
| 0 | 1 | MD5 checksum is present. |
| 1 | 0 | reserved for future use. |
| 1 | 1 | reserved for future use. |
+--------+--------+--------------------------+
Also inside the Flags field, bits 10 and 11 indicate the integrity
checksum for the file to be transferred. If 00, no checksum is
present, and higher-level end-to-end checks within the bundle must be
relied upon. If 01, an MD5 checksum of the file to be transferred is
carried as shown, with a fixed 128-bit field before the varying-
length File Length and File Name information fields. Other flag
combinations are reserved for future use. It is expected that the
MD5 checksum will be used, unless the Saratoga implementation is used
exclusively for small transfers at the low end of the 16-bit file
descriptor range, such as on low-performing hardware, where a weaker
checksum can suffice.
The MD5 Sum field is generated via the MD5 algorithm [RFC1321],
computed over the entire contents of the file being transferred. The
file-receiver can compute the MD5 result over the reassembled
Saratoga DATA packet contents, and compare this to the METADATA's MD5
Sum field in order to gain confidence that there were no undetected
protocol errors or UDP checksum weaknesses encountered during the
transfer. Although MD5 is known to be less than optimal for security
uses, it remains excellent for non-security use in integrity checking
(as is done here in Saratoga), and has better performance
implications than cryptographically-stronger alternatives given the
limited available processing of many DTN use cases.
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+-------+-------+---------------------------------------------------+
| Bit | Bit | Bundles and records |
| 14 | 15 | |
+-------+-------+---------------------------------------------------+
| 0 | 0 | a file is being sent. |
| 0 | 1 | the file being sent should be interpreted as a |
| | | directory record. |
| 1 | 0 | a bundle is being sent. |
| 1 | 1 | invalid. |
+-------+-------+---------------------------------------------------+
Also inside the Flags field, bit 15 of the packet signals whether or
not the file being transferred is a bundle file. This bit is set to
1 if the file's contents are blocks belonging to a DTN bundle,
otherwise this bit is set to 0. Bit 14 of the Flags, if set,
indicates that the METADATA and DATA packets are being generated in
response to a _getdir_ REQUEST, and that the assembled DATA contents
should be interpreted as a sequence of Directory Records, as defined
in Section 4. If bit 14 is set, bit 15 will be 0.
3.4. DATA
A series of DATA packets form the main part of a data transfer
transaction (_get_, _put_, or _getdir_). The payloads constitute the
actual file/bundle data being transferred.
Format
0 1 2 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1| Type | Flags | Id ... /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ ... (Id cont.) [ Offset (descriptor)... /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ ... (Offset cont.) ]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where
+--------+----------------------------------------------------------+
| Field | Description |
+--------+----------------------------------------------------------+
| Type | 3 |
| Flags | bit 15 requests an immediate HOLETOFILL ack to be |
| | generated in response to receiving this packet |
| Id | identifies the transaction that this packet belongs to |
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| Offset | the offset in octets to the location where the first |
| | byte of this packet's payload is to be written |
+--------+----------------------------------------------------------+
Flag bits 8 and 9 are set to indicate the size of the offset
descriptor as described for BEACON and METADATA packets, so that each
DATA packet is self-describing. This allows the DATA packet to be
used to construct a file even when the initial METADATA is lost and
must be resent. The flag values here MUST be the same as indicated
in the initial METADATA packet.
Immediately following the ten-octet DATA header is the payload, which
consumes the remainder of the packet and whose length is implicitly
defined by the end of the packet. The payload octets are directly
formed from the continuous octets starting at the specified Offset in
the file being transferred. No special coding is performed. A zero-
octet payload length is allowable.
Within the Flags field, if bit 15 of the packet is set, the file-
receiver is to immediately generate a HOLETOFILL packet to provide
the file-sender with up-to-date information regarding the status of
the file transfer.
The length of the Offset fields used within all DATA packets for a
given transaction MUST be consistent with the length indicated by
bits 8 and 9 of the transactions METADATA packet. If the METADATA
packet has not yet been received, a file-receiver SHOULD request it
via a HOLETOFILL packet, and MAY choose to enqueue received DATA
packets for later processing after the METADATA arrives.
3.5. HOLETOFILL
The HOLETOFILL packet type is used for feedback from a Saratoga file-
receiver to a Saratoga file-sender to indicate transaction progress
and request transmission (usually re-transmission) of specific sets
of octets within the current transaction (called "holes"). This can
be used to clean up losses (or indicate no losses) at the end of or
during a transaction, or to efficiently resume a transfer that was
interrupted in a previous transaction.
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Format
0 1 2 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 0| Type | Flags | Id ... /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ ... (Id cont.) | Status | Zero-Pad |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
[ Cumulative Acknowledgement (descriptor) ]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
[ In-Response-To (descriptor) ]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (possibly, several Hole fields) /
/ ... /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where
+-----------------+-------------------------------------------------+
| Field | Description |
+-----------------+-------------------------------------------------+
| Type | 4 |
| Flags | defined below |
| Id | identifies the transaction that this packet |
| | belongs to. |
| Status | a value of 0x00 indicates the transfer is |
| | sucessfully proceeding. All other values are |
| | errors terminating the transfer, explained |
| | below. |
| Zero-Pad | an octet fixed at 0x00 to allow later fields to |
| | be conveniently aligned for processing. |
| Cumulative | the offset of the lowest-numbered octet of the |
| Acknowledgement | file not yet received. |
| In-Response-To | the offset of the highest-numbered octet within |
| | a DATA packet that generated this HOLETOFILL |
| | packet, or 0 if this HOLETOFILL is generated |
| | voluntarily. |
| Holes | indications of offset ranges of missing data, |
| | defined below. |
+-----------------+-------------------------------------------------+
The Id field is needed to associate the packet with the transaction
that it refers to. Using the Id as a key, the receiver of a packet
can determine the lengths of the Cumulative Acknowledgement, In-
Response-To, and Hole offsets used within the HOLETOFILL packet, as
this file descriptor size was set in the initial METADATA packet that
established the Id.
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Flag bits 8 and 9 are set to indicate the size of the offset
descriptor as described for BEACON and METADATA packets, so that each
HOLETOFILL packet is self-describing. The flag values here MUST be
the same as indicated in the initial METADATA and DATA packets.
Other bits in the Flags field are defined as:
+--------+-------+--------------------------------------------------+
| Bit | Value | Meaning |
| Number | | |
+--------+-------+--------------------------------------------------+
| 14 | 0 | file's METADATA has not been received. |
| 14 | 1 | file's METADATA has been received. |
| 15 | 0 | this packet contains the complete current set of |
| | | holes at the file-receiver. |
| 15 | 1 | this packet contains incomplete hole-state; |
| | | holes shown in this packet should supplement any |
| | | previous hole-state known to the file-sender. |
+--------+-------+--------------------------------------------------+
If bit 14 of a HOLETOFILL packet indicates that the METADATA has not
yet been received, then the receiver of the packet MUST NOT attempt
to parse the Cumulative Acknowledgement, In-Response-To, or Hole
offset fields because there is an implication that the packet's
sender had no knowledge of the correct width to use for these fields.
Instead session negotiation MUST begin afresh.
Bit 15 of the HOLETOFILL packet is only set when there are too many
holes to fit within a single HOLETOFILL packet due to MTU
limitations. This causes the hole list to be spread out over
multiple HOLETOFILL packets, each of which convey distinct sets of
holes. This could occur, for instance, in a large file _put_
scenario with a long-delay feedback loop and poor physical layer
conditions. When losses are light and/or hole reporting and repair
is relatively frequent, all holes should easily fit within a single
HOLETOFILL packet, and bit 15 will normally be clear.
In the case of a transfer proceeding normally, immediately following
the HOLETOFILL packet header shown above, is a set of "Hole"
definitions. Each Hole definition is a pair of unsigned integers.
For a 32-bit file descriptor, each Hole definition consists of two
four-octet unsigned integers:
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Hole Definition Format
0 1 2 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
[ offset to start of hole (descriptor) ]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
[ offset to end of hole (descriptor) ]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The start of the hole means the offset of the first unreceived byte
in that hole. The end of the hole means the last unreceived byte in
that hole.
For 16-bit descriptors, each Hole definition holds two two-octet
unsigned integers, while Hole definitions for 64- and 128-bit
descriptors require two eight- and two sixteen-octet unsigned
integers respectively.
Since each Hole definition takes up eight octets when 32-bit offset
lengths are used, we expect that well over 100 such definitions can
fit in a single HOLETOFILL packet, given the IPv6 minimum MTU. In
some rare cases of high loss, there may be too many holes in the
received data to convey within a single HOLETOFILL's size, due to
trying to stay within the link MTU. In this case, multiple
HOLETOFILL packets may be generated, and Flag bit 15 should be set on
each HOLETOFILL packet accordingly, to indicate that each packet
holds partial results.
A 'voluntary' HOLETOFILL is sent at the start of each transaction,
once METADATA information has been received. This indicates that the
receiver is ready to receive the file, or indicates an error or
rejection code, described below. A HOLETOFILL indicating a
successfully established transfer has a Cumulative Acknowledgement of
zero and an In-Response-To field of zero.
In the case of an error causing a transfer to be aborted, the Status
field holds a code that can be used to explain the cause of the error
to the other peer. A zero value indicates that there have been no
significant errors (this is called a "success HOLETOFILL" within this
document), while any non-zero value means the transaction should be
aborted (this is called a "failure HOLETOFILL").
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+-------------+-----------------------------------------------------+
| Status | Meaning |
| Value | |
+-------------+-----------------------------------------------------+
| 0x00 | Success, No Errors. |
| 0x01 | Unspecified Error. |
| 0x02 | Unable to send file due to resource constraints. |
| 0x03 | Unable to receive file due to resource constraints. |
| 0x04 | File not found. |
| 0x05 | Access Denied. |
| 0x06 | Unknown Id field for transaction. |
| 0x07 | Did not delete file. |
| 0x08 | File length is longer than REQUEST indicates |
| | support for. |
| 0x09 | File descriptors do not match expect use or file |
| | length. |
+-------------+-----------------------------------------------------+
The recipient of a failure HOLETOFILL MUST NOT try to process the
Cumulative Acknowledgement, In-Response-To, or Hole offsets, because,
in some types of error conditions, the packet's sender may not have
any way of setting them to the right length for the transaction.
4. Directory Entry
Directory Entries have two uses within Saratoga:
1. Within a METADATA packet, a Directory Entry is used to give
information about the file being transferred, in order to
facilitate proper reassembly of the file and to help the file-
receiver understand how recently the file may have been created
or modified.
2. When a peer requests a directory listing via a _getdir_ REQUEST,
the other peer generates a file containing a series of one or
more concatenated Directory Entry records, and transfers this
file as it would transfer the response to a normal _get_ REQUEST,
sending the records together within DATA packets. This file may
be either temporary or within-memory and not actually a part of
the host's file system itself.
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Directory Entry Format
0 1 2 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
[ Size (descriptor) ]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mtime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ctime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Properties | /
+-+-+-+-+-+-+-+-+ /
/ /
/ File Path (max 1024 octets,variable length) /
/ ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where
+------------+------------------------------------------------------+
| field | description |
+------------+------------------------------------------------------+
| Size | the size of the file or directory in octets. |
| Mtime | a timestamp from the operating system's record of |
| | when the file or directory was modified. |
| Ctime | the timestamp of the last status change for this |
| | file or directory, according to the operating |
| | system. |
| Properties | if set, bit 7 of this field indicates that the entry |
| | corresponds to a directory. Bit 6, if set, |
| | indicates that the file is "special". A special |
| | file may not be directly transferable as it |
| | corresponds to a symbolic link, a named pipe, a |
| | device node, or some other "special" filesystem |
| | object. A file-sender may simply choose not to |
| | include these types of files in the results of a |
| | _getdir_ request. |
| File Path | contains the file's name relative within the |
| | requested path of the _getdir_ transaction, a |
| | maximum of 1024-octet UTF-8 string, that is |
| | null-terminated to indicate the beginning of the |
| | next directory entry in _getdir_ results |
+------------+------------------------------------------------------+
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+-------+-------+---------------------+
| Bit 6 | Bit 7 | Properties conveyed |
+-------+-------+---------------------+
| 0 | 0 | normal file. |
| 0 | 1 | normal directory. |
| 1 | 0 | special file. |
| 1 | 1 | special directory. |
+-------+-------+---------------------+
Whether a particular Directory Entry is being interpreted as the
contents of a METADATA packet, or as the result of a _getdir_
transaction, the width of the Size field is the same as that used for
all lengths and offsets within the transfer, given by the METADATA
and DATA Flags bits 8 and 9.
It is expected that files are only listed in Directory Entries if
they can be transferred to the requester. An implementation only
capable of receiving small files using 16-bit descriptors will only
see small files capable of being transferred to it when browsing the
filesystem of an implementation capable of larger sizes. Directory
sizes are not sent, and a Size of 0 is given instead for directories.
The "epoch" format used in the timestamps for Mtime and Ctime in file
object records is the number of seconds since January 1, 2000 in UTC,
which is the same epoch used in the DTN Bundle Protocol for
timestamps. This should include all leapseconds.
5. Behavior of a Saratoga Peer
This section describes some details of Saratoga implementations and
uses the RFC 2119 standards language to describe which portions are
needed for interoperability.
5.1. Saratoga Transactions
Following are descriptions of the packet exchanges between two peers
for each type of transaction.
5.1.1. The _get_ Transaction
1. A peer (the file-receiver) sends a REQUEST packet to its peer
(the file-sender). The Flags bits are set to indicate that this
is not a _delete_ request, nor does the File Path indicate a
directory. Each _get_ transaction corresponds to a single file,
and fetching multiple files requires sending multiple REQUEST
packets and using multiple transaction Ids. If a specific file is
being requested, then its name is filled into the File Path
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field, otherwise it is left null and the file-sender will send a
file of its choice.
2. If the request is rejected, then a HOLETOFILL packet containing
an error code in the Status field is sent and the transaction is
terminated. This HOLETOFILL packet MUST be sent to reject and
terminate the transaction. The error code MAY make use of the
"Unspecified Error" value for security reasons. Some REQUESTs
might also be rejected for specifying files that are too large to
have their lengths encoded within the maximum integer field width
advertised by bits 8 and 9 of the REQUEST.
3. If the request is accepted, then a HOLETOFILL packet MUST be sent
with an error code of 0x00 and an In-Response-To field of zero.
4. Otherwise, if the request is granted, then the file-sender
generates and sends a METADATA packet along with the contents of
the file as a series of DATA packets. In the absence of
HOLETOFILL packets, if the file-sender believes it has finished
sending the file, it MUST send the last DATA packet with the
Flags bit set requesting a HOLETOFILL response from the file-
receiver. This can be followed by empty DATA packets with the
Flags bit set requesting a HOLETOFILL until either a HOLETOFILL
packet is received, or the inactivity timer expires. All of the
DATA packets MUST use field widths for the file descriptor fields
that match what the Flags of the METADATA packet specified. Some
arbitrarily selected DATA packets may have the Flags bit set that
requests a HOLETOFILL packet. The file-receiver MAY voluntarily
send HOLETOFILL packets at other times, where the In-Response-To
field MUST set to zero. The file-receiver SHOULD voluntarily
send a HOLETOFILL packet in response to the first DATA packet.
5. As the file-receiver takes in the DATA packets, it writes them
into the file locally. The file-receiver keeps track of missing
data in a hole list. Periodically the file sender will set the
ack flag bit in a DATA packet and request a HOLETOFILL packet
from the file-receiver, with a copy of this hole list. File-
receivers MUST send a HOLETOFILL packet immediately in response
to receiving a DATA packet with the Flags bit set requesting a
HOLETOFILL.
6. If the file-sender receives a HOLETOFILL packet with a non-zero
number of holes, it re-fetches the file data at the specified
offsets and re-transmits it. If the METADATA packet requires
retransmission, this is indicated by a bit in the HOLETOFILL
packet, and the METADATA packet is retransmitted. The file-
sender MUST retransmit data from any holes reported by the file-
receiver before proceeding further with new DATA packets.
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7. When the file-receiver has fully received the file data and the
METADATA packet, then it sends a HOLETOFILL packet indicating
that the transaction is complete, and it terminates the
transaction locally, although it MUST persist in responding to
DATA packets requesting HOLETOFILLs from the file-sender for some
reasonable amount of time.
Given that there may be a high degree of asymmetry in link bandwidth
between the file-sender and file-receiver, the HOLETOFILL packets
should be carefully generated so as to not congest the feedback path.
This means that both a file-sender should be cautious in setting the
DATA Flags bit requesting HOLETOFILLs, and also that a file-receiver
should be cautious in gratuitously generating HOLETOFILL packets of
its own volition. On unidirectional links, a file-sender cannot
reasonably expect to receive HOLETOFILL packets, so should never
request them.
5.1.2. The _getdir_ Transaction
A _getdir_ transaction proceeds through the same states as the _get_
transaction. The two differences are that the REQUEST has the
directory bit set in its Flags field, and that, rather than
transferring the contents of a file from the file-receiver to the
file-sender, a set of records representing the contents of a
directory are transferred. These can be parsed and dealt with by the
file-receiver as desired. There is no requirement that a Saratoga
peer send the full contents of a directory listing; a peer may filter
the results to only those entries that are actually accessible to the
requesting peer.
For _getdir_ transactions, the METADATA's bits 8 and 9 in the Flags
field specify both the width of the offset and length fields used
within the transfers DATA and HOLETOFILL packets, and also the width
of file Size fields within Directory Entries in the interpreted
_getdir_ results. These Flags bits are set to the minimum of the
file-sender's locally-supported maximum width and the advertised
maximum width within the REQUEST packet, and any file system entries
that would normally be contained in the results, but that have sizes
greater than this width can convey, MUST be filtered out.
5.1.3. The _delete_ Transaction
1. A peer sends a REQUEST packet with the bit set indicating that it
is a deletion request and the path to be deleted is filled into
the File Path field. The File Path MUST be filled in for
_delete_ transactions, unlike for _get_ transactions.
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2. The other peer replies with a feedback HOLETOFILL packet having a
Status code that indicates whether the deletion was granted and
occurred successfully (indicated by the 0x00 Status field in a
success HOLETOFILL), or whether some error occurred (indicated by
the non-zero Status field in a failure HOLETOFILL). This
HOLETOFILL packet MUST have no Holes and 16-bit width zero-valued
Cumulative Acknowledgement and In-Response-To fields.
5.1.4. The _put_ Transaction
A _put_ transaction proceeds exactly as a _get_, except the file-
sender and file-receiver roles are exchanged between peers, and no
REQUEST packet is ever sent. The file-sending end senses that the
transaction is in progress when it receives METADATA or DATA packets
for which it has no knowledge of the Id field. If the file-receiver
decides that it will store and handle this request (at least
provisionally), then it MUST sends a voluntary (ie, not requested)
success HOLETOFILL packet to the file-sender. Otherwise, it sends a
failure HOLETOFILL packet. After sending a failure HOLETOFILL
packet, it may ignore future packets with the same Id field from the
file-sender, but it should, at a low rate, periodically regenerate
the failure HOLETOFILL packet if the flow of packets does not stop.
5.2. Beacons
Sending BEACON packets is not needed in any of the transactions
discussed in this specification, but optional BEACONs can provide
useful information in many situations. If a node periodically
generates BEACON packets, then it should do so at a low rate which
does not significantly affect in-progress data transfers.
A node that supports multiple versions of Saratoga (e.g. version 1
from this specification along with the older version 0), MAY send
multiple BEACON packets showing different version numbers. The
version number in a single BEACON should not be used to infer the
larger set of protocol versions that a peer is compatible with.
If a node receives BEACONs from a peer, then it SHOULD NOT attempt to
start any _get_, _getdir_, or _delete_ transactions with that peer if
bit 14 is not set in the latest received BEACONs. Likewise, if
received BEACONs from a peer do not have bit 15 set, then _put_
transactions SHOULD NOT be attempted to that peer. Unlike the
capabilities bits which prevent certain types of transactions from
being attempted, the willingness bits are advisory, and transactions
MAY be attempted even if the node is not advertising a willingness,
as long as it advertises a capability.
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5.3. Upper-Layer Interface
No particular interface functionality is required in implementations
of this specification. The means and degree of access to Saratoga
configuration settings and transaction control that is offered to
upper layers is completely implementation-dependent. In general, it
is expected that upper layers (or users) can set timeout values for
transaction requests and for inactivity periods during the
transaction, on a per-peer or per-transaction basis, but in some
implementations where the Saratoga code is restricted to run only
over certain interfaces with well-understood operational latency
bounds, then these timers MAY be hard-coded.
We expect that Saratoga instances will often work in conjunction with
DTN bundle agents to fill the role of a convergence-layer adapter
between bundle agents connected via point-to-point links. Saratoga
implementations designed to work this way should have a way of
notifying bundle agents when they receive BEACONs from other nodes,
and when transfers have completed. In order for custody transfer to
function properly, notifications between the Saratoga instances and
bundle agents on both sides of a fully-successful bundle file
transfer is required.
5.4. Inactivity Timer
In order to determine the liveliness of a transaction, Saratoga nodes
may implement an inactivity timer for each peer they are expecting to
see packets from. For each packet received from a peer, its
associated inactivity timer is reset. If no packets are received for
some amount of time, and the inactivity timer expires, this serves as
a signal to the node that it should abort (and optionally retry) any
sessions that were in progress with the peer. Information from the
link interface (i.e. link down) can override this timer for point-to-
point links.
The actual length of time that the inactivity timer runs for is a
matter of both implementation and deployment situation. Relatively
short timers (on the order of several round-trip times) allow nodes
to quickly react to loss of contact, while longer timers allow for
transaction robustness in the presence of transient link problems.
This document deliberately does not specify a particular inactivity
timer value nor any rules for setting the inactivity timer, because
the protocol is intended to be used in both long- and short-delay
regimes.
Specifically, the inactivity timer is started on sending REQUEST or
HOLETOFILL packets. When sending packets not expected to elicit
responses (BEACON, METADATA, or DATA without acknowledgement
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requests), there is no point to starting the local inactivity timer.
For normal file (non-bundle file) transfers, there are simple rules
for handling expiration of the inactivity timer during a _get_ or
_put_ transaction. The file-sender should terminate the transaction
state and cease to send DATA or METADATA packets. The file-receiver
should stop sending HOLETOFILL packets, and MAY choose to store the
file in some cache location so that the transfer can be recovered.
This is possible by waiting for an opportunity to re-attempt the
transaction and immediately sending a HOLETOFILL that only lists the
parts of the file not yet received if the transaction is granted. In
any case, a partially-received file MUST NOT be handled in any way
that would allow another application to think it is complete.
For bundle file transfers, the local bundle agent might be interacted
with in order to perform a reactive fragmentation of the bundle whose
transfer was interrupted by expiration of the inactivity timer.
(Reactive fragmentation is discussed in [RFC4838].) For custody
transfer, there are some complications to making this reactive
fragmentation work properly, and the details required to implement
this functionality are left out of this specification until more
experience with reactive fragmentation in general is obtained.
6. Security Considerations
Saratoga does not in itself provide any services for authentication
of session requests or data, nor does it in itself provide any
privacy or access control for data files transferred. These issues
may be addressed within an implementation or deployment in several
ways that do not affect the file transfer protocol itself. For
instance, IPsec may be used to protect Saratoga implementations from
forged packets, to provide privacy, or to authenticate the identity
of a peer, for instance. Other implementation-specific or
configuration-specific mechanisms and policies might also be employed
for authentication and authorization of requests. Protection of file
data and meta-data can also be provided by a higher level file
encryption facility. Basic security practices like not accepting
paths with "..", not following symbolic links, and using a chroot()
system call, among others, should also be considered within an
implementation.
Security in DTNs is in general considered an open issue. If a
framework of techniques for handling security in DTN scenarios
emerges, Saratoga might be adapted to conform to this.
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7. IANA Considerations
Saratoga runs over UDP, so a single UDP port number allocation is
needed. Some implementations have used port 4000, but there is
nothing special or binding about this value.
8. Acknowledgements
Developing and deploying the on-orbit IP infrastructure of the
Disaster Monitoring Constellation, in which Saratoga has proven
useful, has taken the efforts of hundreds of people over more than a
decade. We thank them all.
Work on this document at NASA's Glenn Research Center was funded by
NASA's Earth Science Technology Office (ESTO).
We thank Stewart Bryant for his review comments.
9. A Note on Naming
Saratoga is named for the USS Saratoga (CV-3), the aircraft carrier
sunk at Bikini Atoll and now a popular diving site. The philosophy
behind the protocol and its use described here can be summarized as
Saratoga Carries Upper Bundles Adequately, or SCUBA.
10. References
10.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
10.2. Informative References
[Hogie05] Hogie, K., Criscuolo, E., and R. Parise, "Using Standard
Internet Protocols and Applications in Space", Computer
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Networks Special Issue on Interplanetary Internet, vol.
47, no. 5, pp. 603-650, April 2005.
[I-D.irtf-dtnrg-bundle-spec]
Scott, K. and S. Burleigh, "Bundle Protocol
Specification", draft-irtf-dtnrg-bundle-spec-09 (work in
progress), April 2007.
[I-D.irtf-dtnrg-ltp-motivation]
Burleigh, S., "Licklider Transmission Protocol -
Motivation", draft-irtf-dtnrg-ltp-motivation-04 (work in
progress), April 2007.
[Jackson04]
Jackson, C., "Saratoga File Transfer Protocol", SSTL
Internal Document , 2004.
[RFC0959] Postel, J. and J. Reynolds, "File Transfer Protocol",
STD 9, RFC 959, October 1985.
[RFC3366] Fairhurst, G. and L. Wood, "Advice to link designers on
link Automatic Repeat reQuest (ARQ)", BCP 62, RFC 3366,
August 2002.
[RFC4838] Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst,
R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant
Networking Architecture", RFC 4838, April 2007.
[Wood07a] Wood, L., Ivancic, W., Hodgson, D., Miller, E., Conner,
B., Lynch, S., Jackson, C., da Silva Curiel, A., Cooke,
D., Shell, D., Walke, J., and D. Stewart, "Using Internet
Nodes and Routers Onboard Satellites", International
Journal of Satellite Communications and Networking Special
Issue on Space Networks, vol. 25, no. 2, pp. 195-216,
March/April 2007.
[Wood07b] Wood, L., Eddy, W., Ivancic, W., Miller, E., McKim, J.,
and C. Jackson, "Saratoga: a Delay-Tolerant Networking
convergence layer with efficient link utilization",
submitted to the International Workshop on Satellite and
Space Communications (IWSSC '07) for review,
September 2007.
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Authors' Addresses
Lloyd Wood
Cisco Systems
11 New Square Park, Bedfont Lakes
Feltham, Middlesex TW14 8HA
United Kingdom
Phone: +44-20-8824-4236
Email: lwood@cisco.com
Jim McKim
RS Information Systems
NASA Glenn Research Center
21000 Brookpark Road, MS 142-1
Cleveland, OH 44135
USA
Phone: +1-216-433-6536
Email: James.H.McKim@grc.nasa.gov
Wesley M. Eddy
Verizon Federal Network Systems
NASA Glenn Research Center
21000 Brookpark Road, MS 54-5
Cleveland, OH 44135
USA
Phone: +1-216-433-6682
Email: weddy@grc.nasa.gov
Will Ivancic
NASA Glenn Research Center
21000 Brookpark Road, MS 54-5
Cleveland, OH 44135
USA
Phone: +1-216-433-3494
Email: William.D.Ivancic@grc.nasa.gov
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Chris Jackson
Surrey Satellite Technology Ltd
Tycho House
Surrey Space Centre
20 Stephenson Road
Guildford, Surrey GU2 7YE
United Kingdom
Phone: +44-1483-803-803
Email: C.Jackson@sstl.co.uk
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